12511 Protective Relaying

The primary function of a protective relay scheme for a grid-parallel DG unit should be to separate the power sources on the occurrence of an anomaly. It is taken on faith at this point in the discussion that both power sources will be adequately protected with a proper scheme (though it remains to be discussed herein). Therefore, if the sources are properly protected as isolated sources, then the only action to take when the sources are paralleled is to separate them so that each may initiate its appropriate protective device. When the anomaly is resolved, the affected circuit will be isolated and the remaining circuitry can restore operation automatically. Such a scenario ensures proper protection, power continuity, and minimum risk.

The protective relay scheme for islanded operation will be presented before exploring the parallel source scheme. Slightly different schemes will be utilized on low and medium voltage power systems, mainly due to the differences in equipment design. For example, instantaneous and time-delayed overcurrent trip functions are an integral part of the circuit breaker trip element in low-voltage equipment. These functions are provided by separate relay packages in medium-voltage equipment. The development of the protection scheme begins with the requirements for operating the power sources as islanded sources. In the islanded mode, the grid provides the source of power to the host facility. The protection scheme at the point of common connection is generally designed to limit exposure of the host facility loads to the damaging effects of excessive current flow from the grid, and provide protection against voltage configurations that could damage loads.

Figure 12.8 presents a single line diagram of the relay scheme at the point of common connection of the grid to the host facility. This is representative of both low-voltage and medium-voltage systems. For the low-voltage system, the wye point of the secondary of the transformer would likely be solidly grounded, as shown on the left of the diagram. In a medium-voltage system, the wye point would likely be resistance- or otherwise grounded, as shown on the right side of the diagram. Devs. 50 and 51 would provide for disconnection of the facility should a fault occur in its immediate zone. If the fault occurs further downstream in the facility, the timing of Dev. 51 would be coordinated with downstream overcurrent protection. Virtually all facilities of any appreciable size have three-phase motors downstream. It is, therefore, desirable to protect them against the heating effects of single phasing. For this reason, the service entrance protective scheme will include Dev. 47, negative sequence voltage function. This function is commonly available in combination with Dev. 27 in the same relay package. Dev. 27 would cause the main breaker to open on loss of voltage from the grid. In the permissive role, these functions would allow reclosure of the main breaker when grid voltage is again within acceptable limits.

Figure 12.8 also includes a single line diagram of a relay scheme for an in-house generator. This type of generator is typically installed for emergency, standby, or optional standby operation of critical loads. The diagram illustrates the same overcurrent protection as provided for the main service. Obviously, it serves the same purpose, although it is often somewhat more difficult to achieve coordination with downstream overcurrent protective devices. This is due to the very limited ability of the generator to supply the level of forcing current into a fault that is available from the grid. Usually, overcurrent protection on the generator will include a short time function to achieve the desired coordination. For generator circuits, Devs. 27 and 81 serve both permissive and protective functions. In the permissive role, these relays prevent the connection of the generator to a load until the output voltage and frequency are at least 95% of nominal. In the protective role, these relay functions will cause disconnection of the generator from a bus if these parameters fall below 90% of nominal. This is for protection in the islanded mode of operation. Sustained low voltage or frequency is indicative of an overload on the generator.

When these power sources are to operate in parallel for extended periods, additional relaying is required. Figure 12.9 illustrates what might be required. The circuit shown is for a soft-load system. A soft-load transfer system will synchronize and parallel the two sources. When in parallel, the loading controls will cause the load to shift from one source to the other at a preprogrammed rate. The objective of such a system is to minimize loading transients. In the case where extended parallel operation is desired, once the sources are paralleled the loading controls will control the amount of real and reactive load each source will carry. As can be seen in this figure, the protective relaying includes additional functions.

Beginning with the DG unit, two additional functions are required. Dev. 32R is applied in the reverse power mode. The reverse power relay serves the protective role to disconnect the DG unit from the power system. Power flow into this source is indicative of an unacceptable condition. Power flow into the source, reverse power, will occur when a malfunction in the prime mover or inverter occurs. For example, should a control or switching component in an inverter circuit fail, it may permit power to flow into the source. Where the prime mover is an engine, it can lose power and become motorized by the source. Under this condition, the DG unit must be separated from the power system. Dev. 40 provides a level of protection against failure of the excitation controls in the DG unit. Where the source of electricity is a synchronous generator, for the size range discussed here it will likely be brushless design. This design makes it difficult to monitor excitation so as to achieve true loss of excitation protection. However, when a brushless synchronous generator loses excitation while operating in parallel with a larger

Grid

Grid

Host Facility Bus Standby Bus

FIGURE 12.8

Single line diagram for grid and generation circuits.

