Abstract
In this paper, a novel active power filter is proposed and implemented by sing a voltage-source power converter with a series connected inductor and capacitor set. The power converter is controlled to generate a compensating voltage that is converted into a compensating current via the series connected inductor and capacitor set. The compensating current flows into the power feeder in order to suppress the harmonic currents
generated by nonlinear loads. The salient advantages of the proposed active
Powerfilter are lower voltage rating of dc capacitor and power switching devices, smaller filter inductor, smaller dimension, light weight, better filter performance and low electromagnetic interference (EMI). A three-phase 100 kVA active power filter is developed to demonstrate the performance of the proposed method. The results show that the proposed active power filter has the expected performance.
I. INTRODUCTION
The power electronic related facilities may generate a large amount of harmonic current due to the nonlinear input characteristic. The harmonic current may pollute the power system causing problems such as transformer overheating, rotary machine vibration, voltage quality degradation, destruction of electric power components, and alfunctining of medical facilities .In order to solve the problem of harmonic pollution effectively, many harmonic limitation standards.
The harmonic current can be suppressed by using a passive or active power filter. Conventionally, the passive power filter is used to solve the problems of harmonic pollution in the industrial power system due to its low cost. However, it has the
Following disadvantages:
1) Sensitive to the variation of power system impedance;
2) Sensitive to frequency variation of the utility;
3) The risk of series/parallel resonance;
4) The filter frequency is fixed, and not easy to adjust.
Among those listed above, the series/parallel resonance is the most serious disadvantage. It may result in over-current/over voltage on the inductor and capacitor causing damage to the passive power filter. Since the system impedance has a significant effect on the performance of passive power filter, it is very hard to obtain an excellent filter performance in practical applications. Moreover, the harmonic current produced by neighboring nonlinear loads may flow into the
Passive power filter and result in the overload of the passive power filter.
Recently, the harmonic suppression facilities based on power electronic technique have been developed. These active harmonic suppression facilities known as active power filter can suppress the different order harmonic components of nonlinear loads simultaneously According to the power circuit configurations and connections, the active power filter can be divided into parallel active filters, series active filters and other filter combinations The parallel active power filter is connected in parallel to the load and the generated compensation current opposes to the load harmonic current to being injected into the power feeder. The parallel active power filter has many configurations .Among these configurations, the standard inverter type is widely used and discussed
Fig.1.system configuration of active power filter (a) standard inverter type parallel active filter, (b) series active power filter, (c) hybrid power filter.
Fig. 1(a) shows the system configuration of the standard inverter type parallel active power filter (conventional parallel active powerFilter). The conventional parallel active power filter can perform the harmonic current suppression, reactive power compensation and balancing three-phase currents. This filter consists of a voltage-source power converter and a filter inductor connected in series. The role of the filter inductor is
used to suppress the high frequency ripple current generated while switching the power electronic devices of the power converter. The inductance of the filter inductor depends on switching frequency, dc voltage, and ripple current limitation. The dc bus voltage must be higher than the peak value of the utility voltage to force the output current of the active power filter under the command of compensating current in the conventional parallel active power filter. The use of high dc bus voltage has many disadvantages such as large filter inductance, and high voltage rating of dc capacitor and power electronic devices. A larger filter inductor will result in significant power loss, more heat dissipation, bulk dimension and weight, and degrades the performance of frequency response. The requirement of high voltage rating of dc
Capacitor and power electronic devices limits high power application of active power filters due to the high power rating of the power converter and cost.
Fig. 1(b) shows the system configuration of the series active Filter. The major advantages of the series active filter over the parallel active power filter are that it can maintain the output parallel active power filter, (b) series active power filter, (c) hybrid power filter
Voltage waveform to be sinusoidal and balance the three -phase voltages. However, the series filter is less popular in the industrial applications due to the inherent drawbacks of series circuits, namely it must handle high load currents, which increases their current rating compared with the parallel active power filters
In some applications, the combinations of several types of filters can achieve greater benefits. The major combinations include parallel active filter and series active filter, series active filter and parallel passive filter, parallel active filter and parallel passive filter, and active filter in series with parallel passive power filter .Among these configurations, the active filter in series with parallel passive filter, also known as the hybrid power filter, is more widely discussed in the literature This
Configurationis shown in Fig. 1(c) where the passive filter filters the dominant harmonic, and the power converter is used to enhance the filter performance and to protect the passive filter from power resonance. Hence, the capacity of the power converter is smaller than that of the parallel active power filter for the same nonlinear load. Besides, the voltage stress applied to the power electronic switches in the power converter is low. As a result, the hybrid filter is suitable to high-power applications. However, the hybrid power filter requires a bulk passive power filter set and a voltage-matching transformer.
