CN1086256C - Combined power filter with both active and passive parts - Google Patents

Abstract

The invention relates to an active and passive integrated power filter device, which is composed of an active filter, a passive filter, a transformer and a control circuit, wherein the control circuit includes a current sensor, a harmonic detection circuit, a regulator, Pulse width modulation control circuit and drive circuit. The current sensor converts the current signal of the power grid into a voltage signal and then inputs it into the harmonic detection circuit to obtain a harmonic current signal, which is sent to the regulator to obtain a command signal for controlling the current, and controls the output voltage of the active filter according to the command signal to realize filtering . The device of the invention has low cost and good economical practicability.

Description

Integrated power filter device composed of active and passive components

The invention relates to an active and passive integrated power filtering device, which belongs to the technical field of electrical engineering.

With the rapid development of power electronic technology, various power electronic devices have been widely used in power system, industry (especially metallurgy, steel, chemical industry, etc.), transportation, building automation and home, which makes the harmonic in the power grid Wave pollution is getting worse. Moreover, the capacitors used to improve the power factor in the grid tend to resonate with the system impedance, resulting in harmonic amplification. This has aggravated the harmonic pollution in the power grid.

Harmonics in the grid not only endanger the grid itself but also endanger its peripheral equipment. On the one hand, harmonics in the power grid reduce the efficiency of production, transmission and utilization of electric energy, cause electrical equipment to overheat, generate vibration and noise, and cause insulation to age, reduce service life, and even malfunction or burn out. At the same time, harmonics can also cause partial parallel resonance or series resonance in the power system, which amplifies the harmonic content and causes equipment such as capacitors to burn out. Harmonics can also cause relay protection and automatic devices to malfunction, causing confusion in electric energy measurement. For the outside of the power system, harmonics will cause serious interference to communication equipment and electronic equipment.

The harmonic pollution in the power grid is becoming more and more serious, so the demand for harmonic control devices is also increasing. Moreover, harmonic control is also an important part of building a "green grid". There are two main types of harmonic control devices: passive filter devices and active filter devices.

The passive filtering device can be found in the book "Harmonic Analysis and Filtering of High-Voltage Direct Current Transmission System" published by Water Conservancy and Electric Power Press in August 1994. The circuit structure diagram is shown in Figure 1. The effective circuit diagram is shown in Figure 2.

According to Figure 2, the system harmonics can be calculated as: $\mathrm{Ish}=\frac{\mathrm{IL}}{\frac{\mathrm{Zs}}{\mathrm{ZF}}+1}+\frac{\mathrm{Vsh}}{\mathrm{Zs}+\mathrm{ZF}}\&Center\; Dot;\&Center\; Dot;\&Center\; Dot;\left(1\right)$

The first item on the right side of the equal sign in the above formula is the result of the harmonic current action, and the second item is the result of the harmonic voltage action. It can be seen from the first item that when the frequency of the harmonic current is consistent with the resonant frequency of the filter, , ZF/Zs→∞, the result of the first term is also approximately equal to zero, which realizes the suppression of the load harmonic current.

In addition to the above disadvantages, passive filters also have the following disadvantages:

1. Poor compensation performance Due to errors in the parameters of capacitors and inductors; during operation, changes will occur due to changes in ambient temperature, self-heating, and aging of capacitor insulation; at the same time, there will be certain fluctuations in system frequency during operation. deviation. These reasons will cause the harmonic frequency to be inconsistent with the resonant frequency of the filter. At this time, the impedance presented by the filter will deviate from its minimum value, which will deteriorate the filtering effect of the filter.

2. Poor dynamic performance When the load current changes dynamically, its suppression effect on harmonic current is poor

3. Compensation performance is affected by system impedance From the formula (1), it can be seen that the filtering effect of the passive filter on the load harmonic current is determined by Zs/ZF, therefore, the change of the system impedance will affect its filtering effect

4. Even under ideal conditions, it can only produce a relatively good suppression effect on harmonic currents of specific orders

5. Huge volume, takes up a lot of space

Existing active filtering devices can be found in the 1995 "Toyo Electric Manufacturing Co., Ltd. Product Manual". Its principle block diagram like chart 3 shows. The basic principle is: through the control, the phase grid of the active filter injects a current that is the same size as the load harmonics but has an opposite phase, so that the grid current can be a sine wave, thereby achieving the purpose of eliminating harmonic currents.

The shortcomings of the current active filter technology can be found in the doctoral dissertation of Tsinghua University in 1997 "Research on New Integrated Power Filter System", mainly including:

1. Since the fundamental wave voltage of the AC power supply is directly applied to the inverter, and the compensation current is completely provided by the inverter, the capacity of the inverter must be large

2. Due to the high cost of power electronic devices, the price of this system is very expensive

3. Since the detection circuit is easily affected by device parameter dispersion and debugging errors, this will affect its compensation performance and needs further improvement

4. Since the active filter device adopts a brand-new structure, it is not suitable for modifying the original passive filter device

The object of the invention is to design a passive and active integrated filtering device. It can not only overcome the shortcomings of using passive filters and active filters alone, but also absorb the advantages of both to obtain a new type of power filter device with good filtering effect and low cost.

