Short circuit strength for medium voltage switchgear
The following selection criteria are important for the short circuit strength of medium voltage switchgear :
- The rated short circuit breaking current I_{sc}: this is the effective value of the breaking current at the terminals of the The following condition must hold true:
Isc >Ik" . ( 1)
Standardized values for middle voltage systems are:
8 kA, 12.5kA, 16 kA, 20 kA, 25kA, 31.5kA, 40 kA, 50 kA and 63 kA.
- The rated short circuit making current I_{ma}: this is the peak value of the making current for a short circuit at the terminals of the The following condition must apply for circuit breakers, load (break) switches and grounding switches of medium voltage switchgear:
Ima > ip . ( 2)
Standardized values for medium voltage systems are:
20 kA, 25kA, 31.5kA, 40 kA, 50 kA, 63 kA, 80 kA, 100 kA, 125 kA and 160 kA.
- The rated short-time current I_{th}: this is the effective value of the short circuit current which the switchgear can carry in the closed state during the rated
short circuit duration t_{th} under the specified conditions for use and operation.
- The thermal short circuit capacity Ithz:
- Isc Rated short circuit breaking current. The symbols have the meanings:
- I" Initial symmetrical short circuit current
- Ithz Thermal short circuit carrying capacity Ithm Thermal equivalent short circuit current Ima Rated short circuit making current
- ip Peak short circuit current
- Ith Rated short-time current
- tth Rated short circuit duration (1s and 3s)
- ik Maximum short circuit duration
Short circuit strength for low voltage switchgear
The following selection criteria must be considered for the short circuit strength of low voltage systems (IEC EN 60 947):
- The rated ultimate short circuit breaking current (breaking capacity) I_{cn}: this is the largest current which switchgear can break without incurring It is given as an effective value. Standardized values for low voltage switch- gear are: 18 kA, 25kA, 40 kA, 70 kA and 100kA.
Icn >Ik" . ( 5)
- The rated short circuit making current (making capacity) I_{cm}: this is the largest current which switchgear can make without incurring It is given as a peak value. The power factor cosf of the short circuit path depends primarily on the reactance of the input transformer. The greater its power, the smaller is the power factor. For this relationship, IEC 947-2 gives a minimum value for circuit breakers:
Icm = nIcn = K2Icn .
According to type, the circuit breakers must be able to interrupt the following short circuit and operating currents:
- Circuit breakers with cosp 2= 1: short circuit currents
- Circuit breakers with cosp 2= 7: load currents
- Circuit breakers with cosp < 1: operating currents
- The rated short-time withstand current I_{cw}: this is the permissible effective value of the AC periodic component of the uninfluenced short circuit current which the switchgear can carry for a certain length of time without noticeable effect (e.g. due to excessive evolution of heat), g. from 0.05s to 1 s.
- In accordance with IEC 60 947, circuit breakers are tested with two currents for breaking the short circuit current.
- The I_{cu} test sequence is 0-t-C0 The I_{cs} test sequence is 0-t-C0-t-C0
- The current I_{cs} represents the more severe condition for the circuit breakers, because switching takes place one additional time and is stipulated as 25%, 50 %, 75% and 100 % of I_{cu} in order to distinguish the function of the switch following a short circuit break.
The symbols have the meanings:
- Icu Rated short circuit breaking current
- Ik" Initial symmetrical short circuit current
- Icm Rated short circuit making current
- ip Peak short circuit current
Switchgear manufacturing demanded standards
“Switchgear short-circuit current withstands are described in the IEC 61439-1 Low-voltage switch- gear and controlgear assemblies - Part 1: General rules. This part of the IEC 61439 series lays down the definitions and states the service conditions, construction requirements, technical characteristics and verification requirements for low-voltage switchgear and controlgear assemblies. This standard cannot be used alone to specify an assembly or used for a purpose of determining conformity. ASSEMBLIES shall comply with the relevant part of the IEC 61439 series; Parts 2 onwards. This standard applies to low-voltage switchgear and controlgear assemblies only when required by the relevant assembly standard as follows:” (IEC61439-1, 2009, p. 12)
- “assemblies for which the rated voltage does not exceed 1 000 V in case of a.c. or 1500 V in case of d.c.;
- stationary or movable assemblies with or without enclosure;
- assemblies intended for use in connection with the generation, transmission, distribution and conversion of electric energy, and for the control of electric energy consuming equipment;
- assemblies designed for use under special service conditions, for example in ships and in rail vehicles provided that the other relevant specific requirements are complied with;
- NOTE: 2 Supplementary requirements for assemblies in ships are covered by IEC 60092- 302.
