Power System Protection and Control Based on LCL Filtering DVR Compensation Transformer Leakage Resistance Determination Wu Fuzhuo, Hou Tingting, Han Liang (Zhongyuan Institute of Technology, Zhengzhou 450007, China) Waves form an LCL filter. According to the design requirements of LCL filter, the method of determining the leakage reactance and impedance drop of the transformer is analyzed. Combined with the characteristics of DVR, the selection principle of each parameter of LCL filter is analyzed. According to the above method, a voltage drop compensation simulation is performed on a DVR having a capacity of 30 kVA. Its working principle is: firstly detecting the grid supply voltage and then controlling the output of the inverter according to a certain compensation strategy by a digital signal processing system, the compensation voltage is superimposed on the input voltage Mf of the load Zf via the filter and the series compensation transformer Up, so that the voltage dropped by the grid can be dynamically compensated.
2 compensation transformer equivalent to the actual series transformer equivalent circuit as shown. In the middle, it is the output voltage of the secondary side of the series transformer; Ms is the power supply voltage; U1 is the load voltage; Uc is the primary side voltage of the series transformer; the leakage reactance and resistance are respectively the primary side of the transformer; the heart and the æ² are respectively reduced to the primary side. The secondary side leakage reactance and resistance; /1, 4 are the primary current of the transformer and the secondary current calculated to the primary side; /m is the excitation current of the transformer; and the sensitive load impedance is calculated. Since the excitation impedance is large and negligible, the simplified circuit of the transformer is as shown.
3LCL filter analysis DVR filter is selected on the output side of the inverter, so it can be considered that the LC filter circuit and the leakage inductance ia(i1+i2) of the transformer form an LCL filter circuit, as shown.
The leakage inductance of the transformer can be used as the LC-side filter inductor of the LCL filter (see), and 厶, Cf constitutes the LCL filter. The basic principle is to endure the impedance shunt of the high-frequency switching ripple contained in the current. The capacitor Cf provides a low-resistance path for the high-frequency component, thus effectively reducing the high-frequency content of the current /1 (ie, the grid current).
3.1LCL filter parameter design Generally, the wind, the formation and the 褚 of the resistance on the inductor are small, which can be neglected for analysis convenience. Simplified as shown.
LCL filter simplified equivalent model diagram from the control requirements analysis, LCL filter parameters can be selected from the following three aspects: meet the requirements of active power and reactive power control; meet the requirements of harmonic current indicators; meet current tracking Requirements.
(L+Za) The design of the LCL inverter's total inductance (+ia) should be limited under steady-state conditions, considering the ability of the inverter to output active (reactive) power. In the fundamental application, for the fundamental current, the filter capacitance of the LCL filter is equivalent to an open circuit, and the circuit can be simplified to an equivalent circuit diagram as shown in (a). Where ik represents the sum of the two inductances.
It is not difficult to see from (b) that when the inverter output voltage is kept constant and the grid voltage value is constant, if the appropriate inductance value and DC voltage parameter are selected, the inverter can be controlled to run at any point on the circumference. If a, b, c, d divides the circumference into four arc segments, the inverters have different operating states and different operating currents on different arc segments, and the design requirements for the inductors are also different. When the design of the inductor satisfies the operating condition of point C, the upper limit of inductance is the smallest; while the operating condition of point A is satisfied, the upper limit of inductance is the largest. When designing the upper limit of the inductance, it is necessary to satisfy the situation in which the working state is the most serious, that is, when working at point C.
According to the requirements of current ripple content, the bridge arm side inductance is actually implemented. The inductor current on the bridge arm side is pulsating with the switching period. In order to limit the ripple of the inductor current to a certain range, it is necessary to analyze the inverter output single. The variation law of the inductor current ripple in the power frequency cycle. The mathematical relationship between the transient inductor currents at different times in the single output power frequency cycle of the inverter can be used to analyze and obtain the current ripple variation law, and obtain the maximum inductor current ripple value, thereby obtaining the bridge arm side inductance. The maximum lower limit of the value.
The variation law of the current ripple is as shown in equation (4), that is, if the switching frequency is constant, when the ratio of /ud. is different, the variation of the current ripple at the peak of the fundamental current and the zero-crossing is as shown.
When 7 is suppressed. When =0.5, the current ripple amplitude is the largest and the maximum value is equation (7).
With current ripple Me '/ Mde variation according to the above findings and the inverter-side arm of the most current control, requires a large current ripple amplitude ~ aw of r, the maximum current ripple in accordance with (11) the amplitude when designing the appropriate inductance value, i.e., 2 smaller due to leakage impedance of the transformer can be considered Lk = Li, static synthesis filter performance requirements of dynamic range side can be drawn from the arm of the inductance of the inductor consideration the cost, the design value of inductance should be as small as possible; consider the design error, the inductor value should leave a certain margin.
Two inductors ratio r is determined as h LCL filter views equivalent single-phase structure of the switching frequency harmonics of the current in the high frequency inverter state is a grid-connected inverter harmonics generator, corresponding to the short-circuit network side . Seen from Norton theorem grid inverter may be equivalent to the side arm by a current source in parallel with the inductor Li.
