**3. An UPQC in high power application**

In many mid-voltage or high-voltage applications, nonlinear load not only produces heavy harmonic current but also is sensitive to harmonic voltage. An UPQC combined a series APF and a HAPF is much suitable for these applications(Khadkikar et al.,2005). Fig.4 shows the detailed system configuration of the high power UPQC, where *sa e* , *sb e* and *sc e* are three phase voltages of generator, *ca e* , *cb e* and *cc e* are the voltages compensated by series APF, *sI* is utility current, *LI* is load current, *FI* is compensating current output from shunt device, *Zs* is impedance of transmission line, C is a big capacitor for DC linker.

Fig. 4. Configuration of high power UPQC

The high power UPQC is composed of series device and shunt device. The series device is mainly for insulating the source voltage interference, adjusting loads voltage etc. The shunt device is mainly for eliminating harmonic current produced by nonlinear load. In series device, *L*<sup>1</sup> and *C*<sup>1</sup> make low-pass filter (LPF) to filter output voltage of Inverter 2 because power electronics devices in Inverter 2 open and close in high frequency and generate high frequency disturbances exerted on expected sinusoidal output voltage of Inverter 2. In series device, transformer *T*<sup>2</sup> not only insulates Inverter 2 from utility but also makes output voltage of Inverter 2 (after LPF) satisfy maximum utility harmonic voltage. In shunt device, *L*<sup>0</sup> and *C*<sup>0</sup> make a LPF to filter output voltage of Inverter 1. The shunt device and series device share the DC capacitor. The shunt device is consisted of an inverter and a PPF. PPF is

Inverter1 Inverter2

In many mid-voltage or high-voltage applications, nonlinear load not only produces heavy harmonic current but also is sensitive to harmonic voltage. An UPQC combined a series APF and a HAPF is much suitable for these applications(Khadkikar et al.,2005). Fig.4 shows the detailed system configuration of the high power UPQC, where *sa e* , *sb e* and *sc e* are three phase voltages of generator, *ca e* , *cb e* and *cc e* are the voltages compensated by series APF, *sI* is utility current, *LI* is load current, *FI* is compensating current output from shunt device,

*T*1

The high power UPQC is composed of series device and shunt device. The series device is mainly for insulating the source voltage interference, adjusting loads voltage etc. The shunt device is mainly for eliminating harmonic current produced by nonlinear load. In series device, *L*<sup>1</sup> and *C*<sup>1</sup> make low-pass filter (LPF) to filter output voltage of Inverter 2 because power electronics devices in Inverter 2 open and close in high frequency and generate high frequency disturbances exerted on expected sinusoidal output voltage of Inverter 2. In series device, transformer *T*<sup>2</sup> not only insulates Inverter 2 from utility but also makes output voltage of Inverter 2 (after LPF) satisfy maximum utility harmonic voltage. In shunt device, *L*<sup>0</sup> and *C*<sup>0</sup> make a LPF to filter output voltage of Inverter 1. The shunt device and series device share the DC capacitor. The shunt device is consisted of an inverter and a PPF. PPF is

UPQ C

*Zs* is impedance of transmission line, C is a big capacitor for DC linker.

Utility

*ZS*

Fig. 3. Unified Power Quality Conditioner

**3. An UPQC in high power application** 

*T*2

Fig. 4. Configuration of high power UPQC

Load

consisted of 3 L-C resonance branches. One is consisted of *L*5 and *C*<sup>5</sup> for 5th harmonic current elimination, the other is consisted of *L*7 and *C*<sup>7</sup> for 7th harmonic current elimination, and the third is consisted of *L*<sup>3</sup> , *C*<sup>31</sup> , *C*<sup>32</sup> for 3rd harmonic current elimination. The resonance frequency of *L*<sup>3</sup> and *C*<sup>32</sup> is set to be the same as the frequency of fundamental component so that most of fundamental reactive current in this series resonance branch goes through *L*<sup>3</sup> and *C*<sup>32</sup> and little goes through inverter through transformer *T*<sup>1</sup> . As a result Inverter 1 suffers little fundamental voltage which helps to cut down its cost and improve its safety. Transformer T1 connects Inverter 1 with the series fundamental resonant branch *L*<sup>3</sup> and *C*<sup>32</sup> to insulate them and fit the difference between maximum output voltage of Inverter 1 and maximum voltage that L3 and *C*<sup>32</sup> needed to generate the maximum compensating current. The 3rd, 5th, 7th harmonic currents can be eliminated by the 3 L-C resonance branches, and Inverter 1 can also inject harmonic current into utility to give a fine compensation to every order harmonic current except 3rd harmonic current.

