A 6.6-kV Transformerless Shunt Hybrid Active Filter for Installation on a Power Distribution...

2004 35th Annual IEEE Power Electronics Specialists Conference Aachen, Germany, 2004

A 6.6-kV Transformerless Shunt Hybrid Active Filter for Installation on a Power Distribution System

Rubin Inzunza, Student Member, IEEE Tokyo Institute of Technology

Hirofumi Akagi, Fellow, IEEE Tokyo Institute of Technology

Email: [email protected] Email: [email protected] Tokyo, Japan Tokyo, Japan

Abstract-This paper presents a fully-digital-controlled shunt hybrid filter for damping of harmonic propagation in power

onance between line inductances and power capacitors installed

harmonic is based on installation of a shunt pure active filter a t the end of a feeder. This nauer urnnoses a shunt

composed of series connection of a 7th-tuned LC filter per phase and a small-rated three-phase active filter. Due to the

dimihution system. The harmonic propagation is caused by res- presence of the LC filter, the fundamental voltage is &coupled

for power from the distribution system, thus reducing the voltage rating A possible solution to danlping of the active filter. This remarkable feature does not require anv transformer for connectine the hvbrid filter to the 6.6-

_ I . . hybrid acbive filter characterized by series connection of a 7th- tuned LC filter per phase and a small-rated three-phase active filter. Like the pure filter, the hybrid filter is connected to the end bus of a feeder. The capacitor of the LC filter imposes a high impedance tu the fundamental frequency, so that the fundamental voltage appears across the capacitor. This unique feature allows us to directly connect the hybrid filter to the 6.6- kV power line without stepdown transformers. Furthermore, the capacitor used in this hybrid filter is lighter, cheaper and smaller than the transformer used in the pure filter. Theoretical analysis, along with experimental results obtained from a ZOO-V, 20-kW laboratory system, verifies the viability and effectiveness of the proposed hybrid filter.

1. INTRODUCTION

In recent years, harmonic propagation has become a se- rious problem in power distribution systems. It consists in harmonic-voltage amplification due to resonance between line inductances and shunt capacitors installed for power factor correction. The worst case occurs under no-load conditions from a theoretical point of view. Thus, i t may happen at night in a real power distribution system under light-load conditions. This harmonic propagation effect has been studied and measured in actual power distribution systems [11-[2].

In order to damp out harmonic propagation, a shunt pure active filter based on voltage detection has been proposed and tested [3]-[8]. This active filter is controlled in such a way as to act as a damping resistor for harmonic frequencies. It has been'found that the best site to install the active filter is at the end bus of a distribution feeder, thus achieving harmonic damping throughout the distribution feeder [3].

The harmonic propagation may appear in several feeders within a power distribution network. This fact makes the treatment of this problem costly and technically difficult. The cost of installing several active filters dispersed on the network should be assumed by the utility company. On the other hand, the physical size of a conventional shunt pure active filter is too large to be installed on electric poles. To overcome the harmonic propagation problem. the size and cost of the present solution must be reduced drastically. One of the main obstacles is the transformer required for the shunt pure active filter.

This paper proposes a new alternative to harmonic damping in a 6.6-kV system. It consists of a shunt hybrid active filter

- kV bus. The replacement of the transformer by the capacitor included in the LC filter makes the hybrid filter compact, light and inexpensive.

Hybrid active filters have been researched and tested [9]- [14]. It is well known that a hybrid filter configuration helps to reduce the active filter rating. Nevertheless, the solutions presented in the available literature are intended, not for harmonic damping in power distribution systems. but for harmonic compensation of non-linear loads. For example, a hybrid filter in [I51 and [I61 is the same in circuit configuration as that in this paper, but different in control strategy.

The hybrid filter in this paper is based on voltage detection, and behaves as a damping resistor for harmonic frequencies. It is directly installed at the end of a 6.6-kV distribution feeder, for the purpose of mitigating harmonic propagation throughout the feeder. Experimental results were obtained with a fully-digital-controlled shunt hybrid active filter connected to a three-phase distribution line simulator rated at 200 V, 50 Hz and 20 kW. Theoretical analysis, along with experimental results, verifies satisfactory damping performance of the proposed hybrid filter.

