One of the Most Important Regulatory Customer-Service Parameters established by the Italian Authority for Electrical Energy and Gas (AEEG) is the cumulative duration of extended supply interruptions. For this network-performance index, interruption is defined as a loss of supply for longer than three minutes, excluding fault-caused outages that are deemed to be beyond a utility's control. AEEG uses the index results to determine financial penalties or premiums, which increases the incentive for utilities to seek ways to minimize the length of circuit outages.

INTERRUPTION DURATIONS

The cumulative customer interruption statistics for ENEL Distribuzione S.p.A.'s (Rome, Italy) medium- and low-voltage (MV/LV) distribution systems and for the LV network are outlined in Table 1. The last line of the table shows the number of interruptions where the fault cause is unknown, which account for 30% to 35% of all interruptions on the LV network. Field staff can quickly restore supplies for this type of fault via switching; however, these particular interruptions have two negative effects:

  • The increasing duration of interruptions per customer, with the imposed penalty of some 3600 euros/minute lost.

  • The cost of the field staff time involved in completing circuit operations.

Table 2 shows the number of LV network interruptions over the past three years where the fault cause was not identified. The time taken to locate and restore supplies following each interruption averaged three hours. Some 70% occur during normal working hours at a cost of 100 euros/hour, the remaining 30% outside normal hours cost 200 euros/hour. Hence, the considerable economic impact on the operational costs of handling these types of LV faults justifies developing technologies for the remote control of LV networks.

MEDIUM-/LOW-VOLTAGE SUBSTATION DESIGN

The standard ENEL MV/LV distribution substation comprises an MV section (a ring main unit or mixed insulation with an SF6-insulated switch-disconnector and air-insulated busbars or an air-insulated switchboard), an MV/LV transformer and a LV switchboard controlling four outgoing circuits, each controlled by an LV circuit breaker (Fig. 1).

ENEL Distribuzione has designed a remote-control facility for the LV motor-driven circuit breakers. The unit employs a global system for mobile (GSM) peripheral unit (UP) equipped with a 24-V battery for the switchgear motor drive. The new components are designed for use in ENEL's standardized MV/LV substations, with standard LV assemblies with the remote-control UP already being used for remote control of the MV system. No additional equipment or operations other than the installation of traditional circuit breakers are required. This LV remote-control system employing motor-driven circuit breakers now forms a new ENEL Distribuzione technical specification.

The remote-control system and motor-driven circuit breakers are designed to send a signal via the GSM to the ENEL Network Operating Centre (COR) in the event of a circuit-breaker operation, and to receive opening and closing commands from COR staff. For transient faults with this system, the supply is restored, field staff intervention is not required and, hence, operating costs are substantially reduced.

CIRCUIT-BREAKER SPECIFICATION

ENEL's motor-driven remote-controlled circuit breakers are connected to the remote-control UP installed in the MV/LV substation. The motor drive is mounted on a 250-A four-pole circuit breaker with thermal and magnetic releases and three operational positions for the main contacts (i.e., open, trip and close).

The isolatable circuit breaker has a moulded-case enclosure (of organic material) with nonaccessible metallic parts designed to be maintenance free. Table 3 outlines the main features of the ENEL circuit breakers used for LV network control.

The motor drive is designed to ensure the duration of motor-driven operations (for both making and breaking operations) are less than 5 sec. Remote operations are executed via the UP with a command that is held on for 300 msec.

The circuit breaker has a three-position — remote-controlled, manual and locked — manual selector that prevents remote operations when the selector is in the manual or locked positions. The locked position is only available when the circuit breaker is in the open position, and for safety reasons, the remote-controlled position allows manual tripping of the circuit breaker. Therefore, the circuit breaker follows the principle that the trip position indicates a network fault while the open and close positions are normal operating conditions. The trip position is retained until the closing command is received that prompts a return to the open position, spring recharging and circuit-breaker closure.

