Electrical Quality Devies

Sizing the power quality problems
Nowadays everybody speaks about power quality. For many reasons, I

would say. Electrical energy is now polluted more than ever, and consumers
are more sensitive to power disruptions and fluctuations than a small flower
to the stronger wind. But, it is as it is. It’s up to us to do something about all
this. There are many levels of implementation of power quality in commercial
plants.
Five
power quality devices that every commercial plant must have installed (photo
credit: nicegrid.fr)
Many plants have some power quality solutions implemented, but how many
of them have done an in-depth analysis of their electrical system and the
actual need for power? The point is that power quality analysis is a smart
job, and it costs a lot.
However, the most important is that the size of power quality problems must
be defined by estimating the cost of disruptions caused to distribution
system equipment (utilities and consumers’ equipment) and the extent of its
sensitivity to power quality. Some plant equipment may be less sensitive to
disturbances, allowing it to ride through the disturbances.
In that case there is no need to apply a solution.
For other equipment that are sensitive, a solution is needed, for example, by
installing power quality devices that suppress or counteract the
disturbances. A cost/benefit analysis of the different solutions is applied to
enable the distribution system planners to make a decision on the most cost-
effective solution.
The power quality devices are used to protect the electrical equipment or to
eliminate the source of disturbances or to mitigate the effect of disturbances.
The devices that are commonly used for this application include the
following: surge protection device, shielding, uninterruptible power supply,
dynamic voltage restorers, series capacitors, capacitor voltage
transformers, wiring and grounding, static var compensator, energy storage
system (ESS), backup generators, isolation transformers and filters.
Examples of Power Quality Devices

Electrical equipment with microprocessor-based controls and power
electronic devices that are sensitive to disturbances are affected by poor
power quality. Control systems can be affected by momentary voltage sag or
small transient voltages, resulting in nuisance tripping of processes.
Furthermore, many of these sensitive loads are interconnected in extensive

network and automated processes. This interconnected nature makes the
whole system dependent on the most sensitive device when a disturbance
occurs.
Let’s see these five devices that can improve the power quality:
1. Surge protection devices

2. Backup generators
3. Uninterruptible power supply (UPS)
4. Isolation transformer
1. How does isolation transformer work?
5. Voltage regulators (stabilizers)
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1. Surge protection devices

The electrical equipment in distribution systems may be exposed to internal
or external surges. Internal surges are generated within a facility by users’
equipment. They result from switching processes, for example, switching
inductive or capacitive loads, fuse or breaker opening in an inductive circuit.
External surges are generated outside a facility and enter the facility through
utility wires. They result from fuse operation, power system switching and
lightning.
Figure
1 – Surge protection device installation (on photo: Surge filter installed to protect the DC
charger to power up generators in Phlippines. Prosurge’s SPD is well-performed under such
high lightning exposure area; credit: Prosurge)
Surge protection devices protect the equipment against these surges by
limiting the amount of undesired surge energy that reaches the equipment.
The surge energy is diverted to a path rather than the equipment itself
(neutral or earth).
The surge protection devices is a nonlinear element acting as an isolating
switch in the normal conditions where its resistance is very high. When the
voltage increases and reaches a certain value called “clamping voltage”,
the surge protection device will change rapidly (in nanoseconds) from a very
high resistance mode to a very low resistance mode.
Then the majority of surge energy is directed through surge protection
device, and most of this energy is dissipated in its internal resistance (Figure
2).
Figure
2 – Function of surge protection device
Go back to Contents Table ↑
2. Backup generators
In large industries and for long-duration interruptions, backup generation is
essential to supply at least the critical loads. It is common to use diesel
generators set with rating sufficient for feeding these critical loads such as
emergency lighting system, electric lifts, industrial processes that cannot
withstand long interruption, hospitals and the like.
Usually, the backup generator is used as a standby unit and connected to

