Power Systems Protection subtransmission, distribution & industrial levels, Part II Faults in electrical power systems can be classified into parallel or series. The first being short circuiting a single phase to ground, two phases or three phases together and may be to ground, too. The last means having an open circuit like fuse blowing or broken terminals. Any of these faults will caause abnormal currents to flow through the system from source to fault. Currents from contributions from squirrel cage, capacitors and synchronous motors will flow through the faults only with varying magnitudes and durations. Abnormal currents can be positive, negative or zero sequence or a combination thereoff. Associated with faults are voltage fluctuations that may affect the connected loads or worsen the fault condition. Thus, measures are taken to reduce the causes of short circuiting like the installation of lightning arresters, to reduce flashovers when lightning (direct or induced) hits, cleaning the insulation periodically to minimize the effect of contamination on short circuiting the phases, monitoring the circuits to remove all overloads/ overvoltages that may be imposed on the system, setting and adhering to the routine maintenance programs and covering the connectors with taps or boots. Despite all these measures, faults still occur on the distribution systems and they have to be removed and the faulty section isolated promptly. The main elements that provide such protection are: relays (protective, timing, auxiliary and programmed) instrument transformers (current and voltage) power supply isolating devices (circuit breakers and load break/disconnect switches) cables/wires for local wiring and remote tripping The causes for such abnormal conditions on power systems are numerous, just to list a few: insulation aging insulation design defects or improper manufacturing improper insulation installation lightning surges switching surges temporary overvoltages wind (may reduce clearances) snow or ice contamination of insulation surfaces long duration overloads or overvoltages rodent,Insects and animals lack of equipment appropriate monitoring and routine maintenance misapplication The classification of electromechanical relays is as follows: the magnetic attraction type, which can be further classified into the plunger, the clapper and the polar. the magnetic induction, which can be further classified into the magnetic induction disc and the
induction cup (cylinder) types. the D'Arsonval types. the thermal types. The elements that build the solid state relays are: the semiconductor components the logic unit operational amplifiers (integrated circuits) multivibrators level detectors For microprocessorbased relays, the main elements are: digital to analog convertors analog to digital convertors multiplexers sample/hold circuit microprocessors RAM & ROM output circuits Electromechanical relays: The Plunger Type: The plunger unit has the following components: the magnetic frame, the cylindrical coil, adjustable core, stationary contacts, the plunger (that carries the disc contacts), the helical spring. The force required to move the plunger is proportional to the square of the current in the coil. The plunger floats in the air gap of the unit, this allows the drop out to pickup ratio to reach 90 percent (units without internal air gap reduce this ratio to 40% or less). These units are used for O/C, U/V and O/V instantaneous relays. They can be applied in d.c. or a.c. circuits. These units can have adjustable flux shunts for more precise settings. The Clapper Unit: The clapper unit for d.c. applications has a Ushaped magnetic frame, coil, core (fixed or adjustable), armature (which is hinged at one end and spring restrained at the other) and fixed and moving contacts. For a.c. applications, a lag loop is used (to create the phase difference between the operating fluxes and reduce the hum). These units are mostly used in auxiliary relays and in protective relays to sealin around the protective relay main contacts. The torque developed on the armature is proportional to the square of the current flowing in the coil. A target for indication can be part of this unit. The Polar Unit: The polar unit has magnetic structures which consist of the coils, a permanent magnet, hinged armature (at the centre of the structure, with the coil around it), magnetic shunts and contacts. These types of units operate on d.c. circuits or on a.c. circuits through the use of a full wave rectifier. Some units use both an operating and a restraint coil. The induction units: Before covering the induction units, the basic operation of the watthour meter will be discussed. The
induction disc relays are based on the watthour meter principles. The rotor of a watthour meter is made up of an aluminum disk. The shaft, which is a metal rod, is free to turn in bearings held in the frame. The bearings may be of the mechanical jewel and pivot type or of the ball bearing. The main parts of the ball bearing support type are: the protective sleeve, the ball, upper and lower jewel. Another type of suspension is the magnetic, of which the main parts are: the guide pins (which maintain vertical alignment of the shaft, but do not support the rotor), guide bearings, inner and outer magnet. The guide bearing has the guide bushing, white vaseline and pin. The stator is an electromagnet with two sets of windings. The shape of the core (electromagnet) provides the desired flux distribution. The disc rotates in the air gap of the electromagnet assembly. The torque that causes the disc to turn, at any given instant, results from the interaction between the flux produced by the current in one coil and the eddy current, which is induced in the disc as a result of the flux produced by the current in the other coil (or set of coils could be the voltage coils). The e.m.f. induced in the disc is always lagging the flux that produced it by 90°C. The disc is considered to be resistive (i.e. the eddy current is in phase with the e.m.f. in the disc) and consequently, the flux of the second set of coils (with unity power factor loadburden) is in phase with the eddy current produced by the first set of coils. The Magnetic Induction Disc This type of relay units can operate on either the watthour principle or the shaded pole (shaded ring) principle. The magnetic induction disc relay unit operating on the shaded pole principle has the magnetic structure, the disc, the coil (operating) and a shaded pole (enclosed ring around a piece of the pole). The shading ring can be replaced by a lag coil (modified principle of watthour operation). The function of the shading ring or lag coil is to create a phase difference between the two fluxes (so that a torque is produced). The basic elements of the magnetic structure are: the electromagnet, the magnet plugs (control the degree of saturation), the permanent fixed magnet, the