Advantages of Static Relays

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Advantages of Static Relays

Static r elays in general possess the fol lowing advantages:

1. Low burden on current and voltage transformers, since the operating power is. in manycases, from an auxiliary d.c. supply.

2. Absence of mechanical inertia and bouncing contacts, high resistance to shock andvibration.

3. Very fast operation and long life.4. Low maintenance owing to the absence of moving parts and bearing friction.5. Quick reset action and absence of overshoot.6. Ease of providing amplification enables greater sensitivity.7. Unconventional characteristics are possiblethe basic building blocks of semiconductor

circuitry permit a greater degree of sophistication in the shaping of operatingcharacteristics, enabling the practical utilization of relays with operating characteristics

more closely approaching the ideal requirements.

8. The low energy levels required in the measuring circuits permit miniaturization of therelay modules.

Differences between Shunt Reactor and Power TransformerPostedSep 18 2012byEdvardinTransformers,Transmission and Distributionwith2 Comments

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Differences between Shunt Reactor and Power Transformer

Main Differences

Shunt Reactor and Transformer both appearsimi lar in construction. Reactors are also oftenequipped with Fans for cooling similar to Power Transformers.

However, there are major differences between the two. While aPower Transformeris designedfor efficient power transfer from one voltage system to another, ashunt reactoris intended only

to consume reactive VArs(or in other words it can be stated as to produce lagging VArs).

Thus, there are more than one windingon a Power Transformer with magnetic core which carry

the mutual flux between the two. In reactor there isjust one windi ng. The core is not therefore

meant only to provide a low reluctance path for flux of that winding to increase the Inductance.

In case of a Power Transformer, primary Ampere-Turns (AT)is sum of exciting AT andsecondary AT. AT loss (in winding resistance, eddy loss and hysteric loss) is kept to as

minimum as possible. Exciting AT is small compared with the secondary AT. Rated current isbased on the load transfer requirement.

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Detailed view of an iron core divided by air gaps

Magnetizing currentis small and is negligible value when compared with the secondary rated

current. Further, since mutual flux is main flux which results in transformation, leakage flux is

kept small and will be based on fault current limitation.

In case of a Shunt Reactor due to absence of other windings, all primary AT is equal to theexciting AT. Similar to a Power Transformer, loss in AT (in winding resistance, eddy current and

hysteresis) are also kept to minimum by design. Magnetizing ATis major component of a Shunt

Reactor. Reactor magnetizing current is its rated current.

Since a Shunt Reactor magnetizing current is large, if it is designed with Iron alone as a Power

Transformer, there will be large hysteresis loss. Air gaps in Iron core are provided in a ShuntReactor to reduce this loss and to minimize the remanent flux in the core.

Thus a Shunt Reactor may also be constructed without iron(air-core).

By construction, a Shunt Reactor can be oil immersedor dry type for both with and without iron core.

Dr y type Reactorsare constructed as single phase units and are thus arranged in a fashion tominimize stray magnetic field on surrounding (in the absence of metallic shielding). When such

an arrangement is difficult, some form of magnetic shielding is required and designed with care

to minimize eddy current loss and arcing at any joints within the metallic loops. One of theadvantages of dry type reactor is absence of inrush current.

Oil immersed reactors can be core-lessor with gapped ir on core. These are either single phaseor three phase design with or without fan cooling. These are installed within tanks which hold oil

& act as metallic magnetic shields.

In some cases, a Shunt Reactor may have additional small capacity winding which can provide

power for small station power loads. Since Shunt Reactor rating is normally based on MVAr

rating, this added station load VA shall be accounted for in designing the Reactor for suchapplications.

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Types of shunt reactors

Shunt reactors are used inhigh voltage systemsto compensate for the capacitive generation oflong overhead lines or extended cable networks.

The reasons for using shunt reactors are mainly two

The first reason is to limit the overvoltages and the second reason is to limit the transfer of

reactive power in the network. If the reactive power transfer is minimized i. e. the reactive power

is balanced in the different part of the networks, a higher level of active power can be transferredin the network.

