Design of a Hybrid Active Filter for Harmonics Suppression in Industrial Facilities

Design of a Hybrid Active Filter for Harmonics Suppression in Industrial Facilities

Tzung-Lin Lee Yen-Ching Wang Jian-Cheng Li Department of Electrical Engineering

National Sun Yat-sen University 70, Lienhai Rd., Kaohsiung 80424, TAIWAN

Email: [email protected]

Abstract—Harmonic voltage amplification, due to uninten- tional series or parallel resonance of power factor correction capacitors, is a significant issue in the industrial power system.

Conventional active filters intended to compensate the harmonic current producing by nonlinear loads would not be an effective solution in this scenario, and the harmonic voltage may still be significant. This paper proposes a hybrid active filter to suppress the harmonic resonance in industrial facilities. The hybrid active filter, which is composed of a capacitor and an active filter in series connection, operates as variable harmonic conductance with dynamically tuning characteristic according to the voltage total harmonic distortion, so the damping performance of the active filter can be adjusted in response to load change and power system variation. Therefore, the harmonic resonance would be avoided as well as harmonic voltage distortion can be maintained at an allowable level. Compared with the pure shunt active filter, the dc bus voltage of the proposed hybrid filter is dramatically reduced since the grid voltage is supported by the series capacitor.

This feature provides a vital advantage of the active filter, in terms of both the kVA rating and the switching ripples. Operation principles are explained in detail, and computer simulations validate the effectiveness of the proposed approach.

K EYWORDS

Hybrid active filter, active power filter, harmonic resonance I. I NTRODUCTION

Compared with active front-end converters, diode or thyris- tor rectifiers still dominate in high-power applications, such as adjustable speed drives, uninterruptible power supply sys- tems, and electrolysis. These equipment always injects a large amount of harmonic current into the power system, which may cause excessive harmonic voltage distortion and even give rise to malfunction of sensitive equipment in the vicinity of the harmonic source. Multiple tuned passive filters are usually installed at the secondary side of the distribution transformer in the industrial facilities to draw dominant harmonic current and provide power factor correction for inductive loads as well [1], [2]. However, unintentional series and/or parallel resonance, due to the passive filters and nonlinear loads and/or the utility, may result in excessive harmonic voltage amplification [3], [4]. Extra engineering work, therefore, must be consumed to calibrate and maintain required filtering performances.

Conventional active filters intended for compensating the harmonic current of nonlinear loads cannot address the har- monic resonance issues resulting from the passive filter or

the power factor correction capacitor [5]. Bhattacharya and his coworkers proposed a hybrid series active filter to isolate harmonics between power system and harmonic source [6].

Fujita and his coworkers proposed a hybrid shunt active filter to suppress the fifth harmonic resonance between the utility and a capacitor bank [7]. Detjen and his coworkers proposed a hybrid filter in series with a capacitor bank by a coupling transformer to suppress harmonic resonance and compensate harmonic current [8]. These methods provide effective harmonics suppression functionality; however, extra added passive components, such as matching transformers or tuned passive filters, becomes a critical issue in terms of installation space and cost. Distributed active filters with voltage detection feature were proposed to cope with the harmonic resonance on the capacitor bank, but a droop- controlled algorithm is required to coordinate the operation of multiple active filters [9].

This paper proposes a hybrid active filter to suppress the harmonic resonance in industrial facilities as well as mitigate harmonic current flowing into the utility. The proposed hybrid active filter is composed of an active filter and a power factor correction capacitor in series connection. The active filter operates as variable damping conductance at harmonic frequencies. The harmonic conductance is determined accord- ing to the voltage total harmonic distortion (THD) at the installation location of the hybrid active filter. Based on this control, the damping performance of the active filter can be dynamically adjusted to maintain harmonic voltage distortion at an allowable level in response to load change and power system variation, where the allowable voltage THD can be regulated according to the harmonic voltage limit in IEEE std. 519-1992 [10]. Since the series capacitor is responsible for sustaining the fundamental component of the grid voltage, the active filter can be operated with a very low dc bus voltage, compared with the pure shunt active filter [11]. This feature is a significant advantage, in terms of both the rated kVA capacity and the switching ripples of the active filter.

