Voltage Sag and Mitigation Using Algorithm for Dynamic Voltage Restorer by PQR Transformation Theory

Transcription

1 International Journal of Engineering Inentions ISSN: , olume, Issue 5 (September0) PP: oltage Sag and Mitigation Using Algorithm for Dynamic oltage Restorer by PQR Transformation Theory Prof. Shianagouda B. Patil, Dr. Shekhappa. G. Ankaliki E & E Engineering Department, Hirasugar Institute of Technology, Nidasoshi-59 36, Karnataka, India. E & E Engineering Department, SDM College of Engineering & Technology, Dharwad , Karnataka, India. Abstract oltage sag is one of the power quality issue and Dynamic oltage Restorer (DR) is used for mitigation of oltage sag. oltage sag is sudden reduction in oltage from nominal alue, occurs in a short time which can cause damage and loss of production in industrial sector. In this paper, focus is gien only on DR using an algorithm and the related implementations to control static series compensators (SSCs). Directly sensed three-phase oltages are transformed to p-q-r co-ordinates without time delay. The controlled ariables in p-q-r co-ordinates has better steady state and dynamic performance and then inersely transformed to the original a-b-c co-ordinates without time delay, generating control signals to SSCs. The control algorithm is used in DR system. The simulated results are erified and mitigation of sag is presented in this paper. Key words oltage Sag, Static Series Compensator (SSCs), PQR Transformation, Dynamic oltage Restorer (DR) I. INTRODUCTION In many recent years, power quality disturbances become most issue which makes many researchers interested to find the best solutions to sole it. There are arious types of power quality disturbances in which oltage sag, oltage swell and interruption. This paper introduces DR and its operating principle for the sensitie loads. Simple control based SPWM technique and pqr transformation theory [] is used in algorithm to compensate oltage sags/swells. A scope of work is DR system will be simulated by using Matlab/Simulink tool box and results were presented. Due to the increasing of new technology, a lot of deices had been created and deeloped for mitigation of oltage sag. oltage sag is widely recognized as one of the most important power quality disturbances []. oltage sag is a short reduction in rms oltage from nominal oltage, happened in a short duration, about 0ms to seconds. The IEC defines the oltage sag (dip) as a temporary reduction of the oltage at a point of the electrical system below a threshold [3]. According to IEEE Standard , defines oltage sags as an rms ariation with a magnitude between 0% and 90% of nominal oltage and duration between 0.5cycles and one minute [4] & [5]. oltage sag is happened at the adjacent feeder with unhealthy feeder. This unhealthy feeder always caused by two factors which are short circuits due to faults in power system networks and starting motor which draw ery high lagging current. Both of these factors are the main factor creating oltage sag as power quality problem in power system. oltage sags are the most common power disturbance which certainly gies affecting especially in industrial and large commercial customers such as the damage of the sensitiity equipments and loss of daily productions and finances. An example of the sensitiity equipments are programmable logic controller (PLC), adjustable speed drie (ASD) and chiller control. There are many ways in order to mitigate oltage sag problem. One of them is minimizing short circuits caused by utility directly which can be done such as with aoid feeder or cable oerloading by correct configuration. The control of the compensation oltages in DR based on dqo algorithm is discussed in [6]. DR is a power electronic controller that can protect sensitie loads from disturbances in supply system. Dynamic oltage restorers (DR) can proide the most commercial solution to mitigation oltage sag by injecting oltage as well as power into the system. The mitigation capability of these deices is mainly influenced by the maximum load; power factor and maximum oltage dip to be compensated [7]. In [-7] oltage Sag Mitigation Using Dynamic oltage Restorer System are discussed under fault condition and dynamic conditions. II. DR SYSTEM Dynamic oltage restorer (DR) is a series compensator which is able to protect a sensitie load from the distortion in the supply side during fault or oerloaded in power system. The basic principle of a series compensator is simple, by inserting a oltage of required magnitude and frequency, the series compensator can restore the load side oltage to the desired amplitude and waeform een when the source oltage is unbalanced or distorted [8]. This DR deice employs gate turn off thyristor (GTO) solid state power electronic switches in a pulse width modulated (PWM) inerter structure. The DR can generate or absorb independently controllable real and reactie power at the load side. The DR also is made of a solid state dc to ac switching power conerter that injects a set of three phase ac output oltages in series and synchronism with the distribution feeder oltages [8]. The amplitude and phase angle of the injected oltages are ariable thereby allowing control of the real and reactie power exchange between the DR and the distribution system [8]. The dc input terminal of a DR is connected to an energy source or an energy storage deice of appropriate capacity. The reactie power exchange between the DR and the distribution system is internally generated by the DR without ac 47