adjustable keeper (shunt) to damp the disc rotation and to prevent its racing. The 3 components: spring tension, the dampening magnet/keeper and the plugs allow relatively independent adjustment of the units inverse time overcurrent characteristics. The Magnetic Induction Cup This unit will have four or more electromagnets, a stationary iron core is placed between these electromagnets. The rotor is a hollow cylinder cup which is free to rotate in the gap between the magnets and the stationary iron core (inside the cup). When the electromagnets are energized, they induce voltages in the rotor cup and consequently eddy currents. The eddy currents due to one flux interact with the flux due to the other pole and a torque is produced. D'ARSONVAL units: It is a moving coil type. It operates from very low d.c. energy inputs, like shunts, bridges or rectifier d.c. output. The main elements of such units are: the frame, the moving coil, the permanent magnet, spacers, mounting blocks. The moving coil in the air gap, when energized, reacts with the air gap flux (from the permanent magnet), to create rotational torque. They are usually of the bimetallic strip. Each metal has a thermal expansion coefficient that differs from the other, fixed at one end and free to move at the other. When this unit is connected to protect
against overload, if the temperature of the strip increases, the free end will bend (move), closing a set of contacts to initiate an alarm and trip open a breaker or energizing an auxiliary relay. If the set of normally closed contacts (i.e. it opens when overheated), are used in a motor contactor circuit, the operation of such contacts will cause the dropping of the contactor and consequently, cutting the supply to the motor. The sequence networks: Such networks (with three phase current or voltage inputs), can provide a single phase output, proportional to zero, positive, or negative sequence quantities. When the secondaries of phase current transformers (1 per phase for 3 phase systems) are connected in parallel, their cumulative output is equal to 3Io (zero sequence current). When the 3 secondaries of the potential transformer (primary connected in grounded wye) are connected in series, the output is equal to 3Vo. The other 2 sequence networks are fed from C.Ts. or P.Ts. The first is fed from the input currents (phase and neutral), a single phase output is produced which is proportional to the positive sequence, or negative sequence current of the circuit monitored or is proportional to a composite positive, negative or zero sequence current. The second has its terminals connected to a potential source from the circuit to be monitored (in a delta configuration), its output will either be proportional to the positive or the negative sequence voltage of the circuit. Semiconductor components: Diode : It is a two terminal PN junction. If a d.c. voltage is applied with positive on the P element and negative on the N element, the free electrons in the N material will flow across the junction. The holes (absence of electrons) move by displacement and a current is established. When the polarity of d.c. is reversed, the depletion of the carriers result in a very low current and the diode is said to be in the blocking phase. Under the reverse bias, breakdown occurs, should the applied voltage exceed the peak inverse voltage. Zener Diode : It differs from the diode in having a sharp and reproducible reverse breakdown voltage called the zener voltage. The junction recovers its nonconducting characteristics when the reverse voltage falls below the zener value. The zener clipper provides effective surge suppression against oscillating transients. Four Layer Diode : It is used to obtain pulses from d.c. sources. Tunnel Diode : It passes current at low voltages reaching a peak, than the current decreases with increasing voltage, afterwards it increases again with further increase in voltage. Transistor : If a single piece of silicon is dopped, with the same type of impurity at either end with the central section having characteristics different from either end, a transistor is produced. The two ends are termed the emitter and the collector, the central base is called the base. PNP or NPN transistors are available depending on the material used as the emitter, collector and base. The emitter is made in a different dimension than the collector and with heavier dopping. The three configurations of transistors
are: the common emitter, the common base and the common collector (emitter follower). Because of the high gain of common emitter circuit, it is most commonly used as an amplifier. Transistors in relays are also used as switches, the common emitter with forward biasing will switch on the transistor and with a reverse biasing, switches it off. Thyristor (SCR) : The voltage drop in the forward direction is 1 to 2 volts. It is robust, small in size and has a long working life. It is a four layer alternate ptype and ntype silicon semiconductor crystals, forming three pn junctions. The effect of increasing the applied voltage with the anode more positive relative to the cathode, at first the forward leakage current reaches saturation value, ultimately a breakover value is reached and the resistance of the thyristor, instantly, falls to a very low value. A resistor is necessary to limit the current to a safe value. When a positive potential is applied to the gate (the value of the breakover voltage of the thyristor depends upon the magnitude of the bias gate current), the breakover voltage falls and the thyristor conducts. The thyristor continues to conduct even after the removal of the gate current, as long as the anode current is equal to or greater than the holding current. Forced, line or load commutation is needed to switch off a thyristor. GateTurnOff Thyristor It is similar to a conventional SCR that can be switched by a positive gate pulse, however, like a transistor, it can also be switched off without power commutating circuits (i.e. the negative current pulse approaching the level of the load current when applied to the gate, is sufficient to switch off the GTO). Unijunction Transistor : It is used for pulsating output circuits. It consists of 2 bases and an emitter. The emitter to base # 1 voltage is applied, when it reaches the peak value, the device conducts and passes the emitter current until the voltage is reduced to the minimum voltage, at which the device ceases to conduct. BiPolar Power Transistor : The control signal is usually applied between the base and the emitter, when the base current is zero, the transistor is switched off and no current flows. When a base current is flowing, the transistor is switched on and the load current flows from the emitter to the collector or vice versa, depending on the type of transistor. To keep the transistor on, the base current should be kept on. Field Effect Transistors : This device includes terminals called gate, drain and source. The current flow, in FETs, is controlled by variation of an electric field in the semiconductor. Principle logic units And Unit: The output is either a 0 or a 1, depending on the input. With two inputs, both should be available to get an output (logic 1) from this device. ICs, circuits with diodes and resistors or transistor/resistors, are used in practice to provide this logic. Or Unit : With two inputs, when any input is present to either terminal, an output will be generated. Not or Inverter Logic Unit :
