There are a number of designs used for substations. However, there are elements common to all:
>> A BUS: is the physical structure to which all lines and transformers are connected. Buses are of two generic types: open air and enclosed. Enclosed buses are used when substations are located in buildings or outdoors where space is at a premium. They involve the use of an insulating gas such as sulfur hexafluoride (SF6) to allow reduced spacing between energized phases. Bus structures are designed to withstand the large mechanical forces that can result from fields produced by high short-circuit currents. These forces vary with the third power of the current. A bus section is the part of a bus to which a single line or transformer is connected.
>> PROTECTIVE RELAYS: are devices that continuously monitor the voltages and currents associated with the line and its terminals to detect failures or malfunctions in the line or equipment. Such failures are called faults and involve contact between phases or between one or more phases and ground. The relays actuate circuit breakers.
>> CIRCUIT BREAKERS: are devices that are capable of interrupting the flow of electricity to isolate either a line or a transformer. They do so by opening the circuit and extinguishing the arc that forms, using a variety of technologies such as oil, vacuum, air blast, or sulfur hexafluoride (SF6). Breakers may be in series with the line or transformer or may be installed on both sides of the bus section where the line connects. They allow individual lines or transformers to be removed from service (de-energized) automatically when equipment (protective relays) detects operating conditions outside a safe range. They must be capable of interrupting the very high currents that occur during fault conditions and are rated by the amount of current they can interrupt. These fault current levels can be 20 or 30 times larger than the current flow under normal operating conditions, that is, thousands of amperes.
To minimize the impact of electrical shocks to the transmission system, minimizing the total time for the relay to detect the condition and the circuit breaker to open the circuit is a critical design issue. Circuit breakers also allow lines or transformers to be removed from service for maintenance. Circuit breakers normally interrupt all three phases simultaneously, although in certain special applications, single-phase circuit breakers can be employed that will open only the phase with a problem.
>> TRANSFORMERS: are devices that are used to connect facilities operating at two different voltage levels. The transformer connects to all three phases of the bus. Physically, the transformers can include all three phases within one tank or there can be three separate tanks, one per phase. Larger capacity units may have three separate tanks because their size and weight may be a limiting factor because of transportation issues.
Transformers can be designed with two mechanisms to adjust the voltage ratio. One mechanism is the provision of more than one fixed tap position on one side of the transformer. For example, a transformer might have a nominal turns ratio of 345/138, with fixed taps on the 345 kV winding of 327.8, 336.7, 345, 353.6, and 362.3. The transformer must be de-energized to adjust the fixed tap ratio. Another mechanism is called tap changing under load (TCUL). In this mechanism, the ratio can be adjusted while the transformer is energized, providing greater operating flexibility. Some transformers have both types of mechanisms, with a fixed tap adjustment in the high voltage winding and the TCUL adjustment in the low voltage winding.
Another type transformer is an autotransformer, which is used when facilities at nearly the same voltage are to be connected, such as 138 kV to 115 kV. Rather than having two separate paths for the electricity, connected only by the magnetic flux through the transformer as in a conventional unit, the winding of autotransformer involves a tap on the higher voltage winding which supplies the lower voltage.
All larger transformers have mechanisms to remove the heat generated within the tank, involving some manner of circulating the transformer insulating/cooling oil through an external heat exchanger with fins mounted on the side of the transformer and fans to circulate air across the fins to maximize heat dissipation.
>> DISCONNECT SWITCHES: are used to open a circuit when only charging current is present. These would be used primarily to connect or disconnect circuit breakers or transformers which are not carrying load current. They are also used in conjunction with circuit breakers to provide another level of safety for workers by inserting a second opening between station equipment out of service for work and the still energized section of line or bus.
>> LIGHTNING ARRESTERS: are used to protect transformers and switchgear from the effects of high voltage due to lightning strikes or switching operations. They are designed to flashover when the voltage at the transformer exceeds a preselected level that is chosen by the station design engineers to coordinate with the basic insulation level of the transformer (BIL).
