**2. Fundamental knowledge**

To show better about the principle and the theory about the high power UPQC, some fundamental knowledge about harmonic and harmonic elimination equipments are list below.

Improve Power Quality with High Power UPQC 155

APF

Fig.2 shows Configuration of shunt APF, where *Zs* is impedance of transmission line, *sh i* is harmonic current trough transmission line, *Lh i* is load harmonic current and *Fh i* is harmonic current from APF. APF employs an inverter to generator a harmonic current that always

Then load harmonic current is intercepted by APF and will not pass through transmission

Usually a voltage source inverter which uses a high capacity capacitor to store energy in DC

Under some conditions, nonlinear load not only produces harmonic current but also produces much more reactive current. In order to avoid reactive current going to transmission line, the shunt equipment needs to compensate also the reactive current. Passive Power Filter (PPF) is usually added to APF to compensate most of reactive current and a part of harmonic current so as to decrease the cost. This hybrid system of APF and PF is called Hybrid Active Power Filter (HAPF) (Wu et al. 2007). In HAPF, APF and PPF are connected in different forms and form many types of HAPF. Because of its low cost, HAPF

Unified Power Quality Conditioner (UPQC) is composed of series APF and shunt APF(Yang & Ren, 2008). It not only protects the critical load from voltage quality problems but also eliminates the harmonic current produced by load. In UPQC, the series APF (usually called its series device) and shunt APF (usually called its shunt device) usually share the energy

attracts more and more eyes and has been developing very quickly.

storage so as to simplify the structure and reduce the cost of UPQC.

**2.3 UPQC: Combined shunt APF and series APF** 

*ZS*

Fig. 2. Configuration of shunt APF

line.

linker is used.

keeps equal to load harmonic current, that is:

Utility *Sh <sup>i</sup> Fh <sup>i</sup> Lh <sup>i</sup>*

Inverter

Nonlinear load

*LhFh ii* (1)

<sup>0</sup> *sh <sup>i</sup>* (2)

### **2.1 Series active power filter**

In power system, voltage out from turbine is promising to be sinusoidal. So if there is no nonlinear load connects to power grid between generator and the nonlinear load in question, a shunt APF is enough to keep both the voltage and the current of transmission line sinusoidal because the transmission line is composed of linear components such as resistances, inductions and capacitors. But in modern power system, power is transmitted for a long distance before delivery to the nonlinear load and power is distributed to many nonlinear loads in many difference places along the transmission line. The transmission of harmonic current causes harmonic voltage in transmission lines which increases possibility of damage to some critical loads such as storage devices and some micromachining devices. Shunt APF can do little with the damage caused by harmonic voltage in transmission line. A series APF is installed between power source and critical load so as to insulate voltage harmonic from the critical load(Kim et al. 2004). It is also promising to eliminate damages to load caused by some other supply quality issues such as voltage sage, instant voltage interrupts, flicks and over voltage.

Fig. 1. Configuration of series APF

## **2.2 Shunt active power filter**

The distortion of current not only brings serious loss of power transmission, but also endangers power grid and power equipments. Harmonic current increases the current flowed through transmission lines and as a result power transmission loss is increased and power grid has to take a risk of higher temperature which threatens the safety of power grid. Harmonic current in transformers will make them magnetic saturated and seriously heated. Much noise is generated because of harmonics in equipments. Besides, harmonics make some instruments indicate or display wrong values, and sometimes make they work wrong.

To eliminate harmonic current produced by nonlinear loads, a shunt Active Power Filter (APF) is expected to connect parallel to power grid(Ahmed et al. 2010). Shunt APF draws energy from power grid and makes it to be harmonic current that is equal to the harmonic current produced by nonlinear load so that harmonic current doesn't go to transmission line but goes between nonlinear load and APF. Usually an inverter is employed to realize this function.

