Introduction to UPQC Control Methods
A Unified Power Quality Conditioner (UPQC) is an advanced device used to improve power quality by mitigating voltage sags, harmonics, and other disturbances in the power supply. UPQC typically uses two main control techniques: DQ0 control and PQ control. These methods control the active filters within the UPQC system to correct disturbances such as voltage sag and harmonic distortion.
In this blog, we will explore the DQ0 control logic for the series active filter, and the PQ control concept for the shunt active filter. We will also compare both methods to understand their performance in real-world scenarios.
Simulation Setup and Conditions
To simulate the performance of these control techniques, a model was developed where:
The source voltage was set to 400V at 50Hz.
A voltage sag was introduced at 4.2 seconds with a magnitude of 0.7 per unit, and a harmonic injection was applied from 2 to 3 seconds using the fifth and seventh harmonics.
The system included a nonlinear load to create harmonic distortion in the grid source current, which was controlled by the series active filter (SAF) and shunt active filter (S filter) of the UPQC system.
DQ0 Control Logic for Series Active Filter
The DQ0 control technique is implemented in the series active filter of the UPQC. This control method converts the source voltage and load voltage into DQ components using a Park transformation. The process involves:
Measuring the source voltage and load voltage.
Converting these voltages into DQ components.
Comparing the D and Q axis components to generate an error signal.
Using a proportional-integral controller (PI controller) to generate control signals based on this error.
The control signal is then used to drive the inverter in the series active filter, effectively mitigating voltage sag and harmonics in the system.
PQ Control for Shunt Active Filter
The PQ control technique is implemented in the shunt active filter (S filter) to address power quality issues like harmonic distortion and reactive power. This control method focuses on real and reactive power calculations to generate the reference current required for harmonic compensation.
The process involves:
Measuring the load current and converting it into DQ components.
Using a low-pass filter to extract the average values of the D-axis current.
Comparing the real and reactive power values to generate the reference current.
The reference current is then compared with the actual current, and a PI controller generates control signals.
The inverter of the shunt active filter is controlled to inject compensating current, reducing harmonic distortion and maintaining the load voltage.
Comparing DQ0 and PQ Control Techniques
Both the DQ0 and PQ control techniques were applied to the simulation model to evaluate their performance. Here’s a comparison of the results:
Harmonic Distortion Mitigation: The DQ0 control method for the series active filter and PQ control for the shunt active filter were able to reduce Total Harmonic Distortion (THD) significantly. For example, THD in the source current was reduced from 20% to less than 5% when using the UPQC with DQ0 and PQ control techniques.
Voltage Sag Compensation: During voltage sag, the UPQC system maintained a stable load voltage and sinusoidal current. The voltage was restored to 230V, and the current waveform remained sinusoidal, indicating effective sag compensation.
Harmonic Injection Compensation: In the case of harmonic injection from the grid, the system was able to inject compensating voltage into the grid, significantly reducing harmonic distortion in both the source and load currents. The system demonstrated that even with harmonic disturbances, the current waveform was close to sinusoidal.
Performance Analysis and THD Reduction
The performance of the UPQC system with DQ0 and PQ control methods was analyzed using the Total Harmonic Distortion (THD) metric. The results showed a substantial reduction in THD in both voltage and current after the application of these control techniques.
For example:
Before harmonic mitigation, the THD in the source current was around 20.98%.
After using the DQ0 and PQ control techniques, the THD was reduced to approximately 2.36%, demonstrating a significant improvement in power quality.
During voltage sag and harmonic injection scenarios, the THD remained consistently below 5%, which meets the required power quality standards.
Conclusion: DQ0 Control vs. PQ Control for Power Quality Improvement
In conclusion, both DQ0 control for the series active filter and PQ control for the shunt active filter have shown promising results in improving power quality. The simulation results indicated that:
The DQ0 control method provides better performance in terms of maintaining sinusoidal source current and reducing THD in the system.
The PQ control technique was effective in handling reactive power and harmonic compensation in the shunt filter.
By combining these two control strategies in the UPQC system, power quality issues such as voltage sag and harmonic distortion can be effectively mitigated. The DQ0 and PQ control methods complement each other, offering a reliable solution for maintaining stable voltage and current waveforms in power systems.
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