## 1251 Power Sources

The interaction of paralleled power sources is largely a function of the disparity in size of the sources. Additionally, this interaction can be separated into the categories of normal operation, fluctuations in parameters, and transient occurrences. The circuit shown in Figure 12.3 is a simplified schematic of a distribution grid, important in understanding the differences in these occurrences. Figure 12.3 will be explored as a power system from the point of generation to the point of utilization.

Utility-scale generation is quite large; the mix will contain base generators rated at several hundreds of megawatts up to 1100 MW. Intermediate generation plants range from 100 to 400 MW. Generators of 25 to 100 MW are used for peaking purposes and are typically disbursed around the grid system at load concentrations to relieve overloading on the transmission system. Since this text concentrates on DG with ratings of 10 MW or less, attention will be focused on utility circuits likely to have generation of this size or less.

The grid is fed by distributed utility generation rated 25 MW and higher. Thus, the grid will have capacity of at least 2.5 times that of the DG unit considered herein. This generation, however, will most likely be connected to the radial distribution feeders. An important consideration in siting this unit is the stiffness of the radial at the point of the connection. This chapter

FIGURE 12.3

Simplified schematic of the power grid from generation to utilization.

FIGURE 12.3

Simplified schematic of the power grid from generation to utilization.

defines stiffness as a measure of the capacity of a radial with respect to its ability to handle load with minimum fluctuation in voltage. A properly sited DG unit below 10 MW will probably have no noticeable impact on the frequency of the grid system. The grid system, however, will control the frequency of the unit.

Assuming a DG unit in parallel operation with the grid, the controls of the prime mover will set fuel flow and excitation to enable the unit to take on real and reactive load up to the desired level for the steady-state condition. Fuel control adjusts the amount of real load (kW) the unit will carry. Excitation control adjusts the amount of reactive load (kVAR) the unit will carry. The response times of these controls (to changes of state on the grid) have time constants between several tenths of a second to a few seconds; these units will be comparatively slow in response to sudden changes in grid voltage at the point of interface. Thus, responses to transients will differ as a function of the type of prime mover and excitation control.

This discussion assumes that the unit is connected to the grid at a distribution radial. Depending on the distance from the substation, conductor size, and other users on the radial, the available mega-volt-amp (MVA) at the point of connection will vary. Generally, the higher the available MVA at the point of connection, the stiffer the circuit. What does all of this mean? When electrical engineers design power distribution systems for buildings, they must determine the available MVA at the point of connection so that circuit protection devices can be adequately sized to interrupt a fault (short circuit) in the building. While a value of available MVA cannot be assumed across the board, for commercial and industrial buildings with service demands higher than 500 kW, one can expect to have 50 MVA available at the point of connection. At residences, one can expect to have approximately 50 times the rating of the pole transformer available. For facilities between these two levels, one can expect about 25 times the transformer rating. Engineers count on this high available MVA to provide for voltage stability in the facility, and they rely on sufficient fault current to permit coordination of circuit protective devices. The objective of this coordination is to clear the fault with the protective device closest to it. This allows unaffected circuits to remain in service.

## Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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