Download If a voltage loaded at auxiliary relay for the operation of trip circuit

Ground Potential Rise Analysis
between Generating Station and Remote
Substation
Yoo Jae-seok, Jung In-choul, and Park Hyun-sung

Abstract— When making a lot of interconnections between
the generating station and substation because of the realistic
problem such as too long distance, the remarkable potential
differences may occur between them. In that case, the significant
potential differences occur between them under fault condition.
This paper reviews the differences of potential when occurring
fault for the safety of any interconnected cables such as control,
communication cables, etc..
Keywords—CDEGS, generating station, grounding system,
potential difference, substation
I.
INTRODUCTION
T
HE grounding system is very important for the generating
station and substation. It is intended to protect personnel
in the generating station against dangerous touch and step
voltages and also to protect property from damage. For
personnel this protection is ensured, in the case of a fault, by
either rapid automatic disconnection of the power supply or
by limitation of the resulting touch and step voltages to
acceptable levels.
Ideally, the generating station and substation should be in
close proximity to each other. And to achieve an efficiency
and equipotential between the two grounding systems, many
ground conductors are connected in parallel between them.
However in reality, it is sometimes impractical to make
interconnections due to long distance, which may give rise to
significant voltage differences between the two grounding
systems. In that case, any interconnected cable such as control
cables, communication cables, etc. is easily influenced under
fault conditions.
This paper analyzes the grounding potential difference
between the generating station and substation located
separately under fault conditions.
This part should contain the authors’ current affiliations, including current
address and e-mail.
Yoo Jae-seok is with KEPCO E&C, M-tower, 8 Gumiro, Bundang-gu,
Sungnam-si, Gyeonggi-do, Korea 463-870 ([email protected])
Jung In-choul is with KEPCO E&C, M-tower, 8 Gumiro, Bundang-gu,
Sungnam-si, Gyeonggi-do, Korea 463-870 ([email protected])
Park Hyun-sung is with KEPCO E&C, M-tower, 8 Gumiro, Bundang-gu,
Sungnam-si, Gyeonggi-do, Korea 463-870 ([email protected])
II. DESCRIPTION ON SITE CONDITIONS
A. General Description
In the Republic of Korea, Shin-Kori Nuclear Power Plant
Unit 1 and 2 hereafter PP are apart from 765kV switchyard
hereafter SWYD about 2.6 km.
Connecting between PP and SWYD grounding system as a
grounding conductor, there are two overhead ground wire
hereafter
OHGW
consisting
of
120mm2
aluminium-conductor steel-reinforced and one 100 mm2 bare
copper conductor embedded in the duct bank.
B. Grounding System of PP
The area of PP is 515 m  466 m approximately and its
grounding system is consistent of 150 mm2 bare copper
conductors in 20 m interval below 0.75 m from the ground
level.
C. Grounding System of SWYD
The area of SWYD is 180 m  160 m approximately and its
grounding system is consistent of 200 mm2 bare copper
conductors in 8 m interval below 0.75 m from the ground
level.
III. SIMULATION
The software CDEGS 1 is used for simulation. For easy
modeling, the locations of PP and SWYD are assumed to lie
in a straight line. Also, OHGW is modeling to the insulated
cables with same size due to reducing the computation time
by using MALZ2 module in CDEGS.
The basic input data for Case I and Case II is shown in
Table I
1
an analyzer program on grounding and lightning system developed by
Safe Engineering Services & technologies ltd.
2
MALZ analyzes the frequency domain performance of buried conductor
networks and calculates the following quantities: earth and conductor
potentials, longitudinal and leakage current distribution in the conductors,
electric field and current density in the soil or at the earth surface, as well as
magnetic fields in the air.
TABLE I.
Input Data
Soil Resistivity
[Ω·m]
Fault Current
[kA]
INPUT DATA FOR SIMULATION
Values
Remark
100
Uniform Layer
50
Circuit Breaker
Rating
A. Case I
Case I is assumed that a fault current occurs at the
transformer area in PP. It may flow through OHGW and
grounding conductor in the duct bank, and soil.
