Power systems Protection ( CT , PT , CB , RELAY)

Instrument Transformers

Instrument transformers are high accuracy class electrical devices used to isolate or transform voltage or current levels. The most common usage of instrument transformers is to operate instruments or metering from high voltage or high current circuits, safely isolating secondary control circuitry from the high voltages or currents. The primary winding of the transformer is connected to the high voltage or high current circuit, and the meter or relay is connected to the secondary circuit. Instrument Transformers are used in AC system for measurement of electrical quantities i.e. voltage, current, power, energy, power factor, frequency.

Advantages of Instrument Transformers

The large voltage and current of AC Power system can be measured by using small rating measuring instrument i.e. 5 A, 110 – 120 V.

By using the instrument transformers, measuring instruments can be standardized which results in reduction of cost of measuring instruments..

Instrument transformers provide electrical isolation between high voltage power circuit and measuring instruments.

Due to low voltage and current level in measuring and protective circuit, there is low power consumption in measuring and protective circuits.

Types of Instrument Transformers

Instrument transformers are of two types –

Current Transformer (C.T.)

    

Potential Transformer (P.T.)

Current transformer is used to step down the current of power system to a lower level to make it feasible to be measured by small rating Ammeter (i.e. 5A ammeter)

Primary of C.T. is having very few turns. Sometimes bar primary is also used. Primary is connected in series with the power circuit. Therefore, sometimes it also called series transformer. The secondary is having large no. of turns. Secondary is connected directly to an ammeter. As the ammeter is having very small resistance. Hence, the secondary of current transformer operates almost in short circuited condition. One terminal of secondary is earthed to avoid the large voltage on secondary with respect to earth. Which in turns reduce the chances of insulation breakdown and also protect the operator against high voltage. More ever before disconnecting the ammeter, secondary is short circuited through a switch ‘S’ as shown in figure above to avoid the high voltage build up across the secondary.

Potential transformer is used to step down the voltage of power system to a lower level to make is feasible to be measured by small rating voltmeter i.e. 110 – 120 V voltmeter. A typical connection diagram of a potential transformer is showing figure below.

Primary of P.T. is having large no. of turns. Primary is connected across the line (generally between on line and earth). Hence, sometimes it is also called the parallel transformer. Secondary of P.T. is having few turns and connected directly to a voltmeter. As the voltmeter is having large resistance. Hence the secondary of a P.T. operates almost in open circuited condition. One terminal of secondary of P.T. is earthed to maintain the secondary voltage with respect to earth. Which assures the safety of operators.

Relay .

A relay is an electromagnetic switch operated by a relatively small electric current that can turn on or off a much larger electric current. The heart of a relay is an electromagnet (a coil of wire that becomes a temporary magnet when electricity flows through it). Relays are the primary protection as well as switching devices in most of the control processes or equipments. A relay is a switching device as it works to isolate or change the state of an electric circuit from one state to another.

Types of relays

1. Electromagnetic Relays

These relays are constructed with electrical, mechanical and magnetic components, and have operating coil and mechanical contacts. Therefore, when the coil gets activated by a  supply system, these mechanical contacts gets opened or closed. The type of supply can be AC or DC.

2. Solid State Relays

Solid State uses solid state components to perform the switching operation without moving any parts. Since the control energy required is much lower compared with the output power to be controlled by this relay that results the power gain higher when compared to the electromagnetic relays. These are of different types: reed relay coupled SSR, transformer coupled SSR, photo-coupled SSR, and so on.

3. Hybrid Relay

These relays are composed of electromagnetic relays and electronic components. Usually, the input part contains the electronic circuitry that performs rectification and the other control functions, and the output part include electromagnetic relay.

4. Thermal Relay

These relays are based on the effects of heat, which means – the rise in the ambient temperature from the limit, directs the contacts to switch from one position to other. These are mainly used in motor protection and consist of bimetallic elements like temperature sensors as well as control elements. Thermal overload relays are the best examples of these relays.

5. Reed Relay

Reed Relays consist of a pair of magnetic strips (also called as reed) that is sealed within a glass tube. This reed acts as both an armature and a contact blade.  The magnetic field applied to the coil is wrapped around this tube that makes these reeds move so that switching operation is performed.

Types of Protective Device 

1. Fuse

It generally means a fuse wire,placed in a fuse holder.It is a safety device,which protects electrical and electronic circuit against over loads,short circuit and earth faults. The fuse link or fuse wire is made of low resistivity material and low melting point.

2. Earth leakage circuit breaker

It is a domestic safety device,which trips the circuit when there is a small leakage to earth or body of the appliance.Thus it protects the operator from shocks and accidents.This is connected in the circuit of the appliance to be protected.

3. MCB & ELCB

It is the combination of both MCB and ELCB palced in one unit.It acts on both the occasion of   earth leakage and overload and protect the circuit,appliance and the operator.

4. Earthing and Grounding

It refers to connecting the metal body of an electrical appliance,machinery or an electrical installation to earth,through a low resistance wire,is called Earthing or Grounding.

