Voltage regulation can be a problem in Norway, especially in rural areas. Voltages outside the statutory voltage level of 230 V ±10% (below 207 V) have been recorded. New planning regulations that came into force in 2005 require the statutory voltage standard be maintained at all times. In city and urban areas, the on-load tap-changers on primary substation transformers, and the off-load tap-change facilities on distribution transformers ensure the maintenance of satisfactory voltages, despite significant differences between the summer and winter system load demands. The situation is different in rural areas where a six-month review of off-load distribution tap-changers is required on the extended medium-voltage overhead line networks.
Customers have become concerned about voltage quality because of their increased use of electronic equipment. Their concerns are no longer limited to supply voltage, but also include flicker, voltage dips and harmonics. Customers are now interested in the most cost-effective solution to address the problem of voltage quality.
Hafslund Nett AS (HNAS; Olso, Norway) is Norway's largest power distributor and the fourth-largest grid company in Scandinavia, serving more than 525,000 customers in Oslo and the surrounding municipalities. System-voltage levels range from 300 kV down to 0.4 kV, with a standard medium voltage of 22 kV, but due to historical reasons, several intermediate voltage levels remain in service.
One of the most common methods to improve voltage quality in rural areas is to increase the cross-section of the low-voltage cables or conductors of the feeder supplying the area. However, on overhead lines, increasing the conductor cross-section is not always an option, because the existing supports are not able to accept the additional mechanical loading. In situations where loadings or line length prohibit increasing the conductor cross-section, HNAS' system reinforcement involved an additional feeder or circuit-voltage upgrading. Although this solves the problem, it is not a cost-effective solution, considering the high investment cost compared with the increased revenue from energy sales. For these situations, a temporary solution satisfies customers, thereby providing time to consider a permanent system-design solution when long-term development plans and financial resources are available.
Conventional motor-driven voltage regulators provide a mechanical solution for HNAS to regulate the voltage automatically. The majority of these time-expired units have been removed from the network because of several shortcomings:
- Slow response to load fluctuations
- Inability to handle unbalanced load conditions
- Poor reliability (errors and breakdowns)
- Need for frequent maintenance.
In the spring of 2004, technology company Magtech AS (Moss, Norway) offered HNAS a way to correct low-voltage problems with its magnetic voltage booster (MVB). This device employs a patented design to regulate the magnetic flux in a coil, enabling constant voltage without any moving parts. The required voltage output is produced by the injection of a small dc current. The robust unit can handle and correct skewed loads, and is resilient to surges and over voltages. The MVB has a 300-msec response time and can lift voltage by 25%; therefore, it can provide voltage regulation for rural areas where low-voltage lines up to 2 km (1.2 miles) supply power to consumers at the remote end.
HNAS was invited to participate in a pilot program with four grid companies to test the operational performance of the MVB, which is currently available for Norwegian utilities. A pilot version was developed in cooperation with Vattenfall Eldistribution AB in Sweden for the European market. The first 400-V unit is installed at DjurÖnäset, VärmdÖ, just outside Stockholm.
The pilot MVB was installed in March 2004 on a low-voltage overhead line at Hvam, Norway, that supplied three farms and two domestic premises. Voltage investigations confirmed that the supply voltage was below statutory limits. The unit that is designed for installation without a supply connection was commissioned during a one-hour pre-arranged supply interruption. The performance of the unit was monitored continuously by on-site instrumentation and transferred to the utility via GSM for a one-year period. The results of this trial confirmed a stabilized satisfactory supply voltage of 235 V. A subsequent customer survey confirmed that the perceived voltage quality corresponded to the measured and recorded values.
The MVB has now been approved as part of HNAS's portfolio for correcting low voltage. The utility has already bought two more units for locations where the cost benefit of an MVB installation was substantial compared to the alternative solutions. Once the MVB is in volume production, the utility expects it to provide a satisfactory means of voltage regulation for several projects. Because HNAS has a clear focus on cost-benefit analysis, the utility conducts cost-efficiency studies prior to all investment decisions linked to projects for the distribution grid. Since the installation of these devices resolve the problems of network voltage-quality problems, the utility has continued to receive favorable responses from those customers who benefit from the trial installations.
Einar Hestetun is an electrical engineer and a graduate in power engineering from GjØvik Technical High School. He then joined Hafslund Nett AS and is currently responsible for voltage quality on the utility's distribution network. Einar.Hestetun@hafslund.no