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Protective Relays Introduction: In a power system consisting of generators, transformers, transmission and distribution circuits, it is inevitable that sooner or later some failure will occur somewhere in the system. When a failure occurs on any part of the system, it must be quickly detected and disconnected from the system. The detection of a fault and disconnection of a faulty section or apparatus can be achieved by using fuses or relays in conjunction with circuit breakers. A fuse performs both detection and interruption functions automatically but its use is limited for the protection of low-voltage circuits only. For high voltage circuits (3.3 kV and above), relays and circuit breakers are employed to serve the desired function of automatic protective gear. Protective relay: A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system. The relays detect the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. The electrical quantities which may change under fault conditions are voltage, current, frequency and phase angle. A typical relay circuit is shown in the fig. below. This diagram shows one phase of 3-phase system for simplicity. The relay circuit connections can be divided into three parts: (i) First part is the primary winding of a current transformer (C.T.) which is connected in series with the line to be protected. (ii) Second part consists of secondary winding of C.T. and the relay operating coil. (iii) Third part is the tripping circuit which may be either a.c. or d.c. It consists of a source of supply, the trip coil of the circuit breaker and the relay stationary contacts. Operation Steps: 1 When a short circuit occurs at point F on the transmission line, the current flowing in the line increases to an enormous value. 2 This results in a heavy current flow through the relay coil, causing the relay to operate by closing its contacts. 3 This in turn closes the trip circuit of the breaker, making the circuit breaker open and isolating the faulty section from the rest of the system. In this way, the relay ensures the safety of the circuit equipment from damage and normal working of the healthy portion of the system.

Fig: Typical Relay Circuit Fundamental Requirements of Protective Relaying The main features of a good protective relaying are; (i) selectivity (ii) speed (iii) sensitivity (iv) reliability (v) simplicity (vi) economy 1- Selectivity: It is the ability of the protective system to select correctly that part of the system in trouble and disconnect the faulty part without disturbing the rest of the system.

Fig: Single line diagram of a portion of a typical power system. The system can be divided into the following protection zones: (a) generators (b) low-tension switchgear (c) transformers (d) high-tension switchgear (e) transmission lines It may be seen in the previous Fig. that there is certain amount of overlap between the adjacent protection zones. For a failure within the region where two adjacent zones overlap, more breakers will be opened than the minimum necessary to disconnect the faulty section. But if there were no overlap, a failure in the

region between zones would not lie in either region and, therefore, no breaker would be opened. For this reason, a certain amount of overlap is provided between the adjacent zones. 2- Speed: The relay system should disconnect the faulty section as fast as possible for the following reasons: (a) Electrical apparatus may be damaged if they are made to carry the fault currents for a long time. (b) A failure on the system leads to a great reduction in the system voltage. If the faulty section is not disconnected quickly, then the low voltage created by the fault may shutdown consumers’ motors and the generators on the system may become unstable. (c) The high speed relay system decreases the possibility of development of one type of fault into the other more severe type. (d) Permit use of rapid reclosure of circuit breaker to restore service to consumers 3- Sensitivity. It is the ability of the relay system to operate with low value of actuating quantity. Sensitivity of a relay is a function of the volt-amperes input to the coil of the relay necessary to cause its operation. 4- Reliability. It is the ability of the relay system to operate under the pre-determined conditions. 5- Simplicity. The relaying system should be simple so that it can be easily maintained. Reliability is closely related to simplicity. The simpler the protection scheme, the greater will be its reliability. 6- Economy. The most important factor in the choice of a particular protection scheme is the economic aspect. Sometimes it is economically unjustified to use an ideal scheme of protection and a compromise method has to be adopted. However, when the apparatus to be protected is of utmost importance (e.g. generator, main transmission line etc.), economic considerations are often subordinated to reliability.

Classification of Relays: In a power system control and protection schemes, various types of relays are used which can be categorized as follows: 1. According to functions: Relays can be divided into three classes according to the function, they are a) Main relays b) Auxiliary Relays c) Signal Relays a) Main relay: These relays operate according to the information received from the power system i.e. they operate when there is a change in the actuating quantity which may be current, voltage, power etc. b) Auxiliary Relays: These relays operate after the operation of main relays and are used to perform some auxiliary functions such as introducing time delay for the operation of the breaker. c) Signal Relays: These are the relays used to indicate the operation of main relays. It is also used to energize a signal or an alarm circuit to make the operators alert to take the necessary actions immediately.

