Keywords: intelligent distribution network; active power filter; static synchronous compensator; unbalanced load
In recent years, for the sake of energy conservation and environmental protection, power electronic conversion devices in the terminal power supply system of the distribution network are more and more widely used, such as lighting, office, air conditioning, elevator and other related power supply systems. At the same time, the power quality of the user side often induces harmonic and reactive current problems on the distribution network side, line loss, overheating of the neutral line and transformer, inaccurate meter measurement, and even protection malfunctions. Although traditional passive filtering and switching capacitor compensation can solve the above problems and have a low cost, they cannot be continuously adjusted in real time. There is the possibility of over-compensation, reactive power reversal and even induced resonance in the distribution network [1-3].
In order to ensure the high-quality customized power supply for the end users of the smart distribution network, with the development of instantaneous power theory and power electronic devices, it replaces passive filtering and capacitor reactive power compensation devices. Its main circuit topology and design, harmonic current detection, Compensation methods, control and modulation strategies, and start-up characteristics are continuing hot topics in industry research and industrial applications [2-6].
Because the power quality problem in the smart distribution network is no longer a single problem, but a very complicated system problem. As shown in Figure 1, there are problems of harmonic current, unbalanced load and low power factor in the distribution system of a public facility. The power quality composite control technology is gradually put on the research agenda by academia and industry [7-8].
Figure 1 Power quality problems of actual distribution network
This paper studies the problem of time-varying harmonic current, unbalanced load and reactive power in the context of intelligent distribution network, and presents a composite compensation strategy for harmonic, negative sequence and reactive current, and its key parameter design method . Relevant simulation, experimental verification and actual measurement results of product field operation verify the correctness and feasibility of the control strategy.
APF-STATCOM circuit structure and working mechanism
Figure 2 Block diagram of parallel APF-STATCOM
As shown in Fig. 2, the parallel APF-STATCOM adopts a two-level three-phase four-arm voltage source inverter topology, in which the first three arms implement harmonic and reactive power compensation, and the fourth arm is used independently to control the neutral current . This is because in a three-phase four-wire system, when the load is unbalanced, a large zero-sequence current often flows through the neutral line, which is different from the three-phase three-wire system. Therefore, adding a fourth bridge arm that is decoupled from the first three bridge arms provides a zero-sequence current path. At this time, APF-STATCOM generates a compensation current C, abc opposite to the sum of the harmonics of the load current iL, abc, fundamental negative sequence and zero sequence components, so that the power supply current iS, abc provides only the positive sequence component of the fundamental current , To ensure that the source outputs a symmetrical three-phase current and improve the power factor.
Among them, the neutral line current separation detection, phase-locked loop, harmonic current detection, DC voltage control, current control and PWM modulation are the keys to achieve high-performance APF-STATCOM. The phase-locked loop, DC voltage control, etc. are the same as the three-phase three-wire system, and will not be described in detail here.
Analysis of key issues
1. The detection and control of the neutral current separation of the fourth bridge arm takes into account that the load current iL in the unbalanced three-phase four-wire circuit and the zero-sequence component iN included in the abc are equal, all
As shown in FIG. 2, at this time, the midline current sampling value iN is input as the fourth bridge arm current controller together with the neutral line zero sequence current component compensation command iNref, and the modulation signal is obtained through the PI regulator to obtain the fourth bridge arm switching signal.
At the same time,
In the formula, i'L, abc contain only positive sequence components and negative sequence components, which is convenient for the subsequent use of the ip-iq harmonic current detection algorithm in the three-phase three-wire system.
2. Harmonic current detection
Figure 3 The principle diagram of using dq transform to detect harmonics
The traditional method of detecting harmonic current based on pq instantaneous reactive power theory is greatly affected by voltage distortion and asymmetry, and it is not applicable in practical situations [9]. In practical occasions, the theoretical detection method of ip-iq instantaneous reactive power added to the phase-locked loop PLL circuit is mostly used, as shown in Figure 3, the relevant transformation is
Extract the current i'L, abc without zero-sequence component, and transform the fundamental component to 0Hz in the dq-0 coordinate through Park transformation (or first The transformation can also be done by dq transformation), and a low-pass filter can be used to extract the fundamental sequence component [5].
