Transformer Simulation Considering Hysteresis And Analysis Of The Incorrect Operation Of Transformer Differential Protection

Transformer is the key instrument for voltage converting, power transmission and distribution in power system. The differential protection for transformer is its main protection, which is responsible for cutting the fault transformer from system as soon as possible. However, due to its intrinsic disadvantage the differential protection for transformer has certain possibilities of misoperation and refusal to operation so that it performs worse than other differential protection such as those for transmission line. To search the current accepted by differential protection under all kinds of power system transients is an important task for improving the level of protection. Basing on a more precise model of transformer the author of this dissertation has done some researches on the machinery of the incorrect operation of differential protection for transformer.The essential problem in the mathematic model of transformer is the description of the hysteresis loops of the ferromagnetic material that constructs the core. We apply the Jiles-Atherton ferromagnetic theory in this case, for the reason that the theory has a good physical interpretation and is validated by the results of experiments. In order to apply the Jiles-Atherton theory to our simulation of power transformer, we make use of the powerful tool S-function in MATLAB/Simulink to write our own model of transformer. Combining this model with other ones in the SimPowerSystems modules library, the research of power transformer in the transient of power system can be carried on.One of the most difficulty in the application of differential protection in transformer is the discrimination of inrush current, internal and external short current. According to the variety of core construction, ferromagnetic material and operating parameters of system, inrush current presents an evident multiformity. Through the simulation of energization of no-load transformer in different case, we have discussed the factors influencing the characteristics of inrush current.Since the differential protection should not operate in any case other than internal fault, the characteristics of the current in some transient condition should be used to block the protection. These include overvoltage, external short circuit and the clear of external short circuit, which are studied in the dissertation.As for the internal fault of transformer, we adopt the method of using a third coil to simulate the short coil. In the analysis of internal fault current, we also discussed the case of slight short circuit before energizing which leads to the operation delay of differential protection.The signals of voltage or current acquired by protection instrument are all transferred through the current or voltage transformer, so there must be an error more or less according to different conditions. The influence of current transformer on secondary current in some cases has been discussed in our work. For differential protection of transformer, a most important cause for the protection misoperation in the case of large current is the disagreement of the characteristics of the current transformers used in the (wo sides of transformer. This situation is also simulated in our work.