Design and Calculation of 66kV Neutral Grounding Resistor for Main
Transformers in Bandar Imam Petrochemical Complex(BIPC) Power Station
Located in South West of IRAN
Hamid.R. Izadfar1, M.R.Farsad2,Davood Andavari3,S.Shokri4
1, 4
Department of Power Engineering, University of K.N.T
2
Energy Farda consultant Engineers
3
Energy Farda consultant Engineers
Abstract— Types of grounding system for Industrial
Networks lower than 45kV and upper than 132kV voltage
levels are clearly recommended in standards, but for some
voltage levels in between, such as 66kV, is not specified.
BIPC’s step up transformers have dY connected winding,
13.8/66kV rating voltage and secondary side of them has
been solidly grounded. The high intensity current of
frequent Earth Faults in 66kV distributed network in BIPC,
caused disruptive effects. In order to limit the short circuit
This substation transforms132KV overall network of Iran
to 66KV.There are different voltage levels in BIPC such as
66KV,
6.6KV,3.3KV
and
low
voltage(400V,220,110,…).most number of electrical faults
in BIPC is line to ground (LG) fault in 66KV voltage
levels. Solidly grounded neutral of main transformers
causes high current fault. High released energy at these
moments has very danger for human and equipment. For
this reason, improvement of main transformers grounding
system has been studied.
current, calculation has been made to change the existing
II. SHORT CIRCUIT ANALYSIS
grounding of step up transformers in BIPC power station.
Obviously, the insertion of a resistor in transformers
neutral grounding system is affected by; short circuit
current protection, unbalance voltages, operation of existing
protections, charging currents, and….This paper introduces
important scientific and applicable notes witch has resulted
from months of studies and discussions by some power
engineers and experts. These results will be good references
for neutral grounding resistor calculation, in general.
I. INTRODUCTION
BIPC has 4 gas turbine units in its power station. This units
produce 13.8KV voltage. There is one transformer for each
unit with 82.1MVA capacity, dY connected winding and
13.8KV/66KV ratio. Secondary side of them has been
solidly grounded. Output feeders have connected to 66KV
Gas Insulated Switchgear (GIS). GIS has 2 bus bars. BIPC's
units have been connected to one of them and the other bus
bar is fed by 2 feeders from KWPA substation. There are 2
parallel transformers in KWPA substation. These
transformers have 30MVA Capacitive; Dy connected
winding with solidly grounded neutral in secondary side.
BIPC existing network short circuit analysis gets the
following results: Maximum currents for 3 phase short
circuit (LLL), 2 phases short circuit (LL), 2 phases to
ground short circuit (LLG) and line to ground short
circuit (LG) in 66KV voltage level with neglecting of
motors injecting motors are: 22KA,19KA, 24.7KA,
25.9KA, respectively. When a LG fault is accrued voltage
drop in 66KV network on faulted phase is 75% up to
100% and in medium voltage(MV) network is 47% up to
78% (depended on this fault is on witch feeder and haw
much distance from GIS). There for trouble voltage drop
is accrued in MV motor buses. Neutral grounding resistor
can improve this subject.
III. NEUTRAL GROUNDING SYSTEMS
According to IEEE standard, the types of neutral
grounding systems are: ungrounded system, solidly
grounded system, resistance grounded, reactance
grounded, resonant grounded. Table 1 shows the standard
grounding systems for different voltage levels of Iran
network [1].
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Table1.Standard grounding system for Iran networks
Nominal Voltage
(kV rms)
Highest Voltage
(kV rms)
400
420
effectively
230
245
effectively
132
145
effectively
66 & 63
72.5
effectively
ineffectively
20
24
effectively
ineffectively
C0 =
Grounding
system
From the different types of systems, reactance
grounded and resonant grounded is not useful for
BIPC network. But resistance grounded system has
good property for it. In this method neutral point is
grounded through one or more resistance. In high
resistance grounding (HRG) short circuit current is
less than 10A. This system is used for lower than
15KV networks [2]. In low resistance grounding
(LRG) is upper than 100A [2] (up to 1000A or
more).HRG is not useful for BIPC network. Because
this system is used when clearing of the first fault is
not necessary. Also it is operational for lower than
15KV networks. LRG system I used for 3.45KV up to
69KV networks [3].
