1
A STUDY OF POWER QUALITY & FUNCTIONING OF FACT DEVICES DVR ,UPFC, UPQC
SANJAY SHRIVASTAVA1 DR BHARAT MISHRA2
DR A K SHARMA3
1(Research Scholar),Deptt. Of Electrical Engineering ,MGCGV Chitrakoot ,Satna (M P) harshilshri@gmail.com
2Associate Prof. and Head, Deptt of Physical Sciences , MGCGV Chitrakoot ,Satna (M P) bharat.mgcgv@gmail.com
3Professor and Head, Department of Electrical Engineering, Jabalpur Engineering College Jabalpur(M.P), India
Abstract:-Power quality is one of major problems in the today’s scenario. It has become important with the introduction of complex devices, whose performance is very sensitive to the quality of power supply. Power quality problem is an occurrence developed as a nonstandard voltage, current or frequency that results in a failure of end use equipments. Some of the major problems dealt here is the power sag and swell. This paper describes the effectiveness of using dynamic voltage restorer (DVR) in low voltage distribution systems. The Dynamic Voltage Restorer (DVR) is a rapid, flexible and resourceful solution to power quality problems. Also, In this paper the performance of Unified Power Flow Controller (UPFC) is studied in controlling the flow of power over the transmission line. And The Unified power quality conditioner (UPQC) is an efficient traditional power electronics device for the improvement of power quality because of its quick response, high consistency and insignificant value. A Unified power quality conditioner is used to suppress misrepresentation, unstable voltage and current situation.
Keywords:- DVR, UPFC , UPQC , Power Quality
I. INTRODUCTION
Power quality is of great importance in all modern environments where electricity is involved, power quality can be
essentially influenced by an important factor like quality service. One of the major concerns in electricity industry today is
2 power quality problems. Presently, most of the power quality problems are due to different fault conditions. These conditions cause voltage sag, voltage swell, transients, voltage interruption and harmonics. These problems may cause the apparatus tripping, shutdown commercial, domestic and industrial equipment, and miss process of drive system.
1.1 POWER QUALITY
PROBLEMS, CAUSES AND EFFECTS
The various power quality problems are as followed:
1. Transients- A transient is a temporary occurrence of a fault which is of a very short duration in a system caused by the sudden change of state.
2. Voltage sags- A voltage sag or voltage dip is a short duration reduction in rms voltage which can be caused by a short circuit, overload or starting of electric motors. A voltage sag happens when the rms voltage decreases between 10 and 90 percent of
nominal voltage for one-half cycle to one minute
3. Voltage swells- Voltage swell, which is a momentary increase in voltage, happens when a heavy load turns off in a power system.
4. Voltage interruption- Interruptions are classified as short-duration or long-duration variation. The term interruption is often used to refer to short- duration interruption, while the latter is preceded by the word sustaine to indicate a long- duration. They are measured and described by their duration since the voltage magnitude is always less than 10% of nominal.
5. Harmonics- Harmonics is the integral multiple of frequencies voltages and currents in an electric power system due to non linear loads. Harmonic frequencies in the power grid are a frequent cause of power quality problems.
3 Fig. 1. Power Quality Problems
1.2 Causes of Power Quality Problems:
Transient – Due to Lightning, Turning major equipment on or off, back to back capacitor energization.
Voltage Sags – Due to starting of large Motors, Energization of heavy
loads, incorrect VAR
compensation.
Voltage Swells – Energizing a large capacitor bank, Switching off a large load, incorrect VAR compensation
Interruption – Faults (Short circuit), Equipment failures,
Control malfunctions (attempting to isolate electrical problem).
Harmonics – IT equipment, Variable frequency drives, Electro Magnetic Interference from appliances, fluorescent lighting, Arc Furnace (Any non linear load).
Effects of Power Quality Problems:
Transient – Tripping, Processing error, Data loss, hardware reboot required, Component failure.
Voltage Sags--Dim lights, Equipment shutdown, Data error, shrinking display screens, Memory loss.
Voltage Swells –Bright lights, Data error, shrinking display screens, Memory loss.
