High-voltage parallel hybrid power grid high-order harmonic active filtering device

0 Introduction

The high-order harmonic currents generated during the operation of nonlinear loads such as electric locomotives, rolling mills, arc furnaces and power electronic equipment are large, three-phase unbalanced, intermittent and impactful. The existing fixed-capacity compensation devices cannot meet the requirements for safe and economical operation of such power loads. In order to control harmonics and improve power factor, many countries have developed various forms of static reactive power compensation and detuning devices over the years to complete different types of reactive power compensation and harmonic control tasks.

(1) A static VAR compensation device (SVC) using thyristor (SCR) elements is connected in parallel with an AC filter (FC). This SVC is composed of a thyristor-controlled reactor (TCR) and a thyristor-switched capacitor bank (TSC). The advantage of this device is that when used in a three-phase system, it can be used to compensate for the imbalance of the three-phase voltage and the reactive output can be continuously adjusted. The disadvantage is that the reactive output of the TCR during operation is achieved by adjusting the conduction angle of the thyristor. The TCR will generate a large amount of high-order harmonic current, so the filtering capacity of the AC filter must be increased accordingly, which greatly increases the loss of normal operation. If the TCR is used for single-phase compensation in an electric railway traction station, its advantages cannot be demonstrated but its disadvantages are more significant.

(2) Use a static reactive generator (SVG) with a high-speed gate turn-off thyristor (GTO) in parallel with an AC filter. This SVG can continuously adjust the output of leading or lagging reactive power, thereby stabilizing the system voltage and balancing the three phases. It also basically does not generate additional high-order harmonics, does not need to increase the filtering capacity of the AC filter, and can reduce infrastructure. At present, large-capacity SVG industrial prototypes can be manufactured in China, but the investment is large and its operating characteristics need to be further developed and put into engineering practice.

(3) Active high-order harmonic filters and passive filter groups are used for mixed parallel operation. The role of the passive filter group is to provide the necessary phase-advancing fundamental reactive power and serve as a low-order (3rd and 5th) harmonic filter circuit; the active high-order harmonic filter does not provide fundamental reactive power, but only compensates for the required harmonic currents. For the system, the active high-order harmonic filter is a high-impedance, high-order harmonic current source. Its access has no effect on the system impedance and can adapt to the harmonic current required by the compensated line. There is no problem of over-compensation and overload; at the same time, it can also prevent the parallel resonance or series resonance that may occur between the system and the capacitor group.

The traditional measure to eliminate high-order harmonics in the power grid is to use LC passive filters. The LC filter is connected in parallel with the harmonic source. In addition to filtering out high-order harmonics, it can also take into account the needs of system reactive power compensation and output a considerable amount of fundamental capacitive reactive power. The LC filter has a simple structure, reliable operation, and easy maintenance, but it occupies a large area and has some shortcomings.

Active filters can not only reduce floor space, but also effectively solve the problems existing in passive filters. The biggest difference between active filters and passive filters is that active filters are active filter devices that inject compensating harmonic currents into the AC grid to offset the harmonic currents generated by the load. They are structurally composed of static power converters and have the high controllability and fast response of semiconductor power converters.

1 Research and development overview

In view of the characteristics of harmonics in electrified railways, the North China Electric Power Research Institute proposed in 1997 a scheme that used a mixed parallel of active and passive filters in the harmonic control project of railway traction stations. The existing filtering and compensation devices have many shortcomings in technical performance when controlling the harmonics and fluctuations of loads such as electrified railways, large rolling mills and induction furnaces, and cannot meet the requirements of modern industry and national economy for power quality. Therefore, it is necessary to develop an active filter device for high-order harmonics of the power grid in direct-hung high-voltage systems. The North China Electric Power Research Institute began to develop this device in June 1999, and the entire set of equipment was successfully put into operation on August 26, 2003. The commissioning of this compensation device effectively filtered out the harmonic currents generated by nonlinear loads, improved the waveform of the 10kV bus voltage, and reduced the total voltage harmonic distortion rate from the original 5.0%.

The passive filter branch also compensates for the reactive power required by the system, raising the power factor from 0.75 to above 0.93. Since the filter device was put into operation, it has been stable and reliable, with significant compensation effect.

