Exercising Control

To help meet Egypt`s medium- and long-term economic objectives, a state-of-the-art regional control centre is to be installed in Cairo that will improve the electricity supply network. PEi highlights some of the features of the Cairo Centre that will be crucial to the ongoing expansion of Egypt`s electricity supply infrastructure.

Michael Jabbar,

Siemens Power Transmission & Distribution Group,

Minneapolis, USA

Expanding the electricity supply network is an important step in the ongoing development of Egypt`s national infrastructure. Energy demand in the country has increased significantly in recent years and is likely to continue to do so well into the next century. A well-regulated and reliable energy network is, therefore, a prerequisite for the sustained economic growth that is so important to this part of the world.

The increasing demand for electrical power by the growing industrial regions in Egypt, particularly around the country`s two major cities – Cairo and the Mediterranean port of Alexandria – coupled with the requirements of better energy management and growing environmental concern, caused the Egyptian Electricity Authority (EEA) to consider the advantages of the latest energy control systems.

After carrying out a careful economic and technical appraisal, the EEA decided to build new regional control centres for both Alexandria and Cairo, and to equip them with modern control and instrumentation systems based on the latest energy management principles.

An important aspect of advanced energy management systems is their open distributed architecture, a facility that brings not only a degree of operational flexibility not previously attainable, but also increased security and the capacity for the systems to be readily updated. Siemens is supplying the Supervisory Control & Data Acquisition (SCADA) systems for both centres on a turnkey basis – the first two of seven state-of-the-art RCCs planned by the EEA. The first, for Alexandria, was ordered two years ago (1997) and is currently near completion; the second, for Cairo, was ordered in 1998.

The Cairo system

An Energy Management System (EMS) is housed in the National Energy Control Centre (NECC) building in Embaba near Cairo. The NECC, as the executive authority, monitors and controls the country`s major power stations, the associated 500 kV transmission lines, and energy transfers between Regional Control Centres (RCCs) over inter-regional tie lines. The centre also directs supply restoration procedures in the event of a total system blackout. In essence, the NECC makes the key decisions affecting the National Unified Power System (NUPS) operations.

The Cairo Regional Control Centre (CRCC) is responsible for the operation of its regional resources that are not controlled by the NECC. This includes control of the 200 kV and 66 kV networks as well as co-ordination and scheduling of equipment outages. Data acquisition and alarm processing for all generation, transmission and sub-transmission is also a CRCC responsibility, along with historical data collection and reporting.

Egypt intends to develop the 500 kV network into a super grid feeding national zones, and to partition the 220 kV network as separate supplies under the control of RCCs. Operations at sub-transmission voltages (i.e. 220 kV and 66 kV) are and will remain co-ordinated from the NECC, but actual day-to-day operations will ultimately be the responsibility of the RCCs. When the Cairo project is completed in 2001, it will provide a comprehensive SCADA system and greatly improved energy control and management for the greater Cairo region. The ultimate goal is to implement hierarchical control that interconnects the EMS at the National Energy Control Centre with computer-based SCADA systems at all RCCs within the Egyptian electricity supply network.

The existing Cairo regional dispatch system uses voice-based communications to monitor and control the manned substations. The requirement for an integrated communications network is necessitated by the demands of hierarchical operation within the overall NECC/RCC control system, plus the benefits to be gained from a single network that can provide common communications services throughout the EEA operations area.

The turnkey project includes a Siemens Sinaut Spectrum SCADA system and the applications software, the construction of a new three-storey control centre building and a state-of-the-art communications network comprising voice/data communications and fibre optic cable that will provide high-capacity connections to 116 Remote Terminal Units (RTUs) and the necessary adaptation of associated substations. The overall project is scheduled to be completed in 2001.

System overview

The CRCC project is being built by Siemens Power Systems Control, a division of the company`s Power Transmission and Distribution Group, for the Egyptian Electricity Authority. It includes over 150 stations within the Cairo area. A modern SCADA system in control of all remote sites will reside at Cairo North station, where all command and control functions of the network are affected.

The communications network enabling the SCADA and RTU interactions is an all-digital transmission and switching system consisting of nine fibre optic rings with full equipment redundancy and alternate route protection. The CRCC communications network provides LAN-to-LAN, asynchronous-to-X.25, X.25-to-X.25 and digital voice trunks supporting networked PABXs.

