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Rotor The rotor of a synchronous machine is an electromagnet.
The effect of the rotating flux on the stator windings
produces an induced voltage.
The principle of voltage generationThe production of voltage in the synchronous machine is based on the induction low. This means if the flux changes in the stator winding of the synchronous machine there will be a voltage induced.
The flux is produced by the current supplied from the excitation system to the rotor winding. The change of flux in the stator winding is caused by the movement of the rotor. This induces the voltage in the stator winding as illustrated in the figure below:
In any excitation system, several components can be identified. Depending on the age and type of the system, the equipment may vary greatly, however the basic components can still be classified.
Rotor Current ProductionThe rotor of the machine must be supplied with a current. For example this could be by:A large power electronic converter (direct), or a small current supply feeding an excitation machine, which in turn produces the large rotor current. (indirect system).
Power SupplyThe excitation system needs a power supply in order to produce a current. There are many different configurations.Shunt Supply – The supply is taken from the machine terminals.Line Supply – The supply is taken from an auxiliary supply.Permanent Magnet Generator – A small permanent magnet generator is mounted on the same shaft as the main machine.
Current ControlNo matter how the current is produced, there must be some method of controlling how much current is produced. In the case of a state of the art control system the rotor current is controlled by semi conductive rectifiers.
Voltage RegulationVoltage regulation is done in the control system by the Automatic Voltage Regulator (AVR). The voltage regulator various the rotor current automatically in order to maintain the terminal voltage of the synchronous machine even in case of load change.
The picture above shows the connections to the excitation system in a power plant. The excitation system is usually located close to the synchronous machine. The main power supply for the production of the rotor current is taken from the generator terminals and fed via the excitation transformer to the excitation system.The output of the excitation system supplies the direct current via slip rings to the rotor winding. The terminal voltage and machine current is measured by means of Potential transformers PT’s and current transformer CT’s. These signals are used to control the generator voltage and reactive power.The excitation system is operated by the operators in the control room via the control interface as illustrated .
The closed loop regulating circuit of the synchronous machine can be represented for the electrical variables as shown in the figure. The output voltage UG of the synchronous machine is picked up by the voltage regulator of the excitation system and compared with the setpoint. The output of the excitation system in the form of the excitation current If is the input to the synchronous machine, which closes the regulating circuit.For a synchronous machine coupled to an electrical network, the network simply acts as a disturbance value. Disturbances in the network such as the shutting down of large consumers or short circuits influence the generator voltage in an undesirable way. It is the function of the excitation system to balance out these undesirable changes immediately and to operate the machine stably on the network.
The solid pole synchronous machineThe solid pole synchronous machine
High speed application for speed range > 1500 rpm
Rotor Stator
The synchronous machine
The synchronous machine essentially consists of two parts: the rotating part, the rotor, and the static part, the stator.In order to cover the wide range of rotational speeds of possible turbines, two different types of synchronous machine are available.
The solid pole machine (Turbogenerators)In thermal turbines, rotational speeds >1500 rpm are usually required. In this case, so-called solid pole machines, as shown in the diagram, are used. The full pole machine is also referred to as a turbogenerator.
The salient pole machineIn river-driven power stations, Kaplan turbines are usually used which have low rotational speeds of < 1500 rpm . In these cases, so-called salient pole machines are used, as shown in the following diagram. In contrast to the full pole machine, in these machines the diameter of the rotor is very large and the length short.
The full pole and salient pole machines basically function in the same way. They only differ, in some cases, in their behaviour under load.
2.2 The Synchronous MachineSynchronous machine triphase representation
URIR
US
IS
UT
IT
UfIf
IDR
IDT
IDS
Rotor
Stator
120° 120°
120°
3-phase representation of the synchronous machine
The diagram shows the synchronous machine with the three phases. Each phase is displaced physically by 120° and, viewed in terms of electrical values, essentially consists of two reactances, the main reactance and the secondary reactance formed by the damper winding. Both reactances are associated with ohmic resistances, which are not of importance in considering the excitation system. A further reactance is found in the rotor winding with the associated winding resistance.
