Electrical Basic Concept
Electrical Surge and protection
Electrical surge is important of understand the safety of electrical devices, here we are explained different definition of electrical surge and there classifications
Electrical Surge
For the purposes of
understanding this standard the following definitions apply.
Urw Required withstand voltage:
test voltage that the insulation must withstand in a standard withstand voltage
test to ensure that the insulation will meet the performance criterion when
subjected to a given class of overvoltages in actual service conditions and for
the whole service duration
1 Surge protection
1.1
Surge protection is provided to protect equipment from damage caused by power
system disturbances. Power system disturbances are increases or decreases in
the system voltage or system frequency beyond the normal tolerances defined in
IEC 60038.
1.2
Surge disturbances are described and classified by two significant physical
characteristics. These include surge duration and surge magnitude. The surge or
change in voltage on the power system can range from complete loss lasting
seconds, minutes or even hours, to very high-magnitude, short-duration impulses
of 50 or more times the normal system voltage lasting for no more than a few
millionths of a second.
1.3
Coordination and protection of distribution apparatus shall be based upon
limiting surge voltages to a suitable margin below equipment basic insulation
levels. By taking proper measures to provide adequate protection against
lightning and switching surges, equipment failures and plant outages due to
these failures will be kept to a minimum. Surge protection shall be provided on
all systems in accordance with IEC 60071-1 & 2 recommendations and IEC
60099 standards
1.4
Surge arresters and capacitors installed in hazardous areas shall meet all
requirements of IEC 60079.
1.5
Proper lightning protection and grounding can prevent or minimize the occurrence
of surges on a power system
2. Surge Sources and
Characteristics
2.1 Surge Origin Location
All surges can be classified as external or internal to the power system. Surge location will impact on installation location, rating and classification of surge protective devices.
2.1.1
External surges are those surges generated outside a facility and brought into
the facility by overhead transmission lines. Lightning and utility switching
surges are the most common external surge sources. Stored energy in
transmission lines, long cable circuits, and large capacitors are the principal
sources of utility switching surge energy. External surges are typically more
severe but less frequent than internal surges.
2.1.2 Internal surges are
generated within a facility by the users own equipment. Switching surges are
the most common type of internal surge. Internal switching transients may be
induced in wire line facilities when inductive equipment is turned off.
i. Nearby lightning strike produces electromagnetic
fields that can induce voltages on the conductors of the primary and secondary
circuits.
ii.
Lightning ground current flow resulting
from nearby cloud to ground discharges couples to facility by way of common
ground impedance paths of the grounding network. This will cause voltage
differences throughout the grounding network.
iii.
Operation of a transformer primary
gap-type arrester that produce surge voltages into the secondary circuit by
normal transformer action.
b.
Direct Lightning Strike
i. Lightning strikes to high-voltage primary circuits
inject high currents into primary circuits. This in turn produces surge
voltages by causing ground potential change, or causing primary conductor
voltage surge. Some of this voltage couples to the secondary of service transformers
and produces surge in low voltage ac power circuits.
ii. Lightning strikes to secondary circuits, resulting in
very high currents and voltages.
a.
In instances where the local industrial plant system is without lightning
exposure, (without overhead lines) lightning surges are likely to be quite
moderate. Application of surge arresters on the transformer primary can provide
effective protection from surges that may come through step down transformers.
b. Properly rated surge
arresters at the plant terminal of the incoming lines will usually reduce the
over voltage to a level the terminal station apparatus can withstand
2.3 Switching Surge
2.3.2 Points of Origin and
Entry to System
a. Switching devices which tend
to chop the normal ac wave, for example thyristors, vacuum switches, current
limiting fuses, and two or three cycle circuit breakers, force the current to
zero. This accelerates the collapse of the magnetic field around the conductor
and generates a transient over voltage.
i.
Internal switching transients may be induced in wire line facilities when
inductive equipment is turned off. In these cases, the parameters, for example
the amplitude of the switching current and the stored energy, are known.
Switching surge voltage magnitudes can be calculated. This information can then
be used to prepare surge protective device requirements.
ii.
External switching transients may be induced in wire line facilities by means
of capacitive or inductive coupling when switching occurs in nearby power
systems.
c.
Examples of switching operations that can produce voltage transients include:
i. Minor switching of loads within the system, for
example process pumps, HVAC equipment, heaters and transformers.
ii.
Periodic transients (voltage notches)
that occur each cycle during the commutation in electronic power converters for
example adjustable speed drives (ASD), and un-interruptible power supplies
(UPS).
iii.
Multiple re-ignitions or re-strikes
during switching operation. Air contactors or mercury switches can produce
surge voltages with amplitudes several times greater than system voltage.
iv. Power system switching, for example capacitor bank,
and grid switching. Examples of switching operations that can produce voltage
transients include switching of loads within the system, for example process
pumps, HVAC equipment, heaters, and transformers.