2 passie reactie components. The real power exchanged at the DR output ac terminals is proided by the DR input dc terminal by an external energy source or energy storage system. DR structure comprises rectifier, inerter, filter and coupling transformer shown in Fig.. Besides, pulse width modulated (PWM) technique is using to control ariable oltage. Filter is using for elimination harmonic generated from high switching frequency in PWM technique. In power system network, DR system is connected in series with the distribution feeder that supplies a sensitie load shown in Fig.. There are two main factors relating to the capability and performance of DR working against oltage sags in a certain power system: the sag seerity leel and the Total Harmonic Distortion (THD). Both of these in turn are mainly decided by the DC source [9] Fig. DR Structure Fig. DR System in Power System The Principle Operation of DR System In normal situation without short circuit in power system, a capacitor between rectifier and inerter (Fig. ) will be charging. When oltage sag happened, this capacitor will discharge to maintain load oltage supply. Nominal oltage will be compared with oltage sag in order to get a difference oltage that will be injected by DR system to maintain load oltage supply. PWM technique is using to control this ariable oltage. In order to maintain load oltage supply, reactie power must be injected by DR system. Practically, the capability of injection oltage by DR system is 50% of nominal oltage. It is sufficient for mitigation oltage sag because from statistic shown that many oltage sag cases in power system inoling less than 0.5 p.u. oltage drop. Mathematical Model for oltage Sag Calculation In this principle the assumption is that the fault current is larger than the load current. The point of common coupling is the point from which both fault and load are fed. Upon the occurrence of the fault which may be a line-line ground fault, a short circuit current flow which leads to reduction in the oltage magnitude at the point of common coupling (PCC) shown in Fig. 3. This oltage sag may be unbalanced and may be accompanied with a phase jump. Fig. 3 Calculation for oltage sag 48

3 I F Z s E Z sag F F F I Z () X X X F S F tan tan RF RS RF Where ZS RS jx S, is the source impedance, Z F RF jx F, is the impedance between the PCC and the fault, and E p. u is the source oltage. The missing oltage which is the oltage difference between pre-sag condition and sagged (faulted) condition which is gien by equation (4) and ectorial representation is shown in Fig. 4. (4) missin g presag sag () (3) Fig. 4 ector Diagram Showing oltage Sag The missing oltage has to be proided by sag compensating deice like Dynamic oltage Restorer (DR) which is injected in series with the supply oltages to bring the oltages to pre-sag condition.. III. PROPOSED APPROACH The objectie of this paper is to implement DR in series with a sensitie load. It is a series compensating deice which injects a set of controllable three-phase AC output oltages, in series and synchronism with the distribution feeder oltages. The DR employs solid state switching deices in a pulse width modulated inerter (PWMI), the magnitude and phase angle of the injecting oltages can be controlled independently. In this work directly sensed three phase oltages are transformed to p-q-r co-ordinates without time delay []. The controlled ariables p-q-r co-ordinates hae better steady state and dynamic performance. Later these are inersely transformed to the original a-b-c co-ordinates without time delay. It must able to respond quickly and experience no oltage sags to the end users of sensitie equipment shown in Fig. 3. I. BASIC CONTROL STRATEGIES OF DR The type of the control strategy mainly depends upon the type of the sensitie or critical load, i.e. it depends on the sensitiity of the load to changes in magnitude, phase shift and wae shape of the oltage wae form. The control techniques adopted should consider the limitations of the DR like oltage injection capability and energy limit. oltage sag compensation can be done by DR using real and reactie power transfer. Reactie power solely will not meet the requirements of dynamic compensation of disturbances that require real power transfer. The three basic control strategies are, a) Pre-sag Compensation b) In-phase Compensation c) Energy Optimal Compensation a) Pre-sag Compensation In this technique, the DR compensates for the difference between the sagged and the presag oltages by restoring the instantaneous oltages to the same phase and magnitude as the nominal pre-sag oltages. In this method, in case there is sag associated with phase shift, full phase compensation is proided so that the load is undisturbed as far as the phase angle and magnitude are concerned. But there is no control of energy injected into the line and often exhausts the rating of the DR. The ector diagram for pre-sag compensation is shown in Fig

4 Fig. 5 ector Diagram of Pre-sag Compensation b) In-phase Compensation In this method, the restored oltages are in-phase with the depressed source side oltages regardless of load current and presag oltages, thus minimizing the magnitude of the injected oltage. Fig.4 shows phase compensation is not proided but has got better performance in compensating a broader range of oltage sags. The ector diagram for inphase compensation is shown in Fig. 6. Fig. 6 ector Diagram for Inphase Compensation c) Energy Optimal Compensation oltages are injected at an angle that will minimize the use of real power since oltage restoration is the only requirement. That means the DR oltage is injected perpendicular to the load current but the current will change the phase according to the new oltage applied to it and energy will be drawn from DR. The ector diagram for optimal compensation is shown in Fig.7. Fig. 7 ector Diagram for Optimal Compensation. COMPUTATION OF COMPENSATING OLTAGES There are many methods to calculate the compensating reference oltage waeforms for dynamic oltage restorers. One of those methods is PQR power theory. An effectie algorithm based on PQR power theory is used to calculate reference compensating oltages. In this algorithm, the directly sensed three phase oltages are conerted into p-q-r coordinates instantaneously without any time delay. Then the reference oltages in p-q-r domain hae simple forms - DC 50

