ABSTRACT:
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.
KEYWORDS:
1. Grid impedance
2. Modeling
3. Power quality
4. Reactive power compensation
5. Transformerless
6. UPQC
7. Voltage control
8. Voltage regulation
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
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.
CONCLUSION:
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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