Fig. 3. GPR of Conductor under Fault Condition
<Power Plant>
<SWYD>
2.6 km
Fig. 1. Modeling of Grounding Systems of SWYD (Left) and PP (Right) with
Two OHGW and Grounding Conductor in Duct Bank
Above Fig. 1, the grounding systems of SWYD and PP are
modeling.
Fig. 4. GPR of Segments in Voltage
Fault Location
In reference to above Fig. 3 and 4, there are about 5443 V
potential differences between SWYD and PP under fault
condition.
B. Case II
Case II is assumed that a fault current occurs at SWYD. It
may flow through OHGW and grounding conductor in the
duct bank, and soil.
Fig. 2. Fault Current Location on Grounding System of PP
The fault current location is assumed at the transformer
area as shown in Fig. 2.
<Power Plant>
<SWYD>
2.6 km
Fig. 6. GPR of Conductor under Fault Condition
Fig. 5. Modeling of Grounding Systems of SWYD (Left) and PP(Right) with
Two OHGW and Grounding Conductor in Duct Bank
Above Fig. 5, the grounding systems of SWYD and PP are
modeling.
Fault Location
Fig. 7. GPR of Segments in Voltage
In reference to above Fig. 6 and 7, there are about 5443 V
potential differences between SWYD and PP under fault
condition
Fig. 6. Fault Current Location on Grounding System of SWYD
IV. COMPARISON
The fault current location is assumed on the center of
SWYD.
The results from analysis for each case are described in
Table II below.
TABLE II.
COMPARISION WITH COMPUTATION RESULTS
Potential
Differences
[V]
Case I
Case II
5443
5443
In two cases, the potential differences are exactly same.
V. REVIEWS & CONSIDERATIONS
When a large potential difference caused under abnormal
conditions occurs between two areas, an unintended voltage
will be induced at the double end grounded cable shield
interconnecting the two areas
An equivalent circuit of induced voltage can be assumed
that it consists of the capacitance between the grounded shield
and conductor in a control cable for a trip command
connecting the control room of generating station to the
control room of remote substation, the impedance of shield,
the impedance of auxiliary relay for the trip circuit breaker
and the impedance of ground fault detector.
Considering the above results, when any cables
interconnected between generating station and substation are
designed, the potential differences under fault conditions
should be taken into account. For the calculation on the
potential differences, it is recommended to use the simulation
tool for the accurate analysis. If the analyzed results are not
acceptable, it is preferable to enhance the design by adding
grounding conductors between the generating station and
substation and then analyze again the modeled ground grids
under fault condition.
REFERENCES
[1]
[2]
Zc
△V
Zs
Zar
Vo
Zgd
△V : Voltage Difference [V]
Vo : Voltage at Aux Relay of C.B. [V]
Zs : Impedance of Cable Shield [Ω]
Zc : Capacitance between Shield and Conductor [Ω]
Zar : Impedance of Aux. Relay of C.B. [Ω]
Zgf : Impedance of Ground Fault Detector [Ω]
Fig. 8. Equivalent Circuit of Induced Voltage
If a voltage loaded at auxiliary relay for the operation of
trip circuit breaker is larger than a minimum operational
voltage, it will be operated accidently.
Therefore, the minimum operational voltage of circuit
breaker should be considered to prevent a malfunction
causing unintended accidents.
VI. CONCLUSIONS
In situations where the generating station and substation
are separated by long distances, multiple interconnections
between the two grounding systems are impractical. Due to
this, significant voltage differences may occur between them.
In general, the generating station and substation are in the
proximity and connected together with many conductors for
efficiency and equipotential. When making a lot of
interconnections between them because of the realistic
problem such as too long distance, the remarkable potential
differences may occur between them.
By using CDEGS software, the grounding systems of
generating station and substation are modeled for analysis on
the potential differences under fault conditions.
It is concluded that in all cases, there might be the potential
differences between two grounding systems. They may cause
some accidents such as the malfunction of electric devices
and insulation breakdown of equipments and others.
IEEE Standard for Generating Station Grounding, IEEE Std 665, 1995.
IEEE Guide for Safety in AC Substation Grounding, IEEE Std 80, 2000