CIRCUIT BREAKER PERFORMANCES SELECTION FOR NEAR-TO-

GENERATOR FAULT CURRENT INTERRUPTION

Copyright Material PCIC energy
Paper No. PCIC energy EUR24_14

Giovanni Gambirasio Mauro Codoni

SELECTY SELECTY
Via Ospedale 70, Via Ospedale 70,
24069, Trescore Balneario 24069, Trescore Balneario
Italy Italy

Abstract - Several installations include synchronous users (e.g. medium voltage induction motors). This is
generators and, in some cases, synchronous motors. the typical case of a mid-size industry.
Under short-circuit conditions, the fault currents generated • Island power system where generator/s is directly
from the synchronous machine are characterized by a high connected to a medium voltage switchgear (e.g. at
level of asymmetry that requires suitable circuit breaker 11 kV level) where are derived the distribution
performances to ensure safe interruption of fault current. transformers (for load supply) and the users (e.g.
To calculate such interrupting performances, which are medium voltage induction motors). This is the typical
important for effective circuit breaker selection (IEC case of a mid-size industry not linked to the National
Standard discussed in this paper), it is necessary to grid, and is also the typical case of large offshore
properly consider: short-circuit current calculation installations, FPSOs, cruise & operating vessels,
Standards that vary depending on installation type etc…
(onshore and offshore), network topology and IEC circuit • Large synchronous motor/s connected to a medium
breaker Standards. voltage switchgear, where commonly other feeders
A systematic steps approach is discussed referring to a (distribution transformers and/or motors) are
real case study, investigating the symmetrical and connected. This is the typical case of a large industrial
asymmetrical components of the fault current and delayed complex such as LNG plants, etc…
zero crossing phenomena. Additionally, a set of sensitivity
analysis is proposed to give the reader an overall feeling All the above-listed types of installations, characterized
about potential critical conditions. by the proximity to the synchronous machine/s, are
potentially exposed to short-circuit currents with a high
Index Terms — Synchronous machine, circuit breaker, degree of asymmetry and delayed current zero crossing
short-circuit breaking capacity, delayed current zero phenomenon. Therefore, the investigation discussed in this
crossing. report is required.

