DG Interconnection Protection Methods and Practices

The functional levels of interconnection protection vary widely depending on factors such as: generator size, point of interconnection to the utility distribution system, type of generator (induction, synchronous, asynchronous) and interconnection transformer configuration. As shown in Table 1, specific objectives of an interconnection protection system can be listed, as well as the relay functional requirements to accomplish each objective. Other than a very simplistic discussion of the detection of loss of parallel with the utility, IEEE 1547 does not address protection areas such as: use of fault current at the DG facility to detect supply tripping, abnormal power flow, and damaging system conditions.

Detection of loss of parallel operation with the utility system

The most basic and universal means of detecting loss of parallel operation with the utility is to establish an over/ underfrequency (81O/U) and over/undervoltage (27/59) "window" within which the DG is allowed to operate. The sensitive setting of both the underfrequency and voltage windows prevent the DG from supporting the power system during emergency conditions. When the DG is islanded from the utility system, either due to a fault or other abnormal condition, the frequency and voltage will quickly move outside the operating window if there is a significant difference between load and dispersed generation levels. If the load and generator are near a balance at the time of separation, voltage and frequency may stay within the normal operating window and under/overfrequency and over/undervoltage tripping may not take place. If this is a possibility, then a transfer trip (TT) using a reliable means of communication may be necessary. As discussed in the previous section of this article, when induction or synchronous DGs are islanded with pole-top capacitors and the generator capacity is near that of the islanded load, a resonant condition that produces a non-sinusoidal overvoltage can occur. For these cases, an instantaneous overvoltage relay (59I) that responds to peak overvoltage needs to be used to detect this situation.

Interconnection protection requirements in the state of California are defined in a filing to the state utility commission called Rule 21. A key provision of this rule is the unique application of a directional power relaying (32) to detect loss of utility parallel operation. This provision is only applicable to DG units that are installed for peak shaving or load following and do not sell power back to the utility.

Supply fault detection

On many small DGs, no specific supply fault detection is generally provided. Induction generators provide only two or three cycles of fault current to external faults―similar to induction motors. Small synchronous machines are typically so overloaded after the utility substation breaker trips that their fault current contribution is very small. For these small generators, as well as asynchronous generators, the detection of loss of parallel operation via 81O/U and 27/59 relays are all the interconnection protection necessary.

The larger the synchronous DG, the greater is the chance that it will contribute significant current to a utility system fault. For this situation, supply fault backfeed detection in addition to loss of parallel operation protection is generally provided. Typically, relay functions such as the 67, 21 or 51V are used to provide phase-fault detection. When developing settings for the 67 and 21 relays, the relay pickup setting must be set above the level of generator current being supplied by the DG to the utility system. Ground fault backfeed removal depends on the primary winding connection of the interconnection transformer. For grounded primary transformer windings, a 51N neutral overcurrent relay or, in some cases, a 67N ground directional relay is used. For ungrounded interconnection transformers, neutral overvoltage relays (59N, 27N) provide the detection for supply ground faults.

Detection of damaging system conditions

The major damaging condition that can be imposed on the synchronous and induction generators by the utility system is single-phasing. Negative sequence current (46) and voltage protection (47) is provided to detect this condition. Negative sequence voltage detection prevents re-synchronizing of the generator after a tripping if an open conductor condition exists. Out-of-step (78) protection is used to detect loss of synchronism for diesel generators, which have very low inertia and can lose synchronism if subjected to sustained low voltage caused by a slow-clearing fault.

Abnormal Power Flow

A directional power relay function (32) is used in peak-shaving and load-flowing DG applications where the DG is operated to supply only a portion of the local load at the installation and there is no intent to sell power back to the utility. The directional power relay is used to enforce the contract that states that no power will flow to the utility.

Typical Protection for Moderately-sized Synchronous DGs

For the ungrounded interconnection transformers, neutral overvoltage relays (59N, 27N) provide the detection for supply ground faults. As previously stated, the functional levels of interconnection protection vary widely depending on factors such as: generator size, point of interconnection to the utility distribution system, type of generator (induction, synchronous, asynchronous) and interconnection transformer configuration. The relay engineer typically selects the protection functions that are appropriate for a specific application.