1、n IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems Part Il-Grounding of Synchronous Generator Systems Energy and Power Sponsored by the Surge Protective Devices Committee of the IEEE Power Engineering Society hbhhed by the InSMute of Electrical and Elecironics Engine
2、em, Inc., 345 East 47th Street. New Y Fig l(b) gives the results with one re- strike followed by a clearing at the next current zero. For the arc extinction voltage assumed, Fig 1 (b) indicates that the ratio of Xo/X1 should not exceed 3 if the transient voltages are to be limited to less than 250 p
3、ercent of normal line-to-neutral crest voltage. However, this voltage is still less than 75 percent of the manufacturers generator high-potential test voltage 7, 8. Each case should be studied using specific characteristics and appropriate modeling techniques. Figure 2 gives peak transient voltages
4、for high- resistance grounding. The voltage is plotted against the ratio of the 3-phase capacitive reactance to ground and the effective neutral resistance of the circuit, Xcg/R, (see ANSI/IEEE C62.92-1987, Figs 1 and 2 6). If this ratio is kept to 1 or greater, the peak voltage can be limited to ab
5、out 260 percent of normal peak line-to-neutral voltage, which is also less than 75 percent of the generator test voltage. This curve applies for any number of restrikes for ratios greater than 1 because each oscillation is damped out and a buildup in transient voltage is prevented. Figure 2 can also
6、 be used to indicate the magni- tude of possible transient voltages on ungrounded machines. The case to be compared is where the ratio of the 3-phase capacitive reactance to ground (X,) and thr neutral resistance (R,) of the circuit is less thai, the 0.1 lower limit of Fig 2. Thus, transient voltage
7、s of 4 to 5 times normal line-to-ground voltage crest may be reached if breaker restriking occurs on the ungrounded system. Temporary overvoltages on a generator can also be caused by a ground fault on the high- voltage side of the main power step-up trans- former. Such an occurrence impresses a neu
8、tral displacement voltage on the generator grounding equipment. The generator neutral grounding in conjunction with the transformer high to low side capacitive coupling forms a voltage divider circuit for the zero-sequence voltage impressed upon the transformer high-voltage winding 23. Consider- ati
9、on must be given to the generator grounding impedance and associated protective features to avoid temporary overvoltages that can damage the insulating systems or cause undesirable gen- erator ground relay operations. The lower the generator system zero-sequence impedance, the lower will be the impr
10、essed neutral displacement voltage. Therefore, this occurrence is a particular consideration for resonant grounded generator systems. The user of this guide should be aware that there is a degree of uncertainty as to the impulse strength of the generator insulation as compared to that of oil-insulat
11、ed apparatus of the same volt- age because of the different types of insulation systems and general construction. Because of this uncertainty, care should be taken in selecting both the class of grounding and the ratings of surge protective equipment. 2.4 Providing a Means of Generator System Ground
12、-Fault Protection. The grounding class chosen for a generator has a significant impact on the sensitivity and speed of ground-fault relaying for the generator and other apparatus connected to the generator voltage system. In general, ungrounded, high-resistance, and resonant- grounded systems allow
13、for the most sensitive ground-fault detection. In systems where genera- tors are bussed together at generator voltage or where feeders are taken out at the generator volt- age, relaying requirements may dictate a ground- ing class other than one which would provide maximum sensitivity for generator
14、stator ground faults. The effects which the choice of grounding class may have on ground relaying are discussed in a general way in Section 3. A complete discus- sion of generator ground-fault protection, includ- ing specific relaying systems, can be found in ANSI/IEEE C37.101-1985 4. 2.5 Coordinati
15、ng with the Other Apparatus at Generator Voltage Level. When a generator is interconnected with other systems, eg, other generators, plant auxiliaries, feeders, etc, at the generated voltage level, the class of generator grounding should not be determined by consider- ing the generators needs alone.