I. INTRODUCTION
B. Analysis objective
It is well known from technical literature that in proximity
of a medium voltage synchronous machine, the short- The aim of the analysis is to accurately predict the short-
circuit current is characterized by a high degree of circuit current that the circuit breaker is required to interrupt,
asymmetry and potential delayed current zero crossing. to qualify its performances adequacy in respect of:
Therefore, special care is required in “near-to-generator • Rated short-circuit breaking current (Isc).
circuit breaker” selection process, to ensure suitable • DC time constant of the rated short-circuit breaking
performances to effectively and safely interrupt the short- current.
circuit current. • Rated short-circuit making current.
Transient recovery voltage (TRV) and out-of-phase • Delayed current zero crossing phenomenon.
current are not discussed in this paper.
C. Theoretical background of a synchronous machine
under short-circuit condition
A. System architecture and Electrical topology
A widely established fact from technical literature is that
The common architecture of an electrical system that the equation below defines the natural asymmetrical short-
includes medium voltage synchronous machine is typically circuit current behaviour of a synchronous machine at no-
one or a combination of the followings: load (here specifically for a turbo generator):
• Generator/s connected to the National grid system
through unit step-up transformer/s. This is the typical 𝑡
√2 ∙ 𝑉𝑚𝑔 ∙ 𝑆𝑛 1 1 − 1 1 −𝑡
case of conventional power plant, synchronous 𝐼𝑎𝑠𝑦𝑚 = − ) 𝑒 𝑇"𝑑 + ( − ) 𝑒 𝑇′𝑑
{[(
3 𝑥" 𝑥′ 𝑥′ 𝑥
compensator plants, and industrial complex with √3 ∙ 𝑉𝑛 𝑑 𝑑 𝑑 𝑑
1 1 𝑡
internal generation capacity. −
𝑇
+ ] cos(𝜔𝑡) − ( ) 𝑒 𝑎 }
• Generator/s directly connected to a medium-voltage 𝑥𝑑 𝑥"𝑑
switchgear (e.g. at 11 kV level) where at the same
voltage level are connected: the incomer from high- Where:
voltage grid (via step-down transformer), the Sn rated generator apparent power.
distribution transformers (for load supply) and the Vmg / Vn maximum and rated generator line-to-
 line voltage. D. Reference IEC Standards
x"d, x’d, xd saturated direct axis subtransient,
transient and synchronous reactance’s. The following IEC Standards define the short-circuit
T"d, T’d short-circuit subtransient and transient current calculation procedure and the circuit breaker
time constants. performances.
Ta armature time constant. • IEC 60909-0 defines the procedure for calculating the
short-circuit currents in high-voltage and low-voltage
The equation describes the behaviour of both the three-phase a.c. systems with a rated frequency of
symmetrical and the DC components, which together 50 Hz or 60 Hz, excluding the installations on board
define the asymmetrical fault current that varies for each of ships and aeroplanes [1].
the three phases as illustrated in the next figure (Fig. 1). • IEC 61363-1 defines a procedure for calculating the
three-phase short-circuit currents of a.c. electrical
installations of ships and mobile and fixed offshore
units with a rated frequency of 50 Hz or 60 Hz [2].
• IEC 62271-100 defines the requirements applicable
to three-phase a.c. circuit-breakers designed for
indoor or outdoor installation and for operation at
frequencies of 50 Hz and/or 60 Hz on systems having
voltages above 1 kV [3].
• IEC/IEEE 62271-37-013 defines the requirements
applicable to three-phase a.c. generator circuit-
Fig. 1: asymmetrical fault current of the three phases breakers (defined as a circuit-breaker installed
between generator and associated step-up
The main factor that causes the high degree of transformer) designed for indoor or outdoor
asymmetry in proximity of a synchronous machine is installation and for operation at frequencies of 50 Hz
displayed in the following figure (Fig. 2). From the envelop and 60 Hz on systems having voltages above 1 kV
of the upper / lower waveform, the combined effect of the and up to 38 kV. It is applicable to generator circuit-
DC current component and the current amplitude reduction breakers that are installed between the generator and
can be observed. The first is the point of wave condition, the transformer terminals with rating equal to or
which is essentially the voltage phase angle at the fault greater than 10 MVA [4]. This Standard defines both
occurrence (in real-life it’s a random and uncontrollable the short-circuit current calculation procedure and the
phenomenon) while the second is determined by the circuit breaker performances.
subtransient and transient (and after a long time by the
synchronous) behaviour of the machine. Summarizing the above, two main types of high-voltage
Those two factors lead to high peak short-circuit current circuit breakers are defined: “circuit breaker” (in the
values and possible delayed current zero crossing (as following named “CB”) and “generator circuit breaker” (in
shown in example of below Fig. 2 not earlier than 5 cycles) the following named “GCB”) that comply with different
by shifting away from the zero value the current waveform. Standards, respectively [3] and [4].
At the same time, three short-circuit current calculation
methods are available. IEC 60909-0 specific for fixed
onshore installations, IEC 61363-1 specific for ships and
offshore installations, IEC/IEEE 62271-37-013 specific for
circuit breaker installed between generator and
transformer. Those methods, even if the physics are the
same and the electrotechnical concepts are invariable,
apply different mathematical equation and process.

II. SHORT-CIRCUIT CURRENT ANALYSIS

Fig. 2: max/min envelop of the worst-case phase
This paper presents a detailed case study, from the
The resulting degree of asymmetry of the fault current whole process of calculating the short-circuit current to the
can easily exceeds 100% value as shown in Fig. 3. comparison of the circuit breaker performance and the
examination of a possible delayed current zero crossing
condition.
The objective is to qualify the required performances of
the circuit breaker with potential alternative solutions.
ETAP power software is used for modelling and
simulations, which enables a detailed representation of the
system and relevant computations.

A. Studied case & topology

Fig. 3: % degree of the fault current
The architecture of the electrical system of this case
study is represented in the Fig. 4. It is composed by a 11 kV
switchgear at which are directly connected: the National
grid network, though a 132/11 kV transformer, one C. Overall short-circuit results
synchronous generator, one synchronous motor and two
distribution transformers for the plant auxiliaries. The data The TABLE II summarizes the rated and calculation
reflect a real-case project and some of them are shown in results of the total bus short-circuit current, which
the following Fig. 4. determine the thermal and peak ratings of the switchgear
[5].

