PERFORMANCE INVESTIGATION OF A DFIG INTERFACED WITH A MICROGRID

Abstract

A sudden surge in the demand and the global warming has paved the way for the renewable energy sources (RES) to take a lead in the generation of electricity across the globe. Among the RES, photovoltaics (PV) and wind energy are playing a key role due to their abundant availability. The PV is present during the day time whereas the wind is active during evening hours. The complimentary behaviour of these two prominent renewable energy sources opens door for the hybrid structure in order to improve the reliability to the customer. Furthermore, the size and cost of the battery can also be reduced. In spite of multiple benefits of the hybrid sources there is ample scope for further research. In this regard, this thesis is focussed to bring out the multiple merits of the PV based DC microgrid (DCMG) and the DFIG wind system when they are present in the distribution system either at the same bus or at the different bus. Further, the age-old problem of DFIG Fault Ride through in the presence of the fault current limiter is also discussed for a specific case where the fault is considered before the fault current limiter. Objective1: A Single Cost-Effective Architecture to Operate DC Microgrid Interfaced DFIG Wind System during Grid Connected, Fault and Isolated Conditions In this work, the authors have proposed a unique architecture of interfacing the DCMG at the DC link of the DFIG wind system. The control strategy engages grid side converter (GSC) to regulate the DC link voltage during grid connected mode. During isolated working conditions, the same GSC is allowed to behave as a simple diode bridge rectifier and therefore the power from the wind system is pumped into the DCMG thereby improving the reliability of the DC loads. In the case of the fault condition, the ultracapacitor which is integrated to the DCMG is engaged to improve the FRT of the DFIG wind system. Henceforth, with the help of the proposed single cost-effective architecture, a feasible solution is presented which can suit for all the working conditions and is also validated through hardware in the loop experimentation. Objective2: Novel Time Delay Based Decentralized Coordinated Voltage Control Schemes for Distribution System With DC Microgrid In this work, two novel time delay based decentralized coordinated voltage control schemes are developed in order to improve the voltage profile of the distribution system during the normal operating conditions. Additionally, the proposed work also aims at improving the reactive power reserve of the fast-acting converters such as the grid side converter and the DSTATCOM so that they could be engaged effectively during the fault condition to reduce the post fault voltage recovery time of the system. In order to achieve the said objectives, a master/slave role is assigned to the different voltage regulating devices such as slow acting on load tap changer transformer (OLTC) and the fast-acting converters such as that of the DCMG converter, Wind Grid Side Converter (WGSC) and the DSTATCOM. A novel time delay concept is introduced before the action of the fast-acting converters such that the slow acting OLTC is given a chance to perform during the normal operating scenario as it is available only during this period. This would indirectly improve the reactive power reserve of the fast-acting converters. With this time delay concept, two CVC schemes are designed, the novel coordinated voltage control scheme (NCVC) and the improvised novel coordinated voltage control scheme (iNCVC). The major difference amongst them is, in the NCVC scheme, the fast-acting converters are allowed to relax during the delay time. Whereas, the iNCVC scheme allows the fast-acting converters to absorb the reactive power mainly during the specific voltage range which is favourable for the OLTC to act reasonably quick. Overall, both the NCVC and iNCVC schemes are found to be effective in improving the overall operating conditions of the grid. Objective 3: PHIL experimentation to evaluate a Cost-Effective Technique for interfacing DFIG Wind System with DC Microgrid Applicable for Various Operating Modes. In this work, the DFIG wind system and the DC microgrid are interfaced at the common AC bus and yet another cost-effective technique is proposed to make the system functional during both the grid connected and isolated working conditions. Unlike the first objective, in this topology the grid side converter is replaced by the diode bridge rectifier. Whereas, the responsibility of regulating the DC link voltage is achieved by the battery present within the DCMG. The DFIG is controlled by the RSC alone where it helps to synchronize the DFIG wind system with the grid initially and further it also facilitates the maximum power extraction from the DFIG wind system. The merit of this work with respect to the literature is that, the isolated topology of the DFIG wind system connected to the DCMG via diode bridge rectifier is extended to operate for the grid connected case. Additionally, this work establishes a hybrid microgrid structure by powering an AC load at the stator terminals. Further, in this work the fault is considered at the DC side and the ultracapacitor situated at the DCMG is utilized to avoid the sudden dip in the DC link voltage thereby improving the FRT of the DFIG wind system. In order to test the effectiveness of the proposed work, the PHIL experimentation is carried out during the grid connected, isolated and fault conditions and it was found that the system is profoundly stable under such disturbances. Objective 4: PHIL Experimentation to Study the Fault Ride Through Behaviour of DFIG Based Wind System in the Presence of Fault Current Limiter The FRT of the DFIG wind system is an age-old topic of discussion where the researchers have extensively used the fault current limiters in order to the FRT improvement of the DFIG wind system due to its added merit of reduction in the overall fault current level. However, the existing FRT solutions of the DFIG wind system have considered fault to occur only after the FCL. However, the literature also recommends the FCL to be placed far away from the bus where the DFIG is connected i.e. prior to the first bus of the distribution system in order to achieve better relay coordination. Under such circumstances there is a possibility of fault to take place prior to the FCL with respect to the DFIG wind system. In order to study the response of the DFIG wind system for the case when the fault takes place before the FCL, the PHIL experimentation is carried out where the 2.2 kW DFIG wind emulator is connected to the IEEE 33 bus distribution system simulated at the RTDS platform. The PHIL experimental results convey an important information that whenever the fault happens before the FCL it worsens the FRT of the DFIG wind system when compared to the case where there is no FCL in the system.