A New Control Structure for Hybrid Power Filter to Reduce the Inverter Power Rating

Lucian Asiminoaei, Wojciech Wiechowski, Frede Blaabjerg Tomasz Krzeszowiak, Bartosz Kedra

Institute of Energy Technology,

Aalborg University, DK-9220, Aalborg SE,

DENMARK las@iet.aau.dk, wtw@iet.aau.dk, fbl@iet.aau.dk

University of Science and Technology Faculty of Electrical Engineering,

30-059 Krakow, POLAND

tkrzeszowiak@op.pl, bartlomiej@vp.pl

Abstract − This paper describes a control structure of a hybrid power filter, which can be used in current harmonic mitigation. The selected hybrid filter topology consists of an active power inverter connected between the capacitor and inductor of a shunt passive filter. In this paper the inverter power rating is seen as the main factor of interest, because it determines the overall cost of the installation. The aim of the proposed control is to reduce the amount of power losses in the inverter by using a current loop that controls the fundamental reactive power. The principles of operation as well as design considerations of the presented harmonic mitigation solution are presented. Several comparisons are given to describe the advantages and limitations of the proposed topology. Experimental results confirm the theoretical analysis on a laboratory setup with a 7 kVA power inverter. Keywords – pulse width modulated inverters; active filters; power system harmonics; reactive power; losses.

I. INTRODUCTION

The harmonic currents are caused mostly by the AC/DC power conversion units, widely used in both home and industry applications. The harmonic currents affect the supply system and determine power losses, possible malfunction of equipments and resonances in the network.

Traditionally, passive filters were used to deal with the harmonics because of their initial low cost and simplicity. However, the disadvantages are increased power losses at fundamental frequency, dependence of the filtering characteristic on the grid impedance, parallel resonances, parameter tolerance and aging. These issues are the main driving forces for the development of the active power filters [1].

The active power filters mitigate the harmonics with much better accuracy [1], but their cost is relatively higher than passive filters. This disadvantage limits them from extensive utilization in industry [2]. To minimize the cost, and to retrofit existing passive power filter installations, various types of hybrid topologies were presented and successfully implemented in the recent years [3]. The hybrid filters mitigate the harmonic currents relatively well while their cost is considerably reduced compared to a pure active power filter solution, because of the lower rating of the power inverter.

Fig. 1. Principle diagram of the analyzed hybrid power filter composed of

a shunt passive filter and an active power filter connected between the capacitor and inductor.

Furthermore, the power inverter and its control structure may solve the issues met in passive filters, as tolerance, temperature variation, etc. One interesting topology is presented in [3] where the inverter is placed in series with the passive filter without an isolation transformer, which reduces even more the cost of the topology.

In [4] the inverter is placed in parallel with the passive filter as it is shown in Fig. 1, which makes the inverter of a smaller size than in [3], because not all the reactive current is flowing through the inverter. The reactive current that circulates through the inverter is in direct relation to the ratio of the inductances LAPF respective LPPF.

This paper presents a solution to reduce the reactive current that passes the power inverter by using a proper control loop for the fundamental frequency current. It also describes the proposed control and its implementation in dq-frame. The implementation of the control is almost of the same complexity as typical hybrid power filter, although an extra set of current sensors is required.

This paper describes also several comparisons of the proposed hybrid filter with the pure shunt active power filter. The results show that the reactive current can be controlled such that it passes mainly in the passive filter while the power inverter is used for controlling the harmonic mitigation. This reduces the power rating of the inverter to less than 10 % compared to a pure active power filter.

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II. REDUCTION OF INVERTER POWER RATING The power switches and the associated passive

components (front-end inductor, dc-capacitor, snubbers) must be rated based on both, on the inverter output current and the dc-link voltage.

A. Active Power Filter (APF)

The pure active power filter (Fig. 2) is a state of the art active solution for current harmonics mitigation. The topology is well covered in literature and it is already used in practical applications. It consists of a power inverter connected to the power system in shunt connection. It may be connected in front of either single non-linear loads or at common buss-bar and it has the task to detect and cancel out the harmonic content of the load current IL. The most important drawback of this topology is the high voltage rating, since the inverter has to operate with the maximum supply voltage. Thus for a line-to-line voltage of 400 V, the dc-voltage is around 700 V (depending on the front-end inductance LAPF and the load current di/dt). This gives high power losses, high voltage rating of the power inverter, and increased EMI into the system, all giving a higher cost of the inverter. Usually a switching frequency filter must be installed at the output to remove the high frequency components generated by the inverter. The advantage of the APF is that the inverter can compensate harmonic currents as high as 50th order if the switching frequency is also of a high value, of 15-20 kHz, although it considerably increases the switching losses. Fig. 3 shows the simulated inverter current IAPF respective the source current IS. The shape of the inverter current shows that the APF compensates a wide harmonic current spectrum. Therefore, there are two design requirements for the power inverter, to cope with the high current gradients and to withstand high output current peaks.