Types of Faults

Electrical faults in three-phase power system mainly classified into two types, namely open and short circuit faults. Further, these faults can be symmetrical or unsymmetrical faults.

1. Open Circuit Faults

These faults occur due to the failure of one or more conductors. The most common causes of these faults include joint failures of cables and overhead lines, and failure of one or more phase of circuit breaker and also due to melting of a fuse or conductor in one or more phases. Open circuit faults are also called as series faults.

2. Short Circuit Faults

A short circuit can be defined as an abnormal connection of very low impedance between two points of different potential, whether made intentionally or accidentally. These are the most common and severe kind of faults, resulting in the flow of abnormal high currents through the equipment or transmission lines. If these faults are allowed to persist even for a short period, it leads to the extensive damage to the equipment Short circuit faults are also called as shunt faults. These faults are caused due to the insulation failure between phase conductors or between earth and phase conductors or both.

3. Symmetrical Faults

Symmetrical fault is also called as balanced fault. This fault occurs when all the three phases are simultaneously short circuited. These faults rarely occur in practice as compared with unsymmetrical faults. Two kinds of symmetrical faults include line to line to line (L-L-L) and line to line to line to ground (L-L-L-G). 

4. Unsymmetrical Faults

The most common faults that occur in the power system network are unsymmetrical faults. These faults are also called as unbalanced faults as it causes unbalanced currents in the system. Unsymmetrical faults include both open circuit faults (single and two phase open condition) and short circuit faults (excluding L-L-L-G and L-L-L). Three types of symmetrical faults occurred due to the short circuit conditions, namely phase or line to ground (L-G) fault, phase to phase (L-L) fault and double line to ground (L-L-G) fault.

Fault level calculations

Circuit Breakers

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. 

Once a fault is detected, the circuit breaker contacts must open to interrupt the circuit; this is commonly done using mechanically stored energy contained within the breaker, such as a spring or compressed air to separate the contacts. 

CT Sizing Calculations

Protective Relays

These are the fault detecting devices. These devices detect the fault and initiate the operation of the circuit breaker so as to isolate the faulty circuit. A relay consists of a magnetic coil and contacts (NC and NO). The fault current energizes the coil and this causes to produce the field, thereby the contacts get operated.

Types of Relay :

Magnitude relays

Impedance relays

Directional relays

Pilot relays

Differential relays

Relay Co-ordination 

The relay co-ordination is nothing but a tripping of protecting relay in a sequence or order in electrical power system. Relay coordination is very difficult task for relay engineers. Relay co-ordination is required to isolate the faulty part with minimized relay & circuit breaker operation.

Consider four number of substation, Substation A, Substation B, Substation C, Substation D. Here Substation A is generation station and B, C and D are distribution stations. In this, if the fault (short circuit or earth fault) occurs in Substation D means, the substation D relay has to operate, instead of that, the substation A relay operated means such system said to be poor relay coordinated power system. It causes the total power system shutdown or unnecessary zone trips. Because, there is no fault on substation A, B and C but the substation A operates unnecessarily. In order to avoid such relay operation, we have to set co-ordination between all 4 substations.

Protection Grading

Protection Grading is an essential part of the electrical network, ensuring discrimination and selectivity of downstream faults. It is of vital importance that the protection system on any distribution network operates swiftly, correctly and that all the various relays grade to ensure that in the event of a fault the affected part of the network is isolated swiftly. The speedy operation of the protection system minimises the stress which the network is subjected to and limits the damage which can be caused to the network resulting from the flow of fault current.

Transmission Line Protection

As the length of electrical power transmission line is generally long enough and it runs through open atmosphere, the probability of occurring fault in electrical power transmission line is much higher than that of electrical power transformers and alternators. Protection of line should have some special features, such as-

During fault, the only circuit breaker closest to the fault point should be tripped.

If the circuit breaker closest to the faulty point, fails to trip, the circuit breaker just next to this breaker will trip as back up.

The operating time of relay associated with protection of line should be as minimum as possible in order to prevent unnecessary tripping of circuit breakers associated with other healthy parts of power system.

The two main methods of transmission line protection are –

1. Distance protection-

 Distance protection relay is the name given to the protection, whose action depends on the distance of the feeding point to the fault. The time of operation of such protection is a function of the ratio of voltage and current, i.e., impedance. This impedance between the relay and the fault depends on the electrical distance between them. The principal type of distance relays is impedance relays and the reactance relays. More specifically, the relay operates depending upon the impedance between the point of fault and the point where relay is installed.

2. Differential protection- Differential protection is based on the fact that any fault within an electrical equipment would cause the current entering it, to be different, from the current leaving it. Thus by comparing the two currents either in magnitude or in phase or both we can determine a fault and issue a trip decision if the difference exceeds a predetermined set value. Principle of Differential Protection scheme is one simple conceptual technique. The differential relay actually compares between primary current and secondary current of power transformer, if any unbalance found in between primary and secondary currents the relay will actuate and inter trip both the primary and secondary circuit breaker of the transformer.