Interruption – Faults, Equipment failures, Control malfunctions
Harmonics – Line current increases, Losses increase, transformer and neutral conductor heating leading to reduced equipment life span.
2. INTRODUCTION TO FACTS DEVICES
Flexible transmission system is akin to high voltage dc and related
4 thyristors developed designed to overcome the limitations of the present mechanically controlled ac power transmission system. Use of high speed power electronics controllers, gives 5 opportunities for increased efficiency.
Greater control of power so that it flows in the prescribed transmission routes.
Secure loading (but not overloading) of transmission lines to levels nearer their required limits.
Greater ability to transfer power between controlled areas, so that the generator reserve margin- typically 18 % may be reduced to 15 % or less.
Prevention of cascading outages by limiting the effects of faults and equipment failure.
Damping of power system oscillations, which could damage equipment and or limit usable transmission capacity.
Flexible system requires tighter transmission control and efficient management of inter-related parameters that constrains today’s
system including – Series impedance- phase angle.
Shunt impedance- occurrence of oscillations at various frequencies below rated frequency.
This results in transmission line to operate near its thermal rating.
Eg- a 1000kv line may have loading limit 3000-4000Mw .but the thermal limit may be 5000Mw[1].
SVC- Uses thyristor valves to rapidly add or remove shunt connected reactors and or capacitors often in coordination with mechanically controlled reactors and/or capacitors.
NGH-SSR damper- a resonance damper:- A thyristor ac-switch connected in series with a small inductor and resistor across the series capacitor[1].
Statcom (static condenser):- A 3 phase inverter that is driven from voltage across a dc storage capacitor and whose there output voltages are in phase with the ac system voltage. when the output voltages are higher or lower than the ac system voltage the current
5 flow is caused to lead or lag and difference in voltage amplitudes determine how much current flows[2]. Reactive power and its polarity can be controlled by controlling voltage.
Phase Angle Regulator:-The phase shift is accomplished by adding or subtracting a variable voltage concept that is perpendicular to the phase voltage of the line
Unified power control:- In this concept an ac voltage vector generated by a thyristor based inverter is injected in series with phase voltage. The driving dc voltage for inverter is obtained by rectifying the ac to dc from the same transmission line. In such an arrangement the injected voltage may have any phase angle relationship to the phase voltage. It is possible to obtain a net phase and amplitude voltage change that confers control of both active and reactive power.
Dynamic Brake:- A shunt connected resistive load, controlled by thyristor switches. such a load can be selectively applied in each
pass, half cycle by half cycle to damp any specific power flow oscillation, so that generating unit run less risk of losing synchronism ,as a result more can be transferred over systems subjected to stability constraints.
2.1 ADVANTAGES OF TCR IN FACT
Accuracy of compensation-Very good
Control flexibility-Very good
Reactive power capacity- Lagging or leading indirect
Control – Continuous
5 Response Time- Fast, 0.5 to 0.2 cycles
Harmonics- Very high(Large size filters are needed)
Losses- Good but increase in lagging mode
Phase balancing ability- good
Cost-moderate
2.2 Dynamic voltage restorer (DVR)
Dynamic voltage restorer (DVR) can provide the lucrative solution to mitigate voltage sag by
6 establishing the appropriate voltage quality level, necessary. It is recently being used as the active
solution for mitigation of power quality problems.
Fig. 2. Basic Structure of A DVR
2.3 DVR (DYNAMIC VOLTAGE RESTORER)
The basic construction of a DVR is shown in Fig. I.
(i) Energy Storage Unit:
This unit is responsible for energy storage in DC form. Flywheels, Batteries, superconducting magnetic energy storage (SMES) and super capacitors can be used as energy storage devices. It is supplies the real power
requirements of the system when DVR is used for compensation.
(ii) Capacitor:
DVR has a large DC capacitor to ensure a proper DC voltage input to Inverter.
(iii) Inverter:
Inverter system is used to convert dc storage into ac. Voltage source inverter (VSI) of low voltage and high current with step up injection transformer is used for this
7 purpose in the DVR Compensation technique.
(iv) Passive Filters:
Filters convert the inverted PWM waveform into a sinusoidal waveform easily. This is achieved by eliminating the unwanted harmonic components generated VSI action. Higher orders harmonic components distort the compensated output voltage.
