电气07K7 栗晓燕 071914030110翻译原文.pdf

Protective Relays ELECTRIC POWER SYSTEMS PART 3 PROTECTIVE RELAYS Charles W Brice August 2002 Protective Relays 3 1 1 3 1 Introduction and Basics Protective relays are the devices that provide protection against faults such as short circuits and abnormal system conditions such as low frequency to avoid serious damage to vital pieces of equipment such as lines transformers and generators The protective relay system detects the fault and sends trip signals to circuit breakers and other switchgear while the switchgear clears the fault by interrupting it and isolating the faulty equipment Although it is desirable to limit damage to the equipment subjected to the fault the overriding concern is to protect the rest of the system from the fault For example a line subjected to a short circuit will often suffer damage if the short circuit is not promptly cleared Relays that are fast selective and reliable along with fast reliable circuit breakers will often prevent such damage and even more importantly will prevent the damage from spreading to the substation bus and the transformer Obviously the protective relay system must be carefully designed to achieve the proper balance among factors such as reliability speed selectivity and economics One aspect of protection namely transient overvoltage protection by application of surge arresters is not covered in this course Rather the present topic focuses on protective relay systems for overcurrent protection and protection from other abnormal conditions An overvoltage relay might be used to protect some apparatus from sustained overvoltage condition but not to protect the apparatus from a transient surge due to switching or lightning 3 1 1 Importance of protection Protection of the system from damaging short circuit currents is obviously important but to underscore its importance consider that an extensive overhead transmission system may be subject to temporary faults due to lightning induced flashover and permanent faults due to physical damage from ice and wind loading as well as accidental destruction of poles and towers The frequency of these faults is obviously a function of the lines overall exposure to the damage but faults may occur several times a day during normal conditions In the worst cases of storm damage hundreds of faults may occur in a few hours Also of great importance is protection from various abnormal system conditions For example severe damage to steam turbine blades can occur at low system frequencies generator step up transformers may be overexcited at very low frequency operation as the unit starts up certain relays may respond to transient generator swings during disturbances resulting in transmission line tripping All these and other conditions must be foreseen by the protective relay engineer 3 1 2 Short circuits When any part of the system is suddenly shorted whether all three phases phase to ground or phase to phase the current that flows just after the occurrence of the short circuit may be quite large compared to normal load current Short circuit protection is the most basic form of protection and is applied at all levels of the system generation transmission substation distribution and utilization Calculation of short circuit currents is a standard problem in power system analysis and many computer programs are available to perform the calculation Obviously this topic is one that goes substantially beyond our current interest but it will prove instructive to review some of the C W Brice August 2002 3 1 2 high points of simple calculations leaving the more complicated cases to the computer When a synchronous generator is subjected to a sudden three phase short circuit it responds with a large transient current that decays first rapidly then more slowly To simply represent this complicated phenomenon we usually calculate the initial value of the AC component although we know from experience that there will be a DC offset that decays as well The initial value of the AC short circuit current is the subtransient current and is calculated by using a constant voltage source behind the generator subtransient reactance which is commonly tabulated by generator manufacturers To calculate the current out in the transmission and distribution systems an extensive network calculation must be performed this is where the computer program comes in To calculate the current drawn by an unbalanced short circuit the theory of symmetrical components