In this article we will discuss about:- 1. Meaning of DVR 2. Basic Configuration of DVR 3. DVR Characteristics 4. Equations 5. Operating Mode 6. Voltage Injection Methods.
Contents:
- Meaning of DVR
- Basic Configuration of DVR
- DVR Characteristics
- Equations Related to DVR
- Operating Mode of DVR
- Voltage Injection Methods of DVR
1. Meaning of DVR:
Among the power quality problems (sags, swells, harmonics) voltage sags are the most severe disturbances. In order to overcome these problems the concept of custom power devices is introduced recently. One of these devices is the Dynamic Voltage Restorer (DVR), which is the most efficient and effective modern custom power device used in power distribution networks.
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DVR is a recently proposed series connected solid state device that injects voltage into the system to regulate the load side voltage. It is normally installed in a distribution system between the supply and the critical load feeder at the point of common coupling (PCC). Other than voltage sags and swells compensation, DVR can also add other features like line voltage harmonics compensation, reduction of transients in voltage and fault current limitations.
Dynamic voltage restorers (DVR) are one of power quality control device that sustains stable load voltages by compensating source voltages when sags or swells occur.
2. Basic Configuration of DVR:
i. Injection/Booster Transformer
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ii. Harmonic Filter
iii. Voltage Source Converter
iv. DC Charging Circuit
v. Control and Protections.
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i. An Injection/Booster Transformer:
The Injection/Booster transformer is a specially designed transformer that attempts to limit the coupling of noise and transient energy from the primary side to the secondary side.
Its main tasks are:
a. It connects the DVR to the distribution network via. a the HV-windings and transforms and couples the injected compensating voltages generated by the voltage source converters to the incoming supply voltage.
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b. In addition, the injection/booster transformer serves the purpose of isolating the load from the system (VSC and control mechanism).
ii. Harmonic Filter:
The main task of harmonic filter is to keep the harmonic voltage content generated by the VSC to the permissible level.
iii. Voltage Source Converter:
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A VSC is a power electronic system consists of a storage device and switching devices, which can generate a sinusoidal voltage at any required frequency, magnitude, and phase angle. In the DVR application, the VSC is used to temporarily replace the supply voltage or to generate the part of the supply voltage which is missing.
There are four main types of switching devices- Metal Oxide Semiconductor Field Effect Transistors (MOSFET), Gate Turn-Off thyristors (GTO), Insulated Gate Bipolar Transistors (IGBT), and Integrated Gate Commutated Thyristors (IGCT). Each type has its own benefits and drawbacks. The IGCT is a recent compact device with enhanced performance and reliability that allows building VSC with very large power ratings. Because of the highly sophisticated converter design with IGCTs, the DVR can compensate dips which are beyond the capability of the past DVRs using conventional devices.
The purpose of storage devices is to supply the necessary energy to the VSC via. a dc link for the generation of injected voltages. The different kinds of energy storage devices are Superconductive Magnetic Energy Storage (SMES), batteries and capacitance.
iv. DC Charging Circuit:
The dc charging circuit has two main tasks:
a. The first task is to charge the energy source after a sag compensation event.
b. The second task is to maintain dc link voltage at the nominal dc link voltage.
v. Control and Protections:
The control mechanism of the general configuration typically consists of hardware with programmable logic. All protective functions of the DVR should be implemented in the software. Differential current protection of the transformer, or short circuit current on the customer load side are only two examples of many protection functions possibility.
3. DVR Characteristics:
i. DVR: represented by voltage sources Vfa, Vfb and Vjc
ii. Supply voltage: represented by sources Vsa, Vsb and Vsc
The DVR is connected between a terminal bus on the left and a load bus on the right. The voltage sources are connected to the DVR terminals by a feeder with an impedance of R + jX. We shall assume that the loads are balanced and the load impedance is given by Z1 = R1 + jX1. It is to be noted that the phase angle φ1 between the load terminal F, and the line current is depends on the load impedance and is independent of the line impedance or the DVR voltage. The objective of the discussion presented below is to regulate the magnitude of the load voltage equal to that of the source voltage through DVR voltage injection.
Further we stipulate the following condition on the DVR:
The DVR does not supply any real power in the steady state. This implies that the phase angle difference between DVR voltage phasor and line current phasor must be π/2 in the steady state. Let us assume that the load current lags the load voltage. To draw a phasor diagram of the steady state operation, we assume that the load voltage is fixed at K per unit and the source voltage is allowed to vary. Since the primary target is to make the magnitudes of V1 and Vs equal, the locus of desirable Vs is the arc NB as shown in Fig. 5.48.
