Static Exitation System -Project Report

ABSTRACTSTATIC EXCITATION SYSTEM FOR SYNCHRONOUS GENERATOR Excitation systems supply and regulate the amount of D.C. current required by generator field windings and include all power regulating control and protective elements. The static excitation system is the most commonly used excitation system for hydro generators. The excitation equipment comprises of excitation transformer, thyristors, field discharge circuit breaker with discharge resistor, field flashing circuit, automatic voltage regulator and protection and control devices and accessories. The main device for static excitation is thyristor.

STATIC EXCITATION EQUIPMENT1 Division Of Electrical Engineering, SOE, CUSAT.

The static excitation equipment consists of regulation cubicales, field flashing and field breaker cubicles, thyristor cubicles and transformer cubicles. The static excitation equipment draws its power from the generator terminal itself through a step down excitation transformer of 850 KVA rating housed inside a cubicle. The initial voltage is build up at the generator terminals by field flashing from the auxiliary ac/dc supply(station supply).The ac input supply of all electronic power supplies are given from the secondary of the excitation transformer through the suitable intermediate transformers. The secondary of the excitation transformer feeds the Thyristor Bridge which consists of parallel connected bridges to meet the field current requirement of the machine. The dc output of the Thyristor Bridge is fed to the generator field through discharge resistance in the field circuit enables faster suppression of stored energy in the field. EXCITATION TRANSFORMER Dry type cast coil excitation transformers are used in static excitation system to feed excitation power to the generator. The primary of the transformer is connected to the generator terminal. The secondary of the transformer is connected to thyristor convertor, which provides controlled excitation to the generator field. POWER RECTIFIER A 3 phase 6 pulse fully controlled thyristor bridge with fuse RC circuit gate circuit and decoupling reactors are provided with conduction monitoring unit to indicate with the help of LEDs the non conduction of any thyristor in the bridge. VOLTAGE BUILD UP As the residual voltage of the generator usually very low, voltage build up circuit is necessary to built up the generator voltage to a level so as the electronic power supplies function. Voltage build (field flashing) can be done either with the help of station battery supply through a dropping resistor and blocking diode stack or station auxiliary supply through a step down transformer and a diode bridge. At 30% of rated generator voltage pulses through the thyristor in the main circuit are released and they take over the build process at about 40% of rated generator voltage. for checking healthiness of the main circuit, the field flashing is kept in circuit upto 70 % of the rated generator voltage after which the field flashing circuit is automatically disconnected LT AUXILIARY SUPPLY2 Division Of Electrical Engineering, SOE, CUSAT.

In the power house there are two 11 KV external supplies available. These feeders are terminated in the 4 pole structure located by the side of the power house. One of the 11KV feeders is kept as external stand by supply which is stable and higher in voltage. This 11KV supply is step down by a 1MVA transformer and the transformer family is controlled by a 11KV outdoor type vacuum circuit breaker. In addition to this external LT supply, a 250KVA diesel engine driven power plant is available. The LT power supply from the diesel power plant is also brought to the LT min switch gear panel through LT breakers normally when the machines are in service. The LT supply is from any of the unit auxiliary. If one supply is not available start the diesel supply and avail the supply.

CONTENTS3 Division Of Electrical Engineering, SOE, CUSAT.

CHAPTER1 INTRODUCTION ........................................................................ 1 CHAPTER 2 STATIC EXCITATION SYSTEM ............................................ 2-16 2.1 Introduction to SEE .......................................................................... 2 2.1.1 Types of Excitation systems ................................................ 2 2.1.2 Rectifier Transformers ........................................................ 4 2.1.3 SCR Output Stage ............................................................... 4 2.1.4 Excitation Start up and Field Discharge Equipment ........... 6 2.1.5 Control Electronics .............................................................. 6 2.1.6 Power Supply ...................................................................... 9 2.1.7 Protections ........................................................................... 10 2.2 Significance of Machine Capability Diagram ................................... 11 2.2.1 Response ............................................................................. 11 2.2.2 Reliability ............................................................................ 11 2.2.3 Field Discharge ................................................................... 12 2.2.4 Procedure for Construction of Capability Diagrams ........... 12 2.2.5 Usefulness of Capability Diagram for Excitation Control System ................................................................................ 16 CHAPTER 3 PERFORMANCE AND CHARACTERISTICS OF SEE.........17-24 3.1Ceiling Voltage .................................................................................. 17 3.2 Response ........................................................................................... 18 3.3 Steady State Equipment ................................................................... 21 3.4 Other Specifications ......................................................................... 21 3.5 Transient and Dynamic Stability Limit ............................................ 21 3.6 Effect of Excitation Systems on Transient Stability ......................... 23 CHAPTER 4 Excitation Transformer ............................................................. 25 -29 4.1 Introduction ......................................................................................... 25 4.2 Standards .............................................................................................. 26 4.3 Voltage and Power Rating ................................................................... 26 4.4 Enclosure and Cooling ........................................................................ 26 4.5 Salient Features .................................................................................... 27 4.6 Operating Conditions ........................................................................... 29 CHAPTER 5 Field Breakers and Field Suppression ........................................ 30-32 5.1Introduction to Field Breaker and Field Suppression ........................... 30

5.2 Essential Features ............................................................................ 30 5.3 Basic Components ............................................................................ 31 4 Division Of Electrical Engineering, SOE, CUSAT.

5.4 Standard ............................................................................................ 31 5.5 Field Suppression Resistor ............................................................. 31 5.6 Selection of Field Suppression Breaker........................................... 32 5.7 Test on Field Breaker ...................................................................... 32 CHAPTER 6 Thyrister Characteristic and Application in SEE ..................... 33-42 6.1Introduction ...................................................................................... 33 6.2 System Description ........................................................................... 33 6.3 Theory of Device .............................................................................. 34 6.4 Selection Procedure of SCR Bridges for SEE .................................. 37 6.5 Parallel Operation ............................................................................ 40 6.6 Conclusion ........................................................................................ 41 CHAPTER 7 DESCRIPTION OF CONTROL SYSTEM ............................... 42 -70 7.1.1 Automatic voltage regulator ................................................ 42 7.1.2 Design specification of automatic voltage regulator ........... 44 7.1.3 Circuit design of the AVR for the synchronous Generation ....... 48 7.1.4 Explanation of the aforesaid features follows ..................... 49 7.1.5 Voltage proportional to frequency signal ............................ 54 7.2 Limiters in Static Excitation System ................................................ 55 7.2.1Limit Controllers .................................................................. 55 7.2.2 Parameters for Limitation.................................................... 55 7.2.3 Mechanism of Limiter Intervention .................................... 56 7.2.4 Power Diagram of the Generator and the Range of Influence of the Limiter Controllers................................57 7.2.5 Current Limiter .................................................................... 64 7.3 Digital Voltage Regulator ............................................................... 65 7.3.1Benefits of Microprocessors ................................................. 65 7.3.2 The Control of Synchronous Machines ............................... 66 7.3.3 Generation of Firing Pulses ................................................. 67 7.3.4 Excitation Monitoring ......................................................... 68 7.3.5 Excitation Protection ........................................................... 68 7.3.6 Control ................................................................................. 68 7.3.7 Micro-terminal .................................................................... 68 7.3.8 Conclusion ........................................................................... 69 CHAPTER 8 RENOVATIONS AND MODERNISATION OF EXCITATION SYSTEMS IN POWER STATIONS ................ 70-79 8.1 Introduction ...................................................................................... 70 8.2 Existing Systems .............................................................................. 70 5 Division Of Electrical Engineering, SOE, CUSAT.

