Abstract - In this paper a novel control method based on Synchronous Reference Frame Theory (SRFT) is proposed to compensate power quality problems through a three- phase Unified Power Quality Conditioner (UPQC) under unbalanced and distorted load conditions. The performance of the proposed system has been verified using MATLAB-SIMULINK and are discussed in detail in this paper.
Keywords- Phase Locked loop (PLL), Power Quality (PQ), Synchronous reference frame (SRF), Unified Power Quality Conditioner (UPQC), voltage and current harmonics mitigation.
ith the recent trend of using non-linear and power electronics loads and electronically switched devices in distribution system results in power quality (PQ) problems, such as voltage sags, voltage swells, flickers, interruption, and imbalance of current have become a serious problem. This has attracted the concentrations of many researchers towards the solution of this problem. The custom power devices based on Voltage source converter (VSC) are becoming popular to mitigate these power quality problems. A series converter (also known as Dynamic Voltage Restorer) is able to mitigate the harmonics and distortion in supply voltage such as voltage sag/swell resulting in a regulated voltage across the sensitive/ critical electronics load. A shunt converter (also known as DSTATCOM) is used to compensate for distortion and unbalance in a load resulting a balanced sinusoidal current can flow through the feeder. One new and very promising solution to power quality problem is Unified Power Quality Conditioner (UPQC). The UPQC is a custom power device and it is a combination of both series and shunt converter connected back to back through a common dc link. UPQC can compensate both voltage and current harmonics such as voltage sag/swell and current disturbances. It can also control the power flow and can improve voltage profile [1-4].
In this paper a new control technique for the SRF based UPQC is presented, without measuring transformer voltage and load current resulting an improvement of system performance. The proposed control technique has been evaluated and tested under unbalanced and distorted load conditions using MATLAB/SIMULINK software.
The three phase UPQC consists of two voltage source inverters (VSI) connected back to back to a common DC link. One of these two VSI is connected in series with the ac line while the other VSI is connected in shunt with the same line. The main purpose of the series active filter is harmonic isolation between a sub transmission system and distribution system. In addition series active filter has the capability to mitigate voltage imbalance, stroke flicker compensation as well as voltage regulation and harmonic compensation at PCC. The block diagram of UPQC is shown in Fig. 1.
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Fig. 1. Unified Power Quality Conditioner configuration
The shunt active filter is used to absorb current harmonic, compensates reactive power and negative sequence current as well as regulates the dc link voltage between two active filters. Two passive filters remove switching frequency harmonics from the output voltage of series converter and output current from shunt inverter.
III. SRF TECHNIQUE
The synchronous reference frame (SRF) method can be used to extract the harmonics contained in supply voltages or currents. For the current harmonic compensation the three phase distorted currents are first converted into two phase stationary coordinated using α-β transformation same as the p-q theory. After that, the stationary frame quantities are transferred into synchronous rotating frame using cosine and sine functions obtained from PLL. The PLL provides the synchronization with supply voltage and current. The harmonics and fundamental components are separated easily by passing the signal from a Low Pass Filter (LPF). After that, the fundamental components transferred back to a-b-c frame using inverse park transformation. The a-b-c to d-q-o transformation is known as park transformation.
IV. UPQC CONTROL ALGORITHM
The proposed UPQC control block diagram in shown in fig.2, sensing three-phase source voltage and current and load voltages along with the dc- link voltages are adequate to compute the reference switching signals in the UPQC.
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Fig.2. Proposed UPQC control block diagrams
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- HFHS - Höhere Fachschule für anthroposophische Heilpädagogik, Sozialpädagogik und Sozialtherapie
- harmonic compensation voltage current using upqc