Dynamic voltage restorer

Dynamic voltage restorer (DVR) operation for grid management in voltage sag and voltage swell conditions.

Introduction

Power quality (PQ) is a critical and growing issue in power grid management, driven by the increase in the share of nonlinear loads in the grid and the growth of microgrids. Common PQ problems which affect industrial consumers include voltage sags, voltage swells, power-line flicker, harmonic distortions, and power supply interruptions. These problems can cause damage to electrical equipment and significant economic losses. One of the commonly available solutions to address these issues is the use of a dynamic voltage restorer (DVR) [1].

DVR is implemented as a voltage source inverter which injects 3-phase voltage into the grid in order to keep load side voltage within prescribed limits. Normally, DVRs are installed in the distribution system between the power supply and a critical load feeder at a point of common coupling (PCC), shown in Figure 1.

Figure 1: DVR schematic diagram

In this application, the three-phase inverter has a higher rated voltage, requiring three single phase transformers to safely connect it to the grid. The transformation ratio of the transformers depends on the three-phase inverter output voltage and the voltage level in the grid. Connecting transformers in open-/star-winding allows the injection of positive, negative, and zero sequence voltage, whereas connecting transformers into open-/delta-winding only allows for injecting positive and negative sequence voltage into the grid. The control technique for the DVR depends on the load type: some loads are sensitive only to magnitude change while others are sensitive both to magnitude change and phase shift. The featured model uses pre-sag compensation as a means to control active and reactive power compensation when a fault occurs, in order to maintain load-side voltage and phase angle under potential fault conditions.

Note: This application note is based on one of the winning models of the 10for10 Typhoon HIL Awards program of 2019. The featured model’s author is Eng. Zeinab Mohamed Kamal from the Electronics Research Institute of Egypt.

Model description

Figure 2 shows the electrical part of the model. On the left side of schematic, the grid is modelled by an ideal voltage source, an inductance, and a resistance for each phase. Grid parameters are set in the Model Initialization

Figure 2: Typhoon HIL schematic of a DVR model

The three-phase inverter is connected in series to the grid through three single phase transformers, which in turn are connected to a DC source which modeled using an ideal voltage source and a resistor. The three phase RL load with grid connection appears on the far right of the schematic. Parameters for the three-phase load are also set in Model Initialization panel.

The control part of the model is represented by a blue color as shown in Figure 3. Both grid-side meters and load-side meters measure the voltage for the controller.

Figure 3: DVR Control Model

Figure 4 shows the controller sub-system block. The controller generates alpha and beta signals which feeds the Space Vector PWM reference generator, generating reference signals for the three-phase inverter.

Figure 4: DVR Controller subsystem

Active and reactive power measurement of the dynamic voltage restorer is done in the DVR power measurement block shown in Figure 5.

Figure 5: DVR power measurement

Table 1. HIL device resource utilization
No. of processing cores 1
Max. matrix memory utilization 51.44 %
Max. time slot utilization 92.5%
Simulation step, electrical 1µs
Execution rate, signal processing 100 µs

Simulation

This application comes with a pre-built SCADA panel shown in Figure 6. It offers the most essential user interface elements (widgets) to monitor and interact with the simulation in runtime, allowing you to further customize it freely.

Figure 6: DVR SCADA panel

Here we consider three DVR application cases: normal operation mode without fault, operation during a voltage sag, and operation during a voltage swell.

  1. Normal operation mode without fault. During this operation mode, voltage in the grid is at a nominal value of 230V. Figure 7 shows the waveforms of injected DVR voltages during stand-by mode operation. The DVR injects a small amount of active power into the grid, shown in Figure 8. The sum of drawn power from the grid and injected power from the DVR equals the active power of the load.

    Figure 7: Injected DVR voltages

    Figure 8: Active power measurement of the DVR

  2. Operation of the DVR during a voltage sag in the grid. Grid voltage is reduced from 230V to 154V. The top graph of Figure 9 shows the waveforms of injected voltage to the grid during fault condition. The drop in grid voltage at 0s shows the modelled fault, which is quickly addressed by DVR voltage injection. The bottom graph of Figure 9 shows that load voltage stays within limits, with a slight decrease in amplitude.

    Figure 9: Injected DVR voltage during voltage dip

    Figure 10 shows the active power response. The DVR compensates for the power decrease in the grid caused by the voltage drop, resulting in a slight decrease in the load-side active power during the transient period.

    Figure 10: Active power response of the DVR during voltage dip

  3. Operation of the DVR during voltage swell. Grid voltage increases from 230V to 268V. During this fault condition, the DVR takes voltage from the grid. The top graph of Figure 11 shows the voltage waveforms following the rise of the grid voltage 0s, representing the fault. The bottom graph shows that the voltage stays within limits, with a slight increase in amplitude.

    Figure 11: DVR voltage waveforms during voltage swell

Figure 12 shows the active power response of the DVR to the voltage swell. The DVR reduces the power increase on the grid side by slightly increasing the active power on the load side during the fault period. The sum of power from the grid and drawn power of the DVR equals the active power of the load.

Figure 12: Active power response of the DVR during voltage swell

Table 2. Minimum requirements
Files
Typhoon HIL files

examples\models\power quality\dynamic voltage restorer

dynamic voltage restorer.tse,

dynamic voltage restorer.cus
Minimum hardware requirements
No. of HIL devices 1
HIL device model HIL402
Device configuration 1

Reference

[1] Dynamic Voltage Restorer for Voltage Sag Compensation

Test automation

We don’t have a test automation for this example yet. Let us know if you wish to contribute and we will gladly have you signed on the application note!

Authors

[1] Kristian Monar