2.2 The Synchronous Machined-q axes representation
UdId
Uf If
IdD
Uq
Iq
IQ2
IQ1
Ψd
ΨfΨq
ΨdD
ΨQ1 ΨQ2
D axis
Q axis
δω
ra
ra
rdD
rf
rQ1 rQ2
Rotor
Stator
D-Q axis representation of the synchronous machine
The D-Q axis representation is used to explain the behaviour of the synchronous machine. The 3-phase system can be transformed into a “single-phase” representation by means of a mathematical operation. The mathematical operation will not be discussed here.In order to explain the behaviour of the synchronous machine, the two resulting axes, the quadrature axis (Q-axis) and the direct axis (D-axis), are given different impedances and reactances, together with the associated resistances, which are given the corresponding index q or d. These impedance values can be found in the detailed data sheets provided by the manufacturer of the synchronous machine.
The meaning of the individual reactances will not be examined here. Rather, we will carry out a substitution of the different reactances in order to explain the behaviour of the synchronous machine.
The simplified equivalent circuit for the synchronous machine
E
Xfσ Xaσ
XmRotor Stator
P
Xd,q
EP
Synchronous Reactance
Uf
Rotor Stator
If
ω
UG
q-axis
d-axis
Fig. a Fig. b Fig. c
The equivalent circuit diagram for the synchronous machine
In order to explain the behaviour of the synchronous machine in stationary operation, we simplify the complex structure of the synchronous machine. Taking into consideration the q-axis and d-axis, one can represent the synchronous machine as bipolar, see Fig.a. The rotor with the field winding is fed from the excitation system. The excitation current generates a magnetic field which induces a voltage in the stator winding through the rotation of the rotor, according to the induction principle. This voltage can be measured at the output terminals of the generator when the machine is in no-load operation.This physical interpretation of the way the synchronous machine functions can be represented as the equivalent circuit diagram Fig. b) with the main reactance Xm and the control reactances Xfσ and Xaσ as shown in the diagram. The voltage source Ep stands for the voltage induced in the stator windings which is determined by the excitation current and the rotational speed of the machine. Ep is also referred to as EMF (electromotive force) or air gap voltage . The structure of this equivalent circuit diagram is also used for transformers. In fact, the synchronous machine acts like a transformer with an air gap. The reactances shown in Fig. b) can be further condensed and transferred to the very simple equivalent circuit diagram Fig. c). This equivalent circuit diagram is sufficient to describe the stationary behaviour of the synchronous machine. Essentially, it simply consists of the “internal“ voltage source and an “internal resistance“ which essentially appears in the form of a reactance, the so-called synchronous reactance Xd or Xq. The synchronous reactance has a great influence on the electrical behaviour of the machine. The value in the direct axis Xd and in the quadrature axis Xq are almost equally in solid pole machines. In salient pole machines, is Xd > Xq.
The operating behaviour of the synchronous machine
Generator no load characteristicsStarting out from the simple equivalent circuit diagram, the generator terminal voltage in no-load operation is essentially determined by the excitation current If and the rotational speed n. In considering excitation, one can assume that the machine rotates at nominal speed. This means the induced voltage Ep is only determined by the excitation current. The relationship between excitation current and generator voltage can be seen from the graphic. If one starts to slowly increase the excitation current, the generator voltage increases in proportion with the excitation current. An important point here is the excitation current required in order to reach the generator nominal voltage. This current is called the no-load field current Ifoand is one of the important characteristic values of the synchronous machine.
2.2 The Synchronous MachineGenerator short circuit characteristic
I g
IfIfo
I G
No load field current
Generatorcurrent at Ifo
Xd
Ep UG = 0
Speed n = constant
If ,n
Ep = UGn
UGn
For If = Ifo ⇒ Xd = IGn/IG IG at (If = Ifo)Example:Measurement at If = Ifo: IGn/IG = 2.43
⇒ Xd = 2.43 pu
Generator short circuit characteristicsFor the short circuit test the machine terminals must be short circuited. Be aware that the machine current can go up the its nominal value. While the machine is running at rated speed the field current will be slowly increased. At the same time the machine current must be read in order to gain the short circuit characteristic of the synchronous machine. The ratio between IG(If
=Ifo)/IGn determines the synchronous reactance Xd of the machine, where Ifo is the no load field current and IG the measured machine current at no load field current.
