## 2 Electromagnetic Interference Characteristics of Converter Station to the Surrounding Environment
### Analysis of Electromagnetic Interference from the Converter Valves  
From this moment on, the term valve will be used also to generally name the full group (matrix) of HV valves, as it is a more compact description.  
The converter valve is the core equipment of the converter station. Its function is to accomplish the conversion between AC and DC. The semiconductor devices widely used in HV valves are mainly thyristors, but recently, the IGBTs made of new semiconductor materials are also used. The converter valve bridge arms consist of thyristors, connected in series and parallel. Each switching element in Figure 1 is a complex set of many thyristors, controlled synchronously to be switched on and off. The valves T1, T2... are connected usually in the form of 6-pulse converters or 12-pulse converters [5]. The classic schematic diagrams are shown in Figure 1.  
The basic principle of the six-pulse converter valve is a typical three-phase full-bridge rectifier circuit. During each period, the trigger circuit sends commands to the six valves, where each switch or valve is a combination of thyristors. In the case of a rectifier, as in Figure 1, a relatively flat and stable DC voltage waveform is obtained after the filter. The flatness of the 12-pulse rectifier is almost perfect and economically it is the preferred configuration for high power converters, although the transformer is more complicated. The number of switching transients per period is also higher in case of 12-pulse converters.  
The thyristor has a cathode, and anode, and a control electrode, a gate, usually commanded by the optical cable, for these HV applications. After the thyristor is turned on, the control becomes impossible. This behavior requires that the thyristor returns to be disconnected by recovering the huge quantity of charges accumulating in it. So, the thyristor needs to suffer from the brutal reverse of the load current, when the voltage of the anode becomes negative and/or the current through the anode is inverted. Since the voltage drop on the thyristor is close to 0 when it is turned on, the terminal voltage will change drastically at the moment of turn-off. In addition, when the thyristor is turned off, the terminal voltage of the converter valve is not only affected by its own high-impedance widescreen characteristics, but also electromagnetic interference from other converter valves, so the terminal voltage fluctuates accordingly. Severe voltage fluctuations produce high-frequency electromagnetic components. During the normal operation of the converter station, the valve, each one a combination of many thyristors, keeps repeating the process of switching on and off, thereby generating a steady stream of high-frequency electromagnetic interference [14].  
Thirteen thyristors are connected in series with two saturable reactors, and then connected in parallel with a voltage equalizing capacitor to form each valve assembly. Two valve components are connected in series to form a valve module. Three valve modules  
Figure 1: Schematic diagrams of converter valve(s), 6 pulse converters and 12 pulse-converters.  
are connected in series to form a converter valve arm. Four converter valves are connected in series to form a valve tower. Three valve towers form a complete converter valve. In the valve hall (Figure 2), the most important component is the converter valve.