II. O PERATION P RINCIPLES

A simplified one-line diagram of the proposed hybrid active

filter and the associated control are shown in Fig. 1(a). The

hybrid active filter unit (HAFU) is composed of an active

filtering part and a power factor correction capacitor C in

PLL

abc to qd

abc to qd

qd

to abc

qd

to abc qd

to abc

iabc

iabc ieqd ˜ieqd

iabc,h iabc,h

i∗abc,h Eabc

Eabc

Eabc

Eeqd Ee˜qd Eabc,h

Vdc

Vdc

V ∗dc

ve∗q,f

ve∗d,f

v∗abc v∗abc,h

v∗abc,f

v∗abc,f

G∗

ω

ω ω

ω ω

ω Ls

C L

HPF

HPF

PI

Kc

Distribution Transformer

Linear Load

Nonlinear Load

Tuning controller

PWM

HAFU

Vs

(a) One-line circuit diagram of the proposed HAFU.

ωf

s+ωf

ωf

s+ωf

PI SQRT SQRT

Ea,h

Eb,h

Ec,h

Ea

Eb

Ec

THD^∗ THD

G^∗

(b) Tuning control of conductance command.

Fig. 1. The proposed HAFU and its associated control.

series connection at the secondary side of the distribution transformer in industrial facilities. The HAFU operates as variable conductance at harmonic frequencies as given,

i

= G

· E

(1) where i

represents the harmonic current command, and E

is harmonic voltage component at the installation point of the HAFU, respectively. The conductance command G

is defined as a variable gain to determine how much harmonic current should be drawn from the grid to suppress voltage

harmonics. Control algorithm is detailed as follows.

Harmonic voltage component E

at the installation location of the HAFU and harmonic current component i

of the HAFU can be obtained by using the synchronous reference frame (SRF) transformation [6] as shown in Fig.1(a).

In the SRF the fundamental component becomes a dc value

, whereas the harmonic component is a ac value. Therefore,

both harmonic voltage component ˜ E

and harmonic current

component ˜ i

in the SRF can be extracted by using high

pass filters (HPFs). After applying the inverse SRF trans-

formation, E

and i

in the three-phase system are derived. Subsequently, the harmonic current command i

is generated by multiplying the voltage harmonics E

and the conductance command G

. Based on the harmonic current command i

, the measured harmonic current i

, the harmonic voltage command v

can be derived by using a proportional controller as follows,

v

= K

· (i

− i

) (2) where K

is a proportional gain. Since the series capacitor draws the fundamental reactive current from the grid, the dc voltage of the HAFU can be regulated by using a proportional- integral (PI) controller to adjust the fundamental reactive volt- age v

of the HAFU. According to the voltage command v

, the space vector PWM is employed to synthesize the required output voltage of the inverter.

Fig. 1(b) shows the proposed tuning control of the HAFU.

The harmonic conductance command G

is determined ac- cording to the voltage THD at the HAFU installation point E

. The derivation of THD can be approximately evaluated by using two low pass filters (LPFs) with cut-off frequency ω

, which are to filter out ripple components in the calculation.

The error between the allowable THD and the measured THD is then fed into the PI regulator to adjust the harmonic conductance command G

. Based on this control, the damping capability of the HAFU can be dynamically tuned, so the HAFU would provide effective damping for harmonic resonant frequencies, and harmonic voltage distortion can be main- tained at an allowable level. The allowable voltage THD can be determined based on the harmonic voltage limit in IEEE std. 519-1992.

A simplified single-phase equivalent circuit of the HAFU at harmonic frequencies is shown in Fig. 2, where v

and i

represent the background harmonic voltage of the power system and the harmonic current producing by nonlinear loads, respectively. Note that linear loadings are not included for the worst-case consideration of the harmonic resonance. When the HAFU is off, i.e. v

=0, the passive filter is directly connected to the load bus as in Fig. 2(a). The harmonic voltage E

and the harmonic current i

can be expressed as follows,

E

= (1 − ω

LC + jωR

C)v

1 − ω

(L

+ L)C + jωR

C

+ (−ω

L

R

C + j(−ω

L

LC + ωL

))i

1 − ω

(L

+ L)C + jωR

C i

= v

1 − ω

(L

+ L)C + jωR

C + (1 − ω

LC + jR

C)i

1 − ω

(L

+ L)C + jωR

C .

