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The rapid emergence of distributed fuel cell powered generations (DFPGs) operating in parallel with utility has brought a number of technical concerns as more DFPGs are connected to utility grid. One of the most challenging problems is known as islanding phenomenon. This situation occurs when a network is disconnected from utility grid and is energized by local DFPGs. It can possibly result in injury to utility personnel arriving to service isolated feeders, equipment damage, and system malfunction. In response to the concern, this dissertation aims to develop a robust anti-islanding algorithm for utility interconnection of DFPGs. In the first part, digital signal processor (DSP) controlled power electronic converters for utility interconnection of DFPGs are developed. Current control in a direct-quadrature (dq) synchronous frame is proposed. The real and reactive power is controlled by regulating inverter currents. The proposed digital current control in a synchronous frame significantly enhances the performance of DFPGs. In the second part, the robust anti-islanding algorithm for utility interconnection of a DFPG is developed. The power control algorithm is proposed based on analysis of a real and reactive power mismatch. It continuously perturbs (±5%) the reactive power supplied by the DFPG while monitoring the voltage and frequency. If islanding were to occur, a measurable frequency deviation would take place, upon which the real power of the DFPG is further reduced to 80%; a drop in voltage positively confirms islanding. This method is shown to be robust and reliable. In the third part, an improved anti-islanding algorithm for utility interconnection of multiple DFPGs is presented. The cross correlation method is proposed and implemented in conjunction with the power control algorithm. It calculates the cross correlation index of a rate of change of the frequency deviation and (±5%) the reactive power. If this index increases above 50%, the chance of islanding is high. The algorithm initiates (±10%) the reactive power and continues to calculate the correlation index. If the index exceeds 80%, islanding is now confirmed. The proposed method is robust and capable of detecting islanding in the presence of several DFPGs independently operating. Analysis, simulation and experimental results are presented and discussed.

This thesis presents a method of detecting electrical islands in low voltage distributed generator networks by measuring negative sequence impedance differences between islanded and utility connections. Extensive testing was conducted on a commercial building and 25 kV distributed generator fed network by measuring naturally occurring and artificially injected negative sequence components. Similarly, this technique was tested using the IEEE 399-1990 bus test case using the EMTP software. The practical measurements have been matched to simulations where further system performance characteristics of detecting power system islands has been successfully demonstrated. Measured results indicate that unbalanced load conditions are naturally occurring and readily measurable while deliberately unbalanced loads can increase the accuracy of negative sequence impedance islanding detection. The typically low negative sequence impedance of induction motors was found to have only a small effect in low voltage busses, though large machines can effect the threshold settings. Careful placement of the island detector is required in these situations. The negative sequence impedance measurement method is an improvement on previous impedance measurement techniques for islanding detection due to its accuracy, and distinctly large threshold window which have challenged previous impedance based islanding detection techniques.

The rapid emergence of distributed fuel cell powered generations (DFPGs) operating in parallel with utility has brought a number of technical concerns as more DFPGs are connected to utility grid. One of the most challenging problems is known as islanding phenomenon. This situation occurs when a network is disconnected from utility grid and is energized by local DFPGs. It can possibly result in injury to utility personnel arriving to service isolated feeders, equipment damage, and system malfunction. In response to the concern, this dissertation aims to develop a robust anti-islanding algorithm for utility interconnection of DFPGs. In the first part, digital signal processor (DSP) controlled power electronic converters for utility interconnection of DFPGs are developed. Current control in a direct-quadrature (dq) synchronous frame is proposed. The real and reactive power is controlled by regulating inverter currents. The proposed digital current control in a synchronous frame significantly enhances the performance of DFPGs. In the second part, the robust anti-islanding algorithm for utility interconnection of a DFPG is developed. The power control algorithm is proposed based on analysis of a real and reactive power mismatch. It continuously perturbs (±5%) the reactive power supplied by the DFPG while monitoring the voltage and frequency. If islanding were to occur, a measurable frequency deviation would take place, upon which the real power of the DFPG is further reduced to 80%; a drop in voltage positively confirms islanding. This method is shown to be robust and reliable. In the third part, an improved anti-islanding algorithm for utility interconnection of multiple DFPGs is presented. The cross correlation method is proposed and implemented in conjunction with the power control algorithm. It calculates the cross correlation index of a rate of change of the frequency deviation and (±5%) the reactive power. If this index increases above 50%, the chance of islanding is high. The algorithm initiates (±10%) the reactive power and continues to calculate the correlation index. If the index exceeds 80%, islanding is now confirmed. The proposed method is robust and capable of detecting islanding in the presence of several DFPGs independently operating. Analysis, simulation and experimental results are presented and discussed.

This thesis presents a method of detecting electrical islands in low voltage distributed generator networks by measuring negative sequence impedance differences between islanded and utility connections. Extensive testing was conducted on a commercial building and 25 kV distributed generator fed network by measuring naturally occurring and artificially injected negative sequence components. Similarly, this technique was tested using the IEEE 399-1990 bus test case using the EMTP software. The practical measurements have been matched to simulations where further system performance characteristics of detecting power system islands has been successfully demonstrated. Measured results indicate that unbalanced load conditions are naturally occurring and readily measurable while deliberately unbalanced loads can increase the accuracy of negative sequence impedance islanding detection. The typically low negative sequence impedance of induction motors was found to have only a small effect in low voltage busses, though large machines can effect the threshold settings. Careful placement of the island detector is required in these situations. The negative sequence impedance measurement method is an improvement on previous impedance measurement techniques for islanding detection due to its accuracy, and distinctly large threshold window which have challenged previous impedance based islanding detection techniques.