is usually used This theory first developed by Fortescue in the early part of the 20TH century states that an unbalanced three phase circuit may be solved by interconnecting three circuits two that are balanced three phase circuits with differing phase sequences and one that is a single phase circuit actually a three phase circuit with all phase currents in phase and all phase voltages in phase These three circuits are called the positive the negative and the zero sequence networks Without developing the theory we state that a single phase to ground short circuit also called a single line to ground short circuit may be solved by interconnecting the positive sequence network the negative sequence network and the zero sequence network in series at the point of the fault Likewise a phase to phase short circuit may be solved by interconnecting the positive and negative sequence networks in parallel at the point of the fault and a double phase to ground fault by connecting all three sequence networks in parallel at the point of the fault Figure 3 1 1 shows the positive sequence network used for calculating the three phase short circuit current and the sequence network interconnections for single phase to ground and phase to phase short circuits on a simple system consisting of two busses and two generators Many cases may be simplified to an equivalent circuit that has that figure For a single phase to ground short circuit the total fault current on the faulted phase is three times the zero sequence current For a phase to phase short circuit the faulted phases deliver 1 73 times the positive sequence fault current For a double phase to ground short circuit the faulted phases deliver 1 57 times the positive sequence fault current and the total fault current is 3 times the zero sequence fault current Phasor diagrams shown in Figure 3 1 2 give approximate results for phase and ground short circuits Protective Relays 3 1 3 GEN 1BUS 1 TRANSF 1 BUS 2 TRANSF 2 GEN 2 POSITIVE SEQUENCE NETWORK NEGATIVE SEQUENCE NETWORK ZERO SEQUENCE NETWORK Io POSITIVE SEQUENCE NETWORK FOR THREE PHASE SHORT CIRCUIT FAULT LOCATION POSITIVE SEQUENCE NETWORK NEGATIVE SEQUENCE NETWORK PHASE TO PHASE SHORT CIRCUIT SINGLE PHASE TO GROUND SHORT CIRCUIT FAULTED PHASE CURRENT IS 3 Io 12 1 1 1 2 2 2 2 2 1 1 Figure 3 1 1 Some fault calculations on a simple system C W Brice August 2002 3 1 4 A B C A B C A B C A B C A B C NORMAL THREE PHASE SHORT CIRCUIT SINGLE PHASE TO GROUND SHORT CIRCUIT PHASE TO PHASE SHORT CIRCUIT DOUBLE PHASE TO GROUND SHORT CIRCUIT Vab Van Ia Vab Ia IbIc Vab Ia Van Ic Ib Ib Ic Figure 3 1 2 Typical phasor diagrams for phase and ground short circuits Typical protective relay practice is to use ground relays to protect against single phase to ground short circuits and phase relays to protect against phase to phase short circuits and three phase short circuits Figure 3 1 3 shows three phase relays and one ground relay connected to current transformers abbreviated CT s Protective Relays 3 1 5 321 AC BUS 51 151 351N 52 DC SOURCE 51 1 51 1 SI 51 1 SI 51 2 51 2 SI 51 2 SI 51 3 51 3 SI 51 3 SI 51N 51N SI 51N SI 51 TIME OVERCURRENT RELAY 52 POWER CIRCUIT BREAKER 51 2 52 TC 52a TC TRIP COIL SI SEAL IN a BREAKER AUX CONTACT closed only when the main contacts are closed RELAY OPERATING COIL RELAY CONTACTS Figure 3 1 3 Three current transformers feeding overcurrent relays Note that the ground relay is device 51N and the phase relays are 51 1 for phase 1 51 2 for phase 2 and 51 3 for phase 3 Three phase and phase to phase short circuits will be detected by the phase relays and phase to ground short circuits will be detected by the ground relay C W Brice August 2002 3 1 6 The device numbers 51 and 52 are examples of ANSI standard device function numbers A few examples of these numbers and the devices that they represent are given in the table below see ANSI C37 2 for a complete list We will have much more to say about this and other connections in the following sections 21 Distance Relay 25 Synchronizing Device 27 Undervoltage Relay 32 Directional Power Relay 46 Reverse Phase Relay 47 Phase Sequence Relay 49 Thermal Relay 50 Instantaneous Overcurrent Relay 51 AC Time Overcurrent Relay 52 AC Circuit Breaker 59 Overvoltage Relay 67 AC Directional Overcurrent Relay 68 Blocking Relay 72 DC Circuit Breaker 76 DC Overcurrent Relay 77 Pulse Transmitter 78 Phase Angle or Out of Step Relay 79 AC Reclosing Relay 81 Frequency Relay 85 Carrier or Pilot Wire Receiver