TABLE II
RATINGS AND OVERALL SHORT-CIRCUIT RESULTS
Rated Rated Calculated initial
Calculated
short-time peak symmetrical
peak short-
withstand withstand short-circuit
circuit current
current current current
40 kA 100 kA 36.6 kA 94.7 kA

D. Current breaking capacity

The system has a short-circuit rating of 40 kA, and the

target is to verify if a circuit breaker “CB” (not specifically a
generator circuit breaker “GCB”) with a rated short-circuit
Fig. 4: simplified single line diagram of the electrical system breaking current of 40 kA can suit one or more of the circuit
breakers listed in TABLE I. Alternatively, for one or some
This analysis covers on all the 11 kV circuit breakers, of them, the next standard ratings or a generator circuit
which are named in this paper as detailed in TABLE I: breaker “GCB” with special interrupting performances may
be required, which would have cost and engineering
TABLE I implications.
INVESTIGATED 11 kV CIRCUIT BREAKERS The following TABLE III summarize the performances of
Name Description the circuit breaker “CB” that will be compared with the next
CB-TR Transformer incomer circuit breaker calculated fault currents, with the aim to confirm where
CB-GEN Generator incomer circuit breaker possible, its adequacy.
CB-AUX Auxiliary transformer feeder circuit breakers Since the making current is higher than the total
CB-MOT Synchronous motor feeder circuit breaker maximum peak current indicated in TABLE II (over-
conservative result), it is validated.
B. Methodology for calculation TABLE III
CIRCUIT BREAKER PERFORMANCES
This case study does not correspond to IEC 61363 Symmetrical Asymmetrical Minimum
Making Time
applications (onshore installation) and to IEC 62271-C37- breaking breaking operating
current constant
13 (generator coupled to the network without a dedicated current current time
step-up transformer and with a synchronous motor at same 40 kA 46.3 kA 100 kA 45 ms 40 ms
voltage level).
Consequently, the only applicable IEC Standard for The first step is to qualify the potential operating
short-circuit current calculation methodology is the IEC condition of the plant:
60909-0. However, this Standard uses simplified equations
to describe the fault current behaviour that don’t take into TABLE IV
account some significant transient aspects. REQUIRED OPERATING CONDITIONS
For such reason, the predictability of the symmetrical Name Description
(and consequently asymmetrical) short-circuit current at Normal Normal scenario. Generator and synchronous motor
the fault clearing time is not optimal for the purpose of the running in parallel with National grid.
No-gen Generator out of service.
case study.
System supplied by National grid.
Even if IEC 61363 and IEC/IEEE 62271-37-013 Island Not allowed scenario. The system cannot operate in
procedures should theoretically both not applicable, those island from the National grid and therefore is not
two methods (that are almost identical from synchronous considered for verification purpose.
machine modelling point of view) provide more suitable and
accurate representation of the effective transient fault The assessment of current breaking capacity requires a
current phenomenon. Consequently, in this case study was different approach than the usual short-circuit current
decided to apply the equations provided by IEC 61363 to calculation used to determine the component sizing (e.g.
determine the symmetrical and asymmetrical current switchgear). Here, it is essential distinguish the effective
behaviour, along with zero crossing condition. fault current to be interrupted, depending on the fault
location and considering exclusively the fault current that
each circuit breaker is required to interrupt.
The second step, then, is to determine the required case
studies for each circuit breaker (see the Tables V, VI, VII
and VIII below) based on the fault locations shown in the
next Fig. 5.
 Practically, the following tables summarize the results of
case studies described in Table V, VI, VII and VIII which
are compared with circuit breaker performances shown in
TABLE III.
For the cases where generator contribution is
considered as “Fault current source”, three extra subcases