11. ACTIVE FILTERS FOR HARMONIC DAMPING A. System Configuration

Fig. I shows a 6.6-kV simplified power distribution feeder under no-load conditions, where a shunt pure active filter for damping out harmonic propagation is connected by a stepdown transformer at the end of the feeder. On the other hand, Fig. 2 shows the simplified feeder having a shunt hybrid active filter proposed in this paper. The transformer in the pure filter is replaced by a capacitor in the hybrid filter. The purpose of the capacitor is to impose a high impedance to the fundamental frequency so that the fundamental voltage appears exclusively across the capacitor. This means that no fundamental voltage is applied across the active filter. Note that the weight of a 6.6-kV capacitor ranges from one-tenth to one-twentieth as light as that of a 6.6-kV/400-V step-down transformer with the same kVA rating as the capacitor.

Fig. 3 shows the detailed power circuit of the hybrid filter, which consists of a three-phase voltage-fed PWM inverter, and a series connection of Lf and Cf per phase. Note that

0-7803-8399-0/M$20.00 02004 IEEE. 4630

2004 35lh Annual IEEE Power Eleclronics Specialists Conference

line inductor resistor

capacitor

Step-down transformer

Lf

L1 = LZ = L3

Rz = RS CI = 6 2 = C,

0.22 mH (3.5%)

0.05 Q (2.5%) 350 f i F (22.0%)

R1 0.02 n (1.0%)

Shunt pure active filter

Fig. I . Shunt active filler with a step-dawn transformer.

Shunt hybrid active filter

Fig. 2. Shunt hybrid filler without transformer.

the tuned frequency of Lf and Cf is not the 5th-harmonic frequency but around the 7th-harmonic frequency. The reason is that the 7th-tuned LC filter is less bulky than the 5th-tuned LC filter as long as both filters have the same inductor as Lf. The dc-bus of the PWM inverter has only a capacitor without external supply, and the dc-bus voltage is controlled by the hybrid filter. The hybrid filter is controlled so as to draw the compensating current ic from the distribution line.

B. Operating Principle The hybrid filter with current control is based on voltage

detection. Three-phase voltages and currents are detected at the installation bus. The harmonic voltage V h in each phase is extracted from the detected three-phase voltage, and then the harmonic voltage is amplified by a control gain Kv. Thus the harmonic current reference iEh is given as follows:

The actual harmonic compensating current i ~ h is extracted from the detected compensating current ic. Assuming that ich is equal to its reference ibh. the hybrid filter behaves as a damping resistor of 1/Kv [V/A] for harmonic frequencies. The optimal value of K v is equal to the inverse of the characteristic impedance of the distribution feeder. With this value, the hybrid filter can damp out harmonic propagation effectively.

C. Experimental System

Fig. 4 shows the experimental system under no-load conditions. A three-phase power distribution feeder simulator rated at 200 V, 50 Hz and 20 kW is used for the laboratory experiments. This feeder simulator has a harmonic generator connected at bus U,, which produces an amount of harmonic voltage at ul. Table I summarizes the line simulator parameters. A 100-fiF, 2-mH LC filter with a tune frequency of 356

~

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Aachen, Germnny, 2W4

Distribution feeder

Fig. 3. Power circuit of the hybrid active filler.

50 Hz

Harmonic voltage generator

Hybrid filter U

Fig. 4. Experimental system.

Hz was used in experiments. Table I1 summarizes the hybrid filter with this LC filter.

The dc-bus voltage of the PWM inverter is controlled to be 40 V. Note that the dc-bus voltage as low as 40 V in the 200-V system corresponds to a dc-bus voltage as low as 1.3 kV in a 6.6-kV system. This means that adopting a diode- clamped three-level inverter allows us to use 1200-V IGBTs that are easily available on the market at low cost. The required capacity of the active filter for the LC filter is given by

( 2 ) 40"

f i x - x 8.1* = 0.4 kVA,

Note that the current in the above equation is calculated as the rms value of ic when a 5th-harmonic voltage of 2% exists at bus 4. The fundamental component of ic takes into account that the fundamental voltage is applied only to the LC filter. On the other hand, the 5th-harmonic component is calculated from ( I ) , considering that the hybrid filter acts as a damping resistor.