Fig. 2. Principle diagram of the shunt Active Power Filter connected to

the power system in front of the non-linear load.

Fig. 3. Steady state simulated waveforms (source current IS, inverter

current IF) of the shunt Active Power Filter.

B. Hybrid Power Filter I (HPF I)

Fig. 4 shows a hybrid power filter [3], referred to in this paper as HPF type I. It consists of a shunt passive filter in series with an active filter. The isolation transformer, which is usually installed between the active and passive parts, is removed to reduce the cost and power losses. The passive filter is designed in such a way that it has low impedance for the 5th harmonic order and at the same time provides reactive power compensation. The active filter improves the harmonic mitigation performance of the PPF, compensates the variation of the parameter due to the temperature, tolerance, aging, and detunes possible parallel resonance between PPF and power system. The advantage provided by HPF I compared to pure shunt active power filter is that the fundamental voltage drops on the passive filter components CPPF and LPPF, which allows the inverter to operate with a significantly lower dc-link voltage (for a 400 V line-to-line, the dc-voltage is typically 20 V − 50 V). This provides significant reduction in the inverter power rating compared to the pure active solution. Nevertheless, there is still a large current flowing through the inverter due to the series connection with the passive filter. All reactive power drained by the passive components circulates within the switching devices, which negatively affects the inverter power rating. Fig. 5 shows the simulated inverter current IAPF respective the source current IS for the HPF I topology. The shape of the inverter current shows that the APF compensates only a narrow harmonic spectrum, in the low range, due to the selectivity imposed by the PPF, which allows a lower switching frequency to be used, 5-10 kHz. The current ripple created by the APF is of a low value (if the inverter is controlled as a voltage source), due to the filtering effect provided by the PPF. A fundamental component can also be seen, which is the reactive current passing through the inverter due to the PPF.

Fig. 4. Principle diagram of Hybrid Power Filter (type I) connected to the

power system. The power inverter is connected in series with the PPF.

Fig. 5. Steady state simulated waveforms (source current IS, inverter

current IF) of the Hybrid Power Filter (HPF I) shown in Fig. 4.

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C. Hybrid Power Filter II (HPF II)

In order to eliminate the issue of the reactive power passing the inverter, a second topology is proposed in [4], as it was shown in Fig. 1. In this case the connection point of the inverter is placed between the passive elements CPPF respective LPPF.

It is required that CPPF is connected to the system side, in order to have the benefits of the previous hybrid topology, i.e. reduced dc-link voltage due to the voltage drop at the fundamental frequency. For the inverter to be able to operate the dc-link voltage has to be higher than the input voltage, which now depends on the voltage drop across the passive inductor LPPF. Depending on application the dc-link voltage is in the range of 50 V − 100 V, although lower values can also be obtained.

The capacitor CPPF allows the APF to compensate a wider harmonic current spectrum than in the case of HPF I due to its decreased reactance. Thus, the APF, besides the role of improving and controlling the tuning characteristic of the PPF, it may also provide mitigation of other higher harmonic currents. On the other hand, the current switching ripple that sinks into the power system becomes higher than in HPF I.

For this reason, a supplementary inductor LAPF is connected in front of inverter acting as a boost inductor, smoothing also the inverter current switching ripple. The value of the inductor is a design parameter that influences the reactive current passing the power switches. A value of LAPF comparable or even higher than the PPF’s inductance LPPF is advisable in order to split the total reactive current flow and reduce the inverter current.

In addition, the presented HPF II has a simpler protection sequence in the case of APF failure or maintenance stop, by simply disconnecting the APF, while the PPF may continue its operation. In HPF I the disconnection of the APF does not allow the PPF to operate unless a crowbar is connected to provide a close path for the PPF currents. This requires a more expensive and complex protection scheme.

