Advanced Control of the Dynamic Voltage Restorer for Mitigating Voltage Sags in Power Systems

The paper presents a vector control with two cascaded loops to improve the properties of Dynamic Voltage Restorer (DVR) to minimize Voltage Sags on the grid. Thereby, a vector controlled structure was built on the rotating dq-coordinate system with the combination of voltage control and the current control. The proposed DVR control method is modelled using MATLAB-Simulink. It is tested using balanced/unbalanced voltage sags as well as fluctuant and distorted voltages. As a result, by using this controlling method, the dynamic characteristics of the system have been improved significantly. The system performed with higher accuracy, faster response and lower distortion in the voltage sags compensation. The paper presents real time experimental results to verify the performance of the proposed method in real environments.


Introduction
Voltage sags are one of the most common events that affect power quality in the distribution system.The major causes of voltage sags are faults of a grid, such as a single line to the ground, phase to phase, twophase to the ground, three-phase faults due to accidents, lightning, wind, animals, and other causes.
Other causes include transformer energizing, switching capacitor banks, and the starting of large induction motors [6] and [12].Voltage sags can be symmetric or asymmetric depending on the causes of sags.
Although voltage sags occur in a short time (from 0.5 cycle to 1 minute) [11], it may affect the operation of equipment (stalling of motors, tripping of sensitive loads, and inaccuracy of control devices).These effects can give a rise in serious production problems, causing huge economical losses for consumers.According to an investigation of Schneider Electric in [13], voltage sags were the largest power quality problems in US distribution system, as shown in Fig. 1.It is also a serious problem in Vietnam electric power system.
There are many methods for mitigating voltage sags, such as UPS (Uninterruptible Power Supply), SVC (Static VAR Compensator), DSTATCOM (Distribution-Static Compensator), DVR (Dynamic Voltage Restorer).The structure of SVC is simpler than DVR but it has the incapability to control active power flow.The energy capacity of the DVR is higher than that of the UPS which has the same power rating.Additionally, the size of a DVR is smaller than that of the DSTATCOM.Furthermore, it is the cheapest in comparison with the UPS and the DSTATCOM [1], [7].Therefore, DVR is widely considered as an effective device in mitigating voltage sags.
This paper is arranged as follows.The proposed vector control with two cascaded loops method is discussed in details in Sec. 2. In Sec. 3. the simulation model using MATLAB-Simulink is illustrated for this control strategy.One part of the real time experimental result is represented in Sec. 4.
Finally, conclusions are given in Sec. 5.

2.
Proposed Control Method

Configuration of DVR
The general structure of a DVR consists of a booster transformer, a harmonic filter, a Voltage Source Con-verter (VSC), an energy storage and a control system.The control system is responsible for creating the injected voltage u inj (t) that can be restored the voltage at the Point of Common Coupling (PCC) with a preset value during the voltage sags.Detailed discussion of principles and operations of the DVR can be found, e.g., in [2] and [3].
The three-phase diagram of a grid with DVR is shown in Fig. 2. Where: • u sa (t), u sb (t), u sc (t) are the three-phase voltages of the source.
• u ga (t), u gb (t), u gc (t) and i ga (t), i gb (t), i gc (t) are the grid voltages and the grid currents at PCC, respectively.
• u inv,a (t), u inv,b (t), u inv,c (t) and i f a (t), i f b (t), i f c (t) are the three-phase voltages and currents of the VSC, respectively.
• u Ca (t), u Cb (t), u Cc (t) and i Ca (t), i Cb (t), i Cc (t) are the filter capacitor voltages and currents, respectively.
• u inj,a (t), u inj,b (t), u inj,c (t) and i inj,a (t), i inj,b (t), i inj,c (t) are the voltages and currents injected by the DVR, respectively.
• u dc (t) is the DC-link voltage.The DVR functions by transferring the voltage sags compensation value from energy source through the booster transformer after the filter to restore the nominal voltage.This means that the differential voltage between PCC and reference voltage is appeared when the voltage disturbance occurs in the grid caused by any events, an equivalent voltage generated by the VSC should be compensated and injected through the booster transformer.The possibility of compensating voltage sags of a particular DVR depends on many factors including different types of voltage sags, different load conditions and the power rating that is supplied to the DVR.

