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Mitigating the effect of voltage sags on

contactors in industrial plant and substations
by Dr. Vladimir Gurevich, Israel Electric Corporation

In this article the difference between electrical equipment in manufacturing plants and that in electrical power substations is discussed. Theses
differences dictate two different sets of requirements for supplying power to large AC contactors. The article offers two technical solutions: one
of which is based on the contactor remaining in the closed position at voltage sag; and the second is based on the instantaneous switching-
off of the contactor at voltage sag with a subsequent switching-on with time delay.

According to IEC 61000-4 [1, 2] voltage sags are brief reductions in These electronic devices use four levels of DC voltages for feeding
voltage, lasting from tens of milliseconds to 15 s (see Fig. 1). The main the contactor coil, which simulates the natural attraction force
cause of voltage sags in the 400 V network of the substation auxiliary characteristics at contactor pickups on AC. These electronic devices
supplies is short circuits in external high voltage grids. In manufacturing with integrated circuits (microchips) are not intended, however, for
plants such voltage sags are frequently associated with the working use with powerful contactors with low resistance coils (10 – 15 Ω) and
modes of high power electrical equipment, e.g. start-up of motors. high inrush currents. For example, the power consumed by the coil of
Voltage sags are an important criterion of the power quality. the 3NTF54 contactor at pickup is 1,6 kVA on AC and 1,2 kW on DC
(with a special starting coil).

For large contactors with powerful coils special devices have been
developed which work on another principle, shown in Fig. 2. The
device consists of undervoltage relay KU, timer K1 for an “Impulse-
on” standard function, and a simple DC power supply consisting of
transformer T, rectifier bridge VD2 and low voltage high capacity
capacitor C. When control switch S1 is closed, the 230 V AC voltage is
applied to the voltage relay KU which operates if the applied voltage
is more than the minimum required (180 V in our case) and closes its
contact in the feeding circuit of the KT timer, which instantly operates
and connects the contactor’s coil to the 230 V AC network through
Fig. 1: Example (from IEC 61000-4-11) of a 70% voltage sag
duration of 25 cycles. rectifier bridge VD1 and limiting resistor R1.

Voltage sags in 400 V networks in manufacturing plants

Sags in the 400 V networks of manufacturing plants may seriously

disturb the production cycle due to mass disconnection of electrical
motors during the sag, followed by auto-starting of motors, which
may cause additional drops in voltage level and exacerbate the
problem [3, 4].

As shown in [5] upon power interruption to a motor of 0,4 – 0,8 s, the

vectors of the residual electromotive force of the motor can occur
in anti-phase with the vector of the network voltage. As a result,
upon network voltage restoration, a high magnitude current pulse
will flow through the motor, causing the tripping of the protective
circuit breaker.

On the other hand, short voltage sags with durations of less than
200 - 300 ms (most frequent in 400 V network) do not harm the motors.
For these reasons the means for ameliorating sags in networks in
manufacturing plants have included technical solutions for keeping
the contactors closed during sag, special dynamic voltage sag
compensators, UPSs, etc. Because such compensators and high Fig. 2: Circuit diagram of the control device for high power AC contactor.

power UPSs are very expensive, different electronic devices [6, 7]

have been developed that guarantee feeding the contactor’s coil
A direct current of approximately 5 A will be carried through the
from a DC power supply during short sags.
contactor’s coil. Such current produces an electromagnetic force
In an AC contactor there is a considerable variation in current equivalent to that of the natural starting current leading to the
consumed by its coil during operation, leading to considerable connection of the contactor to the 230 V AC network. Simultaneously,
variation in the core’s attractive forces needed. When feeding a capacitor C1 is quickly charged. Due to presence of diode VD3,
contactor’s coil from a DC supply, such current variations will not be capacitor C1 is charged from a low voltage (12 V) DC power supply
present and the contactor will not work properly. and is completely isolated from the high voltage of approximately

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70 V applied to the contactor’s coil during start-up. After 2 - 3 s, the with the help of an additional contactor, K2, connected in series
timer’s contact breaks the high starting current in the contactor’s coil. with heavy contacts (for disconnection of the high inductive load at
Diode VD3 will instantly unlock and the low-voltage power supply with 230 VDC) and an additional auxiliary contact block (95, 96) on the
the charged capacitor C1 will connect to the contactor’s coil. The main contactor.
contactor’s coil is fed by the lowered DC current, limited, in addition,
by the low resistance resistor R2. Selection of the value of this resistor
depends on the specific contactor type. For instance, for a 3TF54
type contactor, the resistance of this resistor has to be as low as 5 W.
At this resistance the retention of the contactor in the closed position
is provided over a long period of reduced AC voltage (down to 140
– 130 V) and, at the same time, the allowable temperature of the
coil (50 - 60°) is not exceeded.

