Protective Relays and their functional characteristics

An electric power system should ensure the availability of electrical energy without interruption to every load connected to the system. The most severe electrical failures in a power system are the shunt faults which are characterized by an increase in system current, reduction in voltage, power factor and frequency. 

"Protective relays and relaying systems detect abnormal conditions like faults in the electrical circuits and operate automatic switchgear to isolate faulty equipment/equipments from the system as quickly as possible."

Thus a protective relay is an automatic device which senses an abnormal condition in an electrical circuit and closes its contacts. These contacts in turn close the circuit breaker trip coil circuit, thereby it opens the circuit breaker and the faulty part of the line or equipment is disconnected from the rest of the system. 

The protective relays should operate only the concerned circuit breaker so as to disconnect the faulty equipment from the system as quickly as possible without affecting the healthy section. Relays are tested thoroughly to ensure that they will operate correctly and will assist the related circuit breaker to clear a fault, only within their specified zone. Figure 1 shows the wiring connection of Relay and Circuit Breaker.  

Fig.1: Wiring connection of Relay and Circuit Breaker.

The protective relays do not eliminate the possibility of fault occurrence. It can take action only after the fault has occurred. Situation would be ideal if the protection system could anticipate and prevent fault occurrence but this is next to impossible except where the original cause of fault creates some effect which can operate a protective relay. However Buchholz relay is one of such devices which can anticipate and prevent major faults.

Functional Characteristics of Protective Relays:

A protective relay should have certain qualities. These essential qualities are:

1.      Reliability – A protective should be reliable and must operate when it is required. Every component and circuit which is involved in the operation of the relay is vital and should be considered as a potential source of failure. Failure can be reduced by reliable design well supported by regular and thorough maintenance. Some features of design and manufacture which make relays inherently reliable are good contact material, high contact pressures, dust free enclosures, well braced joints and robust construction.

2.      Selectivity – The protective relay must be capable of selecting the part of the system which is faulty and should isolate it from the healthy one i.e. a relay should differentiate between the faulty part and the healthy part.

3.      Speed – A protective relay must be quick acting and fast otherwise may result in damage to the equipment and the system. The operating time of a relay is of the order of 30 to 100 ms, depending upon the fault level. Also the relays should not be extremely fast (less than 10 ms), otherwise it may result in undesired operation during transient faults such as lightning surges.

4.      Sensitivity – Sensitivity of a protective relay refers to the smallest value of actuating quantity at which the protective relay starts operating in relation with the minimum value of fault current in the protected zone.   

Current Limiting Reactors for Power System application

"A current limiting reactor, also called a series reactor, is a coil which has high inductive reactance as compared to its resistance." 

These reactors are used to limit the short circuit current and the effect of resulting voltage disturbances during fault conditions. The short circuit currents depend upon the generating capacity, voltage at the fault point and the total reactance between the generators and the fault point.

In large interconnected systems, the total rating of the generators is very high and also, when the system is extended by the addition of more generating units, the fault currents are also increased. So the fault current to be interrupted by the same circuit breaker will become greater than the earlier value. These short circuit currents may be large enough to cause damage to the line and other equipments of the power system network.

The short circuit current can be kept within safe limits by increasing the reactance between the source and the fault. Thus, there is a need of providing a protective reactor. By including a reactor or few reactors at strategic locations, the short circuit currents at different points in the power system can be reduced. 

The reactors allow free interchange of power under normal conditions but under short circuit conditions the disturbance is limited to the faulty section. As the resistance of the reactors is very small, thus the efficiency of the system is not affected much.

Main functions of Current Limiting Reactors:

The primary functions of a current limiting reactor are:

1.      To reduce the flow of current into a short circuit so as to protect the power system apparatus and parts of the system from excessive mechanical stress and overheating.

2.      To reduce the magnitude of voltage disturbances caused by short circuits.

3.      To localize the faults by limiting the current that flows into the fault from other healthy feeders or part of the system.

