zer-sequence earth fault currents

7/30/2019 Zer-sequence Earth Fault Currents

1/4

High Order Harmonics In Zero-Sequence Earth Fault Currents Of

Isolated Neutral MV Networks

HANI OBEID

Electrical & Computer DepartmentP.O.Box 950674, Amman 11195

JORDAN

[email protected] www.asu.edu.jo

Abstract: - This paper investigates the effect of high-order harmonics on the value of earth fault current in isolated

neutral MV networks. The relay protection in these networks is intended to give an alarm in case of earth faults.

The selective protection of these networks requires the study of transient character of the current in order to

develop relays that are sensible to small impulses of current during intermittent earth faults.

Key-words:- Earth Fault Current, Zero Sequence, MV Network, Isolated Neutral.

1 IntroductionA large number of public utilities adopted the concept

of isolated networks to maintain availability and

safety. Distribution networks continue to expand,

however and so do zero-sequence fault current [1].

Single phase earth faults are the most frequent and

dangerous faults. The relay protection in these

networks is intended to give an alarm in case of earth

faults. Exception is the case of connecting motors to

these networks, where it is required to trip in case offault currents [2]. If the protection is non-selective the

network collapse is more possible.

There are major contradictions between current

practice of neutral earthing and methods of protection

in these networks [3]. These contradictions are:

a. The requirements of national codes state thatthe capacitive currents should be decreased or

compensated. That leads to difficulties in

construction of efficient and cheap relays to

protect from earth faults.b. Decrease of capacitive currents lead to asituation when most of earth faults have

intermittent character [4]. In that case, the

duration of the fault could be a few

milliseconds (during the period of industrialfrequency), and current in faulty and sound

feeders has transient character. The value of

the fault current depends on the network

configuration, the time of fault occurrence,

and the time of arc extension. Settings of the

relay protection that are based on steady state

condition have not been selected according to

real fault conditions. The same could be

applied to the directional relay, working zone

of which is defined based on current voltage

phasor diagrams in steady state condition;

however, this relay should operate in transient

condition when short term current impulses

occur. Filter of the basic frequency in the

input of some devices does not change the

situation; because there is no way that the

fault could be periodically at each instant.

c. The devices type HSS-3, ZM HSS, which arewidely used in city cable and industrial

networks, react on harmonics of high order,

which contradict the current compensation

principle by coil installed in the neutral. If

earth faults current contains high order

harmonics, capacitive compensation should

not be applied, because compensation is

applied for basic frequency and the harmonics

of high order will lead to opposite.

d. Protection of motors in networks with smallearth fault currents as usual lacks sensibility.

Not only because the earth faults are of small

currents, but also, because protection does not

operate during intermittent faults. Moreover,

protection cant sense the fault if it is being

developed deep down in the winding.
mailto:[email protected]:[email protected]


2/4

It should be taken into consideration that these

problems were discussed by relay protection

developers [5]. Developers also discussed the need for

protection during stable and intermittent faults. Some

of them offered relay protection that reacts on the

magnitude during transients. Others stated that relays

based on harmonics of high order could not be usedduring short term intermittent faults.

However, all devices that are based on transients are

not popular because they are expensive, complicated,

and there are doubts about their reliability.

Many experiments confirm that in networks of large

industrial facilities with dc convertors, selectivity of

protection devices that react on harmonics of high

order is very big. But those experiments do not answer

two important questions, namely: how these devices

(HSS-3, ZM HSS) operate in network without dcconvertors (source of harmonics of high order), and

why these devices are efficient only in networks with

large capacitive currents. This paper tries to answer

those questions.

2 MV network with isolated neutral

Basic analytical results that define currents of

high order harmonics when single phase earth

fault occurs are calculated from network diagram

Fig. 1.

Fig. 1

The source of high order harmonics is transformer

T2 (10/0.4). All other elements of the network:

power transformer T1, current limitation coil P,

neutral coil are taken into account by their line

inductances.

