What is a Dynamic Voltage Restorer?
A Dynamic Voltage Restorer (DVR) is a device used to protect sensitive loads in power distribution systems from voltage sags. Voltage sags occur due to faults in the system, which can lead to fluctuations or reductions in voltage. These fluctuations can be harmful to sensitive equipment that requires a stable voltage supply.
For instance, consider a power distribution feeder that supplies electricity to both domestic and sensitive loads. If a fault occurs in the feeder, it causes a voltage sag that impacts the sensitive loads, potentially damaging them. To mitigate this, a DVR is used to inject voltage into the system, ensuring that the voltage supplied to sensitive loads remains constant at 1 per unit, even during faults.
Key Components of a DVR System
The DVR consists of two main components:
DC Energy Storage: This stores energy for use during voltage sags.
IGBT Converter: This is used to convert the DC energy to AC and inject it into the system as needed.
The DVR continuously monitors the voltage of the distribution feeder and the sensitive loads. When a voltage sag is detected, the DVR injects the necessary compensating voltage into the system to stabilize the load voltage.
Understanding the Control Strategy
In the MATLAB implementation of the DVR system, the first step is to measure the load voltage and convert it into a per-unit system. This involves transforming the measured voltage into a normalized value to simplify control.
Next, the system employs a transformation from the ABC coordinate system to the DQ coordinate system. This allows for easier control by focusing only on two components: the direct axis voltage (VD) and the quadrature axis voltage (VQ). The VD is compared to a reference voltage, and VQ is compared to zero.
Sliding Mode Control in Action
Once the voltage components are compared, sliding mode control is applied to both the VD and VQ voltages. This control strategy is designed to handle system uncertainties and disturbances effectively. By adjusting the control signals based on these comparisons, the system can maintain the voltage at the desired level, even in the presence of faults or harmonic disturbances.
After the sliding mode control is applied, the DQ voltages are converted back into the ABC coordinate system. The final control voltage is then compared with the load voltage, and the result is used to generate pulses for the series filter. This filter injects the necessary voltage into the system to maintain the load voltage at 1 per unit.
Fault and Harmonic Injection Simulation
To test the performance of the DVR system, simulations are run under fault and harmonic conditions. During a fault, the grid voltage and load voltage fluctuate, but the DVR injects the required voltage to maintain the load voltage at 1 per unit. The DVR's ability to respond effectively is demonstrated, as it compensates for the voltage sag within a short time frame, ensuring the sensitive load remains unaffected.
Additionally, harmonics are injected into the system to simulate non-ideal grid conditions. Even in the presence of harmonics, the DVR continues to maintain the load voltage at 1 per unit by injecting opposing voltage to counteract the harmonics.
Conclusion
The MATLAB simulation of the DVR with sliding mode control showcases how this strategy can effectively mitigate voltage sags and harmonics, ensuring that sensitive loads receive a stable supply of voltage. By using a combination of real-time voltage measurement, per-unit conversion, sliding mode control, and harmonic compensation, the DVR plays a critical role in protecting equipment from voltage disturbances. This approach highlights the importance of advanced control strategies in modern power systems to ensure reliability and stability.
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