1. MODERN MOTOR BUS BAR TRANSFER TESTING AND
VERIFICATION – PART I IN-PHASE TRANSFER TESTING.
J.P. van Rooyen*, J.J. Walker**
* Dept. of Power Engineering, Vaal University of Technology, Private Bag X021, Vanderbijlpark,
1911, South Africa
** Dept. of Power Engineering, Vaal University of Technology, Private Bag X021, Vanderbijlpark,
1911, South Africa
Abstract: This paper focuses on the creation of a testing protocol or procedure for verification of an
in-phase transfer method used on a modern motor bus bar transfer scheme. The voltage decaying theory
is shortly discussed and an example of an actual recorded decaying voltage on a motor bus bar system
is given. A short description of the other available transfer methods is given but with emphasis on the
in-phase transfer method. The estimated time of phase coincidence is calculated, with a simulation to
verify this calculation. Similarly, a test protocol to test this transfer method is provided with
recommendations given for future research work and testing.
Key words: In-phase transfer, first coincidence, voltage decay.
1. INTRODUCTION
In today’s modern production driven environment, it is
important to have a continuous supply of electrical power
feeding critical plant loads [11]. This means that a safe and
reliable method of testing and maintaining motor bus bar
transfer schemes needs to be implemented to ensure
continuity of this supply. Alternatively, a generic testing
protocol might be used to test and verify these schemes still
in the installation phase or on an annual basis. A simple
diagram displaying a basic motor bus bar transfer scheme
can be seen in Figure 1-1. Indicators A, B, C and D are
used to indicate circuit breakers in Figures 1-1, 1-2 and 1-
3. The use of these circuit breakers is essential in the
operation of the modern motor bus bar transfer scheme.
Two incoming transformers, one operating as the normal
feeding transformer connected to circuit breaker “A” and
the other as the standby emergency incoming transformer
connected to circuit breaker “C”, provide electrical power
to the motor bus bar load. Motor bus bar load can be seen
as a system of motors which are predominantly inductive
feeding from the same source or system on a bus bar [7].
Normally, only most critical plant motor loads would be
associated with a modern transfer scheme. This implicates
that the critical motors need to be supplied by electrical
power continuously by changing the source from circuit
breaker “A” to circuit breaker “B” as fast and safe as
possible in the event of normal incoming supply failure on
circuit breaker “A”. This operation will only be true if
circuit breaker “C” is running normally closed (N/C) and
circuit breaker “B” is running normally open (N/O) during
normal conditions as displayed in Figure 1-2 [2]. A High-
Speed Transfer Device (HSTD) as seen in Figure 1-2 will
be mainly responsible for the transfer of the motor bus bar
load between circuit breaker “A” and circuit breaker “B”.
Figure 1-1: General Arrangement for Motor Bus Bar
System
Figure 1-2: Motor bus bar transfer system operating in
normal operation
In the event a fault develops on the transformer connected
to circuit breaker “A” the HSTD will issue an open
command to circuit breaker “A” and a closing command
to circuit breaker “B”. This scenario can be seen as a
“break-before-make” transition such that the system will
not feed back into the existing fault. During this transition,
the HSTD will monitor all circuit breaker contacts

2. measuring opening and closing times and the state of each
breaker to determine if the transfer was successful or not
[2]. In the event that circuit breaker “A” does not open
when an open command is issued, suitable protection must
be in place to open the last circuit breaker closed, thereby
preventing a paralleled operation of the two incoming
transformers [9]. A successfully transferred system can be
seen in Figure 1-3.
Figure 1-3: Motor bus bar transfer system operated under
transferable conditions
Section 2.1 of the paper addresses the basic theory behind
motor bus bar transfer and its operating principle as well
as a discussion of an actual motor bus bar voltage run down
recording which is given in section 2.2. Section 2.3 of the
paper briefly describes the different transfer methods
available. A brief testing protocol was discussed during
section 2.4 and the mathematical approach given to obtain
an in-phase transfer verifying parameter. All test results
ran were also displayed in a tabular form with a discussion
in section 2.5. The paper ends off in section 2.6 with a short
discussion of the what was set out to be achieved.