With one input terminal and one output terminal, the 0 logic input will give a one logic output and vice versa. Nand and Nor Unit : Having the output of the AND or the OR circuit as input to the inverter circuit, a nand or a nor logic circuit is produced. Time Delay Unit : It is used in the normal manner, to provide on and/or off delays. The time is adjustable or fixed and is set to have signals outputted, after the delay, when a signal is applied to the input of the device. Operational amplifiers and integrated circuits: Operational amplifiers are d.c. voltage linear amplifiers, with very high gain. The basic function of an OP with feedback, is to perform mathematical operations on voltages for inversion, addition, subtraction, multiplication, division, differentiation, integration and sequence filters (the solid state equivalent of the sequence networks). Integrated circuit chips for operational amplifiers are readily available. OP units that are classified as low power precision style, will have typically, the following parameters: supply voltage +/ 18V, input resistance 6 Mohm, output resistance 500 ohm, open loop gain 120db, input offset voltage 0.5 V (should be 0, theoretically), unity gain bandwidth 0.3 MHZ (should be infinity, theoretically). Integrated circuits have been categorized according to their complexity, as follows: small scale integration (up to 10 complete logic gates), medium scale integration (10100 gates), large scale integration (1001000 gates), very large scale integration (above 1000 gates). The most popular is the plastic encapsulated 14 pin, dualinline (DIP) package. Other packages can have 8, 16, 18, 20, 22, 24, 28 and 40 pins. These packages can be classified according to the type of logic they have (i.e., diode transistor logic DTL, transistortransistor logic TTL, emitter coupled logic and CMOS logic). Compatibility of IC's is as follows:TTL/TTL, TTL/DTL, CMOS/DTL, CMOS/CMOS. Multivibrators and level detectors: A multivibrator is a circuit constructed by coupling two amplifiers together, using strong positive feedback. If in the absence of the triggering pulses, the circuit can remain permanently in only one state, it is said to be Monostable. If it can remain permanently in either state, it is said to be Bistable or FlipFlop. If the circuit cannot remain permanently in either state, it is Astable. Monostable Multivibrators: T2 is coupled to T1 through the capacitor Ct. Under equilibrium conditions, T2 is ON and Vbb causes T1 to be OFF and Ct is approximately charged to Vcc. If a negative input is applied to T1, T2 will be turned OFF and T1 ON and remains ON, as long as T2 is OFF. When T1 is ON, the terminal of Ct towards T1 will be effectively grounded and the base of T2 is driven negative by a value equal to Vcc. This T2 base charge decreases, as Ct charges towards Vcc through Rt. T2 turns ON again when the voltage across Ct reaches 0. This turns T1 OFF and the circuit is reverted to its stable state (which is T2 conducting and T1 blocking). Bistable Multivibrators or FlipFlops : It is a fundamental building block in digital circuits and is used for memories, counters, frequency dividers and shift registers. It is used to register events (being the triggering pulse), by changing its
operating state. It consists of two inverters in cascade with the output of the second inverter supplying the input to the first one. It has 2 stable states, either one of the transistors is OFF and is providing the base drive to keep the other ON. The circuit can be triggered from one state to another by momentarily short circuiting the base emitter terminals of the ON transistor, the OFF one is switched ON. Astable Multivibrator: Due to component tolerances, an abrupt application of power causes one transistor to turn ON first. When a transistor turns ON, the capacitor connected to its collector drives the base of the other transistor negative, thus turning it OFF. When the capacitor voltage falls to zero, the transistor turns ON, turning OFF the other one. One application of such circuit is pulse generation. Level Detectors Circuit: It compares an alternating or unsmoothed rectified signal against a d.c. datum. Whenever the peak input exceeds the reference, an output is produced. Schmidt trigger is normally used as a level detector. When a pair of transistors are directly coupled, it provides a sudden turn On or triggering action by T2 on T1 and this occurs at a selected value, on a slowly changing signal applied to the base of T1. Digital to analog & analog to digital converters: The DACs are either of the voltage output (resistor summing network, resistor ladder network & BCD weighted DAC) or current output. The A/D converters are of any of the following types: ramp voltage and successive approximation (simple counter or sequencing). DigitalToAnalog (DAC): The Resistor Summing Network, Voltage Output : It is implemented by means of binaryweighted summing resistors. Several resistors are connected in parallel, with the o/p voltage available at a common node. The other end of each resistor is connected to a reference voltage or to ground (through an electronic switch). The resistors in this configuration are weighted according to their respective positions in the binary input scheme. The Resistor Ladder Network, Voltage Output : Only two resistor values are used. In this R2R ladder network, the LSB switch is further from the amplifier input. It depends on the principle of the voltage divider network. When a digital input is present on one of the resistors, currents will flow through this leg and get divided at each node by 2 (because of the resistor values and configuration). The BCD Weighted DAC : Both the resistor summing and the resistor ladder networks land themselves nicely to binary coded decimal applications. Each BCD digit requires four binary bits to represent it. Each BCD digit is a decimal digit, so each digit position in a number is decimal weighted. Ro=10n1mR; where Ro is the
value of the desired resistor in the BCD weighted resistor summing network, n is the rank of the BCD digit (1 for Most Significant Digit), m is the rank of the binary bit in the BCD digit, with an MSB (most significant bit) rank of 1, then 2, 4 and 8 is the LSB. Current Output DACs : It is implemented by generating binary weighted currents from active sources, usually transistors and summing them up on a common output node. The transistors have their bases tied to a common node, whose potential is controlled when the circuit is in operation. This voltage turns on all transistors. The