Research has shown that when feeding the contactor’s coil with
lowered DC current, its sensitivity to decreasing power supply voltage
level is sharply reduced. For example, the contactor that was tested
was held in the closed position at a voltage reduction on the coil
from 12 V down to 2 - 3 V, that is in 4 - 6 times. This positive property is
used in this device for holding the contactor for short-term downturns
of the voltage level in AC networks. For very deep voltage sags or
even full voltage loss, the energy of capacitor C1 holds the contactor
operated. From the results of tests it appears that small size capacitors
in the range of 47 000 µF at 40 V are capable of holding the contactor
for periods of 1,3 - 1,5 s. This is quite sufficient for short-term voltage
sags in real-life in AC networks. The rectifier bridge VD needs to be
selected with considerable care because high current pulses flow
through it during capacitor charging.

When the voltage in an AC network falls to a level lower than 160 V,

the under-voltage relay KU opens its contact and disconnects the Fig. 3: CHECK WITH MIKE.
feeding circuit of the timer KT. The position of the output contact of
the timer does not change, and the capacitor continues feeding With the presence of a powerful battery and the possibility of setting
the contactor’s coil from the low-voltage DC power supply until the the DC voltage to the contactor’s location, the problem can be
restoration of the proper voltage level in the AC network or, until the solved by means of a more intelligent method, such as the already
capacitor C1 energy is fully exhausted, at which time the contactor mentioned timers and a small switching type power supply with an
will be disconnected. With the restoration of network voltage to more output voltage of 12 V and current 1,2 A, shown in Fig. 4. Only two
than 180 V, the under-voltage relay KU again will pick up, timer feeding cheap off-the-shelf devices are needed for realizing the solution.
will be applied, and the working cycle of the device will repeat.
Voltage sags in 400 V auxiliary AC networks in substations
The “Impulse-ON” function, which is sometimes called “Interval”,
“Fleeting”, “Single Shot”, “Power ON”, “Single Shot Leading Edge”, The peculiarity of the low voltage auxiliary AC network in power
“Rising Edge Pulse”, represents the standard function. The timers substations is that it does not contain devices that tolerate short
actualizing such functions are widely available in the market. pauses in the power supply, and almost all of the critical auxiliary
Unfortunately, only some from them are suitable for switching coils equipment is fed from the substation battery, while the power
of large contactors. Using timers of the other types will necessitate electronic systems with microprocessor controllers such as invertors,
using an addition auxiliary relay with powerful contacts inserted in
circuit instead of the timer’s contacts.

Voltage relay KU can be used as either an undervoltage relay with

adjustable trip level or hysteresis, which do not require a separate
power supply. The device is assembled in a closed plastic container
with dimensions: 210 x 160 x 90 mm. It is abundantly clear that
the device can be used with medium size contactors and as well
as with small contactors. In both these cases the capacitance of
the capacitor and the power of the transformer will dramatically
decrease.

It needs to be noted that some manufacturers provide the possibility

for feeding AC contactors from a DC power supply In this case the
contactor’s condition is fully independent of sags in the AC network. It
offers one more way of solving the problem, but realising this solution
is not simple because of special starting characteristic which need
to be simulated as mentioned above. Two special windings for 3TF5
series contactors are available: a powerful operate winding (PW),
and a low power holding winding (HW), shown in Fig. 3. Changeover Fig. 4: Device for feeding a large AC contactor from a DC network
from one winding to other after contactor K1 operates is effected with a large capacity battery.

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battery chargers, and power supplies are fed from the auxiliary AC
network. Experience has shown that such devices do not tolerate
short voltage interruptions (50 – 200 ms), although they may have
time to ride through automatic changeover from main to spare
power supply (transformer). Another problem with battery chargers
with large input transformers is the high inrush current at sudden
interruption and subsequent input voltage restoration that causes full
charger disconnection by the input circuit breaker. This state of affairs
is considerably aggravated in some cases when even single voltage
sags with durations of 100 – 200 ms provoke multiple operation and
release of the contactor.

Problematic action of the contactors as changeover from main to

reserved auxiliary AC power supply occurs

Fig. 5: Electromagnetic AC contactor 3TF54 type and oscillograms

for current and voltage on its coil at switch-on.

To increase the reliability of the 400 V AC network in power substations

two auxiliary transformers, fed from different lines, are normally used. Fig. 6: Oscillograms of contactor type 3TF54,
One of them connects to the 400 V AC network permanently and the switching-on and switching-off.

other one automatically connects at failure of the first transformer.