2. THEORY OF VOLTAGE DECAYING TRANSFER
2.1 THE EFFECT OF DRIVE INERTIA
A running loaded induction motor under open circuit
conditions has a certain amount of mechanical and
electrical energy stored in the rotating parts of the motor
and the load inertia it is connected to [14]. The frequency
and residual voltage on the stator terminals decays at a rate
which is directly related to the motor drive speed and
therefore the inertia of the motor load [14]. “Inertia can be
seen as a body’s resistance to a change in velocity “[8].
This decaying characteristic decays exponentially, if a
constant decaying frequency is assumed, according to the
motors time constant. The time constant reduces by the
change in speed which depends on the rate of change of
the decaying bus voltage [5]. A decaying voltage is mainly
produced during an open circuit condition of an induction
motor where a generator-like behaviour will be brought
forward by the load inertia turning the rotor shaft of the
motor [6]. The open circuit voltage characteristic is
comparable to the voltage profile in Figure 2-1 where the
faulted supply bus residual voltage would be maintained
across the motor bus bar terminals but of a decaying form
in voltage magnitude, frequency and phase-angle [14].
Figure 2-1: Typical Characteristic of a Voltage
Decaying Bus
When looking at the motor bus bar as a system connected
to each other for a short period of time it can be seen that
the higher the speed on the motor, due to the load inertia
connected to the motor, compared to its corresponding bus
voltage frequency the more this specific motor acts as a
generator [17]. Similarly, on the same motor bus bar
system the motors running at a lower speed than that of the
corresponding bus frequency are acting as induction
motors. A similar characteristic profile of the decaying bus
voltage will be obtained until the motors are forced to fall
out due to under voltage limitations [1]. Most motor bus
bar transfer schemes are designed and based on the
American National Standards Institute (ANSI) C50.41
standard which dictates that the maximum criterion for a
bus transfer to be successful must be at least 1.33pu V/Hz
[5]. This standard allows a criterion for a safe transfer for
all the relevant transfer methods relying on the decaying
voltage [3]. At different voltage magnitude values
compared to the phase angle and frequency, the Intelligent
Electronic Device (IED) will enable and disable the
specific required zone of transfer required as indicated by
Figure 2-2 [17].
Figure 2-2: Transfer stages based on voltage decaying
theory displayed on an axis which compares voltage
magnitude, phase angle and time.

3. 2.2 MOTOR BUS BAR VOLTAGE DECAY
RECORDING
It was decided that actual decaying voltage measurements
had to be done to prove the voltage decaying theory being
investigated. The opportunity to measure actual motor bus
decay occurred when a fault developed on one of the motor
feeders feeding from a 6.6 kV incomer Similar to the flow
chart displayed in Figure 2-3. The IED on which the
measurement was recorded was on the 525V incomer
feeding from the same bus bar system as the motor feeders
making use of the IED fault recorder, downloading the
fault record and then analysing it with SIGRA Fault
Record Evaluation software. The secondary voltage
recorded by the IED has a nominal Line voltage of
110Vac, thus secondary phase voltage is 63.5Vac.
Figure 2-3: General switchgear arrangement including
protection IED flow chart
This was possible because the design arrangement allowed
the transformer feeder to remain closed during an under
voltage, only when it receives an inter trip or lockout
condition should it be tripped via protection. The faulted
motor feeder didn’t operate when a protection trip was
issued as it was intended to and the way the protection
settings was structured caused the particular incomer to
trip and open thereby locking out the transfer. On the 525V
voltage level which was being supplied by a 6.6/0.525kV
transformer a relay captured the under-voltage condition
which continued for a period of 1.2 Seconds. The motor
feeding buckets on the 525V level was designed to stay
electrically latched and fall out at approximately 80% of
Un (nominal voltage value). During this scenario, all the
525V motors on the relevant incoming section unlatched
and did not contribute to the voltage decay on the still
connected system which must now be viewed as an
asynchronous system. On the 6.6 kV section, the motor
feeder contactors were designed to stay latched until a trip
is issued or mechanically forced to open. These contactors
stayed closed until an under-voltage trip was issued at 1.2
Seconds after the under voltage on each individual relay
picked up at approximately 60% of Un. The 1.2 Seconds
was a pre-determined definite time setting programmed to
each relay used to protect the motors against under voltage
scenarios over a period of time. It is debatably if the setting
of 1.2 Seconds under voltage trip time is correct. Because
of the mechanically latched contactor motor feeders on the
6.6 kV level, the major contributors to the asynchronous
system was the 6.6 kV motors running at that instance of
time. The signal measured by the fault recorder over a
period of 1.2 Seconds was a combination of all the motor
feeders on the relevant side of the motor bus. The complete
voltage decay recording can be seen in Figure 2-4 and
Figure 2-5.
