EEE-6380
Unified Power Quality Conditioner(UPQC)
Prof. Salman Hameed
Department of Electrical Engineering,
AMU Aligarh
Contents
• Introduction
• Power quality
• Power quality problems and issues
• Basic configuration of UPQC
• Operation of UPQC
• Advantages
Introduction
• The power electronic devices due to their inherent non-linearity draw harmonic and reactive power from the supply.
• In three phase systems, they could also cause unbalance and draw excessive neutral currents.
• The injected harmonics, reactive power burden, unbalance, and excessive neutral currents cause low system efficiency and poor power factor.
• The quality of the Electrical power is effected by many factors like harmonic contamination , due to non-linear loads, voltage and current flickering due to arc in arc furnaces, sag and swell due to the switching of the loads etc.
• One of the many solutions is the use of a combined system of shunt and active series filters like unified power quality conditioner (UPQC)
Power Quality
The quality of electrical power supply is a set of parameters which describe the process of electric power delivery to the user under normal operating conditions, determine the continuity of supply (short and long supply interruptions) and characterize the supply voltage (magnitude, asymmetry, frequency, and waveform shape).
Power quality phenomena can be divided into two types :- .
• A characteristic of voltage or current (e.g., frequency or power factor) is never exactly equal to its nominal and desired value. The small deviations are called voltage variations or current variations.
• When the voltage or current deviates significantly from its normal or ideal wave shape.
These sudden deviations are called events. Power quality events are the phenomena which can lead to tripping of equipment, to interruption of the production or of plant operation, or endanger power system operation. This includes interruptions, under voltages, overvoltage, phase angle jumps and three phase unbalance.
Power Quality Problems
• Voltage Sag
• Voltage swell
• Interruption
• Distortion and
• Harmonics.
Power Quality Conditioners
A power conditioner (also known as a line conditioner or power line conditioner) is a device intended to improve the quality of power that is delivered to electrical load equipment.
• Types of Power Quality Conditioners
• Active power filters
-Shunt active power filters -Series active power filters
• Unified Power Quality conditioner (UPQC)
Basic Configuration of UPQC
• UPQC is the integration of series and shunt active power filters ,connected back- to back on the dc side, sharing a common DC capacitor.
• The series component of the UPQC is responsible for mitigation of the supply side disturbances.
• The shunt component is responsible for mitigating the current quality problems
caused by the consumer.
Series active power filter
• It compensate current system distortion caused by non-linear loads.
• The high impedance imposed by the series APF is created by generating a voltage of the same frequency as that of harmonic component that needs to be eliminated.
• It act as a controlled voltage source and
can compensate all voltage related
problems such as voltage harmonics,
voltage sags & swells, voltage flicker etc.
Shunt active power filter
• It compensate current harmonics by injecting equal-but-opposite harmonic compensating current.
• It operates as a current source injecting the harmonic components generated by the load but phase shifted by 180
0.
• They are usually connected across the
load to compensate for all current
related problem such as reactive power
compensation, power factor correction,
current harmonics and load unbalance
compensation.
Operation of UPQC
• The Unified Power Quality Conditioner (UPQC) combines the Shunt Active Power Filter
with the Series Active Power Filter, sharing the same DC Link, in order to compensate both
voltages and currents, so that the load voltages become sinusoidal and at nominal value,
and the source currents become sinusoidal and in phase with the source voltages.
Operation of UPQC
• 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 are 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.
• The UPQC is a relatively new device and not much work has yet been reported on it.
Operation of UPQC
• Taking the load voltage, VL as a reference phasor and suppose the lagging power factor of the load is cos φL we can write;
Operation of UPQC
• 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 φL and iL 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 below:
• Case-I
Fig. 1. Reactive power flow
• The reactive power flow during the normal working condition when UPQC is not connected in the circuit is shown in the Fig. 1(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.
• 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. 1(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
• 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., PL
• P’sh = Power absorbed by shunt APF during voltage sag condition P’sr =P’sh
• If k < 0, i.e., Vt < VL, then PSr 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.
Fig. 2. Active power flow during voltage sag condition
• Case-III
• Ps” = Power Supplied by the source to the load during voltage swell condition
• P”sr = Power Injected by Series APF in such way that sum Ps” – 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., vt > vL, then PSr 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 is will be less than the normal rated current. Since vs 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. above.
Fig. 3. Active power flow during voltage swell
• Case-IV
• If k = 0, i.e., vt = vL, 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. 4.