#### **3.1 Series device of high power UPQC**

Series device of UPQC is mainly to filter utility voltage and adjust voltage exerted on load so as to eliminate harmonic current produced by utility harmonic voltage and provide load a good sinusoidal voltage(Brenna et al. 2009; Zhou et al. 2009).

Series device of high power UPQC has the same topology as series APF whose Configuration is shown in Fig.1. Fig.1shows the single phase equivalent circuit of the series device, where *Zs* is impedance of transmission line. The main circuit and control circuit of the active part are in the dashed box.

From the sigle-pahse system, the voltage of the transformer can be expressed as

$$E\_{\rm C2} = \mathcal{U}\_{inv2} \cdot \frac{Z\_{\rm C1}}{Z\_{\rm L1} + Z\_{\rm C1}} \tag{3}$$

Suppose *E nE C C* 1 2 , then the voltage of the Inverter 2 can be calculated as

$$\begin{split} \mathbf{U}\_{inv2} &= \mathbf{E}\_{\mathbf{C}2} \cdot \frac{\mathbf{Z}\_{L1} + \mathbf{Z}\_{\mathbf{C}1}}{\mathbf{Z}\_{\mathbf{C}1}} \\ &= \frac{\mathbf{Z}\_{L1} + \mathbf{Z}\_{\mathbf{C}1}}{n\mathbf{Z}\_{\mathbf{C}1}} (\mathbf{U}\_{T} - \mathbf{U}\_{L}) \end{split} \tag{4}$$

The voltage of Inverter 2 can be written at another way as

$$
\mathcal{U}\_{inv2} = \mathcal{K}\_V \cdot \mathcal{U}\_{DC} \cdot \mathcal{B}(\mathbf{s}) \tag{5}
$$

Where *KV* is amplitude ratio between *Uinv*2 and *UDC* , *B s*( ) is phase shift between input control signal and output voltage of Inverter 2.

$$\begin{aligned} U\_L &= U\_T - E\_{C1} \\ &= U\_T - n \cdot K\_{\nu} \cdot B(\mathbf{s}) \cdot \frac{Z\_{C1}}{Z\_{L1} + Z\_{C1}} \cdot U\_{DC} \\ &= U\_T - K\_{c2} U\_{DC} \end{aligned} \tag{6}$$

Improve Power Quality with High Power UPQC 159

2 2 sin[ ( )] cos[ ( )] 3 3 *VU k U k U c d <sup>q</sup>* 

M atrix for phase adjusting

M atrix for phase adjusting

M atrix for phase adjusting

a b c\_ d q 0

d q 0 LPF

d q 0

d q 0

Fig. 6. Control scheme of AVR1

a b c\_ d q 0

a b c\_d q 0

*U L e*

Where  0 0 0

P I

P I

P I

Because a delay will unavoidably happen during detecting and controlling, a matrix is used

cos()sin( ) cos( )sin() 0 0 0 0

To check the effect of series device of high power UPQC to harmonic voltage, with MATLAB, a 3-phase 10KV utility supplied to capacitors is set up. Suppose the initial load is a 3-phase capacitor group, a resister valued 0.2 ohm series with a capacitor valued 100uF in each phase. When t=0.04s, series device switches to run. Tab.1 shows the parameters of power source and series device. Comparing the main harmonic voltages and harmonic currents after series run with those before series run, we know that series device reduce much harmonic of load voltage and so load harmonic current is much reduced. Fig.7 shows waveform of load voltage before and after series device run. In Fig.8, the spectrums of load voltage are compared through FFT. Fig.9 shows load current waveform and Fig.11 shows the spectrums of load current before and after series device run. With transformer T2, fundamental voltage produced by Inverter 2 can be added to power source, so it can also compensate voltage sags. When it is concerned, \* *UL* in Fig.6 is set to be expected fundament component of source voltage. Fig.12 and Fig.13 shows this function of series device. At 0.1s, utility voltage suddenly goes below to be 80 percents of previous voltage, as is shown in Fig.12. If series device keep running before voltage sag happen, utility voltage will keep

*k k k k*

to adjust the phase shift of the certain order harmonic. The matrix is described as:

 

 

*q d*

'

'

 

is phase angle for delay.

almost const, as is shown in Fig.13.