(3)

The resonant frequency is

f

= 1

2π 

(L

+ L)C . (4)

Since the HAFU is controlled as harmonic conductance as shown in Fig. 2(b), the equivalent circuit of the HAFU can be simplified as Fig. 2(c). The harmonic voltage E

and the

Passive Filter

vs,h

is,h

Eh

iL,h Ls

L C

Rf

(a) The HAFU is off.

vs,h

HAFU

is,h

iaf =Eh · G Eh

iL,h Ls

L C

Rf

(b) The Operation principle of the HAFU.

vs,h

HAFU

is,h

Eh

G iL,h Ls

(c) The simplified circuit after the HAFU is in operation.

Fig. 2. Simplified single-phase equivalent circuit of the HAFU at harmonic frequencies in the industrial power system.

harmonic current i

, therefore, can be expressed as follows, E

= jωL

· i

+ v

1 + jωL

G i

= i

− Gv

1 + jωL

G .

(5)

As demonstrated (5), the resonance in (3) no longer occurs, and both E

and i

can be reduced by increasing G.

III. S IMULATION R ESULTS

Fig.3(a) shows the simulation circuit of the proposed HAFU and the associated circuit parameters are given as follows.

•

Power system: 220 V(line-to-line), 60 Hz, 20 kVA, Line inductor: L

=1.5 mH(23 %).

•

The passive filter is tuned at seventh harmonic fre-

quency with quality factor 100, and provides power

1.5 mH (23%)

100 μF(9%)

20 mF 1.5 mH

10 Ω 30 mH

10 Ω 5 mH

3 kVA pf =0.66

7 kVA pf =0.83 iL

is

E i

Vdc 220 V

60 Hz 20 kVA

(a) Simulation circuit configuration.

0.4 0.5 0.6 0.7

−300

−150 0 150 300

0.4 0.5 0.6 0.7

−60

−30 0 30 60

0.4 0.5 0.6 0.7

−60

−30 0 30 60

0.4 0.5 0.6 0.7

−60

−30 0 30 60

0.4 0.5 0.6 0.7

0 15 30 45 60

0.4 0.5 0.6 0.7

0 0.2 0.4 0.6 0.8

0.4 0.5 0.6 0.7

0 10 20 30 40 is

iL

i

Vdc

E

G^∗

THD

(b) Voltage, current, conductance command, and voltage THD when the startup of the HAFU.

Fig. 3. Simulation circuit and simulation results.

factor improvement for both linear and nonlinear loads.

L

=1.5 mH(23 %), C

=100 μF (9 %).

•

Linear and nonlinear loads are rated at 3 kVA pf=0.66, 7 kVA pf=0.83, respectively.

•

The AFU is implemented by conventional three-phase voltage source inverter with the switching frequency 10 kHz and the reference dc bus voltage 50 V. The OFF state of the HAFU corresponds to turning on three upper switches, but turning off three lower switches.

•

The reference voltage THD is set as 3% based on the individual harmonic voltage limit of IEEE std. 519-1992.

A. Time domain analysis

Before the HAFU is started (t <0.5s), i.e. in the OFF state, fifth harmonic resonance between the passive filter and the utility causes large fifth harmonic current circulating between the source current i

and the filter current i, as shown in Fig. 3(b). The voltage THD of E is 39% and the current THD of i

is 66%, respectively. This result shows the passive filter thoroughly loses its filtering functionality and even causes excessive harmonic amplification.

The HAFU starts in operation at t=0.5s. Fig.3(b) shows the

dc voltage of the inverter is established and well-controlled

0 5 10 15 20 25 30

HAFU off HAFU on

is

F un 5th 7th 11th 13th

(a) Source current components.

0 5 10 15 20 25 30

HAFU off HAFU on

i

F un 5th 7th 11th 13th

(b) HAFU current components.

0 5 10 15 20 25 30

HAFU off HAFU on

iL

F un 5th 7th 11th 13th

(c) Load current components.

0 25 50 75 100 125 150

HAFU off HAFU on

E

F un 5th 7th 11th 13th

(d) Voltage components.