are investigated to consider different generator pre-loading
conditions: “no-load”, “PF lead” and “PF lag”. The results
highlight that the highest value of asymmetry occurs when,
prior to the fault, the generator is operating in underexcited
mode, with a leading power factor. Under such a condition,
the AC component of short-circuit current is lower than the
assigned AC component of the rated generator-source
short-circuit breaking current. In the case where the
generator is carrying load with a lagging power factor prior
Fig. 5: fault locations identification to the fault, the degree of asymmetry will be lower, but the
AC component will be higher.
TABLE V
CASE STUDY FOR “CB-TR”
TABLE IX
Fault position Scenario Fault current sources RESULTS FOR “CB-TR”
- Generator Fault Isym Iasym % deg.
Point 1, secondary side Normal - Synch. motor Scenario Gen.
pos. [kA] [kA] asym.
11 kV transformer - Aux. (negligible)
no-load 15.2 30.6 124% OK
terminals - Synch. motor
No-gen Normal PF lead 14.9 30.4 125% OK
- Aux. (negligible) Point 1
PF lag 15.8 30.8 119% OK
Point 2, 11 kV switchgear All - National grid
No-gen n/a 9.50 20.0 131% OK
Point 2 All n/a 14.2 17.8 53% OK
TABLE VI
CASE STUDY FOR “CB-GEN”
TABLE X
Fault position Scenario Fault current sources
RESULTS FOR “CB-GEN”
Point 2, 11 kV switchgear Normal - Generator
Fault Isym Iasym % deg.
- National grid Scenario Gen.
Point 3, generator pos. [kA] [kA] Asym.
Normal - Synch. motor
terminals no-load 5.59 10.6 114% OK
- Aux. (negligible)
Point 2 All PF lead 5.36 10.4 118% OK
PF lag 6.20 10.9 102% OK
TABLE VII Point 3 Normal n/a 23.7 36.85 84% OK
CASE STUDY FOR “CB-MOT”
Fault position Scenario Fault current sources
Point 2, 11 kV switchgear All - Synch. motor
TABLE XI
RESULTS FOR “CB-MOT”
- National grid
Normal - Generator Fault Isym Iasym % deg.
Scenario Gen.
- Aux. (negligible) pos. [kA] [kA] Asym.
Point 4, motor terminals
- National grid Point 2 All n/a 9.40 19.9 132% OK
No-gen no-load 19.9 28.0 70% OK
- Aux. (negligible)
Point 4 Normal PF lead 19.7 27.7 70% OK
PF lag 20.5 28.4 68% OK
TABLE VIII
CASE STUDY FOR “CB-AUX”
Fault position Scenario Fault current sources TABLE XII
Point 2, 11 kV switchgear All - Aux. (negligible) RESULTS FOR “CB-AUX”
- National grid Fault Isym Iasym % deg.
Scenario Gen.
- Generator pos. [kA] [kA] Asym.
Normal Point 2 All n/a ~0 ~0 n/a OK
- synch. motor
Point 5, aux. transformer no-load 29.2 47.3 90% NO
- aux. (negligible)
terminals Point 5 Normal PF lead 29.3 47.4 90% NO
- National grid
No-gen - Synch. motor PF lag 29.1 47.2 90% NO
- Aux. (negligible)
The first result is that the degree of asymmetry is greater
Increasing the level of complexity (but it’s necessary), than 100% for all the cases where the fault current only
additional subcases are examined by considering how the includes the synchronous machines contribution, while the
fault current results are affected by the generator pre- combination with National grid short-circuit contribution
loading condition. The three subcases are: no-load and full decreases this value to a maximum of 90%.
load with either rated power factor lead or lag. The used The second result is that the “CB-AUX” shows the
method is suitable for represent all those conditions. highest value of the short-circuit current as related to the
In the following tables, and this we can name third step, total of all the three main fault current sources
are reported the calculated symmetrical and asymmetrical (synchronous machines and National grid).
short-circuit currents, and the degree of asymmetry that The consecutive next step, in term of mitigation and
each circuit breaker is required to interrupt. optimization, is to move to the next rating level if feasible
These results are considered at 40 ms, the minimum (or to a circuit breaker with higher performance) or else,
time at which the circuit breaker may be initiate to open. find the latest possible minimum time for the circuit breaker
Longer times would lead to less critical results. contacts separation initiation, to wait the lowering of the
short-circuit current within the desired performances of
TABLE III.
The following TABLE XIII, investigated the same results
of above TABLE XII at 50 ms (instead of 40 ms).