8.3 Consideration and Selection of New Systems of Excitation ........... 71 8.4 Energy Savings ................................................................................. 75 8.5 Advantage of SEE for Renovation Projects ..................................... 75 8.6 Energy saving/Conservation- Contribution of SEE .......................... 77 8.6.1 62.5MW Rated TG Sets ...................................................... 77 8.6.2 110/129MW Rated TG Sets ................................................ 78 8.6.3 210MW Rated TG Sets ....................................................... 78 8.7 Comparison of Features of Static Excitation System and Brushless Excitation System ........................................................... 79 CHAPTER 9 TESTING AND MAINTAINANCE ........................................ 80 -101 9.1 Functional Testing of SEE ................................................................ 80 9.1.1Introduction .......................................................................... 80 9.1.2Functional Testing ............................................................... 80 9.2 Maintenance Aspect of Static Excitation Equipment ....................... 87 9.3 Operating and Maintenance Instructions of Field Breaker ............... 92 9.3.1 Unelec-Field Contactors ..................................................... 93 9.3.2 Inspection and Setting ......................................................... 93 9.3.3 Recommendations ............................................................... 94 9.3.4 Inspection and Maintenance ................................................. 94 9.3.5 Arc Chutes ........................................................................... 95 9.3.6 Operation ............................................................................. 95 9.3.7 Discharge Contact ............................................................... 95 9.3.8 Commissioning Test ............................................................ 96 9.3.9 OB4 Field Circuit Breaker .................................................. 96 CONCLUSION .................................................................................................... 100 REFERENCES ............................................................................... ...................... 101

CHAPTER 1

INTRODUCTION

6 Division Of Electrical Engineering, SOE, CUSAT.

Excitation control systems have been undergoing improvements/modifications keeping in step with the requirements of larger and larger generating units and complexity of interconnections in power systems. Various types of excitation systems which are in operation are: DC excitation using DC exciter and voltage regulators. AC excitation system using AC exciter, static rectifier & voltage regulator. Brushless excitation systems using AC exciter, rotating diode rectifier system and voltage regulator.

Static excitation system. All these are in operation and as such cannot be called obsolete. But for most

present day application in power plants, static excitation systems using shunt connected, completely static with thyristor control system is being called and here we presently confine to static excitation system. We know, the electrical power has to be produced with some fixed specifications. The parameters related to power are voltage, frequency and phase. These specifications have to be maintained within fixed limitations. This can be also called maintenance of power quality. The Static Excitation Equipment (SEE) is one of the important devices for this job. It senses the change in voltage and varies the excitation of the alternator to keep the voltage within fixed limit. Modern SEE also has many addition and features incorporated in it.


CHAPTER 2

STATIC EXCITATION EQUIPMENT2.1 Introduction to Static Excitation Equipment At present various types of excitation systems, such as, conventional DC. High frequency AC, static & Business are being adopted in India and abroad. The conventional DC exciter was the un-challenged source of Generator Excitation for nearly fifty years till the rating of turbo-generators reached around 100 MW. In the last two decades, alternative arrangements have been widely adopted because of limitations of the DC exciters. With increase in Generator ratings, it is no longer enough to consider the exciter as earlier. Instead, the performance of the whole excitation system including the automatic voltage regulator and the response of the main generator has to be considered. Techno economic considerations, grid requirements, reliability and easy maintenance have become prime considerations. 2.1.1 Types of Excitation Systems a) Conventional DC: The earliest AC turbine generators obtained their excitation supply from the power station direct current distribution system. Each machine had a rheostat in series with its field winding to permit adjustment of the Terminal Voltage and Reactive load. This method was suitable for machines 'which needed small Field power and low internal Reactance. As generator sizes increased excitation power requirements also increased and it became increasingly desirable for each unit to be self sufficient for excitation and thus the shaft driven DC exciter was introduced. b) AC (High Frequency) Excitation System: This system was developed to avoid commutator and Brush Gear assembly. In this system, a shaft driven AC pilot exciter, which has a rotating permanent magnetic field and a stationary armature, feeds the DC field current of the main high frequency AC exciter through controlled Rectifiers. The high Frequency output of


the stationary armature is rectified by stationary diodes and fed via slip-rings to the field of the main turbo generator. A response ratio of about two can be achieved. c) Brushless system: Supply of high current by means of Slip Rings involves considerable operational problems and it requires suitable design of Slip Rings and Brush Gear. In brushless excitation system Diode Rectifiers are mounted on the Generator shaft and their output is directly connected to the field of the Alternator thus eliminating Brushes and Slip Rings. This arrangement necessitates the use of rotating armature and stationary field system for the main AC exciter. The voltage regulator final stage takes the form of a Thyristor Bridge controlling the Field of the main AC Exciter which is fed from PMG on the same shaft. The response ratio of brushless Excitation system is normally above two. d) Static System: In order to maintain system stability it is necessary to have fast Excitation System for large Synchronous Machines which means the Field Current must be adjusted extremely fast to changing operational conditions, besides maintaining the Field Current and steady state stability limits. It is because of these reasons the Static Excitation System is preferred to conventional Excitation Systems. In this system, the AC power is tapped off from the Generator terminal, stepped down and rectified by fully controlled Thyristor Bridges and then fed to the generator field, thereby controlling the generator voltage output. A high control speed is achieved by using an inertia free control and power electronic system. Any deviation in the generator terminal voltage is sensed by an error detector and causes the voltage regulator to advance or retard the firing angle of the thyristors thereby controlling the field excitation of the alternator. Fig.2.1 shows a block diagram for a static excitation system. Static excitation system can be designed without any difficulty to achieve high response ratio which is required by the system. The response ratio of the order of 3 to S can be achieved by this system. This equipment controls the, generator terminal voltage and hence the reactive load flow by adjusting the excitation current. The rotating exciter is dispensed with and the silicon controlled rectifiers (S%'-'R'S) are used which directly feed the field of the Alternator.9 Division Of Electrical Engineering, SOE, CUSAT.

The S.E:E. consists of: 1. Rectifier Transformer 2. SCR output stage. 3. Excitation start up & field discharge equipment 4. Regulator and operational control circuits. 2.1.2 Rectifier Transformer: The excitation power is taken from generator' output and fed through the excitation, (rectifier) transformer which steps down to the required voltage, for the SCR -bridge and then fed through the field breaker to the generator field. The rectifier transformer used in, the SEE should have high reliability a5 failure of this will cause shutdown of power station. Dry type cast coil transformers are suitable for static excitation applications. The transformer is selected such that it supplies rated excitation current at rated voltage continuously and is capable of supplying ceiling current at the ceiling excitation for a short period of ten seconds. . 2.1.3 SCR Output Stage The SCR output stage consists of a suitable number of bridges connected in parallel., Each Thyristor bridge comprises of six thyristars, working as a six pulse fully controlled' bridge. Current carrying capability of each bridge depends on the rating of individual thyristor. Thyristor are C1BSt7ned such that their-unction temperature rise is well within its specified rating. By changing the firing angle of the thyristors, variable output is obtained. Each bridge is controlled by one final stage and is cooled by fan. The bridges are equipped with protection devices and failure of the bridgecauses alarm. If there is failure of one more thyristor bridge the excitation current will be limited to a predetermined value lesser than the normal current. However, failure of the third bridge results in tripping and rapid de-excitation of the generator.


Figure 2.1 Block Diagram of Static Excitation Equipment

2.1.4 Excitation Start up and Field Discharge Equipment:


For the initial built-up of the generator voltage, a field flashing equipment is required. The rating of this equipment depends on the no-load excitation requirement and field time constant of the generator. From the reliability point of view, provisions for both the AC & DC field flashing are made. The field breaker is selected such that it carries the full load excitation current continuously and also it breaks the max. field current when the three phase short circuit occurs at the generator terminals. The field discharge resistor is normally of non-linear type for medium and large machines i.e. voltage dependent resistor. To protect the field winding of the generator against over voltages, an over voltage protector along with a current limiting resistor is used to limit the over voltage across the field winding. The voltage level at which OVP should operate is selected based on insulation level of field winding of the generator. 2.1.5 Control Electronics Regulator is the heart of the system. This regulates the generator voltage by controlling the firing pulses to the thyristors. a) Error Detector & Amplifier: The venerator terminals voltage is stepped down by a three phase PT and fed to the AVR. The a.c. input thus obtained is rectified, filtered and compared against a highly stabilized reference value and the difference is amplified in different stages of amplification. The AVR is designed with highly stable elements so that variation in ambient temperature does not cause any drift or change in the output level. Three CTs sensing the output current of the generator feed proportional current across variable resistors in the AVR. The voltage thus obtained across the resistors, can be added vectorially either for compounding or for transformer drop compensation.

b) Grid-control Unit:


The output of the AVR is fed to a grid control unit, it gets its synchronous a.c. reference through a filter circuit and generates a double pulse spaced 600 el. apart whose position depends on the output of the AVR, i.e. the pulse position varies continuously as a function of the control voltage. Two relays are provided by energizing which, the pulses can be either blocked completely or shifted to inverter mode of operation. c) Pulse Amplifier: The pulse output of the "Grid control unit" is amplified further at an intermediate stage amplification. This is also known as pulse intermediate stage. The unit sets d.c. power supply, from a AC/DC or DC/DC converter unit built in relay is provided which can be used for blocking the 6 pulse channels. In a two channel system (like Auto and Manual), the changeover is effected by energizing/deenergizing the ralay. d) Pulse Final Stage: This unit receives input pulses from the pulse-amplifier and transmits them through pulse transformers to the gates of the thyristors. A power supply, unit provides the required DC supply to the final pulse amplifier. Each Thyristor bridge has its own final pulse stage. Therefore, even if a thyristor bridge fails with. Its final pulse stage, the remaining thyristor bridges can continue to cater to full load requirement . e) Manual Control Channel: A separate manual control channel is provided where the controlling DC signal is taken from a stabilized DC voltage through a D/A/ converter. The DC signal is fed to a separate grid control unit whose output pulses after being amplified at an intermediate stage can be fed to the final pulse stage. When one channel is working, generating the required pulses, the other remains blocked. Therefore, a changeover from "Auto" to "Manual" control or vice versa is affected by blocking or releasing the pulses of the corresponding intermediate stage. A Pulse supervision unit detects spurious pulses or loss of pulses at the Busbar and transfers control from Automatic channel to Manual channel13 Division Of Electrical Engineering, SOE, CUSAT.

of

the

machine

and

thereby

ensure

(n-l)

operation.

f) Flow up Unit: To ensure a smooth ensure a smooth changeover from Àuto' to 'Manual' control; it is necessary that the position of the pulses on both channels should be identical. A pulse comparison unit detects any differences in the position of the pulses and with the help of a follow-up unit actuates the D/A converter on the 'Manual' channel to operate in a direction so as to eliminate the difference. However, while transferring control from `Manual' to Àuto' mode any difference in the two control levels can be visually checked on a balance meter and adjusted to obtain null before change over. g) Limit Controllers: When a generator is running in parallel with the power network; it is essential to maintain it in synchronism without exceeding the rating of the machine and without the protection system tripping. The automatic voltage regulator by itself cannot ensure this. It is necessary to supplement the basic voltage regulator by limiters to limit over-excitation and under- excitation. Limiters do not replace the protection system but only prevent the protection system from tripping unnecessarily under extreme transient conditions. The AVR also has a built-in frequency dependent circuit so that when the machine is running below the rated frequency the regulated voltage should be proportional to frequency. With the help of a potentiometer provided in the AVR, the circuit can he made to respond proportionally to voltage above a certain frequency (fo) and proportional to a voltage below this frequency. The range of adjustment of this cut-off frequency (fo) lies between 40 and 60 Hz. The static excitation system is equipped with three limiters which act in conjunction with the AVR. h) Rotor Current Limiter: This avoids thermal overloading of the rotor winding and is provided to protect the generator rotor against excessively long duration overloads. The ceiling excitation is limited to a predetermined limit and is allowed to flow for a time which is dependent upon the rate of rise of field current before being limited to the thermal limit value.


i) Rotor angle limiter: The unit comprehends the rotor angle and a DC signal proportional to the load or rotor angle by means of a simple analog circuit is formed. When the rotor angle exceeds the limit with the reference potentiometer the excitation is increased immediately to reduce the load angle to the limit value. The rotor angle limiter takes over control by de-coupling the output of the AVR. j) Stator Current limiter: This avoids thermal overloading of the stator windings. Stator current limiter is provided to protect the generator against long duration of large stator currents. For excessive inductive current it acts over the AVR after a certain time lag and decreases the excitation current to limit the inductive current to the limit value. But for excessive capacitive current it acts on the AVR without time delay to increase the excitation and thereby reduce the capacitive loading. This is necessary as there is a risk for the machine falling out of step in the under excited mode of operation. k) Slip Stabilizing Units: The slip stabilizing unit is used for the suppression of rotor oscillations of the alternator through the additional influence of excitation. The slips as well as acceleration needed for the stabilization are derived from active power delivered by the alternator. Both the signals, which are correspondingly amplified and summed up, influence the excitation of the synchronous machine through AVR in a manner as to suppress the Rotor oscillation.

2.1.6 Power Supply: The voltage regulating equipment needs AC supply 384V, 3 phase for its power supply units which is derived from the secondary side of the rectifier transformer through an auxiliary transformer. This voltage is reduced to different levels required for the power packs by means of multi-winding transformers. Power supplies have# been dealt with in detail elsewhere in this volume. A separate transformer supplies the synchronous voltage 3 X380 V for the filter circuit of each channel. The station aux. Supply 3 phase 415V can be temporarily


connected through an step down transformer for testing purpose with the help of a test./service switch. The supply for the thyristor Bridge fan is taken from an independent transformer which gets its input supply from the secondary of the excitation transformer. The control & protection relays need 48V / 24VDC which are delivered from the station battery, and through the DC/DC converters, which are internally protected against overload. 2.1.7 Protections: The following protections are provided in the static Excitation Equipment. Rectifier transformer over current instantaneous and delayed. Rectifier transformer over Temperature Rotor Over-Voltage

Against over-current thyristors dv/dt protection of SCR by snubber net works Rotor earth fault.

Loss of control voltage (48 V & 34V)

2.2 Significance of Machine Capability Diagram:Capability diagrams of Generators give the safe operating regimes and limitations. This is of great help to the operating Engineers to ensure operations of the machines accordingly.16 Division Of Electrical Engineering, SOE, CUSAT.

Their information particularly for limiting zones of operations are useful in setting the various limiters of Automatic Voltage Regulator. One typical procedure for the construction of capability diagram is given at the end of this write-up. Operational requirements of excitation system essentially call for a fast response particularly High Initial Response Excitation System, High degree of Reliability and also suitable arrangement for field discharge. 2.2.1 Response: The fastness of action of an Excitation system is measured/expressed by term "Response Ratio of the Excitation system". The original definition of this by is measuring the rise of excitation of exciter and AVR in Indirect Excitation Systems give the "Excitation System Response Ration". With the ADVENT of very fast acting and high initial response excitation systems, like STATIC systems the same definition is remained for this. But supplemented by additional terms i.e. "HIGH INITIAL RESPONSE SYSTEMS" as given in IEEE STDS-421- as those systems which attain 95% of the ceiling voltage level in 0.1 second or less. Static Excitation Systems comes in this category which thus greatly helps for power system stability considerations. Typical Response time for Static Excitation Equipment is twenty milli-Seconds. 2.2.2 Reliability: For Power Systems application, Reliability is a very important criterion. To ensure this, components are carefully selected, liberal ratings wherever required are used and redundancies built in. For example in indirect excitation system thyristor bridges in Manual & Auto Channels are separate. In Static Excitation Equipment although generally common Thyristor Bridge is used, (n-1) operation is ensured. Even with one of bridges out of operation, full load requirement can be met by balance bridges in parallel. Wherever specified/required by customers, (2 x 100) % bridges are also given. To ensure reliability, various tests/checks are conducted on components, sub-assemblies and functional tests on completely assembled equipment. Temperature cycling for the modules is done in the factory to debug any possible failures due to this. All Logic Cards undergo temperature cycling tests to detect infant failure if any Type tests on Electronic Modules covering High17 Division Of Electrical Engineering, SOE, CUSAT.

frequency and impulse withstand capability were conducted successfully. Site failures are analyzed and improvements to enhance the Reliability of our equipment based on the feed backs are being done.

2.2.3 Field Discharge: In the event of a fault/short circuit say at the terminals of the Generator, fault currents of great magnitude flow, Even when the Generator is isolated from the systems due to action of protective equipments, grid contribution stops, while the Generator continues to feed the fault until its current stops and stored energy in the fie: decays. This causes great damage at the fault point and to the machine particularly in the case of internal faults on fairly large rating utility sets used in the power stations. Field discharge greatly helps to limit the damages. In Excitation Equipments so far supplied b EDN, provision for field-breaker and discharge resistance are made for opening the field circuit and also for quick suppression of stored energy sets: Non-linear field discharge resistance is also used in certain cases, which helps in faster field suppression.