To make the magnitude of the load voltage equal to that of the source voltage, the R is drop must be less than NM. If the drop is less than this limiting value, the DVR must compensate the entire reactive drop in the feeder and provide additional injection such that the source voltage becomes V per unit.
It can be seen from Fig. 5.48 that there are two possible intersection points A and the other at B. This implies that two possible values of DVR voltage and can be obtained for each feeder drop. For the first value, the source voltage will be along OA, while for the other value, it will along OB. It is needless to say that the best choice is the A intersection requiring much smaller voltage injection from the DVR.
4. Equations Related to DVR:
The system impedance ZTH depends on the fault of the load bus. When the system voltage (FTH) drops, the DVR injects a series voltage FDVR through the injection transformer so that the desired load voltage magnitude VL can be maintained. The series injected voltage of the DVR can be written as-
VDVR = VL + ZTHIL – VTH
Where,
VL = Desired load voltage magnitude
ZTH = Load impedance.
IL = Load current
VTH = System voltage during fault condition.
5. Operating Mode of DVR:
The basic function of the DVR is to inject a dynamically controlled voltage VDVR generated by a forced commutated converted in series to the bus voltage by means of a booster transformer. The momentary amplitudes of the three injected phase voltages are controlled such as to eliminate any detrimental effects of a bus fault to the load voltage VL. This means that any differential voltage caused by transient disturbances in the ac feeder will be compensated by an equivalent voltage generated by the converter and injected on the medium voltage level through the booster transformer.
The DVR has three modes of operation which are:
i. Protection mode,
ii. Standby mode,
iii. Injection/boost mode.
i. Protection Mode:
If the over current on the load side exceeds a permissible limit due to short circuit on the load or large inrush current, the DVR will be isolated from the systems by using the bypass switches and supplying another path for current.
ii. Standby Mode (VDVR = 0):
In the standby mode the booster transformer’s low voltage winding is shorted through the converter. No switching of semiconductors occurs in this mode of operation and the full load current will pass through the primary.
iii. Injection/Boost Mode (VDVR > 0):
In the Injection/Boost mode the DVR is injecting a compensating voltage through the booster transformer due to the detection of a disturbance in the supply voltage.
6. Voltage Injection Methods of DVR:
Voltage injection or compensation methods by means of a DVR depend upon the limiting factors such as; DVR power ratings, various conditions of load, and different types of voltage sags. Some loads are sensitive towards phase angle jump and some are sensitive towards change in magnitude and others are tolerant to these. Therefore the control strategies depend upon the type of load characteristics.
There are four different methods of DVR voltage injection which are:
i. Pre-sag compensation method
ii. In-phase compensation method
iii. In-phase advanced compensation method
iv. Voltage tolerance method with minimum energy injection.
i. Pre-Sag Compensation Method:
The pre-sag method tracks the supply voltage continuously and if it detects any disturbances in supply voltage it will inject the difference voltage between the sag or voltage at PCC and pre-fault condition, so that the load voltage can be restored back to the pre-fault condition. Compensation of voltage sags in the both phase angle and amplitude sensitive loads would be achieved by pre-sag compensation method. In this method the injected active power cannot be controlled and it is determined by external conditions such as the type of faults and load conditions.
ii. In-Phase Compensation Method:
This is the most straight forward method. In this method the injected voltage is in phase with the supply side voltage irrespective of the load current and pre-fault voltage. The phase angles of the pre-sag and load voltage are different but the most important criteria for power quality that is the constant magnitude of load voltage are satisfied.
VL = V pre-fault
One of the advantages of this method is that the amplitude of DVR injection voltage is minimum for a certain voltage sag in comparison with other strategies. Practical application of this method is in non-sensitive loads to phase angle jump.
iii. In-Phase Advanced Compensation Method:
In this method the real power spent by the DVR is decreased by minimizing the power angle between the sag voltage and load current. In case of pre-sag and in-phase compensation method the active power is injected into the system during disturbances. The active power supply is limited stored energy in the DC links and this part is one of the most expensive parts of DVR. The minimization of injected energy is achieved by making the active power component zero by having the injection voltage phasor perpendicular to the load current phasor.
In this method the values of load current and voltage are fixed in the system so we can change only the phase of the sag voltage. IPAC method uses only reactive power and unfortunately, not all the sags can be mitigated without real power, as a consequence, this method is only suitable for a limited range of sags.
iv. Voltage Tolerance Method with Minimum Energy Injection:
A small drop in voltage and small jump in phase angle can be tolerated by the load itself. If the voltage magnitude lies between 90%—110% of nominal voltage and 5%-10% of nominal state that will not disturb the operation characteristics of loads. Both magnitude and phase are the control parameter for this method which can be achieved by small energy injection.