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A Unified Power Quality Conditioner (UPQC) is a device that is similar in construction to a Unified Power Flow Conditioner (UPFC). The UPQC, just as in a UPFC, employs two voltage source inverters (VSIs) that connected to a dc energy storage capacitor. One of these two VSIs is connected in series with ac line while the other is connected in shunt with the ac system.

A UPQC that combines the operations of a Distribution Static Compensator (DSTATCOM) and Dynamic Voltage Regulator (DVR) together.

A UPQC is employed in a power transmission system to perform shunt and series compensation at the same time. A power distribution system may contain unbalance, distortion and even dc components. Therefore a UPQC operate, better than a UPFC, with all these aspects in order to provide shunt or series compensation.

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The UPQC is a relatively new device and not much work has yet been reported on it. Sometimes it has been viewed as combination of series and shunt active filters.

Taking the load voltage, V_{L} as a reference phasor and suppose the lagging power factor of the load is cos φ_{L} we can write;

The UPQC is assumed to be lossless and therefore, the active power demanded by the load is equal to the active power input at PCC. The UPQC provides a nearly unity power factor source current, therefore, for a given load condition the input active power at PCC can be expressed by the following equations-

The above equation suggests that the source current is depends on the factor k, since φ_{L} and i_{L} are load characteristics and are constant for a particular type of load. The complex apparent power absorbed by the series APF can be expressed as-

The current provided by the shunt APF, is the difference between the input source current and the load current, which includes the load harmonics current and the reactive current. Therefore, we can write;

**Based on the above analysis the different modes of operation are discussed: **

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**Case I: **

The reactive power flow during the normal working condition when UPQC is not connected in the circuit is shown in the Fig. 5.61(a). In this condition the reactive power required by the load is completely supplied by the source only. When the UPQC is connected in the network and the shunt APF is put into the operation, the reactive power required by the load is now provided by the shunt APF alone; such that no reactive power burden is put on the mains.

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So as long as the shunt APF is ON, it is handling all the reactive power even during voltage sag, voltage swell and current harmonic compensation condition. The series APF is not taking any active part in supplying the load reactive power. The reactive power flow during the entire operation of UPQC is shown in the Fig. 5.61(b). In this case on active power transfer takes place via. UPQC, termed as Zero Active Power Consumption Mode.

**Case II: **

P’_{s} = Power Supplied by the source to the load during voltage sag condition

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P’_{sr} = Power Injected by Series APF in such way that sum P’_{sr} + P’_{sr} will be the required load power during normal working condition i.e., P_{L }

P’_{sh} = Power absorbed by shunt APF during voltage sag condition P’_{sr} =P’_{sh}

If k < 0, i.e., V_{t} < V_{L}, then P_{Sr} will be positive, means series APF supplies the active power to the load. This condition is possible during the utility voltage sag condition. Is will be more than the normal rated current. Thus we can say that the required active power is taken from the utility itself by taking more current so as to maintain the power balance in the network and to keep the dc link voltage at desired level. This active power flows from the source to shunt APF, from shunt APF to series APF via. dc link and finally from series APF to the load. Thus the load would get the desired power even during voltage sag condition.

Therefore in such cases the active power absorbed by shunt APF form the source is equal to the active power supplied by the series APF to the load. Since series APF supplies active power, termed as Active Power Delivering Mode. The overall active power flow is shown in the Fig. 5.62.

**Case III: **

P_{s}” = Power Supplied by the source to the load during voltage swell condition

P”_{sr} = Power Injected by Series APF in such way that sum P_{s}” – P”_{sh} will be the required load power during normal working condition

P”_{sh} = Power delivered by shunt APF during voltage sag condition P”_{sr}=p”_{sh}

If k > 0, i.e., v_{t} > v_{L}, then P_{Sr} will be negative, this means series APF is absorbing the extra real power from the source. This is possible during the voltage swell condition. Again i_{s} will be less than the normal rated current. Since v_{s} is increased, the dc link voltage can increase. To maintain the dc link voltage at constant level the shunt APF controller reduces the current drawn from the supply. In other words we can say that the UPQC feeds back the extra power to the supply system. Since series APF absorbs active power, termed as Active Power Absorption Mode. The overall active power flow is shown in the Fig. 5.63.