Relay 86 Lockout Relay 87 Differential Relay 3 1 3 Phasing and polarity Two of the relay engineer s most valuable and powerful tools are phasor diagrams and relative polarity of currents and voltage These tools go hand in hand that is the phasor diagram is incomplete and somewhat ambiguous unless an elementary schematic diagram is available complete with polarity marks for all voltage and current variables On these diagrams we use the double subscript notation the current flow from a to b is represented as Iab and the voltage drop from b to c is represented as Vbc On circuit diagrams the direction of conventional current flow is indicated by an arrow while voltage polarity is indicated by and marks Figure 3 1 4 gives a simple example of currents and voltages in phasor form Phases will either be labeled with letters or numbers such as ABC xyz or 321 with the order representing the phase sequence Since there is no universally accepted notation bear in mind that one utility may use A to represent the same phase that another calls x Likewise two utilities that use numbers may represent the same phase with 3 and 1 respectively The phase sequence is important since it represents a physical quantity the direction of rotation of the shaft of a three phase AC motor for example There is no standard and the examples will sometimes use ABC and sometimes 321 Figure 3 1 5 illustrates this correspondence Protective Relays 3 1 7 V RXl I Xc pqrs Vqr Vrs IVpq PHASOR DIAGRAM V Vpq VqrVrs PHASOR SUM OF VOLTAGE DROPS Figure 3 1 4 Simple example circuit showing current reference direction voltage polarity and phasor diagram C W Brice August 2002 3 1 8 A B C 3 2 1 N Y Z X N Figure 3 1 5 Phasor diagrams showing phase to neutral voltages of three systems using different nomenclature for the same phases Since the systems are synchronized the phase A in the top figure is identical to the phase 3 in the middle figure and phase Y in the bottom figure The phase sequence is ABC or 321 or XYZ 3 1 4 Grounding systems Formerly many power systems were ungrounded systems using delta delta transformers Ungrounded systems suffer from several problems including transient overvoltages that Protective Relays 3 1 9 necessitate the use of two surge arresters for a single phase transformer one on each side of the winding and trouble with rapid automatic clearing of the very common phase to ground short circuit An ungrounded system is connected to ground only by stray capacitances so a single phase to ground short circuit draws very little fault current To allow rapid detection and automatic clearing of ground faults and to reduce insulation and arrester costs grounded systems have become more common than ungrounded ones In many cases the power transformers in a grounded system are connected delta wye with the wye neutral grounded to the substation ground mat Generators and step down transformers serving only three phase loads are often grounded through a resistance to limit the ground fault current to low values Generator step up transformers and step down transformers serving four wire distribution systems are usually connected delta on the primary and wye on the secondary Remember primary means input and secondary means output so a step up transformer primary is the low voltage side but a step down transformer primary is the high voltage side Distribution transformers on four wire systems are connected phase to neutral single phase or wye three phase on the primary side The secondary side of three phase distribution transformers may be connected in either wye or delta The wye wye distribution transformer is used for four wire three phase conductors and a neutral conductor systems and the wye delta distribution transformer is usually connected with the bank neutral not connected to ground If the neutral is grounded the bank tries to ground the feeder and when ground faults occur on the primary side the distribution banks will supply ground current This usually causes a large number of blown fuses on wye delta distribution transformers with grounded neutrals Floating the bank neutral alleviates the problem but may cause overvoltage problems such as ferroresonance Wye wye banks avoid the ferroresonance problem since they invariably have grounded neutrals on both sides In many cases wye connected autotransformers and wye wye power transformers may be desired These are often provided with a delta tertiary winding so that the transformer will serve as a grounding bank Figure 