Figure 2-4: Complete motor bus decaying voltage
recording displayed in R.M.S. mode
Figure 2-5: Complete motor bus decaying voltage
recording displayed in instantaneous mode
2.3 MODERN TRANSFER METHODS
(1) Fast Transfer: the fast transfer method can be referred
to as simultaneous transfer or the safer method sequential
transfer [3]. During a simultaneous transfer, the opening
signal of the normal circuit breaker feeding the board and
the closing signal of the auxiliary/standby circuit breaker
are sent at the same time [12]. This method is the fastest
method of transferring plant loads to the auxiliary supply,
but there is a chance of paralleling the two sources with the
total transfer time only dependant on the opening and
closing times of the two circuit breakers [15]. It is therefore
advised that a suitable circuit breaker failure scheme be
employed to cater for such a scenario [13]. The sequential
fast transfer will wait for a contact known as an auxiliary
contact which will open in sequence with the “breaker
open” auxiliary contact [10]. Fast-sequential transfer may
be the safest fast transfer method but with a speed
disadvantage of only 5 – 10 cycles rather than the 1 – 2
cycles of the fast-simultaneous transfer [13].
(2) In-phase Transfer: If the fast transfer opportunity is
missed, the IED will select the next suitable transfer
method, which will be the in-phase transfer method. The
in-phase transfer will wait for the first opportunity of the
phase angle rotating the 360-degree axis [15]. Once the
angle is within the predefined closing limits, the transfer
command will be issued [3]. Typically, the relay
parameters are set so that the phase angle limit must not

4. exceed the 25-degree window of phase difference and
voltage decay below 75% for an in-phase transfer to be
successfully initiated [2].
(3) Residual Voltage and Slow Transfer: Residual voltage
transfers can be seen as a safer option to transfer the system
but is with a disadvantage in speed as compared to the fast
and in-phase transfer methods [16]. This time-consuming
transfer ignores the phase angle and only verifies if the
voltage has decayed very fast to a point of at least 30-35%
of the nominal voltage at which point the IED will initiate
a residual voltage transfer. At this stage, load shedding
may be necessary to limit excessive inrush currents due to
large inertia motor loads connected to the motor bus [4].
Whereas the slow transfer will wait for a pre-determined
time and issue a transfer regardless of bus voltage and
phase angle.
2.4 IN-PHASE TRANSFER TESTING
a) Mathematical transfer time estimation:
The mathematical method used, as seen in equation 1 and
2 determines an estimated time of synchronising of two
different sinusoidal waveforms. The values used were
chosen such that a fast transfer could not occur and a
comfortable transfer measuring time would be brought
forward before the slow transfer could be activated. The
two wave form parameters needed for the IPT test will be
110𝑆𝑖𝑛(2𝜋(50)𝑡) and 110𝑆𝑖𝑛 (2𝜋(49.5)𝑡 +
𝜋
4
) of
which the faulted side bus voltage frequency was adjusted
to 49.5Hz and a phase difference starting position of 45°
was introduced to disable the fast transfer opportunity.