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Reactors to limit overvoltages are most needed in weak power systems, i.e. when network short-circuit

power is relatively low.

Vol tage increase in a system due to the capacitive generati on i s:

U(%) = QC x 100 / Ssh.c

where:

Qc is the capacitive input of reactive power to the network

Ssh.c is the short circuit power of the network

With increasing short circuit power of the network the voltage increase will be lower and the

need of compensation to limit over-voltages will be less accentuated.

Reactors to achieve reactive power balance in the different part of the network are most neededin heavy loaded networks where new lines cannot be built because of environmental reasons.

Reactors for this purpose mostly are thyristor control ledin order to adapt fast to the reactive

power required.

Especially in industrial areas with arc furnacesthe reactive power demand is fluctuatingbetween each half cycle.

In such applications there are usually combinations of:

1. Thyristor controlled reactors (TCR) and2. Thyristor switched capacitor banks (TSC).

These together makes it possible to both absorb, and generate reactive poweraccording to the

momentary demand.

Four leg reactors also can be used for extinction of the secondary are at single-phase reclosing inlong transmission lines. Since there always is a capacitive coupling between phases, this

capacitance will give a current keeping the are burning, a secondary arc.

By adding one single-phase reactor in the neutral the secondary arc can be extinguished and thesingle-phase auto-reclosing successful.

Resource:Shunt Reactors and Shunt Reactor Protection - S.R. Javed Ahmed

220V DC System at Thermal Power Station

PostedFeb 15 2012byBipul RamaninEnergy and Power,Maintenancewith0 Comments

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UPS Battery & Critical Reserve Power (photo by Infinity Power Solutions)

The 220V DC system supplies direct current as source of operating power for control, signaling,relays, tripping and closing ofswitchgears, emergency motors of most important auxiliary

systems. Under normal conditions of station generation, the storagebattery unitsare keptfloating in DC bus bars by means of the trickle chargers (also known as float chargers). The

trickle chargers of each battery unit, which is a rectifier with AC input, is normally made to take

all DC requirements of the power station without allowing the battery to discharge. This is

achieved by maintaining the DC output voltage of trickle charger a few volts higher than thevoltage of the battery.

With this, the trickle charger besides meeting all the DC requirements of the power station,supplies a few hundred milliamps of direct current to the battery to compensate the loss in the

capacity of the battery due to action between the plates of the cell. With this arrangement, the

battery remains connected to the DC bus bars as a standby supply source and immediatelysupplies the DC load in the vent of temporary failure of complete AC system.

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The complete AC power system failure in a power station is known as emergency situation. DC

battery units are designed to supply station DC loads for an emergency period of one hour. The

tickle charger normally supplies the station DC load and the momentary loads will also becatered for by the trickle charger and if such a load is more than its capacity, the battery being in

parallel with the trickle charger will supply the excessive load. The trickle charger will normally

be kept operating at around 1152.15 V ie 247 volts. In case of AC mains failure the full batteryof 115 cells will supply the load ie 230 volts. If the emergency lasts for one hour with anappropriate load of 450 Amps, then battery will supply the load for one hour when its end

voltage will drop down to 1.75 volts per cell ie 201 volts.

After the emergency when the quick charger is closed the full battery will receive a boost charge

and at the same time only the voltage of 98 cells will appear across the load.

If a second emergency occurs during quick charging, then immediately all the 115 cells are

connected to the bus by closing the switch meant for the purpose. During routine dailytestingof

emergency DC motors connected to main distribution board middle section, supply has to be

taken from the quick charger and the middle section has to be kept isolated from the left andright sections of main distribution board. This is to test the quick charger.

Types of battery being used:

1. Lead-acid battery tubular2. Lead-acid battery plaint3. Ni-Cd battery

Procedure followed in commissioning a battery

1. The battery is charged initially to its capacity. The lead acid Battery has a capacity of 1000AH ie itmay be charged for 10 hrs with charging current of 100 A or 5 hrs with charging current of 200

A. in case of Ni-Cd battery with a capacity of 2500 AH is charged for 12.5 hrs. with a charging

current of 200A.