2. According to construction: Protective relay can be broadly classified into the following three categories, depending on the technology they use for their construction and operation. i) Electromechanical Relay ii) Static Relay iii) Numerical Relay i) Electromechanical Relay: Electromechanical relays are further classified into two categories i.e. i) electromagnetic relays, and ii) thermal relays. Electromagnetic relays work on the principle of either electromagnetic attraction or electromagnetic induction. Thermal relays utilize the electrothermal effect of the actuating current for their operation. First of all, electromagnetic relays working on the principle of electromagnetic attraction were developed. These relays were called attracted armature-type electromagnetic relays. This type of relay operates through an armature which is attracted to an electromagnet or through a plunger drawn into a solenoid. Plunger type electromagnetic relays are used for instantaneous units for detecting over current or overvoltage conditions. Attracted armature –type electromagnetic relays are the simplest type which respond to ac as well as dc. Initially attracted armature-type relays were called electromagnetic relays. Later on, induction type electromagnetic relays were developed. These relays use electromagnetic induction principle for their operation, and hence work with ac quantities only. Electromagnetic relays were contain an electromagnet (or a permanent magnet) and a moving part. When the actuating quantity exceeds a certain predetermined value, an operating torque is developed which is applied on the moving part. This causes the moving part to travel and to finally close a contact to energize the trip coil of the circuit breaker. Static Relay: A static relay refers to a relay in which there is no armature or other moving element and response is developed by electronic, magnetic or other components without mechanical motion. The solid state components used are transistors, diodes, resistors, capacitors and so on. The function of comparison and measurement are accomplished by static circuits. In a static relay the measurement is carried out by static circuits consisting of comparators, level detectors, filters etc. while in a conventional electromagnetic relay it is done by comparing operating torque with restraining torque. In static relay the relaying quantity such as voltage, current is rectified and measured. When the quantity under measurement attains certain well defined value, the output device is triggered and thereby the circuit breaker trip circuit is energized. Static relays possess the advantages of having low burden on the CT and VT, fast operation, absence of mechanical inertia and contact trouble, long life and less maintenance. Static relays have proved to be superior to electromechanical relays and they are being used for the protection of important lines, power stations and sub-stations. DC supply

Relaying quantity

Secondaryof PT/CT /Transducer

Rectifier

Relay Measuring Circuit

Amplifier

Output device

Fig: Simplified block diagram of a static relay

Trip circuit

The relaying quantity (output of PT, CT or transducer) is rectified by a rectifier. The rectified output is supplied to a measuring unit. The output of the measuring unit is amplified and fed to the output device. The output unit energizes the trip coil. Numerical Relay: Numerical relays are the latest development in this area. These relays acquire the sequential samples of the ac quantities in numeric (digital) data from through the data acquisition system, and process the data numerically using an algorithm to calculate the fault discriminants and make trip decisions. Numerical relays have been developed because of tremendous advancement in VLSI and computer hardware technology. They are based on numerical (digital) devices, e.g. microprocessors, microcontrollers, Digital Signal Processors (DSP) etc. At present microprocessor/microcontroller based numerical relays are widely used. The main features of numerical relays are their economy, compactness, flexibility reliability, selfmonitoring and self-checking capability, multiple functions, low burden on instruments transformer and improved performance over conventional relays of electromechanical and static types. 3. According to speed of operation: Protective relays can be generally classified by their speed of operation as follows: i) Instantaneous relays ii) Time-delay relays iii) High-speed relays i) Instantaneous relays: In these relay, no intentional time delay is introduced to slow down their response. These relays operate as soon as a secure decision is made. ii) Time-delay relays: In these relays, an intentional time delay is introduced between the relay decision time and the initiation of the trip action. iii) High-speed relays: These relays operate in less than a specified time. The specified time in present practice is 60 milliseconds (3 cycles on a 50 Hz system). 4. According to their generation of development: Relays can be classified into the following categories, depending on generation of their development. i) First-generation relays: Electromechanical relays ii) Second-generation relays: Static relays iii) Third-generation relays: Numerical relays. 5. According to their functions: Protective relays can be classified into the following categories, depending on the duty they are required to perform: i) Overcurrent relays ii) Undervoltage relays iii) Impedence Relays iv) Underfrequency relays v) Directional relays

These are some important relays. Many other relays specifying their duty they perform can be put under this type of classification. The duty which a relay performs is evident from its name. For Example, an overcurrent relay operates when the current exceeds a certain limit, an impedance relay measures the line impedance between the relay location and the point of fault and operates if the point of fault lies within the protected section. Directional relay check whether the point of fault lies in the forward or reverse direction. The above relays may be electromechanical, static or numerical. 6. According to the method of connection: They are connected to the power system; relays may be classified into a) primary relays b) secondary relays. Primary relays: primary relays are those whose sensing elements are directly connected to the power lines, which they protect. Secondary relays: Secondary relays are those whose sensing element are connected to the power lines through instrument transformers. Normally, these types of relays are used in power system protection because of high values of voltages and currents of the power circuit. 7. According to the method of action: On the basis of the action of relays on circuit breakers, they are classified into; a) Direct-acting relays b) Indirect acting relays. a) Direct acting relay: These relays are connected mechanically with the tripping mechanism of the breakers and their control elements acts directly to operate the breaker. b) Indirect acting relays: These relays act indirectly i.e. instead of acting directly on the breaker’s operating mechanism, they perform switching actions to supply the power from an auxiliary d.c. source to the trip coil of the operating mechanism. The most relays are used in practice of these kinds.