Fig. 4. Active power flow during normal condition
• 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(a)-(d). Phasor 5(a) represents the normal working condition, considering load voltage vL as a reference phasor φL is lagging power factor angle of the load. During this condition is will be exactly equal to the iL 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(b). The phasor representations during voltage sag and voltage swell condition on the system are shown in the Fig. 5 (c) and Fig. 5 (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.
Fig. 5. The phasor representations of inductive load
• Phasor in Fig. 6(a) represents the normal working condition, considering leading power factor angle of the load. During this condition is will be exactly equal to the iL. 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. 6(b). The phasor representations during voltage sag and voltage swell condition on the system are shown in the Fig. 6(c) and Fig. 6(d) respectively.
Fig. 6. The phasor representations of capacitive load
• Fig. 7 shows variation of angle FSh 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.
Fig. 7 Variation of angle during different modes of operations of UPQC
• 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 Fsh 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 Fsh would be between 90° to 180° leading and between 90° to 180° lagging for inductive and capacitive type of load respectively.
Advantages
• UPQC can compensate both voltage related problems such as voltage harmonics, voltage sags/swells, voltage flicker as well as current related problems like reactive power compensation, power factor correction, current harmonics and load unbalance compensation.
• There is a significant increase in interest for using UPQC in distributed
generation associated with smart grids because of availability of high
frequency switching devices and advanced fast computing devices
(microcontrollers, DSP, FPGA) at lower cost.
Dynamic Voltage Restorer (DVR)
Introduction
• Power Quality: Worthiness of electrical power to consumer devices i.e. not to disturb or damage appliances and devices.
Use of sensitive electronic equipment has increased
• Power quality problems
• Voltage sag
• Voltage swell
• Voltage collapse
• Can cause malfunctioning of sensitive equipments , protection and relay system,
• Can cause Short circuits, lightning strokes.
Parameters considered for Power Quality
• Voltage Magnitude
• Frequency
• Wave-shape
Effects of Power Quality
Power Quality issues cause business problems such as:
• Lost productivity, wastage of manpower and equipments
• Customer and/or management dissatisfaction
• Heavy financial impact
According to Electric Light and Power Magazine, 30% to 40 % of All
Business Downtime is Related to Power Quality issues.
Why Power Quality is Big issue Presently?
• Widespread Usage of Sensitive Loads in all Applications (PLC based equipments, Computers and the like)
• For certain devices, a momentary disturbance may result in
• scrambled data
• interrupted communications
• a frozen mouse
• system crashes and equipment failure Power.
Major Power Quality Problems
• Voltage sags,
• Swells,
• Spikes,
• Outages
Dynamic Voltage Restorer (DVR)
• Series compensation device
• Protects sensitive electric loads from PQ problems
• Implemented through power electronic controllers that use voltage source converters (VSC).
Operating principle of DVR
•
The basic principle of dynamic voltage restoration is to inject a voltage of the magnitude and frequency necessary to restore the load side voltage to the desired amplitude and waveform, even when the source voltage is unbalanced or distorted.
•
Generally, devices for dynamic voltage restoration employ gate turn off thyristors, (GTO) solid state power electronic switches in a PWM inverter structure.
•
The DVR can generate or absorb independently controllable real and reactive power at the load side.
•
The source of the injected voltage is the commutation process for reactive power demand and an energy source for the real power demand. The energy source may vary according to the design and manufacturer of the DVR, but DC capacitors and batteries drawn from the line through a rectifier are frequently used. The energy source is typically connected to the DVR through its DC input terminal.
•
The amplitude and phase angle of the injected voltages are variable, thereby allowing
control of the real and reactive power exchange between the DVR and the distribution
system. As the reactive power exchange between the DVR and the distribution system is
internally generated by the DVR without the AC passive reactive components.
Applications
• Practically, DVR systems can inject up to 50% of nominal voltage, but only for a short time (up to 0.1 seconds).
• However, most voltage sags are much less than 50 percent, so this is not typically an issue.
• DVRs can also mitigate the damaging effects of voltage swells, voltage unbalance and other waveform distortions.
Drawbacks
• DVRs may provide good solutions for end-users subject to unwanted power quality disturbances.
However, they are generally not used in systems that are subject to prolonged reactive power deficiencies (resulting in low voltage conditions) and in systems that are vulnerable to voltage collapse. Because DVRs will maintain appropriate supply voltage, in such systems where incipient voltage conditions are present they actually make collapses more difficult to prevent and can even lead to cascading interruptions.
• Therefore, when applying DVRs, it is vital to consider the nature of the load whose voltage supply is being secured, as well as the transmission system which must tolerate the change in voltage-response of the load. It may be necessary to provide local fast reactive supply sources in order to protect the system, including the DVR, from voltage collapse and cascading interruptions.