0.00736ε
*10 −3
D
log10  
d
(1)
C0=capacitance to ground in µF/Feet
D= diameter over insulation for cable
d = diameter over conductor
ε =specific inductive capacitance of insulation
Size and length of 66KV cables in BIPC is according to
table 2.
Table 2.Charcteristics of 66KV Cables
Y=(ωc)
Charging
Cable
Length
(m)
( µmho / km)
Current
(A)
3(1×95)/Cu/XLPE
13950
0.000041
65.38
3(1×240)/Cu/XLPE
3550
0.000058
23.54
3(1×800)/Cu/XLPE
1400
0.000094
15.4
There for 66KV cable charging current is:
3IC0 = 65.38+23.54+15.4=104.32A
6.6KV & 3.3KV CABLES
Charging currents of BIPC MV cables are very small and
neglected
6.6KV & 3.3KV Electric Machines
Iv. CHARGINC CURRENT CALCULATION
Capacitance between one phase and earth, defines
network charging current .For each of equipment,
charging current calculated and summation of them is
total network charging current. In this section, we
calculated BIPC network charging current.
TRANSFORMERS
Charging current for transformers is about [4]
0.05A/MVA
Total installed main transformer capacity in BIPC is as
follows:
- 66KV/3.3KV and 66KV/6.6KV transformers capacity
= 460MVA.
- KWPA transformers capacity = 2*30=60MVA
- BIPC power station transformers capacity
=4*82.1=328.4MVA
There for total transformers charging current with
neglecting from minor transformers is:
0.05*(460+60+328.4) = 42.42A
66KV CABLES
66KV cables charging current be calculated from
flowing equation [4]
Charging current for electric machines is equal with[4]
P
3I C 0 = 0.05 *
A
(2)
n
P=out put horse power
n= rpm
This current is about 0.4A and neglected.
Total charging current
BIPC charging current is some of above sequences:
3IC0=42.42+104.32=146.74A
MAX. AND MIN. 66KV FEEDERS CHARGING CURRENTS
If neutral is grounded through a resistor, its current under
fault condition must be more than maximum charging
currents of feeders. Although all transformers can be
grounded with communal resistor, it isn't suitable. Because
when a fault accrues on each transformer out put, all
transformers will be tripped. For this reason it is better than
each transformer grounded with particular resistor.
The examination shows that, in worth case, BIPC electric
energy supplied by 3 transformers. There for the current
through each resistance must be bigger than 50A, at least.
2350
200
= 1( A)
200
1
200/1*0.1(Min.tap) =20(A). Minimum pickup current
Using relay 0.1-1(A) ranges will provide satisfactory
pickup range 20-200A and more.
For producing a good protection, earth fault relay
sensitivity and it setting must be so that, it is bigger than
charging current in unfaulted feeders. Also it set on 10% of
maximum short circuit current. Fig 3 in appendix shows
one of different cases for charging currents flow with a LG
fault in BIPC.
V. NEUTRAL GROUNDING RESISTANCE (NGR) DETERMINATION
NGR determination affected by following notes:
- Maximum trip time of network circuit breakers.
- Current transformers ranges.
- Network short circuit levels.
- Allowable maximum fault current for network.
When a LG fault is accrued, real sequence of LG current
must be bigger than the sum of charging current in 3
phases
1
U
, Xc =
(3)
Ic =
Xc
ωc
∑ I c = 3UωC
(4)
U
1
(5)
∑ I R ≥ ∑ I c ⇒ ≥ 3UωC ⇒ R ≤
3ωC
R
Minimum current for each NGR must be 50A.Out put GIS
feeders CT ratio is 200/1A. If each NGR is limited to 200A,
with setting of over current relay instantaneous element on
0.1, perfect protection will produce for LG fault current
more than 20A.
For currents of lower than 150A, with attention to existing
ratio CTs, identification of ground faults in far distance
from GIS with high impedance isn't capable. Also currents
of bigger than 200A, is caused unbalance voltage more than
1% in MV networks and this reduces efficiency of MV
electric motors. There for selection of 200A grounding
resistance for neutral points of transformers is useful.
