2外文翻译(2)

嘉兴学院本科生毕业设计

where Qc is the basic reactive power generated by the power capacitor of the series-connected inductor and capacitor set under the utility fundamental voltage. From (2), it can be found that the proposed active power filter can supply an adjustable reactive power by adjusting the fundamental component of power converter’s output voltage.

Figure 5b shows the equivalent circuit under harmonic frequency.If the frequency is lower than the resonant frequency, then the series-connected inductor and capacitor set is capacitive. However, the series-connected inductor and capacitor set is inductive if the frequency is higher than the resonant frequency. The switching frequency of the power converter is significantly higher than the resonant frequency of the series-connected inductor and capacitor set. As a result, the series-connected inductor and capacitor set acts as an inductor to filter the switching frequency of the power converter.To suppress the load harmonic current, the desired compensating voltage can be derived as: Vah=ZLCILH (3)

where Ilh is the harmonic component of the load current, and ZLC is the impedance of the series-connected inductor and capacitor set. If the power converter can generate a voltage as shown in (3), this voltage is converted into a compensating current that is opposite to the load harmonic current. Hence, the load harmonic current can be suppressed. Equation (3) shows that the desired compensation voltage is dependent on the load harmonic current and the impedance of the series-connected inductor and capacitor set.

From the operation theory of the bridge power converter, the DC bus voltage of a power converter must be higher than the peak value of the utility voltage in a conventional active power filter.However,a series-connected inductor and capacitor set is used to connect in series with the power converter, and it can block the most fundamental component of utility voltage. Hence the DC bus voltage in the proposed active power filter can be smaller than the peak value of the utility voltage, and it is only dependent on the amplitude of the compensating voltage, which is smaller than the peak value of the utility voltage. This means that the DC bus voltage in the proposed active power filter can be significantly reduced as compared with the conventional active power filter. Consequently, the switching power loss and the voltage rating of DC capacitor and power electronics devices can also be reduced. Furthermore, the ripple current of the power converter is dependent on the DC bus voltage and filter inductance. This implies that, the lower the DC bus voltage, the smaller filter inductance required for specified ripple current limitation. Therefore, the filter inductance used in the series-connected inductor and capacitor set is smaller due to the lower DC bus voltage.

- 6 -

嘉兴学院本科生毕业设计

Compared to the conventional active power filter, it can be seen that the proposed active power filter uses three additional AC capacitors to reduce the inductance of the three-phase filter inductor,the voltage rating of the DC capacitor, the voltage rating of the power electronics devices and the size of the heatsink. In practice,the core of an inductor with large inductance is made from the iron alloy, which results in large size, heavy weight and large power loss. In contrast, the core of an inductor with small inductance can be made from ferrite materials, which have small volume, light weight and low eddy current loss [15]. The electromagnetic interference (EMI) generated by the switching of the power converter, is also dependent on the DC bus voltage. Therefore, the salient advantages of the proposed active power filter are low voltage rating of the DC capacitor and power electronic devices, smaller filter inductance, smaller dimension, light weight, good filter performance and low EMI.In addition, the smaller filter inductance can improve the high-frequency response performance of this active power filter. Since the capacity of the DC bus voltage is dependent on the amount of compensation current, rather than the utility voltage, the application of the proposed active power filter can be extended to a wider voltage range. In a limited variable voltage application, such as in the range 220–480 V, the change in the main components is only the voltage rating of the series-connected inductor and capacitor set. However, the voltage rating of both active and passive components must be changed in the conventional active power filter. In addition, the proposed active power filter can be applied in 50/60 Hz power systems by adjusting only the parameters of the control circuit. However, the L-C parameters of the passive power filter must be changed in the hybrid power filter since the dominant harmonic frequency is different in 50/60 Hz power systems.

The above indicates that the proposed active power filter has the performance of suppressing harmonic current and providing an adjustable reactive current. When the active power filter is operated under light load, the harmonic load current is small. The power converter is used mainly to generate a fundamental voltage to reduce the reactive current supplied by the proposed active power filter. When the proposed active power filter is operated under heavy load, the power converter is not used to generate the fundamental voltage but a harmonic voltage. Thus, the entire fundamental component of the utility voltage will drop on the series-connected inductor and capacitor set that produces a maximum reactive power current. To improve the input power factor at the utility current side, the active power filter is able to produce an adjustable reactive current according to either the light or heavy load conditions. Consequently, the proposed active power filter can improve the disadvantage that the reactive power generated from the conventional hybrid power filter is constant. Compared to the

- 7 -

嘉兴学院本科生毕业设计

conventional active power filter, the proposed active power filter has cost advantage due to the lower voltage rating of the DC capacitor, lower voltage rating of power electronic devices and the smaller size of the heat-sink. Hence, the hardware cost of the proposed active power filter is very competitive for nonlinear loads whose input is a diode-rectifier or phase-controlled rectifier with a low-level voltage below 480V.

