I have prepared a report of my investigation into the economic benefits and reliability of common electrical power distribution configurations currently used in the United Kingdom. About 40% of power system investment is in the distribution system equipment (40% in generation, 20% in transmission).
Power systems are comprised of 3 basic electrical subsystems.
• Generation subsystem
• Transmission subsystem
• Distribution subsystem
We distinguish between these various portions of the power system by voltage levels as follows:
• Generation: 1kV-30 kV
• EHV Transmission: 500kV-765kV
• HV Transmission: 230kV-345kV
• Sub transmission system: 69kV-169kV
• Distribution system: ...view middle of the document...
Circuit breakers are always paired with a relay which senses short-circuit condition using potential transformers and current transformers.
c. Reclosers: devices similar in function to circuit breakers, except they also have the ability to reclose after opening, open again, and reclose again, repeating this cycle a predetermined number of times until they lockout.
d. Fuses: devices that can carry a defined load current without deterioration and interrupt a defined short-circuit current.
Circuit breakers, reclosers, and fuses are protection devices. Often, switches are used on the high side of the transformer, and protection devices are used on the low side, but substations supplying large amounts of load may have protection devices on both sides of the transformer. Special substation designs to achieve high reliability may utilize multiple circuit breakers, as shown in Fig. 1. Less expensive designs may use protection only in series with the feeders, as shown in Fig. 2. In these figures, switches and circuit breakers are normally closed unless there is a “N.O.” (Normally open) indicated beside it. The Fig. 1 design provides that all feeders can remain supplied for a transformer outage (caused by maintenance or fault) or a sub transmission line outage. The low voltage breaker scheme of Fig. 1 is called “breaker and a half” because it requires 3 breakers to protect 2 feeders.
3. Voltage regulation: Because current I flows from source to load along the length of the feeder, and because the feeder has some amount of impedance per unit length Z, the feeder will cause a voltage drop IZ volts per unit length. Thus, loads connected along the length of the feeder will see different voltage levels with the load at the far-end of the feeder seeing the lowest voltage of all. This is illustrated by the solid line in Fig. 3. Note that this line indicates the voltage at the substation end of the feeder is 1.02pu. However, the voltage at feeder far-end is about 0.97pu (residential customers would be seeing about 116 volts instead of 120). If the load were to increase, the far-end voltage would drop to an even lower value. As a result, we must regulate the voltage along the feeder as the load varies. Ways to do this include substation load tap-changing transformers, substation feeder or bus voltage regulators (employed in Fig. 3), line voltage regulators, and fixed or switched shunt capacitors.
Fig. 3: Illustration of feeder load variation
4. Metering: Most substations do have some sort of metering device that records, at a minimum, existing current and current max and min that have occurred in the last time period (e.g., 1 hour). Digital recording is also heavily used and capable of recording a large amount of substation operational information.
Most distribution substations carry between 5 and 60 MVA.
The primary distribution system consists of the feeders emanating from the substation and supplying power to 1 or more...