transistors (which differ in number of emitters and their associated resistors), with their binary weighted resistors, constitute the active current sources. The position of the steering switches are controlled by the digital inputs to the circuit. The MSB (Q1 & R), contributes the largest current to the summing node and the LSB contributes the least. The output voltage of the OP AMP eout is the drop across the resistor Rf and represents the digital input combination of 1s and Os. In solid state power system protective relays, it is necessary to convert the digital output of the microprocessor to the analog form, in order to activate output devices. Integrated DAC chips are also commercially available. AnalogToDigital (A/D): They are used in power systems to convert analog signals like current, voltage and frequency to digital form, before feeding it into the microprocessor. IC chips for A/D converters are commercially available. The methods used to achieve such conversion is given hereafter. These methods as well as those given under the DAC headings are also used with digital meters and instruments. The Ramp Voltage Method: The analog voltage to be digitized is fed into one input of the comparator. The reference voltage which is applied to the other input is a ramp voltage. The comparator is a differential amplifier whose output is a function of the difference in voltage between the 2 signals. The comparator triggers whenever the ramp voltage reaches the signal voltage. The Successive Approximation Method: In this approach, a series of digital signals are generated and each signal is successively converted into analog one. Each signal is fed into one of the comparator inputs as a reference signal. The analog input signal is fed into the other input of the comparator. The output is either a 1 or a 0. The successive digital outputs of the comparator are utilized in either a simple counter way or the sequencing way. Simple Counter way: It is very similar to the rampvoltage method except, that instead of using a voltage ramp as the reference i/p, the step output of the DAC is used. Sequencing way: It has an 8bit DAC and an 8bit sequencing register. The digital sequencer is controlled by the clock and sends out a digital 1 on 8 output lines, sequentially (i.e., on one line at a time, starting with the MSB). Microprocessors: It is a digital device which receives information in a digital format, processes the information according to a stored program and give the output information, also in a digital form. Microprocessors have different busses (group of pins), in order to effect the transfer of data and instructions between the microprocessor, memory, i/p and o/p devices. These are the address, data and control busses. The address bus is the one by which the processor indicates from which part of the memory it wants to fetch an instruction. It is a unidirectional bus. The data bus is the one along which information is transmitted to the processor. It is bidirectional. The control bus is used for conveying instructions between the processor and the various i/o or other peripheral devices. The clock circuit (run by a crystal oscillator), provides synchronization among the components of the microprocessor. The clock could supply a single stream of pulses (single phase clock) or it could supply
2 phases. A 2 phase clock could have either overlapping (partial or complete) or non overlapping pulse streams. The time interval between identical points, on 2 adjacent clock pulses of the same clock pulse stream, is called the clock period. The time involved in the combined fetch and execution processes of a single instruction is called a machine cycle. Any portion of a machine cycle, that is identified with a clearly defined activity, is called a phase of the machine cycle. At least one and usually more clock periods are necessary for completion of a phase, of the several phases of the machine cycle. Microprocessors are used in relays, programmable logic controllers and other related data acquisition and control systems in the power distribution and utilization fields. Multiplexers and demultiplexers: These circuits are used when it is necessary to conserve the number of channels required for conveying information. If for example, there are 8 signal sources to 8 receivers, it is possible to use one channel (medium) for conveying all the information. The principle used is of the time sharing or time division multiplexing. At the receiver end data is separated into 8 different channels (demultiplexing). IC packages are available for both functions. Multiplexers can be classified as either single ended or differential ones. The parameters of a multiplexer are d.c. characteristics, accuracy, settling time and switching time. The performance of such circuits is dependent on the output devices characteristics and the input source ch/cs. Sample and hold circuits (Track and hold): It is a circuit that captures an analog voltage at a specific point in time, under control of an external circuit, say a microprocessor. Its primary use is in data acquisition systems, which require that the voltage be captured and held during the analogtodigital conversion process. The ideal amplifier, in its simplest form, contains: the input buffer (amplifier), switch (to gate the buffered input signal to the holding circuit or to remove it), holding capacitor, the output amplifier (presents a high impedance load to the capacitor and a low impedance voltage source to the external loads). Important parameters for such a circuit are: acquisition time, aperture time, aperture jitter, charge offset, droop rate, drift current, hold mode feedthrough, hold mode settling. In practice, this circuit is connected ahead, to allow for the conversion time and accuracy required from the A/D converter, to be achieved. Construction (internals) of protective devices: Over Current Relays: The available designs are the electromechanical, the solid state and the microprocessor based. The main elements of the first type are: the tap block, the electromagnet, the tap screws, the magnetic plugs, the time dial, the stationary contact, the moving contact, the induction disc, the dampening magnet. The solid state inverse overcurrent relay will have the following basic components: input transformer ,current setting (potentiometers), filters, rectifier, level detectors, amplifiers, time circuit, time setting, stabilizer, output relays. The microprocessor based relay has: the microprocessor, the multiplexer, the sample and hold circuit, the analog to digital converter, the RAM, the ROM, the output relay, the input control circuit, the power supply (stabilizer, resistance circuit), the standby battery, the communication port, the input time and current settings. Over/Under Voltage Relays: For over and/or under voltage electromechanical relays, the main parts are: the mounting frame, left
