In Norway, regulations on quality of supply penalize utilities when their customers' electricity is interrupted for routine maintenance. This incentive regulation differs from that imposed on the majority of utilities because the Norwegian utilities' continuity of supply regulation is based on customer interruption costs.
The Council of European Energy Regulators, which performs extensive benchmarking in Europe, stated in its third report: “It is important to be aware that a scheme that allows companies to gain higher revenues by reducing planned interruptions can induce companies to adopt a more efficient maintenance program or, alternatively, create long-term risk due to insufficient maintenance of the network.”
The Norwegian penalty scheme — the cost of energy not supplied (CENS) — was introduced in 2001, and published statistics confirm system performance has improved significantly in terms of energy not supplied, interruption frequency and interruption duration. Customers interruption costs now exert considerable pressure on how maintenance activity is managed to minimize customer inconvenience. As a result, Norway's TrønderEnergi adopted a new technique that enables non-interruptive maintenance of air-insulated load switches (AIS).
To maintain the AIS, TrønderEnergi now uses shunt plates, which are insulated plates with integrated bypass shunt cables. This makes it possible to undertake functional testing of switchgear and several maintenance procedures in medium-voltage/low-voltage (MV/LV) distribution substations without interrupting the supply to customers.
This novel equipment comprises four plates of insulating material. Three plates, one for each phase, are equipped with a shunt cable with clamps at each end to enable connections to the bus bars above the breaker and to the incoming or outgoing cable at the bottom of the breaker. The fourth insulated plate is positioned between the steel cubicle and the phase conductor termination. The shunt plates, tailor made for each type of switchgear, are lifted into position within the cubicle and attached to the cubicle using insulated claw sticks. The screw terminals at the lower end of the shunt cable are then connected using the claw stick to the outgoing feeder. The shunt cables are available with various clamps to suit the different bus bar arrangements.
TrønderEnergi has established specific rules for the AIS procedure to ensure safe operation. Prior to the start of maintenance work, the utility performs partial discharge detection and takes current measurements. After the completion of these operations, the breaker can be operated and maintained. Procedures undertaken include wet and dry cleaning, lubrication and checks on the open-close mechanism, all without interrupting customer supplies.
TrønderEnergi's Maintenance Program
The distribution network in Trondheim has 900 22/0.4-kV and 11/0.4-kV substations, with a population of some 4,000 AIS, which represents about 70% of the switchgear units on the system. Maintenance of the AIS is programmed as a complete work package that also includes all the substation equipment (that is, MV/LV substation overhaul).
Maintenance activities follow a risk-based strategy, with the maintenance frequency being functions of switchgear type, voltage and whether units are subject to environmental exposure because of installation in open outdoor cubicles. Hence, the maintenance frequency varies from one year to 12 years, averaging five years. The total workload remains relatively constant as time-expired units are replaced with new units, so any increase in the workload on switchgear is attributable to the growth of the distribution system.
Changes in Maintenance Programming
Prior to the introduction of CENS in 2001, TrønderEnergi performed substation maintenance using the method known as M1 district outages, whereby each high-voltage/medium-voltage (HV/MV) substation together with the 50- to 150-MV/LV substations supplied from this source were disconnected for an entire night. Maintenance was then undertaken by a large number of field crews.
When the CENS regulation came into effect in 2001, the interruption cost penalties applicable to the M1 method of maintenance programming were too high, so a different approach had to be devised. TrønderEnergi adopted the M2 interruptive MV/LV substation maintenance program, in which single MV/LV substations were subject to prearranged maintenance outages. The work was undertaken during the day or night, depending on the location, and mobile generators were used to limit the outage period.
The main driving force for the development of the bypass switch was the need to establish non-interruptive maintenance procedures. The M3 non-interruptive MV/LV substation maintenance method addressed two objectives:
The increasing need for maintenance of an aging switchgear population
To improve the quality of supply in accordance with the objectives of the CENS regulation.
Impact on Interruption Costs
The most important benefit of using the bypass switch compared with previous substation maintenance practices, there is no interruption to customer supplies and, therefore, no interruption costs.
The M1 district outages method involved customer disconnection for an entire night even though substation maintenance only took 1.5 hours. The result in terms of interruption costs was US$5,000/substation. The M2 interruptive maintenance practice normally resulted in a 1.5-hour outage of an individual MV/LV substation. The cost of this outage in Trondheim was estimated to be $2200 for a daytime shutdown and $1,000 for a nighttime shutdown.
The adoption of the M3 non-interruptive maintenance method discontinued the need to inform customers and the media of impending shutdown. For an individual MV/LV shutdown, the average cost of providing notice was $500.
Effect on Labor Costs
By employing non-interruptive maintenance, the associated labor costs attributable to maintaining a single MV/LV substation are reduced by some $700. This savings is a result of the reduction in network switching and earthing procedures, and also because no nighttime work is involved.
The time taken to maintain a breaker is slightly longer when using the bypass switch. However, the work can be performed in the winter when, in Norway, the available workforce is higher than in the summer. Performing the maintenance during the season of maximum network loading also provides a better opportunity for condition monitoring (that is, use of thermography).
Benefits and Costs
The total benefit in Trondheim is some $500,000 per annum, assuming 180-MV/LV substations are completed per annum. It should be noted these savings exclude the cost of equipment installed for the non-interruptive maintenance method.
TrønderEnergi was faced with regulatory pressure to minimize the cost penalties levied on prearranged interruptions for substation plant maintenance. By adopting the AIS shunt plates, and thereby developing safer work practices, the utility has identified and benefited from the considerable reduction in penalty costs linked to the use of live maintenance procedures. Furthermore, and equally important, is the improvement in the quality of supply for all of the utility's customers.
The authors wish to thank the help and assistance given by SINTEF Energy Research including use of their high voltage laboratories and the support given by Leif Wilhelm Ramslie (ABB Norway).
Geir Solum ([email protected]) holds a MSEE degree from the Norwegian University of Science and Technology. Currently, he is chief engineer at TrønderEnergi Nett, a power grid utility supplying middle Norway. His main focus area is risk-based asset management.
Knut Samdal ([email protected]) holds a MSEE degree and a master's degree in organization and leadership, both from the Norwegian University of Science and Technology. Currently, Samdal is research director at SINTEF Energy Research, where his fields of expertise include smart grid, power system analysis, reliability analysis, Interruption cost assessment and quality of supply regulation.
Costs of Maintaining a Single MV/LV Substation Using M3 Non-interruptive and M2 Interruptive Maintenance Procedures
AIS Shunt Plates
TrønderEnergi Nett is the owner of patent rights already granted in some countries, including Norway, the United States, Australia, New Zealand and South Africa, and pending in Europe and several other countries for the bypass switches. So far, focus has been on internal needs in TrønderEnergi, and the volume of shunt plates and equipment produced has been relatively small and made by a local manufacturer. The utility plans to adapt the method and equipment for use with most common types of medium-voltage switchgear, cells and bus bar designs, and to make the concept available for other grid companies and service providers.
SINTEF Energy Research | www.sintef.no
TrønderEnergi | www.tronderenergi.no