This paper extends the conventional features of a transformerless unified power quality conditioner (TL-UPQC). An enhanced control methodology is presented to allow exchanging reactive power between the system and the grid to provide input grid voltage regulation. Therefore, both load side and input grid side voltages are regulated with one converter. In this regard, a phase angle is created between the input current and the input voltage. Thereby, the system behavior as a capacitive or an inductive reactance is controlled. An additional ac voltage control loop has been designed. The inner current loop has been reformed to receive reference information from two outer voltage loops. The enhanced control strategy takes action based on local information collected by the TL-UPQC with no requirements of additional sensor circuits. Since the conventional functions of the TL-UPQC system have been extended, aspects related to system modeling and control design should be developed. Small-signal model that characterize the dynamics of the power stage and the controller are presented. Design guidelines considering grid impedance to achieve a desired performance are developed. A 500VA / 120V, 60 Hz prototype has been built to verify the models and the overall system performance. Steady-state and transient experimental results are presented and discussed.
- Grid impedance
- Power quality
- Reactive power compensation
- Voltage control
- Voltage regulation
Fig. 1. A block diagram of power flow in a TL-UPQC system.
EXPECTED SIMULATION RESULTS:
Fig. 2. Simulation results adopting conventional TL-UPQC system under random voltage variations in the network.
Fig. 3. Simulation results adopting proposed solution under random voltage variations in the network.
The paper expands the control strategy of a TL-UPQC system to be capable of injecting and absorbing reactive power into and from the input grid in low voltage distribution networks. Employing the proposed enhanced control strategy, the TL-UPQC was able to filter out harmonic components generated by nonlinear loads, compensate all voltage fluctuations across sensitive loads with fast dynamic response and improve the voltage profile of the input grid. A detailed stability analysis and control design criteria employing small-signal models were presented. The experimental results validated the capability of the system to provide voltage regulation at the PCC while supplying linear and nonlinear sensitive loads at the same time. The system was able to reduce the total harmonic distortion of the PCC voltage, the load bus voltage and the grid current. The system was competent to deliver a stable voltage with a constant amplitude to the loads connected to the PCC in the event of voltage sags and swells. Experimental results favorably showed good agreement with the theoretical findings. It is to be noted that adopting a half-bridge topology would increase the voltage stress across the semiconductor switches. Safety mechanisms should be considered if the proposed transformerless system is to be adopted in residential applications.
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