2.2.4 Procedure for Construction of Capability Diagram: Let us take an example of a 100 MW Turbo-Generator of 0.80p.f.(nominal) rating and having a SCR of 0.60. Choosing suitable scale, MW values are marked on Y axis and MVAR values on x-axis. Refer to Fig.2.2 which has been drawn on per unit basis and hence bases must be defined for interpreting actual values. It is usual to define the rated MVA of the machine as Base MVA (i.e. MVA) in which case rated MW are 0.8 MVA. In this case MVA = 125 and rated MW = (0.8 x 125)100MW. The other base unit to define is the per unit excitation and this is usually taken as rotor AMPS to give rated terminal voltage on open-circuit on Air- Gap line. To obtain actual values, the p.u figures from capability!.y diagram must be multiplied by the base values just given. The various MWIMVAR values and the excitation current (Rotor Amps) can also be marked directly for the use of operators. It should he noted that the diagram scaling is only correct for rated machine terminal voltage and that all values must be appropriately adjusted for different, values of terminal voltage i.e., they must be multiplied by V2, so that if the terminal18 Division Of Electrical Engineering, SOE, CUSAT.

voltage is say 90% of normal, then all scaling would have to be multiplied by (0.9)2 = 0.81, although excitation scaling would remain the same. It is obviously undesirable to operate the machine up to theoretical stability limits. Operators have to be informed through this diagram, safe limits for operation to allow for various unpredictable changes such as sudden power increase, a draft in Bus-Bar voltage due to lines or plant tripping etc. It is usual to relate this safety factor to an increase in power demand with no corresponding increase in excitation. The percentage of the power increase used in this way defines the shape and the position of the "Practical Stability Limit line". Referring back to the example stated above, let us assume that we want to have additional 12.5 percent (or 1.125 p.u.) power margin. This depends on the size of the unit and operating practices. On x-axis mark point A such that OA = (MVAb x SCR) i.e. in this case = (125 x 0.6) = 75 MVAR i.e (0.6 p.u From the point A' the dotted line as denotes the theoretical stability line. Horizontal lines parallel to x-axis denote the various MW 'constant power) lines. Power intervals P equal to the required safety margin in this case 0.125 p.u, of rated power i.e, (0.8 x 0.125) = 0.10 p.u. of MVA are marked on the theoretical stability line. As for the loads of Q, 0.20, 0.40, 0.60 and 0.80 p.u. MVAb i.e., at points e,d,c,b and a. With radii Aa, Ab, Ad and Ae arcs of circles are drawn with A as centre to cut the 0.8, 0.6, 0.4, 0.2 and zero power lines. These intercepts are then joined by a continuous curve F ..... G. This will then be the "Practical Stability Line" for a 12.5% power margin. The reasoning behind this construction can be understood by taking the case of Àa' Arcs. This point 1 (or B) would be working point of the machine at 0.8 p.u. MVAb power with an excitation of Àa' Amps. Since the basis of the safety margin is that there should be provision for increase in power without any change in excitation, the working point 1 would move along arc of radius (fixed excitation) towards theoretical pull-out line, so that it is just sufficient to support 0.9 MVAb i.e., 1.125 p:u. power (Presuming turbine has the capability at a rotor angle 900 The same reasoning of course applies to all other points such as 2,3,4 and S in the diagram. Next, with "0" as centre draw a line OE at an angle of Cos- 10.80 (360) (rated p:f. angle) to the Y-axis to cut the rated MW line (Turbine limit line) at E. Rated MVA is denoted by radius OE.


The line AE represents the CMR excitation required. With À' as centre and ÀE' as radius, draw an arc of a circle ED representing excitation (or Rotor heating) Limit. The diagram FBED now is the "Capability Diagram" of the machine.


Figure 2.2 Capability Diagram

2.2.5 Usefulness of Capability Diagram for Excitation Control Systems: As already mentioned, the information given by the capability diagram regarding full load rotor current (excitation) maximum rotor angle during steady state leading p.f. Zone operation (63 in the diagram given) etc., are essential for proper setting of the various limiters in the excitation control system. In power system operation, the importance and necessity of fast acting and reliable excitation control system is well known Capability diagram give the basic information regarding the limiting zones of the operation so that limiters can be set/commissioned suitably for safe operation of the units.


CHAPTER 3

PERFORMANCE AND CHARACTERISTICS OF STATIC EXCITATION EQUIPMENTINTRODUCTION: The steady state and transient behaviours of a synchronous machine copied to an infinite system must be matched to the desired operating conditions by suitable selection of control functions in the entire excitation system. The basic requirement of a closed loop excitation control system is to hold the terminal voltage of a generator at a predetermined value independent of the change in loading conditions. In addition to this, the excitation system has to contribute , a) Maintenance of stable operation of machine under steady state, transient and dynamic conditions. b) Satisfactory operation with other machines connected in parallel. c) Effective utilization of .machine capabilities without exceeding machine operating limits. In order to understand the performance of excitation system and to achieve above mentioned functions, the following parameters are necessary to be studied. 3.1 Ceiling Voltage: It is the maximum voltage that can be impressed on the field under specified conditions. Ceiling voltage ultimately determines how fast the field current can be changed. For normal disturbances, ceiling condition prevails for a few cycles (Ten Seconds maximum) to either increases or decrease the excitation until the system returns to steady operating state. For static Excitation, the ceiling voltage ranges the following functions also.


from 1.6 to 2.0 times the rated one, which is considered to be adequate for a fast system response.

3.2 Response: Response is defined as the rate of increase or decrease) of the excitation system output voltage seen from the excitation voltage time response curve. The starting point for evaluating the rate of change shall be the initial rated value. This is a rough measure of how fast the exciter output circuit voltage will rise within a 'specified time, when the excitation control is adjusted in the maximum increasing direction. Response ratio defined by ASA, is the numerical values which is obtained when the excitation system response in volts per sec. measured over first 0.5 sec. This applied only for the increasing excitation. As the response is nonlinear, the response ratio is determined in terms of equivalent voltage time area for 0.5 second shown: Area abd = Area acd, by approximation (See fig.3.l enclosed) It is being recognized for some times that the modern fast excitation systems may reach ceiling in 0.1sec (as shown in figure3.2) or less and as such the earlier definition is not applicable. These are called high initial response Excitation system.


Figure 3.1


Figure 3.3


3.3 Steady State Accuracy: It is the degree of correspondence between the controlled variable and the ideal value under specified steady state conditions. The accuracy of the excitations system for changing the field parameters to keep the generator terminal voltage at a fixed level depends on its static gain and time constants. By choosing a higher static gain for the system, the steady state error can be minimized appreciably, and thereby improving the steady state accuracy within +0.5%. This can be reduced further with proper integral control. 3.4 Other Specifications: Excitation system performance could be judged by the exciter voltage VS time characteristics in response to a step change in the generated voltage (see fig.3.2). The factors to be studied for optimum performance care a) c) Overshoot Setting Time For ideal performance, it should have one over shoot and one undershoot with quicker rise time to have a smaller steady state error. Details of each the parameters are not discussed here since the requirement varies from case to case. 3.5 Transient and Dynamic Stability Limit: The succession of excitation control lies upon the extent of meeting the requirement of capability of the machine and thereby giving the dynamic performance of the system. A power system is a constant voltage control. Fast excitation helps during disturbances and contributes to the system stability by allowing the required transfer of power even during the disturbances Due to smaller time constants in the excitation control loop; it is assumed that quick control efforts could be achieved through this. In transient stability the machine is subjected to a severe disturbance for a short time. This result in dip in the terminal voltage and power transfer taking one machine connected to infinite bus the equation of power transfer can be written as:26 Division Of Electrical Engineering, SOE, CUSAT.

b) Rise Timed) Damping ratio

P = (Vt X V)sin/X. From the above equation if Vt is reduced 'P' is reduced by corresponding amount. For maintaining the power transfer 'P' the excitation should be fast acting enough to boost up the fielded to ceiling and thereby holding the terminal voltage `Vi at the desired value. Thus it is advantageous to have higher speed and ceiling values in excitation control circuitry. Similarly after the fault is removed, the reactance `X' suddenly changes thereby causing unbalance conditions due to power swings which in turn need fast corrective action through excitation systems to bring the machine to normal operating conditions. Modern fast and high response excitation, systems helps in two ways by reducing the severity of the machines first swing during transient disturbances and also ensuring that the subsequent swings are smaller than the first one. Thus it helps in increasing the transient stability limit. With a typical static excitation system, ceiling level can be achieved within 20 milliseconds due to which it offers an improved transient stability limits. Following a disturbance, the group of machine operating in the same control group experience smaller oscillations. Moreover the oscillating control group of machines react with each other reinforcing these oscillations. Here the change in excitation may not result in a stable operation (for slow acting exciters) because by the time corrective action being taken by the excitation system (due to the inherent system delay) the oscillating system changes causing separate excitation requirement to be met. Though faster excitation system avoids this problem to certain extent, power system stabilizers as mentioned earlier are employed along with the automatic voltage regulators to damp out the subsequent smaller swings in the system. The stabilizer gain is adjusted to a value depending on the negative damping of the system and other network parameters in Power System to damp out the subsequent smaller Swings in the system. The stabilizer gain is adjusted to a value depending on the negative damping of the system and other network parameters. Power System stabilizer helps to damp out inter area oscillations explained above and also local machine system oscillations.


