A. M. El-ZONKOLY ET AL.35

In [5], authors took advantage of the post-fault voltage

and current samples taken synchronously from both ends

of the line to build a recursive optimization algorithm to

find the distance to fault in a transmission line compen-

sated with a series FACTS device. The proposed algo-

rithm in [5] is independent of the FACTS device model.

However, it aimed only to the location of fault without

trying to find its type.

In this paper, we are interested in two of the most im-

portant FACTS devices; the SSSC and the UPFC. The

SSSCs are FACTS devices for power transmission line

series compensation. It is a power electronic-based volt-

age source converter (VSC) that generates a nearly sinu-

soidal three-phase voltage which is in quadrature with

the line current. The SSSC converter block is connected

in series with the transmission line by series coupling

transformer. The SSSC can provide either capacitive or

inductive series compensation independent of the line

current [16]. The UPFC, which has been recognized as

one of the best featured FACTS devices, is capable of

providing simultaneous active and reactive power flow

control, as well as, voltage magnitude control. The UPFC

is a combination of STATCOM and SSSC which are

connected via a common DC link, to allow bidirectional

flow of real power between series output terminals of

SSSC and the shunt terminals of the STATCOM [2].

These two devices are suggested due to some problems

encountered in case of lines compensated with conven-

tional compensators such as fixed series capacitor or

TCSC. Problems encountered in case of series compen-

sated lines are as follows [12]:

1) The steady state current is increased significantly

with series compensation and it may be greater than the

line-to-ground fault current towards the boundary of the

line.

2) In a typical series compensation arrangement, the

metal oxide varistor (MOV) is used to protect the ca-

pacitor from over-voltages during a fault. However, it

acts non-linearly during faults and increases the com-

plexity of the protection problem.

3) Voltage and current inversions.

4) The voltage and current signals produced on the

transmission line contain different frequency components

such as non fundamental decaying as well as decaying

DC components due to resonance between the system

inductance and series capacitor, odd harmonics due to

MOV conduction during faults, sub-synchronous fre-

quencies having frequency components varying around

half the fundamental frequency value, high frequency

components caused by resonance between line capaci-

tance and line inductance and fundamental components

of the steady state fault current.

The proposed algorithm is more general it uses voltage

and current signals recorded at one end of the line with

no need for synchronization and is independent of modes

of operation of FACTS devices. The proposed algorithm

is simple and applied to both symmetrical and unsym-

metrical faults with no need for pre-trained NN.

For the purpose of fault identification and classifica-

tion, the wavelet entropy theory is applied to produce a

simple and accurate algorithm. Wavelet transform (WT)

has good time-frequency localization ability so it par-

ticularly adapted to analyze the singular signals caused

by fault. Wavelet transform provides theory basis for

fault detection. The most effective method for fault de-

tection is using a universal applicable quantity (UAQ) to

describe the system and detect the fault. Shannon entropy

is such a UAQ, and wavelet entropy (WE) is formed by

combining WT and Shannon entropy together [17]. A

combination of wavelet and entropy, could exploit the

advantages of both methods to describe the characteris-

tics of a signal. This is because wavelet meets the de-

mands of transient signal analysis and entropy is ideal for

the measurement of uncertainty.

In [18], the proposed algorithm was applied to a

non-compensated transmission line. Therefore, current

waveforms only are used. In this paper due to the pres-

ence of FACTS devices the steady-state and transient

components of current and voltage signals are much af-

fected which create problems with fault detection, classi-

fication and phase selection. The faulted phase couldn’t

be determined using current waveforms coefficients only.

For this reason, the three phase voltages waveforms are

also needed to determine the phase included in fault in

case of SLG fault after the compensating device. That is

why the proposed algorithm in this paper, although it is

simple, it is more detailed and complicated than that in-

troduced in [18].

In this paper, a test system is built using SIMULINK.

The resulting data under different fault types and posi-

tion with respect to the compensating device are ana-

lyzed using the modified WE algorithm than that in [18]

to consider the system compensation. The test results

show the effectiveness of the proposed algorithm.

2. Wavelet Transform and Entropy

Calculations

Lots of fault information is included in the transient

components. So it can be used to identify the fault or

abnormity of equipments or power system. It can also be

used to deal with the fault and analyze its reason. This

way the reliability of the power system will be consid-

erably improved.

Transient signals have some characteristics such as

high frequency and instant break. Wavelet transform is

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