2. Now the battery is discharged at the rate of 10% of its capacity in case of lead-acid battery and20% or 40% of its capacity in case of Ni-Cd battery.

3. Now the battery is recharged to its capacity.4. Constant voltage charging of battery is called float charging. A lead acid battery of cell voltage

2.2V is float charged upto 2.42 V. A Ni-Cd battery of cell voltage 1.2V is float charged upto 1.41

V.

5. Constant current charging of a battery is called boost charging. A lead acid battery with bankvoltage 237 may be boost charged to 279V. A Ni-Cd battery with bank voltage 242 may be boostcharged to 283V.

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Equipment used in 220V DC supply system

Sources of AC power

Two sources of AC power have been providedfor both quick charger and trickle charger, one isthe normal source and other is standby. AC power supply to the chargers is through transformers

having off-load tap changing arrangement. An AC voltage-signaling relay communicates; AC

voltage low when the supply voltage becomes low.

Voltage level indicating device

A voltage level indicating device in MDB gives audio and visual annunciation when the DC bus

voltage changes beyond set low (180-210) and high limits (240-270).

AVR

The DC voltage is maintained at desired value automatically by means of AVR unit provided atpanel board.

Insulation monitoring device

This device annunciates when the insulation resistance of either positive bus to earth or negativebus to earth falls below 20 kilo ohms and also when the ratio of insulation resistance of positive

bus to earth to negative busto earth is 1.5 or above.

Flickering light device

This has been installed in the MDB, for flicker supply to control and check whether device is inorder or not. Control and signaling panels have two sets of bus bars, one fed by main distribution

board left section and the other by MDB right section. The loads of the first panel should be kept

switched to the set of bus bars fed by MDB. Left section and the loads of the second panelshould be kept connected to the set of bus bars fed by MDB right section.

Electrostatic Precipitator

Dust extraction from industrial gases has become necessity for environmental reasons or forimproving production. Most of the plants in India use coal as fuel for generating steam. The

exhaust gases contain large amount of smoke and dust, which are being emitted into the

atmosphere. This has posed a real threat to the mankind as a devastating health hazard. Hence itbecomes necessary to free the exhaust from smoke and dust.

There are various ways of extracting dust. Electrostatic dust precipitation method is most widely

used as its efficiency is excellent and it is easier to maintain. I ts other advantages are:

Ability to treat large volumes of gases at high temperature Ability to cope up corrosive atmosphere

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Offer low resistance path for gas flow.An electrostatic precipitator is equipment, which utilizes an intense electric force to separate

suspended particles from the flue gases. The process invol ves:

Electrical charging of suspended particles

Collection of charged particles from collecting electrode. Removal of particles from collecting electrode.

The flue gases pass between electrodes and are subjected to an intense electric field. The

emissive electrodes are connected to the negative polarity of HV power supply while collecting

electrodes are connected to positive polarity and grounded.

The HV power supply equipment is supplied in two parts:

The high voltage transformer rectifier (HVR)

The electronic controller (EC)

The EC-HVR equipment provides high voltage DC across the precipitator electrodes. The EC provides

controlled AC voltage through thyristors (SCR) and associated controls to the primary of step up

transformer. The EC has been designed to supply 0 to 415V to the primary of step up transformer

through AC reactor. The equipment operates as constant current controller.

Heaters

Heaters are provided to raise the temperature of flue gases, as they become conductive when

heated. 24 heaters are provided for stage I electrostatic precipitators. Rating: 550W heaters

Zones

The flue gases from the boiler section reach electrostatic precipitator section through ducts. The

flue gases are allowed to pass through various zones each having its own heaters, collecting andemissive electrodes and DC supply. These zones are provided to lessen the burden on a single

zone and to take the load of other zone in case ofmaintenanceor damage of a particular zone.