Where: Xd Sychronous reactance direct axisIG Machine current at no load field currentIGn Machine rated current
If a load is applied to the machine which has been excited in no-load operation, the output voltage Ug drops, because the load current, via the synchronous reactance, results in a voltage drop ΔU. This voltage drop is considerable at machine nominal current. In order to ensure that the generator voltage is also kept stable under load, the voltage drop must be compensated by increasing the excitation current. This compensation takes place automatically if voltage regulators are used. The generator voltage is thereby kept stable through adjustment of the excitation current. This is one of the fundamental functions of the excitation systemIn order to find the excitation current required for a specific load point, a vector diagram (Fig. b) can be drawn for the simple equivalent circuit diagram. Here, the generator voltage UG is left constant and the voltage drop ΔU is drawn in. For a purely ohmic load, this voltage drop ΔU is perpendicular to the load current IG and is applied to the generator voltage. The resulting voltage of the two vectors UG and ΔU in turn represent the induced voltage Ep, which is proportional to the excitation current.This means that a relationship has been found between the excitation current and the generator load current. If one imagines the machine current IG to be reduced to 0, then Ep and UG match. The length of the Ep vector is known to be a measure for the excitation current, which for IG = 0 corresponds to the no load field current, which is determine from the no-load characteristic. In this way, the necessary excitation can be determine for any load point. The broken lines show the vector diagram for inductive load. According to this, the excitation must be increased in order to compensate the voltage drop.
The Power Chart of the synchronous machineThe vector diagram for the synchronous machine was shown with voltage and current vectors. In practice, power vectors tend to be used in order to assess the operating behaviour of the synchronous machine. For this purpose, we can draw the power diagram with the two power axes: the active power axis and the reactive power axis. The nominal apparent power (1 pu) of the synchronous machine thereby appears as a circle. The active or reactive power can thereby assume both positive and negative values. Negative active power means, for example, motor operation. The power vector diagram is obtained from the voltage vector diagram as follows:- All values are expressed in so-called Per Unit (pu) values. For example, thegenerator nominal voltage is 1 pu, the generator nominal power is 1pu etc.
- To obtain the power values from the voltage values, one multiplies the voltagevectors by the value UG/Xd according to Ohm‘s law. This gives us the power vectors.For example, the vector ΔU = IG • Xd becomes the power vector S = IG • Ug . Thepower vector S thus corresponds to the apparent power of the synchronous machine.One can proceed analogously with the other voltage vectors.
The power vectors can be entered in the so-called power diagram with the active power axis and the reactive power axis as shown in the figure above. If the synchronous reactance is expressed in per units, the 1/Xd point is the starting point for the air gap power P(Ep), which for UG=1pu is still proportional to the field current If.The operating point (1) represented in the above diagram only lies in the active power axis, i.e. only active power is output. If the synchronous machine is coupled to the electrical network and the excitation current is increased, reactive power is output into the network in addition to the active power. In this case, the machine operates within the overexcited range. Another important variable is the so-called load angle. This angle also actually occurs as a mechanical angle between the magnetic rotary field generated by the stator windings and the magnetic field generated by the rotor winding. As soon as the machine takes up active power, this angle increases. If active power is present, this angle is also influenced by the excitation current. If, for example, the machine is de-excited, the load angle becomes greater. The question arises here as to how great this angle may become for the machine to still rotate synchronously with the rotary field of the stator.
The torque characteristic of the synchronous machine
The diagram shows the curve of the synchronising torque as a function of the load angle.The maximum torque is achieved at a load angle of 90°, whereby the excitation current determines the value of the maximum. The greater the excitation current, the greater the magnetic flux and thus the synchronisingforce Fsyn in the machine. At a particular active power and excitation current, a particular load angle δ2results. If the excitation current is reduced with the active power of the machine remaining the same, the load angle increases to the value δ1.