The project used power system analysis methods and information processing technology to carry out the overall design of the device; the active filter main circuit adopts a three-phase independent bridge structure; information processing adopts a digital-analog hybrid control technology that combines a high-speed digital signal processor (DSP) and an industrial control computer; the data sampling measurement and processing links adopt the first pre-shaping synchronous sampling technology; high-power insulated gate bipolar transistor (IGBT) components use soft shutdown technology and main circuit buffer discharge technology; high-voltage active filters and passive filters are operated in parallel using simulation and hybrid technology; high-voltage parallel hybrid active filters use electromagnetic compatibility design and its digital simulation technology, etc., to ensure the advanced technical indicators of the entire device and the stability and reliability of the compensation function.

2 Types of hybrid compensation devices

When the power capacity of the parallel active filter is large enough, it can not only quickly compensate harmonics, but also can be used to compensate reactive power, three-phase imbalance, and voltage fluctuations and flickers. Although the active filter has many advantages over the passive filter in terms of technical performance, it is a high-tech, multidisciplinary product. To achieve the same capacity of reactive power compensation and harmonic filtering, the initial manufacturing cost is much higher than that of the passive filter. The solution is to use a hybrid device that combines active filters and passive filters, which can effectively reduce the initial investment cost and improve the efficiency of filtering compensation. There are two types of hybrid compensation devices: a hybrid of a passive filter and a parallel active filter and a hybrid of a passive filter and a series active filter. The system structure is shown in Figure 1.

Figure 1 Circuit structure of hybrid compensation device

In the parallel hybrid compensation device, the main circuit of the active filter adopts a voltage source pulse width modulation (PWM) inverter with high efficiency and low loss. The function of the active filter is mainly to generate the current IC to compensate for the harmonic current, and the passive filter is mainly used to compensate for the reactive power and compensate for the harmonics of a specified order. The advantage of this arrangement is that the capacity of the parallel active filter can be greatly reduced, which is convenient for the application of the parallel active filter. In the series complex (hybrid) compensation device, the function of the active filter is to generate the voltage UC to compensate for the harmonic voltage, so that the power supply side and the nonlinear load can achieve harmonic isolation. Its advantage is that the capacity of the series active filter can be greatly reduced, and the passive filter can compensate for both reactive power and harmonics. Its disadvantage is that the safe isolation and protection of the series active filter applied to high voltage levels still has some technical difficulties, so it can only be applied to some low voltage and small capacity occasions [1]. Therefore, this project selected the development of a parallel hybrid active filter device suitable for high voltage and large capacity occasions.

3. Overall design considerations

The research contents of this project mainly include two points: ① Research on high voltage (10kV level) and large capacity (300kVA and above) active filters, including the study of static and dynamic characteristics of active filters, and the application of intelligent control technology to improve the adaptive ability of active filters and achieve optimal compensation. ② Research on filtering technology of parallel hybrid active filters, including the composite connection technology and dynamic characteristics of passive filters and active filters. The key issues to be solved are the current conversion technology and harmonic current injection technology of high voltage and large capacity power active filters; hybrid parallel technology and intelligent control technology of active filters and passive filters.

3.1 Topology of the main circuit

The main circuit of high voltage active filter generally has two types: high-high direct high voltage structure and high-low transformer structure.

The main circuit of the high-low transformer structure uses a step-down injection transformer to convert the 10kV high voltage into 700-1000V through a step-down injection transformer, and then uses the voltage-type inverter multiplexing technology to achieve high-voltage and high-power output. The biggest advantage of this structure is that it effectively solves the cost problem of high-power switching devices, because modulation and inversion at low voltage can use low-cost power electronic devices, and at the same time can reduce dU/dt, effectively reducing the switching loss of the device. The main circuit of this high-low transformer structure used in this project is shown in Figure 2. This structure can be easily expanded from three-phase 10kV to single-phase 27.5kV voltage levels, providing an efficient compensation device for the harmonic control project of the electric railway traction station.

Figure 2 Main circuit diagram of high and low voltage transformation structure

3.2 Power Switching Devices

Power switching devices should have the following characteristics: ① It can withstand high voltage in the blocking state. ② It has high current density and low conduction voltage drop in the conduction state. ③ It has sufficiently short conduction time and turn-off time, and can withstand high di/dt and dU/dt. IGBT has the advantages of high input impedance and fast switching speed of high-power field effect tube (MOSFET), and high withstand voltage and large current of high-power transistor (GTR); its gate is voltage driven, the required driving power is small, the switching loss is small, and the operating frequency is high. It is currently an ideal high-power switching device used in the main circuit of active power filter. The current application level has reached 3.3kV/1.2kA. More importantly, IGBT has achieved large-scale industrial mass production, and its price is almost the same as GTR, which provides sufficient sources for mass application. The main disadvantage of IGBT is that it has large internal resistance at high voltage and large on-state voltage drop, so the conduction loss is large. For this reason, it is necessary to select a suitable operating voltage to reduce the conduction loss. Therefore, after weighing, this project selects IGBT as the switching element of the main circuit.