Fig. 6 shows the simulated inverter current IAPF respective the source current IS for the HPF II topology. The shape of the inverter current shows that the APF provides only a small amount of harmonic currents. Most of the low order harmonic currents are mitigated by the PPF. Besides the harmonic currents the inverter has also to carry a fundamental component. As it can be seen in Fig. 5 the value of the reactive current passing through the inverter is reduced, which consequently reduces the power losses.

Fig. 6. Steady state simulated waveforms (source current IS, inverter

current IF) of the Hybrid Power Filter (HPF II) shown in Fig. 1.

III. PROPOSED CONTROL FOR HYBRID POWER FILTER

A common control method for hybrid power filters is the feedback current control. The source current IS is detected and the inverter is controlled as a voltage source [3]. Eq. (1) can be written if one considers the notations in Fig. 7a. It allows to solve for the source current IS (2) as a function of the source voltage f(VS(h)) and load current f(IL(h)).

1 2

2

2

1

||

PPF

PPF

APF

PPF PPF APF

S L HPF

HPF APF PPF

HPF C

PPF L

APF APF L

APF APF S

S S S

HPF C L L

I I II I IV V I X

V I X

V V I X

V H IV V I ZZ X X X

= − = + = +

= = + =

= − = +

(1)

( ) ( )( ) ( ) ( )S L SI h f I h f V h= + (2)where:

( )

( )

h>1

( ) ( )

1( ) ( ) 0

HPFL L

PPFHPF APF S

PPF APF

S SPPF

HPF APF SPPF APF

Zf I h I h LZ H ZL L

f V h V h LZ H ZL L

= −+ +

+

= − ≈ + + +

(3)

If it is assumed that the existing harmonic source voltage is negligible, VS(h)≈0 for h>1, then the source current depends mainly on the load current IL. The transfer function f(IL(h)) includes the characteristics of both active and passive power filters, which allows to control the source harmonic currents by proper tuning of both. In [4] HAPF is a proportional constant (with dimension of impedance), which makes the effect of the active inverter similar as a damping resistance in series with the power system.

(a)

(b) Fig. 7. Simplified diagrams of the control loops used for HPF II. a) Close loop control of the harmonic current, where the inverter acts as a classical voltage controlled source. b) Additional current control loops, where the inverter is controlled as a current source at the fundamental frequency to

regulate the reactive current passing trough the APF.

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The approach used in this paper for the transfer function HAPF is generalized harmonic integrators [7].

2 25,7,11,13 5,7,11,13

2( ) ( ) kAPF GIk

k k k

K sH s H ss ω= =

= =+∑ ∑ (4)

where: Kk and ωk are the integral constant respective the angular frequency of the considered resonant controllers.

The harmonic integrator integrates the input signal only if its frequency is inside the bandwidth of tuned orders ωk. Here 4th harmonic orders are set for mitigation, 5, 7, 11, and 13. As the harmonic integrator ideally reaches an infinite magnitude at the selected harmonic, the transfer function of the source current converges to zero:

( )5,7,11,13 5,7,11,13( ) ( ) 0APF Sk k

H s f I h= =

→ ∞ ⇒ → (5)

Regarding the control of the fundamental current (including the reactive current generated by PPF) a different approach is used. The fundamental current is required for the APF to keep its dc-link capacitor charged at the reference value [6]. If the same voltage source control method is used, then the reactive current determined by CPPF splits between LPPF and LAPF according to their ratio. This makes the boost inductor LAPF to be of a large inductance in order to have a small amount of reactive current passing trough the APF. The consequences are increased dc-voltage rating, losses and cost.

The approach used in this paper is to add an inner current loop that changes the behavior of the APF from a voltage source control into a current source control, only at the fundamental frequency. The principle is illustrated in Fig. 7b. Thus, if the APF is current source controlled at the fundamental frequency (i.e. 50 Hz) the reactive current cannot pass through the inverter and is forced to run through the inductor LPPF. The principle is similar as replacing the power inverter in Fig. 7b with an infinite impedance characteristic of the current source model.

The fundamental current loop requires additional current sensors for measuring the inverter output current IF. However, the current sensors may be needed anyway for logic protections in case of over-current or short-circuit.

IV. EXPERIMENTAL RESULTS

The general diagram of the experimental setup is shown in Fig. 8. The setup is realized with a 7 kVA, 400 V, VLT5006 Danfoss inverter.

Fig. 8. Electrical diagram of the experimental setup for testing the

proposed control of the hybrid power filter.