In addition to the above, limiters are generally built into the excitation system for large generators connected to the grid. This helps to extract maximum operating output i.e., optimal utilization of the machine's capability without jeopardizing it's stability. These limit controllers act on both the lagging and leading side in the capability diagram and set below the operating points of the protective relays. Thus they prevent unnecessary tripping by keeping the system parameters well within the safe limits. The limit controllers do not replace the function of the protective relays. These limiters enhance the stability of the machine, thereby increasing its availability to the network. These cannot dispense the protective relays. 3.6 Effect of Excitation System on Transient Stability: Since the transient stability problems deal with the performance of power. system when subjected to sudden disturbance, sometimes leading to loss of synchronism, it is worthwhile to study the behaviour during the first owing as the period is of very short duration. The major factors influencing the outcome are the machine behaviour and the power network dynamic relations. For this it is assumed that the mechanical power supplied by the prime-mover remains constant during the disturbance. Therefore the effect of excitation control on this type of transient depends on its ability to help the generator to maintain its output power in the above period. The main factors that affect the performance during severe transients are: 1) of The disturbance influence of impact. This includes the type disturbance, its location and duration.

2) The ability of the transmission system to maintain synchronizing forces during the transients. 3) Turbine and generator parameters. These factors mainly affect the first swing transient. The system parameters influencing these factors are: i) The synchronous machine parameters. Of these, the most important are: a) The Inertia constant b) The direct axis transient reactance c) The direct axis open circuit time constant28 Division Of Electrical Engineering, SOE, CUSAT.

d) the

The ability of the excitation systems to hold the flux level of synchronous machine and increase the output during

transients. ii) The transmission system impedances under normal, faulted and post-fault conditions. Here the flexibility of switching out faulted section is important such that the large transfer admittances between synchronous machines are maintained when fault is cleared iii) The protective relaying scheme and equipment. The objective is to detect the fault and isolate the faulty sections quickly with minimum disruption. During transients initiated by a fault, the armature reaction has the tendency to reduce the flux linkage. Hence the type of excitation must be so chosen as to have a fast speed of response and a high ceiling voltage (can be referred to the static type) as an aid to the transient stability. With the help of faster boosting up of the excitation the internal machine flux can be offsetted and consequently the machine output power may be increased, during the first swing. This results in the reduction of accelerating power and thereby, effects improvements of transient performance of the system. The subject is not dealt in greater details as the performance are to be evaluated on a case to case basis after obtaining the transfer functions of each element the static excitation and inter-connected network.

CHAPTER 4

EXCITATION TRANSFORMER29 Division Of Electrical Engineering, SOE, CUSAT.

4.1 Introduction: Rectifier transformers directly connected to the generator terminals and feeding power to the field of the machine via thyristor converters plays an important role in an excitation system and in turn power transmission reliability from this transformer has to be ensured in all respects. Important of rectifier transformer has been realized ever since the mercury arc converters came into existence for important applications like large power drives and excitation systems. Following note gives an idea how gradual development has taken place from oil filled transformers to resin cast coil dry type transformers. Oil and clophen filled transformers are still adopted for large rating, However, in urban areas and thickly populated cities where pollution control is also to be thought of ; certain countries like West Germany have brought out regulation that oil immersed transformers can be used only under special circumstances. Further, use of clophen filled-d transformers has already been banned almost in all advanced countries because of poisons gasses emanating in case of damages. Moreover, there has been constant rise;-,-. price of oil in the international market, resulting in substantial increase in th total ;;rice of transformer and its maintenance. Not only the above reasons but other hazards have led the scientists to think of an alternate design which could gradually replace the oil and clophen filled transformers. Accordingly vacuum impregnated dry type transformers were .taken up for large power and high voltage rating. The results were however not satisfactory because of many limitations like effect of atmosphere, over voltages and the need for proper drying out after long break in service. Therefore, the need was felt to have better alternative and cast resin moulding technique came into existence. The development of cast resin transformer has led to the production of dry insulated type transformers up to 36 KV. These transformers have not only been found comparable to oil filled transformers but also proved their superiority in all respects. 4.2 Standards: So far no separate international standard exists to define the various design of the cast resin dry type transformer. The time has come that a new standard be made like oil cooled transformers. However, the various standards to which the cast


resin coil dry type transformers manufactured are BS 171, IEC 726 ANSI, CS7 and VDE 0532 which cover certain requirements of these. 4.3 Voltage and Power Rating The selection of the secondary voltage of excitation transfor.ner depends upon the field forcing voltage. The primary voltage is the same as that of generator terminal voltage. Current rating is dependent on the maximum continuous current in the field winding. TRANSFORMER CUBICAL RATING KVA-850 Primary Voltage - 11 KV Secondary voltage - 380 V NO. of phases - 3 Frequency 50 HZ Basic impulse level 75 KV Connection DYN 5 Type - CAST RESIN COIL DRY TYPE 4.4 Enclosure and Cooling The enclosures are normally designed to ensure natural air cooling to the transformers. These enclosures are made to 11120, IP21 or IP 23 depending upon the requirement. Forced cooling arrangement provides some increase in rating than that with natural air cooled transformer. Normally this arrangement is switched on during peak load period or in summer to deliver more current from the same transformer. The description that follows compares resin cast coil, dry type transformers with other transformers for various characteristics.

4.5 Salient Features: a. Short Circuit Proof: The dynamic short circuit strength' exceeds by far that of oil immersed transformers as well as that of conventional dry-type transformers. -In the event of short circuit the cast31 Division Of Electrical Engineering, SOE, CUSAT.

resin transformer is not endangered mechanically, and only thermal damage can take place. The high mechanical strength is achieved by casting the coils in epoxy resin with a fibre glass filter to form a compact tubular spool. An insulated thickness of 1-2 mm is quite adequate to withstand the force that occurs during operation. b.High Voltage Load Capacity: In certain applications the rectifier transformer is subjected to intermittent loads like rolling mill, furnace, and traction and also in excitation due to high current increase, is followed by low current demand. It results in the windings to be mechanically stressed to a greater extent. In cast resin transformers all the windings are cast and therefore no difficulties concerning mechanical strength due to repeated overloads. Normally H.V. & L.V. Coils are cast separately; all the forces appearing on one winding can be suitably absorbed by it. The resultant forces between primary and secondary winding can be made to absorb by putting suitable support blocks between the coils and frame. Position of the support blocks can be conveniently designed to reduce the forces to a lower value in contrast to conventional type transformer. Conventional type, wound coil transformers consume a considerable amount of insulation material, like paper which absorbs the moisture therefore the expansion of conductor and coils have to be recompressed after certain periods. The cast coils being homogeneous, the coil structure expands and contracts as a whole and the movement is taken care of by means of an elastic support. Recompression of the coils therefore not required. In synthetic liquid cooled transformer there is a rated temperature jump between winding and cooling liquid of the order of 20 to 25 0C with current density 3 to 4 Almm2. In contrast, in these transformers with class F insulation the allowable temperature rise between coil and air is of the order of 100 C with the same current density. This clearly indicates the heating time constant of cast resin, normally f-10 times, higher than that of oil filled transformers. c.Resistant Against Temperature Fluctuation: The selected insulation material as fibre glass reinforced epoxy resin has not got high tensile and bending strength. Therefore the transformer can withstand the wide range of temperature fluctuation.