16、 Requirements for selective relaying, overvoltage control, inductive coordination, etc, in other parts of the system may constrain the choice of a generator ground- ing means. The specifics of these requirements for other systems may be found in the appropriate parts of this guide. The manner in whi
17、ch they may be 12 NEUTRAL GROUNDING IN ELECTRICAL UTILITY SYSTEMS IEEE C62.92 - 1989 reconciled with the generator requirements is discussed in 3.3. 3. Generator Grounding Qpes Various generator grounding classes and types have tended to become associated with particular generator system configurati
18、ons. It is a logical development since configurations that allow com- plete independence of choice of grounding means, ie, the unit generator transformer, are usually associated with grounding classes that maximize protection of the generator. When other equip- ment must be considered, the higher gr
19、ound current schemes are often used. In the following subsections, the various grounding classes are discussed in connection with the configuration with which they are normally employed. This subdivision is not intended to imply that other classes cannot be used, but that the ones dis- cussed are us
20、ed most frequently. 3.1 Unit-Connected Generation Systems. A unit-connected system is one in which a single generator is connected directly to a delta/wye step-up transformer with the delta windings at generator voltage. The unit configuration provides the maximum freedom of choice of a means for ge
21、nerator neu- tral grounding. The delta-connected winding of the unit transformer isolates the generator zero- sequence network from the rest of the system, allowing the neutral grounding of the generator to be chosen for maximum generator protection. The classes commonly used with the unit configu-
22、ration are discussed below. 3.1.1 High-Resistance Grounding. High- resistance grounding normally takes the form of a low-ohmic value resistor connected to the second- ary of a distribution transformer with the pri- mary winding of the transformer connected from the generator neutral to ground. The a
23、dvantage of the distribution transformer resistor combina- tion is that the resistor used in the secondary of Fig 3 Distribution Transformer Neutral Grounding VOLTAGE RELAY in other words, the ratio X,lR, is equal to or greater than 1 (see Fig 2). This prac- tice is described in equivalent terms suc
24、h as: (1) To make the resistive component of ground- fault current equal to or greater than the capacitive component (2 j To increase the power factor of the ground- fault current to at least 0.707 (3) To shift the phase angle of ground-fault current to less than 45” (4) To make the resistor power l
25、oss greater than the generator circuit 3-phase capaci- tive VA This proportioning will prevent high-transient voltages. The primary voltage rating of the distribution transformer in the generator neutral should be equal to or slightly greater than the generator phase-to-neutral voltage. In general,
26、a voltage rat- ing of the nearest standard value below the generator line-to-line voltage is used. For exam- ple, generators rated at 15 kV to 22 kV frequently use distribution transformers with a 14.4 kV primary. The 240 V secondary connection is usually used to provide sufficient voltage to operat
27、e a stan- dard relay. The thermal rating of the transformer is determined by the length of time the primary is expected to carry fault current at the neutral volt- age. Since a single phase-to-ground fault may be allowed to exist for an appreciable period, the thermal rating of the distribution tran
28、sformer (in VAj is usually determined by the product of the transformers rated primary voltage and the neutral current contribution to a solid phase-to- ground fault. However, operational experience and informed engineering judgment have led to Table 1 Permissible Short-Time Overload Factors for Dis
29、tribution Transformers Used for Neutral Grounding 16 Duration of Overload Multiple of Rated kVA 10 s 10.5 60 s 4.7 10 min 2.6 30 min 1.9 2h 1.4 the establishment of overload factors that permit safe and reasonable overloads for various short periods of time. These factors can be applied to the maxim
30、um thermal rating to permit the use of a lower kVA rated transformer. The factor selected depends upon the length of time a fault is allowed to exist before the unit is taken offline. If manu- facturers data is not available, Table 1 may be used as a guide for the selection of short-time overload fa
31、ctors. A detailed example calculation of high- resistance grounding using a distribution trans- former is illustrated in Appendix A. In summary, a generator system grounded through a distribution transformer with second- ary resistor has certain characteristics that may have the following desirable
32、features: (1 j Mechanical stresses and fault damage are limited during line-to-ground faults by re- stricting fault current between 5 and 15 A. (2) Transient overvoltages are limited to safe levels. (3 j Grounding device is more economical than direct insertion of a neutral resistor. (4) Relay sensi
33、tivity is relatively good except sensitivity decreases for faults nearer the neutral end of generator windings. The following features may not be desirable: (1) Surge protective equipment must be se- lected on the basis of higher temporary overvoltages during ground faults. (2 j Longer relaying time
