Research On Transformer Protection Algorithm And Simulation

As one of the most important equipments in the modern power system, the power transformer is in charge of power networks transmission and distribution. In order to ensure the security of power networks, it is necessary to guarantee the reliability of the power transformer. In addition, the voltage level of transformer is high and the price of the transformer is expensive. An unscheduled repair due to fault can be very expensive and time-consuming. Hence, the operation selectivity, rapidity, sensitivity and reliability of the transformer protection are in highly demand. For the sake of understanding the transformer transient process during the fault and improving the performance of the existing methods, it is imperative to study the transformer simulation and novel algorithms.So far, percentage restraint differential protection is still the main protection scheme for power transformer. However, inrush current due to transformer energization can appear as fault to the relay. So many methods must be proposed to discriminate internal fault from inrush current. This paper analyzed the mathematical model of the transformer in order to take in the difference between internal fault and inrush current. Simulation models are also proposed to view the characteristic of them. Results show that inrush current has lots of second harmonic components and the waveform contains dead-angle. The second harmonic restraint method is the most common one used by engineers. Traditional second harmonic restraint method may face security problems. And the wavelet transform is used to advance the performance of the transformer protection.The embedded Matlab function block is an easy and convenient method to write m-code which can be incorporated into a Simulink model. Based on embedded Matlab function block, this paper establishes a simulation model of a simplified power system with transformer differential protection. The model can simulate transformer inrush current, internal fault and external fault correctly. The results from the simulation show that the circuit breaker can realize the correct operation in the transformer internal fault and achieve the good braking effect in external fault. In addition, the protection model can distinguish the inrush current of transformer and avoid mis-trip of the protection.At last, analysis of the transformer over-current protection backup protection is proposed in this paper. By studying the drawback of the traditional transformer backup protection scheme in a substation, a novel backup protection scheme introduced communication technology is studied. In this paper, the block diagram of the backup protection scheme is also given in detail. The novel scheme can compromise the selectivity and the rapidity of the protection, and identify where faults originate.