Over time, the main function of the ENEL high-voltage (HV) network (132-150 kV) has changed from transmission to primary distribution. This change meant transferring the network and the associated supervisory control tasks from the Transmission Division to the Distribution Division and motivated the design of a new control system called the Distribution Control Center (DCC). This center will coordinate the operation of the HV networks and the dispatching of the load. The DCC will carry out tasks related to decision making and not to the direct operation of the networks. Construction of a prototype center is completed and field testing of a pilot application is underway.

Control of the ENEL generation and transmission system is performed by a National Control Center (NCC) and eight dispatching centers (DC). These centers provide control of the generating stations and of the interconnected extra high voltage (EHV) (380-220 kV) grid lines and HV (150-132 kV) still operating for transmission. These control centers are managed by the Transmission Division.

Distribution is organized into three main levels: district, zone and agency. A district is responsible for the HV networks (lines and high-voltage/medium-voltage feeder substations). The zones operate the medium-voltage (MV) networks. The agencies assist the zones for the operation of the MV networks and are responsible for the low-voltage (LV) networks. Telecontrol of the distribution system has been conceived to match the organization structure and is implemented by 25 Standardized Telecontrol Systems (STS) placed at district level, supervising the feeder HV/MV substations (150-132 kV/20-15-10 kV) and the associated HV and MV lines. These telecontrol systems are equipped with operator terminals placed at district as well as at zone level (for the operation of the MV line circuit breakers).

Structure & Operation of Systems The ENEL distribution system (Fig. 1) includes about 1300 HV/MV substations and about 25.000 kM of lines. The HV network has a meshed structure and its operation is also meshed. The HV/MV substations and HV users are supplied, in general, by lines connected end to end between two bulk transforming stations (Fig. 2).Lateral tie lines are sometimes present but are closed only in emergencies, so that the HV supply system is in practice subdivided into a number of "load islands," each typically consisting of four or five lines that can also connect generating stations with ratings up to 200 MW.

The control of an HV network with the above configuration is essentially based on the control of the voltages at the bulk supply bus bars. These voltages are established at the planning stage of operation, evaluating the expected active and reactive power flows and checking compliance with the service requirements on the network and at the bulk supply nodes.

Architecture of the DCC The DCC has been designed to take the maximum advantage in terms of cost/benefit from the available options of computer technology, while maintaining satisfactory reliability. The architecture shown in Fig. 3 includes:

Two main computers of RISC type with a full graphic interface dedicated to operation monitoring and analyses. Each main computer drives a console with two video display units, which allow simultaneous presentation of on-line operation information and the results of analyses or predictive studies. n One front end computer dedicated to the communication with computers belonging to upstream dispatching centers (generation and transmission) and downstream telecontrol centers (distribution), to receive the relevant information (events, measurements and messages) on the operation of the corresponding networks. The communication function is implemented with a single computer, the choice of the type "fault tolerant" (internally redundant), less expensive than a double computer configuration.

Three local area networks (LAN). Two of these connect the main computers to the front end computer. The third LAN is used for the data exchange between the main computers and for a link to external computers dedicated to off-line load flow analyses and operation planning.

Two communication servers, each equipped with two LAN interfaces. To achieve the maximum simplicity of the system, the main computers operate in full parallel, avoiding the intricacies of a warm back-up. However, only one of them can update the dynamic data base and the system log. The loss of a computer implies the loss of an operator console. Maintenance activities, including the updating of the static database of the controlled network and the testing for the activation of a new network configuration, can be carried out without any loss of control. The architecture of the system makes the software and the hardware largely independent from each other so that the system is open to future changes without costly adaptations. For instance it would be easy to substitute the front end computer with two smaller units, should such a choice become competitive in terms of cost and performance.

Functional Characteristics Generalities The operation of HV electric networks can be subdivided into two groups of activities: activities required to support the strategic decision-making, and actions related to the execution of local or remote switching maneuvers. As mentioned before, the latter actions are not carried out by the DCC, but by other control centers.