3 1 6 shows a typical system and Figure 3 1 7 shows the zero sequence networks for delta wye wye wye and wye wye delta transformer banks Note that wye wye banks merely pass through the source system ground if any to the secondary system while the other cases provide a zero sequence connection to ground As Figure 3 1 8 shows this zero sequence connection to ground is quite significant for ground fault calculations The grounded system of Figure 3 16 has a generator grounded through an impedance a delta wye step up transformer with wye solidly grounded delta wye step down transformers with wye solidly grounded and a grounded wye autotransformer with delta tertiary Note that delta wye and wye wye delta banks with grounded wyes are grounding transformers capable of grounding an ungrounded system The grounded wye autotransformer with delta tertiary has the same zero sequence network as the grounded wye wye delta C W Brice August 2002 3 1 10 TRANSF 1TRANSF 2 230 kV 13 8 kV 230 kV 23 kV AUTOTRANSF TERTIARY 230 kV 115 kV 13 2 kV 1 23 4 5 12 5 kV6 7 Figure 3 1 6 Typical grounded system Protective Relays 3 1 11 DELTA GROUNDED WYE GROUNDED WYE GROUNDED WYE GROUNDED WYE GROUNDED WYE DELTA TERTIARY Figure 3 1 7 Zero sequence networks for delta wye wye wye and wye wye delta transformers C W Brice August 2002 3 1 12 12 3 45 12 3 45 12 3 45 3Rn jXgo jXto jX12o jXg2jX12jXt jXtjX12jXg Eg Ifo FAULTED PHASE TO GROUND CURRENT 3 Ifo Figure 3 1 8 Sequence networks connected in series at the point of a single phase to ground short circuit The system is that of Figure 3 1 6 with the fault on bus 5 Note that the zero sequence networks of the transformers have a great effect on the available ground fault current 1 5 Instrument transformers Instrument transformers are the input transducers for protective relays but also metering both for revenue metering and for data acquisition The two main types of instrument transformers are current transformers CT s and potential transformers PT s The latter are also called voltage transformers VT s with no difference intended Some current transformers use the main current carrying conductor as the primary usually one turn through a toroidal iron core that is wrapped by a large number of secondary turns Other current transformers have wound primaries and secondaries in lumped coils on iron core legs In either case the CT acts to step the current down say from 1000 A on the primary to 5 A on the secondary This would give a CT ratio of 1000 5 A or 200 1 A and would require 200 turns on the secondary assuming one primary turn Figure 3 1 9 shows a view of a toroidal current transformer and its circuit diagram showing polarity markings Protective Relays 3 1 13 Current transformer secondaries are typically rated at 5 A although some are rated at 1 A This is the continuous rating and it may be exceeded by a factor of 10 or 20 for a short time especially during a short circuit in supplying relays that provide overcurrent protection TO RELAYS Is Ip SYMBOL FOR CURRENT TRANSFORMER SHOWING POLARITY MARKS PRIMARY CONDUCTOR CURRENT TRANSFORMER WINDOW OR BAR TYPE CURRENT TRANSFORMER Figure 3 1 9 Current transformer and its circuit diagram The polarity marks mean that primary current into the mark induces secondary side current out of the mark Now Figure 3 1 10 shows the equivalent circuit of a transformer which also applies to the case of a current transformer Notice that the magnetizing branch which is often neglected must present a high impedance for the device to produce a secondary output current approximately proportional to the primary input current The shunt branch should be represented as a nonlinear reactance since the iron core may saturate at high input levels If the core saturates the apparent impedance of the shunt branch will drop since very little additional flux is produced by an increment in the current This will cause the secondary current to fail to track the primary current DC offset currents which exist in the short circuit currents of synchronous generators will aggravate the problem since the saturation of the iron core is dependent on the total instantaneous magnetizing current whether AC or DC As we will see CT saturation presents great problems to protective relaying especially differential relays C W Brice August 2002 3 1 14 TO RELAYS CURRENT TRANSFORMER EQUIVALENT CIRCUIT IpIs Im X1X2 Xm Im CORE FLUX CORE MAGNETIZATION CURVE Figure 3 1 10 Equivalent circuit of a current transformer and its magnetizatio