Jz

111. CONTROL SYSTEM A. Digital Conrroller

The active filter is controlled by a digital controller which consists of an A/D unit, a DSP unit, a digital PWM unit and a phase-locked-loop (PLL) unit as shown in Fig. 5. The DSP unit used for processing is a 16-bit fixed-point ADSP-2105 (Analog

2004 35lh A n n u l IEEE Power Elecrronics Specialisis Conference

LC filter inductor LC filter capacitor Resonant frequency Quality factor

TABLE II PARAMETERS OF THE HYIIKII) FIIJER

Lf 2 mH (31.4%) Cf 100 pF (6.3%) f? 356 Hz Q 20

21 - 3@ 2W-V 50-Hz distribution line I

I C, = 100 @F (6.3%)

I/: 0.4-kVA. (2%)

"aIIap:-fed PWM inverter

Fig. 5 . Fully-digitai-conuolled hybrid filter.

Devices) unit. The sampling period of the digital controller is 50 ps (20 kSls). The digital signals in the A/D and D/A units are limited to +IO V.

To cope with the time and phase delays inherent to the digital controller, the signal sampling is synchronized with the 5@Hz line frequency. This also prevents data aliasing from the digital controller. The phase reference is obtained from the fundamental voltage detected at the installation bus, and it is processed in the PLL unit to create both phase signal wSt and triangular carrier wave vt7, used in the PWM unit. The sine waveform used for coordinates transformation is created by using a sine table in the DSP unit.

B. Harmonic Detection There are several calculation methods to extract the har-

monic components from the detected three-phase waveforms in frequency-domain as well as in time-domain. Among them. the time-domain based pq theory has k e n widely applied to the control strategy of active filters. A main advantage of this approach over the frequency-domain based approach is that it does not require data samples of one previous cycle for calculation, so that it can avoid a delay in time response.

The detected three-phase voltage and current are transformed into the d-q coordinates as shown in Fig,, 6. Two high-pass filters (HPFs) with the same cutoff frequency as

~

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Aachen. Germany, 2094

sinwt. C O S W I ~

Fig. 6. Black diagram of the harmonic detection

sinlcos generator

Fig. 1. DC-bus voltage control

17 Hz extract the dc component which corresponds to the fundamental frequency in the d-q coordinates. Finally the obtained harmonic voltage and current are transformed from the d-q coordinates into the three-phase harmonic voltage and current.

C. DC-Bus Voltage Control

A critical issue in this hybrid active filter is the dc-bus voltage control. The dc bus consists of a single capacitor charged from the power supply. During operation, the active filter may absorb an amount of active power into, or release it from, the dc capacitor. Excessive active power absorption will increase the dc-bus voltage, and may damage the active filter. The strategy used to control the dc-bus voltage is based on active-power control. According to the pq theory, a dc component in the d-q coordinates corresponds to active power. No direct-axis current on the d-q coordinates flows in the LC filter. Thus, the active power is controlled by adjusting the quadrature-axis component. The direct axis is set to zero.

Fig. 7 shows a block diagram for the dc-bus voltage control. The dc-bus voltage is detected and compared with a reference, amplifying the error signal by a control gain of I O AN. Finally, the obtained control signal on the d-q coordinates is transformed into a three-phase fundamental voltage reference by using the inverse d-q transformation. The resultant dc-bus voltage control signal is added to the voltage reference WE.

A limiter is included in the dc-bus control loop. It is designed to ensure a smooth transient response and to avoid sudden increments or decrements in the dc-bus voltage. It is also designed to prevent the control loop from numerical saturation in the DSP signals. The limiter is set to f2.5- V in the digital controller which corresponds to 2.5% of the maximum control signal. For a 40-V dc-bus voltage, the maximum dc-bus control signal corresponds to a IO-V peak- to-peak fundamental voltage for the inverter. A low-pass filter (LPF) with a cutoff frequency of IO Hz eliminates ripples from the detected dc-bus voltage. Hence, no harmonic components appear in the dc-bus voltage control signal.