*U U*

0

0

0

   

P I <sup>d</sup> q0 \_ <sup>a</sup> <sup>b</sup>

P I <sup>d</sup> q0 \_ <sup>a</sup> <sup>b</sup>

P I <sup>d</sup> <sup>q</sup> <sup>0</sup> \_a <sup>b</sup>

c

+ +

+

P W M

(14)

c

c

 

*q d*

*U U* (13)

$$\textbf{Where}$$

$$\text{Where}\\
\qquad\qquad\qquad\qquad\qquad\qquad\mathbf{K\_{C1}}=\mathbf{n}\cdot\mathbf{K\_{V}}\cdot\mathbf{B(S)}\cdot\frac{\mathbf{Z\_{C1}}}{\mathbf{Z\_{L1}}+\mathbf{Z\_{C1}}}\tag{7}$$

To make load voltage sinusoidal, load voltage *UL* is usually sampled for control. Control scheme for series device is:

Fig. 5. Control scheme for series device of high power UPQC

Where AVR1 is automatic voltage regulator for *UL* control and AVR2 is for *UC* control. *UDC* is voltage of DC-linker. *K S UC*( ) is transform function of detecting circuit of *UC* which is consisted of a proportion segment and a delay segment. *K S UL*( ) is transform function of detecting circuit of *UL* . \* *U <sup>L</sup>* is reference voltage for load voltage *UL* , when a certain harmonic component is concerned, it is set to zero. AVR1 is automatic voltage regulator for *UL* and it can be divided to 3 parts, one is harmonic extraction, another is PI adjustor and the third is delay array. Control scheme of AVR1 is depicted in Fig.6. A selective harmonic extraction is adopted to extract the main order harmonics. Abc\_dq0 is described as equation (8-10) for a certain k order harmonic and transformation dq0\_abc is described as equation (11-13). LPF is low pass filter that only let DC component pass through.

$$\mathcal{U}\_d = \frac{2}{3} (V\_a \sin(ka\_0) + V\_b \sin[k(a\_0 - \frac{2\pi}{3})] + V\_c \sin[k(a\_0 + \frac{2\pi}{3})] \tag{8}$$

$$\mathcal{U}\_q = \frac{2}{3} (V\_a \cos(ka\_0) + V\_b \cos[k(a\_0 - \frac{2\pi}{3})] + V\_c \cos[k(a\_0 + \frac{2\pi}{3})] \tag{9}$$

$$\mathcal{U}L\_0 = \frac{1}{3}(V\_a + V\_b + V\_c) \tag{10}$$

$$V\_a = \mathcal{U}\_d \sin(ka\_0) + \mathcal{U}\_q \cos(ka\_0) + \mathcal{U}\_0 \tag{11}$$

$$V\_b = \mathcal{U}\_d \sin[k(a\_0 - \frac{2\pi}{3})] + \mathcal{U}\_q \cos[k(a\_0 - \frac{2\pi}{3})] + \mathcal{U}\_0 \tag{12}$$

To make load voltage sinusoidal, load voltage *UL* is usually sampled for control. Control

*<sup>Z</sup> K n K BS*

*UDC*

Where AVR1 is automatic voltage regulator for *UL* control and AVR2 is for *UC* control. *UDC* is voltage of DC-linker. *K S UC*( ) is transform function of detecting circuit of *UC* which is consisted of a proportion segment and a delay segment. *K S UL*( ) is transform function of detecting circuit of *UL* . \* *U <sup>L</sup>* is reference voltage for load voltage *UL* , when a certain harmonic component is concerned, it is set to zero. AVR1 is automatic voltage regulator for *UL* and it can be divided to 3 parts, one is harmonic extraction, another is PI adjustor and the third is delay array. Control scheme of AVR1 is depicted in Fig.6. A selective harmonic extraction is adopted to extract the main order harmonics. Abc\_dq0 is described as equation (8-10) for a certain k order harmonic and transformation dq0\_abc is described as equation

1 1

*Z Z* (7)

*Uinv*<sup>2</sup>

*<sup>q</sup>* (11)

(8)

(9)

(12)

 

> 

*U VVV abc* (10)

  *UL <sup>K</sup> CL*

*L C*

*sK* )( *UL*

00 0 2 22 ( sin( ) sin[ ( )] sin[ ( )] 3 33

00 0 2 22 ( cos( ) cos[ ( )] cos[ ( )] 3 33

( )

> 

> >

0 0 0

( ) *<sup>C</sup>*

Where <sup>1</sup>

\* *Uinv*<sup>2</sup>

Fig. 5. Control scheme for series device of high power UPQC

(11-13). LPF is low pass filter that only let DC component pass through.

