2012 Paris Session
B5 - 105
http : //www.cigre.org
Zero Sequence Compensation Factor Effect on Distance Protection Ground
Reach (Practical study)
Mohamed El-Hadidy(*)
Hayat Farouk
Bahaa Soudy
Egyptian Electricity Transmission Company (EETC)
Egypt
Abstract:
Zero sequence compensation factor
is used for the correction of the distance protection reach in
the case of ground faults.
is essential for the accurate determination of the ground fault location
by distance protection on a transmission line. This is much more critical in the case of underground
cables rather than the case of overhead lines because of small lengths of cables besides their low
should be set to reflect the actual grounding conditions of the protected
impedance per unit length.
cable or the distance protection will suffer from over or under reach [1]. A practical study based on
actual measurements of
for different 52 cables within the Egyptian electricity network shows that
distance protection ground reach is sensitive for the value of
.
The soil nature and the grounding conditions normally affect the value of
well as cables. However, this effect for the underground cables is greater.
of the over head lines as
The paper shows the results of measuring
and its effect on the distance relay reach. It also shows
that the setting of the factor
is a characterizing parameter for each cable feeder even if the feeders
are in the same route. Normally an average value of
is taken for the three phases as all of the
distance protection relays, up till now, deal only with a single valued
factor for the three phases.
(*)
e-mail: dr_alhadidy@hotmail.com
1
1. Introduction
Distance protection, in Egypt, is used on a large scale for the protection of the 66 kV underground
parallel cable feeders. It was noticed that the mal-operation of the distance protection is due to the
over- reach phenomenon in the relay measurement.
Analyzing the reasons behind the mal-operation, it was noticed that the impedance seen by the maloperated relay took place within the first zone for faults existed within its second zone reach. The
mal-operations occur in the cases of single line to ground fault. Studying the fault sheets, it was found
that in most cases, for parallel cable feeders, the mal-operated relay is that of the healthy feeder
protecting the sound cable (phase) parallel to the faulty one. The mal-operated relay sees the value of
impedance to fault less than that seen by the relay responsible for clearing the fault on the faulty cable
phase.
The effect of the
factor on the performance of the distance relay is studied and a relation between
and the error in impedance seen by the relay is deduced. The measured
values are applied to
the deuced formula and the expected error in the impedance value seen by the relay is figured out.
2. Problem Formulation
For the years 2008- 2010, analysis of the incidents of the 66 kV cables tripping in the part of network
under study has pointed out an average percentage equal to about 20% of mal-operation of distance
relays on cables due to unknown reasons. One of the reasons which are discussed was the correctness
of the set values of the zero sequence compensation factors
[2].
To figure out the reasons behind the mal-functioning of the distance relays, a large campaign for
revision of relay setting calculation was performed. This campaign included revision of cable data
sheets, setting calculations and relay testing results. Revision and testing results have shown
everything was right. All settings were made according to the cable parameters which are reviewed
many times besides the good results of testing these relays; another search was needed to find the
reasons of distance relays mal-functioning.
It was noticed that most of the understudy cases of mal-operations took place in the cases of single
line to ground fault. Then, the study was directed to the effect of the zero sequence compensation
factors
on the distance relay reach. It was expected that this factor has a value which may be
different from the real one, this of course affects the actual reach of the distance relays [1]. At the
same time this factor is depending on the conditions of the cable sheath grounding [3]. The findings of
the measuring campaign are discussed below (sec. 5).
3. Zero sequence compensation factor [3]:
Consider a single line to ground fault on phase a as shown in Fig. 1.
Phase a
Phase b
Phase c
Earth
Fig. (1). Single line to ground on phase a
2
In Fig.1, the voltage of the faulty phase at the relay location is:
…….. (1)
Where,
Faulty Phase Voltage
, ,
= Positive, Negative, Zero sequence Impedances of the faulty phase
= Positive, Negative, Zero sequence Currents of the faulty phase
= Positive, Negative, Zero sequence Voltages of the faulty phase
= Faulty Phase Current
Considering:
for overhead lines and cables, we get:
……… (2)
so,
………. (3)
Substituting (3) in (2) gives:
………. (4)
Considering Single Cable single end feed (neglecting effect of load current), then,
:
Substituting,
, then:
……………… (5)
……………… (6)
As
, Therefore
……………. (7)
Calculation of the zero sequence impedance depends on the dimensions of the conductor and also the
grounding of the cable or the overhead line.
