numbered harmonics of the orders and produced on the AC side and even harmonics of order on the DC side. At 12 pulse converter stations, only harmonic voltages or currents of the order and (on the AC side) or (on the DC side) result. Filters are tuned to the expected harmonic frequencies and consist of series combinations of capacitors and inductors. Voltage sourced converters generally produce lower intensity harmonics than line commutated converters. As a result, harmonic filters are generally smaller or may be omitted altogether. 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 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 switch gear of a converter 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 converter station is much larger than a conventional transformer, for example a site 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 Converter stations produce acoustic noise. Converter stations can generate serious levels of radio-frequency interference, so include design features to control these emissions. 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 High-voltage transformer fire barriers References High-voltage direct current Electric power conversion