3.3 IGBT drive circuit design

The working state of IGBT depends largely on the performance of the driving circuit. The driving circuit is often the key to large-capacity PWM technology. This project adopts the design of a dual-channel interlocking driver, which is very suitable for driving IGBTs connected to bridge arms. The soft shutdown function set in the circuit can automatically increase the shutdown time of the IGBT and reduce the overshoot of the DC bus voltage at the same time. The primary and secondary of the driving circuit are isolated by a ferrite transformer, so the driving power supply does not need to be isolated independently, but can share a power supply with the control circuit, simplifying the power supply setting. An interlocking circuit is set in the driving circuit to prevent the two IGBT elements of the half bridge from being turned on at the same time. By adjusting the additional resistor connected, the dead time can be easily adjusted. The driving circuit is also provided with an error signal storage unit. If an IGBT element is short-circuited or the driving power supply is lower than the rated value, the signal can be sent to the external control circuit through the error signal storage unit to achieve system protection action.

The parasitic stray inductance of the circuit wiring is a key issue in all high-current switching power supplies. The fast shutdown process will cause an overvoltage shock proportional to the stored energy and the switching speed. In order to prevent damage from overvoltage, it is necessary to select devices with large redundancy, but this will increase the cost of the whole machine; high switching voltage will also increase system losses and reduce the efficiency of the whole machine. It is impossible to completely eliminate stray reactance, but measures can be taken to minimize the stray inductance of the line, which can reduce the effective loop area of ​​the entire circuit, such as using a layered wiring structure. The gate series resistance Rg can be increased to suppress dU/dt; reducing the switching speed can significantly reduce the overvoltage spike, but increase the switching loss. The method of implementation is to release the gate charge with a gate impedance close to 0Ω when the IGBT is turned off until Uce reaches the main circuit voltage value, and then switch the gate release path to another impedance path.

3.4 Injection transformer

The injection transformer used in the active power filter is responsible for injecting high-power harmonic current into the 10kV line with low loss and no phase shift, so as to compensate the harmonic current generated by the non-linear load at the 10kV common connection point. In order to reduce the loss of the injection transformer to the maximum, corresponding measures should be taken in the selection of core materials, winding structure and winding process.

3.5 Digital-analog hybrid technology for harmonic current detection and compensation signal control

Whether the harmonic components that need to be compensated in the power line can be detected quickly and accurately and whether the dynamic tracking performance is good is the key to the active power filter device, which also directly determines the overall performance of the device.

The instantaneous reactive power p-Q method and its various algorithms can only be used to generate command signals for compensating fundamental reactive power and all harmonic currents; the synchronous rotating coordinate transformation d-Q method and the method of extracting fundamental components based on improved bandpass filters have the same function as the p-Q method; and the method of eliminating fundamental components using a notch filter can also only be used to compensate all harmonics. Considering that the capacity of the active power filter is limited to a certain extent, the hybrid structure environment of the passive filter requires that the harmonic currents of the specified order or specified number of harmonics to be compensated can be selectively compensated. In this regard, the active power filter can exert its optimal harmonic compensation capability, and at the same time, the passive filter will not produce overload or harmonic amplification for a certain harmonic current [2].

By detecting the harmonic current at the nonlinear load end, the harmonic current compensation command signal is obtained after calculation, and the main circuit of the active filter is controlled to generate a current equal to the load harmonic current and opposite in direction to compensate for part or all of the harmonic current in the line. Due to the emergence of high-speed DSP, the price has continued to decline in recent years, so it is no longer a problem to use fully digital sampling, analysis, and calculation to generate the compensation command signal of the active power filter.