Based on the type of compensation, the active power filter can be divided into reactive power compensation, harmonic compensation, balancing of three-phase systems and multiple compensations. The conventional parallel active power filter belongs to multiple compensations, and it can compensate for the harmonic current and reactive power simultaneously .The hybrid power filter belongs to the harmonic compensation, and it only compensates for the harmonic current.
In this paper, a novel active power filter is proposed. The proposed active power filter is implemented by using a voltage-source power converter with a series connected inductor
And capacitorset. The proposed active power filter can be regarded as a new family of the hybrid power filter, combing a parallel active filter and an ac power capacitor. The proposed active power filter has the advantages of lower voltage rating of dc capacitor and power switching devices, smaller filter inductor , smaller dimension, light weight, better filter performance and low EMI. Finally, a three-phase 100kVA prototype is developed to demonstrate the performance of the proposed active power filter.
II. SYSTEM CONFIGURATION AND OPERATION PRINCIPLE
The system configuration of the proposed active power filter is shown in Fig. 2.
.Fig.2.system configuration of proposed active power filter
It consists of a series connected inductor and capacitor set, a power converter and a high frequency ripple filter. The voltage-mode control is used to control the power converter. The power converter generates a compensating voltage that is converted into a compensating current flowing through the series connected inductor and capacitor set, and the compensating current flows into the power feeder in order to filter harmonic currents generated by nonlinear loads. The configuration of the proposed active power filter is similar to that of the hybrid power filter in the first view. However, thefunction and dimension of the passive elements (L-C) are not the same. In the proposed method, the inductor of the series connected inductor and capacitor set is very small, and it is used to filter the switching ripple of the power converter. The capacitor in the series connected inductor and capacitor set is used to supply a fixed reactive power. However, the passive elements (L-C) in the hybrid power filter are used to tune the dominant harmonic component of the load current. The inductance in the hybrid power filter is larger than that used in the proposed active power filter, then, the dimension and weight of the inductor used in the hybrid power filter are also larger than that used in the proposed active power filter. Thehigh frequency ripple filter is configured by a set of capacitorsand resistor, and it uses to further filter out the switching ripple of the power converter.
.Fig.3.Equivalent circuit of proposed active power filter
Fig. 3 shows the equivalent circuit of the proposed active power filter. It consists of two voltage sources, one is the utility and the other is the power converter. The compensating voltage generated by the power converter is a dependent voltage source whose voltage depends on the harmonic components of the load current. The equivalent circuit shown in Fig. 3 can be further divided into the fundamental frequency equivalent circuit and the harmonic frequency equivalent circuit as shown in Fig. 4.
Fig.4.Equivalent circuit of proposed active power filter, (a) fundamental equivalent circuit (b) harmonic equivalent circuit.
Fig. 4(a) shows the equivalent circuit under the fundamental frequency.If the power loss isnegligible; the compensating voltage generated by the power converter only contains harmonic components. Hence, the voltage source of the power converter can be regarded as a short circuit under the fundamental frequency. Also, it is evident that the impedance of the series connected
Inductor and capacitor set is capacitive under the fundamental frequency. Hence, the series connected inductor and capacitor set performs the fixed reactive power compensation. Since the inductance of the series connected inductor and capacitor set is
Very small in the proposed active power filter, the compensating reactive power can be approximated as
(1) Is the capacitance of the series connected inductor and capacitor set, and isthe RMS value of the utility line voltage is. Fig. 4(b) shows the equivalent circuit under harmonic frequency. If the frequency is lower than the tuned frequency, the
Seriesconnected inductor and capacitor set is capacitive. On the contrary, the series connected inductor and capacitor set is inductive if the frequency is higher than the tuned frequency.