and/or right stationary contacts, time dial, the induction disc, the electromagnet. The major components for a solid state relay are: the input transformer, the rectifier ,the smoothing circuit, the level detectors (one to detect the over and one to detect the below settings), the integrated circuit amplifiers, the output relays. The same basic elements used to build the overcurrent relays, are utilized in building microprocessor based voltage relays. Differential Relays: They are used to protect generators, motors, transformers and station buses. The electromechanical type will have the following components: the induction disc, the moving contact, the stationary contact, the time dial, the operating coil, the restraining coils, the electromagnet, with poles above and under the disc. There is another type of electromechanical differential relay, this is the moving coil type, which has: the coils (operating, restraining and blocking), the rectifier, the output contacts, the setting knobs, the d'arsonval unit (moving coil). The solid state type is built of a few printed circuit boards (cards), supported by the chassis. The main elements are: the rectifiers, the nonlinear circuits, the harmonic filters (if required), filters, the level detectors, the integration circuit (it could be an operational amplifier), diode circuits, setting devices, indicators, resistor circuits. When bus differential protection is applied, a modified differential relay may be used if the application requires a high impedance bus differential relay. The main elements for such electromechanical relays are: the overvoltage cylindrical (cup) induction unit, the overcurrent unit (a clapper type magnetic attraction), the nonlinear resistor, the adjustable air gapped reactor, the target. The high impedance differential bus relay is used if under fault conditions (on one of the feeders fed from the bus), one of the current transformers connected to the relay saturates. If a C.T. saturates, nuisance tripping will occur. Distance Relays: The construction of the electromechanical distance relay is as follows: air gapped transformers (compensators), tapped autotransformers, cylinder type (induction cup), operating units, targets. The operating units will have: the frame, the electromagnet, the moving element assembly, a moulded bridge. The electromagnet has four poles with twoseries connected coils, mounted diametrically opposite one another, to excite each set of poles. The moving element assembly consists of a special spring, a contact carrying member, aluminum cylinder assembled on a hub (inner core), which holds to the shaft. The shaft has top and bottom jewel bearings. The bridge (secured to the electromagnet and the frame) holds the upper (pin) bearing and the stationary contact. The solid state distance relays have: a signal preparation circuit (convertor), the starting relays (overcurrent and underimpedance), selecting circuit, measuring element (direction, reactance, resistance units), timing circuit, indicator, output relay, operating logic, power supply. The starting relays select the appropriate quantities for fault measurement and the solid state selecting circuit applies them to the measuring element. The measuring element is built up of comparators. Comparators are classified into phase and amplitude. Comparators are either electromechanical, (e.g. induction disc or cup, with operating and restraint electromagnets, polarized moving iron or moving coil with two coils) or static (coincidence circuit, vector product, rectifier bridge, transductor and sampling). Negative Sequence Current relays:
The major components of the negative sequence current relays are: the negative sequence current filter (sequence network), the tap block ,the time dial, the electromagnet with coil. Certain designs may have the operating of the magnetic attraction polar unit in place of the induction disc. For the solid state relay, the major components are :the negative sequence filter (operational amplifier negative sequence), rectifier, smoothing filter, measuring circuit (a Schmidt trigger),a control circuit and an output relay. Reverse Power Relays: The reverse power relay is used, for example, to protect the prime mover of generators, in case the system starts to motor the generator, upon loss of input to the prime mover. The electromechanical types have the directional unit (product type, operates as a result of interaction of flux, created by an operating current circuit and polarizing voltage circuit). Maximum torque is produced, when the current/flux in the current coil leads the flux produced by the current in the voltage circuit by 90°. The unit, also, has the stationary contact, the electromagnet, with the series coils (two operating, two polarizing, with each pair on alternate poles) and the moving contact on the cylinder assembly. If the relay is of the time delay, the timer unit (induction disc voltage unit) will have the following parts: the time dial, timer moving contact, induction disc, dampening magnet. The main coil of the timer unit is connected in series with the directional unit contact. For the solid state device, the components are: the converter unit (the signal conditioning and the comparator), the filter, the measuring element (for example Schmidt trigger or any type of level detector), the timer, the output relay. Loss of Field Relays: The generator field protection (loss of field relay) is applied to protect the generator from thermal damage, due to the decrease or loss of field. It is, also, used to protect the system from instability, due to decrease in the voltage caused by the generator operating at low excitation. The major components of such a relay are: the compensators, the autotransformers, the distance unit, the directional unit, the undervoltage unit, the time delay unit. The directional and the distance units are usually used to sound an alarm, during low field excitation conditions. The voltage unit is set to trip the generator when low excitation persists, in order to prevent undervoltage and possible instability conditions. The major elements for a solid state loss of field relay are: the converter, the phase comparator (offset MHO ch/cs), the timer and the output relay. Frequency Relays: The electromechanical frequency relay is built from the following major parts: frequency setting rheostat (connected to the upper pole coil), the mounting frame, the time setting dial, the moving and fixed (stationary) contacts, the induction disc element, with upper and lower pole (has a capacitor in series with the coil). This type of relays accept a voltage input (i.e. 120V from a potential transformer). The solid state frequency relay is built up of: a voltage transformer (as part of the converter), d.c. auxiliary supply unit (including the stabilizer), quartz oscillator, wave shapers, multivibrators, frequency selector plug board, level detector, counter, timelagsetting, indicator (target), filters and output relay. The principle of operation of the electromechanical unit is based on the phase angle between the upper pole coil and the lower one. The closing contact effect is produced when the lower pole current starts to lead the upper one. For the solid state relay, the principle of operation is based on counting the number of cycles of the oscillator during one period of the system cycle. The relay decides