The safe operating area of the synchronous machine
Drive Limit
Sn~Ifn
ϕ
Rated Power
δmax= 90°
Stability Limit
Field CurrentLimiter
The safe operating area of the synchronous machine
If the machine is operated at the nominal operating point, an excitation current is present which we call the nominal excitation current Ifn. The rotor windings and the power units of the excitation system are designed for this current, because it must be possible to operate permanently at this point. In order to prevent the rotor or the excitation from being overloaded, an excitation current limiter is used which is implemented in the excitation system. The result of this is that the operating range is limited within the overexcited range.In the active power axis, the operating range is limited by the maximum turbine ouput, which usually lies between 80% and 90% of the output of the synchronous machine.Within the underexcited range, the operating range is limited by the machine current or by the stability limit of the synchronous machine. The theoretical stability limit is reached at a load angle of δ=90°. This means that the safe operating range of the synchronous machine is determined by the turbine and the two limiters in the overexcited and underexcited range. Why the stability limit is reached at a load angle of 90° will be explained in greater detail in the following.
2.6 Transient behaviour of the synchronous machine
Overvoltage relay
IQ x Xd "
t = 0 1 Sec.
t
Ug
Uo
with AVR (static excitation system)with constant field current
Generator voltage in case of reactive load rejection
Load rejection
By opening of the main circuit breaker of the machine the load will be dropped off immediately. The early invention of the automatic voltage regulator is certainly caused by the consequences of this event. It is also an important quality mark for a voltage regulator how the generator voltage varies with the time after the breaker has opened. The drop of the reactive load current to zero inevitably causes an immediate voltage rise ΔU=Ireactive • Xd”. If for instance the subtransient reactance Xd’’=0.2 p.u. the rejection of 0.5 p.u. reactive current gives an instantaneous rise of 10%. If the load on the synchronous machine is changed through connection of an additional load, then the electrical active power changes suddenly, which can not be reduced by any control action.Without AVR the voltage then rises further till the maximum value is reached defined by the synchronous reactance. The time delay corresponds to the no –load time constant Tdo’. With an AVR this further rise is more or less completely eliminated and the voltage is brought back to the initial value. How quickly this is achieved depends on whether or not the additional time constant of an exciter machine has to be overcome. Without a voltage regulator the over voltage relay of the generator protection would be activated and deexcite the generator.
2.6 Transient behaviour of the synchronous machine
1 sec
UO
with constant field current
with voltage regulatorUG
t = 0t
Generator voltage in case of long distance short circuit
Long distance short circuit
In case of a short circuit in the grid system away from the power plant the voltage will drop immediately. The voltage regulator tries to keep the machine voltage on its setpoint. After a certain time the fault in the grid will be cleared by the line protection and the system voltage will recover. This leads to an overshoot of the machine voltage. The voltage regulator will reduce the voltage to normal again.
2.7 Definition of Excitation SystemsGlossary and Definitions (IEEE STD. 421.2)
Ifo No load field or excitation currentRequired field current to achieve 100% generator terminal voltage at rated speed
Ifn Nominal field or excitation currentRequired field current to operate the synchronous machine at rated power
Icl Ceiling field currentMaximum field current that excitation system is able to supply from its terminals for a specific time
Ufo No load field voltageRequired field voltage to obtain the no load field current considering the field resistance
Ufn Nominal field voltageRequired field voltage to obtain the rated field current considering the field resistance
Ufcl Ceiling field voltageRequired field voltage to obtain the ceiling field current
KPl Excitation Ceiling factorCeiling field voltage divided by no load field voltage Ufcl/Ufo
δ Load anglePhysical angle between rotor field and stator field
Glossary and Definitions
The diagrams show the most important abbreviations and definitions of physical values in connection with excitation, as defined in the IEEE STD. 421.2 standards.
