ANALYSIS OF UNSYMMETRICAL FAULTS USING BUS IMPEDENCE MATRIX

INTRODUCTION OF UNSYMMETRICAL FAULTS Most Of the faults that occur on power systems are unsymmetrical faults, which

may consist of unsymmetrical short circuits, unsymmetrical faults through impedances, or open conductors.

Unsymmetrical Faults occur as single line--‐to--‐ground faults, line--‐to--‐line faults, or double line--‐to--‐ground faults. The Path of the fault current from line to line or line to ground may or may not contain impedance. One Or two open conductors result in unsymmetrical faults, through either the breaking of one or two conductors or the achon of fuses and other devices that may not open the three phases simultaneously

Connection of sequence network for

LG fault bus

Consider that there are n number of buses in a

System and a LG fault occurs bus of this system The positive sequence network is replaced by its

Thevenin’s equivalent i.e. the prefault voltage.

Of bus I in series with the passive positive sequence

Network. As there is no prefault negative and zero sequence

Voltage ,both are passive network only

for passive positive sequence network

Where

According to sequence network connection , current is injected only at the faulted bus of the positive sequence network. Hence,

Hence the positive sequence voltage at the bus of the passive positive sequence network is

Thus, the passive positive sequence network present an impedance to the positive sequence current

For a negative sequence network.

The negative sequence network is injected with current at the bus only.hence, ∴

Thus negative sequence network offers an impedance to the negative sequence current

For zero sequence network

Thus zero sequence network offers an impedance to the zero sequence current

From sequence network connection, we can write,

Similarly we can compute sequence current for LL & LLG fault. For passive positive sequence network, the voltage developed at bus k due to

injection of current at bus I is ,

Hence the postfault positive sequence voltage at bus K is, ; k=1,2,…n.

Where, =prefault positive sequence voltage at bus K

component of

As the prefault negative sequence bus voltage is zero, the postfault negative sequence bus voltage is,

Where = component of

The postfault zero sequence bus voltage is ;k=1,2,…n.

Where = component of

After computing postfault sequence voltage , the sequence current in the line can be computed as , for line pg. having sequence admittance , and

After computing sequence voltage and current, phase volage and current can be easily computed as ,

As this method requires computation of bus impedance matrices of all the three sequence network, it seems to be more tedious and time consuming . But once the bus impedance matrices have been formed, fault analysis can be easily can be easily carried out for all the buses which is the aim of fault analysis.

Also for any changes in power network bus impedance matrix can be easily modified

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