*U Vk Vk Vk da b <sup>c</sup>*

*U VkVk Vk qa b <sup>c</sup>*

3

0 1

0 00 sin( ) cos( ) *VU k U k U a d* 

2 2 sin[ ( )] cos[ ( )] 3 3 *VU k U k U b d <sup>q</sup>* 

scheme for series device is:

*U <sup>T</sup>*

\* *UL*

*CL V*

*KCC*

$$V\_c = \mathcal{U}\_d \sin[k(a\_0 + \frac{2\pi}{3})] + \mathcal{U}\_q \cos[k(a\_0 + \frac{2\pi}{3})] + \mathcal{U}\_0 \tag{13}$$

Fig. 6. Control scheme of AVR1

Because a delay will unavoidably happen during detecting and controlling, a matrix is used to adjust the phase shift of the certain order harmonic. The matrix is described as:

$$
\begin{bmatrix} U\_d^\cdot \\ U\_q^\cdot \end{bmatrix} = \begin{bmatrix} \cos(k\alpha\_0 + \theta) & -\sin(k\alpha\_0 + \theta) \\ \sin(k\alpha\_0 + \theta) & \cos(k\alpha\_0 + \theta) \end{bmatrix} \begin{bmatrix} U\_d \\ U\_q \end{bmatrix} \tag{14}
$$

Where is phase angle for delay.

To check the effect of series device of high power UPQC to harmonic voltage, with MATLAB, a 3-phase 10KV utility supplied to capacitors is set up. Suppose the initial load is a 3-phase capacitor group, a resister valued 0.2 ohm series with a capacitor valued 100uF in each phase. When t=0.04s, series device switches to run. Tab.1 shows the parameters of power source and series device. Comparing the main harmonic voltages and harmonic currents after series run with those before series run, we know that series device reduce much harmonic of load voltage and so load harmonic current is much reduced. Fig.7 shows waveform of load voltage before and after series device run. In Fig.8, the spectrums of load voltage are compared through FFT. Fig.9 shows load current waveform and Fig.11 shows the spectrums of load current before and after series device run. With transformer T2, fundamental voltage produced by Inverter 2 can be added to power source, so it can also compensate voltage sags. When it is concerned, \* *UL* in Fig.6 is set to be expected fundament component of source voltage. Fig.12 and Fig.13 shows this function of series device. At 0.1s, utility voltage suddenly goes below to be 80 percents of previous voltage, as is shown in Fig.12. If series device keep running before voltage sag happen, utility voltage will keep almost const, as is shown in Fig.13.

Improve Power Quality with High Power UPQC 161

(a) Before series device run (b) After series device run

(a) Before series device run (b) After series device run

Fig. 8. FFT analysis for load voltage

Fig. 9. Waveform of load current

Fig. 10. FFT analysis for load current


Table 1. Parameters for series device


Table 2. Harmonics before and after series device run

Fig. 7. Waveform of load voltage

3-phase in positive sequence; line to line voltage: 10KV; Initial phase: 0 deg.

3-phase in negative sequence; line to line voltage: 250V; Initial phase: 0 deg.

3-phase in zero sequence; line to line voltage: 600V; Initial phase: 0 deg.

3-phase in negative sequence; line to line voltage: 1500V; Initial phase: 0 deg.

3-phase in positive sequence; line to line voltage: 1300V; Initial phase: 0 deg.

Resister: 0.2 ohm; capacitor: 100uF

2nd (%) 3rd (%) 5th (%) 7th (%) THD(%)

Items Parameters

transmission line Resister: 0.04 ohm; Inductor : 1uH;

Load 3-phase series resister and capacitor

Voltage before run 3.07 7.35 12.24 9.79 17.58 Voltage after run 0.88 1.55 3.55 2.37 4.66 current before run 6.09 21.93 60.48 66.96 93.05 current after run 1.99 4.86 17.72 16.44 25.67

Low Pass filter L1: 4mH; C1: 15uF

Transformer T2 n=10

Utility fundamental voltage

Utility 2nd harmonic voltage

Utility 3rd harmonic voltage

Utility 5th harmonic voltage

Utility 7th harmonic voltage

Impedance of

Table 1. Parameters for series device

Fig. 7. Waveform of load voltage

Table 2. Harmonics before and after series device run

Fig. 8. FFT analysis for load voltage

Fig. 9. Waveform of load current

Fig. 10. FFT analysis for load current

Improve Power Quality with High Power UPQC 163

( )

5 57 7 ( )( ) *L CL C LCLC*

1 57 1 *C C T P L F ZZ Z U UI nZ n*

57 <sup>11</sup> *Z*

*Z <sup>U</sup> InI <sup>L</sup> PT*

*L TP F C <sup>U</sup> U nU I Z*

*PT U*

*<sup>Z</sup> <sup>I</sup>* ) <sup>1</sup> ()1(

*Z*

57 1

))(( *LCLC LCLC ZZZZ ZZZZ <sup>Z</sup>*

For completely compensating load harmonic current, IF is controlled to be the same as IL, so