Connecting and disconnecting the 400 V AC network to these to 150 – 135 V causes vibration of the contactor magnetic system of
transformers is affected by means of two contactors at currents of a magnitude that is sufficient to close and open its main contacts.
200  –  400 A with AC control coils. These contactors are the major The same phenomenon arises when the AC voltage across the coil
elements of the auxiliary network on which reliability of all substations increases from 0 up to 160 – 185 V. A contactor working in such
depend. conditions, even at a single voltage sag (to level 135 – 150 V during
100 – 200 ms), can cause multiple interruptions of the main voltage
As an example, contactor type 3TF54 with a switching capability of
in the auxiliary AC network. The same result appears when connecting
300 A, which is commonly used as changeover in the auxiliary 400 V AC
the contactor’s coil to a power supply with a voltage of 150 – 170 V.
network in substations, was tested. See Fig. 5, Oscillograms of operation
and releases of the contactor were recorded at the connection to Offered solution for the problem
the control coil from the AC supply, see Fig. 5 and 6. The oscillogram
In view of character of the loads fed from the auxiliary AC network
in Fig. 5 shows the presence of high starting (inrush) current caused
in substations, the technical solution offered for contactors intended
by the low impedance of the coil until the moment of the closing of
for use in manufacturing plants, is not the correct technical solution
the contactor’s magnetic circuit. Oscillograms shown in Fig. 6 allow
for a 400 V substation network, because, through closed contactor’s
determining the operate time and release time of the contactor, i.e.,
contacts, the short sags will affect sensitive equipment.
the reaction time of the contactor for voltage sags.
In our opinion the problem must be solved not by means of retention
Analysis of the oscillograms shows that full switch-on time, t, is about of the contactor in the closed position at voltage sags, but by means
20 ms,and the full release time is 15 – 18 ms, compared to values of the rapid (during 10 – 12 ms) disconnection of the contactor’s coil at
given in the data sheet of 10 – 30 ms for nominal voltage applied voltage levels dropping below 160 V and returning it to initial condition
before disconnection, and 10 – 15 ms for a voltage 0.8 of the nominal at voltage level restoration up to 185 V with time delay 5 – 10 s. A single
value. Such small time delays for large contactors means that during interruption of 5 – 10 s duration in the auxiliary 400  VAC substation
typical voltage disturbances with alternate voltage level sags and power network does not cause serious disturbances in the substation
restorations, the contactor will have time to connect and disconnect equipment due to a power battery feeding most of the important
the main power circuit several times. Moreover, as shown in [8], the substation consumers.
reaction of the contactor during a 75% voltage sag is more serious
For fast contactor disconnection at voltage network drops most
than 100%, since the release time for first case is shorter by 40 – 50%
electronic relays available in the market are not suitable because their
than in second case and may be 10 ms even for large apparatus.
minimal reaction time is of the order of 100 ms. During a time interval
One more problem with the AC contactor was discovered during the of this duration, the contactor will make several disconnections and
research stage. It was found that reducing the voltage across the coil connections of the main power.

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Fig. 7: Device for fast forceful disconnection of the

main contactor at voltage sag.

As a result of our research only a few types of the devices

suitable for contactor’s control were found, Fig.7. One of them
is an under-voltage relay combined with timer: Brown-out timer
type GBP2150. The reaction time of this device for voltage drops
of 30% is only 5 ms. The release time after voltage restoration
up to 80% can be adjusted to intervals of 1 to 10 s. Another
good example is the under voltage relay type PKH-1-3-15. Such
devices are ideal solutions for our purposes. For decreasing loads
on the output contacts in these devices an additional auxiliary
fast electromagnetic relay, type 58.32.8.230, with heavy output
contacts and a release time 3 ms is employed.

Conclusion

For manufacturing plants with electrical motors as dominant

consumers and for power substations with power electronic
equipment as dominant consumers in 400 V AC networks different
methods must be used in contending with voltage sags. In the
first case devices with capacitor or compact switching power
supply for retention of the main contactor in closed position
during short voltage sags, as described above, can be used. In
the second case it is recommended using a simple device for
fast forceful disconnection of the main contactor at voltage sags
below 25 - 30%.

References
[1] IEC 61000-4-11 Ed. 2.0 b:2004. Electromagnetic compatibility (EMC)
- Part 4-11: Testing and measurement techniques - Voltage dips, short
interruptions and voltage variations immunity tests.
[2] IEC 61000-4-34 Ed. 1.0 b:2005. Electromagnetic compatibility (EMC)
- Part 4-34: Testing and measurement techniques - Voltage dips, short
interruptions and voltage variations immunity tests for equipment with
input current more than 16 A per phase.
[3] C J Melhorn , T D Davis, G. E. Beam. “Voltage Sags: their impact
on the utility and industrial customers”, IEEE Transactions on Industry
Applications, vol. 34, No. 3, 1988, pp. 549-558.
[4] M F McGranaghan, D R Mueller, M J Samotyj. “Voltage sags in industrial
systems”, IEEE Transactions on Industry Applications, vol. 29, No. 2,
1993, pp. 397-404.
[5] Fishman V, “Voltage sags in industrial networks”, Electrical Engineering
News, 2004, No. 5 (29), and No. 6 (30), Rus.
[6] A Kelley, J Cavaroc, J Ledford, L Vassalli. “Voltage regulator for
contactor ridethough”, IEEE Transactions on Industry Applications,
vol. 36, No. 2, 2000, pp. 697-703.
[7] P Andgara, G Navarro, J I Perat. “A new power supply system for AC
contactor ride-throuhg”, 9th International Conference “Electric Power
Quality and Utilisation”, Barcelona, 9-11 October, 2007.
[8] I Iyoda, M Hirata, N Shigei, S Pounyakhet, K Ota. “Affect of Voltage
Sags on Electro-magnetic Contactor”, 9th International Conference
“Electric Power Quality and Utilisation”, Barcelona, 9-11 October,
2007.

Contact Vladimir Gurevich, Israel Electric,

vladimir.gurevich@gmail.com v

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