Keeping the frequency of both sine waves constant an
estimated in-phase time can be obtained. By using the k
method solving 𝑡 in terms of 𝑘 to obtain equation 1:
110𝑆𝑖𝑛(2𝜋(50)𝑡) = 0
Cancelling 110𝑆𝑖𝑛 = 0 and equating 𝑘𝜋 to the
remaining equation we have that:
(2𝜋(50)𝑡) = 𝑘𝜋
Solving 𝑡 we have that:
𝑡 =
𝑘
100
(1)
Where:
𝑡 = time
𝑘 = Non-Negative Integer
By using the k method solving 𝑡 in terms of 𝑘 to obtain
equation 2:
110𝑆𝑖𝑛 (2𝜋(49.5)𝑡 +
𝜋
4
) = 0
Cancelling 110𝑆𝑖𝑛 = 0 and equating 𝑘𝜋 to the
remaining equation we have that:
99𝜋𝑡 +
𝜋
4
= 𝑘𝜋
Solving 𝑡 we have that:
𝑡 =
𝑘
99
−
1
396
(2)
Where:
𝑡 = time
𝑘 = Non-Negative Integer
Solving Simultaneous equation to obtain the value of k:
𝑘
99
−
1
396
=
𝑘
100
4𝑘 − 1 = 396𝑘
𝑘 = 25
Substituting 𝑘 into (1) or (2) we have that 𝑡 = 0.25𝑆𝑒𝑐.
Figure 2-5 indicates the simulation run with SIMetrix to
verify this calculation and it can be seen that estimated
time for the two wave forms to be in-phase is 250ms. It
should be noted that a suitable tolerance would have to be
incorporated when measuring signals secondary to the IED
as there will be a relatively small time delay between the
actual signal and measured signal.
Figure 2-5: Synchronising of 50Hz and 49.5Hz injection
for in-phase transfer testing starting at a 45° phase
difference within estimated 250ms
2.5 IN-PHASE TRANSFER TESTING AND
VERIFICATION PROTOCOL
The method of testing will rely on the mathematical
transfer time estimation as indicated. It is important to do
this calculation with the individual and appropriate
parameters unique to the installation and breaker operating
times. With the in-phase transfer synchronising time
calculated the tests can begin.
(a) In-phase transfer time test: The three-phase injection
set must be programmed in such a way that two steps for
each individual test will be obtained. Step 1 will only be
an unhealthy transfer condition where by step 2 will allow
for a gradual change in frequency still at an unhealthy
condition. By adjusting the phase-angle maximum limit
outwards of the tolerable window the fast transfer
opportunity is disabled. Both the frequency and phase-
angle parameters are needed for the synchronising time
calculation. These two values are crucial when
programming the injection set. The protection initiate
signal must be applied on the instant step two begins to
obtain the most accurate transfer time. Once all the
relevant parameters are programmed to the test bench the
test can be started and the values noted down in tabular

5. form. The constant difference in frequency will cause a
“spiral effect” of the phase angle thereby simulating a
constant rate of voltage decay. Figure 2-6 displays the
voltage injection scenario, the total transfer time must be
measured from the protection initiation signal up to the
definite close signal from the breaker.
Figure 2-6: In-phase transfer injection diagram at
constant 110V and 45° phase shift with varying
frequency
Table 2-1 displays the actual recorded times when the test
unit was connected to actual circuit breakers controlled by
the modern transfer device using the test protocol as
discussed. A relative small difference between the actual
and expected transfer time can be observed and is due to
the relaying unit used to change the signal measured from
the circuit breakers to voltage free contacts not to damage
any measuring equipment. These results are based on
testing a modern motor bus bar transfer device.
Table 2-1 In-phase transfer protection initiated operation
time test results
2.6 CONCLUSIONS AND RECOMMENDATIONS
There were some minor challenges when computing the
actual transfer time when testing the in-phase transfer
method where a calculation should be done to ascertain the
first-phase coincidence and hence some marginal errors
could be introduced. The practical experimentation results
obtained closely matched the calculated and simulated
values achieving the goal of creating a testing protocol for
the in-phase transfer method. It was noticed that the testing
protocol, might have to be adjusted from time to time to fit
different plant philosophies and scenarios. This is also the
case for different transfer electronic devices which may
differ in the algorithmic approach the electronic device
uses when interpreting different voltage signals. Further
research and testing can be done on the main-tie-main
based schemes as well as the remaining transfer methods
which may greatly improve practicality of factory
acceptance and maintenance testing.
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