From equation (24), we can find control rule for shunt device of UPQC. If Inverter 1 is controlled to work as a current source, we can make it linear to load harmonic current and a fore-feed controller of load harmonic voltage is expected to add to the harmonic current controller. Control scheme for shunt device of high power UPQC is shown in Fig.14. To support DC linker voltage, shunt device should absorb enough energy from utility. Because it is easier for shunt device to absorb energy from utility, the DC linker voltage controller is placed in control scheme of shunt device. A PI conditioner is used here to adjust fundamental active current so as to keep DC-linker voltage const. ACR1 and ACR2 are the same as that of series device. Current out of active part is detected and form a close-loop controller. ACR3 is a hysteresis controller which makes Inverter 1 work as a current source.

Fig.15 shows the effect of this control scheme for shunt device of UPQC. The simulation parameters are shown in Tab.3. Suppose at 0.04s, passive part of shunt device is switched on

7755 7755

31 57 31

332 57 1 57 332 <sup>1</sup> (1 ) ( ) *C C T P L L Z Z <sup>Z</sup> I I nn U Z Z nZ Z*

*Z ZZ Z <sup>Z</sup> ZZZZ* 

*PT F*

Where 5 57 7 57

Besides 1 11 <sup>31</sup>

So 57 31 <sup>31</sup>

From equation (16) and (20), we get

1

1

*C*

332 31

*Z*

57

UL is also added to control scheme as a fore-feed controller.

*F*

1 1 1

*nI*

332 11

57 ( ) *<sup>L</sup>*

*n*

1

(16)

<sup>332</sup> <sup>3</sup> *ZZZ CL* <sup>32</sup> (18)

*<sup>Z</sup>* (19)

(20)

*C*

(22)

<sup>332</sup> *<sup>C</sup>*<sup>32</sup> *ZZZ <sup>L</sup>*<sup>3</sup> (23)

*ZZn ZZ*

 (24)

 (21)

332571 57 31

*L*

, (17)

*Un*

And

Where

Fig. 12. Load voltage when series device run

#### **3.2 Shunt device of high power UPQC**

Fig.13 shows the single phase equivalent circuit of the shunt device of high power UPQC. The active part of the shunt device could be considered as an ideal controlled voltage source Uinv1, the Load harmonic source is equivalent to a current source IL. The impedance of the output filter L0 and C0 are ZL0 and ZC0.

Suppose <sup>1</sup> 1 1 *T S T P U n <sup>U</sup>* and transformer *T*1 is a ideal transformer, we can learn

$$\begin{aligned} U\_{1\text{w1}} &= U\_{T1P} + Z\_{L0}(I\_{T1P} + \frac{U\_{T1P}}{Z\_{C0}}) \\ &= U\_{T1P}(1 + \frac{Z\_{L0}}{Z\_{C0}}) + Z\_{L0}I\_{T1P} \end{aligned} \tag{15}$$

And

162 Power Quality Harmonics Analysis and Real Measurements Data

Fig.13 shows the single phase equivalent circuit of the shunt device of high power UPQC. The active part of the shunt device could be considered as an ideal controlled voltage source Uinv1, the Load harmonic source is equivalent to a current source IL. The impedance of the

> C31

L0

+


*Uinv*<sup>1</sup>

(15)

C0

C32

> L3

+

+

*PTI* <sup>1</sup>


*T*1


*U*1*ST*

*PTL*

10

*IZ*

*C PT*

0 1

*Z*

( )

*C L*

0 0

*Z*

)1(

Fig. 11. Voltage sag at 0.1s

Fig. 12. Load voltage when series device run

**3.2 Shunt device of high power UPQC** 

output filter L0 and C0 are ZL0 and ZC0.

Is

+


1 *T S T P U*

1

*n*

es+ec1

Suppose <sup>1</sup>

Zs

+

UL


IL

Fig. 13. The single phase equivalent circuit of the shunt device of UPQC

L5 L7

*<sup>U</sup>* and transformer *T*1 is a ideal transformer, we can learn

*inv PTLPT*

11 10

*<sup>U</sup> IZUU*

*<sup>Z</sup> <sup>U</sup>*

*PT*

1

C7

IF

C5

$$I\_{T1P} = \mathfrak{n}\_{\mathfrak{l}} (I\_F - \frac{U\_L}{Z\_{\mathfrak{S}\mathfrak{I}}} - \frac{\mathfrak{n}\_{\mathfrak{l}} \cdot U\_{T1P}}{Z\_{332}}) \tag{16}$$

