The growing installation of distributed generation (DG) units in low voltage distribution systems has popularized the concept of nonlinear load harmonic current compensation using multi functional DG interfacing converters. It is analyzed in this paper that the compensation of local load harmonic current using a single DG interfacing converter may cause the amplification of supply voltage harmonics to sensitive loads, particularly when the main grid voltage is highly distorted. To address this limitation, unlike the operation of conventional unified power quality conditioners (UPQC) with series converter, a new simultaneous supply voltage and grid current harmonic compensation strategy is proposed using coordinated control of two shunt interfacing converters. Specifically, the first converter is responsible for local load supply voltage harmonic suppression. The second converter is used to mitigate the harmonic current produced by the interaction between the first interfacing converter and the local nonlinear load. To realize a simple control of parallel converters, a modified hybrid voltage and current controller is also developed in the paper. By using this proposed controller, the grid voltage phase-locked loop and the detection of the load current and the supply voltage harmonics are unnecessary for both interfacing converters. Thus, the computational load of interfacing converters can be significantly reduced. Simulated and experimental results are captured to validate the performance of the proposed topology and the control strategy.
1. Parallel converters
2. Active power filter
3. Dynamic voltage restorer
4. LCL filter
5. Resonance; power quality
6. Harmonic detection
7. Phase-locked loop.
Fig. 1. Diagram of the proposed topology.
EXPECTED SIMULATION RESULTS:
Fig. 2. Only the local load harmonic current is compensated. (From upper to lower: 𝑉𝑠𝑢𝑝𝑝𝑙𝑦, 𝐼𝑔, 𝐼2, 𝐼𝐿𝑜𝑎𝑑)
Fig. 3. The harmonic spectrum of grid current 𝐼𝑔 in Fig. 11.
Fig. 4. The harmonic spectrum of supply voltage 𝑉𝑠𝑢𝑝𝑝𝑙𝑦 in Fig. 11.
Fig. 5. Only the supply voltage harmonic component is compensated. (From upper to lower: 𝑉𝑠𝑢𝑝𝑝𝑙𝑦, 𝐼𝑔, 𝐼2, 𝐼𝐿𝑜𝑎𝑑)
Fig. 6. The harmonic spectrum of grid current 𝐼𝑔 in Fig. 14.
Fig. 7. The harmonic spectrum of supply voltage 𝑉𝑠𝑢𝑝𝑝𝑙𝑦 in Fig. 14.
When a single multi-functional interfacing converter is adopted to compensate the harmonic current from local nonlinear loads, the quality of supply voltage to local load can hardly be improved at the same time, particular when the main grid voltage is distorted. This paper discusses a novel coordinated voltage and current controller for dual-converter system in which the local load is directly connected to the shunt capacitor of the first converter. With the configuration, the quality of supply voltage can be enhanced via a direct closed-loop harmonic voltage control of filter capacitor voltage. At the same time, the harmonic current caused by the nonlinear load and the first converter is compensated by the second converter. Thus, the quality of the grid current and the supply voltage are both significantly improved. To reduce the computational load of DG interfacing converter, the coordinated voltage and current control without using load current/supply voltage harmonic extractions or phase-lock loops is developed to realize to coordinated control of parallel converters.
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