Principle and Application of Commutation Converter in Power Network

High voltage direct current (HVDC) transmission is one of the solutions to reduce transmission and distribution losses. Why is HVDC more efficient than conventional AC transmission? The loss of HVDC transmission line is 30-50% less than that of AC line with the same voltage. When the voltage and current become out of phase, HVDC can improve the power factor. Because DC has no frequency associated with it, it is not affected by skin effect and can reduce the total power transmitted through the line. When the current density is concentrated on the surface or "skin position", the skin effect will occur, and it will gradually become sparse when it moves towards the center of the conductor. The higher the current density along the surface, the higher the effective resistance of AC. HVDC also improves the reliability of the network. Some types of HVDC stations can help stabilize asynchronous networks.

So how can such a large amount of electricity be transmitted to your home from all over the country? Power is first transmitted from the source to the converter station, where AC is upgraded to the required voltage before rectification to DC voltage. Power can then be transferred as HVDC over a long distance to another converter station, where it is re converted to ac. some types of converter stations have the added benefit of controlling active and reactive power. The transformer then raises the AC power to the required voltage to transmit and distribute the voltage to the home and / or factory as needed. Figure 1 shows the complete process.

Figure 1: transmission process (picture of transmission line provided by Duke)

The most common converter station types are grid commutation converter (LCC) and voltage source converter (VSC).

Grid commutation converter

Most HVDC systems currently in operation use LCC topology. LCC is slightly more efficient than VSC and can transmit a larger amount of power. Its typical voltage level is 450kv or 500kV; However, China has several 800kV lines. Due to the use of pulse width modulation (PWM) technology, LCC will not have switching loss like VSC. LCC uses thyristors as switching devices. Multiple thyristors are connected in series into a single line of three-phase rectifier, which constitutes the so-called "valve".

Since the thyristor can only be turned on and cannot be turned off, the AC voltage will reverse bias the thyristor and stop conduction. Therefore, the bias of the thyristor in the LCC depends on the power used for commutation on the AC side of the grid. The delay of conduction after thyristor forward bias determines the phase angle delay (trigger angle). The phase angle delay of thyristor realizes the phase angle control of AC wave.

LCC has two typical architectures: 6-pulse bridge and 12 pulse bridge. Figure 2 shows a 6-pulse bridge using six thyristor valves: two valves for each phase to conduct positive and negative voltage waveforms. The harmonic response capability of LCC is very poor. In order to make up for this, the harmonic can be improved by connecting two 6-pulse bridges in series to form a 12 pulse bridge.

Figure 2: LCC configuration (picture provided by EE WEB)

By analyzing the signal, the waveform in and out of the converter can be controlled. Properly analyzing the signal can let the system know the voltage and current levels and power factor, and help to determine whether there is any fault on the line. The protection relay or intelligent electronic device (IED) analyzes the signal. See Figure 3.

Figure 3: signal interpretation

Ti has several design guidelines that introduce signal analysis methods. The reference design of using delta sigma chip diagnosis to measure AC voltage and current in protection relay discusses how to collect output signal by using current transformer, voltage divider or Rogowski coil. The signal is then adjusted by isolated and non isolated operational amplifiers to increase amplitude and suppress any common mode voltage and noise. The adjusted signal is then analyzed by the ADC. The digitized information obtained from ADC is transmitted to MCU for interpretation. The information determined according to the waveform is fed back to the control device of the converter, so that the changing phase and voltage level will be adjusted to maintain stability.