The calculation of zero sequence or positive sequence impedances contains many approximations
which lead to the inaccuracy of the results which causes the inaccuracy of the value of the resulting
compensation factor.
There are some other forms of Zero sequence compensation factor. Each relay developer use different
form of
. The main forms are:
1- The most famous form:
2-
The form
= Form of reactive zero sequence compensation factor
Xo = Zero sequence reactance of the conductor
X1 = Positive sequence reactance of the conductor
3- The form
= Form of resistive zero sequence compensation factor
Ro = Zero sequence resistance of the conductor
R1 = Positive sequence resistance of the conductor
4- Ratio form:
, the simple form of the ratio between impedances Zo and Z1.
3
4. Effect of zero sequence compensation factor on the relay reach
To study the effect of the zero sequence compensation factor on the reach of the distance relay in case
of single line to ground fault according to the following simplified equation is used [4]:
Where:
Is the actual (impedance) seen by the distance protection relay.
Is the measured value of impedance by the distance without compensation (uncompensated).
Is the zero sequence compensation factor.
Therefore,
= The real (measured) zero sequence compensation factor
=The false zero sequence compensation factor
= Difference in zero sequence compensation factor
=The actual relay reach according to
=The actual relay reach according to the
=Relay impedance difference
Error in zero sequence compensation factor =
Error in relay impedance =
(1+
)
So
Let:
= A = Constant , and
= B = Constant
The relation between the error in impedance seen by the relay and the value of
following:
is as
It is clear that the zero sequence compensation factor
is used through the above equation to
modify the value of the measured impedance
to the correct value considering the grounding
conditions . At the first glance it is clear that as the factor
increases than the correct value the
impedance seen by the relay
decreases and so the relay Over-reaches! and vice versa.
Table (1) shows the relation between the error in impedance and error in the zero sequence
compensation factor at certain compensation factor
= 0.8
Ko
Table (1) The relation between the error in impedance and
0.1
0.4
0.7
1.0
1.3
1.6
1.9
0.338
0.222
0.055 -0.111 -0.277 -0.444 -0.61
at
2.1
-0.721
2.4
-0.888
2.8
-1.11
4
0.6
0.4
ƐZr
0.2
0
ko'=0.6
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
-0.2
1
1.1 1.2 1.3 1.4 1.5 1.6
Ko'=0.8
Ko'=1.0
-0.4
-0.6
-0.8
Ko
Fig. (2). Error in Relay Impedance against (Ko) Factor
From Fig. (2) it is clear that error in impedance is to cause under reach in the first part of the line i.e.
positive error as false
is less than actual one
and then after that the error is negative i.e. the
relay will over reach Fig. (2) could be used to give the value of the error in
at any incident without
measuring Ko only if the error in the measured impedance and the set value of
are known.
5. Actual measurements analysis:
The total number of 66 kV cables that have been measured to get their actual positive and zero
sequence parameters and so their actual zero sequence compensation factor
are 52 cables. Table
(2) shows the distribution of zero sequence compensation factor measured values for the 52 cables of
the network under study. The analysis of results shown in the Table (2) indicates the following:
51.98 % of the total number of the measured zero sequence compensation factor in ranged between
0.5 and 0.8. So, the majority of the zero sequence compensation factor components values are ranged
in that margin:
= 0.5 - 0.8.See Fig. (3).
Table (2) Distribution Measured values Zero sequence compensation factor
0-0.3
Value of
0.3-0.5
19.2
13.46
0.5-0.8
51.98
0.8-1.0
3.84
1.0 -1.5
11.53
1.5-2.8
---------
60.0%
Cable %
50.0%
40.0%
30.0%
20.0%
10.0%
0.0%
0.3-0.0
0.5-0.3
0.8-0.5
1.0-0.8
Ko Value
Fig. 3. Distribution of
1.5-1.0
2.8-1.5
Value for all cables
5
Table (3) shows the results of measuring the zero sequence compensation factors against the aging
factor. Analyzing the results of the zero sequence compensation factor total and components values
according to the aging factor it is noticed that:
For the aging from 5-10 years: 28.5%of the total number of the total zero sequence compensation factor
is ranged between 0.5 and 0.8. The above values could be taken as reference guide values as they are
the significant percentages of the measured values in this aging range. See Fig. (4).
For the aging from 10-15 years: 60%of the total number of the total zero sequence compensation factor
is ranged between 0.5 and 0.8. See Fig. (5).