Mathematical Modelling of DVR
From structural diagram of DVR, to reduce the complication of modeling, assuming the injection transformer is considered ideal with a 1:1 turn ratio.The one phase diagram of the grid connected VSC is shown in Fig. 3, where u invx and i f x are voltage and current of VSC, u Cx and i Cx are voltage and current of the filter, u injx and i injx are injected voltage and injected current.Applying Kirchhoff's law: By applying Clarke's transformation, Eq. ( 1) and Eq. ( 2) can be written in the α, β -coordinate system as: After manipulation of these equations, the following state-space model is obtained: where The block diagrams for Eq. ( 5) and Eq. ( 6) are shown in Fig. 4. Re-writing the Eq. ( 5) and Eq. ( 6): By transforming coordinate system from αβ -to dqwith a PLL (Phase-Locked Loop) synchronized with the grid voltage vector, Eq. (3) and Eq. ( 4) becomes: where: • i f are d and q components of current of the VSC; inj are d and q components of injected voltages; inv are d and q components of voltages of VSC in dq-coordinate system; inj are d and q components of injected current in dq-coordinate system.Equation (9) and Eq. ( 10) can be rewritten as follows: The block diagrams for Eq. ( 11) are shown in Fig. 5.
To derive the voltage controller to be implemented in a digital controller, it is necessary to discretize Eq. (9) and Eq. ( 10).This is done by integrating the equation over one sample period T s and then dividing by T s , thus obtaining: Vector controller analysis and designs, its problems and possible solutions can be found, e.g., in [4], [8], [9], [10] and [14].

Voltage Controller -Outer Loop
Using PI controller, Eq. ( 12) can be written as follows: • with positive components: • with negative components: where G P P I and G N P I are integral gain of the voltage controller for positive and negative components, respectively.The output of the voltage controller is the reference current i dq * .Figure 8 shows the schematic structure of voltage controller.

Pulse Width Modulation abc/αβ and αβ/dq transform
Fig. 7: Schematic structure of DVR in the rotating dq-coordinate system.

Current Controller -Inner Loop
Using PI controller, Eq. ( 13) can be written as follows: • with positive components: • with negative components: The output of the current controller is the reference voltage u dq .The schematic structure of the current controller is shown in Fig. 9. Finally, cascaded double loop vector controller based on combining two separated controllers implemented in the positive and negative sequence, respectively, as shown in Fig. 7.This means that voltage and current on dq-and αβ -coordinates system are separated into positive and negative sequences, the voltage controller and current controller are used independently, after that by transforming them into the fixed αβcoordinate system.

Simulation and Discussion
Three cases of distribution system based on real data are simulated using MATLAB-Simulink.The numerical data are taken from case study of co-author in an existing publication [5].The cases of study can be represented as follows: • Case I: Balanced voltage sags.
• Case III: Fluctuations and distortion voltages caused by Switching capacitor ON or OFF.

Case I: Balanced Voltage Sags
The balanced voltage sag occurs while three phases fault in the grid, during the period from 2 to 2.1 second.The voltage of the sensitive load is reduced by 50 % with respect to the reference voltage.Figure 10, shows grid voltage, injected voltage and load voltage respectively in three phase and d, q components in rotating dq -coordinate system.The error between d and q components of designed injected voltage and real time experimental injected voltage is shown in Fig. 11.The information of voltage at PCC and load voltage in this case is presented in Tab. 1.It can be clearly seen that the DVR is doing nothing during normal operation but it quickly injects voltage components to restore the load voltage during voltage sag.Tab.1: RMS voltage of case I.

