Abstract:The increasing growth in power demand and the penetration of renewable distributed generations in competitive electricity market demands large and flexible capacity from the transmission grid to reduce transmission bottlenecks. The bottlenecks cause transmission congestion, reliability problems, restrict competition, and limit the maximum dispatch of low cost generations in the network. The electricity system requires efficient utilization of the current transmission capability to improve the Available Transfer Capability (ATC). To improve the ATC, power flow among the lines can be managed by using Flexible AC Transmission System (FACTS) devices as power flow controllers, which alter the parameters of power lines. It is important to place FACTS devices on suitable lines to vary the reactance for improving Total Transmission Capacity (TTC) of the network and provide flexibility in the power flow. In this paper a transmission network is analyzed based on line parameters variation to improve TTC of the interconnected system. Lines are selected for placing FACTS devices based on real power flow Performance Index (PI) sensitivity factors. TTC is computed using the Repeated Power Flow (RPF) method using the constraints of lines thermal limits, bus voltage limits and generator limits. The reactance of suitable lines, selected on the basis of PI sensitivity factors are changed to divert the power flow to other lines with enough transfer capacity available. The improvement of TTC using line reactance variation is demonstrated with three IEEE test systems with multi-area networks. The results show the variation of the selected lines' reactance in improving TTC for all the test networks with defined contingency cases.
In grid-connected power converter applications, the phase-locked loop (PLL) is probably the most widely used grid synchronization technique, owing to its simple implementation. However, in power grids some very common problems, such as voltage distortion, voltage unbalance, and frequency instability make synchronization a challenging task. The performance of the conventional synchronous reference frame PLL (SRF-PLL) is greatly reduced in the presence of distorted grid conditions. For a polluted grid some advance PLL techniques have been proposed, such as moving average filter PLL (MAF-PLL) and cascaded delayed signal cancellation PLL (CDSC-PLL). These techniques have been mostly evaluated in the presence of odd and even harmonics but the effects of interharmonics on these synchronization techniques still needs to be investigated. In this paper, a detailed performance comparison has been made between SRF-PLL, MAF-PLL, and CDSC-PLL for grid voltages contaminated with interharmonics in the presence of different grid disturbances, such as frequency jump, phase angle jump, and dc offset. The techniques are simulated using Matlab/Simulink. The CDSC-PLL shows excellent performance as compared with other techniques in terms of dynamic response as it settles to frequency step change in a half cycle but the presence of interharmonics greatly reduces its filtering capability. On the other hand, MAF-PLL gives a ripple free behavior in frequency estimation but with a much slower dynamic response as it settles to a frequency step change in more than three cycles. SRF-PLL only performs well under harmonics free grid voltages. INDEX TERMS Grid synchronization, phase locked loop, interharmonics and power quality improvement.
The increased use of non-linear loads results in more harmonics content in the source current. Shunt active power filters are used for correction of distorted source current by injecting a compensating current in parallel to the load current. The injected current will shape the supply current to a sinusoidal. In this paper an improved performance of shunt active power filter is achieved by using sliding mode controller for generation of gate pulses of voltage source inverter. The implemented control technique has been evaluated and compared with hysteresis current control. The performance parameters used for evaluating the shunt active power filter are total harmonic distortion, power factor and harmonics compensation ratio. The simulation results show excellent performance of active power filter in terms of harmonics mitigation and dynamic response. Experimental tests are also performed using NI DAQ card in Labview environment to validate the simulation results. Both the simulation and experimental results prove good performance of sliding mode control for accurate injection of reference current.
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