Abstract
A zigzag transformer is a key segment of the electric power system. The optimal design of the zigzag transformer is important for transformer designers to provide a required return path for earth faults and to ensure proper operation of a power system. The two most important parameters of the zigzag transformers are no-load losses and leakage impedance. The accurate calculation of both factors helps to minimize the overall cost of the transformer. Therefore, the prediction of leakage reactance in the zigzag transformer using analytical or numerical methods is an essential part of the early designing stages of the transformer. This paper provides several two- and three-dimensional finite element models. The main purpose of these models is to evaluate the accuracy of the different models for the calculation of the leakage reactance. An analytical formula and a complete procedure for the calculation of the leakage reactance in the zigzag transformer are also provided, which will help the researchers and transformer designer to optimize this type of transformer. The prototype is also manufactured and tested to verify the accuracy of the analytical method and finite element models for the calculation of the leakage reactance. The simulation and experimental results show that the finite element calculation cannot only obtain accurate leakage reactance values (magnetostatic analysis), but also provides better accuracy in the no-load losses (transient analysis).
Highlights
Transient overvoltages, short-circuit withstands capability, basic insulation level, and other important factors of the power system depend on the neutral grounding
Zigzag transformers have a low leakage impedance value, which allows the triple-harmonics of the excitation current and the zero-sequence currents to flow through it because there is no opposition to their flow except the small value of the leakage reactance of the zigzag transformers
Appropriate calculation of the leakage reactance is vital for transformer designers because the short-circuit current of the transformer is mainly dependent on the leakage reactance
Summary
Short-circuit withstands capability, basic insulation level, and other important factors of the power system depend on the neutral grounding. The leakage reactance in the transformer can be calculated with analytical techniques, numerical methods, and experimental tests. In [11]–[18], different analytical methods and finite element models are presented to evaluate leakage reactance in the different types of winding connections. A comparison of the different methods for evaluating leakage reactance applied to zigzag configuration transformers for determining the accuracy of these methods is still missing. This work explains the determination of the leakage reactance in the zigzag transformer in detail, i.e., the calculation of the leakage reactance by analytical methods and the finite element method. Several finite element models are compared for the accurate calculation of the leakage reactance. Different magnetostatic analysis and transient analysis are performed to evaluate the accuracy of the finite element analysis for values of the leakage reactance and no-load losses in the zigzag transformers. The analytical method is derived from the available analytical equations and the results of the analytical method are compared and investigated by finite element simulations and experimental tests
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