TABLE XIII
RESULTS FOR “CB-AUX” with CPT = 50 ms
%
Fault Isym Iasym Fig. 6: National grid only
Scenario Gen. deg.
pos. [kA] [kA]
Asym.
no-load 28.6 44.7 85% OK
Point 5 Normal PF lead 28.7 44.7 85% OK
PF lag 28.5 44.5 85% OK

The results show that applying an additional 10 ms delay

to the CB-AUX minimum opening time is enough to
achieve satisfactory results. Therefore, for all cases, the
circuit breaker “CB” with the performances described in
Fig. 7: Generator only
TABLE III are validated, in terms of breaking current
(symmetrical and asymmetrical). In other words, in term of
magnitude of fault current values, there are not restrictions
under short-circuit condition to use a standard circuit
breaker “CB” instead of a specific generator circuit breaker
“CGB”.

E. Delayed current zero crossing

Fig. 8: Synchronous motor only
The fourth step is the delayed current zero crossing
assessment.
As well-known from the technical literature, the
generator circuit breaker “GCB” is designed with the
capability to interrupt short-circuit current with delayed zero
by forcing it to cross the zero. On the contrary, a circuit
breaker “CB” is not designed for that and is essential to
ensure that, at the opening initiation, the short-circuit
current naturally crosses the zero.
Therefore, the actual natural zero crossing for each Fig. 9: National grid + Generator
circuit breaker (shown in TABLE I) will be calculated for
faults on both source side and load side, based on the
potential combination of fault current sources and system
configurations: results are summarized on TABLE V to
TABLE VIII.
In next TABLE XIV are summarized all the plotted
current waveforms investigated for this assessment.

TABLE XIV
INVESTIGATED DELAYED CURRENT ZERO CROSSING Fig. 10: National grid + Synchronous motor
Figure Description of short-circuit current sources
Fig. 6 National grid only
Fig. 7 Generator only
Fig. 8 Synchronous motor only
Fig. 9 National grid + Generator
Fig. 10 National grid + Synchronous motor
Fig. 11 Generator + Synchronous motor
Fig. 12 National grid + Generator + Synchronous motor

Fig. 11: Generator + Synchronous motor

Fig. 12: National grid + Generator + Synchronous motor

 The results of the above plots are translated in individual that may occurs or not, on the same installation, in the
case studies result (from TABLE XV to TABLE XVIII). same configuration, for the same fault type, depending on
the voltage waveform at the instant of fault occurrence.
TABLE XV
ZERO CROSSING TIME FOR “CB-TR”
Fault position Scenario Zero crossing F. Impact of protective philosophy
Point 1, 11 kV transformer Normal 120 ms
terminals No-gen 140 ms The assessment of the circuit breaker performance
Point 2, 11 kV switchgear All <20 ms cannot be disregarded from the deep understanding of the
protective system and philosophy.
TABLE XVI The achievement of fast acting protection system (short
ZERO CROSSING TIME FOR “CB-GEN” fault clearing time) combined with a proper selectivity
Fault position Scenario Zero crossing (coordination) is a desirable condition for a series of
Point 2, 11 kV switchgear Normal 80 ms
reasons (minimize the damages, increase human safety,
Point 3, generator terminals Normal <20 ms
mitigate arc-flash effects, reduce system disturbances,
increase rotor angle stability, etc…).
TABLE XVII The moder protection technology allows to achieve
ZERO CROSSING TIME FOR “CB-MOT”
satisfactory both fast fault clearance and coordination in
Fault position Scenario Zero crossing
medium voltage system, using zone differential protection
Point 2, 11 kV switchgear Normal 140 ms
Point 4, motor terminals All <20 ms
and logical selectivity (independently or combined).
These techniques typically allow to get fault clearance in
less than 150-200 ms with logical selectivity, and less than
TABLE XVIII
ZERO CROSSING TIME FOR “CB-AUX” 100 ms with differential (zone) protections.
Fault position Scenario Zero crossing The protection scheme of this case study is displayed in
Point 2, 11 kV switchgear Normal n/a the following Fig. 14 where are exclusively shown the
Point 5, aux. transformer protective function relevant to phase faults. The main
All <20 ms equipment (transformer, generator and synchronous
terminals
motor) are equipped with a differential protection
The first important result is that, in all the cases where (respectively ANSI code 87T, 87G, 87M). The auxiliary
the fault current includes the short-circuit contribution of the outgoing feeders are equipped with phase overcurrent
National grid, the current crosses zero within the first cycle. (ANSI code 50). Generator and incomer from National grid,
With a conservative approach, further analysis is are moreover equipped with phase overcurrent (50) for
performed by considering the maximum reduction of the busbar fault clearance (and for coordination back-up).
short-circuit power of the National grid (20% lower than the
provided value). The results, shown in the following figure
(Fig. 13), consider the worst-case scenario (generator +
motor + National grid). The observed conclusion is that the
zero crossing moves from 20 ms to 40 ms: not a big impact
considering the effective expected trip time of the “CB-
AUX” (50 ms) that’s the one for which this case study apply.

