Typical Power System

An electrical power system is a network of electrical components used to supply, transmit and use electric power.  An example of an electric power system is the network that supplies a region’s homes and industry with power – for sizable regions, this power system is known as the grid and can be broadly divided into the generators that supply the power, the transmission system that carries the power from the generating centers to the load centers and the distribution system that feeds the power to nearby homes and industries. Smaller power systems are also found in industry, hospitals, commercial buildings and homes. The majority of these systems rely upon three-phase AC power- the standard for large-scale power transmission and distribution across the modern world. Specialized power systems that do not always rely upon three-phase AC power are found in aircraft, electric rail systems, ocean liners and automobiles.

The layout of a power system has following components:

·         Supplies

·         Loads

·         Conductors

·         Capacitors and reactors

·         Power electronics

·         Protective devices

·         SCADA(Supervisory Control And Data Acquisition) systems

1.      Supplies

           All power systems have one or more sources of power. For some power systems, the source of power is external to the system but for others it is a part of the system itself. Direct current power can be supplied by batteries, fuel cells or photovoltaic cells. Alternating current power is typically supplied by a rotor that spins in a magnetic field in a device known as turbo generators. There have been a wide range of techniques used to spin a turbine’s rotor, from steam heated using fossil fuels or nuclear energy, falling water and wind.

        The speed at which the rotor spins in combination with the number of generator poles determines the frequency of the alternating current produced by the generator (typically of 50 Hz in case of Nepal). If the load on the system increases, the generators will require more torque to spin at that speed and, in a typical power station, more steam must be supplied to the turbines driving them. Thus the stream used and the fuel expended are directly dependent on the quantity of electrical energy supplied.

2.      Loads

              Power systems deliver energy to loads that perform a function. These loads range from household appliances to industry machinery. Most loads expect a certain voltage and, for alternating current devices, a certain frequency and number of phases. At any one time, the net amount of power consumed by the loads on a power system must equal the net amount of power produced by the supplies less the power lost in transmission. Making sure that the voltage, frequency and amount of power supplied to the loads is in line with expectations is one of the great challenges of power system engineering. A final consideration with loads is to do with power quality. Power quality issues occur when the power supply to a load deviates from the ideal value. For an AC supply, the ideal is the current and voltage in-sync fluctuating as a perfect sine wave at a prescribed frequency with the voltage at a prescribed amplitude. For DC supply, the ideal is the voltage not varying from a prescribed level.

3.      Conductors

     Conductors carry power from the generators to the load. In a grid, conductors may be classified as belonging to the transmission system, which carries large amounts of power at high voltages from the generating centers to the load centers, or the distribution system, which feeds smaller amounts of power at lower voltages from the load centers to nearby homes and industry. Choice of conductors is based upon considerations such as cost, transmission losses and other desirable characteristics of the metal like tensile strength.

       

Conductors in exterior power systems may be placed overhead or underground. Overhead conductors are usually air insulated and supported on porcelain, glass or polymer insulators. Large conductors are stranded for ease of handling; smaller conductors used for building wiring are often solid. Conductors are typically rated for the maximum current that they can carry at a given over ambient conditions. As current flow increases through a conductor it heats up. For insulated conductors, the rating is determined by the insulation.

4.      Capacitors and reactors

        The majority of the load in a typical AC power system is inductive; the current lags behind the voltage. Since the voltage and the current are out of synchronization, so we need a form of power called reactive power. Reactive power does no measurable work but is transmitted back and forth between the reactive power source and load every cycle. This reactive power can be provided by generators themselves but it is often cheaper to provide it through capacitors, hence capacitors are often placed near inductive loads to reduce current demand on power system. Reactors consume reactive and are used to regulate voltage on long transmission lines. Reactors installed in series in a power system also limit rushes of current flow, small reactors are therefore almost always installed in series with capacitors to limit the current rush associated with switching in a capacitor. Series reactors can also be used to limit fault currents.

5.      Power electronics

      Power electronics are semi-conductor devices that are able to switch quantities of power ranging from a few hundred watts to several hundred megawatts. Despite their relatively simple function their speed of operation means they are capable of a wide range of tasks that would be difficult or impossible with conventional technology. The classic function of the power electronics is rectification, or the conversion of AC-to-DC power. High-powered power electronics can also be used to convert AC power to DC power for long distance transmission.

6.      Protective devices

    Power systems contain protective devices to prevent injury or damage during failures. The quintessential protective device is the fuse. When the current through a fuse exceeds a certain threshold, the fuse elements melts, producing an arc across the resulting gap that is then extinguished, interrupting the circuit. Circuit breakers devices are used as fuse which can reset after they have broken current flow. In higher powered applications, the protective relays are used that detect a fault and initiate a trip that is separate from the circuit breaker.

7.      SCADA systems

      In large electric power systems, Supervisory Control And Data Acquisition is used for tasks such as switching on generators, controlling generator output and switching in or out system elements for maintenance.