Sizing and Selection of Grounding Transformers­
Decision Criteria
George Eduful
Godfred Mensah
Electricity Company of Ghana
Electricity Company of Ghana
P.O. Box 5278, Accra-North, Ghana
P.O. Box 5278, Accra-North, Ghana
System Planning Division
System Planning Division
georgeeduful@yahoo.com
Abstract-
godmens@ieee.org
Within a period oHwo years, the Electricity t:ompany
of Ghana (ECG) lost a total of six grounding transformers in a
particular substation. The situation created a lot of instability
and resulted in huge productivity losses to both the company
and its customers. The failures were believed to be related to
wrong selection of
grounding
transformer
rating. However,
using the concept of capacitive charging current of a system, it
was
found
that
the
short
time
rating
of
the
grounding
transformers were rightly selected. Analysis of the phenomenon
strongly linked the damages to protection deficiency. This paper
duty transformer of equal kVA rating. For this reason,
grounding transformers are often not sized by "kVA" but by
their continuous and short time current ratings. They are
usually oil immersed and may be installed outdoor.
Grounding transformer is used for direct grounding or
through a current limiting resistor. Zero sequence impedance
of grounding transformer is quite low, but it can be increased
if the purpose is to limit current through the transformer
during earth fault. The reasons for limiting current may be:
discusses analysis of the problem and proposes decision criteria
a.
for selecting a grounding transformer.
b.
Keywords: Grounding transformer, Capacitive charging current,
Zero sequence impedance, short time rating current
I.
To reduce transient over voltage incursion from
phase-to-earth fault.
To
reduce
mechanical
stresses
apparatus carrying fault currents.
in
circuits
and
As a rule of thumb, grounding transformers are designed
INTRODUCTION
A proposal for a change in specification of grounding
transformer was presented in response to persistent failure of
grounding transformer in a particular substation of the
Electricity Company of Ghana. Among others, the proposal
with a continuous current rating equal to approximately 10%
of its short-time rating. For example, a grounding transformer
rated 1000A for 10 seconds may carry 100A (10% of 1000A)
continuously. In practice, the size of a grounding transformer
is based on capacitive charging current of a system. This is
suggested a reduction in flow of earth fault current from 3180
because the charging capacitive current is the lowest level of
seconds to 10 minutes.
be effectively reduced.
A to 1245 A and an increase in short time rating from 10
The role of grounding transformer in power systems is so
critical that issues relating to its quality and reliability are
earth fault current at which system transient overvoltage can
As discussed above, grounding transformers can safely
carry about 10% of it short time rated load. Temperatures
treated with the utmost seriousness. As a holistic approach to
during its continuous rating should not damage the windings.
of grounding transformer in power systems to put the subject
short duration currents. Temperatures that cause excessive
broader context. Based on technical analysis, it was proposed
temperature for the windings in direct contact with the oil
solving the problem, the report first looks at the basic concept
in perspective. Thereafter, the proposal is examined in a
that the existing grounding transformer specification be
maintained. This paper presents report of the analysis and
proposes
decision
transformer.
criteria
for
selecting
a
grounding
Heating of grounding transformers are caused by random
gas
development
in
the
oil
should
be
avoided.
The
should not exceed 140°C. For this reason, Bucholz relay and
temperature protection are provided. Neutral C.T is also
installed at neutral point of grounding transformers to ensure
that in an event of severe earth-fault, it signals the appropriate
II. BASIC CONCEPT OF GROUNDING TRANSFORMER
earth-fault relay to initiate tripping to protect the transformer.
IN POWER SYSTEMS
III.
Grounding transformer is used to provide a ground path to
an ungrounded delta connected system. As a short-time rating
device, its size and cost are less compared with a continuous
With
the
DISCUSSION OF PROPOSAL
brief
overview
of
the
general
concept
the proposal in detail.
978-0-9564263-4/5/$25.00©2011
IEEE
of
grounding transformers in power system, we now examine
45
Proposal 1: Reduce the thermal stress on network
components, and hence failure rates, by reducing currents
that flow during earth faults from the current maximum of
3180A to 1245A. ................it is being proposed that the
existing zero sequence impedance of 19.2Q be changed to
50Q.