Fig. 13: National grid + Generator + Synchronous motor, Fig. 14: simplified protection single line diagram
sensitivity analysis
Summarizing, for this specific case, the following fault
Vice versa, in all the cases where the fault current is clearance times are expected:
composed by the synchronous machines only (generator
or the synchronous motor or the combination of the two), TABLE XIX
the short-circuit current zero crossing occurs in a range PROTECTION TRIP COMMAND TIME
from 80 ms to 140 ms. For delayed current zero crossing, Fault position Trip by Trip command
Point 1, 11 kV transf. terminals 87T 30 ms
the “CB-AUX” is the less critical one as there are no
Point 2, 11 kV switchgear 50 300 ms
restrictions to use a standard circuit breaker “CB” while, for
Point 3, generator terminals 87G 30 ms
all the others, needs to be verified the effective opening
Point 4, motor terminal 87M 30 ms
time by adding the protection trip time: if it will be lower than Point 5, aux. transformer terminals 50 20 ms
the calculated 80-140 ms, the installation of a generator
circuit breaker “GCB” may be required.
For Point 2, 11 kV switchgear, the considered trip
As a general note, the calculated delayed current zero
command time of 300 ms for the operation of transformer
crossing refers to the worst-case for one phase only based
on point of wave. Therefore, is a “random” phenomenon
incomer and generator phase overcurrent protection (50) III. SENSITIVITY ANALYSIS
ensure coordination with the other protections.
On breaker performance evaluation, is not intentionally The following additional technical consideration and
listed above the arc-detection system (that’s effectively sensitivity analysis are discussed.
installed on busbar compartment of the switchgear) even if
are even faster than a differential protection. However, A. Impact of armature resistance
since the trip command results where arc is released in air,
being the arc-plasma is characterized by high-resistivity One of the key factors determining the degree of
component, the level of asymmetry and delayed zero asymmetry and delayed current zero crossing is the
crossing are naturally mitigated. armature time constant of a synchronous machine.
With the target to provide a sensitivity analysis on this
parameter, sets of results (plots) are provided for a range
G. Final results of values of armature time constant from 50 ms to 300 ms,
by considering the same electrical parameters of the
In this case study, are investigated the current breaking generator.
capacity and the delayed current zero crossing condition The first plot provides the DC short-circuit current profiles
for the four circuit breakers investigated, with following that determine the increase of both the peak value of the
results: short-circuit current and the related zero crossing.
“CB-TR”: Symmetrical, asymmetrical, and making
currents are within the circuit breaker ratings in all the
cases, at time 40 ms. Delayed current zero crossing exists
for source side fault (Point 1, TR terminal 11 kV), where the
current crosses zero in approx. 140 ms (worst-case).
Differential protection 87T is expected to operate in this
case and send the trip command in 30 ms. The following
two alternative solutions can be applied:
1. select a circuit breaker “CB” type with performance
as per TABLE II and impose to the protection setting
a minimum intentional delay time of 120 ms.
2. select a generator circuit breaker “GCB” type, Fig. 15: DC short-circuit current [kA]
without any extra delay requirement.
“CB-GEN”: Symmetrical, asymmetrical, and making The following plot shows the profile of the degree of
currents are within the circuit breaker ratings in all the asymmetry (as a percentage) where is evident that a higher
cases, at 40 ms. Delayed current zero crossing exists for a values of armature time constants determine a level of
system side fault (Point 2, 11 kV switchgear), where the asymmetry above 100%.
current crosses zero in approx. 80 ms. In this case, the
protection system will not act before 300 ms and thus, a
circuit breaker “CB” type with performance as per TABLE
II is adequate and there is no need to install a generator
circuit breaker “GCB”.
“CB-MOT”: Symmetrical, asymmetrical, and making
currents are within the circuit breaker ratings in all the
cases, at 40 ms. Delayed current zero crossing exists for a
source side fault (Point 2, 11 kV switchgear), where the
current crosses zero in approx. 140 ms but in this case, the
protection relay of the synchronous motor will not directly
operate on it (eventual intertrip in long time ≥300 ms). Fig. 16: Degree of asymmetry of short-circuit current [%]
Consequently, also in this case, a circuit breaker “CB” type
with performance as per TABLE II is adequate and there is The last plot describes the instantaneous current profile
no need to install a generator circuit breaker “GCB”. on the worst-case phase, with relevant impact in term of
“CB-AUX”: Symmetrical, asymmetrical, and making delayed zero crossing. Higher values of armature time
currents are within the circuit breaker ratings in all the constant postpone the zero crossing: 20 ms (1 cycle) for a
cases, at 50 ms (for 40 ms the requirements are not Ta = 50/100 ms, 40 ms (2 cycle) for a Ta = 150 ms, 60 ms
satisfied). No delayed zero crossing condition subsists, and (3 cycle) for a Ta = 200 ms, 100 ms (5 cycle) for a Ta =
thus, a circuit breaker “CB” type with performance as per 250 ms, 140 ms (7 cycle) for a Ta = 300 ms.
TABLE II is adequate with a minimum trip time of 50 ms.

