Design and implementation of a DC active filter for high voltage DC transmission

HVDC (high voltage direct current) is a technology developed by ABB more than 50 years ago to improve the efficiency of long-distance power transmission. Power generated by power plants is alternating current (AC); most transmission lines - whether high-voltage, medium-voltage or low-voltage distribution networks - transmit AC that oscillates at 50 or 60 cycles per second; and AC is also what reaches the end user, that is, homes, factories and offices. DC does not produce oscillations, so DC transmission has less energy loss. In a DC transmission system, AC power is converted to DC power at a converter station and then transmitted to the receiving point via overhead cables. At the receiving point, another converter station converts DC power to AC power and connects it to the AC grid. ABB built the world's first HVDC transmission line in 1954 and has built more than half of the world's HVDC projects. In 1997, ABB built the first HVDC Light (light high voltage direct current) transmission line. This technology generally uses underground or underwater lines to transmit power, and its emergence has opened up new possibilities for improving the power supply quality of AC grids. With the development of power electronics and computer technology, active filtering technology has been gradually applied to the filtering of the DC side of HVDC transmission systems. Compared with the passive filter on the DC side, the DC active filter (ADF) has many advantages, mainly:

(1) In the audio range (300Hz~3000Hz), the DC active filter has a strong suppression effect on each harmonic;

(2) The smaller the equivalent harmonic interference current required to be suppressed, the higher the performance-price ratio of the ADF;

(3) When the frequency changes or the resonance is out of tune, the ADF can also accurately track and compensate without any overload problem;

(4) Small footprint.

At present, active and passive hybrid DC active filters are mostly used in projects. This can reduce the insulation level and capacity of ADF, give full play to the advantages of passive filters in absorbing high-power harmonic currents and the high efficiency of active filters in suppressing changing frequencies and multiple harmonic currents, and achieve the best filtering effect and economic benefits.

The filtering effect of a DC active filter depends on whether it can accurately track harmonic currents and generate compensation currents in real time. This requires that the DC active filter not only has good detection accuracy, but also requires it to have a fast dynamic response speed. Reference [4] studied the tracking PWM control method. The simulation and experimental results show that the tracking effect of the hysteresis instantaneous value comparison method is the best. This paper adopts the hysteresis instantaneous value PWM comparison method in the DC active filter, and conducts experimental and simulation research on the tracking and compensation effects of this control method.

1 System structure of DC active filter

The harmonic voltage and harmonic current on the DC side of the high-voltage direct current transmission system are mainly 12th, 24th, 36th, etc. There are two ways to install active filters in the system: series or parallel. The DC filter in this article is shown in Figure 1. It is a hybrid filter system composed of a 12/24th double-tuned passive filter and an active filter in series, which are connected in parallel between the DC buses. In the system, the active filter is connected in series to the bottom of the passive filter, and the other end is connected to the DC zero line to reduce the insulation level of the active filter. The role of the active filter here is to reduce the impedance of the double-tuned filter at the resonance point and improve its filtering effect. In addition, it filters out other harmonics that cannot be filtered out by the passive filter (such as the 18th harmonic and some higher harmonics that interfere seriously with the communication line). The content of these harmonic components is relatively small.

2 Working principle of DC active filter

The DC active power filter (DCAPF) and the AC active power filter, which is what we generally call the active power filter (APF), both use active rather than passive methods or means to absorb or eliminate harmonic (ripple) waves. Therefore, the working principles of the DC active power filter and the AC active power filter are the same or similar. However, due to the different objects of action, the DC active power filter also has its own characteristics. Similar to the AC active power filter, according to its connection method with the DC load, the DC active power filter can also be divided into a series DC active power filter and a parallel DC active power filter, as shown in Figure 1 and Figure 2 respectively. The working principle of the series DC active power filter is: detect the output voltage of the rectifier after the smoothing reactor (passive filter), as shown in Figure 1, separate the ripple voltage through a low-pass filter, use this signal to control the output voltage of the DC active power filter, and make it equal to the magnitude and opposite in phase, so as to achieve the purpose of significantly reducing the ripple current in the DC load. In theory, it is completely possible to make the DC load free of ripple voltage. Here, the DC active power filter is equivalent to the inverter of the voltage controlled voltage source (VCVS). Strictly speaking, when a series DC active power filter is used, it is not necessary to connect a series smoothing reactor.

