Unit 9: An electrical Substation I READING AND COMPREHENSION What is an electrical substation An electrical substation is a subsidiary station of an electricity generation, transmission and distribution system where voltage is transformed from high to low or the reverse using transformers Explanation Transformation may take place in several stages in sequence, starting at the generating plant where the voltage is increased for transmission purposes and is then progressively reduced to the voltage required for household or industrial use The range of voltages in a power system varies from 110 V up to 765 kV depending on the country A substation that has a step-up transformer increases the voltage while decreasing the current, while a step-down transformer decreases the voltage while increasing the current for domestic and commercial distribution The word substation comes from the days before the distribution system became a grid The first substations were connected to only one power station where the generator was housed, and were subsidiaries of that power station Substations generally contain one or more transformers, and have switching, protection and control equipment In a large substation, circuit breakers are used to interrupt any short-circuits or overload currents that may occur on the network Smaller distribution stations may use recloser circuit breakers or fuses for protection of branch circuits Substations not (usually) have generators, although a power plant may have a substation nearby A typical substation will contain line termination structures, highvoltage switchgear, one or more power transformers, low voltage switchgear, surge protection, controls, and metering Other devices such as power factor correction capacitors and voltage regulators may also be located at a substation Substations may be on the surface in fenced enclosures, underground, or located in special-purpose buildings High-rise buildings may have indoor substations Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions 67 Where a substation has a fence, it must be properly grounded to protect people from high voltages that may occur during a fault in the transmission system Earth faults at a substation can cause Earth Potential Rise at the fault location Transmission substation A transmission substation is one whose main purpose is to connect together various transmission lines The simplest case is where all transmission lines have the same voltage In such cases, the substation contains high-voltage switches that allow lines to be connected together or isolated for maintenance Transmission substations can range from simple to complex The largest transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, and a large amount of protection and control equipment (capacitors, relays, switches, breakers, voltage and current transformers) Distribution substation Underground distribution system: Upgraded predominantly by CAB or C.C.Box methods Underground distribution system: C.C.Box method is positively adopted (C.C.Box method)〉 (CAB method〉 70cm 150cm 50 cm 150 cm A distribution substation is one whose main purpose is to transfer power from the transmission system to the distribution system of some area It is uneconomical to directly connect electricity consumers to the main transmission network (unless they use large amounts of energy); so the distribution station reduces voltage to a value suitable for connection to local loads The input for a distribution substation is typically at least two transmission or subtransmission lines Input voltage may be, for example, 115 kV, or whatever is common in the area The output is a number of feeders Distribution voltages are typically medium voltage, between 2.4 and 33 kV depending on the size of the area served and the practices of the local utility 68 The feeders will then run overhead, along streets (or under streets, in a city) and eventually power the distribution transformers at or near the customer premises Besides transforming the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching, and switching and backup systems on the low-voltage side More typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side Task Checking facts and ideas Transformation may happen in several stages in sequence, starting at the generating plant where the voltage is increased for transmission purposes and is then progressively reduced to the voltage required for household or industrial use The range of voltages in a power system range from 110 V up to 765 kV depending on the country A substation that has a step-up transformer increases the voltage while decreasing the current, while a step-down transformer reduces the voltage while increasing the current for domestic and commercial distribution The first substations were connected to only one power station where the generator was located, and were subsidiaries of that power station In a large substation, circuit breakers are used to pause any short-circuits or overload currents that may occur on the network Other devices such as power factor correction capacitors and voltage regulators may also be placed at a substation Where a substation has a fence, it must be properly earthed to protect people from high voltages that may occur during a fault in the transmission system Task Checking facts and ideas Substations always increases the voltage while decreasing the current for domestic and commercial distribution (F) The minimum level of voltage in a power system is 110 kV and the maximum one is 765 kV depending on the country (T) Circuit breakers are used to interrupt any short-circuits or overload currents that may occur on the network in every substation (F) 69 Indoor substations are hardly built in rural areas, but urban ones to decrease the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions (T) Recloser circuit breakers or fuses for protection of branch circuits are dedicatedly used for smaller scale distribution substations Transmission Substations There is a wide range of transmission substations from simple to complicated one The big transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, and a large amount of protection and control equipment (capacitors, relays, switches, breakers, voltage and current transformers Distribution Substations It is not cost-effective to indirectly connect electricity consumers to the main transmission network (unless they use large amounts of energy); so the distribution station reduces voltage to a value suitable for connection to local loads (F) The functions of the distribution substation is not only to isolate faults in either the transmission or distribution systems but also to transform the voltage.(T) 10 Complex distribution substations are usually built in the urban areas, with highvoltage switching, and switching and backup systems on the low-voltage side (T) II GUIDED WRITING Sentence Building If The frequency of the power system rises about 0.5 [Hz] and more or falls about 1.5 [Hz] and more The frequency of the power system rises about 0.5 [Hz] and more or falls about 1.5 [Hz] and more In order to Secure stable power supply in the power system New installation, extension, improvement, repair and inspection of power facilities are indispensable, and stop of facilities become necessary in the process of these construction works So that As a result of system supervision, control of power flow is carried out by switching power system or by controlling power generation The power system can be operated under the condition of standard value for safety operation, if necessary 70 In order to In the case of loop system, in addition to switching of power system and control of power generation, series capacitor and phase shifting transformer may be used Change the power flow distribution So that In the case of abnormal weather or thunderstorm, switching of power system and control of power generation are carried out to control the power flow supplied for the region Influence may become as small as possible even when fault occurs When Transmission line is interrupted by a fault Recovery operation is performed aiming at power supply as early as possible on the assumption that the fault has disappeared When Wide-area outage occurs Comprehensive recovery control of the whole system is needed Since Reactance of the circuit is larger than resistance in general Reactive power affects greatly voltage fluctuation rather than active power Which The system processes load dispatching work automatically The system is called automatic load dispatching system 10 Since Large-scale information transmission system and computers are needed The application of this system is limited to extra-high voltage system or above in some power companies 71 ... the substation contains high-voltage switches that allow lines to be connected together or isolated for maintenance Transmission substations can range from simple to complex The largest transmission... fuses for protection of branch circuits are dedicatedly used for smaller scale distribution substations Transmission Substations There is a wide range of transmission substations from simple to... fault location Transmission substation A transmission substation is one whose main purpose is to connect together various transmission lines The simplest case is where all transmission lines