The output value of the DC voltage regulator in Figure 2 generates part of the active current command, which is used to stabilize the DC bus voltage and compensate for the power loss. If it is to improve the power factor, the reactive current can be compensated at the same time. At this time, the fundamental negative sequence reactive current command value is set to 0. Finally, subtract the positive sequence component of the fundamental current from the load current to obtain the command current amount that compensates for the harmonic component in the load current and the negative sequence component, zero sequence component, and positive sequence component of the reactive current caused by the load imbalance. Command current to realize APF-STATCOM function.
3. Design of current PR resonant controller. Because the current command tracked by APF-STATCOM is a superimposed signal of sinusoids of various frequencies, the traditional SPWM modulation using PI control must have steady-state error and phase offset. Hysteresis modulation, but frequency conversion modulation inevitably brings filter design and noise control problems [9].
By rotating the coordinate transformation, the sinusoidal signal can be changed into a DC signal, so that the PI controller is used in the new coordinate system. However, in the field of APF-STATCOM control, coordinate transformation must be performed at multiple frequencies, and the calculation is complicated, which is not conducive to practical application. In recent years, the proposed PR controller for sinusoidal signals can avoid the rotation coordinate transformation and greatly reduce the amount of calculation, while obtaining the same control effect as the PI controller in the synchronous coordinate system: it can track a specific frequency without steady-state errors The sinusoidal signal, more importantly, can be selectively compensated for the harmonics of the specified frequency.
In the formula Is the resonant frequency.
It can be seen from equation (7) that for the DC system, due to the existence of the integration link, the gain at 0 Hz is extremely high, so that the system can achieve no static adjustment; for AC systems, 50 Hz and its multiple harmonics, equation ( 7) The gain is limited. Equation (8) has a higher gain in the corresponding frequency band due to the introduction of the resonance link. If the tracked target is fundamental ; If you need to compensate the 5th harmonic of higher amplitude, there is
. Usually the harmonic order is up to 20 or 50 times, especially the odd harmonics with higher amplitude. So there are,
Figure 4 shows the Bode diagram of the fundamental resonance and the third, fifth, and seventh harmonic compensation for the PR resonant controller. It can be seen that the current controller gain is higher in the corresponding frequency band, which helps reduce tracking error.
Figure 4 Bode diagram of PR resonant controller
Simulation and experimental verification
In order to verify the proposed composite compensation strategy for harmonics, negative sequence and reactive current, this paper establishes a simulation platform under the environment of Matlab Simulink. The relevant parameters are set as follows: input three-phase four-wire system voltage 380V / 50Hz, three-phase diode rectifier nonlinear load DC side filter inductance 1mH, resistance 3.2Ω, three-phase diode rectifier AC reactance 0.4mH, APF-STATCOM grid-connected reactance 0.4mH , DC side support capacitance 4000μF, AC side unbalanced RL load star connection, inductance values ​​are 8mH, resistance values ​​are 5Ω, 50Ω, 500Ω, switching frequency 10kHz.
Figure 5 shows an example of phase A, which shows that the APF-STATCOM injection current after compensation well cancels the harmonic current of the load current, which makes the grid current sinusoidal and realizes the APF harmonic compensation function; The grid voltage is in the same frequency and phase, and the power factor is close to 1, which realizes the STATCOM reactive power compensation function. Figure 6 shows the results of three-phase compensation. The symmetrical three-phase current waveforms verify its ability to suppress unbalanced loads.
Figure 5 Voltage and current waveforms after phase A compensation (from top to bottom are grid voltage / V, grid current / A, compensation current / A, load current / A, time axis t / s)
Figure 6 Three-phase voltage and current waveforms of the grid after compensation (from top to bottom are three-phase grid voltage / V, three-phase grid current / A, time axis t / s)
Figure 7 further shows the DC-side bus voltage waveform. It can be seen that after the APF-STATCOM completes the harmonic compensation, the bus voltage fluctuates slightly, but is stable near the 750V set value.