>> METERING EQUIPMENT: is provided to measure line and transformer loadings and bus voltages so operating personnel can ensure that these facilities are within acceptable limits. Metering equipment also is provided at some locations to measure the flow of energy for the billing that is required for sales and purchases of energy between various participants in the electric energy market.
>> SCADA: is an acronym for system control and data acquisition. It covers the measurement, telecommunications, and computing technologies that allow more and more automation of substation operations.
Depending on the electrical characteristics of a particular part of the transmission system, other types of equipment that may be located at a substation are:
>> SHUNT REACTORS: (reactors connected from the energized bus to ground) are installed to control high voltages that occur especially at night due to the capacitive effect of lightly loaded transmission lines. These reactors can be energized always or they can be energized only at specific times. Shunt reactors are also used to reduce or control the high voltages that can occur when a sudden loss of a block of customer load occurs. The windings, insulation, and the external tank are similar to those used for transformers.
>> SERIES REACTORS: are installed in a transmission line to increase the impedance of the line, to decrease current levels in the event of short circuits, or to reduce its loading under various operating conditions.
>> SHUNT CAPACITORS: are installed to provide mVArs to the system to help support voltage levels.
>> SERIES CAPACITORS: are installed to reduce the effective impedance of a transmission line. These would be installed in very long transmission lines to effectively reduce the electrical angle between the sending and the receiving parts of the system, enabling more power to flow over the line and increasing stability limits.
>> PHASE ANGLE REGULATING TRANSFORMERS: are installed to control power flow through a transmission line, causing more or less power to flow over desired lines. They use a variant on the design of a normal transformer, in which, due to the specialized way they are wound, they electrically inject an angular phase shift into the line. The angle can be made to either increase or decrease power flow on the line. Since they are expensive, they are usually used only on cable systems where, because of the cost and limited capacity of cables, maximum utilization of all parallel cable capacity is essential. In recent years, many of them are being installed in transmission lines to control parallel path flow, when power flows over paths in other systems not involved in transactions or which do not have adequate capacity.
>> FACTS (FLEXIBLE AC TRANSMISSION SYSTEMS): is a generic name used for a variety of devices intended to dynamically control voltage, impedance, or phase angle of HVAC lines.
These devices mirror and extend the benefits of the fixed series and shunt inductors and capacitors previously discussed in that the FACTS devices allow rapid and precise adjustments. Depending on the device, they provide a number of benefits: increased power transfer capability, rapid voltage control, improved system stability, and mitigation of sub-synchronous resonance. There are many devices made by many manufacturers, some of which are in the development stage and a few of which are in service. The names of the devices vary somewhat, depending on the manufacturer. The following lists some of the devices:
>> STATIC VAR COMPENSATORS (SVCS): These devices employ fixed banks of capacitors, controlled with thyristors, which can switch them on and off rapidly. In many instances, there are also thyristor-switched inductors to prevent system resonance.
>> THYRISTOR CONTROLLED SERIES COMPENSATORS (OR SERIES CAPACITORS) (TCSCS): A thyristor controlled reactor is placed in parallel with a series capacitor, allowing for a continuous and rapidly variable series compensation system.
>> STATIC COMPENSATORS (STATCOMS): These are gate turn-off type thyristor (GTO)-based SVCs. They are solid-state synchronous voltage generators that consist of a multi-pulsed, voltage-sourced inverter connected in shunt with a transmission line. They do not require capacitor banks and shunt reactors but rely on electronic processing of voltage and current waveforms to provide inductive or capacitive reactive power. They have the added advantage of output that is not seriously impacted by low system voltage.
>> UNIFIED POWER FLOW CONTROLLER (UPFC): This device has a shunt-connected STATCOM with an additional series branch in the transmission line supplied by the STATCOM’s DC circuit. The device is comparable to a phase shifting transformer. It can control all three basic power transfer parameters: voltage, impedance, and phase angle.