VI. ICALCULATION OF RESISTANCE
Resistance must be installed in secondary side of
transformers, where line voltage is 66KV.Resistance
calculated as follows:
- Voltage rating of ground resistor =
66
V L− N =
= 38.1KV
(6)
3
For 82.1MVA,13.8/66KV and 60MVA, 132/66KV, deltastar connected transformers, the current must be limited to
200A.Hence:
38100
R=
≅ 190 A
(7)
200
Using a 200/1A current transformer:
VII. NEW EQUIPMENT INSTALLATION
Grounding system modification needs to new equipment
for installation in BIPC network. This equipment is:
-NGR for each main transformer in BIPC and KWPA
-equipping of all GIS output feeders with earth fault relay
(50N) if there isn't enough protection under new
conditions.
-equipping of input feeders in KWPA switchgear and GIS
with Restricted Earth Fault (REF) relay.
- equipping of all input feeders in GIS with standby earth
fault relay such as 51N relay if there isn't.
Also some of relay settings must be modify. Because of
LG fault current reduced about 20 times with respect to
solidly grounding system.
VIII. LIMITTER SUBJECTS IN RELAYS SETTING
NGR causes that unfaulted phases voltages increase when
a fault is accrued. In fault position this voltages may
reach to line voltages (i.e. 1.732 times).Hence one major
limiters are existing voltage relays on HV and MV levels.
We must aware these setting relays and new protection
must be set so that before voltage relay operation, fault
cleared.
IX. VOLTAGE UNBALANCE
Insertion of NGR in grounding system has effects
on equipment and network electrical parameters.
Although using of NGR causes increase voltages when a
LG fault accrued, balancing of voltage in MV and LV
networks will improve. Figs. 1 and 2 compare unbalance
voltages in 2 different case of grounding system on
several positions of BIPC network with a LG fault in
66KV level.
This figures show unbalance voltage in 66KV networks a
few increases. But in MV networks unbalance voltage
very reduces and reaches to zero, nearly. Hence transient
stability in this level of network fortifies. Main electrical
motors work in this voltage level.
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Unbalance Voltage %
45
40
35
30
25
20
15
10
5
0
1
3
5
7
9 11 13 15 17 19 21 23 25 27
66 kV Short Circuit Buses
Solid Grounding
Resistance Grounding (200 A)
Fig1. BIPC 66KV buses unbalance voltage
Un b alan ce V o ltag e %
35
30
25
20
15
10
between neutral point of transformer and NGR its nominal
voltage would be 38.1KV.
In 1st case, although CT is cheaper but if NGR and earth
connection omitted CT will destroy. Because of in LG fault
moment, neutral point voltage increases to 38.1KVand CT
will see this voltage.
Voltage and current fault analysis is done in many papers.
Readers can refer to [5] and [6].Our goal in this paper was
introduction the basic and important operational notes and
avoided from mention of done searches and studies.
REFERENCES
[1] Ghods Niroo consultant Engineers." standard for
132/20(33) KV substation,"
[2] IEEE Std141-1993.
[3] Dr.Luke, L.Henriks,"Selection of System Neutral
Grounding Resistor and Ground Fault Protection
for Industrial Power Systems," IEEE. Paper No.
PCIC-91-51.
[4] www.ipc-resistor.com"Ground Fault Protection on
Ungrounded and High Resistance Grounded
Systems Application Guide"
[5] "Resistance Grounded Systems," Copyright 2002
Kilowatte classroom,LLC.
[6]
5
0
1 3 5 7 9 11 13 15 17 19 21 23 25 27
66 kV Short Circuit Buses
Solid Grounding
Resistance Grounding (200 A)
Fig2. BIPC 3.3KV buses unbalance voltage
X. CONCLUSION
With NGR, LG fault current and released energy reduces.
There for safety for personal and equipment is improved.
Damping of over voltage and unbalance voltage in MV
reduces. Adding of new equipment may be impossible
because switchgear is GIS type.
Also radio interferences may increase with respect to
solidly grounding systems. Charging currents, voltage relay
settings, sensitivity of current relay for high impedance and
far away fault identification, CTs ratio,…are major
parameter for resistor calculation.
With NGR network needs to new protections. In NGR
cubicle CT is located between NGR and earth. In this case
the nominal voltage of CT is low (400V).If it located
2352
J.Roberts, Dr.Hector,J.Altuve,Dr.Daqing
Hou"Review"Reviw of Ground Fault Protection
Methods for Ground,Unground and Compensated
Distribution Systems"scheitzer engineering
labratoies,Inc
APPENDIX
Fig.1.charging current flow in BIPC network
2353