الفهرس 
Chapter 1 : IntroductionOnly 14 pages are availabe for public view
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Abstract
The increasing widespread of the Distributed Generators (DG) connected to Medium Voltage (MV) and Low Voltage (LV) distribution networks has bring a series of new challenges related to the integration of new generators types, mainly interfaced through power electronic converters, into the networks. The areas involved in these new problems are very different and cover, for example: Power Quality aspects; secure operation of the supply system; regulation and standardization of service and market.
The aim of this thesis is to investigate the challenges and possibilities of reactive power control in distribution networks with high penetration level of DG regarding the steady state and transient operations. For steady state operation, reactive power control based on the DG units can be performed using decentralized or centralized methods. Decentralized (Local) control of DG reactive power is achieved based on local information such as voltage at the Point of Common Connection (PCC) and active power generated from DGs. Centralized (coordinated) control is achieved based on collaboration of all DGs and other VAR compensation, (i.e. shunt capacitors and OLTC) to control system voltage with optimum sharing of reactive power. In case of faults, the low voltage fault ride-through capability of the DG need to be identified through grid codes. These grid codes need to be developed and analyzed as the penetration levels of DG is increased. Therefore, this thesis is divided into three parts.
In the first part, the local control of DG reactive power was investigated. Different reactive power control strategies which were presented in the German grid code (Cosϕ(P) and Q(U)), also control strategies which were proposed in the literature (Cosϕ(P,U) and Modified Cosϕ(P)), were analyzed. Then a new voltage/reactive power control (Q(P,U)) was developed. The new strategy was implemented on a simple test system, a CIGRÈ benchmark system, and real system. The implementation on real system was performed for different scenarios to check the performance of the control strategies under different DG penetration levels. The results of the proposed method were compared with those of the standard and published methods. from the results, it has been found that the proposed method has the ability to keep the voltage within the limits, can reduce the amount of reactive power consumed from the High Voltage (HV) network, can improve the utilization factor of the DG power, and can increase the hosting capacity of the DG power.
In the second part, the thesis proposed a new centralized reactive power control based on coordination between the On-Load-Tab-Changer (OLTC) of the main HV/MV transformer in the network and reactive power from DGs. The proposed method was developed based on an accurate and fast-flowing scheme based on predefined procedures. The results of the proposed method were compared with those of two decentralized control methods. The proposed method was implemented on a real network and the results show that the number of OLTC switching was minimized; system losses and the reactive power consumed from the HV network can be reduced. The drawbacks of local control of DG reactive power in case of high penetration level were investigated. Moreover, the importance of coordination between DGs and other VAR compensation in this case was highlighted.
In the third part, Low Voltage Ride-Through (LVRT) capability of converter based DG units was investigated. The LVRT requirements of DGs in German grid codes were discussed. As the national grid codes are basically valid for transmission networks where the X/R ratio is high; it will be unsuitable to be implemented in case of active distribution networks. Therefore, a simple modification to the German code was developed. The modification is based on supplying the available active power from the DG in case of fault in addition to providing reactive power. A comparison between the German grid code and the modified model was performed and the results showed that the voltage can be supported using the modified model. In order to verify the ability of the modified model, an implementation on a PV system in a simple grid was performed.