The zero sequence capacitance of transformer is negligible.
However, for over headlines, zero sequence capacitance can
be high if considerable lengths are involved. As a general
rule, the following approximate capacitance values are used:
Transformer
Although the proposal did not give detail on the technical
Over headline
consideration that influenced the choice of the 1245A, it is a
general knowledge that zero sequence impedance determines
the value of earth-fault current. The desired value of the zero
Co = O.OlflFltransformer
Co = 0.00625 flFlkm
As indicated above, value of 3Ico is critical for sizing and
selecting grounding transformers. For good approximation of
sequence impedance is dependent on the system charging
3Ico value, we considered all cables and the overhead lines
charging current before the zero impedance value can be
capacitor bank of 1O.8MVar at the station.
capacitive current. Therefore, it is necessary to determine the
selected.
The generally accepted criterion for determining the size of
zero sequence impedance (Zo) is that
length in the system. Also considered are transformers and a
Based on equation (2), the zero sequence capacitance of the
cables are calculated, see the table-I. SIC value used for the
capacitance calculation is 3.5.The capacitance values of the
transformer
and
the
overhead
lines
approximate values as indicated above.
At this condition, the destructive voltage build up on the
charging capacitance of the un-faulted phases cannot occur
[1, 2, 3, and 4]. Where, Xco is the line-to-earth capacitive
reactance of the system. Stated in another way, the current in
the zero sequence impedance IN during a line-to-earth fault
is
=
..fi(2
31
ro
Therefore,
=
.J3
(
2 x 1T x 50 x104.3 x 33
fx C
103
o
X
ELL)
Amperes
(1)
3Ieo = 1872.308 Amps
33000
IV. GROUNDING TRANSFORMER SELECTION CRITERIA
SIC
insulation shield, d is the diameter of the conductor, system
Co is zero sequence capacitance of the system
Based on equation (1), the system charging currents for the
(33kV
network)
1
can
be
calculated
and
624
=30.5Q
is dielectric constant, D is the diameter of cables over the
frequency and
)
=--x-
Where, ELL is the system line-to-line voltage in kilovolts,
system
103
.J3
x 1[ x
the
Accordingly, using equation (2), 3Ico for the 33kv system
current (3Ico) is given as
co
on
From Table 1, total zero sequence capacitance of the 33kV
must be equal to or greater than three times the line-to-earth
31
based
system is 104.3IlF.
system charging current, 3Ico.
According to [4, 5], during line-to-earth system, charging
are
hence,
determine the appropriate zero sequence impedance. The
charging current is calculated by summing the zero-sequence
capacitance of all the cable and equipment connected to the
system.
Criterion
1: Based on the general rule that Z0
::; X0' it
can be said that grounding transformers with values of Zo up
to 30.50 is appropriate for selection.
In relation to the above criteria, the 500 zero sequence
impedance value suggested by the proposal does not match
the property of the system. Hence, the proposed 500 zero
sequence impedance is not appropriate.
Criterion 2: It appears that the existing specification of
19.40 at short time current rating 3180A also satisfies the
The zero sequence capacitance of any type of cable can be
calculated using the following formula:
o
C
(2)
fl
978-0-9564263-4/5/$25.00©2011
necessary to compute the values of transient over voltage
under the existing specification and the calculated one (30.50
= 0.00736 x SIC
j1F 1l000
D
logd
general criteria. However, to take an informed decision, it is
IEEE
at rating of 1872A). For comparative analysis, transient
overvoltage for Zo=500 is also computed.
46
I
Table 1: Components of the Substation
Variables
Diameter over
insulation D (mm)
Diameter over
conductor d (mm)
Number
Length(J,.'ll)
Capacitance in
J1
Total capacitance
(Co) in I!