2.7 Definition of Excitation SystemsGlossary and Definitions cont…
ϕ Phase angleElectrical angle between machine voltage and machine current
cosϕ Power factorRatio of machine’s active power to apparent power
Xd Machine synchronous reactance in direct axeXq Machine synchronous reactance in quadrature axeRs System nominal response
The rate of increase of the excitation system output voltage divided by the nominal field voltage
Tv Excitation system voltage response timeThe time in second for the excitation voltage to attain 95% of the difference between ceiling field voltage and nominal field voltage
Generally speaking, two basic configurations of excitation systems are used nowadays.
Indirect excitation system (brushless excitation system)This excitation system basically consists of a voltage regulator with power unit, the alternating current machine and the rotary diodes for converting the alternating current generated by the exciter machine into the direct current required by the main machine.The voltage regulator output therefore first controls the field current of the exciter machine. In this machine, the field winding is in the stator. The 3-phase alternating current windings in which an AC voltage is induced through the rotation of the rotor lie on the rotor. This AC voltage is converted by means of the diodes which are rotating on the shaft. The direct current is fed, without slip rings, directly into the exciter winding of the main machine. No brushes are therefore necessary, for which reason this type of excitation system is called “brushless excitation”.
Direct excitation system (static excitation system)The static excitation system essentially consists of the voltage regulator, the power unit, a switch and the brushes with slip rings. The power supply to the excitation system is usually taken directly from the generator terminals and transformed in the power unit by means of thyristors into a direct current which is fed via a switch and slip rings to the rotor winding of the main machine. These systems are distinguished by very fast regulating performance.
• Exciter response limited by theexciter machine time constant(>200ms)
• Field discharge with natural timeconstant
• Supply from PMG possible providing supporting of shortcircuit currents
• Relative large size of exciter machine for low speed generators
• No sliprings (less maintenanceand dust)
• Positive and negative ceiling voltagecapabilities
• Fast response (<20 ms) in both directions
• Fast field discharge by discharge resistoror inverter operation
• Size of excitation of transformer dependson field requirements only
• Shorter shaft (torsional oscillations)
• Maintenance on power rectifier themachine must not be at standstill
• Direct measurements of field quantitiesUf, If possible
Comparison: Indirect - Static Excitation System
Brushless excitation Static Excitation System
Comparison
The diagram shows a comparison of the most important advantages and disadvantages of both systems. It cannot be said straight away which is the “better” system. The most suitable system has to be determined from case to case.
This type of excitation system is often used for hydrogenerators and large turbogenerators larger than about 50 MVA with exceptions to clients requirements. The power for the excitation system is taken from the generator terminals. The automatic voltage regulator works through a semiconductor output stage, which is mostly a thyristor converter or an integrated gate bipolar transistor (IGBT) stage.
The voltage regulator with the power converter and excitation transformer as well as the field circuit breaker complete the number of the main components of a static excitation system.
3.2 Excitation System: Basic ConfigurationsTrue-Dual Channel UNITROL AVR with 2 x Automatic & Manual modesfor Indirect Excitation Systems
SM =
Supply
~
Voltagesetpoint
A
M
==
Field currentsetpoint
Follow-up
~=
VOltagesetpoint
A
M
==
Field currentsetpoint
follow-up
~=
Autom.Mode
ManualMode
ManualMode
Autom.Mode
Channel I
Channel II
True dual-channel system
The so-called dual-channel system increases the availability of the excitation system significantly. The dual-channel system is equipped with two identical channels. Each channel includes the regulator functions present in a single-channel system, as described above.If a channel fails, the system switches over automatically to the other channel. Only one channel is in operation at one time (active channel), the other channel is in standby position (passive channel) and is continually matched to the active channel so that a smooth switchover is possible at any time. The operating personnel can select which channel is the active channel. There is no preference as to whether channel I or channel ll is the active channel.
Single Channel Excitation System for Static Excitation System
System variants for static excitation systems with higher field currents
For higher power outputs it is neither economic nor technically sound to double the thyristor power stage. A solution with variable ac transformer stays out of consideration. Instead the converter is built redundant. Details will be explained later.
For this kind of equipment the electronic control channels can be designed as a single channel or double channel.