Fig. 17: Instantaneous short-circuit current [kA]

 The above results show also that the impact of the
armature time constant is not linear and generally
recommends attention selecting the circuit breaker in
proximity of a synchronous machine with a high value of
armature time constant.
For statistical data, the typical values of the armature
time constant, based on more than 75 synchronous
generator datasheet of 50/60 Hz are in a range from 1 to
70 MVA, are here below displayed. The value of armature
time constant is not linear with the generator rating,
however it tends to increase for larger rating, as well known Fig. 20: impact of the pre-loading condition on the
from technical literature. generator delayed current zero crossing

C. Problem propagation through a transformer

The near-to-generator problem, embed in the wording a

non-scientific boundary. The meaning and the numerical
value of the word “near” is not of obvious interpretation and
arises the natural questioning, if all the above problems can
(and how much) propagated from the generator terminals
to the various parts of the plant.
The typical case of interest for potential propagation of
the near-to-generator concern is through a transformer
Fig. 18: armature time constant for various generator (e.g. a generator step-up transformer).
rating As intuitive, normally a power transformer is enough to
mitigate the problem. The two reasons in behind are: the
X/R ratio (that in a transformer is commonly lower than in a
B. Synchronous machine pre-loading condition generator) speed-up the DC current decay, and the series
impedance circuit (that reduces the ratio between the
As shown in this paper, the pre-loading condition of the equivalent subtransient, transient and synchronous
generator significantly affects the calculation results. reactance’s) mitigation effect is discussed in initial Fig. 2.
Since the generator’s short-circuit current contribution is The following two plots, obtained with a step-up
a direct consequence of its electromotive force, directly transformer with a size 10% greater than the generator
correlated to both the excitation condition and the machine one, qualify both phenomena combined. Apart the numeric
reactance’s, any operating point, that determines a higher value itself, that’s obviously lower with the series between
excitation condition, causes a higher magnitude of the generator and transformer, the concept to be read is the
short-circuit current. The following Fig. 19 describes the trend and the qualitative effect. Also important, the fault
symmetrical short-circuit current of a generator at two current zero crossing move from 80 ms (4 cycle) to 20 ms
different pre-loading conditions (extreme cases) at no-load (1 cycle).
and at full-load with 0.80 lag power factor.

Fig. 21: impact of the unit-transformer on the generator

Fig. 19: impact of the pre-loading condition on the symmetrical short-circuit current
generator symmetrical short-circuit current

Therefore, a reduced magnitude of the short-circuit

current, even with almost the same degree of asymmetry,
conditions also the delayed current zero crossing. The
following Fig. 20 confirms that the no-load case (zero
crossing at 80 ms) is a worst-case respect to the full load
with lead power factor (zero crossing in first cycle).