Cable
Cable
Cable (I x630)
(3x240)
48
28.2
Cable (lx500)
62
for
for
Zo=19.40,
Zo=300,
Zo=500,
Transformers
Capacitors
NA
NA
52
NA
NA
NA
18
I-bank
32.6
21.1
NA
NA
NA
NA
17.2
39.85
12.02
69.3
0.153311763
0.204498567
0.337224215
8.702845932
26.89527355
13.3776735
From ASPEN One-liner modeling of the substation,
for
Overhead line
NA
(10.8MYar)
NA
0.430610236
NA
NA
NA
0.18
54.66854076
Criterion 3: The third criterion is to consider sensitivity of
the relaying system and the thermal stress that will be
imposed on the system in an event of earth-fault. At this
Xo/X\=33.6177
Xo/X\=62.5156
stage, system engineers are normally guided by protection
Xo/X1=168.732
philosophies. The general philosophy is that in an event of
Where, Xo/X\ is the Thevenin's ratio of zero sequence
fault, enough current should be allowed to flow such that
protective devices can detect earth-fault current and trip off­
reactance to positive sequence reactance of the location of the
line but not so much current to cause major damage.
The transient over voltage is then calculated from the
on ft. Using Zo = 19.40 will result in the following:
grounding transformer.
following relation [6]:
Thermal stress rating of power system equipment depends
Higher earth fault current and faster operating times
a.
for the existing IDMT protection schemes at the
station.
Effects of high earth current will affect;
b.
a.
During a line-to-earth fault on one phase, the transient
voltages on the healthy phases in relation to the Zo values are
0
b.
Transient
P.U
48.56
30
1.48
48.99
50
1.49
49.31
Cable and Overhead lines if the damage
IDMT
(kV)
1.47
Where IFfault
curve of these equipment are lower than the
Overvoltage value
19.4
•
current, 1tnp=relay operation time.
given in P.U and kV as:
Zo Values in
Grounding transformer
2
2
ifl t.
. n xtd eSl
.gn
Irlp >Id eSl
g
J
curve
of
protection
scheme
protecting these equipment.
From
IDMT
protection
schemes
at
the
station,
the
protection curves are all far lower than the damage curve of
the cables and feeders, see the Fig.1.
As can be seen, the transient voltage values presented by
the respective zero sequence impedances to the healthy
phases under line-to-earth condition are lower for Zo=19.40
and for Zo=30.50 as compared to Zo=500. This confirms that
Zo=500 does not satisfy the general condition of Zo:::Xco.
V.
CASE STUDY: THERMAL STRESS ANALYSIS ON THE
RECENT GROUNDING TRANSFORMER FAILURE AT THE
STATION
This case uses typical earth-fault data, obtained from the
transient
protection relays and technical data as specified on the most
regarding the selection of the grounding transformer can still
thermal stress, if any, on the system during the recent failure
However,
relative
to
the
closeness
of
the
overvoltage values for Zo=19.40 and Zo=300, decision
recent failed grounding transformer, to examine impact of
not be made at this stage.
at the station.
978-0-9564263-4/5/$25.00©2011
IEEE
47
10
HXXl
2
3
4
5
7
1�
. -1-'
2
3
4
5
7
lOll
111191 r--
c;w.l�IId<\c>'OM�
..,
�
+'
__
2
3
4
5
7
;
lCDX1
2
3
4
5
7
f-' �
I c.
r-�
f:::
..i!���="""'_ko(lDl9J'5IP':zo:o:nocmls
m _
f-'
CmiU:b'�Cuwtr630"'JIlR'tmo!tlHI>91B
·
,oo
·
,
·
t
-
'-1-'
·
·
\
f
·
·
,
\
.t-\
,
\
+
\
.
lr-'"
1\
\
$
;
-
"
�:
1-:
1+-
+
.
,
"------
,
Condu:I><Oo""rC"",el><:�CU)Q.A:tom�HI>S1E
�:
f-' .
+
t:t
,
�c=:�c"",e ko(l0497S_736470c"*
,
r- "
r-
"
ID
f- llX1ll00010
,
,
,
Ie:
·
I�
,
,
,
I,:
++
·
+
++
m
�
+
m
.1-- '
"
,
m
,
..
,
,oo
,
,
..
,
(a)3X240
,
,�
Cu
,
..
XLPE
,
, ..
,
,
..
,
�
3
4
5
7
I.
100
:
3
4
5
7
1000
CURR9(f(Aj
:
3
4
5
(a)IX630 AI XLPE
+
1 X500 Cu
1+
+
.-
.