d.Moisture Proof: The cast resin coils are impregnated and cast under vacuum which ensures the void less embedding of all windings into a system of uniform glass fibre-epoxy laminate. This process helps the coil to offer an increased protection against moisture. Conventional dry type transformers are not moisture proof. The windings absorb humidity and there is, a danger of flash-over once they are put in service after a long period. e.Immediate Switch On: Because of the cast resin coil, the coils are homogeneously built in all respects. Here is no possibility of any effect of moisture and a:nbiennc temperature fluctuations over coils. Under such cases the transformer can be directly switched-on without pre drying i.e. same after long interruption from service. f.Impulse Strength: Impulse strength of these transformers is higher than that of conventional dry type transformers and is comparable to that of oil cooled transformers according to any 'international standard. g) Non-Inflammable: Due to high quality of non-hygroscopic material, it has been proved that neither with welding/cutting torches nor with welding electric arc the cast coil resin could be induced to burn and as such is almost non-inflammable.

h)Partial Discharge: During operation, there is no partial discharges inside the winding, exceeding narrow band 10 p.c. i.e. transformers are designed for long life. Presently EDN has been supplying mainly, three phase cast resin coil transformers but depending upon customer requirements, three numbers of single phase transformers can also be provided.


4.6 Operating Condition: In spite of all advantages of the cast coil resin transformers mentioned above, it is recommended that this transformer should be mounted in an enclosure installed away from water, oil leaking sources, away from sun rays and heat dissipating equipments. Care has to be taken that sufficient free space all around is available to maintain the ambient temperature and proper ventilation. It has been observed in many sites that this equipment is kept in a dusty place and never cleaned. In fact installation of the transformer has to be thought of in the beginning itself to avoid dust. However, dust/carbon particles must be removed during periodical shut downs.

CHAPTER 5

FIELD BREAKER & FIELD SUPPRESSION5.1 Introduction To Field Breaker & Field Suppression Use of field suppression circuit breaker in a synchronous machine is not a new concept. Following the occurrence of a fault on the generator terminals the fault current has to be reduced as quickly as possible to limit the resulting damages. In order to do this it is necessary to disconnect the excitation source to reduce the fault current quickly and also to avoid high voltage across the rotor which may feed fault34 Division Of Electrical Engineering, SOE, CUSAT.

current. Normally the field windings of synchronous machines are fed via special suitable DC breaker so that under any disturbance/fault condition, the field is disconnected from the source and simultaneously the magnetic energy is discharged. 5.2 Essential Features: Because of the above mentioned requirement, the field suppression breakers are having main poles through which the field current is fed, and the discharge poles which short circuit the field when main poles opens. Normally under ideal conditions the breakers used with synchronous machines are make before break type and accordingly following requirement could be met by them. While closing, the discharge poles opens first and the main poles close later to avoid current flow to the discharge circuit from the excitation source. While opening, the discharge pole closes first and the main poles open later. Thus the magnetic energy is discharged and the over voltages on the field is also avoided. As the rise and decay in field current cannot be sudden because of the large time constant of the field, even a small negative overlapping time between the main pole and discharge contact will not matter. The overlapping time is mainly dependent on the design of a particular breaker.

5.3 Basic Components: Field suppression breakers consist of main poles, discharge poles with their arc chutes, closing coil, trip coil and auxiliary contacts. Provision of resistance in the closing coil circuit depends on the design of coil and hence it may or may not be provided. As per standard USAC37.18.1968 series over current tripping devices are generally omitted from the field circuit breaker since they would cause undesirable tripping of the breaker from transient over current in the field circuit due to disturbances on the AC side of the machine.


5.4 Standard: There are no international standards existing except USA C37.18.1968 for field suppression circuit breaker. However, certain manufacturers have followed standards IEC158, VDE0660 and NEMA SG 3 for a few items of the breaker. 5.5 Field Suppression Resistor: The value of the discharge resistor selected should be such that the rapid de-excitation takes place and the voltage developed across the discharge resistance following the 3 phase fault current is less than the insulation level of field winding. A part of field energy is converted into heat within the discharge resistance. The size and dimension of this resistor is determined by its heat storing capacity. As the peak discharge current would be flowing through the field discharge resistance the conductor and the structure has to be designed for dynamic condition accordingly. Normally for smaller generators linear resistor are employed and for large generators voltage dependent resistor made of silicon carbide discs are used to enable faster de-excitation. The initial resistance value is made equal to the one of the linear discharge resistance, being limited by the admissible voltage. But this voltage falls slowly owing to the increased resistance and the current is quickly reduced.

5.6 Selection of Field Suppression Breaker: For the optimum selection of field breaker in static excitation, following conditions are required to be fulfilled: Rated continuous current of the main contacts must be equal to or greater than the maximum continuous field current. Rated nominal voltage must be equal or greater than the maximum field voltage at maximum continuous current. Rated interrupting current at rated maximum interrupting voltage must be equal to or greater than the possible excitation current occurring following three phase fault current on the generator terminals.


Maximum interrupting voltage must be greater than the sum of supply voltage and the voltage drop across the field discharge resistance due to the excitation current following fault current at stator terminals. For direct static excitation system under this condition, supply voltage is taken as negligible.

5.7 Tests on Field Breaker Following routine tests are normally conducted on field breakers. Mechanical operation Dielectric withstand test on main circuit Dielectric withstand on control and auxiliary circuit DC resistance measurement of closing and tripping coils Following type tests are required for field suppression breakers Continuous current test i.e. temperature rise test Short time current test Interrupting test on main contacts at rated short time voltage at maximum interrupting voltage Interrupting test on discharge contacts Over time test Making current on discharge contacts Load current interrupting tests on main contacts Mechanical endurance test

CHAPTER 6

Thyristor Characteristics and Application in SEE

6.1 Introduction to Thyristor Characteristics and Application in SEE The advancement of high power semiconductor technology and the availability of SCRs have revolutionised the sphere of excitation system. In the latest trends of excitation system neither the rheostatic mode of excitation control nor the magnetic amplifier type of control system is used as these are sluggish in action and have an inherent dead bank of operation because of37 Division Of Electrical Engineering, SOE, CUSAT.

their low loop gains. With the use of SCRs as the power stage for the excitation system for the voltage regulator control the response of the system is much faster than the conventional one. The modern excitation systems incorporating SCRs as their power stage have a very low dead band as the power stage does not involve any delay. The selection and application of SCRs for this system assume considerable importance as improper selection can lead to frequent failures of the equipment. Hence proper care should be taken during the design of the SCRs and its associated circuits. 6.2 System Description The excitation power being fed from the generator terminals or auxiliary supply through normally a step down transformer and then to the input of the SCRs bridge. The voltage regulator having close loop controls compares the actual terminal voltage of the machine with that of the set reference value and forms an error signal which controls the firing angle of the Thyristor Bridge. Subsequently, the variable controlled DC voltage is applied to the field of the generator through a field circuit breaker. The SCR Bridge forms an important integral part of the excitation system by providing an accurate and fast DC voltage control. 6.3 Theory of Device Basically, the SCR consists of four layers of P & N material and three junctions between layers as shown in fig.7.1. This has got two blocking states. When the anode terminal is biased positively with respect to cathodes, the junctions J1 & J3 are forward biased whereas J2 would be reverse biased so that current flow is blocked and the SCR is set to be in the forward blocking state. Similarly with the negative voltage applied to the anode with respect to cathode, junction J1 & J3 are reverse biased and junction J2 is

J blocking state or high impedance state. The SCR can be driven into conduction state 1 when blocking characteristic is erased and the SCR continues to conduct until theforward biased and the device will not switch. This state of SCR is called as reverse current level falls below a certain lower value termed as holding current of the SCR. The SCR configuration is shown in fig.7.1.