4.      To reduce the duty imposed on switching equipments during short circuits.

Construction of Current Limiting Reactor:

Reactors are normally of two types:

1.      Dry type, and

2.      Oil immersed type.

In air cored dry type reactor, the core is of air and the whole construction of the reactor is free from ferromagnetic materials. The winding of the reactor is rigidly placed on glass-reinforced synthetic resin supports. Due to the absence of iron, the reactance remains fairly constant during the flow of heavy current. Dry type reactors are usually cooled by natural ventilation and sometimes provided with forced air and heat exchanger auxiliaries. These reactors occupy relatively large space and are used only up to 33 kV.

Oil immersed reactors are used for voltages above 33 kV. These reactors are similar to power transformers in several aspects. They are either without iron core or have gaped iron core. Cooling is similar to that of power transformers. The coil assembly is oil immersed and is enclosed in a tank. Laminated iron shields are provided around the outer conductors so as to avoid the entering of magnetic flux in the surrounding iron parts. Oil immersed reactors have the advantage of smaller size, high thermal capacity and higher safety against flash-overs.

Drawbacks of Reactors:

The main drawbacks of a reactor are:

1.      The total percentage reactance of the system is increased, thus causing an increase in the reactive voltage drop.

2.      The power factor is decreased.

Location of Current Limiting Reactor:

By including a reactor or few reactors at strategic locations, the short circuit currents at different points can be reduced. The reactors may be connected in –

1.      Series with the generator (Generator Reactor),

2.      Series with each feeder (Feeder Reactor),

3.      Between bus-bar sections (Bus-bar Reactor)

When a reactor is connected in between the generator and the bus, the reactor is known as generator reactor. Modern alternators are designed to have sufficiently large reactance (may be 2.0 p.u.) to protect themselves against dead 3-phase short circuits at its terminals. With such a large reactance the current during short circuits at terminals may be less than full load current therefore, externals reactors are not required. Current limiting reactors are only used with old generators having low values of reactance.

Feeder reactors are connected in series with the feeders. The advantage is that the voltage of the bus does not drop substantially in the event of fault on any one feeder. Thus, other generators continue to supply the load and other feeders are also not affected. But there is constant voltage drop and power loss during normal operation in case of feeder reactors. Cost-wise feeder reactors are expensive.

Bus-bar reactors are connected between bus sections. Two systems of bus-bar reactors are common. They are Ring system and the Tie-bar or the Star system. In the ring bus-bar system, under normal operation each generator supplies to the feeder connected to its own section and thus there will be no current through the reactors (under normal operation). Therefore there is no voltage drop or power loss in the reactor during normal operation. In case of fault on any one feeder, only one generator feeds the fault while the current from the other generator is limited because of the bus-bar reactor. This system facilitates parallel operation of systems and is extensively employed for plants of moderate output.

The tie-bar system is better and more flexible than the ring system. In this system, the generators are connected to the common bus-bar (tie-bar) through the reactors but the feeders are fed from the generator side of the reactors. 

In the ring system the short circuit current due to a fault on any bus bar section, is fed from the generators connected to other sections through one reactor, whereas in the tie-bar system the current flows through two reactors in series. Therefore this system requires only half the reactance compared to the ring system. 

In the tie-bar system, the short circuit MVA or the short circuit current is independent of the number of bus-bar sections. Thus, extra generators may be added to the system without addition of extra circuit breakers or without increasing the existing reactance.

Operating Cost of a Thermal Power Plant

India is among the nations where power sector is booming up. Of its total installed power capacity more than 60% share is of coal-fired power plants. Coal-fired or the thermal power plants have a significant operating cost and a matter of concern for the agencies that care for the environment.

The optimal power system operation involves factors such as economy of operation, system security, emission of fossil-fuel fired plants, etc. All the above considerations are conflicting and hence a compromise has to be made for the optimal operation of the power system. 

The main aim of economic dispatch problem is to minimize the total cost of generating the real power of various power plants while satisfying the constraints. The operating cost is insensitive to reactive loading on a generator, and hence the manner in which the reactive load on a station is shared among various on-line generators is not going to affect the operating economy. 