Stage I have 16 zones eight belonging to PASS A and rest to PASS B. Stage II has 20 Zones fivebelonging to each PASS A, PASS B, PASS C and PASS D.

Diodes

These are provided to rectify the AC voltage to the required DC voltage for electrostatic

precipitators to work. The required DC voltage is 70 kV, 1000 mA. Type: BY 127

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Motors

Rapping motors are provided along with each zone. A hammer is coupled to each of the motorsshaft. Due to rotary motion of motor these hammers hit the collecting electrodes after a certain

time delay and the ash is allowed to flow down through outlet in form of slurry. Rating: .5A

motors

A GD screen (gas diverting) motor is also provided in electrostatic precipitator to provide a

zigzag motion of flue gas so as to allow the heavy dust particles to settle down and removed.

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Features:

Spark regulation

Flashovers of extremely low intensity are difficult to detect using the comparator technique. Nondetection results in sustained arcing which may damage the collecting electrode. For such digital

detection system is adopted.

Fast ramp control

In case of fast changes in operating conditions of precipitator many sparks may occur within a

short time reducing current to a low value, when the disturbance disappears, it may take a

relatively long time before the current can assume its normal value. This is the case particularly

if selected rate of rise is low.

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Modes of operation

Back corona mode

In this mode the precipitator voltage decreases with increase in precipitator current. This reducesthe efficiency of precipitator and consumes unnecessary power.

Charge ratio mode

In a high resistive dust a potential gradient is created within the dust layers which causes

occurrence of local sparks in dust layer. This spurious discharges or BACK CORONA occurs assoon as potential gradient is high. This has negative impact on efficiency.

Charge ratio

This mode supplies current in pulses and provides a dense corona for a short circuit time and atsame time gives a low current to avoid back corona.

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Sources: Internet and several books of Electrical Engineering

Historical Review of Power System Stability Problems

PostedDec 8 2010byEdvardinEnergy and Power,Transmission and Distributionwith1 Comment

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Historical Review of Power System Stability Problems

As electric power systems have evolved over the last century, different forms of instability haveemerged as being important during different periods. The methods of analysis and resolution ofstability problems were influenced by the prevailing developments in computational tools,

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stability theory, and power system control technology. A review of the history of the subject is

useful for a better understanding of the electric power industrys practices with regard to system

stability.

Power system stability was first recognized as an important problem in the 1920s (Steinmetz,

1920; Evans and Bergvall, 1924; Wilkins, 1926). The early stability problems were associatedwith remote power plants feeding load centers over longtransmission lines.

With slow exciters and noncontinuously acting voltage regulators, power transfer capability wasoften limited by steady-state as well as transient rotor angle instability due to insufficient

synchronizing torque.

To analyze system stability, graphical techniques such as the equal area criterion and power circle

diagrams were developed. These methods were successfully applied to early systems which could be

effectively represented as two machine systems.

As the complexity of power systems increased, and interconnections were found to beeconomically attractive, the complexity of the stability problems also increased and systems

could no longer be treated as two machine systems. This led to the development in the 1930s ofthe network analyzer, which was capable of power flow analysis of multimachine systems.

System dynamics, however, still had to be analyzed by solving the swing equations by hand

using step-by-step numerical integration. Generators were represented by the classical fixedvoltage behind transient reactance model. Loads were represented as constant impedances.

Improvements insystem stabilitycame about by way of faster fault clearing and fast actingexcitation systems. Steady-state aperiodic instability was virtually eliminated by the

implementation of continuously acting voltage regulators. With increased dependence on

controls, the emphasis of stability studies moved from transmission network problems togenerator problems, and simulations with more detailed representations of synchronousmachines and excitation systems were required.

The 1950s saw the development of the analog computer, with which simulations could be carried

out to study in detail the dynamic characteristics of a generator and its controls rather than the

overall behavior of multimachine systems.