Fig. 4. Harmonic components before and after the HAFU is in operation.

at 50V after t=0.6s, and the tuning controller adjusts the conductance command G

to draw the harmonic current for damping harmonic resonance. At the steady state, the voltage THD at E is significantly improved to 3.0% and the current THD of i

is also reduced to 5.1% with the conductance command G

=0.57Ω

. The harmonic resonance and the circulating harmonic current no longer occur. Fig. 4 illustrates the harmonic components of i

, i, i

, E. Fifth harmonic

1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4

0 0.2 0.4 0.6 0.8 1

1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4

0 1 2 3 4 5 G^∗

THD

Fig. 5. Conductance command and voltage THD as the nonlinear loading is increased at 2s and 3s, respectively.

current of i

is reduced from 16A to 2A, and fifth harmonic voltage of E is reduced from 50V to 4V. Since the inverter is simply operated at V

=50V, the HAFU consumes about 300 VA, which is approximately 1.5% of the system rating.

This feature is a significant advantage, in terms of the active filter kVA capacity and the associated switching ripple.

Fig. 5 shows the conductance command and the voltage THD under the variation of the nonlinear loading, which is added from 1.75kVA to 3.5 kVA at 2s and subsequently increased from 3.5kVA to 7.0 kVA at 3s. Since voltage distortion is enlarged, G

is raised to maintain the voltage THD at 3%. At the steady state, G

=0.07Ω

for 1.75kVA and G

=0.38Ω

for 3.5kVA, respectively.

B. Frequency domain analysis

Damping performances based on frequency domain analysis are shown in Fig. 6. The resonant peaks, due to the line impedance and the passive filter, is located at fifth harmonic frequency. After the HAFU is engaged, the resonant phe- nomenon would fully disappear and the filtering performance is strongly dependent on the damping conductance provided by the active filter. As demonstrated in Fig. 6(a) and Fig. 6(b), the magnitude of the harmonic impedance

L,h

and the magnitude of the harmonic current amplification

L,h

are effectively sup-

pressed with increasing conductance command. Note that the

HAFU exhibits high impedance for harmonic frequencies and

the passive filter simply provides reactive power compensation

when G

=0.

10² 10³ 10⁴

−100

−80

−60

−40

−20 0 20 40 60 80 100

Frequency (Hz)

HAFU OFF G^∗=0 pu G^∗=0.5 pu G^∗=1.0 pu G^∗=1.5 pu G^∗=2.0 pu

| Eh iL,h|

(a) Magnitude plot of harmonic impedance

L,h

.

10² 10³ 10⁴

−100

−80

−60

−40

−20 0 20 40 60 80 100

Frequency (Hz)

HAFU OFF G^∗=0 pu G^∗=0.5 pu G^∗=1.0 pu G^∗=1.5 pu G^∗=2.0 pu

|is,h iL,h|

(b) Magnitude plot of harmonic current amplification

L,h

. Fig. 6. Damping performance analysis of the HAFU.

IV. S UMMARY

This paper presents a hybrid active filter to suppress the har- monic resonance in industrial facilities. The proposed hybrid filter, which is composed of an active filter and a power factor correction capacitor in series connection at the secondary side of the distribution transformer, operates as variable harmonic conductance with dynamically tuning feature in response to load change and the parameter variation of the power system.

Therefore, the harmonic resonance would be avoided and harmonic voltage distortion can be reduced and maintained at an allowable level. Since the series capacitor sustains the fundamental component of the grid voltage, the active filter can be operated with a reduced kVA capacity, compared with its counterpart of the pure shunt active filter, which is the significant advantage of the proposed method.

In most power electronics applications, low-pass filters or EMI filters are required to install at the grid side of the inverter for alleviating switching ripples into the power system. These filters usually present capacitive characteristic and may cause unintentional harmonic resonance with the leakage inductance

of the power system. Fig. 7 shows the equivalent circuit of the HAFU considering a capacitive filter C

installed at the loading bus. At this situation, the harmonic voltage E

and the harmonic current i

can be expressed as:

E

= jωL

· i

+ v

1 − ω

L

C

+ jωL

G i

= i

− (G + jωC

)v

1 − ω

L

C

+ jωL

G .

(6)

Obviously, both E

and i

can also be suppressed by controlling the harmonic conductance G when the harmonic resonances occur between C

and L

.

vs,h

HAFU

is,h

Eh

G iL,h Ls

Cemi

Fig. 7. The simplified circuit of the HAFU with the capacitive filter C

.

A CKNOWLEDGMENT

This research is funded by the National Science Council of TAIWAN under grant NSC 98-2221-E-110-078.

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