whether the frequency of the system is normal or abnormal. If it is abnormal (either high or low), after the time delay set, the output relay unit is energized and a signal comes out of the relay to trip or alarm. Pilot Wire Schemes: They employ a communication channel of one type or another in conjunction with protective relays to ascertain in the minimum possible time whether the fault is in the protected zone or external to it. The most commonly used communication channels are: power line carrier, microwave channel or wire line channel. Each type of channels has its own coupling equipment beteen the one end (terminal station) and the transmitter/receiver point in the same station. The protective schemes applied in this protection are: directional comparison, phase comparison, current differential and transferred tripping. The major components of such scheme beside the transmission medium and transmitter/receiver are: sequence network, relay equipment, starter circuit, filters, carrier control and modulation devices. Reclosers and sectionalizers: Reclosers can be classified according to their interrupting medium (oil, vacuum or sulphur hexafluoride). Reclosers are used on overhead distribution systems to split a long feeder and to minimize the tripping of the transformer station feeder breakers. Coordination is needed between the feeder breaker and the reclosers. A sectionalizer is another device that can be found in overhead distribution systems, it must, always, be backed up by a recloser of the proper size. The sectionalizer is installed on taps or branches off the main lines or somewhere along the feeder. When a fault occurs beyond the sectionalizer, the recloser will operate. If a fault is permanent, the sectionalizer will count the number of operations of the recloser and trip/lock itself, after a predetermined number. The recloser continues on its final operation, restoring power up to the sectionalizer. Other recloser/sectionalizer combinations depend on the sensing of voltage and time coordinated, rather than counting the number of operations. Reclosers, as mentioned, are designed to interrupt and reclose alternating current circuits. The number of reclosings is adjustable up to 4 times. It recloses after a pre determined time. After the fourth reclose, the device locks itself in the open position. It has to be reset through an operator and then closed. A fuse link interrupts temporary and permanent faults alike. Reclosers give temporary faults repeated chances to clear or be cleared, by a downstream fuse or sectionalizer. The control of reclosers is achieved either hydraulically or electronically. Reclosers can be set for a number of different operation sequences, for example: two instantaneous (trip and reclose) operations, followed by two time delayed trips. one instantaneous plus three time delayed operations. four instantaneous or time delayed operations. Recloser ratings range for series coils, from 5 to 1120A and for nonseries (current transformers are used), from 100 to 2240A. The minimum pickup for all ratings is set, usually, to trip instantaneously at twice the current rating. The points of installation of reclosers depend on the amount of exposure of the line, the operating experience and the degree of importance of not tripping the transformer station feeders. Reclosers can be considered equivalent to the following: a circuit breaker, overcurent relay, reclosing relay. A typical recloser will have the following major components: interrupting chamber, main contacts, a control mechanism and a lockout mechanism plus all safety covers, hoods and enclosures. Reclosers are built in either single phase or three phase units. To define a recloser, the
following data, as a minimum should be available: rated maximum voltage, continuous current, BIL, maximum interrupting capacity, voltage withstandability and the duty cycle of the reclosing philosophy. Fuses for distribution transformers: In distribution systems, three phase transformers and three phase banks (i.e. 3 single phase connected to provide a delta or a Y 3phase configuration) are common. In general, the protection of the power transformers is provided through the use of protective relays (o/c or differential and over current ground) and gas relays. The distribution transformers are protected by fuses (current limiting and expulsion types). The distribution transformers are either ovehead (pole mounted) transformers or installed in above or below grade vaults or pad mounted. The connection and protection to each type differ significantly. Pad mounts can be classified into radial feed and loop feed. The pole mounted transformers have ahead of them current limiting fuses and distribution cutouts with fuse links with speed T or K as defined in ANSI C37.100 other speeds are also available to achieve proper co ordination between the fuses and upstream/downstream protective devices. The pad mounted transformers will have load or fault sensing (expulsion) type fuse that is accessible from outside the transformer to remove and replace and in series with these fuses are current limiting backup fuses under the oil and is inaccessible without denergizing the transformers and removing the transformers from the site and probably breaking the welds of the cover. The partial range current limiting (C.L.) fuse operates without discharging flame, gases or other byproducts of expulsive action. This series of fuses provides the current time characteristics of a coordinated full range. C.L. fuse is selected to operate only on internal failure of the transformer (permanent shorts). For vault mounted transformers, a series of current limiting and expulsion type (with power fuses or fuse links) mounted on the pole or the wall of the vault are considered as primary protection to. Medium voltage fuses (2.4 to 72kV) can be classified according to the following, they either fall under the distribution fuse cutouts or power fuses. The power fuses can further be classified into expulsion type and current limiting. Distribution fuse cutouts were developed for use in overhead distribution circuits (a connection to distribution transformers, supplying residential areas or small commercial/industrial plants). Pole mounted capacitor banks, used for voltage regulation or power factor