Static inverter plant

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:

• three-phase alternating current switchgear
• transformers
• capacitors or synchronous condensers for reactive power
• filters for harmonics and
• direct current switchgear

Components

Direct current switchgear

The direct current equipment often includes a coil (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 insulators. 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.

Inverter

Main article: Inverter (electrical)

The inverter is usually installed in a building called the valve hall. 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 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 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 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.

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 condensers. 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 situated on Volga Hydroelectric Station.

AC switchgear

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.

Others

Required area

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). Lower-voltage plants may require somewhat less ground area, since less air space clearance would be required around outdoor high-voltage equipment.

Location factors

Static inverter stations produce acoustic noise and radio-frequency interference. Walls may provide noise protection. As with all AC substations, oil from equipment must be prevented from contaminating ground water in case of a spill. Substantial area may be required for overhead transmission lines, but can be reduced if underground cable is used.

See also

• List of HVDC projects
• Rotary converter plant