2004 35th A n n u l IEEE Power Eleclronics Specialists Conference Aachen, Gennan>~, 2004

isampling penod mgulator - of Vd,

, ............ ~~~~.~ ~ I

i C h

V

Fig. 8. Cantml block diagram of the hybrid filter

D. Current Control Fig. 8 shows a control block diagram of the hybrid filter.

The detected harmonic voltage Uh is multiplied by a gain KV to produce the current reference igh. The current reference is compared with the detected harmonic compensating current iCh , and the difference between i;h and ich is multiplied by a control gain K c . This results in producing the inverter voltage reference vi.. The effect of a time delay, that is, one sampling period T (=Sops) inherent to the digital controller is represented as eCsT in the control block diagram.

On the other hand, the gain K v is calculated as the inverse of the characteristic impedance of the line simulator in Fig. 4. According to Table I, K v is set to be 1.25 AN.

Iv. STABILITY ANALYSIS OF CURRENT CONTROL LOOP The stability of the current control loop is analyzed based

on the experimental setup shown in Fig. 4 and the parameters contained in Tables I and 11. An arithmetic software package Mathematica, is used for analysis. Fig. 9 shows a current- control block diagram of the hybrid filter. The supply voltage U and the dc-bus voltage control signal U; can be assumed as disturbance signals in the current control. Since the current control emphasizes to control the harmonic current, these disturbance signals having the fundamental frequency can be ignored.

The harmonic current iCh is obtained from the detected hybrid filter current ic, as shown in Fig. 6. The transfer function for the harmonic detection scheme can be generally expressed as:

where Tc is the time constant of the two identical high-pass filters which extracts the dc component in the d-q coordinates, and w1 is the angular frequency of the extracted frequency which in this case corresponds to the fundamental frequency.

The PWM inverter is assumed to be an ideal inverter ( H ( s ) = 1). Therefore, the current-control stability depends on the control gain KC. The system becomes marginally stable when the open-loop transfer function W,(s)

Fig. 10 shows the frequency response of the open-loop transfer function WC(S) when Kc = I O VIA. To observe

(3)

~

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harmonic delay LC filter detection

I I

Fig. 9. The cunentSnntml block diagram

gain margin

} 1 5 d B

frequency [Hz]

Fig. IO. Gain margin of the current cnntrol system

the effect of the harmonic detection transfer function on the current loop stability, the open-loop transfer function is calculated with and without the harmonic detection transfer function. In both cases, the system is stable with a gain margin of 15 dB. The harmonic detection scheme does not affect the performance of the current controller.

A gain margin of 15 dB means that K c could he increased to 31.6 V/A without instability. In this system, however, K c is set to 10 VIA in order to keep a 15-dB stability margin. Experimental verifications show that the gain of Kc=lO V/A is high enough to keep the steady-state error below 5% of the reference signal while it is low enough to prevent the DSP signals from numerical saturation.

v. ANALYSIS OF DAMPING PERFORMANCE Fig. 1 I shows an equivalent circuit for analysis. In order to

analyze harmonic damping performance, the supply voltage is assumed to he a harmonic voltage of v& = VI. This means that no fundamental component exists. According to Fig. 8,

Fig. 11. Equivalent circuit for analysis

2004 35th Annual IEEE Power Electronics Specialists Conference Aachen, Germany. 2004

200 400 600 800Ik 2k frequency [Hz]

Fig. 12. Frequency characteristics of harmonic propagation and damping.

the ac output voltage of the active filter. Vc for harmonic frequencies can be expressed as

where ZJ is the impedance of the LC filter. Kv ancl K c are the control gains of the hybrid filter. Note that the positive direction of Vc is to draw the compensating current IC from the distribution system. From (5), i t is clear that the hybrid filter is characterized as a voltage-controlled voltage source. K v , KC, 2, and the delay time eCBT are constant parameters whereas the hybrid filter voltage varies according to the harmonic voltage at the installation bus V,.