The active and passive integrated power filtering device designed by the present invention is composed of active filter, passive filter, transformer and control circuit. One end of the passive filter is connected to the power grid, and the other end is connected to the secondary side of the transformer. One end of the transformer is connected, the other end of the transformer secondary is shorted together, one end of the primary side of the transformer is connected to the midpoint of the inverter bridge arm in the active filter, and the other end of the primary side is shorted together. The control circuit includes a current sensor, a harmonic detection circuit, a regulator, a pulse width modulation control circuit and a drive circuit. The current sensor converts the current signal of the power grid into a voltage signal and then inputs it into the harmonic detection circuit to obtain the harmonic current signal of the power grid, which is sent to the regulator and controlled together with the feedback signal of the capacitor voltage in the active filter. The command signal of the current is sent to the pulse width modulation control circuit, and the corresponding control signal is generated according to the sign of the command signal, that is, when the command signal of one phase is greater than zero, a signal is generated, sent to the drive circuit, and shaped and amplified , After isolation, make the switching tube of the upper bridge arm of the inverter bridge corresponding to the phase turn on, and the switching tube of the lower bridge arm turn off; if it is less than zero, a signal is generated and sent to the drive circuit , after shaping, amplification, and isolation, the switching tube of the upper bridge arm of the inverter bridge corresponding to the phase is turned off, and the switching tube of the lower bridge arm is turned on to control the output of the active filter voltage to achieve filtering.

The device of the present invention has the following advantages:

1, low cost, good economical practicability: the power of the active part in the present invention is less, with reference to IEEE Trans.Ind.Appl.Vol (33) No.4 (1997) " Effectiveness of Harmonic Mitigation Equipment for Commercial Office Buildings " According to the price standard calculation given in the article, the price of this filter device is $227.3/kVA, which is almost the same as the price of passive filter devices ($200/kVA). If the original passive filtering device is transformed, the transformation cost is only $50/kVA.

2. Good filtering performance: For rectifier loads, after the equipment is put into operation, the harmonic current contents of the 5th, 7th, 11th, and 13th harmonics in the grid current are 1.7%, 1.4%, 0.8%, and 0.9%, respectively. The current waveform is an ideal sine wave. Refer to Figure 7 for the comparison of current waveforms in the power grid before and after the filtering device is put into use. The actual operation results show that the filtering device has the following characteristics: the filtering effect of the filtering system is less affected by the impedance of the power system; there will be no series or parallel resonance between the impedance of the power system and the passive filter.

Description of drawings:

Figure 1 is a schematic diagram of a passive filter circuit.

Figure 2 is a single-phase equivalent circuit diagram of a passive filter under the action of harmonics.

Figure 3 is a schematic diagram of the active filter circuit.

Fig. 4 is a schematic circuit diagram of the device of the present invention.

Fig. 5 is the equivalent circuit diagram of the device of the present invention under the action of harmonics, wherein Fig. 5a is a single-phase equivalent circuit diagram, Fig. 5b is an equivalent circuit diagram under the single action of load harmonic current Ilh, and Fig. 5c is a grid harmonic voltage Ush Equivalent circuit diagram under single action.

Figure 6 is a schematic diagram of the pulse width modulation (PWM) control circuit.

Fig. 7 is a comparison of current waveforms in the power grid before and after the device of the present invention is put into operation, wherein Fig. 7a is the current waveform in the power grid before the device of the present invention is put into operation, and Fig. 7b is the current waveform in the power grid after the device of the present invention is put into operation.

Below in conjunction with accompanying drawing, introduce the content of the present invention in detail.

The working principle of active and passive integrated filtering device of the present invention is as follows:

Assume that the active filter is an ideal controllable voltage source, and the harmonic load is a current source. In the case of three-phase symmetry, its single-phase equivalent circuit diagram is shown in Figure 5(a). Among them, U _sf is the fundamental wave part in the power supply voltage, U _sh is the harmonic part in the power supply voltage; I _sf is the fundamental wave part in the power supply current, I _sh is the harmonic part in the power supply current; I _Lf is the load current In the fundamental part, I _Lh is the harmonic part in the load current; Z _F is the passive filter impedance; Z _s (L _s ) is the system impedance.

