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Reply To: معمای حفاظت شماره 110 – حفاظت ارت فالت دیزل ژنراتور


    در این خصوص با عجله قضاوت نکنید، بهتر است قبل از هر چیز با سازنده ژنراتور تماس بگیرید و در مورد حد تحمل اتصال کوتاه فاز- زمین سوال کنید،چون ممکن است سازنده امکان زمین کردن مستقیم ژنراتورش را تأیید کند. به هر حال برای جزئیات مطلبی که خدمتتان عرض کردم می توانید به استاندارد IEEE 142 که حاوی ارجاعات لازم به استاندارد NEMA است مراجعه فرمائید. در زیربندهای مرتبط برای ارجاع سریع آمده است. 1.7 Grounding of industrial and commercial generators 1.7.1 Industrial and commercial generator characteristics Generators have several characteristics that are significantly different from transformers, the other common source of power. As compared to the transformer, the generator has little ability to withstand the heating effects or mechanical forces of short circuits. The generator may be required by standards to withstand a less than 10-per-unit short circuit, and the imposition of higher currents is defined as unusual service by the National Electrical Manufacturers Association (NEMA) MG 1, whereas a transformer may be required to withstand a 25 per-unit current. The generator may be capable of withstanding less than 25% of the heating effect of this current as compared to the transformer. If the current is unbalanced, this capability may be reduced to less than 10% of the transformer capability (see NEMA MG 1; Nichols). Unlike the transformer, the three sequence reactances of a generator are not equal. The zero-sequence reactance has the lowest value, and the positive-sequence reactance varies as a function of time. Thus, a generator will usually have higher initial ground-fault current than a three-phase fault current if the generator has a solidly grounded neutral. According to NEMA, the generator is required to withstand only the three-phase current level unless it is otherwise specified (see NEMA MG 1). Also, NEMA states that the negative-sequence current thermal withstand limit is a product of time in seconds and the square of per-unit negative-sequence current (I2t) equaling 40 (see NEMA MG 1). With a solidly grounded neutral, the steady-state ground-fault current will be about eight times that of full-load current, while the steady-state three-phase fault current is three times fullload current; but, because of the negative-sequence content of the ground-fault current, the generator has less thermal withstand capability than it would for a three-phase fault. 1.7.2 Single unparalleled generator This configuration may offer the most options for grounding. The distribution system may be particularly designed for flexibility in applying grounding by having only three-wire loads connected directly to the generator or even having only a single transformer connected to the generator (unit bank). Thus the design may employ high-resistance grounding to minimize damage from internal ground faults, or low-resistance grounding if needed to operate selective ground relays. In either case the ground-current level should be substantially less than the phase-current fault levels. The generator may also be applied to a four-wire load without transformation. If the generator is rated for solidly grounded service, the neutral may be connected directly tothe grounded circuit conductor. If a standard generator is used, a reactor should be connected between neutral and the grounded circuit conductor so as to limit the momentary ground-fault current to no more than the momentary three-phase fault current (see Beeman; NEMA MG 1). When 3i0 = i”d the value of this neutral reactor, XN, should be as shown in Equation (1.1): XN=1/3(2X”d-X2-X0) (1.1 Where 3i0 = Ground-fault current = 3Vin/(X”d + X2 + X0 + 3Xn) i”d = Three-phase subtransient fault current = Vin/X”d X”d = Generator subtransient reactance X2 = Generator negative-sequence reactance X0 = Generator zero-sequence reactance Vin = Phase to neutral voltage Note that a resistor should not be used for this purpose, since its impedance is in quadrature with the machine reactance and thus would require a much larger value of resistance than reactance. This resistance would incur large losses from the flow of either fault or load current. The zero-sequence load current would also produce an objectionable voltage drop, since the load is primarily resistive. On the other hand, the neutral reactor will cause little voltage drop to be produced by inphase zero-sequence load current. The total zero-sequence current will be a small value because the generator has limited unbalanced current capacity. The continuous negativesequence current capability of generators covered in ANSI C50 standards is 8% or 10%. For salient-pole generators covered under NEMA MG 1, the limit is 10% at full load. The use of the reactor between the generator neutral and the neutral circuit conductor does not affect the NEC requirement that the neutral circuit conductor be solidly grounded. If generators are solidly grounded, the system