Fig. 12 shows the frequency characteristics of harmonic propagation and damping, calculated as the ratio of the voltage at the installation bus, u4 with respect to the voltage at the source, u1 for the system shown in Fig. I I . The hybrid filter voltage is calculated according to (5). Note that the line simulator is designed to emulate a real distribution line, including the line impedance and the shunt capacitors normally installed for power factor correction.

According to Fig. 12, when no filter is connected, the characteristic impedance of the system, which corresponds to the characteristic impedance of the line simulator only, has three resonance frequencies. The lowest resonance frequency which happens to have also the highest amplification ratio is at 250 Hz, that is, the 5th-harmonic frequency in a 50- Hz system. A 5th-harmonic voltage can be found easily in real distribution systems, as well as 3rd- and 7th-harmonic voltages. Theoretically, the 5th-harmonic voltage is amplified by 21 times, the 3rd- and 7th-harmonic voltages are amplified by 4 times.

When only the LC filter is connected to the feeder simulator, the characteristic impedance of the system changes, and con- sequently the frequency response of the harmonic propagation changes substantially. The presence of the LC filter makes the system resonant at 350 Hz, where the terminal voltage is attenuated. The LC filter can be considered as a passive

filter tuned to the 7th-harmonic frequency. The 5th-harmonic voltage amplification is considerably lower than that in the previous case, having an amplification ratio of about 12 times. This value is still unsafe, because when the 5th harmonic frequency becomes high, the terminal voltage become highly distorted.

When the hybrid filter is connected, the harmonic propagation is completely eliminated, so that no amplification occurs at any frequency. Thus, the hybrid filter effectively damps out the harmonic propagation throughout the feeder.

VI. EXPERIMENTAL RESULTS

In the following experiments, a harmonic voltage generator injects a 5th-harmonic voltage of 2% at v1 under no-load conditions, as shown in Fig. 4. Experimental waveforms of bus voltages u l p ~ 4 and compensating current ic for three different operating conditions are shown in Fig. 13. Additionally, Table 111 summarizes the 3rd-, 5th- and 7th-harmonic voltages and the THD values for the operating conditions indicated in Fig. 13.

When no filter is connected, the 5th-harmonic voltage is amplified by 8 times due to harmonic propagation, distorting severely the voltage waveform. The 7th-harmonic voltage as well as the 3rd-harmonic voltage are amplified slightly. This result shows the effect of harmonic propagation on the distribution system.

When only the LC filter is connected, the harmonic propagation is reduced considerably, because its connection changes the characteristic impedance of the system. However, harmonic amplification still occurs so that the voltage waveform is still distorted. The THD value reaches 6. I %. This does not comply with harmonic mitigation standards and guidelines. However, the LC filter reduces considerably the harmonic propagation, compared with the previous case.

When the hybrid filter is connected, the harmonic propagation is completely eliminated. None of the harmonic frequencies are amplified. The THD value in v4 is kept as low as 2.1%. because the harmonic components at the installation bus or bus 4 are reduced. The fundamental component is the same as that in case of the LC filter connected because the active filter presents a high impedance to the fundamental frequency. These experimental results conclude that the hybrid filter effectively damps out harmonic propagation.

Fig. 14 shows the transient response of the dc-bus controller, where is the dc-bus voltage reference and u,jC is the actual dc-bus voltage. Due to the presence of the limiter in Fig. 7, the transient response is smooth enough to reach the steady state in 0.2 sec. Fig. 15 shows a steady-state characteristic of the current control, where ibh corresponds to the harmonic current reference, and iCh to the actual harmonic current. The steady- state error is small in amplitude, and moreover no phase delay exists between the current reference and the actual detected current. Note that Fig. 15 uses the same experimental data as Fig. 13(c). It is clear that iCh is the same as ic when the fundamental component is extracted from ic . The current control loop as well as the dc-bus control loop works properly and stably.

