IEEE C62.92.2-2017 pdf free download.IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part II—Synchronous Generator Systems.
ANSI C50.10™-1977, American National Standard General Requirements for Synchronous Machines. 1 IEEE Std C50.12™-2005, IEEE Standard for Salient-Pole 50 Hz and 60 Hz Synchronous Generators and Generator/Motors for Hydraulic Turbine Applications Rated 5 MVA and Above. 2, 3 IEEE Std C50.13™-2014, IEEE Standard for Cylindrical-Rotor 50 Hz and 60 Hz Synchronous Generators Rated 10 MVA and Above. IEEE Std C62.92.1™-2000, IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part 1—Introduction. IEEE Std 519™, IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems. 3. Definitions For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary Online should be consulted for terms not defined in this clause. 4 4. Objectives of generator grounding The principle objective of grounding a synchronous generator system is the protection of the generator and associated equipment against damage caused by abnormal electrical conditions. The specific objectives in the protection of the generator are as follows: a) Reducing damage for internal ground-faults b) Limiting mechanical stress in the generator for external ground-faults c) Limiting temporary and transient overvoltages on the generator insulation The choice of grounding class is largely determined by the relative importance to the user of each of the above objectives. Also under consideration are providing a means of generator system ground-fault detection, coordinating the protection of the generator with the requirements of other equipment connected at generator voltage level and considering the level of reliability required. The degree to which each of the possible grounding methods accomplishes the desired objectives is discussed in 4.1.
It should be noted that the ground-fault current depends not only on the generator grounding, but also on other sources of ground current available to the generator. The last three classes have substantially higher fault current levels than ungrounded, resonant grounded, and high-resistance grounded classes (see IEEE Std C62.92.1 [B18], Table 1, for more information). In addition to the normal shutdown sequence (e.g., tripping the generator breaker, prime mover, and excitation) initiated by the machine protective relays, other measures are sometimes used to reduce the magnitude and duration of the fault current after the ground-fault relay has operated. These measures include forced field reduction and the use of automatic neutral circuit breakers. Automatic neutral circuit breakers are used for comparatively small machines. When a neutral generator breaker is used, the Transient Recovery Voltage (TRV) characteristics of the breaker shall be considered. Where automatic neutral circuit breakers are applied, it is their function to interrupt heavy currents during a single phase-to-ground fault within the generators, thereby minimizing damage in the generator. Opening of the neutral ground connection will change the system parameters of X 0 /X 1 and R 0 /X 1 and result in higher than normal temporary overvoltages on the un-faulted phases that can cause serious damage in other parts of the system. After the main circuit breaker has been tripped, the fault current will continue to flow as long as the fault circuit exists and until field flux in the generator decays to zero. Reduction of armature fault current can be accomplished with forced field reduction of the excitation system. Forced field reduction can be accomplished in several different ways (Berdy, Crenshaw, and Temoshok [B4]). The decay rate of the generator field flux determines the rate of reduction in generator fault current.
Meeting the limitation of IEEE Std C50.12-2005 and IEEE Std C50.13-2014 requires that at least a minimum value of impedance, either a resistance or a reactance, be installed in the neutral of all wye- connected grounded generators where the zero-sequence reactance is less than the positive-sequence subtransient reactance. In calculating the maximum currents that can flow in the generator windings during an external fault, it is usually sufficient to consider the generator impedances alone. It can be shown that, if sufficient neutral impedance is used to make the phase-to-ground fault current less than or equal to the three-phase fault current with the machine isolated from the system, the winding currents for any fault will be less than or equal to the winding current for a three-phase fault (Brown [B5]).