08-16-2017, 09:36 PM
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ABSTRACT
Today everyone is aiming at a reduction in greenhouse gas emissions, the requirements for adding new generation capacity can no longer be met by traditional power generation methods of burning the primary fossil fuels such as coal, oil, natural gas, etc. This is why distributed generators (DG) have significant opportunity in the evolving power system network. Both consumers and power utilities can benefit from the widespread deployment of DG systems which offer secure and diversified energy options, increase generation and transmission efficiency, reduce greenhouse gas emissions, improve power quality and system stability, cut energy costs and capital expenditures.
This paper proposes operation and control of converter based single phase distributed generators (DG) in a utility connected grid. A common utility practice is to distribute the household single-phase loads evenly between the three phases. The voltage unbalance between the phases remains within a reasonable limit. The single phase sources are operated to deliver available maximum power generated while the rest of the power demands in each of the phases are supplied by utility (and if available, three phase DG sources). However the voltage unbalance can be severe if single-phase rooftop mounted PVs are distributed randomly between the households. Moreover, there can also be single-phase nonlinear loads present in the system. The cumulative effect of all these will cause power quality problem at the utility side. The problem can be macabre if three-phase active loads (e.g., induction motors) are connected to the utility feeder.
A DSTATCOM can compensate for unbalances and nonlinearities, while providing reactive power support. The size of the dc capacitor determines how much reactive power support the DSTATCOM can provide without any drop in voltage. The choice of this capacitor is thus a trade-off between the reactive support and system response distribution static compensator (DSTATCOM) is connected at the utility bus to improve the power quality. The DSTATCOM only supplies reactive power and no real power. Alternatively a three phase DG-compensator can be connected at the PCC to share the real and reactive power with utility and to compensate for the unbalance and nonlinearities in the system.
The imbalance in three phase power is compensated two ways either through a DSTATCOM or through a DG-compensator. With the proposed structure of distribution system, it is possible to operate single phase DG sources in a utility connected grid and this might become a useful tool as their penetration in distribution systems increases.
I. INTRODUCTION
More countries are aiming at a reduction in greenhouse gas emissions, the requirements for adding new generation capacity can no longer be met by traditional power generation methods of burning the primary fossil fuels such as coal, oil, natural gas, etc. [1]. This is why distributed generators (DG) have significant opportunity in the evolving power system network. Both consumers and power utilities can benefit from the widespread deployment of DG systems which offer secure and diversified energy options, increase generation and transmission efficiency, reduce greenhouse gas emissions, improve power quality and system stability, cut energy costs and capital expenditures, and alleviate the bottleneck caused by distribution lines [2]. Properly sited DG can increase the feeder capacity limit, but this does not necessarily produce an improvement in system reliability or power quality, as quantified by standard indices [3]. With improving reliability of the owner, the DG may reduce the severity of voltage sags near the DG. The DG often has a negative impact on reliability indices through sympathetic tripping, required changes to utility over current device settings, and increased fuse blowing. The utility cannot assume DG automatically improves system reliability, and action may be required to ensure that reliability does not actually degrade for other customers [3].
Application of single phase converter based DGs are very common in distribution level and with the increasing number of single phase micro sources in a utility connected grid has raised concern about power quality. For a microgrid, a common practice is to isolate the microgrid from the utility grid by an isolator if the voltage is seriously unbalanced [4]. However
when the voltages are not critically unbalanced, the isolator will remain closed, subjecting the microgrid to sustained unbalanced voltages at the point of common coupling (PCC), if no compensating action is taken. Unbalance voltages can cause abnormal operation particularly for sensitive loads and increase the losses in motor loads.
This paper proposes the operation and control of single phase micro-sources (DG) in a utility connected grid. While the DGs supply their maximum generated power, rest of the power demand of each phase is supplied by the utility and three phase DG connected at the PCC, if any. To counteract this problem, we have proposed two different schemes. In the first scheme, a distribution static compensator (DSTATCOM) is connected at the point of common coupling (PCC) to compensate the unbalance and nonlinear nature of the total load current and to provide the reactive power support. In the second scheme, a three phase DG, connected at the PCC, in place of the DSTATCOM to share both real and reactive power with the utility. The DG also compensates the system and makes the PCC voltage balanced. The efficacies of the controllers and improvement in power quality have been validated through simulation for various operating conditions using PSCAD.