High-voltage switchgear and controlgear
Part 200: AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV
Switchgears are important nodal points in modern power distribution. Correspondingly important is their reliable functioning, a clearly defined switching behavior according to specified parameters as well as the protection of personnel and protection against operational interruptions when an overload occurs.
The International Electrotechnical Commission (IEC) has taken up the task of developing the required specifications, their worldwide standardization and further development.
The same applies to IEC 62271-200 – the new standard for medium-voltage switchgear.
IEC 60298 – for four decades this abbreviation was the decisive factor for the type testing of metal-enclosed switchgear. In the meantime there are tens of thousands of switchgear panels of the primary and secondary distribution level based on this standard in use – certified according to the mandatory part of the standard and, if required, according to optional tests.
- Dielectric test to verify the insulation level of the switchgear (tests with rated lightning impulse withstand voltage and rated short-duration power frequency withstand voltage with the specifi ed values depending on the respective rated voltage).
- Temperature rise tests to verify the current carrying capacity with rated normal currents. Peak and short-time withstand current tests to verify the dynamic and thermal current carrying capacity of the main and earth circuits; the tests are performed with rated peak short-circuit current or rated short-circuit making current or rated shorttime current or rated short-circuit breaking current.
- Switching capacity test to verify the making/breaking capacity of the installed switchgear.
- Mechanical function test to verify the mechanical functions and interlocks.
- Degree of protection tests to verify the protection against electric shock and foreign objects.
- Pressure and strength tests to verify the gas tightness and pressure resistance for gas-fi lled switchgear.
There is also the possibility of voluntarily certifying switchgear for resistance to internal arc faults and for personal protection. Manufacturers and operators can select the criteria which are relevant to them from the following six criteria and have them tested.
- Criterion 1: Doors and covers must not open.
- Criterion 2: Parts of the switchgear must not fl y off.
- Criterion 3: Holes must not develop in the external parts of the enclosure.
- Criterion 4: Vertical indicators must not ignite.
- Criterion 5: Horizontal indicators must not ignite.
- Criterion 6: Earth connections must remain effective.
In order to guarantee safe access to the individual switchgear components, e.g.the incoming cable, without isolating the busbar, the IEC 60298 standard differentiates between three types of compartmentalization that serve exclusively as protection against electric shock.
Division of the switchgear panel into four compartments (busbar compartment, switching-device compartment, connection compartment and low-voltage compartment); partitions between the compartments made of sheet steel, front plate made of sheet steel or insulating material.
Division of the switchgear panel same as for metal-clad switchgear, but with the partitions between the individual compartments made of insulating material.
All other types of construction that do not meet the above features of the metal-clad or compartmented designs.
In this context, access to the then common minimum-oil-content circuit-breakers for maintenance work without longer operational interruptions was of prime importance because of the limited number of operating cycles. Therefore, with switchgear in metalclad or compartmented design, the busbar in the busbar compartment and the incoming cable in the connection compartment could remain in operation. With a cubicletype design, the incoming cable had to be isolated, but the busbar itself could remain in operation.
Overview of IEC 62271-200
Although the old IEC 60298 standard was very helpful, in time it was superseded by the technological progress. Above all, the appearance of maintenance- free vacuum circuit-breakers, with operating cycles far exceeding the normal number, made frequent access to this circuit-breaker no longer of prime importance.
The vacuum arc-quenching principle is technologically so superior to other arc-quenching principles that the circuit-breaker can be fixed-mounted again. This resulted in the first-time use of gas insulation with the important features of climatic independence, compactness and maintenance-free design. However, both technologies – the vacuum arc-quenching principle and gas insulation – were not adequately taken into account in the existing standard. Therefore, at the end of the nineties, the responsible IEC committees decided on the reformulation of the switchgear standard, which finally came into effect as IEC 62271-200 in November 2003. At the same time the old IEC 60298 standard was withdrawn without any transition period.
Four key features are of special note with the new IEC 62271-200 standard:
1. Changed dielectric requirements
According to IEC 60298, two disruptive discharges were permitted in a series of 15 voltage impulses for the test with rated lightning impulse withstand voltage. According to the new standard, the series must be extended by another fi ve voltage impulses if a disruptive discharge has occurred during the fi rst 15 impulses. This can lead to a maximum of 25 voltage impulses, whereas the maximum number of permissible disruptive discharges is still two.
2. Increased demands on the circuit breaker and earthing switch
In contrast to the previous standard, the switching capacity test of both switching devices is no longer carried out as a pure device test. Instead, it is now mandatory to carry out the test in the corresponding switchgear panel. The switching capacity may get a negative infl uence from the different arrangement of the switchgear with contact arms, moving contacts, conductor bars, etc.
For this reason, the test duties T100s and T100a from the IEC 62271-100 standard are stipulated for the test of the circuitbreaker inside the switchgear panel.
3. New partition classification
The new partition classes PM (partitions metallic = partitions and shutters made of metal) or PI (partitions nonmetallic = partitions and shutters made of insulating material) now apply with respect to the protection against electric shock during access to the individual components.
The assignment is no longer according to the constructional description (metalclad, compartmented or cubicle-type design), but according to operator-related criteria.
4. Stricter internal arc classification
Significantly stricter changes have also been implemented here. The energy flow direction of the arc supply, the maximum number of permissible panels with the test in the end panel and the dependency of the ceiling height on the respective panel height have been redefined.
In addition, the five following new criteria must always be completely fulfilled (no exceptions are permitted):
- Covers and doors remain closed. Limited deformations are accepted.
- No fragmentation of the enclosure, no projection of small parts above 60 g weight.
- No holes in the accessible sides up to a height of 2 meters.
- Horizontal and vertical indicators do not ignite due to the effect of hot gases.
- The enclosure remains connected to its earthing parts.
For the internal arc classifi cation of substations with and without control aisle, the testing of the substation with installed switchgear is mandatory in the new IEC 62271-202 standard. The classification of the substation is only valid in combination with the switchgear used for the test.
The classification cannot be transferred to a combination with another switchgear type as each switchgear behaves differently in the case of an internal arc (pressure relief equipment with different cross-sections and pickup pressures, different arcing conditions because of different conductor geometries).