Host Facility Bus Standby Bus

FIGURE 12.8

Single line diagram for grid and generation circuits.

FIGURE 12.9

Power and relaying circuit for soft loading and extended parallel operation.

power source, it will generally draw reverse VARs. Therefore, directional power relays connected in quadrature are typically applied as loss of excitation protection on brushless generators.

At the CTTS, Devs. 27, 59, and 81 U/O act in the permissive role for permitting transfer and in the protective role for initiating transfer. Dev. 25 acts as previously described. At the interconnect point of the circuit to grid-derived power, there is another set of protective relays. Up to this point in this discussion, only Dev. 27 has been discussed for voltage check. In the relay scheme shown at this interconnect point, over- and undervoltage and over- and underfrequency relaying are shown. The issue is whether the DG unit can drive the grid at the point of connection to overvoltage or over- or underfre-quency. Under the most likely scenario for DG unit application, where the DG unit is less than 10% of the MVA rating of the grid, the only way the unit can cause a rise in voltage at the grid is by taking on host facility load so that the voltage drop in the grid system is reduced by the reduction in demand. However, many utility companies require these protective functions at this interface. Given that these functions are in the CTTS relaying, they are not required at the point of interconnect. Devs. 47 and 46 provide for protection of the DG unit generation process against single phasing of the grid connection. If this connection or the grid itself becomes single phased, the feeder breaker will be tripped, allowing the in-house source to carry the load. Negative sequence voltages and currents have detrimental thermal effects on induction and synchronous generators. While the decrement curve of these generators can tolerate this condition for some short period, it must still be cleared. Therefore, it is common to find the time setting of these relays to be in the two- to five-second range. In actuality, these devices will operate as backup to overcurrent relays for single-phase fault clearing most of the time.

Some discussion on the remaining protective devices is necessary. Only one or the other of Devs. 32 and 67 should be required at the point of interconnection. Dev. 32 is a directional overpower relay, and Dev. 67 is a directional overcurrent relay. It is this author's opinion that directional overcurrent, Dev. 67, should be used. This has to do with the fault current-driving ability of the unit in comparison to the grid. Recall that it has been demonstrated that the grid is many times larger than the DG unit. AC power is a vector dot product. It is the product of the voltage, current, and power factor at the point of measurement. The nature of a fault on an AC circuit is inductive. Since circuit conductors are selected to reduce voltage drop along their path, their resistance is quite low in comparison to their inductive reactance. As the ratio of XL:R increases, the power factor decreases. The voltage at the location of a fault is zero. The voltage drop in the power system to the point of fault is equal to the source voltage less the voltage drop across its internal impedance. Since power is a function of voltage and power factor and since both voltage and power factor reduce under fault conditions, it is therefore concluded that current must increase to accommodate these reductions in order to operate a protective device that operates at a fixed power setting. For example, a directional power relay is set at 5%, 0.5 pu. At unit voltage and current and 0.5 power factor, the relay will trip (.05 = 1.0 x .1 x .5; note that power relays may be less sensitive at power factors below 0.5). Under fault conditions in the grid, the power factor and voltage at the point of connection of the DG unit will be quite low. If the voltage were as high as 0.05 and the power factor as high as 0.2, 5 pu current flow through the measuring point would be required to cause the relay to trip. It is quite difficult for DG units to produce this current flow at the point of connection to the grid. Dev. 67 uses a small voltage sample for polarization and acts directly on current flow. A Dev. 67 set to the correct value of reverse current flow would be more reliable than a directional power relay when acting as the primary device to separate the power sources when an anomaly occurs in the combined power system.

In the control strategy where the DG unit acts to power in-house loads only while operating in parallel with the grid, Dev. 67 would be set for a very low current level, 0.05 pu. In a scenario where the DG unit will feed power to the grid, the Dev. 67 trip point is set at a current level equivalent to a slightly higher than expected value and at a slightly lower power factor. This relay is an instantaneous trip relay. Accordingly, on the occurrence of a fault, disturbance, or anomaly on the grid distribution radial to which the DG unit is connected, within six to nine cycles of that occurrence, the DG unit will be separated and disconnected. The recloser will not close before the DG unit is disconnected. Reconnection of the distribution radial feeder at the substation will clear the fault or open again and lock out. Whichever of those two states occurs, the DG unit cannot be reconnected to the grid until the voltage permissive relays enable the auto synchronizing control, Dev. 25, to produce synchronism and reconnection.

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