>> SVC LIGHT STATCOM: This is based on voltage source converter technology equipped with insulated gate bipolar transistors (IGBTs), which are power switching components. It provides reactive power as well as absorption purely by means of electronic processing of voltage and current waveforms.
>> PROTECTIVE RELAYS: are devices that continuously monitor the voltages and currents associated with the line and its terminals to detect failures or malfunctions in the line or equipment. Such failures are called faults and involve contact between phases or between one or more phases and ground. The relays actuate circuit breakers.
>> CIRCUIT BREAKERS: are devices that are capable of interrupting the flow of electricity to isolate either a line or a transformer. They do so by opening the circuit and extinguishing the arc that forms, using a variety of technologies such as oil, vacuum, air blast, or sulfur hexafluoride (SF6). Breakers may be in series with the line or transformer or may be installed on both sides of the bus section where the line connects. They allow individual lines or transformers to be removed from service (de-energized) automatically when equipment (protective relays) detects operating conditions outside a safe range. They must be capable of interrupting the very high currents that occur during fault conditions and are rated by the amount of current they can interrupt. These fault current levels can be 20 or 30 times larger than the current flow under normal operating conditions, that is, thousands of amperes.
To minimize the impact of electrical shocks to the transmission system, minimizing the total time for the relay to detect the condition and the circuit breaker to open the circuit is a critical design issue. Circuit breakers also allow lines or transformers to be removed from service for maintenance. Circuit breakers normally interrupt all three phases simultaneously, although in certain special applications, single-phase circuit breakers can be employed that will open only the phase with a problem.
>> TRANSFORMERS: are devices that are used to connect facilities operating at two different voltage levels. The transformer connects to all three phases of the bus. Physically, the transformers can include all three phases within one tank or there can be three separate tanks, one per phase. Larger capacity units may have three separate tanks because their size and weight may be a limiting factor because of transportation issues.
Transformers can be designed with two mechanisms to adjust the voltage ratio. One mechanism is the provision of more than one fixed tap position on one side of the transformer. For example, a transformer might have a nominal turns ratio of 345/138, with fixed taps on the 345 kV winding of 327.8, 336.7, 345, 353.6, and 362.3. The transformer must be de-energized to adjust the fixed tap ratio. Another mechanism is called tap changing under load (TCUL). In this mechanism, the ratio can be adjusted while the transformer is energized, providing greater operating flexibility. Some transformers have both types of mechanisms, with a fixed tap adjustment in the high voltage winding and the TCUL adjustment in the low voltage winding.
Another type transformer is an autotransformer, which is used when facilities at nearly the same voltage are to be connected, such as 138 kV to 115 kV. Rather than having two separate paths for the electricity, connected only by the magnetic flux through the transformer as in a conventional unit, the winding of autotransformer involves a tap on the higher voltage winding which supplies the lower voltage.
All larger transformers have mechanisms to remove the heat generated within the tank, involving some manner of circulating the transformer insulating/cooling oil through an external heat exchanger with fins mounted on the side of the transformer and fans to circulate air across the fins to maximize heat dissipation.
>> DISCONNECT SWITCHES: are used to open a circuit when only charging current is present. These would be used primarily to connect or disconnect circuit breakers or transformers which are not carrying load current. They are also used in conjunction with circuit breakers to provide another level of safety for workers by inserting a second opening between station equipment out of service for work and the still energized section of line or bus.
>> LIGHTNING ARRESTERS: are used to protect transformers and switchgear from the effects of high voltage due to lightning strikes or switching operations. They are designed to flashover when the voltage at the transformer exceeds a preselected level that is chosen by the station design engineers to coordinate with the basic insulation level of the transformer (BIL).
>> METERING EQUIPMENT: is provided to measure line and transformer loadings and bus voltages so operating personnel can ensure that these facilities are within acceptable limits. Metering equipment also is provided at some locations to measure the flow of energy for the billing that is required for sales and purchases of energy between various participants in the electric energy market.
