Improvements in protection of medium voltage resonant grounded networks with distributed sources
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- Institutt for elkraftteknikk 
The increasing penetration of distributed generation into medium voltage networks creates challenges for traditional protective schemes developed for conventional radial systems with unidirectional power flows. Moreover, the internal protection of embedded generators must guarantee necessary decoupling in case of abnormal events in the network or prevent from unnecessary disconnection. This becomes especially important during earth faults in resonant grounded networks because the reliable identification of damaged feeders or sections is a challenge. Dependability enhancement of feeder and ground protection to overcome adverse effects from distributed generation, and the improvement of generator protection are important tasks in ensuring reliable and secure network operation. The current PhD research work focuses on developing new protective schemes and algorithms for application in distribution networks with embedded dispersed generation in order to resolve these issues. Since the Smart Grid concept is inseparable from the involvement of communication networks in future power system operation and protection, the thesis actively utilises this benefit. The research method includes: electromagnetic transient program simulations of various faulty scenarios in a medium voltage network; processing of acquired data, analysis and development of protective algorithms; laboratory verifications utilising real-time simulations with hardware in the loop (commercially available relays and self-developed prototypes). Firstly, the work employs impedance protection against phase-to-phase faults as an alternative to the standard overcurrent relays and proposes a new communication-assisted method for its dependability improvement as impedance measurements are affected by remote infeed currents from local generations and non-zero fault resistances. The method compensates for both these negative impacts by using multi-point synchrophasor measurements. Impedance measurements with error compensations can be further utilised for fault location. Secondly, the research work develops new algorithms for the identification of the faulty feeder (or section on the feeder) during earth faults in resonant grounded networks. Two methods are proposed: the utilisation of two-end measurements and one-end measurements. The two-end method is universal since it works independently of network configuration and is suitable for any earth fault type. It also provides possibilities for exact faulty point location, suitable for persistent and permanent earth faults. The one-end method is based on system transients and is especially useful for intermittent faults in mixed networks with cable sections. The results of impedance protection laboratory tests demonstrate improved relay dependability with application of the compensation method. Accuracy of fault location estimation based on reactance is also investigated and it is acceptable for fault discrimination. Imperfections in communication channels (jitters and data losses) have negative influence on the method; however, it is still better than relay performance without the compensation. Tests of the two-end method for earth fault location are conducted offline on simulated fault records and show good dependability and security for various fault and network parameters. Laboratory verification of the one-end method is done on a prototype with simulated and real fault records, and shows improved dependability compared to the standard steady-state approach. Both methods are capable of handling adverse effects arising from the large capacitive imbalance of phases, cable penetration, high fault impedances and insufficient natural watt-metric contribution (the parallel resistor can be excluded). Protective schemes for fast anti-islanding protection of embedded generators or prevention of their unintentional decoupling based on these methods are designed with communication links. The application of the developed method allows increased reliability of power supply in future distribution networks with large penetration of renewable sources, and improves their security.