Figure 7 DC-side bus voltage / V (time axis t / s)
Figure 8 and Figure 9 further give the internal test results of the industrial prototype. Limited by the experimental conditions, the load is only a rectifying nonlinear load at this time, so the load current and compensation current are different from the simulation, which mainly reflects the APF compensation function. Figure 10 shows the results of the product on-site operation. Compared with Figure 1, the neutral current is reduced from 37A to 5A, the maximum three-phase current THD does not exceed 3.4%, and the symmetry is good, which fully validates APF-STATCOM Compound compensation function.
Figure 8 Grid-side current and load current after phase A and phase B compensation (from top to bottom are phase A voltage, phase B current, phase A load current, phase B load current)
Figure 9 Grid-side current, reverse harmonic current and load current after phase A compensation
Figure 10 The actual site APF-STATCOM after compensation results
Conclusion
Based on the idea of ​​power quality compound control, a harmonic, negative sequence and reactive current compound compensation strategy is proposed for the problems of harmonic current, unbalanced load and low power factor in the intelligent distribution network.
Simulation, engineering prototype test and field operation results verify the APF-STATCOM composite compensation function based on this strategy.
Article Source: "Electrical Application" 2014 Issue 6
references
1. State Technical Supervision Bureau. China National Standard GB / T 14549-93 Power Quality Harmonics of Public Grid [S]. Beijing: China Standard Press, 1994.
2. Hirofumi Akagi (translation by Xu Zheng). Instantaneous power theory and its application in power regulation [J]. Beijing: Mechanical Industry Press, 2009.
3. Wang Zhaoan, Yang Jun, Liu Jinjun. Harmonic suppression and reactive power compensation. Machinery Industry Press, 1998.
4. Ma Li, Zhou Jinghai, Lu Zhengyu, Qian Lighting. Research on an improved harmonic detection scheme based on dq transform [J]. Chinese Journal of Electrical Engineering, 2000, 20 (10): 55-63.
Ma Li, Zhou Jinghai, LüZhengyu, Qian Zhaoming. An improved harmonic detecting approach based on dq rotating coordination transformation [J]. Proceedings of the CSEE, 2000, 20 (10): 55-63 (in Chinese).
5. Zeliang Shu, Yuhua Guo, and Jisan Lian. Steady-state and dynamic study of active power filter with efficient FPGA-based control algorithm [J]. IEEE Transactions on Industrial Electronics, 2008, 55 (4): 1527-1536.
6. Zhao Guopeng, Lin Shaobo, Han Minxiao. Analysis of DC side voltage value of parallel active power filter based on compensation characteristics [J]. Power System Automation, 2012, 36 (14): 83-87.
Zhao Guopeng, Lin Shaobo, Han Minxiao. Design of voltage in DC link of parallel-type active power filter based on compensation characteristics [J]. Automation of Electric Power Systems, 2012, 36 (14): 83-87 (in Chinese).
7. Single Ren Zhong. Parallel compound power quality disturbance and compensation control method and implementation [D]. Doctoral Dissertation, Beijing: North China Electric Power University, 2010.
8. Liu Haibo, Mao Chengxiong, Lu Jiming, Wang Dan. Four-arm three-phase four-wire parallel type APF-STATCOM [J]. Power System Protection and Control, 2010, 38 (16): 11-17.
Liu Haibo, Mao Chengxiong, Lu Jiming, Wang Dan. Three-phase four-wire shunt APF-STATCOM using a four-leg converter [J]. Power System Protection and Control, 2010, 38 (16): 11-17 (in Chinese).
9. Cao Wu. Research on key technology of harmonic independent compensation active filter [D]. Master's degree thesis, Nanjing: Southeast University, 2011.
About the Author:
Yu Jing, female, undergraduate, engineer of Wuhan Ankerui Electric Co., Ltd., the main research direction is intelligent power monitoring and power management system
Innovation of MCU control system was adopted to realize stepless, four color temperature of light output, single control/group control, timer switch, smartprofiles, ect
Wireless control, easy to install. 86 controller, light sensor is compose of simple control system can compatible with our remotecontrol.
CCT Changeable & Dimmable Downlight
Dimmable LED Downlights,CCT Changeable Downlight,CCT Dimmable Downlight,Cut Out 160-180mm Downlight
SHENZHEN KEHEI LIGHTING TECHNOLOGY CO.LTD , https://www.keheiled.com