Customer Demands and Expectations for Quality and Security Improvements of electricity supplies have accelerated in recent years, increasing faster than improvements in the design and performance of electricity networks. In Sweden, the 10-kV to 20-kV distribution network supplying rural areas is comprised mainly of overhead lines. Therefore, the network is exposed to weather-related disturbances from storms and snow that result in trees falling on overhead-line conductors.

Swedish utility Vattenfall BU Distribution Nordic Sweden has launched a large-scale project to refurbish its entire 10-kV to 20-kV distribution network to make the system weather robust by replacing overhead lines with underground cables. The conversion to an underground cable network, in combination with substation modules, also will improve the working environment and security of field personnel. Furthermore, the fieldwork for fault location and repair during extreme weather conditions will be significantly reduced.

LIFE-CYCLE OBJECTIVES

The introduction of insulated plug-in cable connectors to simplify connection to substation equipment will improve personnel safety. And, network stations, designed as modules, will simplify and reduce fieldwork. These 10/0.4-kV modules will also prove easy to replace in the event of a fault. However, this major change in approach to rural networks in Vattenfall's system has introduced a need to change the design regarding earth-fault currents to comply with the existing Swedish legislation. Thus, a new module was developed in cooperation with manufacturers.

The design of a simple and straightforward power distribution network in the rural parts of Sweden will result in a simplified network structure, enabling a reduction in resources. Although the investment cost for an underground-cable system may be higher than for a conventional overhead-line system, the lifetime cost over time, considering all aspects, will be similar or less than that for the traditional network.

THE DRIVING FORCES

Historically, the medium-voltage (MV) systems in Sweden's rural areas have comprised overhead lines, with the majority routed through wooded areas. When these lines were erected, a large number of small utilities managed the electricity industry, but changes have created fewer, larger centralized utilities. In some areas, this has resulted in increased outage times due to longer fault repairs.

More importantly, however, are the customer demands at a time when energy usage is increasing, together with the range of applications for electricity that has led to an increasing gap between the customer expectation and actual levels of supply reliability. This factor has been one of the main driving forces for Vattenfall's launching of a large-scale program for the redesign of the MV system.

The considerable media interest in supply reliability promotes awareness among all customers. Hence, customers are aware of supply failures, increasing the damage to the utility's reputation. At present, widespread network damage caused by a major storm results in a large number of staff being simultaneously required for fault-repair work. Vattenfall expects there will be a reduction in both the cost and maintenance of network fault repairs with the introduction of the new technology, which should address the following issues:

• Unacceptable supply reliability due to weather-related disturbances

• Increasing and changing customer demands

• High repair and maintenance costs

• Media focus adversely affecting the Vattenfall brand

• Extreme and difficult environment for field staff.

THE PROPOSED SYSTEM

The distribution network should be designed simply with the minimum number of components required to achieve high availability with minimum investment. Three types of modules have been identified: transformer, switch and branching. A transformer module incorporating a Petersen-coil (P-coil) has been developed to provide compensation for ground-fault currents.

The circuit between modules should comprise underground cables to reduce fault rates. Also, the design of rural distribution networks is normally radial; however, for certain sections, it may be preferable to arrange for an alternate source of supply during fault repair conditions.

The transformer module includes a transformer and a cable cabinet in a single unit. The transformer has internal fuses on the MV side and three-phase disconnection facilities for internal faults and for flashovers in the low-voltage (LV) winding to ground. The MV fuses and the isolating circuit breaker are enclosed within the transformer. The cables are connected with touch-proof connectors, and some of the transformer modules include P-coil in the transformer tank. Mounted on a concrete plinth or concrete foundation, the transformer module is designed for simple installation and removal by a single operative with a truck-mounted mobile crane.

Like other modules, the switch module is compact, has touch-proof parts, and is easy to install and operate. Functionally, this module should have the ability to open and close on load current to allow for network reconnection during normal operation. The device is designed for manual operation in the field, with the option for remote control in specific locations.

The branching module consists of a touch-proof cable cabinet (630 A, 12 kV to 24 kV) and is used to connect three branches. The touch-proof cable connectors are equipped with a capacitive test point (voltage test) and the modules include facilities for grounding and easy disconnection of a branch.

The cost of laying cables is a major component of the total investment cost, which preferably requires cost-effective laying techniques. The cable should be designed to withstand tough installation conditions in air, ground or water. The cable should be easy to bend and particularly suitable for installation by mole plough, which is an inexpensive method.

PROTECTION AND FAULT CLEARANCE

Swedish legislation specifies certain standards on maximum ground-fault currents and touch voltages; hence, proper functionality regarding ground faults was a key consideration. The maximum touch voltage allowed in Sweden is rather low (100 V) because the LV system is a TN-C system.