Later in the 1950s, the digital computer emerged as the ideal means to study the stability

problems associated with large interconnected systems. In the 1960s, most of the power systems

in the U.S. and Canada were part of one of two large interconnected systems, one in the east and

the other in the west. In 1967, low capacity HVDC ties were also established between the eastand west systems. At present, the power systems in North America form virtually one large

system. There were similar trends in growth of interconnections in other countries.

While interconnections result in operating economy and increased reliability through mutual

assistance, they contribute to increased complexity of stability problems and increasedconsequences of instability. The Northeast Blackout of November 9, 1965, made this abundantly

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clear; it focused the attention of the public and of regulatory agencies, as well as of engineers, on

the problem of stability and importance of power system reliability.

Until recently, most industry effort and interest has been concentrated on transient (rotor angle)

stability. Powerful transient stability simulation programs have been developed that are capable of

modeling large complex systems using detailed device models. Significant improvements in transientstability performance of power systems have been achieved through use of high-speed fault clearing,

high-response exciters, series capacitors, and special stability controls and protection schemes.

The increased use of high response exciters, coupled with decreasing strengths oftransmission

systems, has led to an increased focus on small signal (rotor angle) stability.

This type of angle instability is often seen as local plant modes of oscillation, or in the case ofgroups of machines interconnected by weak links, as interarea modes of oscillation. Small signal

stability problems have led to the development of special study techniques, such as modal

analysis using eigenvalue techniques (Martins, 1986; Kundur et al., 1990). In addition,

supplementary control of generator excitation systems, static Var compensators, and HVDCconverters is increasingly being used to solve system oscillation problems.

There has also been a general interest in the application of power electronic based controllers

referred to as FACTS (Flexible AC Transmission Systems) controllers for damping of power

system oscillations (IEEE, 1996).

In the 1970s and 1980s, frequency stability problems experienced following major system upsets

led to an investigation of the underlying causes of such problems and to the development of longterm dynamic simulation programs to assist in their analysis (Davidson et al., 1975; Converti et

al., 1976; Stubbe et al., 1989; Inoue et al., 1995; Ontario Hydro, 1989). The focus of many of

these investigations was on the performance of thermal power plants during system upsets(Kundur et al., 1985; Chow et al., 1989; Kundur, 1981; Younkins and Johnson, 1981).

Guidelines were developed by an IEEE Working Group for enhancing power plant response

during major frequency disturbances (1983).

Analysis and modeling needs of power systems during major frequency disturbances was also

addressed in a recent CIGRE Task Force report (1999). Since the late 1970s, voltage instabilityhas been the cause of several power system collapses worldwide (Kundur, 1994; Taylor, 1994;

IEEE, 1990). Once associated primarily with weak radial distribution systems, voltage stability

problems are now a source of concern in highly developed and mature networks as a result of

heavier loadings and power transfers over long distances. Consequently, voltage stability is

increasingly being addressed in system planning and operating studies.

Powerful analytical tools are available for its analysis (Van Cutsem et al., 1995; Gao et al., 1992;Morison et al., 1993), and well-established criteria and study procedures are evolving (Abed,

1999; Gao et al., 1996).

Present-day power systems are being operated under increasingly stressed conditions due to the

prevailing trend to make the most of existing facilities. Increased competition, open transmission

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access, and construction and environmental constraints are shaping the operation of electric

power systems in new ways that present greater challenges for secure system operation. This is

abundantly clear from the increasing number of major power-grid blackouts that have beenexperienced in recent years; for example, Brazil blackout of March 11, 1999; Northeast USA-

Canada blackout of August 14, 2003; Southern Sweden and Eastern Denmark blackout of

September 23, 2003; and Italian blackout of September 28, 2003. Planning and operation oftodays power systems require a careful consideration of all forms of system instability.

Significant advances have been made in recent years in providing the study engineers with anumber of powerful tools and techniques.

A coordinated set of complementary programs, such as the one described by Kundur et al. (1994)makes it convenient to carry out a comprehensive analysis of power system stability.

SOURCE: Electric Power Generation, Transmission, and Distribution by Leonard L.