J 2 J

38

Division Of Electrical Engineering, SOE, CUSAT.

fig.6.1. SCR configuration The SCR can be turned on by increasing the anode voltage sufficiently to exceed the break-over voltage so that the reversed biased junction J2 breaks down because of large voltage gradient across the depletion layers and the forward current increases. It is limited only by the external resistance of the circuit. The most convenient method of switching the SCR is by applying a positive trigger pulse to the gate of the SCR with a lower positive anode voltage than the breakdown voltage. This is known as the gate control. Once the SCR is ON, the forward current is to be maintained above a certain value known to be latching current so as to enable the SCR to hold on to conducting stage. For turning off a SCR, it is essential that the forward current through it should be brought down below the holding current value by reversing the anode potential when the junction J2 is again reversed biased because of reduced no. of carriers. Fig.7.2 shows the V/I characteristic of the SCR.


Fig.6.2- V/I characteristic of the SCR.

For using gate control methods to turn on the SCR following conditions are to be fulfilled for safer operation: Appropriate anode to cathode voltage must be applied to bring the device to the forward blocking state. The gate signal must be removed once the device is turned ON. The gate pulse duration is to be maintained in such a way that the gate loss is less than that specified for the device. No gate signal should be applied when the device is in the reversed blocking state. When the device is in the OFF state, a negative voltage applied to the gate cathode junction will improve the reverse blocking characteristic of the device. Turn ON time is dependent upon the load current and rate of rise of gate pulses. Turn OFF time depends on the recombination of charges near junction J2. Some typical values of turn ON & OFF times are 1-4 microseconds and 10-250 microseconds respectively. For power frequency applications these turn ON & OFF times do not pose any problems. Like any other semiconductor device reliable operation of the SCR can be ensured only when rating are not to exceed during ON or OFF states. Junction temperature has a direct bearing on the performance of the device since it affects the carrier densities of the P & N layers. With high temperature both the forward and reverse break-over voltage will be lower, the turn OFF time will increase for a given forward current and the min. gate current required for turn ON for the SCR will be lower. The junction temperature depends on the losses generated in the device and the efficiency of heat transfer mechanism. The following factors contribute to the terminal loss of the device. On state forward voltage drop across the device.


Off state forward and reverse currents. The gate current.

6.4 Selection Procedure of SCR Bridges For SEE For SEE applications SCRs are used in a 3-phase full wave bridge configuration as shown in fig.7.3.

Fig.6.3 - 3-phase full wave bridge configuration Six SCRs are connected to form one bridge and thereby each SCR conducts for 120 electrical degrees in a cycle with commutation at every 60 electrical degrees. For selecting SCRs for the bridge to be used for SEE applications, the following factors are taken into account. PIV: The device must not only withstand the peak value of the line but

also the repetitive commutation spike voltages that occur in the parallel connected bridge circuits. These spikes are kept lower by connecting RC snubber circuits across the device. Hence the peak inverse voltage of the device should be much higher than that normally expected across the device during normal operation. As such, the PIV selected is equal to or more than three times the highest working AC rms voltage at the SCRs input. Junction temperature: the junction temperature of the device shall be under the manufacturers specified limit for that device under the41 Division Of Electrical Engineering, SOE, CUSAT.

following conditions. For this criterion, the nature of cooling and ambient temperature must be taken into account. Rated load condition followed by ceiling condition for duration as specified by the generator. Rated load condition followed by a current equivalent to that which will be induced in the field during a three phase short circuit at the generator terminals for 0.2secs. The thyristor bridges are generally designed for a continuous load which is either the excitation equipment rating or maximum continuous excitation. Although the bridges are designed for the ceiling conditions for the specified duration, the occurrence of the same cannot be dictated as it depends upon the terminal voltage variation of the generator. Hence, the most appropriate load cycle is chosen as shown in fig.4.1 taking into account the field forcing condition and stator short circuit condition. It is so chosen that the thyristors get sufficiently cooled before taking the subsequent over-load. The loads are considered for the calculations of junction temperature for the rest of the bridges other than the redundant bridges which adds to the reliability factor for the system.

dv/dt rating: During commutation process the turn ON of the thyristor which is to take over is accompanied by high rates of rise of voltage dv/dt that may cause undesirable turn ON of the other thyristors and sometimes might destroy them. For this reason the selected device dv/dt value should be higher than that expected in the circuit. For further safety, RC snubber circuits are usually adopted to suppress the voltage surges dv/dt generated in the circuit during the commutation process.

di/dt rating: Owing to the finite time for the entire surface of the thyristor to turn ON, in the initial phase of partially conducting devices has to withstand the rising load current and the RC discharge current. The resultant high current density may give rise to localise a hot spot leading to failure of the device.


For selecting a device, the di/dt of the device must be higher than that encountered in the circuit under worst operating conditions. To improve this capability, considerable overrating in the initial instant helps to create large conducting cathode area than one produced by just a minimum gate trigger.

Gate firing requirement: The firing circuit is to be so designed as to match the firing pulse in synchronism with the AC input voltage to the device. The power dissipation and pulse height at the gate must be kept below the manufacturers specified limit. Current rating: The SCRs must be provided with series fuses to protect in the event of over current. In that case, the SCRs should be fully co-ordinated with the fuse. The major selection criterions for the fuses are as follows: Voltage rating of the fuse must be higher than the highest system voltage. Fault level of the fuse must be higher than that expected in the circuit. Fuse must withstand the rated rms current flowing through the

thyristor. I-t chara for both should match throughout the operating ranges. Several other derating factors as applicable are to be considered

during the selection process of the fuse.


6.5 - Parallel Operation For certain high current applications or for redundancy for the power stage paralleling of the devices are required. For such cases, following points must be carefully observed while designing the entire system.

For paralleling, the connections which are done by bus bars and cables etc., are to be kept symmetrical as far as practicable.

Cooling for the devices are to be kept almost similar i.e. the positions and type of mounting of the bridges and the cooling fans are to be mounted identical. RC circuit should be so designed to keep the RC discharge current through the device within the specified limit under all circumstances. In addition to the above, precautions are to be taken to limit the rate of rise of RC discharge current by providing decoupling reactors in series with the device.

The above series decoupling reactors with proper tolerances also serve the purpose of reducing the missharing factor for the parallel connected devices. While designing this, the missharing factor is to be taken into account for the junction temperature calculation.


6.6 - Conclusion: Dry cast resin transformers have become very popular in all countries. Already many manufactures have manufactured them up to 36 KV systems. The days are not far off when oil transformers in many installations would be replaced. Because of the large demand of dry transformers, research for basic material like insulation and resin is also constantly done. In excitation system, practically all the leading manufacturers of the equipments have started using cast resin dry type transformers and three is no necessity presently of using oil cooled transformers with its inherent disadvantages of fire-risk etc., as already mentioned. Also the field breaker is a vital part in an installation. Care has to be taken to see that field breakers are operated only when they are required. This is equipment which normally does not require regular maintenance. However on various adjustments/gaps and contact conditions are necessary. Field suppression through non-linear field discharge resistor is greatly effective. The discharge resistors are normally short time rated.Various methods are being attempted for effective field suppression. However, the above mentioned method by DC breaker is presently quite popular and used.


CHAPTER 7

DESCRIPTION OF CONTROL SYSTEM7.1.1 AUTOMATIC VOLATAGE REGULATOR (AVR) The automatic voltage regulator type UN 2010 is an electronic control module specially designed for the voltage regulation of synchronous machines. It primarily consists of an actual value converter, a control amplifier with PID characteristics which compares and actual value with the set reference value and forms an output proportional to the difference. The output of this module controls the gate control circuit UN 1001. The module does not have an inbuilt power supply and derives its power from Auto Channel Power Supply. AVR CIRCUIT

Fig.7.1-Automatic Voltage Regulator(AVR)


The main features of this module are listed below: The AVR comprises of an input circuit which accepts 3 phase voltage signal of 110VAC and 3 phase current signals of 5A or 1A AC. It is thus necessary to use intermediate PTs and CTs to transform the generator voltage and current to the above mentioned values. The module itself contains PTs and CTs with further step down the signals to make them compatible with electronic circuit. A circuitry is available in the module for adding the current signals vectorialy to the voltage signals for providing compensation as a function of active or reactive power flowing in the generator terminals.

An actual value converting circuit for converting the AC input signal to DC signal with minimum ripple with the aid of filter network. A reference value circuit is using temperature compensated zener diodes. The output of which is taken to an external potentiometer that provides 90-110% range of operation of generator voltage.