correction, can be protected by such fuses. A distribution fuse cutout consists of a special insulating support and fuse holder. The disconnecting fuse holder engages contacts supported on the insulating support and is fitted with a fuse link (with speed Kfast or Tslow as defined in ANSI 37.100). The operation of the fuse is goverened by two curves: the minimum melting and the total clearing. The fuse holder is lined with an organic material. In fuse cutouts, the interruption of an overcurrent takes place inside the holder. The gas ionized (liberated), when the liner is exposed to the heat of the arc (as a result of the melting of the link), is then deionized (at current zero). Power fuses have characteristics that diffeentiate them from distribution fuse cutouts, these characteristics are: they are available in higher voltage ratings, the can carry higher load currents, they can interrupt higher fault curents and they can be installed indoors. Power fuses consist of a fuse holder, which accepts a refill unit or fuse link. The power fuse (expulsion
type) interrupts currents, like the distribution cutout. The current limiting type interrupts overcurrents when the arc established by the melting of the fusible element, is subjected to the mechanical restriction and cooling action of powder or sand filler, surrounding the fusible element. There are three features for the medium voltage current limiting fuse: 1.Interruption of overcurrents is accomplished quickly, without the expulsion of arc products or gases, as all the arc energy is absorbed by the sand filler and, subsequently, released as heat. 2.Current limiting action that occurs through the fuse is substantial, if the overcurrent exceeds, significantly, the continuous current rating of the fuse. 3.Very high interrupting ratings are achieved by virtue of the current limiting action of the fuse. Current limiting fuses can reduce the mechanical forces exerted on the components (in series) from the source up to the fault point due to the peak short circuit current. They can,also, reduce the thermal overloading due to the integration of the short circuit current over the period of the fault existence. They may impose an overvoltage condition on the equipment connected due to the current chopping effect (forcing current to zero before natural current zero). The typical refill construction of the distribution fuse cutout unit: current transfer bridge (connects the lower fusible end to the lower ferrule), fusible element, auxiliary arcing rod, auxiliary bore (where the arc is drawn and is interrupted for low fault currents), main arcing rod, main bore (where the arc is drawn and interrupted for moderate to high fault currents above 100A), solid material arc extinguishing medium (boric acid for example), outer tube (of epoxy), fuse tube plug and upper terminal. The major components of the fuse holder of a fuse refill type fuse: the pull ring, upper and lower ferrule, glass epoxy fuse tube, blown fuse indicator, window and silencer. The typical parts that constitute a power fuse link are: the exhaust ferrule, the current transfer bridge, the fusible element, the arcing rod, the bore with the solid arc extinguishing material, drive spring, actuating pin, glass epoxy tube, the upper contact, the upper seal, arcing rod retainer. The typical parts for the fuse holder are: the formed steel base, the upper and lower contacts, the insulators mounted onto the steel base, terminal pads (upper and lower line and load sides), latch and guide (to secure engagement of fuse unit and provide self guiding action during opening and closing of the unit). The fuse units can have the following attachments to assist in the proper operation and installation of the unit: upper fitting (pull ring), blown fuse target and silencer (filter). The typical ratings for the fuse/fuse holder combination are: nominal voltage, maximum voltage, BIL, current rating and speed and interrupting rating. Low voltage direct acting tripping devices and fuses: Direct acting trip devices for low voltage circuit breakers can be classified into electromechanical, solid state or microprocessor based devices. The electromechanical device has a heavy copper coil on each pole of the circuit breaker, which is an integral part of the trip unit. It is capable of carrying the full load current. The force produced during the passage of currents through the coil acts on an armature. This force overcome the dashpot (restraining force) which consequently trips the circuit breaker. This protection provides against overloads. Protection against S.C. is provided by the magnetic forces proportional to the S.C. current, which overcomes the spring restraining forces. Although current
transformers and O/C relays have been used on low voltage switchgear assemblies, integral electromechanical and now solid state and microprocessor based devices are more acceptable as a means of protection on L.V. circuits. The solid state (S.S.) trip devices offer longtime, shorttime and instantaneous trip elements which perform essentially the same protective functions as provided by the electromechanical trip devices. The ground trip function can be part of the S.S. device, for those applications that require the protection against ground faults or to meet the Code requirements. Such trip devices consist of sensors, logic box, latch release and interconnecting wires. Generally, there are two sensors on the primary conductor. One is supplying the logic box (through a rectifier circuit) with the power required to operate the latch release and solid state circuitry. The other supplies the logic box with a current signal, proportional to the primary current through the signal input circuit adjustable resistor, pickup circuit, timing circuit and to the output circuit. The power supply sensor output is switched to the latch release on command from the output circuit. The output circuit operates when the primary current exceeds the selected current magnitude and duration (i.e., time delay). These devices can have operation indicators and load alarms (adjustable from 50% to 100% of the tap setting). With the increased number of loads that have nonlinear characteristics, harmonics can be generated from these loads and injected into the network. Harmonics can confuse S.S. tripping devices. With microprocessor based (true RMS sensing vs. peak) units, this problem can be alleviated. These devices provide an efficient means of highly repetitive sampling (wave capturing) to calculate the RMS value of the current wave. By proper selection of asynchronous sampling frequency (twice the maximum harmonic frequency available on the system) and RMS software algorithm (FFT), the RMS value of the distorted wave can be determined. The major blocks that build such devices are: the input transformers (analog to digital circuit), the sample and hold circuit, the multiplexer, the ROM, the microprocessor, the