Application of Distribution Static Compensator ( D-STATCOM ) to Voltage Sag Mitigation

This paper presents a study of a D-STATCOM (Distribution Static Compensator) used for mitigating voltage sag. The basic idea of the voltage sag mitigation, using a D-STATCOM is to dynamically inject a current of desired amplitude, frequency and phase into the grid line. The proposed method extracts the active and reactive parts of the positiveand negative-sequence component for generating reference values of current that need to be injected into the point of connection D-STATCOM in order to compensate the voltage errors. The proposed method offers structural simplicity and less calculation complexity. Simulation results indicate that this method is effective and D-STATCOM has good performance to mitigate the voltage sag.


Introduction
Voltage sag is a reduction between 10 and 90%in rms voltage with a duration between 0.5 cycles and 1 min (although voltage sag depth and duration typically range from 80 to 90% and 0.5 to 30 cycles, respectively) [1].Voltage sags are the most important power quality (PQ) problems that many industries and utilities face it.Among different types of disturbances occurring in power system, voltage sag is known to produce the most devastating impact on the loads [2].Studies show that 92% of all disturbances in the electrical power distribution systems are voltage sags, transients, and momentary interruptions [3,4].Voltage sags are not tolerated by sensitive equipment used in modern industrial plants such as process controllers; programmable logic controllers (PLC), adjustable speed drive (ASD) and robotics [5].Various methods have been applied to reduce or mitigate voltage sags.The conventional methods are by using capacitor banks, introduction of new parallel feeders and by installing uninterruptible power supplies (UPS).
Recently D-STATCOM has emerged as a promising device to provide not only for voltage sag mitigation but also for a host of other power quality solutions such as voltage stabilization, flicker suppression, power factor correction, and harmonic control [6].D-STATCOM is a shunt connected device that generates a balanced set of three sinusoidal voltage or current at the fundamental frequency [7].DSTATCOM configuration consists of a VSC that converts voltage across the storage device into a set of three-phase ac output voltages.These voltages are in phase and coupled with the ac system of network through the reactance of the coupling transformer [8].In [9], a repetitivebased controller for a D-STATCOM is presented that compensate reactive power, but it offers more calculation complexity.[10], presented a D-STATCOM control algorithm which enables seperate positive and negative sequence currents, but the presented method needs more time than the control method that perenetd in this paper.
This paper presents a study of a D-STATCOM (Distribution Static Compensator) used for mitigating voltage sag.The control object of D-STATCOM is that the line voltages at the normal operation only include positive sequence.The control method is made up of DC voltage controller and current controller.
The rest of the paper is organized as follows: section II present the D-STATCOM description.Section III explains the control method.The obtain simulation results are discussed in section IV and finally, section V concludes the paper.In the unbalanced conditions the line voltage is equal to the sum of negative and positive sequence so we will have: 1 2 cos( ) 2 cos( )

Description of the D-STATCOM
In the above equation sp V and Sn V are the RMS of positive and negative sequence component of the line voltages, respectively.By applying Kirchoff's Voltage Law (KVL) we will have: Applying Kirchoff's Current Law (KCL) satisfy the fallow equations: By combining the Eqs (4-6) with Eqs (7-9) we will have: The model transformed from 'abc' frame into 'dq' frame is given by: In ( 13) the parasite resistance f R is very small so that we have: The switch frequency is much higher than line frequency, so the changes of ld i and lq i in a switch period can be omitted.Thus, we have: According to ( 14) and (15) we have:

Extraction the Positive Component
In order to extraction the positive component; first consider the line voltage by a 3 π delay, as the below: 2 cos( ) 2 cos( ) 3 The negative component of 1 2 s s V − and 2 3 s s V − are equal to: Universal Journal of Electrical and Electronic Engineering 1(2): 11-15, 2013 Eq (22) shows the relationship between line to line negative voltage and line to neutral negative voltages The line voltages are equal to sum of negative and positive sequence, so that : Finally we can calculate the positive component by Eq (23).

Application of 'dq' Controllers
The proposed D-STATCOM controller consists of current and dc voltage controller.The dc voltage controller regulates the dc voltage at the required level and the current controller forces the D-STATCOM current to follow the references.
The transformation matrix from 's1s2s3' into 'dq' frame can be defined as follows [11]: We need the transformation of the Eqs (23) into conventional stationary ' ' αβ coordinates and the transformation matrix is equal to [11]: From the Eq (25) we will have: We need to decouple d i and q i for proper control design.
Decoupling can be satisfied by introducing new parameters d u and q u .
Fig. 2 shows the d i and q i controller; the transfer function of the closed loop of the controllers can be illustrated as below:

DC Voltage Controller
Regulation the DC voltage at desired value in the application of the D-STATCOM is very important.It is obvious that the regulation of DC voltage is carried out by charging and discharging of the capacitor.The relationship between dc V and d i can be written as the below: where is the d-axis component of point of common connection (PCC).By rearranging the Eq (30) we have: ) The schematic of the DC voltage controller is shown in Fig. 3.

Reactive Power Controller
In the case of balanced three-phase voltage, the direct transformation of abc voltages into the dq reference frame will result in dc-quantities.Hence, the DSTATCOM can use a conventional PI-controller to control the injected reactive currents.However, if the grid voltage or the load voltages are unbalanced, a ripple of double the grid frequency will occur in the d − q reference frame.In the case of unbalance three-phase voltages, breaking the voltage signals into positive-and negative-sequence components and then transforming into dq synchronous reference frame results in dc-quantities and these dq feedback signals allow in control design reactive power control.Transformation of three-phase balance voltages with unity magnitude into the positive and negative dq synchronous reference frame results in dc-quantities with following values.

Test System
The simulation system is constructed according to Fig.During the simulation, the unbalance load is operated with between phase A-C and between phases A-B.

Results and Discussions
Application of D-STATCOM to voltage sag mitigation is simulated in MATLAB/SIMULINK.Figure .4shows the uncompensated voltage, from Fig. 4 can be seen that the balanced sag (70%) occurs at 0.3 (sec).Fig. 5 shows the compensated voltage which the voltage remains at 1 pu during the sag.Fig. 6 shows the positive dq component and Fig. 7 shows the negative dq component.

Conclusion
This paper presented the operation of D-STATCOM to voltage sag mitigation.In order to mitigate voltage sag the positive and negative sequence is separated and the proposed method extracts the active and reactive parts of the positiveand negative-sequence component for generating reference values of current that need to be injected into the point of connection D-STATCOM in order to compensate the voltage errors.Simulation results show that D-STATCOM can be used for voltaje sag mitigation and has a good performance under voltaje sag.

Figure. 1
Figure.1 shows the schematic representation of D-STATCOM.The basic electronic block of the D-STATCOM is the voltage source inverter that converts an input dc voltage into a three-phase output voltage at fundamental frequency and the dc link voltage is provided by capacitor.

−
are the negative sequence of voltage.The controller's operation are based on above values, in other words the voltage sag in power network can be corrected by regulating positive and negative sequences to the above values.