Fault Analysis Method Considering Dual-Sequence Current Control of VSCs under Unbalanced Faults

Voltage source converters (VSCs) are able to provide both positive- and negative-sequence short circuit currents under unbalanced faults. Their short circuit responses can be significantly different from those of conventional synchronous generators. This paper developed a static fault analysis method by considering dual-sequence current control of VSCs under unbalanced faults where VSCs are treated as voltage-dependent current sources in both positive- and negative-sequence networks. Since the control strategy of VSCs varies, flexible parameters are included in the model to reflect their diverse short circuit behaviours. The proposed method is verified through a modified IEEE 9-bus system and a simplified western Danish power system with real time simulations. This analytical method can be used to help understand and evaluate the impact of dual-sequence current control of VSCs on future converter-dominated power systems

Edge-of-grid voltage control in distribution networks

As the electric power supply systems are undergoing major changes with the integration of renewables, the issues related to voltage regulation and system protection are arising. In this scenario, advanced voltage regulation technologies that provide voltage control at the grid-edge, that is at the low-voltage secondary side of the distribution circuit, have emerged as a potential solution to address the shortcomings of traditional voltage control practices in distribution systems. In this work, these technologies are modeled and algorithms are developed to strategically deploy them, tune their control parameters, and evaluate their voltage regulation performance. A two-stage optimization framework is proposed for optimal placement and real-time control of the low-voltage static var compensators to minimize the energy losses while maintaining the voltage regulation. Integration of high levels of distributed generation such as photovoltaic (PV) systems impacts the voltage regulation by causing steady-state voltage variations and transient voltage fluctuations. This work further develops a procedure to tune the control parameters of PV smart inverters to mitigate these voltage issues. Furthermore, the PV penetration levels in a distribution network can be increased without creating voltage problems by dynamic controlled reactive power absorption at several strategic buses. This concept is modeled and demonstrated in this work. Furthermore, the high levels of PV generation can interfere with the overcurrent protection schemes prevalent in distribution networks. An analytical approach is proposed in this work to estimate the distribution feeder PV accommodation limits with respect to overcurrent protection issues as the impact criteria, without needing to simulate numerous PV screening scenarios to assess the impactElectrical and Computer Engineerin

Short-circuit calculation with considered production and consumption uncertainties

U disertaciji je predložen algoritam za proračun kratkih spojeva zasnovan na korelisanim intervalima. U savremenim distributivnim mrežama postoje različiti tipovi generatora koji proizvode električnu energiju iz energije obnovljivih izvora. Za takve generatore, kao i za potrošače, karakteristično je to što je njihova proizvodnja i potrošnja neizvesna. Predloženi algoritam u disertaciji uvažava te neizvesnosti, kao i korelacije između navedenih elemenata. Neizvesnosti su modelovane intervalima i direktno su uvažene u predloženom algoritmu za proračun kratkih spojeva. Algoritam je prvenstveno razvijen za proračun kratkih spojeva savremenih distributivnih mreža sa velikim brojem distribuiranih generatora i potrošača. NJime je moguće proračunavati režime sa kratkim spojevima distributivnih mreža velikih dimenzija, što je numerički verifikovano u disertaciji. Predloženim algoritmom se dobija režim distributivne mreže sa kratkim spojem koji je realističniji od režima dobijenih algoritmima sa determinističkim pristupom. Proračuni kao što su: koordinacija, podešenje i provera osetljivosti relejne zaštite, provera kapaciteta prekidača i osigurača, lokacija kvara itd. mogu na osnovu realističnijeg režima, dobijenog predloženim algoritmom, da daju kvalitetnije rezultate, što je numerički potvrđeno na primeru koordinacije prekostrujne zaštiteIn this dissertation an algorithm for correlated intervals-based short-circuit calculation is proposed. In modern distribution networks there are various types of generators that produce electric energy from renewable energy resources. For these generators, as well as loads, uncertain production and consumption is characteristic. The proposed algorithm in the dissertation deals with above-mentioned uncertainties, as well as correlations among them. The uncertainties are modeled with intervals and directly taken into account in the proposed algorithm for short-circuit calculation. The algorithm is primarily developed for short-circuit calculation in modern distribution networks with a great number of distributed generators and consumers. The proposed algorithm enables calculation of short circuits states of large-scale distribution networks, which is numerically verified in the dissertation. The proposed algorithm provides short circuit state of distribution network which is more realistic than the one obtained with algorithms with deterministic approach. Calculations such as: coordination, settings and sensitivity check of relay protection, breaker and fuse capacity check, fault location, etc. can give better results, on the basis of the more realistic state obtained by the proposed algorithm for short circuit calculation, which is numerically confirmed by the example of coordination of overcurrent protection