5 alues. The controller in p-q-r domain is ery simple and clear, has better dynamic and steady state performance than conentional controllers. The algorithm based on PQR power theory is used to get the reference compensation oltages in p- q-r domain and transformed back to 0 domain to drie the space ector modulator to generate the firing signals for inerter. PQR Transformation Theory The 3-phase oltages of three-phase a-b-c coordinates can be transformed to o coordinates as gien below, o a b c (5) If sinusoidal balanced oltages a, b the reference waes in o coordinates can be and c are selected for the reference waes in the a-b-c coordinates, calculated and shown below, a (6) b c Since the reference waes, b and c are sinusoidal balanced, the 0-axis component oltage o a does not exist and the reference wae Using the reference waes become sinusoidal and orthogonal on the plane. in the o coordinates in a mapping matrix, the oltages ino coordinates can be transformed to p-q-r coordinates as gien by (). 0 p o q 0 r 0 0 Where (8) (7) Combining (5) and (7) the oltages in a-b-c coordinates can be transformed to p-q-r coordinates as shown below, C C C Where C a b c (9) 5

6 0 C From p-q-r domain to a-b-c domain can be achieed by taking inerse of C, a p b C q c r (0) () Where C ( C C ) C C C () C (3) PQR Transformation Fig. 8 Physical Meaning of PQR Transformation When three-phase oltages are sinusoidal and balanced, the locus of the sensed oltage space ector SEN is a circle on the plane. If the three-phase oltages are in-phase with the three-phase reference waes, the sensed oltage space ector SEN becomes aligned with the reference wae space ector. In this case q and comprises only a dc component that is equal to DR. SEN r do not exist while. This condition will be a target for the oltage sag compensation by a p 5

7 DR Model Description The block diagram of oerall control flow is shown in Fig. 9. Here open loop feed forward control technique is adopted. Upon the occurrence of sag, there is a reduction in the phase oltages on the downstream of the DR. The data is acquired by data acquisition system and detected. The speed with which the sag is detected depends upon the sensors and sag detection algorithms. Then the three phase oltages are processed using PQR algorithm. The reference compensating oltages are generated in PQR domain and transformed back to alpha-beta domain. The alpha and beta axis reference oltages are gien as inputs to drie the space ector modulator. The generated firing pulses are used to fire the switches of the oltage Source Inerter (SI). The topology of the SI used is conentional -leel, three-leg inerter and three-leel (multi-leel configuration) diode clamped inerter. Depending upon the topology of the inerter, the number of firing pulses may be 6/. Fig. 9 Simulation Model of Dynamic oltage I. SIMULATION RESULTS The simulation results carried out using Matlab/Simulink tool box for conentional two-leel inerter using space ector modulation scheme is presented. The data used for simulation is, sa = 30 0 ; sb = 30-0 ; sc = 30 0 ; ab = 350 The three phase output oltages generated by the SI can be controlled both in magnitude and phase indiidually. Finally the 3-phase oltages are filtered out by a low pass filter before injecting into the line ia booster transformer. All the simulations are carried out using MATLAB/SIMULINK tool box. The phase and line oltages in conentional two-leel inerter are shown in Fig.0. The sag compensation by DR for double line to ground fault is shown in Fig.. There is oltage sag in phase b and phase c. The DR proides full magnitude and phase compensation instantaneously. 53

8 Fig. 0 Phase and Line oltages in Conentional Two Leel Inerter Fig. Compensation of Sag by DR for Phase b and c. II. CONCLUSION In this paper, a complete simulated DR system has been deeloped by using MATLAB/SIMULINK tool box. The proposed scheme for DR uses the PQR algorithm to generate the reference compensation oltages without time delay applied to space ector modulator to drie the conentional two-leel topology which generates the required compensation oltages. It is shown that the simulated DR deeloped, works successfully without lacks in its performance when applied to a simulated power system network. By introducing DR in the power network, it can help to improe power quality. It is important to hae a good deliery power quality in electrical power systems especially to the critical areas, such as in the industrial sectors, in order to ensure the smoothness of the daily operations. ERENCES. Hyosung Kim, Frede Blaabjerg, Biritte Bak-Jensen, Jacho Choi, Instantaneous Power Compensation in Three Phase Systems by Using P-Q-R Theory IEEE Journal 00, PP

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