A

Fig. 28-16 Grounded Wye/Op en Delta.

Open-Circuit Single-Phase

Open-Circuit Two-Phase

Open-Circuit Three-Phase

Fig. 28-13 Open-Circuited Phase Faults.

circuits and to re-energize the line by reclosing to restore service. Because most faults are temporary (lasting only a few cycles or seconds), reclosers momentarily trip open and closed, after which most faults will be cleared. Reclosers are set for one or several reclosing operations. Typically, with multiple reclosing settings, the initial response is instantaneous, lasting a couple of cycles. If the fault is not cleared, subsequent responses will be time delayed (for lower levels of current) to allow for a slightly longer interruption in an effort to clear the fault. Finally, if the fault is not cleared, the recloser will lock open. Figure 28-17 illustrates a typical recloser operating sequence.

Sectionalizers are used in conjunction with a circuit breaker or recloser to isolate more permanent faults. After a circuit breaker or recloser opens, sectionalizers open, isolating the fault and allowing the breaker or recloser to close and restore normal load current to downstream circuits. If a recloser is used and set for pre-set multiple operations, the sectionalizer will count the operations and lock open after the second-to-last operation to allow the recloser to close.

Several problems may occur when a utility system breaker opens and the local on-site generator is isolated from the network. If the generator is undersized relative to the connected load, it will be overloaded and begin to drop voltage and frequency. If the generator is oversized relative to the remaining load (meaning that it had been exporting power), it may be subject to overspeeding. In either case, the customer's main breaker must trip whenever utility power is disconnected. Under-frequency or over-frequency relays (set at 1 Hz +/-), undervoltage and overvoltage relays, and directional current relays can be used.

When generators are being used for isolated operation, a synchronizer is used at the main breaker to allow reconnection without stopping. The facility breaker cannot be reclosed onto a dead or live unsynchronized utility line. Reconnection of the generator should only occur when trip conditions have been absent for a period of time greater than the longest reclose time for the main breaker at the feeding substation. A synchronizing breaker should only close when the frequency difference across the breaker is within the generator tolerance level. If an automatic synchronizer is not used, a synchronism check relay is required to prevent manual reclosing out of synchronism. A synchronizer is used at the generator breakers to allow the generators to be brought off and on without interruption of utility service.

Design of a facility's distribution network includes analysis of the available short-circuit current at various points in the network. Switchgear must be specified with sufficient short-circuit capacity. While an on-site generator may decrease the actual load drawn from the utility, it will usually increase the available short-circuit current at the facility, which may necessitate the use of heavier switchgear.

When an on-site generator and the utility network feed the same bus, the short-circuit current available from each must be added together to determine the capacity requirements of the circuit breakers. When several buses are used for in-house distribution, short-circuit current considerations limit the ability to transfer loads from one source to another in the event of a failure. Reactors are sometimes used to tie facility buses together to allow transfer of power from one bus to another without increasing short-circuit current in the event of a failure.

Representative Protection Configurations for Parallel Operation

The previous discussion has presented some of the relay functions. The following discussion will center on a series of examples. These examples assume that on-site generation is to be operated in parallel with the utility-derived service. They represent a medium- and a low-voltage system.

With the medium-voltage system, it is assumed that the facility has two utility-derived feeds and a number of on-site generators. The generators serve as emergency power for the facility as well. Whether these generators are run occasionally

Fault Current

"Fast" Operations (Contacts Closed)

"Time-Delay" Operations (Contacts Closed)

Load Current

AAAAAAA (Contacts Closed)

Fault Initiated

Reclosing Intervals (Contacts Open)

Recloser Lockout

"Fast" Operations (Contacts Closed)

"Time-Delay" Operations (Contacts Closed)

Fault Current

Load Current

AAAAAAA (Contacts Closed)

Fault Initiated

Recloser Lockout

(Contacts open)

Reclosing Intervals (Contacts Open)

(Contacts open)

Fig. 28-17 Typical Recloser Operating Sequence.

as peak-shaving units or continuously as baseload units, the relay scheme would remain the same. The fact that parallel operation is to last for more than 1 minute dictates the need for a coordinated protective scheme. If the parallel operation was solely for the purpose of closed transition transfer of the load from one source to the other, a very simple scheme could be used because the operation would typically last less than 30 seconds.

Medium Voltage Examples

Figure 28-18 represents a facility with two service feeders from the utility and a number of on-site generators. This example is representative of a campus with several buildings. The switchgear would be medium-voltage, 5 or 15 kV, and is divided into two line-ups: utility and generator. The utility switchgear is a typical double-ended design with a tie. It can be operated with any two of the main tie breakers closed at any time. All three breakers are never closed simultaneously to prevent feed-through of fault current from one utility feeder to the other.

In this example, 52A and 52T are closed and 52B is open. When the generators are to operate in parallel with the utility, 52T1 and 52T2 are closed and the selected generators are then started. When synchronized with the utility service, their respective breakers are closed. When operation is completed, the generator breakers are opened and 52T1 is opened. The feeders from the utility switchgear provide normal power to the campus and the generator switchgear provides emergency power to these buildings.

Figure 28-19 represents a minimum protective relay scheme for the utility main breakers shown in Figure 28-18. The transformers are connected and, to limit ground fault current, the neutral of the secondary winding is low-resistance grounded. A current relay is connected to a current transformer in series with the grounding resistor and has the designation of 50G. Upon establishing a ground fault, the 50G trips the associated main. This relay will typically have a time delay of 12 to 18 cycles to permit a downstream device to detect and respond to the fault by opening the breaker closest to the fault. This coordination avoids shutdown of the entire distribution system for faults down stream.

Each main breaker would have the same protective devices. Device 27/59 allows closure of the main when the utility voltage is within acceptable limits. Most utility companies require a frequency relay at the connection point, as well, to ensure that the on-site generation will not change the frequency of the utility grid. However, this relay serves as backup protection for isolating the generators from the bus to prevent overload when the utility system has an upstream failure.

Device 50/51 is an overcurrent relay that opens the main for various overcurrents. The 51 includes a delay operation relay that allows brief in-rush currents for motor starting, etc., while the 50 is an instantaneous relay that acts at much higher current levels to respond to fault currents. Device 46 acts as a current balance and phase sequence relay. Device 67 is a direction overcurrent relay and would typically be set at the full-load current rating of the generators where normal operation includes export power. If planned operation does not include export, the overcurrent relay setting would be low to operate immediately upon detection of flow into the utility system. Unlike the other relays, Device 67 would trip 52T1 to isolate the generators from the utility.

Fig. 28-18 Line Diagram for Parallel Operation of Local On-Site Generation with Utility-Derived Service at Medium Voltage. Source: Automatic Switch Company

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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