Eskom Distribution experienced several incidents on metal-clad switchgear when an internal arc was not contained or vented appropriately. These incidences posed a serious safety hazard to personnel, and the damage to the switch-gear was excessive, resulting in long restoration times. In 1999, the utility launched an investigation of its specifications for metal-clad switchgear, emphasizing bus bar protection. This cause of action resulted in significant changes to protection philosophy.
Eskom's Existing Standards
Metal-clad switchgear. Prior to 1999, Eskom Distribution never specified an internal arc rated time, even though this was a requirement of IEC 298 for metal-clad switchgear. Typically, switchgear could withstand internal arc rated times of 100 msec.
Medium-voltage (MV) bus protection. The bus protection standards for metal-clad switchgear differed according to the application. The standards include:
Transformer back-up overcurrent protection. (Protection trip times typically 1.2 to 1.4 seconds.)
Bus-blocking arrangement using the transformer overcurrent function interlocked with the MV feeder overcurrent functions. (Protection trip times typically 300 to 400 msec.)
High-impedance bus protection based on the circulating current principle. (Protection trip times typically between 35 to 60 msec.)
The first two standards are too slow to ensure a total fault clearance time of less that 100 msec. The third standard limits the zone of protection to the physical location of the current transformers. The majority of faults are in the switchgear cable compartment and, therefore, outside this protective zone.
MV feeder protection. The MV feeder protection standard impacts the cable connection to the switchgear as it forms part of the protective zone. The overcurrent settings for cable networks are chosen to obtain a protection trip time between 0.8 to 1.0 seconds for close in faults with a magnitude typically 20 times plug setting multiplier (PSM). No auto reclosing is applied to cable networks.
For overhead line distribution networks, two stages are used. Line faults with a magnitude typically between 10 and 20 times PSM are cleared by the instantaneous element. Lower fault levels are cleared by IDMT elements with a protection trip time of 1.0 second; both elements will initiate auto-reclosing. With these arrangements, a fault in the switchgear cable box would allow an auto reclosure.
New Bus Bar Protection Policy
Eskom Distribution decided to pursue a new form of bus protection for metal-clad switchgear that would satisfy the internal arc specification without significantly increasing the cost of the switchgear. Upon further investigation, Eskom discovered that the cost of switchgear increased by 10% to increase the internal arc rating from 100 msec to 200 msec, and by 100% to achieve a 1.0-second internal arc rating. The utility opted to specify an internal arc rated time of 200 msec for its switchgear with the provision that the applied protection would be able to detect and clear all faults within this specified time.
In general, the internal arc protection provides a trip output within 15 msec; therefore, a total fault clearance time between 80 to 100 msec is obtained for faults in any compartment of the switchboard. Following technical evaluation of the products available, Eskom Distribution decided to specify the “VAMP” arc protection, manufactured by VAASA Electronics Oy from Finland.
Internal Arc Protection
The VAMP arc protection measures two different quantities associated with a fault within the switchboard, namely the light emitted by an arc and the rise in current. Figure 1 shows a schematic diagram of this protection unit. The light sensors are placed in each compartment within the switchboard and this information is fed back to the master units that are normally located on the switchboard's incoming feeders, for example transformers. The master units have an unrestrained overcurrent element that supervises the light sensor providing security, as both conditions must be satisfied before a trip command is issued.
Protection zoning is achieved by the physical positioning of the arc sensors. Two sensors are positioned in the bus-section circuit breaker, being the point of common coupling. The exact fault position is indicated on the master unit by means of the slave addressing system. This indication is critical to reduce downtime, as no physical damage should be visible on the exterior of the metal-clad switchgear. The sensor information for each zone is routed to each master unit. This will ensure that a trip is issued for a circuit breaker fail condition and also will ensure that the appropriate circuit breaker is tripped when an incoming feeder circuit is not in service. The protection time to signal trip is less than 10 msec.
Eskom's Field Experience
Eskom Distribution has a program to install VAMP arc protection on 11-kV switchboards and the details on three faults experienced in the past year effectively demonstrate the benefits of this improved level of protection.
Bus fault without ARC detection protection. Figures 2 and 3 show a circuit breaker on an 11-kV switchboard in North Substation without arc protection that faulted because of continuous overload and a downstream through fault. The circuit breaker and the complete 11-kV bus bar chamber experienced extensive damage. This fault caused a 24-hour outage with 50% of the circuits out of service for an undetermined period; adjacent substations supplied the remaining load.
Ascot North Substation. The 17-panel 11-kV switchboard at Ascot North 66/11-kV Substation is equipped with VAMP bus bar protection. A cable termination faulted in the cable box causing a single phase-to-earth fault as shown by the waveform from the recorder installed on the 66-kV incoming feeder (Fig. 4). The substation fault level was 12,000 A with a total fault clearance time of 85 msec. Figure 5 shows the damage limited to the cable box, the fault location being determined from the sensor address. The cable termination was repaired and the circuit returned to service with the minimum disruption to the power supply. Traditionally, it would take 300 msec to 1.2 seconds to clear a fault of this nature, depending on the bus protection arrangement. The damage to the switchboard and the disruption to power supplies would have been severe.
Rietvlei Substation. A 21-panel 11-kV switchboard installed at Rietvlei 66/11-kV Substation contains VAMP bus bar protection. A contractor digging in the vicinity ripped through a three-phase cable, resulting in a 16,000-A, three-phase cable fault. The fault was cleared by the feeder protection, and when the circuit breaker interrupted the fault current, the arc protection operated. The sensor address indicated the location was in the feeder circuit-breaker truck chamber.
Further investigation showed the circuit breaker was constructed with vacuum interrupters made of a translucent material. This increased the probability that the sensor was able to detect sufficient light above the set threshold. The arc protection overcurrent function detected the through fault current at the time of fault interruption.
The Western Region has 323 protected substations with some 2000 circuit breakers installed. Fault statistics for the past four years indicate a failure rate of 0.65%. Because of the total number of units installed throughout the country, Eskom Distribution initiated a survey to determine the total number, location and specifications of the switchgear units installed.
To address this problem, the policy for the future will be as follows:
New installations. The new switch-gear specifications will specify arc detection protection.
Existing switchboards. Arc detection protection will be retrofitted in the event additional units are installed, provided the minimum internal arc rating exceeds 200 msec.
Existing switchboards with internal arc protection of less than 200 msec will be evaluated in terms of strategic importance and risk to human life. Cost-wise it may be more beneficial to provide a controlled environment, for example, in remote operation, no personnel have access to the substation when bus bar is live.
Internal arc protection offers a marked improvement over traditional bus protection philosophies for metal clad switchgear as faults in all compartments can be detected. The speed of fault detection is such that the damage to switchgear and the substation is limited, thereby reducing the outage times to repair any fault damage. Applying internal arc bus protection to metal-clad switchgear reduces the internal arc specification for this switch-gear with a resultant reduction in the capital cost of these units.
André de Jongh, who has the MSEE and is registered with the Engineering Council of South Africa, joined Eskom in 1984. He is currently responsible for power system protection planning and technology within Eskom's Western Region. Additionally, de Jongh is responsible, at a national level, for various protection technologies including bus protection for metal-clad switchgear.