XLPE
Figure 1. Protection curves for cables at the station
978-0-9564263-4/5/$25.00©2011 IEEE
48
VI.
Calculated design stress:
2
I � x t 4800 X 10 230,400,000A2 S
Frequent damage of grounding transformer at the station
is attributed to inability of temperature protection system to
Relay on Grounding transformer
IN>140A,
td=
O.3Sseconds,
Inverse Curve (LTI)
IFI7.7SkA
(anticipated
assuming no CT saturation)
Calculated
thermal
177S0
2
detect overheating of the grounding transformers possibly
curve:
trip
Long
Time
time=0.34secs,
on
the
2
x 0.34 = 107,12 1,2S0A
grounding
imposed on the system during the fault condition. Ideally, the
fault stress should not damage the transformer. The high level
of the fault current could be attributed to a short in the
transformer winding due to insulation breakdown. Insulation
breakdown might be due to the following:
the total capacitive charging current of the system. To avoid
transient over-voltages, grounding transformers must be sized
Grounding transformers should be selected to limit phase­
to-ground fault current such that the thermal stress imposed
on the system will be less than the equipment design stress.
Grounding transformer should be selected such that in an
event of fault, enough current will flow to allow protective
device to detect ground fault.
REFERENCES
Inability of temperature protection system
to
detect
transformer
overheating
possibly
of
from
grounding
the
flow
of
1977.
the grounding transformers.
[2] J.P. Nelson, "System Grounding and Ground Fault Protection in the
Petrochemical Industry: A Need for a Better Understanding," IEEE
Transactions on Industry Applications, vol 38, pp 1633-1640, NovlDec 2002.
fault current.
Our analysis was also extended to the previous failures at
the station. It was confirmed that the thermal stress from the
phase-to-earth faults were all far lower compared to the
equipment designed stresses.
Based on the above analysis, it obvious that the existing
(Zo
[1] J.R. Dunki-Jacobs, "The Reality of High-Resistance Grounding," IEEE
Transactions on Industry Applications, vol IA-13, pp 469-475, Sept/Oct
current exceeding the continuous rating of
Poor CT sensitivity to the flow of ground­
2.
frequent
neutral CT to ground fault current.
exceeds the electrical system's charging current.
grounding transformer is about 200% greater than the stress
the
rating of grounding transformer and poor sensitivity of
so that the amount of the earth-fault current allowed to flow
s
As shown from the calculation, the design stress of the
specification
resulting from the flow of current exceeding the continuous
Sizing of zero sequence impedance depends entirely on
stress
transformer due to the fault:
1.
CONCLUSION
=
=
= 19.4n at 3180A) has no connection with
damages.
The
existing
specification
even
provides room for future growth of the substation compared
with the proposed rating of SO n at
[3] W.C. Bloomquist, KJ. Owen and R.L. Gooch, "High-Resistance
Grounded Power Systems - Why Not?" IEEE Transactions on Industry
Applications, vol IA-l2, pp 574-580, Nov/Dec 1976.
[4] D.S. Baker, "Charging Current Data for Guesswork-Free Design of High
Resistance Grounded Systems," IEEE Transactions on Industry Applications,
vol IA-15, pp 136- 140, Mar/Apr 1979.
[5] B. Bridger, Jr., "High-Resistance Grounding," IEEE Transactions on
Industry Applications, vol IA-19, pp 15- 21, JanlFeb 1983.
[6] Electricity Company of Ghana Distribution Planning Manual, Revised
Edition 2011.
1245A.
Proposal 2: Prolong the life span of the grounding
transformers by increasing the short time duration rating
from the current 10 seconds to 10 minutes.
Line-to-earth is undesirable condition and must not be
allowed
to
persist
for
long
time.
Short-time
rating
is
necessary to limit damage in an event that the system earth­
fault escalates into
a double line-to-earth
fault
or the
impedance of the transformer becomes shorted. The standard
rating allowed for grounding transformer ranges from 10 to
60seconds. However, where grounding transformers are used
to establish a neutral point to enable connection of phase-to­
neutral loads, continuous neutral current rating of the device
is allowed because of the attendant load imbalance.
978-0-9564263-4/5/$25.00©2011
IEEE
49