- assemblies designed for electrical equipment of machines provided that the other relevant specific requirements are complied
- NOTE: 3 Supplementary requirements for assemblies forming part of a machine are covered by the IEC 60204 series.” (IEC61439-1, 2009, p. 12)
“This standard applies to all assemblies whether they are designed, manufactured and verified on a one-off basis or fully standardised and manufactured in quantity. The manufacture and/or assembly may be carried out by other than by the original manufacturer. This standard does not apply to individual devices and self-contained components, such as motor starters, fuse switches, electronic equipment, etc. which will comply with the relevant product standards.” (IEC61439-1, 2009, p. 12)
The IEC 61439 series, is structured into following parts:
- IEC 61439-1: General rules
- IEC 61439-2: Power switchgear and controlgear assemblies
- IEC 61439-3: Distribution boards
- IEC 61439-4: Assemblies for construction sites
- IEC 61439-5: Assemblies for power distribution
- IEC 61439-6: Busbar trunking systems
Short-circuit calculation for three phase electrical systems are defined in the standard series IEC- 60909-0, 60909-1, 60909-2, 60909-3 and 60909-4. These different parts are listed below.
“IEC 60909-0, Short-circuit currents in three-phase a.c. systems – Part 0: Calculation of currents is applicable to the calculation of short-circuit currents in low-voltage three-phase a.c. systems and in high-voltage three-phase a.c. systems operating at a nominal frequency of 50 Hz or 60 Hz.” (IEC60909-0, 2001, p. 13)
“IEC TR 60909-1, Short-circuit currents in three-phase a.c. systems – Part 1: Factors for the calculation of short-circuit currents according to IEC 60909-0. This technical report aims at showing the origin and the application, as far as necessary, of the factors used to meet the demands of technical precision and simplicity when calculating short-circuit currents according to IEC 60909-0.” (IEC60909-1, 2002, p. 15)
“EC TR 60909-2, Short-circuit currents in three-phase a.c. systems - Part 2: Data of electrical equipment for short-circuit current calculations. IEC 60909-2 comprises data of electrical equipment collected from different countries to be used when necessary for the calculation of short-circuit cur- rents in accordance with IEC 60909-0.” (IEC60909-2, 2008, p. 6)
“EC 60909-3:2009/COR1:2013, Corrigendum 1 - Short-circuit currents in three-phase AC systems - Part 3: Currents during two separate simultaneous line-to-earth short circuits and partial short-cir- cuit currents flowing through earth.” (IEC60909-3, 2009)
“IEC/TR 60909-4:2000, Short-circuit currents in three-phase a.c. systems - Part 4: Examples for the calculation of short-circuit currents This part of IEC 60909 is a technical report intended to give help for the application of IEC 60909-0 for the calculation of short-circuit currents in 50 Hz or 60 Hz three-phase a.c. systems.” (IEC60909-4, 2000, p. 11)
“IEC 60865-1, Short-circuit currents – Calculation of effects – Part 1: Definitions and calculation methods. This International Standard is applicable to the mechanical and thermal effects of short- circuit currents. It contains standardized procedures for the calculation of the effects of the short- circuit currents in two sections as follows: – Section 2 - The electromagnetic effect on rigid conductors and flexible conductors. - Section 3 - The thermal effect on bare conductors and electrical equipment.” (IEC60865-1, 1993, p. 9)
Reference:
- Short Circuits in Power Systems: A Practical Guide to IEC 60909
- SAVONIA