Building Load

Generator C.B.

G Generator

Fig. 28-4 Example of Connection for Parallel Operation with Utility System Capable of Standby Operation.

• Safety. Protection of customers, line workers, and the general public is paramount.

• Quality of power. The ability of the utility system is designed to assure that a certain level or range of power quality (i.e., proper voltage, frequency, etc.) to customers is not compromised.

• Equipment protection. The formidable fault currents available from electric utility systems always pose a risk to connected equipment and on-site generators, like other major equipment at the host facility requiring protection.

• Unencumbered system control by the utility. The utility must be able to control and respond to all events (i.e., faults, etc.) that interrupt operation or present risks, including network subsection isolation and dynamic repair functions.

Interconnection requirements for parallel operation can vary widely, depending on a number of factors. The capacity of the generating plant (kVA or kW) compared with the host facility's distribution system and the utility substation feeder will be major determinants of system requirements and cost. Most utilities place a maximum limit on the rating of a generator that can be connected to their distribution systems.

Power exportation adds complexity and cost, particularly if the utility's local distribution system is inadequate to handle additional exported power. While the utility will generally make the needed modifications, the cost usually must be absorbed by the self-generating facility. Interconnection for small induction generators is usually simpler than for larger synchronous units.

Generator voltage is also an important factor in terms of in-house distribution and power exportation. Voltage selection for on-site generation at existing facilities depends on the existing distribution system and any reasonable retrofits that can be made. In some cases, existing transformers may require replacement or major modification, or the distribution system may require modification. At higher voltages, current is lower and generator internal losses are less. At low voltage, a higher ampacity interconnect is required and the system may not be able to export power without significant voltage variances. For export, generator voltage must be raised to account for transformer impedance and voltage drop, and the Fig. 28-5 Primary Distribution Feeder with Step Voltage and Voltage Profile.

resulting overvoltage may have an undesirable effect on plant equipment. For an on-site generating plant of 2,000 kVA (or slightly larger), 480 volts may be well matched. Above that, consideration should be given to using higher voltages.

Voltage regulation is the primary means for utility network feeder system control. In contrast, current and power factor (PF) depend on customer loads and frequency is set by central utility prime movers. Utility-supplied voltage is rarely constant and typically varies by 5%, depending on the size of the load and the distance to the nearest substation. When an on-site generator is large with respect to the capacity of the utility intertie, it may be preferable to let the on-site generator set the facility voltage rather than the network.

Voltage is adjusted by the utility using load tap changers or induction voltage regulators. A load tap changer is a motor-driven switching device that can adjust transformer ratio in response to voltage variation. A voltage regulator is an automatic transformer connected in series with the main transformer. Capacitors that can be switched in and out of the circuit are used to control voltage changes resulting from PF variations. Figure 28-5 illustrates a primary distribution feeder with step voltage regulator and voltage profile.

By controlling on-site generator excitation with a PF or kVAR controller, the utility network PF can be improved and better regulated. Over-excitement produces excess kVARs, thereby reducing the kVARs drawn from the utility. Because PF is improved, the utility has an incentive to cooperate with the on-site generator. A communication link can be installed to vary excitation with respect to utility network load changes. Figures 28-6 and 28-7 are block diagrams of a self-excited and a separately excited synchronous generator, respectively.

Distribution Substation

First Distribution Transformer

Last Distribution Transformer

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