IEC TR 62544:2011 pdf – High-voltage direct current (HVDC) systems – Application of active filters.
4.2 SemIconductor devices available for active filters
Three types of power semiconductor devices, suitable for use in an active filter, are available at present:
• metal-oxide-semiconductor field-effect transistor (MOSFET);
• insulated gate bipolar transistor (IGBT):
• gate turn-off thyristor (GTO) and other thyristor-derived devices such as the gate commutated thyristor (GCT) and integrated gate commutated thyristor (IGCT).
The MOSFET is an excellent switching device capable of switching at very high frequencies with relatively low losses, but with limited power handling capability.
The IGBT has a switching frequency capability which, although very good and sufficient to handle the frequencies within the active d.c. filter range, is inferior to the MOSFET. However the IGBT power handling is significantly higher than the MOSFET.
The GTO-type devices has the highest power handling capacity, but with a relatively limited switching speed far below the required frequency range for active d.c. filter. The use of GTO. type devices will probably be limited to handle frequencies below a few hundred of hertz.
The relatively high frequency band for active d.c. tittering excludes the use of thyristors and GTO. Even though the MOSFET and IGBT are suited as switching elements in a power stage. the limited power handling capacity on MOSFET and the installed cost evaluations tend to point on the use of IGBT in future power stages.
5 Active d.c. filters
5.1 HarmonIc disturbances on the d.c. side
The main reason tor specifying demands on the d.c. circuit is to keep disturbances in nearby telephone lines within an acceptable limit, which will vary depending on whether the telephone system consists of overhead lines or underground cables which are generally shielded and therefore have a better immunity [2j, A summary is given below to illustrate the demands wtilch made It feasible to install the active filters. As described, the demand on disturbances can appear as an harmonic current on the d.c. line or as an Induced voltage In a fictive telephone line. It should be kept in mind that the harmonic demand, the specific HVDC system and surroundings (earth resistivity, telephone system. etc.) all together define the d.c. filter solution.
The specified requirements:
• The induced voltage Lfld in a theoretically 1 km telephone line situated I km from the d.c. overhead line shall be below 10 mV I or monopolar operation.
• A one minute mean value of the equivalent psophometrlc current fed into the d.c. pole overhead line shall be below 400 mA.
The mentioned induced voltage and the equivalent psophometric current are defined as:
5.2 DescriptIon of active d.c. titters
5.2.1 General
Active d.c. filters use a controllable converter to introduce currents in the network, presenting a wavetorm which counteracts the harmonics. This subclause describes types of power stages. converters to be used in active fillers and the possible connections in HVDC schemes.
5.2.2 Types of converters available General
Two basic types of switching converters are possible in an active d.c. filter: the current-source converter using inductive energy storage and the votage-sourced converter (VSC) using capacitive energy storage Current source converters
In a current-source converter, the d.c. element Is a Current source, which normally consists of a dc voltage source power supply in series with an inductor. For correct operation, the current should flow continuously in the inductor. Hence if ac. current is not required current must be by-passed within the converter. This fact restricts the switching actions. A simple current- source converter is shown in Figure 4. Voltage sourced converters (VSC)
In the VSC, the d.c. element is a voltage source. This may be a d.c. power supply or, in the case of an active d.c. filter application, an energy storage unit. in practice, the voltage source for an active d.c. tilter power stage Is usually a capacitor with a small power supply to offset the power slage losses. A VSC also has the property that its a.c. output appears as a voltage source.
A Circuit of simple VSC is shown in Figure 5.