A control amplifier which compares the reference and actual value and provides an output proportional to deviation. Apart from this, it has the facility to accept other inputs for operation in conjunction with various limiters and power system stabilizer. A voltage proportional to frequency network which reduces the excitation current when frequency falls below the set level, thus keeping the air gap flux constant. This prevents saturation of connected transformers and possible over voltage.


7.1.2 DESIGN SPECIFICATION OF AUTOMATIC VOLTAGE REGULATOR An automatic voltage regulator can be designed, and it is Necessary to know certain factors about the input and the Required accuracy of the output voltage, together with, Certain information on the load A. Supply-type Automatic Voltage Regulator In the supply-type automatic voltage regulator, it is necessary to state the type of input, whether direct or alternating, its nominal voltage and, if alternating, its nominal frequency. Most automatic voltage regulators are operated over a limited range of input voltage. If the frequency of input is likely to vary, the range of variation of the frequency may have a considerable influence. The output voltage is to be variable; the range of variation must be stated. The maximum output current must be known and also the range of variation of output current over which the regulator is to operate. When the output is alternating it is necessary to specify the power factor of the load, as certain designs will only operate over a small range of power factor, around unity. In three phase regulators it may be necessary to maintain the three phase voltage at 120 degrees to each other, as well as maintaining them constant in magnitude. Certain information may also be specified concerning the maintenance, operation and reliability. The accuracy of maintenance of the output voltage may be divided into two general classes; (1) Short period accuracy: this is, the accuracy over a period of minutes, due to changes of input or load and (2) long-period accuracy: this is the accuracy over a period of hours or days, due to changes in ambient temperature, ageing of components, vibration instability of components.

There are two other factors connected with the output voltage that may be important.


(1) Response time: All regulators take a finite time to effect a change in the supply voltage or load. This time is referred to as the time constant of the regulator, but in most cases it is termed as response time. In some cases, the response time is depending on the magnitude of the change of output voltage, but the rate of change remains constant. The maximum allowable response time depends upon the type of application. It is always desirable to make the response time as small as possible to reduce the transients in the output voltage. (2)Waveform distortion: It is important in AC voltage regulators and the ripple voltage in DC voltage regulators Care should be taken to reduce the distortion as much as possible. The distortion is expressed as the total percentage of harmonics relative to the pure sine wave. B. Generator-type Automatic Voltage Regulator It is a control device which automatically regulates the voltage at the exciter of an alternator, to hold the output voltage constant within specified limits. Probably due to the fact that this part of the equipment is often of different manufacture from the generator. One can only express the performance in terms of the whole equipment as this is determined by the characteristics of the generator (and exciter, if used). When referring to the performance which is used by the term automatic voltage regulators will imply the whole equipment and not just that part which controls the field current. In the specification for an automatic voltage regulator of this type it is necessary to bring in the characteristics of the machine. The design of the regulator will depend on; (1) The characteristics of the driving source since changes in speed cause variations of voltage (2) The maximum and minimum load on the generator (3) In the case of alternating current, the power factor of the load, since this, in conjunction with (2), will determine the range of field current required (4) The regulation of the generator (5) The magnetization curve of the generator (6) The characteristics of the exciter (if used).


In the case of small machines most of this information may be given by stating the field current at minimum speed and maximum load, and the field current at maximum speed and minimum load. When a regulator is being designed for a large machine (e.g. an alternator in a large power station) more information is required, and the designer of the machine and of the regulator must work in closed harmony if a successful result is to be achieved. The short period accuracy of the output voltage is usually specified as the percentage change of load, speed and power factor. The long period accuracy may not be so importance


Fig.7.2 Over all Circuit of AVR for Synchronous Generator


7.1.3 CIRCUIT DESIGN OF THE AVR FOR THE SYNCHRONOUS GENERATOR The circuit arrangement of the field control circuit of the synchronous generator is shown in. In this system, the output voltage of the generator is sampled through the transformer and is rectified by simple circuit and the bridge rectifier. In the initial state condition, the output of the generator may be 25V or 30V which depends on the electromagnetic field in the machine, at the time, the 12V relay is normally close position. At the time, the gate voltage is fed to the synchronous generator field coil until the output voltage is 230V. Now, 12V relay is normally open position, When the mains supply voltage falls, Q2 produce negative current to the bridge circuit and the bridge circuit supplies positive current to the gate of SCR and the required current is fed to the field coil and the output voltage of the synchronous generator is increased. When the output is 230V, the output positive current of the bridge is balanced with the output negative current of the Q1. When the main supply voltage raises, Q2 will give a little current is fed to the gate of SCR and the required field current is fed to the field coil and absorbs the required reactive power from the supply line. This is a typical automatic voltage regulator which can be used for 10 kVA alternators field control applications. The advantages of this AVR card is that the system cost is decreased and system reliability and design flexibility are increased. This AVR card is well suited to the high production requirements of mass production.

7.1.4 Explanation Of The Aforesaid Features Follows:52 Division Of Electrical Engineering, SOE, CUSAT.

1.

IR Drop Compensation Techniques: The current signals at the secondary of the internal CTs are dropped across

ganged variable resistor R330 in the secondary of the internal voltage transformers T611, T612 and T613 (fig.7.3). With this vectorial addition of current to voltage different compensation can be achieved depending on the system configuration and requirement. A few of the possibilities are described below. The description is with reference to phase voltage and the current vector which can be extended to the three phase network as is the case in UN 2010. Compensation as a function of reactive current for a network having low external reactance. For such a network a voltage proportional to the current in the T-phase is added to the R-S phase voltage, as can be seen through a figure . the influence of purely active current is insignificant whereas a purely reactive current has the effect of increasing the actual value. The control amplifier sees a higher voltage as compared to reference value and acts immediately to decrease the terminal voltage by reducing the excitation current, such compensation makes the reactive power less dependent on voltage variations. This kind of compensation is provided where many generators are connected in parallel without intermediate transformers. This is generally termed as cross current compounding or droop characteristic. Maximum compensation that is possible in UN 2010 is 15% of UN. Reactive current compensation for systems with high external reactance. In such applications it is necessary to compensate the drop in voltage due to high reactance of the transformer and line up to the load centres is within acceptable limits. By reversing the current direction at the CTs inputs, it can be observed that he resultant voltage vector is smaller in magnitude as compared to the set value. The AVR acts in such a way as to boost the excitation so that the terminal voltage is increased. As in previous case purely active current does not have significant effect but only causes a phase shift. A maximum compensation of 15% can be achieved by adjusting the plot, R330 on the front plate of UN 2010. Compensation as a function of active power.


Instead of T-phase current, if the R-phase current is passed in CT the vectorial difference of this voltage and R-S phase voltage is less than the R-S phase voltage. As a result the terminal voltage of the generator is increased and so also the inductive reactive current. Thus the power factor is maintained constant, as an increase in active causes a proportional increase in reactive current and vice versa. With this kind of connection there is a certain amount of compensation for reactive current also.


Figure 7.3


2.

Actual Value Formation:

The actual value signal after current compensation is fed to a 3 phase diode rectifier bridge which converts the AC voltage to DC voltage. This voltage is taken across a potentiometer, so signal is fed to low pass filter in order to smoothen the signal. The output from the filter is fee from ripples. 3. Reference Value Circuit:

The reference value for the AVR is formed by a zener diode and a resistor. The maximum value is set at 7.7 at works. This voltage across an external 500 ohms potentiometer provides a variable reference which is normally calibrated from 90100% of rated generator voltage. This reference voltage is fed to the summing point of the control amplifier. The actual voltage and the reference voltage are compared and if there is any deviation the control amplifier provides an output proportional to this deviation. Digital reference value units are also provided which provide the reference value in the range mentioned above. However, in place of motors for remote operation up/down counters are used. 4. Control Amplifier and PID:

The objective of any closed loop control system is to keep the following parameters in check. The overshoot after any step change in controlled variable should be within the acceptable limits for the system to be stable. The response time should be as possible so that the corrective action to any change in the controlled variable is taken by the controller without any appreciable lapse of time. Steady state error should be as small as possible. The settling time should be as small as possible. The system should remain stable for a wide range of operating points. It is not possible for the control amplifier alone to take care of all the said points. Hence to improve the performance compensation networks are provided in the control amplifier.


These compensation networks are mainly proportional controllers that provide an outpu

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