Single channel for static excitation systemsThe control signal within the automatic operating mode is supplied by the voltage control amplifier. Within the manual mode the signal comes from a closed loop field current control. Both out put signals are forwarded to the change-over switch to select from Auto to Manual operating mode. The control signal is fed to the pulse generation which generates the firing pulses for the thyristor stage. An automatic follow up functions is balancing the output of the non active regulator to the active one.
Power Converter ConfigurationsEconomy Configuration(Single Channel)
Twin Configuration(Double Channel)
Power converter configurationsThe converter lay-out is defined by the excitation needed for the synchronous machine and the corresponding redundancy requirements.The following standardized designs are available:
Simple converter configurationFor systems with a low excitation current demand (i.e. a single thyristorconverter is sufficient) or if there are no redundancy requirements, the combination of a single channel AVR and integrated pulse amplification with a single thyristor output stage is fully sufficient as shown in the figure above.
Redundant converter configurationFor excitation systems with large output currents or higher availability requirements the following two designs are available:
Redundancy concept (1+1): There are two identical converters connected in parallel of which only one is in operation at a time. By alternatively blocking and releasing the firing pulses to the corresponding converter switch-over is effected in case of failure.
TWIN Configuration(Dual channel system without converter redundancy)
Gate controlunit
Pulse amplifiers
Channel2
Channel1
3.2 Excitation System: Basic Configurations
Converter without redundancy
The size of the converter is at least the rated field current of the synchronous machine.
This means that if there is any failure in the converter, then the excitation system must generate a trip.
Example Failures may be for example:Fan failure.Thyristor Failure.Electronic PCB Failure (e.g. CIN board).Converter Over Temperature.Converter Current Measurement Failure.Snubber Fuse.
3.2 Excitation System: Basic ConfigurationsParallel bridges with n-1 redundant configuration
Supply
To the field circuit
M
M
Final pulsestages
M
Gate controlunit ofchannel I
M
1
2
3
nPulsebus
Gate controlunit ofchannel II
Channel1
Channel2
Redundant converter configuration
With even higher output currents where the parallel connection of several thyristors is necessary the reliability of the converter is secured by theredundancy concept (n-1). This means that one more parallel converter than necessary is provided.
The two channels work through gate control set and intermediate pulse stage on a common pulse busbar. The different redundancy concept (1+1) and (n-1) is chosen because of selectivity reasons of the thyristor fuses. If two converters would operate in parallel, and if a thyristor looses its blocking capability, then a short circuit current starts to flow when the next thyristorsare fired. In this case the two new fired thyristors drive a short circuit current into the defective one. And as a consequence we have a series connection of two thyristor fuses in parallel with one thyristor fuse (of the defective thyristor). This arrangement does not assure that the single fuse will blow first. Therefore the converters must be changed over from one to the other. With more than 2 converters in parallel this selectivity is assured.
3.3 Excitation System: Field FlashingField flashing feature
AVR
Generator
Auxiliaryvoltage
~ +
DiodeBridge
Thyristorbridge
Usyn
U>40%
Field flashing breaker
Ug
t
Field flashing off
Thyristor bridgestarts to conduct
U>40%
U>10%Field flashing characteristic
Field flashing failedFCB Trip
10sField flashing OFF
5s
100%Softstart
Ug
• Order Fieldbreaker CLOSE• Order Excitation ON• Pulses to the thyristors are released • Field Flashing breaker closes if residual machine voltage is too low
• Stator voltage raises• Field flashing breaker opens• The softstart function raises the generator voltage smoothly up to its nominal value.
Sequence:
Field flashing feature
If the excitation system is supplied by shunt supply, i.e. directly from the generator terminals, then the residual voltage of the generator is sometimes not sufficient to build up the voltage. In such cases, when the excitation is switched on the excitation current is built up with the aid of field flashing. The field flashing consists of a diode bridge and a switch which connects an external auxiliary voltage to the field. It is dimensioned in such a way that the generator voltage is built up to approx. 20%. Once the generator voltage reaches approx. 30-40% of the nominal value, this switch is switched off again.The generator voltage is then built up to nominal value by the main converter. The slow build-up is thereby controlled by means of a softstart ramp implemented in the voltage regulator.