$$\text{Where}\tag{17}$$

$$\text{Where}\tag{18}$$

$$\text{V}\_{\text{S7}} = \frac{(\text{Z}\_{\text{L5}} + \text{Z}\_{\text{C5}})(\text{Z}\_{\text{L7}} + \text{Z}\_{\text{C7}})}{\text{Z}\_{\text{L5}} + \text{Z}\_{\text{C5}} + \text{Z}\_{\text{L7}} + \text{Z}\_{\text{C7}}},\tag{17}$$

$$Z\_{332} = Z\_{13} + Z\_{C32} \tag{18}$$

$$\text{Besides} \tag{18.35} \\ \text{S}(\text{Z}\_{L1} - \eta\_1 \text{LI}\_{T1P} = (\text{I}\_F - \frac{\text{LI}\_L}{\text{Z}\_{\text{S7}}}) \text{Z}\_{\text{C31}} \tag{19.36}$$

$$\mathbf{S}$$

So 57 31 <sup>31</sup> 1 1 57 1 *C C T P L F ZZ Z U UI nZ n* (20)

From equation (16) and (20), we get

$$I\_{T1P} = (1 + \frac{Z\_{C31}}{Z\_{332}})I\_F - n\_1(\frac{1}{Z\_{S7}} + n\_1 \cdot \frac{Z\_{S7} + Z\_{C31}}{n\_1 Z\_{S7} Z\_{332}})U\_L \tag{21}$$

Where

$$Z\_{S\uparrow} = \frac{(Z\_{C\uparrow} + Z\_{L\uparrow})(Z\_{C\uparrow} + Z\_{L\uparrow})}{Z\_{C\uparrow} + Z\_{L\uparrow} + Z\_{C\uparrow} + Z\_{L\uparrow}} \tag{22}$$

$$Z\_{332} = Z\_{C32} + Z\_{L3} \tag{23}$$

For completely compensating load harmonic current, IF is controlled to be the same as IL, so

$$I\_{T1P} = (1 + \frac{Z\_{\gets31}}{Z\_{332}})I\_L - n\_1(\frac{1}{Z\_{57}} + n\_1 \cdot \frac{Z\_{\gets7} + Z\_{\gets31}}{n\_1 Z\_{57} Z\_{332}})\mathcal{U}\_L\tag{24}$$

From equation (24), we can find control rule for shunt device of UPQC. If Inverter 1 is controlled to work as a current source, we can make it linear to load harmonic current and a fore-feed controller of load harmonic voltage is expected to add to the harmonic current controller. Control scheme for shunt device of high power UPQC is shown in Fig.14. To support DC linker voltage, shunt device should absorb enough energy from utility. Because it is easier for shunt device to absorb energy from utility, the DC linker voltage controller is placed in control scheme of shunt device. A PI conditioner is used here to adjust fundamental active current so as to keep DC-linker voltage const. ACR1 and ACR2 are the same as that of series device. Current out of active part is detected and form a close-loop controller. ACR3 is a hysteresis controller which makes Inverter 1 work as a current source. UL is also added to control scheme as a fore-feed controller.

Fig.15 shows the effect of this control scheme for shunt device of UPQC. The simulation parameters are shown in Tab.3. Suppose at 0.04s, passive part of shunt device is switched on

Improve Power Quality with High Power UPQC 165

(a) Before shunt device run (b) After PPF switched on (c) After APF switched on

High power UPQC is composed of series device and shunt device. Its control scheme combined control of series device and shunt device, as is shown in Fig.17. From above discussion, we know that load harmonic current is a bad disturb to series device controller because it influences load harmonic voltage. With shunt device, utility harmonic current is cut down and it does help to series device controller. On the other hand, load harmonic voltage is also a bad disturb to shunt device controller which will produce additional harmonic current and influence effect of shunt device. With series device, load harmonic voltage is cut down and it does help to shunt device controller. Cycling like this, effects of shunt device and series

device are both improved. Tab.4 shows parameters for high power UPQC.

*UDC*

*K CC*

 3/2 

*SK* )( *IF*

*sK* )( *UC*

*sK* )( *UL*

*Z IF* /1

)( <sup>1</sup> *inv SK*

*Uinv*<sup>2</sup>

*UL*

*IL ZK IF* /

*Uinv*<sup>1</sup>

*UL <sup>K</sup> CL*

*F I*

Fig. 16. Spectrums of utility current

*UDC* \* *UDC*

*U <sup>T</sup>*

*LI*

 2/3 

**3.3 Entire control of high power UPQC** 

\* *U C*

Fig. 17. Control scheme for high power UPQC

and at 0.1s active part is started. Fig.15 shows waveform of utility current during shunt device is switched on. Fig.16 shows spectrums of utility current. Before shunt device switched on, THD of utility current is 28.53%. after passive part is switched on, it is cut down to be 18.25% and after active part is also switched on it is further cut down to be 11.97%.

