Implementation of DVR Sliding Mode Control Strategy of Dynamic Voltage Restorer
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What is a Dynamic Voltage Restorer?
A Dynamic Voltage Restorer (DVR) is a series-connected power electronic device designed to protect sensitive loads from voltage sags and distortions. It operates by injecting a compensating voltage into the distribution feeder whenever a disturbance is detected, thereby ensuring that the load voltage remains at its rated value (typically 1 per unit).
Practical Scenario
Consider a distribution feeder supplying both domestic and sensitive industrial loads. When a fault occurs upstream—such as a short circuit or sudden load change—the feeder voltage drops. This voltage sag propagates to the sensitive load, which may not tolerate such variations. The DVR mitigates this issue by injecting an appropriate voltage in series with the feeder, restoring the load voltage to its nominal value.
Key Components of a DVR System
A typical DVR system consists of the following main components:
1. DC Energy Storage System
The DC energy storage unit (battery, capacitor, or DC link) provides the required energy during voltage sag conditions. This stored energy is used to generate the compensating voltage.
2. Voltage Source Converter (IGBT-Based)
An IGBT-based voltage source converter converts the DC energy into AC voltage. This converter is responsible for injecting the required compensating voltage into the distribution system through a series transformer.
DVR Control Strategy Overview
The performance of a DVR depends heavily on its control strategy. In the MATLAB/Simulink implementation discussed here, an advanced sliding mode control (SMC) approach is adopted due to its robustness against system uncertainties and disturbances.
Step 1: Voltage Measurement and Per-Unit Conversion
The load voltage is continuously measured and converted into a per-unit (p.u.) system. This normalization simplifies control design and allows easy comparison with reference values.
Step 2: ABC to DQ Transformation
The measured three-phase load voltage (ABC frame) is transformed into the DQ reference frame. In this frame:
Direct-axis voltage (VD) represents the active voltage component.
Quadrature-axis voltage (VQ) represents the reactive component.
The VD component is compared with a reference value of 1 p.u., while VQ is compared with zero, indicating balanced operation.
Sliding Mode Control for DVR
Sliding Mode Control is applied independently to both VD and VQ components. This control method is particularly effective in DVR applications due to its:
High robustness
Fast dynamic response
Strong disturbance rejection capability
The controller generates appropriate control signals that compensate for voltage sags, unbalances, and harmonic distortions.
After control action, the compensated DQ voltages are transformed back into the ABC frame, which serves as the reference voltage to be injected by the DVR.
Pulse Generation and Voltage Injection
The reference compensating voltage is compared with the actual load voltage, and the resulting error is used to generate PWM switching pulses for the IGBT converter. These pulses control the series filter and injection transformer, allowing the DVR to inject the required voltage precisely and dynamically.
Fault and Harmonic Injection Simulation
To validate the effectiveness of the DVR, MATLAB simulations are performed under the following conditions:
Voltage Sag Due to Fault
During fault conditions, the grid voltage experiences a significant drop. Without compensation, the load voltage also sags. With the DVR in operation:
The DVR detects the sag instantly
Injects the missing voltage component
Maintains the load voltage at 1 p.u.
The compensation occurs within a very short time, ensuring uninterrupted operation of sensitive loads.
Harmonic Disturbance Injection
Harmonics are introduced into the grid to simulate non-ideal conditions caused by nonlinear loads. Even under harmonic distortion:
The DVR injects counteracting harmonic voltage
The load voltage remains sinusoidal and regulated at 1 p.u.
Conclusion
The MATLAB simulation of a Dynamic Voltage Restorer using Sliding Mode Control demonstrates the effectiveness of this approach in mitigating voltage sags and harmonics. By combining real-time voltage measurement, per-unit normalization, DQ transformation, and robust sliding mode control, the DVR ensures stable voltage delivery to sensitive loads.
This study highlights the critical role of advanced power electronic controllers in modern distribution systems and emphasizes the importance of DVRs in enhancing power quality, system reliability, and equipment protection.
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