3 Control method

Conventionally, the active power filter has been controlled by the current mode. However, this is very difficult to implement under low filter inductance because of the high switching ripple, and it may generate multiple crossings during a carrier period of the pulse-width modulator. This phenomenon of multiple crossing will result in more than one switching operation during a carrier period. To resolve this, the proposed active power filter uses voltage-mode control. The three-phase power converter controlled by voltage-mode control acts as a voltage amplifier with the gain represented by:

Kcon=Vdc/2Vtri (4) where Vdc is the DC bus voltage and Vtri is the amplitude of the carrier signal of the pulse-width modulator. Hence, the control circuit of the voltage-mode controlled power converter is used to determine a reference voltage by dividing the desired compensating voltage by the gain shown in(4). From the above, it can be found that the desired compensating voltage generated by the power converter for filtering the load harmonic current is derived in(3). Hence, the first control signal v1(t) can be further derived from (3), and it is represented as:

(5)

Where L and C are the inductance and capacitance of the series-connected inductor and capacitor set,and R is the component stray loss of the active power filter.

If the power converter can generate a harmonic voltage equal to the first control signal v1(t) and convert it into a compensating current by the series-connected inductor and capacitor set, then theoretically the harmonic components of the load current can be compensated. However, the filter performance is degraded due to the parameters of the series-connected inductor and capacitor set, which may vary due to age, variable frequency, production and temperature in practice. To improve the compensating performance, a second control loop must be used to modify the error of the compensating

- 8 -

嘉兴学院本科生毕业设计

result. The concept of the second control loop is based on the theory of the conventional hybrid power filter[10

–12]

. The second control signal v2(t) is obtained by detecting the harmonic components of the

utility current and then amplifying with a gain (k1), and it can be represented as: v2(t)=k1ish(t)

(6)

where ish(t) is the harmonic component of the utility current.If the power converter can generate a voltage equal to the second control signal v2(t), the utility harmonic current can be derived from Fig.4b and represented as:

(7)

where ILh is the harmonic component of the load current.From (7), it can be found that a term k1 is added to the denominator when the power converter generates a voltage as the second control signal v2(t). Hence, the second control loop is used to control the power converter to act as a virtual harmonic resistor k1. The virtual harmonic resistor k1 is in series with the utility to block the uncompensated harmonic components of load current flowing back to the utility[12]. In the proposed active power filter, the first control loop acts as the rough tuning, and the second control loop is used for the fine tuning. Owing to the use of voltage-mode control in the proposed active power filter, the series-connected inductor and capacitor set may result in high-frequency oscillation between the utility and the active power filter. Hence,a third control loop is applied to avoid high-frequency oscillation.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.The virtual harmonic resistor acts as a harmonic damper. This can be realised by using the power converter to generate a harmonic voltage that is proportional to the harmonic components of the active power filter current.Hence, the third control signal v3(t) can be represented as:

v3(t)=k2iah(t) (8)

where iah(t) is the harmonic current component of the active power filter. Hence, the power converter can act as a virtual harmonic resistor (k2).

A DC capacitor located at the DC bus of the voltage-source power converter is used to supply a DC voltage to the power converter and act as an energy buffer. The DC bus voltage is expected to be a constant voltage. However, the virtual harmonic resistor in the second and third control loops and the switching loss of power converter will consume the real power. Then, the voltage variation in the DC bus cannot be avoided. To maintain a constant DC bus voltage, the fourth control loop is used. Hence,

- 9 -

嘉兴学院本科生毕业设计

the voltage regulation of the DC bus voltage can be obtained by using the power converter to generate a fundamental voltage in phase or out of phase with the fundamental component of the active power filter current. The fourth control signal v4(t) can be represented as:

v4(t)=k3ia1(t) (9)

Whrer ial(t) is the fundamental component of the active power filter current.Then,the power converter acts as a positive/negative fundamental resistor absorb/regenerate the real power from/to the utility,to maintain the DC bus voltage at a constant value.

To adjust the reactive power supplied from the active power filter, the power converter must generate a fundamental voltage that is in phase with the utility fundamental voltage. Equation (2) shows that controlling the amplitude of this fundamental voltage can control the compensating reactive power.The fifth control signal v5(t) can be represented as: v5(t)=k4vs1(t)

(10)

where vs1(t) is the fundamental component of the utility voltage. The major function of the proposed active power filter is the harmonic suppression, and the reactive power compensation is the minor function. Hence, the priority of the harmonic suppression is higher than the compensating reactive power, and the compensating reactive power is proportional to the amplitude of load harmonic current. The maximum compensating reactive power is the basic reactive power for the power capacitor of the series-connected inductor and capacitor set under the utility fundamental voltage,and the power converter generates no fundamental voltage that is in phase with the utility voltage under full-load condition.

Finally, the reference signal for suppressing harmonic current and adjusting the reactive power can be obtained and represented as:

Vref(t)=v1(t)+v2(t)+v3(t)+v4(t)+v5(t) (11)

4 Control Block Diagram

Figure 6 shows the control block diagram of the proposed active power filter. It consists of five control loops. From Fig. 6, it can be found that five feedback signals, namely the load current, the utility current, the output current of power converter, utility voltage and the DC bus voltage, are used in the control circuit of the proposed active power filter to calculate the reference voltage of the power converter. The first control loop is used to implement the product of harmonic components of the load current and the impedance of the series-connected inductor and capacitor set shown in (5). The load current is detected

- 10 -

百度搜索“77cn”或“免费范文网”即可找到本站免费阅读全部范文。收藏本站方便下次阅读，免费范文网，提供经典小说综合文库2外文翻译(2)在线全文阅读。