This thesis presents a method of detecting electrical islands in low voltage distributed generator networks by measuring negative sequence impedance differences between islanded and utility connections. Extensive testing was conducted on a commercial building and 25 kV distributed generator fed network by measuring naturally occurring and artificially injected negative sequence components. Similarly, this technique was tested using the IEEE 399-1990 bus test case using the EMTP software. The practical measurements have been matched to simulations where further system performance characteristics of detecting power system islands has been successfully demonstrated. Measured results indicate that unbalanced load conditions are naturally occurring and readily measurable while deliberately unbalanced loads can increase the accuracy of negative sequence impedance islanding detection. The typically low negative sequence impedance of induction motors was found to have only a small effect in low voltage busses, though large machines can effect the threshold settings. Careful placement of the island detector is required in these situations. The negative sequence impedance measurement method is an improvement on previous impedance measurement techniques for islanding detection due to its accuracy, and distinctly large threshold window which have challenged previous impedance based islanding detection techniques. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

As distributed generation penetrates the electric power grid at higher power levels, grid interface issues with distributed generation must be addressed. The current power system consists of central power generators, while the future power system will include many more distributed resources. The centralized power generation system is controlled by utility operators, but many distributed resources will not be controlled by utility operators. Distributed generation must use smart control techniques for high reliability and ideal grid interface. This thesis discusses the grid interface issue of anti-islanding. An electric island occurs when a circuit breaker in the electric power system trips. The distributed resource should disconnect from the electric grid for safety reasons. This thesis will give an overview of the possible methods. Each method will be analyzed using the ability to detect under the non-detection zone and the economic feasibility of the method. This thesis proposes two addition cases for analysis that exist in the electric power system: the effect of multiple methods in parallel in the non-detection zone and the possibility of a false trip caused by a load step. Multiple methods in parallel are possible because the islanding detection method is patentable, so each grid interface inverter company is likely to implement a different islanding detection method. The load step represents a load change when a load is switched on. / Master of Science

Electrical reliability assurance is a very important aspect of electrical power systems; significant consideration should be given to reliability at both the planning and operation stage of power systems. A decrease in reliability levels can lead to enormous economic losses, especially for certain industrial facilities, and utilities could be penalized for violation of the mandatory reliability standards. Besides the traditional methods for electrical reliability enhancement, it is highly recommended to consider the adoption of innovative technologies, such as the integration of Distributed Generation (DG) units into the electrical network, especially those which are based on renewable energy source (wind and photovoltaic). Distributed Generation technologies can be beneficial to the electrical distribution system performance. However, these pose certain technical challenges to the reliable operation of the system. In this work, we also focus on the micro-grid operation security during islanding mode of operation in the presence of DG units. In this thesis, the unique aspects of reliability evaluation for an electrical distribution system has been performed using system-independent analytical expressions, considering probabilistic load and DG unit modeling, under different scenarios including dispatchable and renewable DG units with reasonable penetration levels. Further, a modified adequacy formulation has been adopted during the islanding mode of operation in order to consider micro-grid load correlation and an additional load curtailment level introduced in this work. The extra curtailment is needed to ensure adequate technical constraints and allow successful micro-grid operation, when the dispatchable DG units rating in a micro-grid is less than a defined percentage of the micro-grid peak load at time of islanding. Afterwards, during islanding, a second load curtailment level is adopted as needed to ensure service continuity under different operational conditions. A distribution test system is considered, and accordingly reliability indices are evaluated for both the worst case load scenario (islanding occurs at peak load), and for a realistic case (islanding might occur at any load level). Further, Expected Energy Not Served is evaluated. In conclusion, the impacts of DG units and islanded operation of micro-grids have been analyzed for the enhancement of the overall reliability of the distribution system and the successful islanding mode of operational conditions.

This thesis experimentally validates the performance of an active islanding detection method under various scenarios. It is also analytically shown that the islanding detection method has a non-detection zone (NDZ), and a method to eliminate the NDZ is proposed. Moreover, the performance of an autonomous mode controller for islanded DG units is experimentally evaluated. Based on a robustness analysis, it is shown that the controller, which is basically designed for the nominal plant, can maintain the stability of the system despite of significant load uncertainties. The feasibility of the islanding detection method for islanding detection in two-DG systems is also experimentally investigated. Moreover, a control strategy for autonomous operation of two-DG systems is proposed, and its performance is experimentally evaluated. Then, adopting the islanding detection method and the proposed control strategy, the viability of smooth transitions from grid-connected modes to autonomous (islanded) modes in two-parallel DG systems is experimentally validated.

This thesis experimentally validates the performance of an active islanding detection method under various scenarios. It is also analytically shown that the islanding detection method has a non-detection zone (NDZ), and a method to eliminate the NDZ is proposed. Moreover, the performance of an autonomous mode controller for islanded DG units is experimentally evaluated. Based on a robustness analysis, it is shown that the controller, which is basically designed for the nominal plant, can maintain the stability of the system despite of significant load uncertainties. The feasibility of the islanding detection method for islanding detection in two-DG systems is also experimentally investigated. Moreover, a control strategy for autonomous operation of two-DG systems is proposed, and its performance is experimentally evaluated. Then, adopting the islanding detection method and the proposed control strategy, the viability of smooth transitions from grid-connected modes to autonomous (islanded) modes in two-parallel DG systems is experimentally validated.