The boost inductor has LAPF=5.82 mH inductance and RAPF=0.35 Ω series resistance. The dc-link capacitor has Cdc=2.2 mF capacitance. The switching frequency is 10 kHz. The control algorithm is implemented in Matlab/ Simulink, and executed on a dSpace DS1103 platform [10].

A 5.5 kVA three-phase dc-smoothed diode rectifier that replicates the behavior of a typical Non-Linear Load produces the harmonic currents.

The passive power filter, i.e. components CPPF and LPPF is tuned at 230 Hz for mitigation of the 5th harmonic current, CPPF= 65 µF, LPPF= 7.3 mH. Since the active filter can control possible dangerous resonances between the PPF and the power system inductance LS the design procedure is highly simplified. The classical issues of parameter tolerance, variation with temperature and aging are automatically compensated by the APF, though they still have to be considered if the APF stops and the PPF must operate alone.

The control algorithm is developed in the synchronous dq-reference frame. The input signals (IS, IF, VS), which are initially achieved in the stationary system, are transformed into the dq-rotating reference frame by means of the Park transformation. The frame rotates at fundamental angular frequency that makes the fundamental current as dc-component and the harmonics as ac-signals. Thus, the harmonic detection becomes a matter of removing the dc-signal by means of a high pass filter [5].

The block diagram of the proposed control (Fig. 9) is a typical implementation of an APF having the current controller in the inner loop and the voltage controller in the outer loop [6]. The current control is realized as explained in §III, in a combined structure with a classical proportional-integral (PI) controller for fundamental current and resonant controllers, one for each harmonic pair k = 6n±1 [7]. In dq-frame rotating with the fundamental frequency, the characteristic harmonics 5th and 7th are folded in the 6th order, 11th and 13th in 12th, etc. This reduces the number of harmonic current controllers from 4, i.e. 5th, 7th, 11th, 13th as in (4) to only 2, i.e. 6th and 12th:

2 26,12 6,12

2( ) ( )dq dq kAPF GIk

k k k

K sH s H ss ω= =

= =+∑ ∑ (6)

A more detailed description of the inner harmonic current controllers and the controller parameters are given in [8]. The total voltage reference is the superposition of commands produced by all current controllers [9].

Fig. 9. Control diagram of the hybrid power filter control. Two inner current loops are implemented, one for the harmonic source current

mitigation and one for the fundamental current control.

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Fig. 10. Measured waveforms in steady state conditions of the PPF connected alone for mitigation of the harmonic currents from ASD.

Regarding the inner current controller set for the fundamental frequency to charge the dc-capacitor, two tests are performed. In the first test the inverter is controlled as a voltage source by opening the feedback connection of the output current IF in position “Vc” (see Fig. 9); at the same time setting to zero the integral component of the PI controller in PICC.

In the second test the inverter is controlled as a current source by closing the feedback connection of the output current in position “Cc”.

The dc-voltage control loop is a typical PI controller. Its output is the dc-current references in the q-axis, which determines a real power to be drained by the APF for keeping the dc-voltage at the imposed value. Usually in a pure active power filter the dc-reference appears on the d-axis, which has the significance of real power, but in this topology the capacitor introduces a 90° phase-shift, which is equivalent of moving the dc-reference into q-axis.

Fig. 10 shows the measurement of the load current from the Non-Linear Load and the effect of the PPF alone. The filter current is the 5th harmonic as the PPF is tuned for the 5th harmonic order, and also a large reactive current, drained by the filter capacitor. The total RMS current in the PPF is around 5 A. As the PPF cannot remove other higher harmonic orders, they sink into the source, IS.

Fig. 11a shows the effect of the entire hybrid passive filter for the APF controlled as a voltage source at the fundamental frequency. It can be seen that the inverter

output current IAPF, contains harmonic currents of higher orders, in accordance with the designed controller structure. The quality of the source current is improved, reaching a THDi of 3 %. It is also noted that the magnitude of the PPF current is reduced to 3.8 Arms (Table I), which is an indication that the initial drained reactive current of 5 A now divided between the PPF and APF.

Fig. 11b shows the second test when the inverter is controlled as a current source at the fundamental frequency. The source current remains almost the same, which proves that the HPF does not change its overall performance. However, the amplitude of the inverter current IAPF is reduced, while the amplitude of the passive filter IPPF is increased (see Table I). This proves that the reactive current closes mainly through the PPF inductor LPPF, and not through the APF. For the tests performed in this paper, the power of the inverter controlled as a current is 40 % lower than the voltage controlled source topology.

