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嘉兴学院本科生毕业设计

and sent to the band-rejection filter to filter out its fundamental component.

Fig.6 Control block of proposed active power filter

From(5), it can be found that the product of harmonic components of the load current and the impedance of the series-connected inductor and capacitor set can be obtained by feeding the harmonic components of load current to a proportional integral differential (PID)controller. The proportional, integral and differential coefficients are the resistance R, capacitance C and inductance L, respectively,as shown in (5). Then, the output of the first control loop is obtained. To avoid the effect of noise, a low-pass filter is used in the front of the differential controller and a high-pass filter is inserted at the end of integral controller to reject the DC component due to the initial condition. Since, the series-connected inductor and capacitor set is located at the output of the power converter, the capacitor also can block the DC component owing to the initial condition. Hence, the effect of the initial condition caused by the switch-on of the proposed active power filter can be suppressed.

To improve the compensating performance, the second control loop is used to modify the error of the compensating results of the first control loop. In the second control loop, the detected utility current is sent to the band-rejection filter to filter out the fundamental component.Then,the uncompensated harmonic components of the utility current are obtained. The output of the band-rejection filter is fed to the amplifier to obtain the output of the second control loop.

The third control loop is used to generate a virtual harmonic resistor to be connected in series with the series-connected inductor and capacitor set to act as a damper. The output current of the power

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嘉兴学院本科生毕业设计

converter is sent to a bandpass filter to obtain the fundamental component, and then the detected output current of the power converter and its fundamental component are fed to a subtractor to obtain the harmonic components. The harmonic components are fed to an amplifier to obtain the output of the third control loop.

The fourth control loop is used to regulate the DC bus voltage. The fourth control loop is composed of a lowpass filter to filter out the DC bus voltage ripple and a subtractor to subtract a setting value from the output of low-pass filter. After this,the subtractor result is sent to a PI controller. The output of the band-pass filter is the fundamental component of the active power filter current, and the output of the fourth control loop is the product of the output of the PI controller and the output of the band-pass filter. The fifth control loop is used to adjust the compensating reactive power.First, the reactive power of load is calculated. The utility voltage is sent to a phase shift circuit to shift its phase by 90°and then multiplied to the load current; the product is sent to a low-pass filter to obtain the reactive power of the load. Since,the DC bus voltage is constant and the priority of the harmonic suppression is higher than the compensating reactive power in the proposed active power filter, the compensating reactive power must be limited. Hence,a limit circuit is used to restrict the compensated reactive power. The limit value of the limit circuit is varied and depends on the amount of compensated harmonic current. Under the no-load or light-load condition, the amount of compensated harmonic current is small, and the value of the limit circuit is large. This means that the power converter can supply a large reactive power in this condition. However, the value of the limit circuit is nearly zero under the heavy load condition. This means that the maximum reactive power is the reactive power supplied from the series-connected inductor and capacitor set in this condition. The utility voltage and the output of limit circuit are sent to a multiplier to obtain the output of the fifth control loop.

Finally, the modulated signal can be obtained by summing the outputs of the first, second, third, fourth and fifth control loops. Then, the modulated signal is sent to a pulse-width modulator in order to drive the power electronics devices of the power converter.

5 Experimental results

To demonstrate the performance of the proposed active power filter,a three-phase 20KVA prototype was devel-oped. The major parameters of the prototype are shown in Table 1. The utility power is supplied by a three-phase three-wire utility system operating at 380V and 60Hz. A comparison of the proposed active power filter and the conventional parallel active power filter is shown in Table 2. Because the

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嘉兴学院本科生毕业设计

inductance of the series-connected inductor and capacitor set is only 200H, a ferrite core can be used to reduce the power loss, weight and volume. Hence, the volume and weight of the proposed active power filter is clearly smaller than that of the conventional parallel active power filter. In addition, the hardware cost is also reduced significantly owing to the low-voltage rating of DC capacitor and power electronic devices, small inductance of the filter inductor and small dimension of the overall system. The tested load is a six-pulse rectifier charger. Table 1: Major parameters of prototype

Table 2: comparison results

Fig. 7 Test results for proposed active power filter at steady state a Utility voltage b Utility current c Load current

d Active power filter current

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嘉兴学院本科生毕业设计

Fig. 8 Test result of proposed active power filter under transient of applying nonlinear load a Utility voltage b Utility current c Load current

d Active power filter current

Fig. 9 Test results under the condition with large system impedance before applying proposed active power filter a Utility voltage b Utility current.

Figure 7 shows the experimental results of the proposed active power filter in the steady state. The load current shown in Fig. 7c is rich in harmonics and its total harmonic distortion(THD)is 26%. The waveform of the utility current shown in Fig.7b is nearly sinusoidal, and its THD is only 3% after compensation by the proposed active power filter. The test results show that the harmonic suppression

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嘉兴学院本科生毕业设计

performance of the proposed active power filter is excellent.

Figure 8 shows the experimental results of the proposed active power filter under the transient of applying nonlinear load. As seen in Fig. 8b, the proposed active power filter has an excellent transient response.

In an industrial distribution power system, a turbine generator is often used for backup power. However, the power source of the turbine generator can be regarded as a weak power source because the capacity of the turbine generator is not large enough. The system impedance of a weak power source is large, and the utility voltage will be clearly degraded under the condition of nonlinear load. The experimental results shown in Figs.9 and 10 are obtained under the condition of large system impedance. An inductor with 0.6mH is inserted into the utility power feeder to simulate the large system impedance condition. As seen in Fig.9, the voltage waveform of the utility is seriously distorted owing to the nonlinear load. The THD of the utility voltage and the utility current are 9% and 39%, respectively.The distorted utility voltage may disturb the normal operation of the power equipment itself or the neighbouring load in the same power feeder. Fig.10 shows that both the voltage and current waveforms of the utility are nearly sinusoidal after applying the proposed active power filter. The THD of the utility voltage and current are 1.4% and 4.9%, respectively. Hence, this verifies that the proposed active power filter can not only suppress the input current harmonics but also avoid voltage waveform distortion under nonlinear loads.

Fig. 10 Test results under the condition with large system impedance after applying proposed active power filter a Utility voltage b Utility current

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