the distribution system of industry at the main low voltage switchboard (see
Figure 3). An automatic transfer switch (ATS) can be used to automatically
transfer the power source from the utility incoming feeder to the backup
generator in emergency cases.
An electrical interlock is provided between both sources to avoid parallel
operation.
Figure 3 – Low voltage

panel supplied via Utility source or backup generator connected with automatic transfer
switch (ATS)
Some of the outgoing feeders of non-essential loads may be disconnected to
keep the power demand within the generator rating that is mostly less than
the rating of the utility source.
In many cases, the main LV busbar is sectionalized into two sections with a
bus coupler to increase the reliability. Each section is fed from a different
utility source (see Figure 4). The outgoing feeders of critical loads are
preferred to be connected to one of these two sections that is supplied by the
utility source in normal operation and backup generator in emergency
operation.
Figure 4 – Sectionalized busbars with

two utility sources and backup generator
The circuit breakers A, B, C and D must be controlled to satisfy the truth
table given in Table 1. This table indicates on/off positions of generator
circuit breaker D at different combinations of breakers A, B and C positions.
Position “on” is represented by “1,” while “0” represents “off
position”.
Table 1 – Circuit breakers positions ‘0’ for Off and ‘1’ for On
A B C D
0 0 0 1
0 0 1 0
0 1 0 1
0 1 1 0
1 0 0 1
1 0 1 0
1 1 0 1
1 1 1 0
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3. Uninterruptible power supply (UPS)

The UPS is an alternative power source to supply power to the load during
interruption or outage of main power source (e.g., utility source). The UPS
includes a rectifier circuit to convert AC input power into direct current (DC)
power. The DC power charges a set of batteries to store energy and an
inverter to convert the DC stored energy back onto AC power for the load
(see Figure 5).
A 6- or 12- or 24-diode bridge can constitute the rectifier circuit depending on

the desired level of wave distortion. From the point of view of frequency
stability as well as voltage stability, the inverter that constitutes the UPS
generator has performance superior to that of the mains.
It is designed to generate sinusoidal voltage even when supplying nonlinear loads, that is,
dealing with highly distorted currents.

Figure 5 – Main components of uninterruptible power supply (UPS)
For instance, in a single-phase unit with half-bridge converter, the square
wave voltage appearing between A and B (see Figure 6) is filtered so as
to obtain in the output of the unit a sinusoidal voltage wave.
During normal operation, the utility supplies power both to the load directly
bypassing the UPS unit and to the UPS to charge its batteries via the rectifier
circuit. In emergency operation, for instance an outage of utility power
source, the UPS supplies power to the load fast enough (few milliseconds) to
avoid any damage resulting from load interruption.
This necessitates using an electronic transfer switch to change the power

source to load.
Figure
6 – Principles of half-bridge converter
UPSs are effective for digital electronics-based loads, such as computer
systems and PLCs where the loss of data is avoided. On the other hand,
they have deficiencies where the transfer switch and rectifier are exposed to
line disturbances in normal operating conditions. In an emergency, the
operation time of UPS is limited by the capacity of batteries.
The design of UPSs and, generally, ESS depends on the required mode of
operation. Three modes of operation are considered: standby (offline), online
and line interactive. (Learn more about UPS devices )
The standby mode of operation means that the ESS operates only during the
interruption time, while it operates full time in the case of online mode of
operation. Line-interactive mode of operation includes both of these two
modes.
Figure 7 – Galaxy VM 160, 1125

kVA, 480 V 3-phase UPS power protection that seamlessly integrates into medium data
centers, industrial or facilities applications
4. Isolation transformer
They are generally composed of two separate windings with a magnetic
shield between these windings to offer noise control. The noise can be
transported to the electric device by electromagnetic coupling (EMC) in two
basic ways: a differential mode noise and a common mode noise (see Figure
8).
The isolation transformer is connected between the power source and the
electric device. Therefore, it carries the full load current and thus must be
suitably sized.
The main benefit offered by isolation transformers is the isolation between two circuits by
converting electrical energy to magnetic energy and back to electrical energy, thus
acting as a new power source.
Figure 8 – Isolation
transformer differential and common-mode noises
4.1 How does isolation transformer work?

