114 108

Electrical Power Output

Electrical Power Output

Fig. 28-6 Block Diagram of Self-Excited Synchronous Generator.

Protective devices — typically combinations of relays and circuit breakers — guard the power system against damage. Relays sense electrical parameters of voltage, current, power (real, apparent, and reactive), frequency, and phase angle. The relays respond to over, under, and differential

Electrical Power Output



Set Reference

Voltage Regulator


Main Stator

Mechanical Rotational Power input

Main Field (rotor)

Exciter Rotor (armature)

Rotating Diodes

Exciter Field (Stator)

Fig. 28-7 Block Diagram of Separately Excited Synchronous Generator.

limits, direction of flow, sequence of phases and currents, etc., with or without time functions to permit coordination with other devices.

The basic principle of device coordination is to identify and isolate only the faulted circuit, leaving the rest of the distribution system operating. When a disturbance occurs in the system, the cause is typically immediately indeterminate and the optimum approach is to isolate the power sources and let their respective relaying schemes segregate the affected circuits.

Given the need to customize each protection system, the selection and application of protective devices to power systems is often referred to as the "art of protective relaying." The protection scheme used at a given installation will depend on the physical installation, available fault currents, distribution voltage, and skill of the designer. A protective relaying and control system is required for any type of utility tie. The system must function automatically in a safe, logical sequence. Certain manual controls will also be required for system testing, calibration, and maintenance procedures.

A primary safety requirement for interconnection is safe access to the circuits for maintenance personnel. There must always be a locking provision on the tie-point disconnect that is accessible to utility or ISO company service personnel. The local on-site generator must be able to be separated from the utility grid immediately upon an unsafe power system disturbance.

When on-site generators are connected to the utility grid, they must be prevented from reclosing to a dead bus. This prevents generators from backfeeding the utility system when it is down. Additionally, when utility service is restored, it may not be synchronized with the on-site system. Therefore, it must be ensured that the two systems remain isolated. When the utility system re-establishes stable conditions, the generators can then be synchronized and paralleled with the grid.








Host facilities are connected to the utility-derived power supply via transformers, which reduce utility distribution or transmission voltage to a usable level and provide isolation and phase matching. When facilities have on-site generation systems, the transformer matches the generator voltage with the utility voltage while limiting the effects of transients on the generator.

Fig. 28-8 Wye/Delta Connection.

For retail power purchase arrangements, decisions must often be made regarding transformer ownership and metering point. Often, the utility owns and maintains the main transformer and the metering point for billing is on the secondary (or low-voltage) side of the transformer. In this arrangement, the utility absorbs capital and maintenance costs as well as the power losses through the transformer. Facilities are often afforded the opportunity to own and maintain the transformer and take power metered at primary voltage. There are various methods by which the utility provides incentives or compensation to such facilities. This usually involves some type of rate discount calculated as a percentage reduction of use or cost.

There are several different transformer types, designated according to the connections on the primary and secondary side. The critical difference between wye- and

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