Static inverter plant
Encyclopedia
A static inverter station, also known as an HVDC Converter Station, is the terminal equipment for a high-voltage direct-current
transmission line, in which direct current is converted to three-phase alternating current
, and, usually, the reverse. Besides the static inverter
itself, the station will also contain:
(called a reactor) that adds inductance
to help smooth the direct current. The inductance amounts to between 0.1 H and 1 H. The smoothing coil can have either an air-core or an iron-core. Iron-core coils look like oil-filled high voltage transformers. Air-core smoothing coils resemble, but are considerably larger than, carrier frequency choke coils in high voltage transmission lines and are supported by insulator
s. Air coils have the advantage of generating less acoustical noise than iron-core coils, they eliminate the potential environmental hazard of spilled oil, and they do not saturate under transient high current fault
conditions. This part of the plant will also contain instruments for measurement of direct current and voltage.
Special direct current filters are used to eliminate high frequency interference. Such filters are required if the transmission line will use power line carrier
techniques for communication and control, or if the overhead line will run through populated areas. These filters can be passive LC filters or active filters, consisting of an amplifier coupled through transformers and protection condensers, which gives a signal out of phase to the interference signal on the line, thereby cancelling it. Such a system was used on the Baltic Cable
HVDC project.
. Since the mid 1970s solid-state devices such as thyristors are used. Many thyristors are assembled into a module, the thyristor tower. The thyristor towers can stand on insulators on the floor or hang from insulators from the ceiling. Thyristor inverters require voltage from the AC network for commutation, but insulated gate bipolar transistors used in some HVDC designs can provide power to a de-energized AC system.
s step up the voltage of the AC supply network. By using a star-delta (US: wye-delta) connection, the inverter can operate with 12 pulses in each cycle of the AC supply, which eliminates numerous harmonic current components. The insulation of the transformer windings must be specially designed to withstand a large DC potential to earth. Inverter transformers can be built as large as 300 MVA as a single unit. It is impractical to transport larger transformers so when larger ratings are required, several individual transformers are connected together. Either two three-phase units or three single-phase units can be used. With the latter variant only one type of transformer is used, making the supply of a spare transformer more economical.
Inverter transformers operate with high flux Power Steps In the Four Steps of the Inverter per cycle, and so produce more acoustic noise than normal three-phase power transformers. This effect should be considered in the siting of an HVDC static inverter plant. Noise-reducing enclosures may be applied.
s. The demand for reactive power can be reduced if the inverter transformers have on-load tapchangers, with a sufficient range of taps for ac voltage control. Some of the reactive power requirement can be supplied in the harmonic filter components.
Beside the harmonic filters, equipment is also provided to eliminate spurious signals in the frequency range of power-line carrier equipment, in the range of 30 kHz to 500 kHz. These filters are usually near the alternating current terminal of the static inverter transformer. They consist of a series coil which passes the load current, with a parallel capacitor to form a resonant circuit.
In special cases, it may be possible to use exclusively machines for generating the reactive power. This is realized at the terminal of HVDC Volgograd-Donbass
situated on Volga Hydroelectric Station
.
. It will contain circuit breaker
s for overcurrent protection of the converter transformers, isolating switches, grounding switches, and instrument transformers for control, measurement and protection. The station will also have lightning arrester
s for protection of the AC equipment from lightning
surges on the AC system.
High-voltage direct current
A high-voltage, direct current electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses...
transmission line, in which direct current is converted to three-phase alternating current
Three-phase electric power
Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...
, and, usually, the reverse. Besides the static inverter
Inverter (electrical)
An inverter is an electrical device that converts direct current to alternating current ; the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits....
itself, the station will also contain:
- three-phase alternating current switchgear
- transformerTransformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s - capacitorCapacitorA capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s or synchronous condenserSynchronous condenserIn electrical engineering, a synchronous condenser is adevice identical to a synchronous motor, whose shaft is not connected to anything but spins freely. Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid...
s for reactive power - filterElectronic filterElectronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
s for harmonics and - direct current switchgear
Direct current switchgear
The direct current equipment often includes a coilInductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
(called a reactor) that adds inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
to help smooth the direct current. The inductance amounts to between 0.1 H and 1 H. The smoothing coil can have either an air-core or an iron-core. Iron-core coils look like oil-filled high voltage transformers. Air-core smoothing coils resemble, but are considerably larger than, carrier frequency choke coils in high voltage transmission lines and are supported by insulator
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...
s. Air coils have the advantage of generating less acoustical noise than iron-core coils, they eliminate the potential environmental hazard of spilled oil, and they do not saturate under transient high current fault
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
conditions. This part of the plant will also contain instruments for measurement of direct current and voltage.