The switching frequency of the power converter is significantly higher than the tuned 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. For suppressing the load harmonic current, he desired compensating voltage can be derived as
(2) Where is the harmonic component of the load current, is the impedance of the series connected inductor and capacitor set. If the power converter can generate a voltage as shown in (2), then this voltage is converted into a compensating current that is opposite to the load harmonic current. Hence, the load harmonic current can be suppressed. As shown in Fig. 4(a), the fundamental component of utility voltage drops on the series connected inductor and capacitor set, hence, the compensating voltage generated by the power converter consists only the harmonic components. In addition, (2) shows that the desired compensation voltage is dependent on the load harmonic current and the impedance of the series connected inductor and
Capacitorset. This value is smaller than the peak value of the utility voltage. 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 compensating voltage. Because the peak value of compensating voltage is smaller than that of theutility voltage, the dc bus voltage in the proposed active power filter can be reduced significantly as compared with the conventional parallel active power filter whose voltage must behigher than the peak value of the utility voltage. Consequently, the voltage rating of dc capacitor and power electronic devices can also be reduced. Besides, 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. Besides, the high frequency response of the proposed active power filter is better than that of the conventional parallel
Active power filters due to the smaller filter inductance. Compared to the conventional parallel active power filter, it shows that the proposed active power filter uses three additional ac capacitors to reduce the inductance of filter inductor. In practice, the core of an inductor with large inductance is made from the iron alloy, which results in the bulky volume, heavy weight and large loss. The core of an inductor with small inductance can be made from the ferrite materials, which have the characteristics of small volume, light weight and low eddy current loss .The EMI, generated by the switching of 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 dc capacitor and power switching devices, smaller filter inductance, smaller dimension, light weight, good filter performance and low EMI. Besides, 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 and not the utility voltage, the application of the proposed active power filter could be extended to a wider voltage range. In the limited variable voltage application, such as 220 V to 480 V, the change in the main components is only the voltage rating of series connected inductor and capacitor set. In addition, the proposed active power filter can be applied in50/60 HZ power systems only adjusting the parameters of the control circuit. For the conventional active power filter, the voltage rating of both active and passive components must be changed. The hardware cost of theproposed active power filter is very competitive in the nonlinear loads whose input is a diode-rectifier or phase-controlled rectifier with a low level voltage below 480 V. The conventional parallel active power filter can supply the reactive power as the variation of the load, and performs the unity power factor compensation. Nevertheless, the hybrid active power only supplies a fixed reactive power. This results in the leading power factor, as the load condition is light. The reactive power compensation performance of the proposed active power filter is similar to the hybrid power filter supplying a fixed reactive power.
III. CONTROL THEORY
Conventionally, the active power filter was controlled by the current-mode. However, it is hard to be implemented under low filter inductance due to the high switching ripple, and it may generate multiple crossing during a carrier period of pulse-width modulator. The phenomenon of multiple crossing will result in more than one switching operation during a carrier period. In the proposed active power filter, the voltage-mode control is used. The three-phase power converter controlled by the voltage-mode control acts as a voltage amplifier with the gain represented by
(3)
Is the dc bus voltage and is the amplitude of the carrier signal of the pulse-width modulator. Hence, the control circuit of the voltage-mode controller is used to determine a reference voltage by dividing the desired compensating voltage by the gain shown in (3). From the above section, it can be found that the desired compensating voltage generated by the power converter is derived
From(2). Hence, the first control signal where can be further derived from (2), and it is represented as
(4)
Where L and C are the inductance and capacitance of the seriesconnected inductor and capacitor set respectively, and R is the stray loss of active power filter. If the power converter can generate a harmonic voltage equal to the first control signal and convert into a compensating current by the series connectedinductor and capacitor set, the harmonic components of the load current can be compensated theoretically. In practice, the filter performance is degraded due to the parameters of the series connected inductor and capacitor set that may be varied due to age, variable frequency, production and temperature. For improving the compensating performance, the second control loop must be used to modify the error of compensating result. The concept of the second control loop is based on the theory of conventional hybrid power filter .The second control signal is obtained by detecting the harmonic components of the utility current and then amplifying with a gain , and it can be represented as