Considering a high voltage, high current transient is introduced into a power
line by the direct and indirect (induced) effects of lightning activity or a
switching surge.
If these transients are differential mode, then the isolation transformer will
effectively pass these transients with little or no attenuation.
This occurs because the isolation transformer is designed to pass power

frequencies in the differential mode, and the frequency makeup of a
lightning transient is such that most of the energy content is in the frequency
components below a few tens of kilohertz, that is, within the pass band of
most isolation transformers.
If, on the other hand, these transients are common mode, then a suitable
shielded isolation transformer will provide effective protection against such
surges. This is because a common-mode transient is split in two between a
pair of power lines, and they proceed in the same direction. They flow into
the transformer from its terminals and run in the primary coil, finally going out
to the earth through the shield plate.
At this moment, they run in the coil in opposite directions, canceling their

inductive effects in the secondary side. Therefore, a common-mode
transient does not reach the secondary side (see Figure 9).
Figure 9 – Common-mode
transient propagation
It can thus be seen that the shielded transformer will provide effective
protection against common-mode surges provided the peak voltage does not
exceed the insulation rating of the transformer. It also effectively provides no
attenuation of differential mode surges.
Figure 10 – Isolation transformer placed inside panel

5. Voltage Regulators
The function of voltage regulators is to maintain the voltage at load within
preset limits. During voltage sags voltage regulators increase the voltage to
a desired level for sensitive loads and, conversely, during overvoltages or
swells they decrease the voltage. Mostly, the usage of voltage regulators is
to mitigate the effect of sag events .
The common type used for regulating the voltage is a motor-driven variable-
ratio autotransformer. A motor is used to change the location of a slider on
transformer winding, providing a change of transformer ratio to increase or
decrease the voltage levels (see Figure 11).
The response time is slow, which may be inadequate for some loads and
may not correct large short-term voltage variations.
Figure 11 – Motor-driven voltage regulator

Servo voltage stabilizer (it works on servomechanism) uses a servo motor
to enable the voltage correction. It is mainly used for high output voltage
accuracy, typically ±1 percent with input voltage changes up to ±50 percent.
Internal circuit of a servo stabilizer incorporates servo motor,

autotransformer, buck boost transformer, motor driver and control circuitry.
Figure
12 – Servo voltage stabilizer (it works on servomechanism) – photo credit: Civilization
Diamond Co. Ltd
Sources:
1. Electric distribution systems by ABDELHAY A. SALLAM and OM P.
MALIK
2. Protection against lightning effects by Legrand

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Sizing the power quality problems

Nowadays everybody speaks about power quality. For many reasons, I

In that case there is no need to apply a solution.

Examples of Power Quality Devices

Furthermore, many of these sensitive loads are interconnected in extensive

1. Surge protection devices

1. Surge protection devices

Go back to Contents Table ↑

Usually, the backup generator is used as a standby unit and connected to

Figure 3 – Low voltage

Figure 4 – Sectionalized busbars with

3. Uninterruptible power supply (UPS)

A 6- or 12- or 24-diode bridge can constitute the rectifier circuit depending on

dealing with highly distorted currents.

This necessitates using an electronic transfer switch to change the power

Figure 7 – Galaxy VM 160, 1125

Go back to Contents Table ↑

acting as a new power source.

4.1 How does isolation transformer work?

This occurs because the isolation transformer is designed to pass power

At this moment, they run in the coil in opposite directions, canceling their

Figure 10 – Isolation transformer placed inside panel

Go back to Contents Table ↑

Figure 11 – Motor-driven voltage regulator

Internal circuit of a servo stabilizer incorporates servo motor,

Go back to Contents Table ↑

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