The operating cost of hydro power plants is negligible, but there is limitation of availability of water in case of such power plants.   

Factors affecting the Operating Cost of a Thermal Power Plant:

The operating cost of a thermal power plant depends on the operating efficiency of generators, fuel cost, and transmission losses. In many cases the plants are located far from the load centre and hence the transmission losses to transmit the power to the major substations may be considerably high. Under such a situation, even the most efficient generator in the system does not guarantee the minimum operating cost. 

Therefore, the operating cost a thermal power plant plays a very important role in the economic scheduling of generators.

Incremental fuel-cost curve

The input to the thermal power plant is usually measured in Btu/h, and the output is given in MW. A simplified input-output curve of a thermal generating unit is called the heat-rate curve. The fuel input in Btu/h can be replaced by cost of the fuel. Normally the fuel cost of a generator is expressed as a quadratic function of real power generation. The incremental fuel-cost curve is obtained by plotting the derivative of the fuel-cost curve and the real power. This incremental fuel-cost curve is a measure of how costly it will be to produce the next MW of power.

Impact of Transmission losses

In the simplest economic dispatch case the transmission losses are neglected i.e. the model assumes that the generator and the load is at the same bus. This is the case when the transmission distance is very small and the load density is very high. As the line or transmission losses are neglected, the total demand is the sum of all the generation. When the transmission losses are neglected, the optimal dispatch of generation is obtained with all plants operating at equal incremental production cost.

However, in an actual power system, thermal plants are usually pit-heads plants located far away from the load centre. Hence, it is necessary to consider the transmission losses and in this case the generation should equal the total demand plus the transmission losses.  

Per Unit Representation in Power System Studies

In a large interconnected power system, the various sections of the system are connected through transformers and have different voltage levels. In the computation of power system problems it is more convenient to express impedance, current, voltage and power in terms of per-unit (p.u.) value rather than in ohms, amperes, volts, and watts.

"The per-unit value of any quantity is defined as the ratio of its actual value to another arbitrarily chosen value of the quantity of the same dimension." 

This arbitrarily chosen value is called the base or reference value. Hence p.u. values are dimensionless.

Voltage, current, apparent power and impedance are so related that the selection of base values for any two of them determines the base values for the remaining two. 
Usually base power in kVA or MVA and base voltage in kV are the quantities selected to specify the base. 

For example; if a base voltage of 10 kV is selected, voltages of 8, 10 and 11 kV would be specified as 0.8, 1.0, and 1.1 p.u. respectively.

Advantages of p.u. system:

Some of the advantages of p.u. system are:

1.      The current, impedance, losses etc. vary considerably with the variation of terminal voltage, power rating etc., whereas the p.u. values of machines of the same type and widely different rating usually lie within a narrow range.

2.      The p.u. impedance referred to either side of a transformer, whether 1-phase or 3-phase, is the same when expressed on the proper base. For a 3-phase transformer the p.u. impedance for both the sides is the same regardless of the 3-phase connection i.e. delta or star.

3.      The computational effort required is very much reduced with the use of p.u. representation.  

Selection of Base values:

The selection of base values of apparent power and voltage is made in order to reduce the work required in computation. Normally the p.u. values of various equipments are given in terms of their own voltage and kVA rating. 

In a large power system, the various sections of the system having different equipments are connected through transformers and have different voltage and capacity levels. It is necessary to refer all the per unit values to a selected base value. At first a convenient value of MVA is selected as the base value and the same value is used as base MVA in all the sections of the power system. The base MVA so selected may be the total MVA of the system, the largest MVA of a section or any round figure as 100, 500 MVA etc. 

After the selection of base MVA, the base voltages for each section are to be selected. The rated voltage of the largest section may be taken as the base voltage of that section. The base voltages for remaining sections are then assigned according to the transformation ratio of the transformers. When a common base MVA and base voltages of different sections are made, the p.u. impedance of various sections can be calculated to draw the single-line diagram.