3. Generator protection- An electrical generator can be subjected to either an internal fault or external fault or both. The generators are normally connected to an electrical power system, hence any fault occurred in the power system should also be cleared from the generator as soon as possible otherwise it may create permanent damage in the generator. The number and variety of faults occur in a generator are huge. That is why generator or alternator is protected with several protective schemes. The various forms of protection applied to the generator can be categorized into two manners :-

Protective relays to detect faults occurring outside the generator.

Protective relays to detect faults occurring inside the generator.

Other than protective relays, associated directly with the generator and its associated transformer, there are lightning arrestors, over speed safe guards, oil flow devises and temperature measuring devises for shaft bearing, stator winding, transformer winding and transformer oil etc. Some of these protective arrangement are of non-trip type i.e. they only generate alarm during abnormalities. But the other protective schemes ultimately operate master tripping relay of the generator. This should be noted that no protective relay can prevent fault, it only indicates and minimizes the duration of the fault to prevent high temperature rise in the generator otherwise there may be permanent damage in it.

4. Switchgear protection-

 A switchgear is a generic term which includes all the switching devices associated with power system protection. It also includes all devices associated with control, metering and regulating of electrical power systems. Assembly of such devices in a logical manner forms switchgear. In other words systems used for switching, controlling and protecting the electrical power circuits and different types of electrical equipment are known as switchgear. The circuit breakers can be operated manually as when required and it can also be operated automatically during over current and short circuit or any other faults in the system by sensing the abnormality of system parameters. These power system parameters can be current, voltage, frequency, phase angle etc. The circuit breaker senses the faulty condition of system through protection relays and these relays are again actuated by faulty signal normally comes from current transformer or voltage transformer. The switchgear has to perform the function of carrying, making and breaking the normal load current like a switch and it has to perform the function of clearing the fault in the power system. In addition to that, it also has the provision of metering and regulating the various parameters of electrical power systems. Thus the switchgear includes circuit breakers, current transformers, voltage transformers, protection relays, measuring instruments, electrical switches, electrical fuses, miniature circuit breaker, lightning arresters or surge arresters, electrical isolators and other associated pieces of equipment.

4. Transformer protection- There are different kinds of transformers such as two winding or three winding electrical power transformers, auto transformer, regulating transformers, earthing transformers, rectifier transformers etc. Different transformers demand different schemes of transformer protection depending upon their importance, winding connections, earthing methods and mode of operation etc. It is common practice to provide Buchholz relay protection to all 0.5 MVA and above transformers. While for all small size distribution transformers, only high voltage fuses are used as main protective device. For all larger rated and important distribution transformers, over current protection along with restricted earth fault protection is applied. Differential protection should be provided in the transformers rated above 5 MVA. Depending upon the normal service condition, nature of transformer faults, degree of sustained over load, scheme of tap changing, and many other factors, the suitable transformer protection schemes are chosen.

4. Relay Co-ordination- Relay coordination is an important aspect in the protection system design as coordination schemes must guarantee fast, selective, and reliable relayoperation to isolate the power system faulted sections. Thus the relay coordination problem is formulated, for a real time distribution system is simulated using ETAP. In transmission networks, any increase of the operation speed of the protection will allow the loading of the lines to be increased without increasing the risk of losing the network stability. 

Testing of Relays

It is generally accepted that protective relays and their trip circuits should be periodically checked in order to ensure that they will always be ready to operate with certainty. The recommended practice is to carry out three types of test: 

(a) Acceptance tests at the installation or commissioning of the relays.

(b) Periodic tests to check the calibration and condition of the relay. 

(c) More frequent tests of a simple nature to cause movement of the parts, and to check the continuity of the trip circuit.

Each relay unit should be given a mechanical inspection to see that the armature moves freely and that the contacts have the necessary travel and wipe to ensure reliable operation, checking the manufacturer's settings, if given, in the instruction book. An inspection light and dental mirror should be used to see that the magnetic gaps are clean before the relay is left in service. Suitable electrical tests to check the performance characteristics of the relay are usually described in some detail in the manufacturer's instruction book accompanying the relay. The contacts of each relay should be closed electrically or manually to see that the trip circuit is complete and that the proper alarms are actuated. In order to check the current-transformers, voltage-transformers and wiring associated with the relays it is usual also to make overall tests from the primary circuit. The primary current is usually supplied by a test transformer of about 5 kVA supplied from a low voltage lighting or power source, such 230.A.C. Primary injection test circuit Relay as a 240 volts, 30 amperes source, and tapped for various voltages (say 1 to 10 volts) necessary to give line currents up to 1000 A depending on the impedance of the circuit; this current is sufficient to check the polarity of the connections but not to simulate fault currents, the latter being done in the secondary injection tests to check the relay characteristics. The secondary wiring can be further checked if necessary by a low reading ohmmeter or by the ringing method using a bell and battery.  

Types of protection for busbars

Busbars play an important role in power transmission and distribution. They are employed as a central distribution point for all feeders. In the case of a fault, current on the busbar becomes high, resulting to mechanical destruction which would affect all feeders.