In power system, voltage out from turbine is promising to be sinusoidal. So if there is no nonlinear load connects to power grid between generator and the nonlinear load in question, a shunt APF is enough to keep both the voltage and the current of transmission line sinusoidal because the transmission line is composed of linear components such as resistances, inductions and capacitors. But in modern power system, power is transmitted for a long distance before delivery to the nonlinear load and power is distributed to many nonlinear loads in many difference places along the transmission line. The transmission of harmonic current causes harmonic voltage in transmission lines which increases possibility of damage to some critical loads such as storage devices and some micromachining devices. Shunt APF can do little with the damage caused by harmonic voltage in transmission line. A series APF is installed between power source and critical load so as to insulate voltage harmonic from the critical load(Kim et al. 2004). It is also promising to eliminate damages to load caused by some other supply quality issues such as voltage sage, instant voltage

Critical load

C

C1

+

The distortion of current not only brings serious loss of power transmission, but also endangers power grid and power equipments. Harmonic current increases the current flowed through transmission lines and as a result power transmission loss is increased and power grid has to take a risk of higher temperature which threatens the safety of power grid. Harmonic current in transformers will make them magnetic saturated and seriously heated. Much noise is generated because of harmonics in equipments. Besides, harmonics make some instruments indicate or display wrong values, and sometimes make they work

To eliminate harmonic current produced by nonlinear loads, a shunt Active Power Filter (APF) is expected to connect parallel to power grid(Ahmed et al. 2010). Shunt APF draws energy from power grid and makes it to be harmonic current that is equal to the harmonic current produced by nonlinear load so that harmonic current doesn't go to transmission line but goes between nonlinear load and APF. Usually an inverter is employed to realize this


+ -

EC2

EC1-

L1

*UT* +

**2.1 Series active power filter** 

interrupts, flicks and over voltage.

Fig. 1. Configuration of series APF

**2.2 Shunt active power filter** 

wrong.

function.

Zs

#### Fig. 2. Configuration of shunt APF

Fig.2 shows Configuration of shunt APF, where *Zs* is impedance of transmission line, *sh i* is harmonic current trough transmission line, *Lh i* is load harmonic current and *Fh i* is harmonic current from APF. APF employs an inverter to generator a harmonic current that always keeps equal to load harmonic current, that is:

$$\dot{\mathfrak{u}}\_{Fh} \equiv \dot{\mathfrak{u}}\_{Lh} \tag{1}$$

Then load harmonic current is intercepted by APF and will not pass through transmission line.

$$
\dot{a}\_{sh} \equiv 0 \tag{2}
$$

Usually a voltage source inverter which uses a high capacity capacitor to store energy in DC linker is used.

Under some conditions, nonlinear load not only produces harmonic current but also produces much more reactive current. In order to avoid reactive current going to transmission line, the shunt equipment needs to compensate also the reactive current. Passive Power Filter (PPF) is usually added to APF to compensate most of reactive current and a part of harmonic current so as to decrease the cost. This hybrid system of APF and PF is called Hybrid Active Power Filter (HAPF) (Wu et al. 2007). In HAPF, APF and PPF are connected in different forms and form many types of HAPF. Because of its low cost, HAPF attracts more and more eyes and has been developing very quickly.

#### **2.3 UPQC: Combined shunt APF and series APF**

Unified Power Quality Conditioner (UPQC) is composed of series APF and shunt APF(Yang & Ren, 2008). It not only protects the critical load from voltage quality problems but also eliminates the harmonic current produced by load. In UPQC, the series APF (usually called its series device) and shunt APF (usually called its shunt device) usually share the energy storage so as to simplify the structure and reduce the cost of UPQC.

Improve Power Quality with High Power UPQC 157

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

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

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

1

(3)

(4)

1 1 *C*

*L C*

1 1

*L C*

*C*

*Z Z Z U U*

1

( )

*CL C*

11 1

*ZZ*

*T L*

<sup>2</sup> ( ) *U K U Bs inv V DC* (5)

*DC*

)( (6)

*U*

1 1 1

*C*

Where *KV* is amplitude ratio between *Uinv*2 and *UDC* , *B s*( ) is phase shift between input

*L C*

*nZ*

compensation to every order harmonic current except 3rd harmonic current.

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

2 2

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

*C inv*

2 2

*DCCLT*

1

*<sup>Z</sup> sBKnU*

*T V*

*UKU*

*CTL*

*EUU*

*Z Z U E*

*inv C*

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

a good sinusoidal voltage(Brenna et al. 2009; Zhou et al. 2009).

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

control signal and output voltage of Inverter 2.

**3.1 Series device of high power UPQC** 

the active part are in the dashed box.

Fig. 3. Unified Power Quality Conditioner