>> SCADA: is an acronym for system control and data acquisition. It covers the measurement, telecommunications, and computing technologies that allow more and more automation of substation operations.
Depending on the electrical characteristics of a particular part of the transmission system, other types of equipment that may be located at a substation are:
>> SHUNT REACTORS: (reactors connected from the energized bus to ground) are installed to control high voltages that occur especially at night due to the capacitive effect of lightly loaded transmission lines. These reactors can be energized always or they can be energized only at specific times. Shunt reactors are also used to reduce or control the high voltages that can occur when a sudden loss of a block of customer load occurs. The windings, insulation, and the external tank are similar to those used for transformers.
>> SERIES REACTORS: are installed in a transmission line to increase the impedance of the line, to decrease current levels in the event of short circuits, or to reduce its loading under various operating conditions.
>> SHUNT CAPACITORS: are installed to provide mVArs to the system to help support voltage levels.
>> SERIES CAPACITORS: are installed to reduce the effective impedance of a transmission line. These would be installed in very long transmission lines to effectively reduce the electrical angle between the sending and the receiving parts of the system, enabling more power to flow over the line and increasing stability limits.
>> PHASE ANGLE REGULATING TRANSFORMERS: are installed to control power flow through a transmission line, causing more or less power to flow over desired lines. They use a variant on the design of a normal transformer, in which, due to the specialized way they are wound, they electrically inject an angular phase shift into the line. The angle can be made to either increase or decrease power flow on the line. Since they are expensive, they are usually used only on cable systems where, because of the cost and limited capacity of cables, maximum utilization of all parallel cable capacity is essential. In recent years, many of them are being installed in transmission lines to control parallel path flow, when power flows over paths in other systems not involved in transactions or which do not have adequate capacity.
>> FACTS (FLEXIBLE AC TRANSMISSION SYSTEMS): is a generic name used for a variety of devices intended to dynamically control voltage, impedance, or phase angle of HVAC lines.
These devices mirror and extend the benefits of the fixed series and shunt inductors and capacitors previously discussed in that the FACTS devices allow rapid and precise adjustments. Depending on the device, they provide a number of benefits: increased power transfer capability, rapid voltage control, improved system stability, and mitigation of sub-synchronous resonance. There are many devices made by many manufacturers, some of which are in the development stage and a few of which are in service. The names of the devices vary somewhat, depending on the manufacturer. The following lists some of the devices:
>> STATIC VAR COMPENSATORS (SVCS): These devices employ fixed banks of capacitors, controlled with thyristors, which can switch them on and off rapidly. In many instances, there are also thyristor-switched inductors to prevent system resonance.
>> THYRISTOR CONTROLLED SERIES COMPENSATORS (OR SERIES CAPACITORS) (TCSCS): A thyristor controlled reactor is placed in parallel with a series capacitor, allowing for a continuous and rapidly variable series compensation system.
>> STATIC COMPENSATORS (STATCOMS): These are gate turn-off type thyristor (GTO)-based SVCs. They are solid-state synchronous voltage generators that consist of a multi-pulsed, voltage-sourced inverter connected in shunt with a transmission line. They do not require capacitor banks and shunt reactors but rely on electronic processing of voltage and current waveforms to provide inductive or capacitive reactive power. They have the added advantage of output that is not seriously impacted by low system voltage.
>> UNIFIED POWER FLOW CONTROLLER (UPFC): This device has a shunt-connected STATCOM with an additional series branch in the transmission line supplied by the STATCOM’s DC circuit. The device is comparable to a phase shifting transformer. It can control all three basic power transfer parameters: voltage, impedance, and phase angle.
>> SVC LIGHT STATCOM: This is based on voltage source converter technology equipped with insulated gate bipolar transistors (IGBTs), which are power switching components. It provides reactive power as well as absorption purely by means of electronic processing of voltage and current waveforms.
Comments