The first group belongs to what in the following will be called "supervisor control" and constitutes the purpose of the DCC. The objective of supervisory control is to obtain the best compromise between economy and security of operation and can be subdivided into prediction, operation and assessment of performance.

Prediction is based on load forecasts and on assumptions of availability of means to meet the anticipated demand (generators, lines, transformers). By ENEL practice, this evaluation is carried out by classifying the states of the supply system as normal, critical and emergency. In the normal state, the supply system is unperturbed and each element operates within its rated characteristics. In the critical state, the conditions are those of the normal state, but the loss of an additional element might place the supply system in an emergency condition. In an emergency state, one or more elements are operated outside the correct range of performance, and it is possible that circuit breakers may be tripped by their protections.

Operation controls the state of the supply system (voltages and power flows) and requests to the telecontrol centers or to on-site agents the switching maneuvers necessary to face unpredicted or planned conditions, which require changes in the configuration of the networks.

The assessment of performance is carried out by a comparison of the real operating conditions with the predictions, in order to improve the existing planning.

Main Functions (see sidebar) The main functions supported by the DCC are: Load analysis and forecasting Definition of network configurations Voltage control Network calculations Coordination of protections and automatic devices Planning for work programs Contingency analyses Relations with HV users

To carry out the functions of the DCC, certain supervisory activities must take place, which in turn require knowledge of the state of the supply system based on data being fed into the center computers.

Supervisory control activity consists of: Verifying operation configurations, power flows and voltages at the supply bus bars and supply security analyses

Identifying and executing the actions necessary to bring the network back to safe operating conditions under an abnormal situation

Coordinating the switching maneuvers on the network

Coordinating supply restoration and re-energization of the network after an outage, in cooperation with the control centers of the generation and transmission system

Verifying the compatibility of the generators connected to the HV grid in case of variations of the network from the normal configuration, in order to request changes in the generation schedules.

To perform the supervisory control activities, a certain amount of information is required, including the connection scheme of the HV lines; the state of circuit breakers in all substations; values of voltages and of active and reactive power and contingencies related to particular events such as the tripping of load shedding devices. This information is received from the distribution telecontrol computers.

The generation and transmission control computers contribute data on EHV/HV transformers; the transforming ratios of the EHV/HV transformers; the state of HV line circuit breakers; data for determining equivalent circuits of the generation and transmission system; load forecasts and operator-to-operator messages.

Considering that the territory covered by a distribution control center will be only a part of the total supplied area, it is necessary to exchange data with similar neighboring control systems.

Interactive Procedures The control system must fulfill two principal objectives: supervisory on-line control and analysis of the performance of the supply system. Supervisory control displays to the operator the network diagrams with the relevant signals, alarms and measurements. Signals are acquired with priority as soon as they are available. Measurements are refreshed every 20 sec.

Network analysis procedures are initiated by the operator and can be carried out for two time scenarios: present time and short term (typically up to three days). Therefore, the control system includes facilities to store and retrieve field information and the results of fault analyses and operation case studies.

Handling Capacity The handling capacity of the new control system has been tailored to the size of the largest existing HV distribution network, which could be considered self contained but for a limited number of interconnections to adjacent HV supply systems, identified with the same criterion of mutual independence.

Taking into account the necessity of an adequate margin for the growth of the supply system in time, this criterion led to a target handling capacity of 500 substations, 800 HV lines, 50 bulk supply transformers, 400 HV/MV transformers, 50 generating units, 3800 measurements and 2200 signals.

Considering the links with other control centers, the data exchange capacity has been designed for a maximum of 21 remote computers.

The control center represents a cost-effective solution to the supervisory control of HV supply systems. Its design has been based on a broad systematic approach that has covered all aspects, including telecommunications, in order to reach the optimum compromise in terms of economy, efficiency and reliability. TDW Editor's Note: This article is based on a presentation by the authors at the 1995 CIRED Conference.