**Case IV: **

If k = 0, i.e., v_{t} = v_{L}, then there will not be any real power exchange though UPQC. This is the normal operating condition. The overall active power flow is shown in the Fig. 5.64.

**Case V:**

If the terminal voltage is distorted one containing several harmonics, in such cases the series APF injects voltage equal to the sum of the harmonics voltage at PCC but in opposite direction. Thus the sum of voltage injected by series APF and distorted voltage at PCC will get cancelled out. During this voltage harmonic compensation mode of operation the series APF does not consume any real power from sources since it injects only harmonics voltage. Here UPQC works in zero active power consumption mode.

**Case VI: **

If the load is a non-linear one producing harmonics, in such cases the shunt APF injects current equals to the sum of harmonics current but in opposite direction, thus cancelling out any current harmonics generated by nonlinear load. During this current harmonics compensation mode of operation them shunt APF does not consume any real power form the source since it injected only harmonics currents. Here UPQC works in zero active power consumption modes.

The phasor representations of the conditions are shown in the Fig. 5.65(a) (a)-(d) and Fig. 5.65(a)-(d) for inductive and capacitive type of load respectively. Phasor 5.65(a) represents the normal working condition, considering load voltage v_{L} as a reference phasor φL is lagging power factor angle of the load. During this condition i_{s} will be exactly equal to the i_{L} since no compensation is provided.

When shunt APF is put into the operation, it supplies the required load vars by injecting a 90° leading current such that the source current will be in phase with the terminal voltage. The phasor representing this capacitive effect is shown in Fig. 5.65(b). The phasor representations during voltage sag and voltage swell condition on the system are shown in the Fig. 5.65 (c) and Fig. 5.65 (d) respectively. The deviation of shunt compensating current phasor from quadrature relationship with load voltage suggests that there is some active power flow through the shunt APF during these conditions.

Phasor in Fig. 5.66(a) represents the normal working condition, considering leading power factor angle of the load. During this condition i_{s} will be exactly equal to the i_{L}. When shunt APF is put into the operation, it cancels ut the vars generated by load by injecting a 90° lagging current such that the source current will be in phase with the terminal voltage. The phasor representing this inductive effect is shown in Fig. 5.66(b). The phasor representations during voltage sag and voltage swell condition on the system are shown in the Fig. 5.66(c) and Fig. 5.66(d) respectively.

Fig. 5.67 shows variation of angle F_{Sh} during different modes of operations of UPQC, represented by zones. Figure consists of seven zones of operations. The x axis represents the reference load voltage whereas the shunt APF compensating current can vary form 0° to 360°. Zone I, II and III represents the case of pure resistive, inductive and capacitive load respectively. If the load is pure resistive, shunt APF des not inject any compensating current since there is no reactive power demand from the load, this condition is represented by zone I.

Considering the case of inductive load, the load var requirement is supplied by shunt APF by injecting 90° leading current. The magnitude of the compensating current would depend on the vars to be compensated. This condition is represented by zone II. Now, if the load is capacitive one, theoretically, the load would draw leading current from the source, i.e., load generates vars. This load generated vars are compensated by shunt APF by injecting 90° lagging current. The magnitude of compensating current depends on the vars to be cancelled out, represented by zone III. During the operation of UPQC in zone II and III larger the var compensation more would be the compensating current magnitude.

Zone IV and zone V represents the operating region of UPQC during the voltage sag on the system for inductive and capacitive type of the loads respectively. During the voltage sagas, shunt APF draws the required active power from the source by taking extra current from the source. In order to have real power exchange between source, UPQC and load, the angle F_{sh} should not be 90°.

For inductive type of the load, this angle could be anything between 0° to 90° leading and for capacitive type of the load, between 0° to 90° lagging. This angle variation mainly depends on the % of sag need to be compensated and load var requirement. Zone VI and zone VII represents the operating region of UPQC during the voltage swell on the system for inductive and capacitive type of the loads respectively. During the voltage swell, shunt APF feeds back the extra active power from the source by taking reduced current from the source. In order to achieve this angle F_{sh} would be between 90° to 180° leading and between 90° to 180° lagging for inductive and capacitive type of load respectively.