In Common with Most Other Utilities in European Union Countries, Danish utilities must adhere to the strict limitations, imposed by deregulation, on investments linked to quality of supply. This is now being followed by benchmarking based on the system average interruption index (SAIDI) and system average interruption frequency index (SAIFI).

Over the past decade, the quality-of-supply indices for DONG Energy has remained more or less unchanged following a period of investment in the traditional methods used to improve system performance. These included refurbishing or replacing time-expired assets, replacing overhead lines with cables and improving response times by implementing outage management and mobile workforce management systems. DONG Energy Sales and Distribution operates the electricity supply network that supplies some 980,000 customers in and around Copenhagen, the capital of Denmark.

A 20-year asset management study of DONG Energy's distribution assets has confirmed that, without increasing investment, the existing quality of supply will start to decline beyond 2010. Due to the utility's expenditure limitations, automation of the distribution grid offered an economic solution.

However, the utility's market investigations highlighted weaknesses in the commercially available distribution automation (DA) schemes. They were generally only suitable for new substations equipped with ring main units, and they mainly focused on outage management.

DONG Energy believed that the DA scheme should add value to the entire business and offer a positive cost-benefit. So, in 2003, the utility decided it would develop a new solution to improve fault management and optimize the use of the medium-voltage (MV) system.

THE DISTRIBUTION GRID

DONG Energy's 10-kV distribution system comprises 100% underground cables and is designed to operate radially. The system neutral is Petersen-coil grounded, which allows sustained operation with a single earth fault. All 10-kV low-voltage (LV) transformers are 3-phase units, and balanced feeder loading is assumed. Field staff supervision, operational planning and outage management are subject to central control using a supervisory control and data acquisition (SCADA) distribution management system (DMS).

System faults are located by field staff, aided by mechanical or electromechanical short-circuit indicators (lacking directional sensitivity) that are installed in most distribution substations. All MV switch disconnectors in MV/LV substations are manually operated, but SCADA is used to remotely control the circuit breakers in high-voltage (HV)/MV substations.

Distributed generation facilities, including co-generation heat and power plants (CHP) and wind turbines that are being connected to the majority of the utility's MV feeders, will increase the risk to misinterpret short-circuit indications due to the lack of directional sensitivity.

THE AUTOMATION SYSTEM

The key requirements for the automation system sought by DONG Energy were:

• Sensors suitable for installing on all switchgear regardless of type and age

• The ability to provide information on MV fault currents, fault direction and distance to fault

• Continuous measurement of operational currents, voltages and power.

DONG Energy selected the Discos system that was developed by NESA, one of the six Danish power businesses merged into DONG Energy in 2005. In addition to NESA and DONG, these included Elsam, ENERGI E2, Copenhagen Energy's power activities and Frederiksberg Forsyning. In late 2006, PowerSense A/S was established, jointly owned by DONG Energy and a venture capital company. All commercial rights and technical information were then transferred to PowerSense A/S, who developed, manufactures and markets the system.

SYSTEM STRUCTURE AND SCOPE OF FUNCTIONS

A unique feature of the Discos solution is the fiber-optic current sensors, which have a 0-A to 20,000-A range and are very compact at 50-mm (2-inches) long. These properties enable the sensors to be used in almost all types of MV and LV switchgear.

DISTRIBUTION SYSTEM APPLICATIONS

The fiber-optic current sensors are based on Faraday's principle, which states that the plane of polarized light undergoes a rotation relative to the magnitude of the applied magnetic field. Since the current produces a magnetic field, it may be measured by determining the angle of rotation of the polarization plane. The sensors are attached directly on the cable or bare conductor and the fiber optics guide the polarized light from the sensor to the control module. This eliminates the risk of electromagnetic interference while ensuring galvanic separation between the HV and control circuits.

Voltage measurement is taken on the LV side of the transformer and, by using the transformer ratio and the vector group, the control modules are able to continuously calculate the MV voltage.

The system is adapted for each substation, selecting the modules for current and voltage measurement, battery and charger, and global system for mobile communication with the control center. The system offers the user a choice of communication channels, including short message service (SMS), TCP/IP and general packet radio services (GPRS). DONG Energy uses an SMS security system for alarms and commands and GPRS for on-line measurements. The table summarizes the measurements, indications, alarms and controls. In addition, the modules have several extra analog inputs, binary inputs and command relays (e.g., temperature or pressure monitoring).

During normal operation, the Discos system is used to determine voltage, and real and reactive power, and transmit them to the control center on request. Similarly, in the event of a short-circuit fault, the system determines the current, fault direction and electrical distance to the fault, providing control staff with much-faster fault location. The system also is used for remote control of circuit breakers and switches, again improving fault-restoration times and status monitoring of all operational plant.

The sensors on the MV radial distribution system are installed with three different operational features:

• The Type A sensor system significantly reduces the extent and duration of outages for the majority of customers from 30 to 90 minutes to 5 to 10 minutes.

• The Type B sensor system can further identify the fault position and reduce the time required for circuit isolation and supply restoration.

• The Type C sensor system is used to provide information on MV and LV outages, transformers' loads and voltage quality data on individual feeder MV/LV stations.

System operational data available from a Type C sensor installation can show that the use of 3-phase transformers does not necessarily ensure balanced load conditions. Further investigation indicates that these conditions are attributable to the unbalanced load on the LV network as recorded on the LV side of the transformer.