31-05-2012, 03:52 PM
input surge current
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INTRODUCTION
Inrush current or input surge current refers to the maximum, instantaneous input current drawn by an electrical device when first turned on. When a transformer is first energized a transient current, much larger than the rated transformer current can flow for several cycles. This is caused due to the residual magnetism of the transformer and when the transformer is reenergized the incoming flux will add to the already existing flux which will cause the transformer to move into saturation and produce transient current. This transient current is termed as inrush current. Transformer differential relay is based on the comparison of the balance of the currents flowing into an energizing winding with those flowing out to through-fed loads. The transformer differential relay could mistaken the inrush current as a fault current, which causes the transformer to be wrongly taken out of service by the relay. Therefore, some techniques are needed to detect transformer inrush and block differential relays.
DETECTION METHODES
There are several inrush detection techniques. The important transformer inrush detection techniques are short window filter algorithm, neuro-fuzzy method, and wavelet transform method. These methods accurately detect the inrush current in a transformer. Short window filter algorithm offers better performance in capturing the characteristics of inrush current in a transformer. In this algorithm a data window of short width is used for the detection purpose. The simulation results of short window filter algorithm give the accurate detection of inrush current. The neuro fuzzy algorithm is based on the different behaviors of second harmonic component of inrush and fault currents. In most of fault conditions, phase angle of proportion of second harmonic to the fundamental component of differential currents is near zero, 180 or 360 degree and in various switching or inrush conditions these values are near to 90 or 270. In this method, magnitude and difference of phase angle of proportion of second harmonic to the fundamental component of differential current is calculated simultaneously. Wavelet transform and correlation factor method is another important method of inrush current detection. A discreet wavelet
transform has been used to identify the inrush currents of power transformers from the fault current.
PREVENTION TECHNIQUES
There are several inrush current prevention techniques. The current limiting devices like series resistor, triac and inrush current limiters are used for the prevention of inrush current. The inrush current limiter is the most commonly used inrush current prevention techniques. An inrush current limiter is a temperature dependent resistor with a negative temperature coefficient (NTC), which means that the electrical resistance decreases with increasing temperature. Whereas NTCs are frequently used as a temperature sensor measuring temperature applied from external environments an ICL uses the self-heating effect due to electrical current flow through the component. When the device is turned on, the inrush current is limited by the high resistance of the ICL in the cold state. During the initial transient sequence of switching on capacitive or inductive loads which typically take a few milliseconds depends on the ICL will heat up in the range of approximately 10 K to 30 K. Additional further warming may come from steady state current during normal operation of the device. The steepness of the R/T characteristic of ICLs results in low residual resistance during operation in this mode meaning that the component has practically no effect on the application. However, industrial applications may utilize a relay, which short-circuits the ICL path after the inrush sequence.
ORGANIZATION OF THE REPORT
The seminar topic is divided into five chapters. In the first chapter the necessity of the inrush current detection and prevention is described. Second chapter describes the effects and classification of inrush current. In the third chapter the important detection methods; short window filter algorithm, neuro fuzzy method and wavelet transform method are described. In the fourth chapter the prevention techniques are described. The report is summarized in chapter five.
INRUSH CURRENT
INTRODUCTION
The maximum, instantaneous input current drawn by an electrical device when
first turned on is termed as inrush current. Inrush current has a value which is 50 to 100
times greater than the rated current of transformer, which is occurred only for few cycles.
When a transformer is first energized a transient current, much larger than the rated
transformer current can flow for several cycles. This is caused due to the residual
magnetism of the transformer and when the transformer is reenergized the incoming flux
will add to the already existing flux which will cause the transformer to move into
saturation and produce transient current.