Science Fair Project Encyclopedia
Static inverter plant
A static inverter 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:
- three-phase alternating current switchgear
- capacitors or synchronous condensers for reactive power
- filters for harmonics and
- direct current switchgear
Direct current switchgear
The direct current equipment often includes a coil for smoothing of the direct current. The inductance amounts to between 0,1 H and 1 H. The smoothing coil can be implemented as air coil or as coil with iron core. In latter case it looks similar to an oil-filled high voltage transformer. Air-core smoothing coils resemble carrier frequency choke coils in high voltage transmission lines, but are substantially larger. Air-core coils are usually supported by insulators. Air coils have the advantage of less acoustical noise than iron-core coils, and also eliminate the environmental hazard due to spilled oil. This part of the plant will also contain instruments for measurement of direct current and voltage.
Special direct current filters are used for the elimination of high frequency interference. Such filters are required if the transmission line will use power line carrier techniques for communication, 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 over transformers and protection condensers, which gives a signal out of phase to the interference signal on the line and thus cancelling it. Such a system was used on the HVDC "Baltic Cable".
The inverter is usually installed in a building called the valve hall. Since the mid 1970's solid-state devices such as thyristors are used. While mercury arc rectifiers are individually connected, with thyristor static inverters 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.
The inverter transformers 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 currents components. The insulation of the transformer windings must be specially designed to withstand a large DC potential to earth. Inverter transformers can 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 densities, 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.
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 condensers. The demand for reactive power can be reduced if the inverter transformers have on-load tapchangers, with a sufficent 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 in direct proximity of the three-phase 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.
The three-phase alternating current switchgear of a static inverter station is similar to that of an ac substation. It will contain circuit breakers 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 arresters for protection of the AC equipment from lightning surges on the AC system.
The area required for a static inverter plant with a transmission rating of 600 megawatts and a transmission voltage of 400 kV is approximately 300 x 300 metres (1000 x 1000 feet).
Static inverter station produces acoustic noise and radio-frequency interference signals. Walls may be built for noise protection. As with at ac substations, oil from electrical equipment must be prevented from contaminating ground water in case of a spill. Substantial land area may be required for transmission line entry and exit if overhead conductors are used, but this space can be reduced if connections are made by cable.
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