3.4 Excitation System: Field SuppressionField Suppression Circuit (Crowbar)
5 6 3 4
7 8
LfRf
RE
If (field suppression)
If (operation)
inverter (WR)
+
- +
-+
- +
--Lf.dIf/dt
WR
Q02
Uarc
Uarc
UarcU
disc
harg
e
Components of field suppression equipmentThe main elements of a field discharge circuit are the field breaker with discharge contact or DC breaker with electronic discharge circuit, the discharge resistor and the overvoltage protection. In addition there is a certain amount of control means.
Field breaker (field discharge contactor), DC breakerGenerally the field breaker has to interrupt a direct current in a circuit with high inductance. Due to the inductive load the change of current depends on the discharge voltage which is defined by the arc voltage of the breaker.Field breakers are designed specially for this duty. They are equipped with arc chambers and electromagnetic quenching.Modern field breakers are equipped with limiting means such as auxiliary arc gaps, limiting resistors and the important distribution of the grown arc into a row of partial chambers. The result is a much quieter and more constant arc voltage. The most important criterion of a breaker is its interrupting capability. It is determined by several factors.• maximum arc voltage• maximum interrupted current• maximum arc energy.
3.4 Excitation System: Field SuppressionStatic Field Suppression Circuit (Crowbar) with Firing units
1011
K1
K2
K3
+
T T
-R02
I>
BOD
-V334
12
567
1 3
2 4
Firing PCB
Fielddischarge I
Fielddischarge II
Freewheeling
F iel
dw
i ndi
ng
+
DCbreaker
_
V1 positive overvoltagethyristor
V2 discharge and negative overvoltagethyristor
V3 redundant discharge or free wheeling thyristor
CROWBARDC Breaker
-V2
-V1
Current Measurement
Discharge resistor
Crowbar design
Overvoltage protection (Crowbar)
The voltages occur during the deexcitation process should with a safety margin always remain below the insulation level of the winding, i.e. below the test voltage, whereby a reduction caused by aging has to be taken in account. The same voltages also appear at the converter output and stress the blocking capability of the thyristors. In addition with salient pole machines inverse induced voltages (back emf) occur during asynchronous operation, that is after falling out of step. Since an inverse current can not pass the thyristors, such voltages rise fast to high amplitudes.
Therefore practically without exception a fast acting overvoltage protection is provided, which discharges the field winding over a resistor, if a well defined voltage level is exceeded. Mostly the normal field discharge resistor is also used for this purpose. As sensing devices special pre-selected avalanche diodes (BOD) are used.
Current sensors are used to detect current in the field discharge circuit. This criteria is used to supervise the discharge circuit.
Methods for field suppressionThere are several kinds of deexcitation circuits which are partly only of historical interest. Some of them will be described in short below for the better understanding of the whole matter.Basically a field suppression circuit must accelerate the current decrease in the field winding. If we just reduce the voltage of the feeding source to zero, the current will decrease in accordance with the well-known exponential function and with the natural time constant T = L/Rf of the field circuit.By insertion of a discharge resistor in series with the field, e.g. by opening the switch Q02, the effective time constant of the circuit is reduced.We want a fast decrease of the flux. It is important to realize that we can force the flux decrease in the direct axis only. The time constants in the quadratureaxis cannot be influenced at all.If we connect a suppression resistor RE equal to the field resistance Rf in series, the effective time constants at no-load Tdo’ and with load Td’ are reduced to half the natural value. The time constant of the core TA and the one of the quadrature axis Tqo remain unchanged. The quicker the field decrease in the direct axis is achieved, for instance with the help of a non-linear suppression resistor, the more significant delayed field decrease in the quadrature axis becomes.This is a passive field suppression method where part of the magnetic energy stored in the field w = ½L• If2 is converted into heat in the discharge resistor. The supply voltage UG must be reduced quickly to zero. Otherwise the field current does not come down to zero and the resistor RE is overloaded. The arrangement is simple and uses a normal dc-breaker.