High-speed DSP (TMS320F2407A) is used to complete fast multi-channel A/D conversion. Through digital plus analog calculations such as FFT, a PWM signal corresponding to the compensation current can be obtained to drive the switch devices of the main circuit. The measurement controller is realized by combining high-speed DSP and industrial control computer, in which DSP is used for data acquisition, compensation calculation analysis and composition; industrial control computer (MIC-2000) is used for regulation, control, communication and protection. Compared with the full digital controller, this digital-analog measurement controller has the advantages of accurate measurement, sensitive regulation and fast response speed. The detection and analysis of load current is realized by digital methods, which can ensure the accuracy and stability of the detection and analysis of the system. The compensation current instruction generation is realized by analog circuit, which can realize the fast response of compensation current tracking and better eliminate the circulation between the switch modules.

3.6 Digital data sampling measurement and control using pre-shaped synchronous sampling technology

The pre-shaping synchronous sampling technology can strictly guarantee the high accuracy of harmonic measurement and detection, and ensure that the output compensation signal is strictly synchronized with the system voltage, thereby ensuring the fast response and accuracy of the active filter compensation current.

Using a phase-locked loop to control the timing and rate of the sampling pulse is a relatively practical synchronous sampling method. In order to eliminate the influence of the distorted waveform on the operation of the synchronous sampling circuit, pre-shaping measures can be adopted before the synchronous signal enters the phase-locked loop to ensure a higher timing accuracy when comparing in the phase-locked loop. The use of pre-shaping synchronous sampling technology to reduce the synchronization error in synchronous sampling can easily eliminate the truncation error in the integral mean operation or the error caused by the leakage effect of FFT processing. It is particularly suitable for the measurement and real-time processing of any non-sinusoidal periodic signal. Its characteristics are that the amplitude variation of the synchronous source signal is wide, the high-precision timing and synchronization performance is not affected by the distortion of the signal waveform, and a variety of synchronous signal sources can be easily selected by the program.

3.7 Device software system

The software system of the device consists of two parts: the running program and the debugging program. The running program is the main part of the software, and most of the functions of the device are realized by the running program. To facilitate program development and management, the software is designed in a modular way. The debugging program mainly realizes the performance debugging of the main functional modules of the software and hardware and the writing and modification of the setting values ​​[4].

3.8 Electromagnetic compatibility management in active filter design process

The structure of the high-voltage parallel active filter itself is composed of AC and DC power systems, measurement, calculation and control and other weak current (electronic) systems. In order to avoid various interferences to the electronic system, corresponding electromagnetic measurements and analyses should be carried out during the design and development process so as to take measures for the power system or electronic system.

The management of electromagnetic compatibility is mainly carried out around the three elements that constitute electromagnetic interference (i.e., electromagnetic interference sources, interference coupling paths, and sensitive equipment). The management content includes: ① The mechanism of electromagnetic interference generation, how to suppress the emission of electromagnetic interference sources. ② In what way and through what channels electromagnetic interference is coupled (or conducted), how to cut off the transmission path of electromagnetic interference. ③ What kind of impact does sensitive equipment have on electromagnetic interference, and how to improve the anti-interference ability of sensitive equipment. During the operation of the high-voltage parallel active filter, the closing of the main circuit AC contactor, the pull-in of the control relay, the conduction and cut-off of the IGBT element, etc., will generate electromagnetic interference in different forms and different channels, and the measurement and control electronic circuit loop is most sensitive to this type of interference. Therefore, the problems of these three elements should be reasonably and effectively solved according to the standards and regulations related to electromagnetic compatibility.

4 Simulation operation of filter compensation device

Using the Electromagnetic Transients Program (EMTP) and PSpice to perform mixed digital-analog signal simulation, the efficiency of the active power filter and the function of the control system can be evaluated. Without physical testing, the structure of the active power filter can be evaluated by using software [5]. Using the CHP harmonic power flow calculation program, the simulation operation and safety verification of the 10kV parallel hybrid active filter device can be carried out, and the parameters and structure can be optimized.

The compensation effect of the hybrid structure of active filter and passive filter is analyzed according to three different installation positions and control methods. For each structure, only the harmonic filtering effect and the suppression of series and parallel resonance are analyzed. The simulation results show that the advantages of the harmonic compensation device using a combination of active and passive filters are: ① It can effectively compensate for the harmonic current with a wide range of frequency changes generated by the load during operation. ② It can effectively suppress the parallel resonance and series resonance that may be generated in the system.