Energy source that works as a system generates

only basic frequency. The no-load current of

transformer T2 could be changed. There is no

point to consider a large number of those

transformers in the network, because the result is

defined by an equivalent source of the high orderharmonics, specter of which is always changed

due to changes in the transformer load, changes in

phases between high order harmonics of the

transformers, and changes of the network

diagram. Only odd harmonics are presented in

magnetizing current of the transformer ( k=5, 7,11) when phase windings are connected in delta

(if there is unsymmetrical magnet system, is, there

could be odd harmonics k=3,9 ). Maximumvalues of harmonics in transformers current are in

over voltage modes when transformer load is

little. If we suppose that there is a symmetry ofphase parameters of the network and if all phase

currents of the network generate high order

harmonics with positive zero sequence,

current will be equal to zero until single phaseearth fault occurs. When fault occurs, currentscould be detected by devices, however, these

currents are only a small part of the currents that

are generated by the source (a few amperes). Thatis why if there is no resonant enhancing of these

currents, sensitivity of the protection will not be

good enough.

The equivalent circuit of network shown in Fig. 1is shown in Fig. 2.

Fig. 2

The faulted cable is simulated by four line

capacitance diagram. The difference between

positive and zero sequence capacitances is

System

Coil

/Y

10/0.4 KV

/Y

110/10 KV

R


3/4

considered by capacitance to neutral thatcould be calculated by the following formula:

The inductive connections between cables areignored. Earth fault current and in faultedcable are defined in branches as shown in Fig. 2.

The same model is applied to sound cables with

capacitances and. Zero sequence current inthe sound feeder is .

If the source of harmonics of the order k is

presented (

and based on

equivalent diagram in Fig.2, the following

equations may be used to determine the value of

earth fault current:

( )

( )

( )

Where: S=1,2

Solving these equations, yields the value of current atplace of short circuit and triple value of zero sequence

in faulted feeder () and in sound feeder ().

Where:

current of k-order harmonic in phase a of

transformer (current source).

- rotational speed of k-oreder harmonic. the sum of phase to earth and phase tophase capacitances.

zero-sequence capacitance of faulted andsound feeders.

Where:

are positive sequence capacitances ofrespective feeders.

inductance of the source network.number of sound feeders.

It is easy to show that the value of current calculated

by the first equation of equations system (1) may be

obtained from the following equivalent circuit (the


4/4

shunt resistance which is shown as dashed line is not

included), which is shown in Fig. 3.

Fig. 3

We can conclude from studying equivalent circuit

of Fig.3, that the increae of k-order harmonic of

the current

in place of fault is possible when

the following condition is fulfilledthe total sum

of admittances of the branch with L inductance of

k-order harmonic is approximately equal the sum

of admittances . If their absolutevalues are equal then the equivalent admittance of

parallel branches to the current source will beequal to zero. That leads to current resonance,

when voltage of capacitance will be increasedto infinity and the harmonic current will beincreased at place of fault.

In real conditions this can not happen as the

elements of the network have active resistance

and admittance, and the active resistance of the

current source is connected in parallel.

3 Conclusions

The value of high-order harmonic of zero-sequence

earth fault current was obtained based on equivalentcircuit diagram shown in Fig. 3.

The currents

in faulted and sound feeders may

be calculated as part of the total current

.

The value of current in faulted feeder

is always

greater than current in sound feeder

by .

Current resonance will take place if

References

[1] A.A.Meer, M.Popov. Directional relay

coordination in ungrounded MV radial distributionnetworks using RTDS.International conference on

power system transients (IPST 2009), Koyto, Japan.

2009.

[2] M.A. Shabad single phase earth fault protection in

6-35 KV networks. , 2007. (Russian

Language).

[3] G.A.Evdokunen, S.S. Titenkof. The systems of

neutral earthing in MV networks (6-35KV).

Overvoltage and Reliability of Equipment. Collective

works: , 2006.

[4] Jozef Lorenc, Kazimierz Musierowicz, and

Andrzej Kwapisz. Detection of the intermittent earth

fault in compensated MV networks.IEEE Power Tech

Conference, 2003, Bolonga-Italy.

[5] L.E. Dudarof, B.B. Zubkov. Problems of

protection against earth faults in 6-35 KV networks.

Electrechestvo, No.2, 1999. (Russian Language).

zer-sequence earth fault currents

Documents