Fig. 14. Control scheme for shunt device of UPQC


Table 3. Parameters for shunt device

Fig. 15. Utility current waveform

and at 0.1s active part is started. Fig.15 shows waveform of utility current during shunt device is switched on. Fig.16 shows spectrums of utility current. Before shunt device switched on, THD of utility current is 28.53%. after passive part is switched on, it is cut down to be 18.25%

and after active part is also switched on it is further cut down to be 11.97%.

 3/2 

*SK* )( *IF*

Reactor: 1mH; resister: 10 ohm;

3*rd L*<sup>3</sup> 15 *mH C*<sup>31</sup> 334

*LPF L*<sup>0</sup> 4*mH C*<sup>0</sup> 15*uF*

*T*<sup>1</sup> 10 *n*<sup>1</sup>

5*th L*<sup>5</sup> 4.3 *mH C*<sup>5</sup> 120

7*th L*<sup>7</sup> 5.1 *mH C*<sup>7</sup> 140

Power source 3-phase; line to line voltage:10KV;

line Resister: 0.04 ohm; Inductor : 1uH;

Load Rectifier with series reactor and resister;

*Z IF* /1

*F C*<sup>32</sup> 669

*F*

*F* *F*

)(1 *inv SK*

*UL*

*IL ZK IF* /

*Uinv*<sup>1</sup>

*F I*

*UDC* \* *UDC*

Fig. 14. Control scheme for shunt device of UPQC

Items Description

 2/3 

Impedance of transmission

Shunt device of UPQC

Table 3. Parameters for shunt device

Fig. 15. Utility current waveform

*LI*

Fig. 16. Spectrums of utility current

#### **3.3 Entire control of high power UPQC**

High power UPQC is composed of series device and shunt device. Its control scheme combined control of series device and shunt device, as is shown in Fig.17. From above discussion, we know that load harmonic current is a bad disturb to series device controller because it influences load harmonic voltage. With shunt device, utility harmonic current is cut down and it does help to series device controller. On the other hand, load harmonic voltage is also a bad disturb to shunt device controller which will produce additional harmonic current and influence effect of shunt device. With series device, load harmonic voltage is cut down and it does help to shunt device controller. Cycling like this, effects of shunt device and series device are both improved. Tab.4 shows parameters for high power UPQC.

Fig. 17. Control scheme for high power UPQC

Improve Power Quality with High Power UPQC 167

(a) Before UPQC run (b) Switched on series device

(c) Switched on passive part (d) Switched on active part

Fig. 19. Spectrums of utility current

Fig. 20. Utility voltage waveform


Table 4. Parameters for high power UPQC.

Suppose at 0.04s, series device is switched on, at 0.1s passive part of shunt device is switched on and finally at 0.16s active part of shunt device is also switched on. Fig.18 shows the utility current waveform and Fig.19 shows its spectrums. Fig.20 shows the utility voltage waveform and Fig.21 shows its spectrums. The harmonics during switching on the whole UPQC are shown in Tab.5. We can see that power quality is improved step by step.


Table 5. THD comparison during switching on UPQC

Fig. 18. Utility current waveform

Load Rectifier load in Tab.3 paralleled with 3-phase series resister and capacitor listed in Tab.1

Suppose at 0.04s, series device is switched on, at 0.1s passive part of shunt device is switched on and finally at 0.16s active part of shunt device is also switched on. Fig.18 shows the utility current waveform and Fig.19 shows its spectrums. Fig.20 shows the utility voltage waveform and Fig.21 shows its spectrums. The harmonics during switching on the whole

> Series device only

voltage 17.7 8.26 4.78 4.77

current 40.36 31.10 11.97 8.90

Switch on passive part

Switch on active part

UPQC are shown in Tab.5. We can see that power quality is improved step by step.

2nd, 3rd, 5th, 7th harmonic voltage listed in Tab.1

Items Description

Shunt device Same as Tab.3 Series device Same as Tab.1

Table 4. Parameters for high power UPQC.

