Motor contribution to three phase fault currents in a

power intensive industry with CPP: A case study

Abstract— Addition of motor-contribution to the total fault ratings of these motors, individually, can be as high as 10 MW
current during a three-phase fault is of interest to all power or more.
intensive industries with dominant motor loads. Such results are
analyzed for a petrochemical industry where majority of loads To model an induction motor for short circuit study, the
are induction motors. Three phase fault currents are computed value of impedance used for calculation is same as the
at five buses of 6.6 kV and also at the main distribution bus of 33 impedance which limits the locked rotor current when voltage
kV. In this case study, the fault currents at 6.6 kV buses are is applied to the motor at rest [2].
estimated due to the contributions from (i) captive generating
The effect of increase in current on circuit breakers
units alone (ii) motor(s) connected to the faulty bus only (to that
of generating units) (iii) all the major motors in the network
depends upon the duration of breaker operation relative to the
including those connected to faulty bus (to that of generating rate of decay of the current supplied by the induction motors
units). The results indicate that for this industry, with captive [3]. Developments in protection systems and switchgear
generation, the motor-contribution to fault currents can be as designs have reduced tripping times, while the time constants
high as 40%. The transformer rating (and its impedance) of motors have increased because of their increased size and
feeding the motors play an important role in the motor- the special characteristics (high efficiency motors) [4], [5]. As
contributions to fault currents. the total fault current magnitude is increased, the same must
be considered while selecting ratings of over current
Index Terms--Captive Power Plant, Generators, Induction protective devices [6]. Reference [7] demonstrates a classic
motors, Locked rotor current, Three phase faults, Transformer example of an under-designed power system where motor
impedance. contribution is not considered for fault calculations. Thus, the
effect of fault current contributed by motors becomes an
I. INTRODUCTION important part in the short circuit study while designing a
system.
Short circuit analysis is for determining the fault currents
magnitudes at various system locations in the electrical For longer duration (more than 4 s) after the short circuit
network. At a particular given location in power system fault, the smaller induction motor fault contribution decays
network, all the sources (Thevenin’s equivalent) and network rapidly and is almost negligible although motors remain
impedance limiting the current determine magnitude of fault connected [5]. In a particular case study carried out in [8], it is
current. The power system components such as synchronous emphasized that in interval (0.05-0.11) s, all motors participate
generators act as the major sources of fault currents. in the total fault current.
Components such as transformers affect the magnitude of fault
current due to their impedance and restrict it to some extent. Literature states that [3] the parameters such as
capacitance of the cables feeding the motor would have no
When there is a three phase fault at a particular bus in an appreciable effect on fault currents induced by motors.
electrical network, in addition to synchronous generators, the
motors connected to the bus act as generators of fault current The objective of the present study is to compute fault
due to the stored energy in the motor [1]. current at various buses of the power intensive industry with
motor load and captive generation by simulation. The effort is
Along with synchronous generators, induction motors are to understand the percentage of motor contribution to the short
important equipment in any captive power plant (CPP). In circuit current at a given bus in this electrical network (case
general, in power intensive industries, induction motors load study).
constitutes a major component of the total load [1]. The


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 Figure 1. Simplified single line diagram of a captive power plant
II. DESCRIPTION OF THE CPP WITH MAJOR MOTOR LOADS full load rated current whereas for motors M5 and M6, the
This CPP under study is a petrochemical industry locked rotor current is 450% of full load rated current.
consisting of three generators generating at 11 kV. Generators This CPP has three different grounding systems, right from
G1, G2 are gas turbine units of 22 MW each and G3 is a steam generation (11 kV), transmission (33 kV) and distribution
turbine unit of 28 MW; the total CPP generating capacity is of voltages (6.6 kV). The generators G1, G2 and G3 are high
72 MW. The maximum connected load is 40 MW. The resistance grounded. The generator transformers, TR-1, TR-2
voltage from each generator is stepped up to 33 kV using the and TR-3 are solidly grounded. The distribution transformers,
associated generator transformer, TR-1, TR-2 and TR-3, as TR-4 to TR-8 are low resistance grounded. All the
shown in figure 1. Bus A, Bus B and Bus C are 33 kV buses transformers considered in this study are of delta – star
from where power is distributed to localized distribution configuration. Advantages such as continuity of service,
substations within the industrial premises. The circuit breakers minimum equipment damage due to lesser fault current are
C1, C2 and C3 are normally closed such that there is one 33 shifting grounding mechanism towards high resistance
kV distribution bus (Bus-A). The voltage is further stepped grounding [9]-[12]. Some of the grounding related aspects will
down using 33/6.9 kV transformers - TR-4 to TR-8 to feed be part of presentation, though not included as the part of this
some of the important loads of 6.6 kV. paper. All the motors considered in the study are delta
The ratings of generator transformers, TR-1, TR-2 and TR- connected except motor M5, which is star connected and
3 are 31 MVA, 31 MVA and 42 MVA respectively with grounded. This star connection shall have a significant effect
percentage impedances of 12.5 % each. The distribution on single line to ground fault current levels due to presence of
transformers ratings vary from 10 MVA (TR-4 & TR-5), 15 a ground. Motor grounding plays an important role in single
MVA (TR-7), 20 MVA (TR-6), and 35 MVA (TR-8). The phase fault currents. Reference [13] explains the reduction in
percentage impedances of these transformers are 8.35 %, 10%, single phase to ground fault current on removal of a motor
10 % and 13 % respectively based on the MVA ratings. ground through a case study.