Without considering the system loss, the output voltage of the active filter does not contain the fundamental wave component. Figure 5(a) can be decomposed into Figure 5(b) and Figure 5(c) according to the superposition principle. For the harmonic current source ILh, the equivalent circuit under its single action is shown in Fig. 5(b). At this time, the active filter can be regarded as a harmonic impedance Zc. When Uc=ZF×ILh, the harmonic impedance Zc at each harmonic frequency is always equal to -ZF, that is, ZF+Zc=0, so all the load harmonic current ILh will flow into the passive filter branch, without The filtering effect of the source filter has been improved. For the harmonic component Ush in the system voltage, the equivalent circuit under its single action is shown in Fig. 5(c). At this time, the active filter can be regarded as a controlled voltage source. When UC=Ush, the harmonic current Ish generated by Ush will be zero, thus suppressing the harmonic current caused by the system harmonic voltage. Combining the above two situations, when the active filter output voltage UC=Ush+ZF×ILh, all harmonic currents in the system will be suppressed.

Since the active filter voltage UC=Ush+ZF×ILh is much smaller than the grid voltage, this makes the capacity of the active filter in the integrated power filter system much smaller than that of the parallel active filter, and more It is suitable for compensating large-capacity harmonic loads, and its initial investment and operating costs are lower than those of parallel active filters. Therefore, it has the advantages of low cost of the passive filter and good filtering performance of the active filter at the same time, and has strong practicability and economy.

See Figure 6 for the schematic diagram of the PWM (pulse width modulation) control circuit, and take phase a as an example to introduce its working process. When the phase a command signal Ia ^* is greater than zero, the comparator Comp-A outputs a low level to the input of the NOT gate IC1:A, and the NOT gate IC1:A outputs a high level to the D flip-flop IC3:A D terminal, when the clock terminal CLK of the D flip-flop has a rising edge, the high level will make its non-inverting output terminal Q output a high level, and the high level will be sent to the drive circuit, so that the switch The tube S1 is turned on; at the same time, the high level also makes the negative phase output terminal Q output a low level, and the low level is sent to the driving circuit, so that the switch tube S2 is turned off. On the contrary, when the a-phase command signal Ia ^* is less than zero, the output of the D flip-flop is a low level, so that the switch tube S1 is turned off and S2 is turned on. The control process of the other two phases is similar to phase a. In this way, the output voltage of the active filter can be guaranteed to be equal to (Ush+ZF×Ilh) ^* N, thereby ensuring that the integrated filter device has a relatively ideal filtering effect.

The non-gate IC2 in the figure: A, IC2: B, IC2: C, RY and CY form a square wave oscillator. Assume that initially the output terminal of the NOT gate IC2:A is low level, and the high level is input to IC2:B to make its output high level, and the high level is positively charged to the capacitor CY through the resistor RY, when B When the point potential is higher than the threshold voltage of IC2:C, IC2:C outputs a low level, which is input to the input terminal of IC2:A to make it output a high level, and at the same time, the output terminal of IC2:B is a low level In this way, IC2:A discharges the capacitor, and when the potential of point B is lower than the threshold voltage of IC2:C, IC2:C outputs a high level. In this way, a square wave pulse with a certain period can be obtained at the output terminal of IC2:C by repeated charging and discharging, and the square wave pulse is sent to the clock terminal of the D flip-flop after being positive and reversed by IC2:D. From the above analysis, it can be known that the state of the output terminal Q of the D flip-flop may change only when a rising edge occurs at the clock terminal, thereby causing the switching state of the switch tube to change. Therefore, the state change of the switch tube is synchronized with the rising edge of the square wave pulse. In this way, the switching frequency of the active filter can be kept constant.

Claims (1)

1, a kind of combined power filter with both of active and passive formation, it is characterized in that this device is made up of active filter, passive filter, transformer and control circuit, one end and the electrical network of passive filter link, one end of the other end and transformer secondary output links, the mutual short circuit of the other end of transformer secondary output together, the mid point of inverter bridge leg links in one end on the former limit of transformer and the active filter, and the mutual short circuit of the other end on former limit together; Control circuit comprises current sensor, harmonics detection circuit, adjuster, control circuit for pulse-width modulation and drive circuit; Current sensor is transformed into input harmonics testing circuit behind the voltage signal with the current signal of electrical network, obtain the mains by harmonics current signal, this signal is sent into adjuster, the command signal of the feedback signal of the capacitance voltage in active filter controlled electric current after regulating, and send into control circuit for pulse-width modulation, symbol according to command signal produces control signal corresponding, promptly when the command signal of a phase wherein greater than zero the time, produce a signal, send into drive circuit, through shaping, amplify, after the isolation, make the switching tube conducting of brachium pontis on the inverter bridge with this mutually corresponding active filter, the switching tube of following brachium pontis turn-offs, if less than zero the time, produce a signal, send into drive circuit, through shaping, amplify, after the isolation, the switching tube of brachium pontis on the inverter bridge with this mutually corresponding active filter is turn-offed, the switching tube conducting of following brachium pontis with the output voltage of control active filter, realizes filtering.

Images