The base selected should be one that gives p.u.values of rated voltage and current approximately equal to one in order to simplify the calculations. Time required will be less if the base values is so selected that few p.u. quantities already given need not to be converted to a new base. 

Normally the p.u. impedance of various equipments of a power system is given corresponding to their own voltage and kVA rating. Since all impedance in any one part of a system must be expressed on the same impedance base when doing calculations, therefore it is desired to convert p.u. quantities from one base to another. It is possible to change the base of p.u. quantities with the help of a very simple formula. 

Single-Line diagram representation of a Power System

The complete circuit diagram for a 3-phase power system is rarely used to represent and convey the information about the system. 

"It is much more practical to represent a power system by means of simple symbols for each component resulting in the so called single-line diagram."

A balanced 3-phase system is studied on per phase basis. It is always solved as a single-phase circuit consisting of one of the three lines and a neutral return. The loop impedance of a single phase circuit is supposed to be concentrated in one conductor only with the impedance of the return conductor assumed to be zero. Hence the transmission line is represented by a single line and the main components such as generators, transformers, motors etc are indicated by standard symbols. Such as simplified diagram of an electric power system is called single-line diagram. 

Thus a 3-phase balanced system is effectively replaced by a single line or one line diagram.

Purpose of single-line diagram:

The purpose of single-line diagram is to provide the important information about the power system. The importance of different features of a system varies with the problem under consideration. Any particular component may or may not be shown in a single-line diagram depending on the information required in a system study. 

For instance, the line diagram for load flow studies may not include circuit breakers. On the other hand, in stability studies position of circuit breakers and relays are of extreme importance. In a short circuit studies, three separate diagrams to represent positive, negative and zero sequence networks are shown.

In single-line diagrams, the connection of generators and transformers (star /delta connection, neutral grounding etc) are indicated by symbols drawn by the side of the representation of these elements. Generators and transformers are specified in 3-phase MVA, line to line voltage and per phase reactance (or impedance in case of transformer). Loads are specified in 3-phase MW, line to line voltage and power factor.

Impedance and reactance diagram:

To compute the performance of a power system under load conditions or during short circuits, the single-line diagram must be converted into an impedance diagram. 

In an impedance diagram, the equivalent circuit of the transmission line is represented by 'nominal pi' model with the total resistance and inductive reactance of the line in series and the total capacitance to neutral divided between its parallel arms at the two ends. The generators are represented as voltage source with appropriate values of series resistance and reactance. 

Loads are assumed to be passive and are shown by resistance and inductive reactance in series. Since the magnetizing current of a transformer is usually very small when compared to its full load current, the magnetizing circuit is usually omitted in the equivalent circuit of a transformer.

The impedance diagram can be further simplified by making certain assumptions. The inductive reactance of a system is much larger than its resistance. Hence, in many power system studies the resistance of generator and transformer winding, resistance of transmission lines, line charging and magnetizing circuits of transformers are neglected. Loads which do not contain rotating machines have negligible effect on the total line current during a fault and are normally omitted. However, synchronous motor loads are always included in fault calculations as their generated emfs contribute to the short circuit current.         

To simplify the calculations, if all the static loads, all resistances, the magnetizing circuit of each transformer and the capacitance of the transmission line are omitted, the impedance diagram becomes the reactance diagram. 

These simplifications apply to fault calculations only and not to load flow studies. The impedance and reactance diagrams are also called as positive sequence diagrams since they show impedance to balanced currents in a symmetrical 3-phase system. 

Classification of Electrical Faults

As mentioned in my previous blog 

"a fault is the defect in the electrical circuit due to which the current in the circuit is diverted from the intended path."

Because of a fault the value of current and voltage at various points in the network changes giving rise to abnormal operating conditions.

Electrical faults may be broadly classified into two groups:

1.      Symmetrical faults, and

2.      Unsymmetrical faults.

Symmetrical faults

In symmetrical faults, also called three phase short circuits, all the three phases are short circuited to each other and often to earth also. Such faults are balanced and symmetrical as the system remains balanced even after the occurrence of the fault. During such a fault the fault current in the three lines of a 3-phase circuit are equal in magnitude and displaced by 120 electrical degrees from one another.