The attenuation ratio a of the inverter side harmonic current to the grid side harmonic current i1 is defined as the number of ZLc switching harmonics.
The resonant frequency formula of the LCL filter dictates that the general primary attenuation ratio is usually about 20%. It is possible to determine the inductance ratio r. The determination of the filter capacitor Cf In the grid-connected inverter LCL filter design, the reactive power generated by the capacitor is generally limited to a system rated power of no more than 5%. The available capacitance range is the design limit of the resonant frequency s. For the LCL filter, the resonant frequency of the filter is generally required to be designed between 10 times the fundamental frequency and 0.5 times the switching frequency, that is, 10/nS/resS0.5/sw , fn is the fundamental frequency of the grid, and fsw is the switching frequency of the inverter.
3.2 The impedance drop is determined according to the impedance drop of the available transformer, L1+L2=La, and La=rLi. 4 is simulated by the above analysis. The first curve is the grid voltage after DVR compensation, and the second The curve is to compensate the output voltage of the secondary side of the transformer, the third curve is the compensated grid current, and the fourth curve is the grid change curve. It is not difficult to see from the figure that the grid voltage drops at 0.10.3s, and the DVR can compensate the voltage in time to ensure the voltage at both ends of the load is stable, thus ensuring the normal operation of the sensitive load. In 0(a1), the filter inductor Li takes 3mH, the filter capacitor Cf takes 15垆, the transformer leakage inductance La takes 0.04mH. The impedance drop is 0.19V, the voltage drop percentage is about 1%, and the harmonic distortion rate of the compensation voltage is 16.34. %, the harmonic distortion rate THD of the power grid is 1.80%; 0(a2) is the simulation waveform diagram when the grid voltage drops by 30%, and other parameters are unchanged, the harmonic distortion rate of the compensation voltage is 5.71%. The harmonic distortion rate in the power grid is 1.65%. It can be obtained from the simulation results that the harmonic content of the compensation voltage is higher when the grid drop voltage is small, but because the compensation voltage accounts for a small proportion in the grid, The harmonic distortion rate will also be small.
Curve 1 in 0(b) is the grid voltage curve, the value is 220V, curve 2 is the voltage curve after the grid falls 10%, the value is about 198V, 0(c) is the output voltage curve of the compensation transformer, the value is about 23.4 V, After the grid voltage and the compensation voltage are superimposed, the voltage across the load is guaranteed to be around 220V. It is obvious that the filter inductor and transformer leakage inductance designed by this method have good practical results.
5 Conclusion From the filter performance of the filter circuit and the economics of the transformer, the transformer leakage inductance and LC filter circuit are combined to form the LCL filter circuit. By analyzing the stability, current tracking and filtering performance of the LCL filter circuit, the filter is designed. And related parameters of the transformer. The rationality of the method is verified by simulation and experimental results.
2 compensation transformer equivalent to the actual series transformer equivalent circuit as shown. In the middle, it is the output voltage of the secondary side of the series transformer; Ms is the power supply voltage; U1 is the load voltage; Uc is the primary side voltage of the series transformer; the leakage reactance and resistance are respectively the primary side of the transformer; the heart and the æ² are respectively reduced to the primary side. The secondary side leakage reactance and resistance; /1, 4 are the primary current of the transformer and the secondary current calculated to the primary side; /m is the excitation current of the transformer; and the sensitive load impedance is calculated. Since the excitation impedance is large and negligible, the simplified circuit of the transformer is as shown.
3LCL filter analysis DVR filter is selected on the output side of the inverter, so it can be considered that the LC filter circuit and the leakage inductance ia(i1+i2) of the transformer form an LCL filter circuit, as shown.
The leakage inductance of the transformer can be used as the LC-side filter inductor of the LCL filter (see), and 厶, Cf constitutes the LCL filter. The basic principle is to endure the impedance shunt of the high-frequency switching ripple contained in the current. The capacitor Cf provides a low-resistance path for the high-frequency component, thus effectively reducing the high-frequency content of the current /1 (ie, the grid current).
3.1LCL filter parameter design Generally, the wind, the formation and the 褚 of the resistance on the inductor are small, which can be neglected for analysis convenience. Simplified as shown.
LCL filter simplified equivalent model diagram from the control requirements analysis, LCL filter parameters can be selected from the following three aspects: meet the requirements of active power and reactive power control; meet the requirements of harmonic current indicators; meet current tracking Requirements.
(L+Za) The design of the LCL inverter's total inductance (+ia) should be limited under steady-state conditions, considering the ability of the inverter to output active (reactive) power. In the fundamental application, for the fundamental current, the filter capacitance of the LCL filter is equivalent to an open circuit, and the circuit can be simplified to an equivalent circuit diagram as shown in (a). Where ik represents the sum of the two inductances.