(v) By-Pass Switch:
It is used to protect the inverter from High current in the presence of unwanted conditions. During the occurrence of a fault or a short circuit, DVR changes it into the bypass condition where the VSI inverter is protected against over current flowing through the power semiconductor switches. The rating of the DVR inverters is a limiting factor for normal load current seen in the primary winding and reflected to the secondary winding of the series insertion transformer.
(vi) Voltage Injection Transformers:
In a three-phase system, either three single-phase transformer
units or one three phase transformer unit can be used for voltage injection purpose. Basic principal of DVR is to transfer the voltage sag compensation value from DC side of the inverter to the injected transformer after filter.
The compensation capacity of a particular DVR depends on the maximum voltage injection capability and the active power that can be supplied by the DVR.
When DVR’s voltage disturbance occurs, active power or energy should be injected from DVR to the distribution system A DC system, which is connected to the inverter input, contains a large capacitor for storage energy. It provides reactive power to the load during faulty conditions. When the energy is drawn from the energy storage capacitors, the capacitor terminal voltage decrease. Therefore, there is a minimum voltage required below which the inverter of the DVR cannot generate the require voltage thus, size and rating of capacitor is very important for DVR power circuit.
8 2.4. DVR OPERATING STATES
1. During a voltage sag/swell on the line:
The DVR injects the difference between the pre-sag and the sag voltage, by supplying the real power requirement from the energy storage device together with the reactive power. The maximum injection capability of the DVR is limited by the ratings of the DC energy storage and the voltage injection transformer ratio. In the case of three single-phase DVRs the magnitude of the injected voltage can be controlled individually. The injected voltages are made synchronized (i.e. same frequency and the phase angle) with the network voltages.
2. During the normal operation:
As the network is working under normal condition the DVR is not injecting any voltages to the system. In that case, if the energy storage device is fully charged then the DVR operates in the standby mode or otherwise it operates in the self-charging mode. The energy storage device can be charged
either from the power supply itself or from a different source.
3. During a short circuit or fault in the downstream of the distribution line:
In this particular case the by-pass switch is activated to provide an alternate path for the fault currents. Hence the inverter is protected from the flow of high fault current through it, which can damage the sensitive power electronic components.
2.5 DVR COMPENSATION TECHNIQUES
The compensation control technique of the DVR is the method used to track the supply voltage and synchronized that with the pre-sag supply voltage during a voltage sag/swell in the upstream of distribution line. Generally voltage sags are associated with a phase angle jump in addition to the magnitude change. Therefore the control technique adopted should be capable of compensating for voltage magnitude, phase shift and thus the wave shape. But
9 depending on the sensitivity of the load connected downstream, the level of compensation of the above parameters can be altered.
Basically the type of load connected influences the compensation strategy. For example, for a linear load, only magnitude compensation is required as linear loads are not sensitive to phase angle changes.
Further when deciding a suitable control technique for a particular load, the limitations of the voltage injection capability and the size of the energy storage device should be considered. Compensation is achieved through real power and reactive power injection.
Depending on the level of compensation required by the load, three types of compensation methods are defined and discussed below namely pre-sag compensation, in-phase compensation and energy optimization technique.
3. OVERVIEW UPFC
The power-transfer capability of long transmission lines are usually
limited by large signals ability.
Economic factors, such as the high cost of long lines and revenue from the delivery of additional power, give strong incentives to explore all economically and technically feasible means of raising the stability limit. On the other hand, the development of effective ways to use transmission systems at their maximum thermal capability has caught much research attention in recent years. Fast progression in the field of power electronics has already started to influence the power industry. This is one direct outcome of the concept of flexible ac transmission systems (FACTS) aspects, which has become feasible due to the improvement realized in power- electronic devices. In principle, the FACTS devices could provide fast control of active and reactive power through a transmission line. The unified power-flow controller (UPFC)is a member of the FACTS family with very attractive features.
This device can independently control many parameter, so it is the combination of the properties
10 of a static synchronous compensator (STATCOM) and
static synchronous series compensator (SSSC)
Figure 3: UPFC constructions .