Fig. 22: impact of unit-transformer on delayed zero

crossing from a generator
 IV. CONCLUSIONS V. REFERENCES

The proximity (near-to) of a synchronous machine [1] IEC 60909-0 “Short-circuit currents in three-phase
requires careful selection of the circuit breaker a.c. systems – Part 0: Calculation of currents”, 2016.
performances. [2] IEC 61363-1 “Electrical installations of ships and
Such verification requires several analyses discussed mobile and fixed offshore unit – Part 1: Procedures
step-by-step in this paper, where the electrical system was for calculating short-circuit currents in three-phase
modelled and studied in ETAP software. a.c.”, 1998.
The investigation of the short-circuit current magnitude [3] IEC 62271-100 “High-voltage switchgear and
compared with the target circuit breaker performances, the controlgear – Part 100: Alternating-current circuit-
delayed current zero crossing phenomena and the breakers”, 2021.
required circuit breaker opening time for the various [4] IEC/IEEE 62271-37-013 “High-voltage switchgear
locations of a fault, leads to a non-intuitive result. and controlgear – Part 37-013: Alternating current
In the studied network (Fig. 4 on page 3), for the circuit generator circuit-breakers”, 2021.
breakers located at the generator incomer and at the [5] IEC 62271-1 “High-voltage switchgear and
synchronous motor feeder, a circuit breaker “CB” controlgear – Part 1: Common specifications for
responding to the requirement of IEC 62271-100 is alternating current switchgear and controlgear”,
adequate. On the contrary, the outgoing feeder to the 2017.
auxiliary transformer is the one with the highest short-circuit
current breaking performance request and is II. VITA
recommended to apply an extra delay time to the protection
settings to prevent the needs of increase the circuit breaker Giovanni Gambirasio, M.Eng., P.Eng., is the Director of
rating. However, also in this case and with this additional SELECTY, a consulting company with worldwide business
delay (minor) a circuit breaker “CB” responding to the specializing in Power System studies and protection relay
requirement of IEC 62271-100 is adequate. The incomer coordination, across all the energy sectors. He has +10
from the transformer (from National grid) requires more years of professional experience in conducting and
performances in term of delayed zero crossing, up to reviewing different types of electrical power system studies
require a minimum delay to be applied to the transformer such as Load Flow, Short-circuit, Motor acceleration,
differential protection of 120 ms (to allow the use of circuit Transient Stability, Load Shedding, Generator Dynamic
breaker “CB” responding to the requirement of IEC 62271- model validation, Power Harmonics, Protection relay
100), or to select a generator circuit breaker “GCB” Coordination, CT & VT verification, and Arc-Flash hazard.
responding to the requirement of IEC 62271-C37-13. Since 2018, ETAP Representative for Italy and certified
Under this specific case study, the presence of the ETAP instructor for advanced class.
National grid connection (for which a sensitivity analysis gambirasio.giovanni@selecty.it
has been done) and the not-allowed island operation,
significantly simplify the calculation process and provide Mauro Giuseppe Codoni is a Senior Electrical Engineer
many benefits in term of results. In this sense, it is obvious focused on Power System Studies part of SELECTY’s
that any system operating in electrical island, is normally team, based in Bergamo, Italy. He has 15 years of
more subjected to this phenomenon, and for example in a professional experience in electrical engineering of
case like this one, will require special performances and industrial plants (oil & gas, cement, and power generation
potentially the requirement of a “GCB” an all the circuit application). Before joining Selecty he worked for an EPC
breakers, with a potential exception of the generator Company active in the oil & gas industry and thermal power
incomer circuit breaker “CB_GEN” only. plant, a major cement Company and an ORC power
This paper also includes a sensitivity analysis of the main generation unit Manufacturer. Mr. Codoni received his
parameters affecting the circuit breaker selection, along M.Sc degree (Electrical Engineering, 2006) at the
with the discussion of the concept of “near-to-generator” Polytechnic University of Milan and Certified professional
with a more scientific approach. Engineer for the State of Italy since 2007.
codoni.mauro@selecty.it