Fig. 12a shows the measured current spectra for the case of PPF connected alone to the system, while Fig. 12b shows the measured current spectra for the case of HPF with current source controlled inverter at the fundamental frequency. The spectrum of the inverter current IAPF indicates only a small amplitude of the 5th harmonic current. The main part of the 5th harmonic sinks into the PPF. The inverter provides also the compensation of other higher harmonic orders, i.e. 7th, 11th and 13th.

It is noticed that the APF changes the harmonic spectrum of the passive filter current IPPF. The 7th harmonic current is higher compared to the case of PPF alone. The explanation resides in the way the harmonic current generated by the APF propagates. An illustration is provided in Fig. 13. The harmonic current splits depending on the impedance ratio of both current paths, towards the power system and the inductance LPPF, as in (7).

TABLE I. Measured currents with APF’s fundamental current control set to voltage, respective current control mode. The load current is 6.8 Arms.

Current [Arms] Passive filter

alone

APF with fundamental

voltage controller

APF with fundamental

current controllerSource: IS 8.83 8.78 8.77

Active filter: IAPF 0* 2.16 1.29 Passive filter: IPPF 5.07 3.78 4.69

* APF is disconnected

(a)

(b)

Fig. 11. Measured waveforms in steady state conditions of the HPF II. The control of the fundamental frequency of APF is done as a) fundamental voltage controller, b) fundamental current controller.

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(a)

(b)

Fig. 12. Measured spectrum (the scale is the same for all plots, set as 1 A/division for y-axes as 250 Hz/division for x-axes) of the source current IS, inverter current IAPF, passive filter current IPPF and ASD current IL for a) APF disconnected and PPF operating alone, b) APF connected and operating with the

proposed fundamental current controller.

Fig. 13. Illustration of the split of APF harmonic current between the power

source and the passive filter inductance LPPF.

,

, 1

( ) PPF

PPF PPF

S C SPPF

S C L S PPFh

Z LI hZ X L L

>>

= ≅+ +

(7)

Assuming that the capacitor has a much smaller reactance compared to the source inductance LS, then the part of the current that sinks into the passive inductor LPPF can be limited in the design stage by a proper selection of the LPPF inductance. A value of 5-10 times higher than the system inductance LS provides a satisfactory result.

V. DISCUSSION

An evaluation of the power losses is done based on the measured data in §IV. Considering the same non-linear load with a current distortion THDi of 26 %, four cases of harmonic mitigation are compared: a) pure active power filter, b) hybrid power filter HPF I, c) HPF II controlled as voltage source at fundamental frequency, d) and HPF II controlled as a current source at fundamental frequency.

The inverter power is calculated in (8), assuming the modulation index is 1.

,3 3APF LL APF APF dc APFP V I V I= = (8)The inverter current of the pure APF is estimated based

on the harmonic current distortion, i.e. IAPF= IL⋅THDi. For the case of HPF I, the inverter current is assumed the same as of PPF alone (see §IV), while its dc-voltage is taken of 50 V. The dc-voltage of HPF II is taken of 100 V, recognizing that HPF II requires a higher dc-voltage than HPF I. Both hybrid topologies may operate with even lower dc-voltages, but here a worst-case scenario is assumed. Table II shows the estimated inverter power in each case. The power of HPF II inverter (voltage controlled) is relatively close to HPF I because even if the reactive current is reduced, the inverter requires higher dc-voltage. However, the HPF II inverter controlled as current source lowers the power rating to 8 % compared to a pure APF.

TABLE II. Comparisons of different active power filtering topologies.

Parameters of active inverter Pure APF HPF I

HPF II fundamental

voltage controller

HPF II fundamental

current controller

IAPF [Arms] 2.3 A 5.1 A 2.2 A 1.3 A Vdc [V] 700 V 50 V 100 V 100 V

SAPF [W] 2788 VA 442 VA 381 VA 225 VA SAPF [%] 100 % 16 % 14 % 8 %

VI. CONCLUSION

This paper describes a hybrid power filter suitable for reducing the power rating of the inverter, which is placed in between the passive components and controlled in a classical approach as a voltage source the inverter. A supplementary reduction of the inverter power rating is achieved in this paper by using a proper current control loop at the fundamental frequency to force the reactive current circulating through the passive part of the HPF. Practical experiments on an existing setup sustain the advantage of the proposed control method. The results show that the reactive current can be controlled such that the inverter power is less than 10 % compared to a pure APF.

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