Figure 2 shows the equivalent circuit diagram of the DC side of the HVDC transmission system. In the figure, is the harmonic voltage source in the system, ZS is the internal impedance of the harmonic source, ZL is the DC transmission line impedance, Zf is the impedance of the passive filter (including the leakage reactance of the coupling transformer), is the harmonic voltage source injected into the system by the active filter, and this voltage source is related to the harmonic current on the DC bus.

As can be seen from Figure 2, the harmonic current on the line consists of two parts: the harmonic current generated by and the harmonic current generated by. According to the superposition principle,

When in effect, the harmonic current on the line is:

=

=·(1)

When in effect, the harmonic current on the line is:

=

=·(2)

Therefore, the total harmonic current on the line is:

=+=(3)

By detecting the current on the line, the signal for PWM control of the inverter of the active filter is output, and then the equivalent voltage source required for compensating the harmonic current on the line and the active filter injection system is obtained.

=-k(4)From equations (3) and (4), we can get:

=·(5) It can be seen from formula (5) that when k=0, the active filter does not work, and the harmonic current on the line depends on the filtering effect of the passive filter; when k>0, the active filter and the passive filter form a hybrid filter to jointly suppress the harmonic current on the line. It can also be seen that when k is large enough, the harmonic current on the line can be completely filtered out. In fact, k cannot be large enough, so the harmonic current can only be suppressed and eliminated to a certain extent.

3 DC active filter control system structure

According to the above analysis of the working principle of the active filter, the control system structure diagram of the active filter can be obtained as shown in Figure 3. The current on the line is detected by the current detection circuit, and the harmonic current signal on the line is obtained through the Butterworth low-pass filter circuit. The harmonic current signal is used as a given signal to generate a PWM control signal, and a deviation signal is obtained by comparing it with the active filter feedback signal. The hysteresis comparator obtains the PWM signal for controlling the on and off of the main circuit switch device according to the deviation signal, thereby obtaining a voltage that suppresses the line harmonic current, so as to achieve the purpose of suppressing harmonics by the active filter.

4 DC active current using hysteresis instantaneous value comparison method

This paper uses the power electronic circuit automatic control system simulation software PECS developed by our research group for simulation. The voltage on the DC bus of this system is about 500V, the load is a resistive load, the current on the bus is about 51A, and the loop width of the hysteresis comparator in the control circuit is about 5% of the input error signal (the difference between the given signal and the feedback signal). The simulation results of the active filter are given below. Figure 4 shows the current waveform and spectrum of the line when only the passive filter is working, and Figure 5 shows the current waveform on the line when the passive and active filters are working at the same time, the compensation voltage waveform output by the active filter, and the spectrum of the current waveform.

Figure 1 Structure diagram of hybrid DC active filter system

Figure 2 Equivalent circuit diagram of DC active filter

Figure 3 DC active filter control system structure diagram

Figure 4 Current waveform and spectrum of the line when only the passive filter is working

(a) Current waveform on the line (b) Spectrum of the current waveform

Figure 5 Simulation results when passive and active filters work simultaneously

(a) Compensation voltage waveform output by active filter

(b) Current waveform on the line when passive plus active filter

(c) Current waveform spectrum of the line when passive plus active filter is used

Figure 6 Experimental waveform with only passive filter

Figure 7 Experimental waveform of passive plus active filter

5 DC active filtering using hysteresis instantaneous value comparison method

Using Tektronix's TD340 digital oscilloscope, the experimental waveforms of passive filter only and passive plus active filter are obtained, as shown in Figure 6 and Figure 7. It can be seen from the figure that the addition of active filter has a significant effect on eliminating low-order harmonics, but increases the high-order harmonic components, which is consistent with the simulation results.

6 Conclusion

This paper aims at the hybrid filter system of passive-active filter, and uses the hysteresis loop instantaneous value comparison method to obtain the PWM signal to control the on and off of the switch device of the active filter inverter. The working principle of the DC active filter is analyzed in this paper, and the corresponding control circuit is designed. The simulation experiment results show that the DC active filter using the hysteresis loop instantaneous value comparison control method has a fast dynamic response speed, good tracking effect, and obvious compensation effect for low-order harmonics, but it increases the component of high-order harmonics. The biggest disadvantage of this control method is that the switching frequency of the device is not fixed, and further research is needed in this regard.