Voltage at PCC Balanced Voltage
Load voltage before sag (V) sag at PCC (V) during sag (V) 6320 3180 6220

Case II: Unbalanced Voltage Sags
The unbalanced voltage sags are simulated and the results are shown in Fig. 12 and Fig. Fig. 12: Case II-The difference of d, q components between designed injected voltage and real time experimental injected voltage.In this simulation, the capacitor was switched ON at 2 second and switched OFF at 3 second, the period of fluctuations and distortions is 1 second.The result of simulation is shown in Fig. 14 and Fig. 15 where   Fig. 14 shows source voltage, injected voltage and load voltage respectively in three phase and d, q components in rotating dq -coordinate system, Fig. 15

Experimental Results
The experiment of proposed methods for mitigating balanced voltage sags was performed at the laboratory.Fig. 16 and Fig. 17 show the experimental setup and its elements.In this experiment, voltage at the sensitive load reduced by 50 % due to three-phase fault, during the period from 1.68 to 3.21 seconds.The result of experimental is shown in Fig. 18 and Fig. 19 where Fig. 18 shows source voltage, injected voltage and load voltage respectively in three phases, Fig. 19 shows d, q components in rotating dq -coordinate system.

Conclusion
In this paper, the summary of the mathematical representation and configuration of DVR for mitigating balanced voltage, unbalanced voltage sags, fluctuant voltage and distorted voltages was presented.The paper discussed the vector control with two cascaded loops technique to improve the properties of DVR.The proposed DVR control method was modeled using MATLAB-Simulink and tested in balanced, unbalanced voltage sags and fluctuant voltage and distorted of voltages.As a result, the system performed with higher accuracy, faster activated ability and lower distortion in the sags voltage compensator.
Experimental results from laboratory proved the correctness and effectiveness of proposed solutions.

Fig. 2 :
Fig. 2: Three-phase diagram of a grid with the dynamic voltage restorer.

Fig. 3 :
Fig. 3: One phase diagram of the grid connected VSC.

Fig. 5 :
Fig. 5: Block diagrams of controller in the dq -coordinate system.

Figure 6 Fig. 6 :
Figure6shows the proposed control strategy is developed based on vector controller method performed on dq-and αβ -coordinates system with double loops, the outer loop is voltage controller and the inner loop is current controller.Both voltages and currents are analyzed by their sequence components and two separated controllers are used, the voltage controller can restore the load voltage both under balanced and unbalanced conditions of the grid voltage, the current controller can regulate the injected currents and improve response and operate properly of DVR.

Fig. 8 :
Fig. 8: Schematic structure of voltage controller in the rotating dq-coordinate system.

Fig. 9 :
Fig. 9: Schematic structure of Current controller in the rotating dq-coordinate system.

Fig. 10 :
Fig. 10: Case I-The voltage in three phases and in the dqcoordinate system: from top to bottom traces are grid voltages, injected voltages, and load voltages.

Fig. 11 :
Fig. 11: Case I-The difference of d, q components between designed injected voltage and real time experimental injected voltage.

Fig. 14 :
Fig. 14: Case III-The voltage in three phases: from top to bottom traces are grid voltages, injected voltages, and load voltages.

4Fig. 15 :
Fig.15: Case III-The difference of d, q components between designed injected voltage and real time experimental injected voltage.

Fig. 18 :
Fig. 18: Experimental results for balanced voltage sags: from top to bottom traces are source voltages, injected voltages, and load voltages.

Fig. 19 :
Fig. 19: Experimental results for balanced case: from top to bottom traces are voltage components on the dq -coordinate system of source, injected and load.

VSC Filter Lf Cf Booster Transformer
• u La (t), u Lb (t), u Lc (t) are the load voltages.L Fig. 13: Case II-The voltage in three phases and in the dqcoordinate system: from top to bottom traces are grid voltages, injected voltages, and load voltages.c 2018 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING phase C and little increased 10 % in phase B with respect to the reference voltage (see Tab. 2).Observing that, the injected voltage was produced by DVR can restore balanced voltage at the PCC despite of unbalanced voltage sags occurring in the grid.
shows the difference between d and q component of designed injected voltage and real time experimental injected voltage.It can be seen that the DVR is capable to nearly maintain normal operation of load voltage after about 0.04 second (see Tab. 3).