The conversion from DC voltage to AC voltage is obtained by using standard bridge circuits. These bridge circuits use GTO as their building blocks. Since these circuits convert DC voltage to AC voltage, they are termed as voltage source converters (VSC). The control system associated with VSC allows it to adjust its magnitude and phase angle. The
term "inverter" has also been used to denote the VSC. Consider now the connection of two VSC connected back to back with a common DC Link capacitor 'C' as shown in Fig. 3. Such an arrangement allows for al1 the three functions namely series, shunt and phase angle compensation to be unified in one unit. Inverter 1 is connected to a shunt transformer and the inverter
11 2 is connected to a series transformer. It is readily seen that the VSC connected to the shunt transformer can perform the function of a variable reactive power source similar to that of shunt compensator. In addition, the inverter 1 can charge the DC link capacitor. Inverter 2 can provide series or phase angle compensation by injecting a series voltage of proper phase relationship. In the case of series compensation, inverter 2 can function independent of the inverter 1, as inverter 2 supplies/consumes only reactive power and does not have any real power exchange with invener 1. In such a case. the DC link capacitor voltage will ideally be constant
4. UPFC Modeling 4.1 Load Flow Models
Different load flow models have been used to mode1 the UPFC in varying degree of complexity and have been discussed here briefly.
As mentioned in chapter-l, a UPFC consists of two inverters connected back to back with a DC link
capacitor. One inverter is connected in shunt and the other in series with the transmission line as shown in Fig.4. The early modelling efforts for a UPFC were focussed on the series inverter modelling. The reason being that commercial software did not have series voltage source models.
American Electric Power (AEP) and Westinghouse came up with a load flow model [8]. The requirement for the inclusion of the model was that the Load flow should be a solved one. Basically, what was required was that the voltages and the angles of the power system buses had to be known in advance to inchde the UPFC model. The Load flow model for UPFC consisted of two generators, one representing the shunt inverter and the other the series inverter. Different configurations of these generators were needed to model different operating conditions. Fig.4. shows the model that was used to include the UPFC into Load flow studies [8]. Here the process of solving starts with the opening of the series line, and the generator G2
12 generates the scheduled real and reactive power. The scheduled power in the transmission line is converted into an equivalent load at the terminal where the
generator G l is connected. The generator G1 generates the required reactive power to maintain scheduled bus voltage.
Figure 4.: A UPFC connected to a Transmission line.
Figure 4.2: Coupled source mode1 for UPFC Generator G2 also supplies the
real power demand of the series inverter. The series injected voltage
is the phase difference between𝑉𝑙𝑖𝑛𝑒 and 𝑉𝑢𝑝𝑓𝑐𝑢𝑠 .The product of series injected voltage
13 and the current 𝐼𝑠𝑒 gives the amount of volt-ampere of the series inverter. The real part of the volt-ampere (𝑃𝑠𝑒 ) of the series inverter is added as a load at the shunt inverter bus. The algorithm to perform the addition of equivalent loads at the shunt inverter bus, to open the appropriate lines, have been included in their program. The problem is that it needs a solved load £low case. The idea of solving a load flow with an UPFC is to obtain the shunt and the series inverters' injected voltages for a given operating condition. This procedure is crude for solving a load flow with UPFC.
6. MATHEMATICAL MODELLING OF UPFC
In this model, we have considered the UPFC is placed at the centre of a 300km transmission line. This model was derived with to study the relationship between electrical transmission system and UPFC in steady state conditions. The basic scheme is shown in fig.3 [26]. A UPFC can be represented by two voltage sources representing fundamental components of output voltage waveforms of the two converters and impedances being leakage reactance of the two coupling transformers [27].
Figure 5. Equivalent circuit of UPFC
14 Based on the basic principle of UPFC and network theory, the active and reactive power flows in the line, from bus-i to bus-j, having UPFC can be written as [8],
6.1 Importance of UPQC
UPQC Every Power electronics device has its own advantages and disadvantages. But The UPQC is the most powerful electronics device for high loads and very sensitive to line voltage and load current disruptions. The most effective type of device is to be considered as the Unified Power Quality Conditioner (UPQC). There are many reasons as why the UPQC is selected over the others.
