A power distribution system can be an interface between the power transmission network and electricity end-customers. For example, a power distribution system can include a number of primary substations which are connected to secondary substations via power lines and switches. The primary substations can contain transformers that reduce the voltage from the HV (high voltage) level of the transmission or sub-transmission grid down to MV (medium voltage) levels suitable for regional transmission. Distribution level network control can involve pieces of secondary equipment interacting with the primary equipment of the substations and power lines. Primary equipment or devices can include switches, tap changers, capacitor banks, and the like. Distribution areas, (also termed regions or cells herein) can be assigned to one single primary substation and defined by electrically unambiguously connected primary equipment. For example, the primary equipment can be connected in a tree or feeder structure. However, a distribution area is subject to changes during reconfiguration of network topology. These changes can potentially lead to a discrepancy between the distribution area and a virtual domain of associated secondary equipment. In addition, distributed generation of electric power on lower voltage levels of the distribution system can generate some considerable coordination tasks for distribution level network control.
By way of example, the paper by Per Lund entitled “The Danish Cell Project—Part 1: Background and General Approach”, IEEE 2007, Power Engineering Society General Meeting, June 2007, describes a Cell Controller Pilot Project which aims at developing a new solution for optimal management and active grid utilisation of the large amount of distributed generation present in Western Denmark. For this purpose, the 60 kilo Volt (kV) network parts below each 150/60 kV transformer at the primary substations can be operated as radial networks by opening a sufficient number of 60 kV line breakers in selected substations and thus sectioning the otherwise meshed networks of the 60 kV distribution systems. Each of these radially operated 60 kV networks can then define a 60 kV distribution cell, to be controlled by a cell controller with a number of functions and a link to the Supervisory Control And Data Acquisition (SCADA) system at the Network Control Center (NCC) of the Distribution Network Operator (DNO).
Cell or distribution controllers, also termed Intelligent Substation Control Systems (ISCS), may comprise one or several physical devices and can be located in a primary substation of a distribution area. An ISCS is capable of functioning as a substation gateway to the NCC by providing gateway functions for mapping signals between secondary equipment for protection and control and higher-level systems. For example, the ISCS can translate internally process data from various master protocols into a standard protocol, e.g. the IEC 61850 standard data model, and can translate the data from the standard data model into a common slave protocol.
According to one example, an ISCS is connected through the existing communication infrastructure to an NCC, the ISCS and the NCC communicating via a tele-control protocol of the master-slave type, for instance IEC 60870-5-101. A number of other protocols, such as SPA, LON-LAG and IEC 60870-5-103, are used to connect the ISCS to the secondary or process devices for protection, control and monitoring purposes. The process devices can, for example, be located in the vicinity of the primary devices and execute local decision logic. On the other hand, the IEC 61850 standard protocols are client-server based, which allows several clients to access data from a same server or process device. They define the semantics of the data within the substation in a standardized object-oriented way, and offer a standardized method to transfer data between different engineering tools in a standardized format.
The ISCS can include, for example, OPC (Object-Linking and Embedding (OLE) for Process Control, also referred to as “OPen Connectivity”) Data Access client and server components. OPC Data Access is a group of standards that provides specifications for continuously communicating real-time data from data acquisition devices to process or interface devices and for synchronizing process measurements with mirror entries at an OPC server. OPC also allows a client application to access several data items with one single request. OPC clients are used for slave/server protocol stacks to enable external systems to access data available on OPC Servers. OPC servers in the ISCS can be used, for example, for master/client protocol stacks in order to provide access to the data in the data acquisition or process devices connected via a particular protocol. Different types of OPC server instances, depending on the process devices connected and/or the protocols (LON, SPA, or IEC 61850) used to communicate with the process devices are instantiated in the ISCS.