This article is for surge protection of high voltage electrical systems, medium voltage equipment, motors, and low voltage sensitive equipment. Main keywords for this article are Surge Protection – Lightning Surge – Switching Surge. Surge Sources and Characteristics. Surge Protection References.
Surge Protection References
American National Standards Institute (ANSI)
C84.1 Electric Power Systems and Equipment – Voltage Ratings
Institute of Electrical and Electronics Engineers (IEEE)
C62.11 Standard for Metal-Oxide Surge Arresters for AC Power Circuits (>1 kV)
C62.22 Guide for Application of Metal-Oxide Surge Arresters for Alternating Current Systems
C62.41 Recommended Practice on Surge Voltages in Low-Voltage AC Power Circuits
C62.43 Guide for the Application of Surge Protectors Used in Low-Voltage (Equal to or Less than 1000V rms or 1200V dc) Data, Communications, and Signaling Circuits
141 Recommended Practice for Electric Power Distribution for Industrial Plants
1313.1 Standard for Insulation Co-ordination-Definitions, Principles and Rules
1313.2 Guide for the Application of Insulation Co-ordination.
National Electrical Manufacturers Association (NEMA)
LS-1 Low Voltage Surge Protective Devices
MG-1 Motors and Generators
National Fire Protection Association (NFPA)
70 National Electrical Code, 2008
Underwriters Laboratories (UL)
1449 Standard for Transient Voltage Surge Suppressers
Definitions / Terms
For the purposes of understanding this standard the following definitions apply.
Basic Lightning Impulse Insulation Level (BIL). The electrical strength of insulation expressed in terms of the crest value of a standard lightning impulse under standard atmospheric conditions.
Basic Switching Impulse Insulation Level (BSL). The electrical strength of insulation expressed in terms of the crest value of a standard switching impulse.
Clamping Voltage. The peak voltage across the surge-protective device measured under conditions of a specified surge current and specified current waveform.
CWW. Chopped wave withstand
Crest Value. The maximum absolute value that a wave, surge or impulse attains.
Discharge Current. The surge current that flows through the surge-protective device when conduction occurs.
Discharge Voltage. The voltage that appears across other terminals of a surge protective device during passage of discharge current.
Flashover. A disruptive discharge around or over the surface of a solid or liquid insulator.
FOW. Front of wave.
LPL. Lightning protective level.
Maximum Continuous Operating Voltage (MCOV). The maximum designated root-mean-square value of power frequency voltage that may be applied continuously between the terminals of the over voltage protective device.
SPL. Switching protective level.
Surge Protection
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 ANSI C84.1.
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.
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 IEEE 141 recommendations.
Surge arresters and capacitors installed in hazardous areas shall meet all requirements of NFPA 70.
Proper lightning protection and grounding can prevent or minimize the occurrence of surges on a power system. See SES E11-S03 for grounding and lightning protection requirements.
https://www.youtube.com/watch?v=t3URwh_wB54
Surge Sources and Characteristics
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.
- 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.
- 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.
Lightning Surge
- Surge Characteristics
A lightning stroke current surge will have the form of a steep front wave that will travel away from the stricken point in both directions along the power system conductors. The surge is typically very short in duration and high in magnitude. - Common Points of Entry and Impact to System.
The mechanisms by which lightning surges enter a facility include: a. Indirect Lightning Strike
(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. - Surge Mitigation Techniques
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.
Switching Surge
- Surge Characteristics
A surge generated by switching action will have the form of a steep wave-front transient over voltage when circuits are switched from one steady state condition to another. - 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. b. Switching phenomena can be grouped into two categories; internal switching transients and external switching transients.
(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.
Surge Protection Equipment
Capabilities
- Surge protection equipment is selected and installed to reduce surge magnitude and modify the wave shape of electrical system surges to levels below equipment insulation ratings. On medium and high voltage systems, surge protection equipment is limited to surge arresters and capacitors. For sensitive low voltage equipment, more sophisticated ‘black box’ type surge protection devices are used.
- Surge arresters are specifically applied to reduce magnitude of surge. Surge arresters dissipate switching surges by absorbing thermal energy. The selected arrester shall have an energy capability greater than the energy associated with the expected switching surges on the system, as specified in IEEE C62.22 and 141.
- Surge capacitors are used to dampen or decrease rate of rise of system voltage as a surge approaches equipment. This helps to reduce internal stresses in sensitive equipment until surge arresters can operate to reduce magnitudes to acceptable levels.
Material Technologies
In all applications, arresters are exposed to continuous voltages at system fundamental frequency. Arresters shall exhibit high resistance at these voltages. Low resistance at surge voltage is desirable for the arresters to achieve satisfactory surge protection. The predominant technology for new applications is metal-oxide technology. Metal Oxide Valve Arresters (MOV) shall be manufactured and tested in accordance with IEEE C62.11. MOV arrester applications shall conform to IEEE C62.22.
Equipment Ratings
Note: Arresters are rated on the basis of the associated applied system voltage and not in relation to their surge-protective characteristics.
- Arrester ratings. MOV arrester characteristics are in terms of the maximum voltage associated with discharging a specified magnitude of surge current through them. Three categories of protective voltage characteristics are established by industry standards that relate to three specific discharge current wave shapes. These categories include:
a. Front of Wave (FOW) protective level
b. Lightning impulse protective level (LPL) also referred to as discharge voltage of the arrester (IR)
c. Switching impulse protective level (SPL) - Arrester classes. Four classes of arresters are recognized by industry standards that specify ‘lightning impulse’ classifying and ‘switching surge’ classifying current requirements for the respective classes. In order of decreasing cost and overall protection and durability, these classes are as follows:
a. Station class
b. Intermediate class
c. Distribution class heavy duty / distribution class normal duty
d. Secondary class - Metal oxide valve arresters (MOV) shall be rated and classified in accordance with IEEE C62.11.
- Short circuit currents may produce explosive pressure build-up due to the associated rapid heating effects and gas generation inside the arrester. Station and intermediate arrester designs shall incorporate pressure-relief devices to ensure safe containment of otherwise possibly dangerous arrester disintegration during the passage of system high short-circuit current through them, as required by IEEE C62.11.