The comprehensive use of underground cable in the rural distribution network increases problems related to ground-fault currents, especially for mixed networks comprising cable and overhead lines. To secure the necessary selectivity and sensitivity for ground faults, studies confirm the need to compensate for the capacitive currents locally (distributed compensation). Therefore, a combination of compensation devices is installed in the source substation and locally in the 10/0.4-kV substations. These compensation devices require, for example, a low R-X ratio and a linear voltage-current characteristic. The distributed P-coil is enclosed in the transformer tank.

The transformer module is equipped with fuses on the LV side to protect for transformer overload and to clear external faults. The fuses — located in the transformer tank — on the MV side provide protection in the event of internal faults in the transformer. All faults on the MV system are disconnected from the source substation. This technical concept also brings other benefits as the module approach with fewer components reduces the need for maintenance and increases the time interval between inspections.

NETWORK OPERATIONAL FEATURES

The system will require no changes to current operational practice following a fault. The branch and switch modules play a central role for fast fault location as well as limiting the number of customers affected by the fault. Transformers subjected to a fault can simply be replaced in an easy and fast procedure aided by standardized snap-on connections, simple mechanical mounting on concrete foundations and the use of a standard mobile mounted crane.

Although the equipment is designed with touch-proof connectors, a special tool has been developed for use to snap off and snap on cable connectors, a feature designed to improve operational safety.

ECONOMIC ASSESSMENT

Six alternative system design networks for a typical section of the MV network were compared and evaluated on the basis of life-cycle costs (shown at right). These alternatives were as follows:

1. Existing overhead lines (with bare conductors)

2. Overhead line with covered conductors

3. Underground cable and traditional techniques

4. Underground cable and traditional techniques (no disconnecting switch)

5. Underground cable and new techniques based on current costs

6. Underground cable and new techniques based on future costs.

The economic analysis confirmed that the cable network is economical if customer costs are included and the use of the new substation technique does not significantly affect the overall economy. However, the analysis conservatively assumes that the cost for the new substation technique is equal to that of traditional techniques. Based on the simplified approach in this new technique, there are reasons to assume there is a significant potential for cost reductions of the modular components with increasing competition and mass production of the units. This new design concept has fewer components, reducing maintenance, the periodic inspection workload and fault outages, thereby improving network reliability and availability. Also, the design features in the new modules and use of underground cables will substantially improve utility and third-party safety.

PILOT INSTALLATIONS

Vattenfall has two pilot projects on trial to develop the concept and test the theoretical approach. Two different manufacturers (Transfix and ABB) are involved, one on each project. The projects are similar in size, each including some 25 10/0.4-kV substations. The operational experience to date has been very good with no problems regarding the substation and modular technique. The conversion from an overhead-line to an underground-cable network on one project has already substantially improved system availability by reducing the SAIDI from 1400 minutes to 70 minutes.

On the Värmlandsnäs pilot project, covered overhead-line conductors have been retained due to rocky terrain in the area. Also, a section of the existing overhead line with covered conductors has been retained so that performance of the grounding arrangements and overvoltage lightning protection on an underground-cable/overhead-line network can be monitored.

FULL-SCALE IMPLEMENTATION

So far, the experience from the pilot projects has been helpful in verifying this design concept. Therefore, Vattenfall has decided to move ahead with implementation on a larger scale. During the spring of 2007, Vattenfall introduced the new concept in the updated frame agreement with several manufacturers. Conventional transformer stations and the new compact design are in parallel. The choice between the concepts will be based mainly on economics.

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Bernt Hansson has 29 years of professional experience. For the last five years, he has been responsible for network optimisation within Vattenfall Eldistribution, focused on an investment programme linked to the rehabilitation of the distribution network. Previously, Hansson worked with consultancy services on an international basis within the transmission and distribution sectors. Currently, Hansson is a director for Harryda Energi, a network company located close to Gothenburg in Sweden. bernt.hansson@harrydaenergi.se

Kristina Olofsson, who has a BSEE degree, joined Vattenfall at the beginning of 2004, working for Vattenfall Nordic Distribution. Following responsibilities for power quality, Olofsson is now involved in studies linked to long-term network planning and network analysis. kristina.olofsson@vattenfall.com

Arne Berlin was awarded his master's degree in electric power systems before joining Vattenfall. He has vast experience in network studies, managing network contracting and metering. For the past seven years, Berlin has worked on fault and power system analysis and the fault registration system in Vattenfall Nordic Distribution. arne.berlin@vattenfall.com