THD(%) Before

Table 5. THD comparison during switching on UPQC

Utility

Utility

Fig. 18. Utility current waveform

UPQC run

Impedance of

Power source 3-phase; line to line voltage:10KV;

transmission line Resister: 0.04 ohm; Inductor : 1uH;

Fig. 19. Spectrums of utility current

Fig. 20. Utility voltage waveform

Improve Power Quality with High Power UPQC 169

combined series APF and shunt APF can not only eliminate harmonic current but also

In some applications, the equipment needs to compensate high power reactive power produced by load. In this case, An UPQC with current-injection shunt APF is expected to be installed. This chapter discussed the principle of UPQC, including that of its shunt device and series device, and mainly discussed a scheme and control of UPQC with currentinjection shunt APF which can protect load from almost all supply problems of voltage

In high power UPQC, load harmonic current is a bad disturb to series device controller. Shunt device cuts down utility harmonic current and does help to series device controller. On the other hand, load harmonic voltage is also a bad disturb to shunt device controller and series device does much help to cut it down. With the combined action of series device and shunt device, high power can eliminate evidently load harmonic current and harmonic

Terciyanli, A., Ermis, M.& Cadirci, I. (2011). A Selective Harmonic Amplification Method for

Wen, H., Teng, Z., Wang, Y. & Zeng, B.(2010). Accurate Algorithm for Harmonic Analysis

Ahmed, K.H., Hamad, M.S., Finney, S.J., & Williams, B.W.(2010). DC-side shunt active

Wu, L.H., Zhuo, F., Zhang P.B., Li, H.Y., Wang, Z.A.(2007). Study on the Influence of

Yang, H.Y., Ren, S.Y.(2008), A Practical Series-Shunt Hybrid Active Power Filter Based on

Kim, Y.S., Kim, J.S., Ko, S.H.(2004). Three-phase three-wire series active power filter, which

Khadkikar, V., Chandra, A., Barry, A.O., Nguyen, T.D.(2005). Steady state power flow

Brenna, M., Faranda, R., Tironi, E.(2009). A New Proposal for Power Quality and Custom

Filters, Power Delivery, Vol.6., No.1, pp.65-78, ISSN: 0885-8977

Automation , Vol.1., No.13-14, pp.386-389, ISBN: 978-1-4244-5001-5

Reduction of kVA Rating of Current Source Converters in Shunt Active Power

Based on Minimize Sidelobe Window, Measuring Technology and Mechatronics

power filter for line commutated rectifiers to mitigate the output voltage harmonics, Proceeding of Energy Conversion Congress and Exposition (ECCE), 2010 IEEE, pp.151-157, ISBN: 978-1-4244-5286-6, Atlanta, GA, USA, Sept.12-16,

Supply-Voltage Fluctuation on Shunt Active Power Filter, Power Delivery, Vol.22,

Fundamental Magnetic Potential Self-Balance, Power Delivery, Vol.23, No.4,

compensates for harmonics and reactive power, Electric Power Applications,

analysis of unified power quality conditioner (UPQC), ICIECA 2005. Proceeding of International Conference, pp.6-12, ISBN: 0-7803-9419-4, Quito,

Power Improvement: OPEN UPQC, Power Delivery, Vol.24, No.4, pp.2107-2116,

quality and eliminate harmonic current transferred to power grid.

voltage and improve power quality efficiently.

No.3, pp.1743-1749, ISSN: 0885-8977

Vol.153, No.3, pp.276-282, ISSN: 1350-2352

pp.2089-2192, ISSN:0885-8977

May 10-14, 2005

ISSN:0885-8977

**5. References** 

2010

guarantee a good supply voltage.

(c) Switched on passive part (d) Switched on active part

Fig. 21. Spectrums of utility voltage

## **4. Conclusions**

To eliminate harmonics in power system, series APF and shunt APF are adopted. Series APF mainly eliminate harmonic voltage and avoid voltage sag or swell so as to protect critical load. It also helps to eliminate harmonic current if power source voltage is distorted. Shunt APF is to eliminate harmonic current avoiding it flowing through transmission line. UPQC combined series APF and shunt APF can not only eliminate harmonic current but also guarantee a good supply voltage.

In some applications, the equipment needs to compensate high power reactive power produced by load. In this case, An UPQC with current-injection shunt APF is expected to be installed. This chapter discussed the principle of UPQC, including that of its shunt device and series device, and mainly discussed a scheme and control of UPQC with currentinjection shunt APF which can protect load from almost all supply problems of voltage quality and eliminate harmonic current transferred to power grid.

In high power UPQC, load harmonic current is a bad disturb to series device controller. Shunt device cuts down utility harmonic current and does help to series device controller. On the other hand, load harmonic voltage is also a bad disturb to shunt device controller and series device does much help to cut it down. With the combined action of series device and shunt device, high power can eliminate evidently load harmonic current and harmonic voltage and improve power quality efficiently.