By analyzing the different combination structures of three active and passive filters, the simulation results show that both the total harmonic current compensation method and the total current feedback compensation method have good harmonic compensation characteristics and the ability to suppress system parallel or series resonance. The simulation also found that the harmonic components of the power supply voltage may cause current oscillation between the active filter and the passive filter. This phenomenon should be solved in practical applications.

The simulation results also show that the best connection method for the injection transformer used in the active filter is D, d or Y, y. In this way, there is no need to specify the connection method of the distribution transformer of the load, and any form of connection method can achieve the expected compensation effect. Otherwise, the compensation effect will be damaged due to the phase shift of the compensation current caused by the injection transformer, resulting in an increase in harmonic current.

5. Device characteristics and industrial operation effects

The compensation device is installed in a 35kV substation in an industrial area. The 35kV line main power supply is provided by the upper 110kV substation. The substation is equipped with two SZ9-8000/35 main transformers running separately. According to the long-term monitoring of the local power supply bureau, the total distortion rate of the 10kV bus voltage of the substation often exceeds the harmonic voltage limit (4%) specified in the national standard GB/T 14945-1993, and sometimes even reaches 5% to 7%. The nonlinear loads of the substation are mainly DC rolling mills and high-frequency induction heating furnaces in some steel pipe factories. The main characteristic harmonics of DC rolling mills and high-frequency induction heating furnaces are 5th, 7th, 11th and 13th harmonics, in addition to 2nd, 3rd, 4th, 6th and other harmonics.

5.1 Harmonic Control and Reactive Power Compensation Scheme

A set of passive 5th order filters is designed with an installed capacity of 1500kvar to filter out the 5th order harmonic current and compensate for the required fundamental reactive power. At the same time, a high-voltage active filter BHY480/10 is connected in parallel to filter out other harmonic currents and not output fundamental reactive power to avoid over-compensation of reactive power.

5.2 Measurement results of device performance and industrial use effects

The expert test group of the appraisal committee conducted characteristic measurements on the device and the results are as follows: ① The design value of the three-phase output compensation capacity of the active filter device is 480kVA, and the measured output compensation capacity reaches more than 508.1kVA. ② The airdrop loss of the active filter device during airdrop is 563.8W, which is 0.12% of the rated compensation capacity. ③ The active filter device has a fast response tracking compensation characteristic, and the dynamic response time is less than 0.3ms. ④ The total distortion rate of the 10kV bus voltage of substation No. 5 was 4.9% to 5.9% before the filter compensation device was put into operation; the measured value after the device was put into operation was 1.3% to 1.5%. It can be seen that the 10kV bus voltage waveform has been significantly improved, and the total distortion rate of the voltage waveform has dropped to less than 1.5%. ⑤ The total compensation rate of the 10kV load harmonic current is 76.4%. ⑥ After the device was put into operation, the power factor of the system increased from 0.75 to more than 0.93.

The spectrum analysis of the line current on the 10kV side before and after the compensation device is put into operation is shown in Figure 3. The change curve of the total harmonic distortion rate of the bus voltage on the 10kV side before and after the compensation device is put into operation is shown in Figure 4.

Figure 3 Spectrum analysis of the 10kV line current before and after the compensation device is put into operation

Figure 4 Changes in the total harmonic distortion rate of the 10kV bus voltage before and after the compensation device is put into operation

6 Conclusion

The high-voltage parallel hybrid power grid high-order harmonic active filter device is a new type of filter compensation device that can be directly hung in a 10kV system. The passive filter equipment has the function of filtering out harmonics and taking into account the reactive power compensation required by the system. The active filter equipment actively compensates for the high-order harmonics of the system and has a high degree of adaptability. At the same time, it can suppress the parallel or series resonance caused by changes in system parameters and prevent reactive power overcompensation. It is a device for modern enterprises to control power harmonics and reasonably compensate reactive power for nonlinear loads containing various commutation devices.

The State Grid Corporation of my country held a scientific and technological achievement appraisal meeting for the project in Beijing in December 2004. The appraisal opinion believes that this device is the first 10kV large-capacity power grid high-order harmonic active filter device independently developed and put into industrial operation in China, filling the gap in the domestic 10kV direct-connected power grid high-order harmonic active filter device (GAPF). Its main technical indicators have reached the current international advanced level and have broad application prospects. It can be easily expanded from three-phase 10kV to single-phase 27.5kV voltage level, providing an efficient compensation device for the harmonic control project of the electric railway traction station.