The LV network must remain operational at all times, so if the remote-control system fails, the circuit breaker can be manually operated via closing, opening and trip facilities. For security reasons, the circuit breaker is equipped with an anti-pumping device that prevents repetitive operations should a micro-switch fail. For safety, the operational lever is not accessible when the circuit breaker is equipped for remote-controlled motor-driven operations. Table 4 identifies the main characteristics of the motor drive.

LOW-VOLTAGE SYSTEM CONFIGURATION

The LV network remote-control operations are effected by means of the same system already in use for the MV network remote control called Sistema Telecontrollo Media Tensione (STM), which translates to "MV network remote-control system."

To avoid confusion between the MV and LV sections, the LV section is configured as a new substation busbar linked with the MV section by means of a "fictitious" link as shown in Fig. 2.

In the STM system, each circuit breaker is identified by a reference number, and a GSM channel is configured to receive, in the case of a protection operation, the signal sent by the circuit breaker. The COR staff can then remotely control the circuit breaker and disable the remote-control facility. This facility must be used during field staff work on live LV lines, thereby preventing a closing operation after a circuit trip.

The STM system can record fault data and, in particular, the duration times, and the start and finish times of supply interruptions.

FIELD TRIALS

The first four pilot circuit breakers produced by two different manufacturers, ABB and Bticino, were successfully installed in Lametia Terme (Calabria) and Ginostra (Stromboli Island-Sicilia). To date, the use of remote control has been 100% successful for transient faults, and supplies are remotely restored for some 50% of the LV network faults. Hence, there has been a significant reduction in field staff intervention and a significant reduction in the length of LV system outages.

The further installation of some 1000 remote-controlled circuit breakers in the customer-dense metropolitan areas is part of a major evaluation study that will extend across Italy. This installation program is 50% complete. The major study will be reviewed after a 12-month period to evaluate the benefits of introducing this technology in terms of the reduction in the duration of LV supply interruptions and the reduction in the cost of field staff switching.

Antonio Cammarota received a BSEE degree from Federico II University of Naples in 2001. He joined ENEL in Rome in 2002, where he has been working in the headquarters R&D department on HV, MV and LV switchgear design and product development engineering. He is chairman of the Italian National Committee of CIRED, the Italian representative within the CIRED Directing Committee and a member of various IEC committees and working groups. antonio.cammarota@enel.it

Luca Giansante joined ENEL in 1987 and has 21 years experience working in many engineering disciplines on HV, MV and LV networks. Beginning with the coordination of field activities on MV and LV networks, he then began working on the maintenance of primary substations and protection systems. For the past 10 years, he has worked in ENEL's headquarters R&D department on secondary substation components in infrastructures and networks. He is currently a member of the CEI 17A/C Committee. luca.giansante@enel.it

Table 1. Annual cumulative duration of supply interruptions per customer in minutes for ENEL Distribuzione.

Year 2004 2005 2006
Total value (MV and LV network) 88.77 78.58 58.27
Total value (MV and LV network) excluding exceptional events and faults not caused by ENEL 60.05 62.58 50.86
Total value (LV network) 17.02 17.03 16.62
Total value of interruptions where the fault cause was unknown (LV network) 5.32 6.13 5.52

Table 2. Low-voltage network interruptions with unidentified fault causes.

Year 2004 2005 2006
Number of interruptions 68,335 77,151 72,658

Table 3. Traditional low-voltage circuit breaker characteristics.

Rated voltage 400 V
Rated insulated voltage 690 V
Rated impulse withstand voltage 8 kV
Rated uninterrupted current at 40°C 250 A
Rated short-circuit breaking capacity at Ue 400 Vac 16 kA

Table 4. Main characteristics of the motor drive.

Motor-drive voltage 24 Vdc ±20%
Maximum allowable value of current (excluding the peak) 13 A
Maximum operation duration 5 sec
Degree of protection (as per IEC-529 standard) IP4X
Other motor characteristics CEI EN 60034
Minimum operating temperature -5°C
Maximum operating temperature (with average value in 24 hours less than 35°C) +40°C
Storage temperature -25°C to +70°C
Humidity (without condensate and ice) ≤ 95%