Special direct current filters are used to eliminate high frequency interference. Such filters are required if the transmission line will use power line carrier
SCADA
SCADA generally refers to industrial control systems : computer systems that monitor and control industrial, infrastructure, or facility-based processes, as described below:...
techniques for communication and control, or if the overhead line will run through populated areas. These filters can be passive LC filters or active filters, consisting of an amplifier coupled through transformers and protection condensers, which gives a signal out of phase to the interference signal on the line, thereby cancelling it. Such a system was used on the Baltic Cable
Baltic Cable
The Baltic Cable is a HVDC power line running beneath the Baltic Sea that interconnects the electric power grids of Germany and Sweden.The Baltic Cable uses a transmission voltage of 450 kV – the highest operating voltage for energy transmission in Germany...
HVDC project.
Inverter
The inverter is usually installed in a building called the valve hallValve hall
A valve hall is a building which contains the valves of the static inverters of a HVDC plant. The valves consist of thyristors, or at older plants, mercury arc rectifiers. Mercury arc rectifiers are usually supported by insulators mounted on the floor, while thyristor valves may be either...
. Since the mid 1970s solid-state devices such as thyristors are used. Many thyristors are assembled into a module, the thyristor tower. The thyristor towers can stand on insulators on the floor or hang from insulators from the ceiling. Thyristor inverters require voltage from the AC network for commutation, but insulated gate bipolar transistors used in some HVDC designs can provide power to a de-energized AC system.
Inverter transformer
The inverter transformerTransformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s step up the voltage of the AC supply network. By using a star-delta (US: wye-delta) connection, the inverter can operate with 12 pulses in each cycle of the AC supply, which eliminates numerous harmonic current components. The insulation of the transformer windings must be specially designed to withstand a large DC potential to earth. Inverter transformers can be built as large as 300 MVA as a single unit. It is impractical to transport larger transformers so when larger ratings are required, several individual transformers are connected together. Either two three-phase units or three single-phase units can be used. With the latter variant only one type of transformer is used, making the supply of a spare transformer more economical.
Inverter transformers operate with high flux Power Steps In the Four Steps of the Inverter per cycle, and so produce more acoustic noise than normal three-phase power transformers. This effect should be considered in the siting of an HVDC static inverter plant. Noise-reducing enclosures may be applied.
Reactive Power
A static inverter will require between 40 and 60% of its megawatt rating as reactive power. This can be provided by banks of switched capacitors or by synchronous condenserSynchronous condenser
In electrical engineering, a synchronous condenser is adevice identical to a synchronous motor, whose shaft is not connected to anything but spins freely. Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid...
s. The demand for reactive power can be reduced if the inverter transformers have on-load tapchangers, with a sufficient range of taps for ac voltage control. Some of the reactive power requirement can be supplied in the harmonic filter components.
Filter for harmonics
Filter for harmonics are necessary for the elimination of the harmonic waves and for the production of the necessary reactive power. At plants with six pulse inverters complex harmonic filters are necessary, because there are odd-number harmonics from the 5th order upwards. At 12 pulse static inverter plants, only harmonic waves of the order 12*n + 1 and 12*n - 1 (n = natural number) result. Filters are tuned to the expected harmonic frequencies and consist of series combinations of capacitors and inductors.Beside the harmonic filters, equipment is also provided to eliminate spurious signals in the frequency range of power-line carrier equipment, in the range of 30 kHz to 500 kHz. These filters are usually near the alternating current terminal of the static inverter transformer. They consist of a series coil which passes the load current, with a parallel capacitor to form a resonant circuit.
In special cases, it may be possible to use exclusively machines for generating the reactive power. This is realized at the terminal of HVDC Volgograd-Donbass
HVDC Volgograd-Donbass
The HVDC Volgograd–Donbass is a long bipolar 400 kV high voltage direct current powerline used for transmitting electric power from Volga Hydroelectric Station at Volgograd to Donbas and vice versa...
situated on Volga Hydroelectric Station
Volga Hydroelectric Station
The Volga Hydroelectric Station or Volga GES also known as the 22nd Congress of the CPSU Stalingrad/Volgograd Hydroelectric Power Station , is the largest hydroelectric station in Europe and is the last of the Volga-Kama Cascade of dams, before the Volga River flows into the Caspian Sea...
.
AC switchgear
The three-phase alternating current switchgear of a static inverter station is similar to that of an AC substationElectrical substation
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions...
. It will contain circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
s for overcurrent protection of the converter transformers, isolating switches, grounding switches, and instrument transformers for control, measurement and protection. The station will also have lightning arrester
Lightning arrester
A lightning arrester is a device used on electrical power systems to protect the insulation on the system from the damaging effect of lightning. Metal oxide varistors have been used for power system protection since the mid 1970s. The typical lightning arrester also known as surge arrester has...
s for protection of the AC equipment from lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
surges on the AC system.