Note: First winding is utility primary, second is IPP secondary

The first three transformer winding configurations provide a focus on interconnection protection, (and all configurations provide an ungrounded utility primary winding,) but they require a different utility system ground fault protection method than the last two transformer winding configurations, which provide a grounded utility primary winding.

When employing a grounded utility primary winding, if the utility opens its substation breaker or line recloser, the IPP's DG can backfeed the distribution line, and a ground current is available at the interconnection transformer which is detectable by employing ground overcurrent elements. On the primary (utility) side of the transformer, a transformer neutral ct may be the source for directional or non-directional ground current protection (51N or 67N).

On the secondary (IPP) side of the transformer, phase undervoltage elements may be applied to detect utility ground faults, as the resultant voltage drop is measurable across the interconnection transformer while the utility has not yet cleared the ground fault. The measured secondary (IPP) side voltage will also drop if the IPP is sourcing the fault. For delta secondaries, in addition, voltage controlled or restrained overcurrent elements (51VC and 51VR), sometimes directionalized for greater sensitivity (67 supervision) may be employed. For grounded wye secondaries, ground overcurrent (51N) or directional ground current (67N) elements may be employed as the zero sequence current commutates across the grounded wye-grounded wye transformer.

When employing an ungrounded utility primary winding, if the utility opens its substation breaker or line recloser, the IPP's DG can backfeed the distribution line. As the ungrounded delta winding does not commutate zero sequence current to the secondary, conventional ground relays applied on the primary neutral will not detect ground fault current. This is because the ungrounded winding does not provide a ground source. The phase and ground protection on the secondary (IPP) side of the interconnection transformer will not be able to detect and clear the utility feeder ground fault supplied by the ungrounded source. Fortunately, there are methods employing voltage protection that can detect a ground fault supplied from an ungrounded source.

To detect and clear the utility ground feeder ground fault sourced from the ungrounded primary (utility) side winding, a protection scheme is applied that uses one of two options:

The displacement voltage across a broken delta transformer on the utility's system (primary side of the interconnection transformer): This method utilizes the fact that when a corner of the delta system is grounded, the normally balanced voltage triangle is shifted as shown in Figure 4. The resultant voltage across the broken delta potential transformer is three times the line-to-ground voltage (secondary).

Over/under voltage of a single phase measured line-to-ground voltage on the utility's system (primary side of the interconnection transformer): This method takes advantage of the fact that when a phase of the delta system is grounded, the grounded phase falls to zero volts, as it is now the ground reference. The other two phase voltages rise to line-to-line values. The resultant voltage across a single line-to-ground connected potential transformer will be a detectable undervoltage or overvoltage (1.73 times the line-to-ground secondary value) depending on which phase has the ground fault. Both of these methods are shown in Figure 5.

When used together with the other protections typically employed for IPP interconnection protection, we have two basic schemes, one for grounded interconnection primary (utility) windings, as shown in Figure 6, and one for ungrounded interconnection primary (utility) windings, as shown in Figure 7.

To summarize, the interconnection transformer winding arrangement applied has implications on the protection utilized at the IPP's facility; it also has possible impacts on distribution system protective elements. There is no single 'best' connection type or universally applied arrangement.

Attention must be paid to winding and utility side grounding so the proper IPP ground fault backfeed protection may be applied and other coordination issues can be realized.

References

1. ANSI/IEEE Std. 1001-1988, 'Guide for Interfacing Dispersed Storage and Generation Facilities with Electric utility Systems.'
2. IEEE P1547, Draft Standard for Distributed Resources Intercon-nected with Electric Power Systems.
3. Mozina, C.J., 'Interconnection Protection of Dispersed Generators in the New Millennium', Texas A&M University Conference for Protective Relay Engineers, College Station, Texas, April 11-13, 2000.
4. M-3410 Intertie/Generator Relay Instruction Manual, Beckwith Electric, 2001.