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Chapter 5—Electrical Network 5–50 voltages. The substation may feed a loop where every point in the loop can be reached from two directions. This organization allows most of the loads along such a primary feeder to be served even if a section of distribution line is not operable, due to storm damage, for example. There will also be single-phase lateral or radial lines extending out from the distribution substation. These are connected to distribution transformers to supply 120/240 volts to homes along the line. These lines generally operate at voltages between 2.4 and 34.5 kV. These circuits may be overhead or underground, depending on the load density and the physical conditions of the particular area to be served. Substation transformer ratings may vary from as small as 1000 to 2500 kVA for small rural applications up to 50,000 or 60,000 kVA. Distribution transformer ratings are typically between 5 and 50 kVA. The first responsibility of the design engineer is to protect all the utility equipment from faults on the system. These faults may be caused by lightning, storm damage, or equipment malfunction. When a fault occurs, line currents will be much larger than normal and a circuit breaker will open the line. A simple distribution substation protection scheme is shown in Fig. 27. The circuit breakers are adjusted so only the one nearest the fault will open. That is, if a fault occurs on the load side of circuit breaker CB4, only CB4 will open. The others will remain closed. Many of the circuit breakers in use are of the automatic reclosing variety, where the circuit breaker will automatically reclose after a fraction of a second. If the fault was caused by lightning, as it normally is, the electric arc between conductors will have time to dissipate while the breaker is open, so service is restored with minimum inconvenience to the customer. If the fault is still present when the breaker closes, it will open again, wait a fraction of a second, and close a second time. If the fault is still present, the breaker will open and remain open until the maintenance crew repairs the problem. The larger transformers will be protected by differential current relays. This is a relay which compares the input and output current of a transformer and opens a breaker when the current ratio changes, indicating a fault within the transformer. There may be an underfrequency relay which will open a breaker if the utility frequency drops below some specified value. A number of other protective devices are used if required by the particular situation. Once the system is properly protected, the quality of electricity must be assured. Quality refers to such factors as voltage magnitude, voltage regulation with load, frequency, harmonic content, and balance among the three phases. The electric utility goes to great lengths to deliver high quality electricity and uses a wide variety of methods to do so. We shall mention only the methods of controlling voltage magnitude. The voltage in the distribution system will vary with the voltage coming in from the transmission lines and also with the customer load. It is adjusted by one or more of three possible methods. These are transformer load tap changing, voltage regulators, and capacitor banks. In the tap changing case, one of the transformer windings will have several taps with voltage differences of perhaps two percent of rated voltage per tap. The feeders can be connected to different taps to raise or lower the feeder voltage. This is done manually, perhaps on a seasonal basis, but can also be done automatically. Automatic systems allow the taps to Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
Chapter 5—Electrical Network 5–51 Figure 27: Distribution substation protection. be changed several times a day to reflect changing load conditions. The voltage regulator acts as an autotransformer with a motor driven wiper arm. This can adjust voltage over a finer range than the tap changing transformer and responds more rapidly to changes in voltage. It would be continually varying throughout the day as the loads change. Capacitor banks are used to correct the power factor seen by the substation toward unity. This reduces the current flow in the lines and raises the voltage. They are commonly used on long distribution lines or highly inductive loads. They may be located at the substation but are often scattered along the feeder lines and at the customer locations. They may be manually operated on a seasonal basis or may be automatically switched in or out by a voltage sensitive relay. All of these voltage control devices operate on the assumption that power flows from the substation to the load and that voltage decreases with increasing distance from the substation. A given primary feeder may have 102 percent of rated voltage at the substation and 98 percent Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
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