Earth fault detection and protection When the fault current flows through earth return path, the fault is called earth fault. Other faults which do not involve earth are called phase faults. Since earth faults are relatively frequent, earth fault protection is necessary in most cases. When separate earth fault protection is not economical, the phase relays sense the earth fault currents. However such protection lacks sensitivity. Hence separate earth fault protection is generally provided. Earth fault protection senses earth fault current. Following are the method of earth fault protection. Connection of CT's for earth fault protection 1 Residually connected earth fault relay Referring to Fig. below. In absence of earth fault the vector sum of three line currents is zero. Hence the vector sum of these secondary currents is also zero. Īas + Ībs + Īcs = 0 The sum (Īas + Ībs + Īcs) is called residual current (IRS). The earth fault relay is connected such that the residual current flows through it (Ref Fig below)

In the absence of earth fault Īresidual = Īas + Ībs + Īcs = 0 Therefore, the residually connected earth fault relay does not operate. However, in presence of earth fault the condition is disturbed and (Īas + Ībs + Īcs) is no more zero. Hence residual Iresidual flows through the earth fault relay. If residual current is above the pick-up value, the earth fault relay operates.

Fig: Earth fault relay connected in residual current

Fig: Earth fault protection by earth fault relay connected in neutral to earth circuit

In the scheme discussed here the earth fault at any location near or away from the location of CT's can cause the residual current flow. Hence the protection zone is not definite. Such protection is called unrestricted earth fault protection. For selectivity directional earth fault protection is necessary. 2 Earth fault relay connected in neutral to earth circuit Another method of connecting an earth fault relay is illustrated in Fig below. The relay is connected to secondary of a CT whose primary is connected in neutral to earth connection. Such protection can be provided at various voltage levels by connecting earth fault relay in the neutral to earth connection of that of voltage level. The fault current finds the return path through the earth and then flows through the neutral to earth connection. The magnitude of earth fault current is dependent of type of earthing (resistance, reactance or solid) and location of fault. In this type of protection, the zone of protection cannot be accurately defined. The protected area is not restricted to transformer/generator winding alone. The relay senses the earth faults beyond the transformer/generator winding. Hence such protection is called unrestricted earth fault protection. The earth fault protection by relay in neutral to earth circuit depends upon the type of neutral earthing.. In case of large generators, voltage transformer is connected between neutral to earth. The earth fault relay is connected to secondary of VT. Combined earth fault and phase fault protection It is convenient to incorporate phase fault relays and earth fault relay in a combined phase fault and earth fault protection. The increase in current of phase causes corresponding increase in respective secondary currents. The secondary current flows through respective relay units. Very often only two

phase relays are provided instead of three, because in cause of phase faults current in any at least two phases must increase. Hence two relay units are enough.

Fig: Earth fault protection combined with phase fault protection

Fig: Principle of core balance CT for earth fault protection

Earth fault protection with core balance current transformer (Zero sequence ct) In this type of protection as shown in figure below, a single ring shaped core of magnetic material, encircles the conductors of all the three phases. A secondary coil is connected to a relay unit. The cross section of ring core is ample, so that saturation is not a problem. During no earth fault condition, the component of fluxes due to the fields of three conductors is balanced and the secondary current is negligible. During earth faults, such a balance is disturbed and current is inducted in the secondary. Core balance protection can be conveniently used for protection of low voltage and medium voltage system. The burden of relay and exciting current are deciding factors. Very large cross section of core is necessary for sensitivity less than 1A. Thus form of protection is likely to be more popular with static relay due to the less burden of the latter. Instantaneous relay unit is generally used with core balance schemes. Theory of core balance CT. Let Īa, Īb and Īc be the three line currents and Φa¯, Φb and Φc be corresponding components of magnetic flux in the core. Assuming linearity, we get resultant magnetic flux Φr as, Φr¯ = k(Īa + Īb + Īc) Where k is constant Φr¯= k Ia. Referring to theory of symmetrical components Īa + Īb + Īc = 3Īc = Īn Where, Io is zero sequence current and In is current in neutral to ground circuit. During normal condition, when earth fault is absent, Īa + Īb + Īc = 0 Hence Φr¯= 0 and relay does not operate During earth fault the earth fault current flows through return neutral path. For example for single line ground fault.

If = 3Ia0 = In

Fig: Mounting of core balance CT with cable terminal box

Fig: Principle of frame leakage protection of metal clad switchgear

Hence the zero sequence current component of Io produces the resultant in the core. Hence core balance current transformer is also called as zero sequence current transformer (ZSCT)

Application of core balance CT's with cable termination joints The termination of a three core cable into three separate lines or bus-bars is through cable terminal box. Ref. Fig. the core balance protection is used along with the cable box and should be installed before making the cable joint. The induced current flowing through cable sheath of normal healthy cable need particular attention with respect to the core balance protection. The sheath current (Ish) flow through the sheath to the cover cable box and then to earth through the earthing connection between cable box. For eliminating the error due to sheath current (Ish) the earthing lead between the cable box and the earth should be taken through the core of the core balance protection. Thereby the error due to sheath current is eliminated. The cable box should be insulated from earth. Frame leakage protection The metal-clad switchgear can be provided with frame leakage protection. The switchgear is lightly insulated from the earth. The metal framew...