output circuit (dry or transistorized contacts) and the watchdog circuit. These devices can have any of the following capabilities: load indicator/alarm, thermal memory (for motor protection), zone interlocking, microprocessor status and fault type indicator. Low voltage fuses have a voltage rating of 125V to 600V and they are either current limiting or non current limiting. They can also be classified into standard code that has the minimum interrupting capacity (10KA) or HRC (high rupture capacity, with an interrupting capacity of 50, 100 or 200 KA, minimum). Fuses can also be classified into Class I or II, class I for protection against overload and short circuit and Class II against only short circuit. Class II are used mainly with motor protection circuits (the overload elements: melting alloy, bimetallic strip or solid state provide for O/C protection). The standards that govern the design and testing of L.V. fuses are :C22.2 No. 59 covers the dimensions, the minimum interrupting rating (10kA) and the overload blowing parameters for standard and time delayed characteristics equivalent to Class H (UL 198.1) and plug fuses (UL 198.5). The plug fuses are rated for maximum 125V a.c., 30 A. The Class H fuses can be rated for 250V or less and for 600 V or less, with a maximum continuous current of 600A. CSA C22.2 No. 106 specifies: the fuse designations HRCI.R, HRCII.C, HRC I.J, HRC.L, the fuse dimensions, the current limiting levels (I2 t and Ip) and
time delay fuses. The minimum interrupting ratings for the above mentioned designation is 200 kA RMS, symmetrical. Other designations as given in C22.2 No. 106 are the HRCI.Misc, which covers any HRCI fuses having non standard dimensions and a minimum interrupting capacity of 100 kA, which is
equivalent to UL 198.2 (198.C) Class G or Class, CC. HRCI.T has available voltage ratings of 300V (0 1200A) livetoground or 600V (0800A) and the I.C. is 200kA. HRC II.Misc. has no equivalent in UL. HRCI.R, HRCI.J and HRC.L are equivalent to UL class R (198.E), UL Class J (198.C) and UL Class L (198.C), respectively. For Class R, the voltage rating is 250 or 600V and the current range is 0600 A. Class J is rated for 600V and the current ranges from 0600A. The voltage for Class L is 600V and ranges from 6016000 amps. Class K in UL, does not have a counterpart in #106 and is rated 250 or 600V and currents of 0600 amp., with three permitted I.C. (50, 100 or 200kA). In #106, HRCII.C has
dimensions non compatible with UL, but the I.C. is 200kA with the peak current and the I2 t limits in line with UL RK5 (one of the 2 subclassifications of Class R). Multifunction motor and feeder protection relays: With the availability of solid state and microprocessor based technology, the load/system protection can be achieved through the use of single, multifunction relay (vs.discrete multiple relays). The couple of examples that will be covered hereafter are: the motor protection and the feeder protection relays. The basic blocks for a typical multifunction squirrel cage induction motor protective relays are: the current to voltage transducers, phase protection overload/overtemperature, ground fault function card (which is fed from a donut type C.T. through which all power conductors to the motor are passed), annunciator, relay driver cards, trip and alarm output relays. Among the most frequent conditions causing thermal damage to a motor are: prolonged overload conditions, too frequent starts of the motor and severe duty cycle loading. The RTD protects against the condition when the machine is started while it is still hot plus the last two conditions given in the previous sentence. The thermal overload function (sensing the line current and ambient temperature) provides protection for the overload condition. For openphase or phase reversal function, the three signals from the transducers are amplified and shaped into 60 HZ square waves. Sequence sensitive circuits compare the relative timing of the square waves. If a square wave is detected at the wrong time, relative to another, or if one is missing, the relay trip circuit is energized and the trip relay is operated. Unbalanced motor line voltages produce unbalanced line currents into the motor. The negativephasesequence current (one of the three symmetrical components of an unbalanced current), produces a rotating field opposite in direction to motor rotation. This causes an undesirable reverse torque and considerable heating in the rotor of the machine. A negative sequence network or by finding the difference between the maximum current in one phase and the phase carrying the minimum current, the amount of unbalance is known. A comparison is made between this value and the setting. If the setting is exceeded, the output relay will operate. Some designs will provide an alarm when the level of unbalance reaches a certain percentage of the setting, say 70%. The ground fault function operates when the vectorial sum of the line currents is not equal to zero (i.e. a ground fault exists). If this level, after being converted, rectified and amplified, exceeds the setting, the output relay will be energized. The relay driver serves as an interface between the protection functions circuits and the output relays. Usually, the annunciator card has the indication for both trip and alarm, for all protective functions available on the multifunction relay. The feeder protection relay offers the following functions in one unit: overcurrent protection, event recording, reclosing capability, operational counter, total KA interrupted for the breaker controlled by the relay, communication capabilities and certain designs will have a voltage input to sense under/over
voltage conditions. In general, these devices consist of printed circuit boards carrying subassemblies, interconnected by flat cables. The transformer (i/p) board, provides electrical insulation and brings the variable to be processed to levels suitable for the next step. On each channel, a low frequency filter shapes the signal before sampling. The multiplexer is connected between the filter and the sample and hold circuit (S/H), the output of the S/H is fed into an analog to a digital converter (10 or 16 bit). The other major building blocks of such relays are: the microprocessor, the input/output module (controlling the output module relay), input contacts, the keypad on the relay and integral display. The microprocessor provides the control and the decision making process through communicating with the ROM, RAM and I/O module. The relay checks, continuously, whether the fault criteria programmed by the user has been reached or exceeded. If it is exceeded, the programmed time delay will start counting. If the fault still exists after the elapse of the time delay set, a trip is initiated. These relays are periodically monitored by the builtin watchdog circuits to give a warning of any hardware or software failure.