34、 may be required. 3.1.2 Ungrounded. A system is considered to be ungrounded when no intentional connection to ground is made, except for potential trans- formers connected from generator neutral to ground and supplying only relays or instruments. If the neutral potential transformer secondary is loa
35、ded with a substantial resistive load, the sys- tem takes on the character of high-resistance grounding. The advantages of this class are essentially the same as for high-resistance grounding except 14 NEUTRAL GROUNDING IN ELECTRICAL UTILITY SYSTEMS IEEE C62.92 - 1989 that the maximum fault current
36、is somewhat less and transient voltages are not controlled well. A disadvantage is that excessive transient overvolt- ages may result from switching operations or intermittent faults. 3.1.3 Resonant Grounded. The ground-fault neutralizer is a neutral reactor having character- istics such that the ca
37、pacitive charging current during a line-to-ground fault is neutralized by an equal component of inductive current contri- buted by the ground-fault neutralizer. The net fault current is thus reduced by the parallel resonant circuit to a low value, which is essen- tially in phase with the fault volta
38、ge. After extinc- tion of the fault, the voltage recovery on the faulted phase is extremely slow with an exponen- tial time constant of Q/.rrf sec. (Q is the ratio of inductive reactance to the effective resistance of the transformer/reactor combination.) Accord- ingly, any ground fault of a transie
39、nt nature would automatically be extinguished on a resonant-grounded system. The application of generator resonant neutral grounding in the United States has been applied to some unit-connected generators supplying delta-connected, low-voltage windings of step-up transformers. The purpose of this gr
40、ounding scheme is to provide an extremely sensitive means of detecting phase-to-ground faults on the gener- ator voltage system and to limit the fault current to a very low value so that iron burning associated with generator insulation faults to ground is mini- mized 21, 26. A distribution transfor
41、mer and a reactor con- nected as in Fig 3, Note B, comprise the basic components of a ground-fault neutralizer. The reactor is selected so that the resultant reactance as seen from the high side of the distribution grounding transformer just matches the 3-phase capacitive reactance of the generator
42、windings, generator leads, step-up transformer, station serv- ice transformers, and all other equipment con- nected directly between the generator terminals and the low side of the step-up transformer. Single phase-to-ground faults are detected by the voltage or current in the secondary of the dis-
43、tribution transformer. For a phase-to-ground fault on the generator terminals, full generator phase- to-neutral voltage is impressed across the fault impedance in series with the primary winding of the grounding transformer. Because the net impedance of the tuned parallel LC circuit (tank circuit) c
44、onsisting of the generator system capaci- tive reactance and the inductive reactance of the neutralizer is essentially a very high resistance, a detectable voltage will result even if a rather high fault resistance is present. The fault resistance and the tuned LC circuit are in series and form a vo
45、ltage divider. Fault detection sensitivity is very high because of the effective amplification of the resonant tank cir- cuit. The equivalent impedance of the LC circuit in series with the fault is QX,. A voltage sensing device set at some ratio, lln, of the full line- neutral secondary voltage will
46、 detect a fault resistance at generator voltage of (n- 1) times the tank circuit impedance, QX,. Since X, is usually several thousands of ohms, detection sensitivity is very high. Fault resistance sensitivity decreases for faults near the neutral end of the generator winding, which reaches 0 at (100
47、/n)% of the winding length from neutral. Resonant grounding creates a highly tuned cir- cuit, and amplified zero-sequence voltages will possibly be impressed on the generator windings from the high-voltage system because of the capacitive coupling through the windings of the step-up transformer. Thi
48、s voltage can be kept to within reasonable limits by selecting a value of Q in a range of from 10 to 50 without excessively reducing the sensitivity of the fault detection sys- tem 20, 23. Zero-sequence and third harmonic voltages, which are inherently present in the generator output, can cause ampl
49、ified zero-sequence and third harmonic currents to flow in the generator system. They are injected by the generator voltage between the neutral connection to the neutralizer inductance and the generator system capacitance to ground. This series-resonant circuit permits amplified zero-sequence currents and accentu- ates the harmonic voltages across the neutralizer inductance. The magnitude of the neutral voltage depends upon the magnitude of the zero-sequence voltage and the losses in the circuit and ap- proximately equals E,/ therefore, testing can be done using 1
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