Though the symmetrical faults are rare, but when occurs they generally lead to most severe fault current flow. Balanced short circuit calculations are performed to find these large currents. A power network comprises of synchronous generators, transformers, transmission and distribution lines and loads. Loads are often neglected during faults, as the voltage drops down to such a low value that current drawn by loads can be neglected in comparison to fault currents.

Unsymmetrical faults

The majority of faults that occur in a power system are unsymmetrical faults involving only one or two phases. The most common type of unsymmetrical fault is a short circuit between a phase and the earth. In case of unsymmetrical faults, voltages and currents in the network become unbalanced and each phase is to be treated individually for computational purpose.

The magnitude of fault currents in the three lines is different having unequal phase displacements. The calculation procedure called as “method of symmetrical components” is used to find the currents and voltages during this type of fault.       

Electrical faults can also be classified as:

1.      Shunt faults or short faults, and

2.      Series faults or open faults.

Shunt or the short faults involve short circuit between power conductors or power conductors to earth. Shunt fault in a 3-phase line or system can be classified as:

1.      Single line to ground (LG) fault,

2.      Line to line (LL) fault,

3.      Double line to ground (LLG) fault,

4.      Three phase short circuits (LLL), and

5.      Three phase to ground (LLLG) fault.

Of the above mentioned faults, single line to ground, line to line and double line to ground faults are unsymmetrical faults, whereas three phase short circuits and three phase to ground faults are symmetrical faults. These faults may occur at the terminals of the generator and or transformer, on the conductors of a line or any other part of the power system. Often the path to earth contains resistance in the form of arc. Shunt faults are characterized by increase in current and fall in voltage and frequency.

A series fault is an unbalance in the line impedances mainly due to open conductors. It does not involve any connection between lines or between lines or between line and ground at the fault point and therefore called as series fault. These faults disturb the symmetry in one or two phases and are therefore unbalanced faults.   

Faults in Power System

"A fault in an electrical circuit is defined as the defect in the electrical circuit because of which the current in the circuit is diverted from the intended path."

For example suppose a circuit has two parallel paths, opening up of a path will divert the current to the other path and in the process it may damage the path or the conductor. Thus, faults can damage or disrupt the power system in many ways.

Causes:

In a power system, the faults occur because of insulation failure which may be because of a system over-voltage such as switching surges or lightning stroke. Faults may also be due to a broken insulator or a conductor. Various other reasons such as improper operating habits may also lead to a fault; for example, loading a distribution transformer beyond its normal rated capacity. 

Nearly one half of the faults occur on power lines which are widely branched, have greater length, operate under variable weather conditions and are more exposed to atmospheric disturbances.  

Also read:

• Classification of  Electrical Faults

Effects of fault

Faults give rise to abnormal operating conditions. When a fault occurs at any point in the power system large currents, large forces and or abnormal voltages are developed. The excessive current because of the fault is determined by the internal e.m.f.s of the machines in the network, their impedances, and the impedance in the network between the machines and the fault.

Faults currents, also called short circuit currents, are many times greater than the normal currents. Large voltage stresses the insulation of the various equipments, which are on the way, beyond their breakdown value causing the failure. 

Similarly large currents overheat the equipment or the element of the power system. Sometimes faults lower the system voltage below the permissible voltage limit causing unwanted and teasing interruption of various equipments and components. Faults can also cause a three-phase system to become unbalance.

Action to be performed during a fault

It is necessary that the faults or the faulty section should be removed immediately so that the normal operation of the rest of the system is maintained. The protective relays employed in the power system or network should immediately detect the faults or the faulty section without fail and send trip signal for the operation of circuit breakers.

To obtain proper setting of the protective relays and the interrupting capacities of circuit breakers, the values of these fault currents and voltages should be known with great accuracy. Short circuit studies and calculations provide currents and voltages on a power system during fault conditions.