UPQC is more flexible than any single Converter/inverter based
device. It can alternately correct for the imbalance and disruption in the supply voltage and current where as other devices either correct the current or the voltage distortion only. Hence the purpose of two devices at a time will be served by UPQC alone.
6.2 Unified Power Quality Conditioner (UPQC) The major parts of a UPQC are series power converter, shunt power converter, capacitors, low-pass & high-pass passive filters, series and shunt transformers etc.
1. Series converter is a voltage- source converter connected in series with the AC supply line and acts as a line voltage source to compensate voltage disruptions. It is used to minimize line voltage fluctuations from the load supply voltage and feeds to shunt branch of the device to consume current harmonics produced by unbalance load. The pulse-width modulation (SPWM) is used to Control the series converter output voltage.
The gate pulses which required for converter are produced by
15 comparing the fundamental voltage and reference signal voltage with a high-frequency triangular waveform.
2. Shunt converter is a voltage- source converter (VSC) which is connected in parallel with the same AC supply line and acts as a current source to eliminate current disruption and mitigates the reactive current of in the load circuit, and enhances the load power factor. It also acts as DC- link voltage regulator for the reduction of the DC capacitor rating. The resultant current of shunt converter can be saturate by the application of dynamic hysteresis band (DHB) on the basis of the status semiconductor switches control so that resultant current follows the reference signal and remains in predicted hysteresis band (PHB).
3. The DC capacitor bank connected between Midpoint-to- ground is divided into two parts, which are arranged in series together. The neutral point‟s secondary transformer is
connected to the DC link midpoint directly. Since both three-phase transformers are connected in Y/Yo form, therefore the zero- sequence voltage will appear in primary winding of transformer which is connected in series to mitigate the zero-sequence voltage of the supply power system. There would not be any zero-sequence current flow in the primary side of both transformers. This assures the balancing of system current when the voltage disturbance occurs.
4. The Low-pass filter (LPF) is made use to to get high attenuation in high frequency components at the output side of series converter which are produced by high-frequency switching mode.
5. High-pass filter (HPF) can applied at the output of shunt converter to consume the ripples produced while in current switching mode .
6. Series and shunt transformers are used to inject the mitigating voltages and currents for the purpose of electrically seperation
16 of UPQC converters. The UPQC is very useful for steady-state analysis and dynamically control of series and shunt active and reactive power mitigation at fundamental as well as harmonic frequencies. Any how the UPQC is
concerned about the good quality load voltage and the line current at the point of its application, but it does not enhance the quality of power for entire power system unit.
Figure 6.: Block diagram of UPQC 7. Equivalent circuit of UPQC
In this circuit, Voltage Source (VS) represents the voltage at power supply. VSR represents the series Active Power Filter (APF) for voltage mitigation, VL shows the load voltage and ISh stands for Current of shunt Active Power Filter (APF) and VSR is for mitigation purpose.
The negative phase sequence and harmonic components may occur
due to voltage Distortion. The source voltage in Figure 7 can be written as: Vs + Vsr = VL To get a balanced sinusoidal load and line voltage with fixed amplitude V, the output voltages of the Series-APF should be given by; Vsr = (V –V1p) sin (𝜔𝑡+𝜃1𝑃) – VLn (t) - (𝑡)∞𝐾=2 ,where, V1P: Positive sequence voltage amplitude fundamental Frequency 𝜃1 : initial phase of
17 voltage for positive sequence V1n:
negative sequence component.
Figure 7. equivalent circuit for UPQC
7.2 Functions performed by UPQC
• Convert the feeder (system) current into balanced sinusoids through the shunt compensator.
• Convert the load voltage VL to balanced sinusoids through the series compensator.
• Ensure zero real power injection (and/or absorption) by the compensators.
• Supply reactive power to the load (Q compensation).
8. CONCLUSION
Transmission capability of existing transmission line is highly improved with the presence of
different fact devices. But the difference between the sending end real power and receiving end real power is high in the transmission line. In this research we studied about the functioning of different fact device like DVR, UPFC, UPQC with power quality issues.
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