It is not difficult to see from (b) that when the inverter output voltage is kept constant and the grid voltage value is constant, if the appropriate inductance value and DC voltage parameter are selected, the inverter can be controlled to run at any point on the circumference. If a, b, c, d divides the circumference into four arc segments, the inverters have different operating states and different operating currents on different arc segments, and the design requirements for the inductors are also different. When the design of the inductor satisfies the operating condition of point C, the upper limit of inductance is the smallest; while the operating condition of point A is satisfied, the upper limit of inductance is the largest. When designing the upper limit of the inductance, it is necessary to satisfy the situation in which the working state is the most serious, that is, when working at point C.
According to the requirements of current ripple content, the bridge arm side inductance is actually implemented. The inductor current on the bridge arm side is pulsating with the switching period. In order to limit the ripple of the inductor current to a certain range, it is necessary to analyze the inverter output single. The variation law of the inductor current ripple in the power frequency cycle. The mathematical relationship between the transient inductor currents at different times in the single output power frequency cycle of the inverter can be used to analyze and obtain the current ripple variation law, and obtain the maximum inductor current ripple value, thereby obtaining the bridge arm side inductance. The maximum lower limit of the value.
The variation law of the current ripple is as shown in equation (4), that is, if the switching frequency is constant, when the ratio of /ud. is different, the variation of the current ripple at the peak of the fundamental current and the zero-crossing is as shown.
When 7 is suppressed. When =0.5, the current ripple amplitude is the largest and the maximum value is equation (7).
With current ripple Me '/ Mde variation according to the above findings and the inverter-side arm of the most current control, requires a large current ripple amplitude ~ aw of r, the maximum current ripple in accordance with (11) the amplitude when designing the appropriate inductance value, i.e., 2 smaller due to leakage impedance of the transformer can be considered Lk = Li, static synthesis filter performance requirements of dynamic range side can be drawn from the arm of the inductance of the inductor consideration the cost, the design value of inductance should be as small as possible; consider the design error, the inductor value should leave a certain margin.
Two inductors ratio r is determined as h LCL filter views equivalent single-phase structure of the switching frequency harmonics of the current in the high frequency inverter state is a grid-connected inverter harmonics generator, corresponding to the short-circuit network side . Seen from Norton theorem grid inverter may be equivalent to the side arm by a current source in parallel with the inductor Li.
The attenuation ratio a of the inverter side harmonic current to the grid side harmonic current i1 is defined as the number of ZLc switching harmonics.
The resonant frequency formula of the LCL filter dictates that the general primary attenuation ratio is usually about 20%. It is possible to determine the inductance ratio r. The determination of the filter capacitor Cf In the grid-connected inverter LCL filter design, the reactive power generated by the capacitor is generally limited to a system rated power of no more than 5%. The available capacitance range is the design limit of the resonant frequency s. For the LCL filter, the resonant frequency of the filter is generally required to be designed between 10 times the fundamental frequency and 0.5 times the switching frequency, that is, 10/nS/resS0.5/sw , fn is the fundamental frequency of the grid, and fsw is the switching frequency of the inverter.
3.2 The impedance drop is determined according to the impedance drop of the available transformer, L1+L2=La, and La=rLi. 4 is simulated by the above analysis. The first curve is the grid voltage after DVR compensation, and the second The curve is to compensate the output voltage of the secondary side of the transformer, the third curve is the compensated grid current, and the fourth curve is the grid change curve. It is not difficult to see from the figure that the grid voltage drops at 0.10.3s, and the DVR can compensate the voltage in time to ensure the voltage at both ends of the load is stable, thus ensuring the normal operation of the sensitive load. In 0(a1), the filter inductor Li takes 3mH, the filter capacitor Cf takes 15垆, the transformer leakage inductance La takes 0.04mH. The impedance drop is 0.19V, the voltage drop percentage is about 1%, and the harmonic distortion rate of the compensation voltage is 16.34. %, the harmonic distortion rate THD of the power grid is 1.80%; 0(a2) is the simulation waveform diagram when the grid voltage drops by 30%, and other parameters are unchanged, the harmonic distortion rate of the compensation voltage is 5.71%. The harmonic distortion rate in the power grid is 1.65%. It can be obtained from the simulation results that the harmonic content of the compensation voltage is higher when the grid drop voltage is small, but because the compensation voltage accounts for a small proportion in the grid, The harmonic distortion rate will also be small.
Curve 1 in 0(b) is the grid voltage curve, the value is 220V, curve 2 is the voltage curve after the grid falls 10%, the value is about 198V, 0(c) is the output voltage curve of the compensation transformer, the value is about 23.4 V, After the grid voltage and the compensation voltage are superimposed, the voltage across the load is guaranteed to be around 220V. It is obvious that the filter inductor and transformer leakage inductance designed by this method have good practical results.
5 Conclusion From the filter performance of the filter circuit and the economics of the transformer, the transformer leakage inductance and LC filter circuit are combined to form the LCL filter circuit. By analyzing the stability, current tracking and filtering performance of the LCL filter circuit, the filter is designed. And related parameters of the transformer. The rationality of the method is verified by simulation and experimental results.
Battery Management And Control Systems
Shenzhen Sunbeam New Energy Co., Ltd , https://www.sunbeambattery.com