For the aging of 15 years Plus: 47.5%of the total number of the total zero sequence compensation
factor
is ranged between 0.5 and 0.8. See Fig. (6).
The above results shows that the aging factor affects the distribution of the total zero sequence
compensation factor and its resistive and reactive components. Table (3) shows the measured values
of the zero sequence compensation factors.
Table (3) Measured values of Zero sequence compensation factor against aging
Ko Value
Cables % (5-10 Years)
Cables% (10-15 Years)
Cables% (5-10 Years)
0.0-0.3
28.5
--------17.5
0.3-0.5
28.5
40
10
0.5-0.8
28.5
60
50
0.8-1.0
--------------5
1.0-1.5
14.25
---------17.5
1.5-2.8
-------------------------
Cable %
30.0%
20.0%
10.0%
0.0%
0.3-0.0
0.5-0.3
Fig.(4) Distribution of
0.8-0.5
1.0-0.8
Ko Value
1.5-1.0
2.8-1.5
Value Aging from 5-10 years
70.0%
60.0%
Cable %
50.0%
40.0%
30.0%
20.0%
10.0%
0.0%
0.3-0
0.5-0.3
Fig.(5) Distribution of
0.8-0.5
1.0-0.8
Ko Value
1.5-1.0
2.8-1.5
Value Aging from 10-15 years
6
60.0%
Cable %
50.0%
40.0%
30.0%
20.0%
10.0%
0.0%
0.3-0.0
0.5-0.3
0.8-0.5
1.0-0.8
Ko Value
Fig (6) Distribution of
Factors affecting the value of
1.5-1.0
2.8-1.5
Value Aging 15 years plus
:
The factors that are expected to affect the value of
can be summarized in the following:
1-Agging of the cable
2- The number of heavy short circuits that the cable is exposed to.
3-Grounding conditions of the substations and the sheath of the cable
4-The type of the soil that the cable is lied in
Table (4) and figure (7) shows the distribution of the zero sequence factor in case of the clay soil.
The analysis of results shown in the Table (4) indicates following: 53% of the total number of the total
zero sequence compensation factor
is ranged between 0.5 and 0.8
Table (4) Measured values of zero sequence compensation factor in Clay Soil
Ko Value
0-0.3
0.3-0.5
0.5-0.8
0.8-1.0
1.0 -1.5
1.5-2.8
Cable %
11
11
53
7.4
7.4
11
60.0%
Cable %
50.0%
40.0%
30.0%
20.0%
10.0%
0.0%
0.3-0.0
0.5-0.3
0.8-0.5
1.0-0.8
Ko Value
Fig.(7) Distribution of
1.5-1.0
2.8-1.5
for Clay Soil
7
Conclusions
1- The Measured (not the calculated) values of zero sequence compensation factor
for setting the distance protection.
should be used
2- The measured value of
directly after cable installation is considered as a reference value and
then a Periodical measuring of the zero sequence compensation factor is done at the following
situations:
- Periodicaly, e.g. every 5 Years.
- After each case of maintenance of the cable (Making a new joint for example).
- Changing the root of the cable.
3- Establishing a dedicated database for the measured values of the zero sequence compensation
factor (associated with the reasons behind the new measurements). Use this data base for studying
changes in its value and the affecting factors causing these changes.
4- The suitable protection for cables is the differential protection. If it is not available to use the
differential protection so the distance protection is used for protecting cables should be
associated with using directional earth fault with communication.
5- Using Compensation factor for each phase and each stage in the distance protection.
6- The effect of the load current in the calculation of the zero sequence compensation factor especially
in cases of low fault current should be considered.
References
[1]- K-Factor & Mutual Coupling Correction On Asymmetrical Overhead Lines For Optimum
Reliability Of Distance Protection, U. Klapper, A. Apostolov ,D. Tholomier Cigre2008 –B5,
Paper No.212.
[2]- Cairo Zone Fault Reports Years 2008-2010, EETC.
[3]- "Protection of High-Voltage AC Cables", Demetrios A. Tziouvaras, Power Systems Conference:
Advanced Metering, Protection, Control, Communication, and Distributed Resources, Madren
Center, Clemson University,Clemson, SC, USA 14-17 March 2006, pp 316-328.
[4]- Applied Protective Relaying, GEC,1997.
[5]- Siemens AG 2004 7SA6 Manual C53000-G1176-C156-4.
8