Motors are connected to 6.6 kV buses. In the present III. DESCRIPTION OF CASES STUDIED
study, eight motor loads (M1 to M8) having their power rating The present study is aimed at
greater than 1 MW are considered. The motors from M1 to
M4 are of rating 1 MW each. The ratings of motors, M5 and x Understanding the contribution to the fault current at a
M6 are 7.6 MW and 2.6 MW respectively. Motors M7 and bus due to motors connected to that bus only, at 6.6 kV
M8 are identical motors of 1.9 MW each. The locked rotor buses.
current of motors M1 to M4, M7 and M8 is 550 % of motor
 x Understanding the contribution to the fault current at a Also, in CASE-3, when fault current contribution from all
bus due to all motors in the system at 6.6 kV buses. the motors in the system (including that on the faulty bus) are
considered, the total fault current increases marginally as
x Understanding the contribution to the fault current at 33 compared to CASE-2 where the total fault current was
kV bus due to all the motors at 6.6 kV buses. computed due to contribution of motor at that bus only.
Hence in the present study, symmetrical short circuit fault TABLE 1: COMPUTED FAULT CURRENT MAGNITUDES AT 6.6 KV BUSES FOR
currents are computed for a three-phase short circuit fault at (i) CASE-1, CASE-2 AND CASE-3
6.6 kV buses (Bus 1, Bus 2, Bus 3, Bus 4 and Bus 5) for three Motor(s) connected CASE-1 CASE-2 CASE-3
Bus
cases and (ii) 33 kV bus (Bus A) for two cases. to Bus (kA) (kA) (kA)
Bus 1 M1+M2+M3 (3 MW) 7.01 8.68 9.06
The details of these cases are:
Bus 2 M4 ( 1 MW) 7.01 7.57 8.00
a) CASE-1: Computation of three phase short circuit fault
Bus 3 M5 (7.6 MW) 9.93 13.31 13.94
currents for each of the 6.6 kV buses (Bus 1 to Bus 5)
separately with no motor contributions. Bus 4 M6 (2.6 MW) 8.23 9.40 9.96
b) CASE-2: Computation of three phase short circuit fault Bus 5 M7+M8 (3.8 MW) 11.8 13.90 15.0
currents for each of the 6.6 kV buses (Bus 1 to Bus 5)
separately with contribution from motor(s) connected to Table 2 gives the quantitative feel of percentage increase
that bus only. For example, in case of Bus 1, fault in total fault current at a particular bus due to addition of
contribution is calculated with motors M1+M2+M3. motor contribution from that bus (Column 3), and further
Similarly, for Bus 5, combination of motors M7+M8 is addition of all motors in the network (Column 4).
used for calculation. Fault current contribution of motors
TABLE 2: PERCENTAGE INCREASE IN CASE-2 AND CASE-3 IN COMPARISON
connected to other buses is not taken into account. WITH CASE-1
c) CASE 3: Computation of three phase short circuit fault Motors connected to % increase in % increase in
currents for each of the 6.6 kV buses (Bus 1 to Bus 5) Bus
Bus CASE-2 CASE-3
separately with contribution from motor connected to all 1 M1+M2+M3 (3 MW) 23.78% 29.31%
buses including the faulted bus.
2 M4 (1 MW) 7.93% 14.11%
d) CASE 4: Computation of three phase short circuit fault
currents for 33 kV bus (Bus A) without any motor 3 M5 (7.6 MW) 34.05% 40.42%
contribution. 4 M6 (2.6 MW) 14.17% 20.92%
e) CASE 5: Computation of three phase short circuit fault
5 M7+ M8 (3.8 MW) 17.74% 26.65%
currents for 33 kV bus (Bus A) with contribution from all
the motors (M1 to M8) at 6.6 kV buses. As seen in the Table 2, for fault at Bus 3, there is an
increase of 34% in total fault current due to addition of motor
IV. COMPUTATIONS OF FAULT CURRENTS fault current of that particular bus. Also, there is an additional
The three-phase short circuit fault currents are computed in increase (6.37%) in total fault current due to contribution of all
ETAP software. Cable impedances are neglected in the study. the motors leading to a net increase of 40%.
The data related to transformers and induction motors is
mentioned in section II. In this case study, motors of ratings In CASE-2, though, the rating of motors on Bus 5 (3.8
above 1 MW only are considered. Motor load currents are not MW) is higher than Bus 1 (3x1 MW) and also, they have same
considered in total fault contribution. locked rotor current, the percentage increase in bus fault
current is higher in Bus 1. This can be attributed to the fact
The study is done with respect to three phase short circuit that the transformer feeding Bus 5 is 35 MVA (with its Z =
fault only. Single line to ground fault is not part of this study. 13%) as compared to transformer feeding Bus 1 10 MVA
(with its Z = 8.35%). It leads to higher fault current
V. RESULTS AND DISCUSSION contribution when compared to generators-only case (CASE-
Three phase short circuit fault current magnitudes 1). Though the total motor load on Bus 1 and Bus 5 is
computed for CASE-1, CASE-2 and CASE-3 described in comparable, the percentage increase in total fault current at
section III are given in Table 1. individual bus due to addition of motor contribution is lesser
in Bus 5 compared to Bus 1 (CASE-2).
As seen from Table 1, there is a substantial increase in
total fault current at a particular bus due to motors connected In CASE-2, though the ratings of motor on Bus 1 (3 MW)
to the faulted bus (CASE-2) as compared to fault currents due and Bus 4 (2.6 MW) and their transformer ratings are
contribution of generators only (CASE-1; no motor comparable, the percentage increase in three phase fault
contributions). current due to addition of motor current is higher in Bus 1 as
compared to Bus 4. This is attributed to the difference in Motors at the 6.6 kV bus have shown an increase of 27%
locked rotor current of motors connected to Bus 1 (550%) as in total fault current at 33 kV bus in this case study.
compared to motors connected to Bus 4 (450%).
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