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2004 35rh Annual IEEE Power Elecrronics Specialirrs Conference

No Filter (Fig. 13(a))

Aachen, Germany, 2004

LC Filter (Fig. 13(b)) I Hybrid Filter (Fig. 13(c))

200 v U1 0

iC IS : +. 7iiZ

(a) No filter connected (b) LC filter

svr U& 0

L

SO V r

Fig. 14. Transient response of the dc-bus voltage control

VII. CONCLUSIONS

This paper has discussed control and performance of a trans formerless shunt hvbrid active filter for installation on 6.6-

10 A

i& 0

10 A

iCh 0

c__

IO ms Fig. 15. Experimenlal waveforms for the current control.

at a low cost. No phase delay occurs in the compensating current with respect to the current reference. The hybrid filter is stable and robust. beine a solution recommended IO deal . -

-

kV power distribution systems. The replacement of a 6.6-kV transformer by a 6.6-kV capacitor contributes to a significant reduction in cost, size and weight. Moreover it allows us to use 1200-V IGBTs that are easily available on the market and

with harmonic propagation. The viability and effectiveness of the proposed shunt hybrid active filter have been confirmed theoretically and experimentally, achieving good performance in damping out harmonic propagation in power distribution systems.

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Task Force on Harmonics modeling and Simulation. ''Mmleling and simulation of the propagation of harmonics in electric power networks. II. Sample systems and examples," Tmns. Power Delivery, vol. 11. "0.1. pp, 466474, Jan 1996. S . M. William, G. T. Brownfield, I. W. Duffus, "Harmonic propagalion on an electric distribution system: field measurements compared with computer simulation," Truns. Power Delivery. vol. 8. 110.2, pp,. 547-552, Apr 1993. H. Akagi, "Conuol strategy and site selection of a shunt active filter for dumping of harmonic propagation in power distribution systems," IEEE Trans. Power Delivery, vol. 12, no.1. pp. 354-363. 1997. H. Akagi, H. Fujita, and K. Wada. "A shunt active filter based On voltage detection for harmonic termination of a radial power distribution line:' IEEE Trans. lndusrry Applicalionr, vol. 35, "0.3. pp. 638-645. 1999. P. Jintalrosonwil. H. Fujita, and H. Akagi. "Control and perfrmnance of a fully-digital-contmlled shunt active filter for installation on a power distribution system," IEEE Trans. Power Elecmmnics, vol. 17. no.1. pp. 132-140. Jan. 2002. P. Jinmkosonwit. H. Akagi. H. Fujita. and S. Ogasawam. "Implementation and performance of automatic gain adjustment in a shunt active filter for harmonic damping throughout a power distribution system:' IEEE Tmns. Power Electronics. vol. 17, "0.3. pp. 438-447, May. 2M12. K. Wada. H. Fujita, and H. Akagi, "Considerations of a shunt active filter based on voltage detection for installation on a long distribution feeder:' IEEE Trans. Industry Applicalims, vol. 38. "0.4. pp. 1123-1 130, JuVAug. 2002. P. Jintakosonwit. H. Fujita. H. Akagi. and S. Ogasawaira, "Implementation and performance of cooperative contml of shunt active filters fcr harmonic damping throughout a power distribution system," IEEE Trans. lndurrry Applicalionr. vol. 39. no.2. pp. 556-564, MadApr. 2003.

402-408, Aug. 1995. [I41 S . Bhaltacharya. Po-Tai Cheng, D. M. Divan, "Hybrid Solutions for

improving passive filter performance in high power applications." IEEE Tram. lndusrrv Anolicafions. vol. 33. no.3. PD. 732-747. MavNun. 1997. , .. .. ~~

[I51 S . Srianthumrong, H. Akagi,"A medium-voltage transformerless AUDC power conversion system consisting of a diode rectifier and a shunt hybrid filter:' IEEE Trans. Indusrry Applicarions. vol. 39, no.3. pp. 874-882. MaylJun. 2003.

[I61 H. Akagi. S . Srianthumrong. Y. Tamai. "Comparisons in circuit configuration and filtering performance between hybrid and pure shunt aclive filters," in Pruc. IEEWIAS Cunf.. 2W3. pp. 1195-1202.

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A 6.6-kV Transformerless Shunt Hybrid Active Filter for Installation on a Power Distribution...

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