The essential functions of the SCADA/RTU data communications are to provide:

• two-way data interchange

• concentration of traffic from any number of RTUs into fewer high-speed connections to the SCADA system

• transparent transfers of data blocks of any size from the RTU to the SCADA system

• alternate network routes from the backup SCADA workstations to the same port of an RTU

• path diversity from the backup SCADA workstations to a second port of the same RTU

• alternate SCADA system access to the RTUs from another network

• virtually error-free RTU traffic over the transmission network

To achieve the necessary functions, a connection-oriented packet-switched data network is implemented. This network uses the ITU X.25 standard interfaces over a wide area network. The packet-switched network is a two-tier implementation minimising queuing delays while providing multiple alternate routes.

The CRCC multi-ring fibre network is shown in Figure 2. This diagram shows the location of the packet switches over the fibre rings, some with integral PADs (Packet Assembler/Disassembler). Part- icipating RTUs are directly attached to the rings with a minimum of two ports. Each ring has access to at least two packet switches. RTU ports are distributed over these packet switches (with PAD functions) using path and equipment diversity rules. Therefore, each RTU may be accessed from a totally different path. More importantly, an RTU can be reached from a different SCADA workstation.

Multiple RTU ports are connected to different PADs where conversion from the asynchronous format IEC 870-5 protocol to X.25 packets takes place. The asynchronous data blocks from the RTU are assembled into one or more packets and forwarded to the SCADA system. Each RTU port at a PAD uses a V.24/V.28 interface and can be reached via a unique X.121 address at call set-up time.

A reverse scheme brings commands from the SCADA system to the RTU. For each RTU data block transfer to the SCADA system, a request from the SCADA is required. By this mechanism, a request/ response transfer mode is implemented over the dedicated circuits.

One of the more important aspects of data block assembly into packets is the forwarding condition in the PAD. Packet forwarding is achieved in two conditions: a maximum packet size of 128 octets or end of RTU transmission sensed by a 50 ms timer. In order to reduce delays in packet assembly, the RTU speed is set at 9.6 kbit/s or higher as necessary.

Multiple simultaneous connections and data transfers are possible from an RTU. In effect each port of the RTU operates independently with its own X.121 address. The SCADA system calls to an RTU port are of the Switched Virtual Connection (SVC) type. The SVC calls are initiated by the SCADA system at RTU port connection times and the established virtual connections `stay up` indefinitely unless a call-clearing condition has occurred. The same process is used for SCADA workstation switchover. In this case, a new call to the RTU port is made by the SCADA system.

The use of SVCs to an RTU port and alternate access to another port of the same RTU can provide four data connection possibilities to an RTU. In addition, the same RTU may be accessed from a wholly different SCADA system at a different location at the NECC.

The essential facilities afforded by the master station include data processing, despatch simulation for training, a switching procedure management system, outage management and automatic trouble analysis. In common with the latest energy management thinking, the Sinaut-Spectrum system is based around distributed hardware using fully graphical man-to-machine interfaces comprising IBM workstations and servers with Reduced Instruction Set Computer architectures making use of high-speed parallel processing.

The workstations and servers are connected through local area networks with high redundancy for maximum reliability. Each interface will have a particular function, e.g. power system management, load despatch and simulation training. A standard feature of the work stations is the use of accelerated graphics to provide PC-like features such as smooth zooming and panning, de-cluttering and windowing for easier multi-tasking.

As would be expected in any computer-controlled system, the heart of the SCADA system is high-speed data acquisition. This vital sub-system has a great deal of built-in redundancy to ensure both high reliability and high availability and, among other functions, provide the master station/telecontrol interfaces for communicating with the plant via the RTUs.

An important facility is the flexible redundancy concept that allows any critical function to be assigned priority status by switching to servers carrying out time-critical duties and relegating less urgent tasks to non-critical servers.

Remote terminal units

The CRCC will have monitoring and control responsibility for its own RTUs. There will also be a local RTU at the centre to handle local control centre status inputs and analogue inputs, and outputs. The CRCC`s responsibility is expected to increase to a total of 200 RTUs by the year 2005.

The RTUs will obtain generating station and substation data used for power system monitoring and accounting purposes. Supervisory control of circuit breakers, load tap changer (LTC) transformers and other devices is a required RTU function and output control modules and relays will be provided for the LTC transformers and circuit breakers of line and bus couplers.

The RTUs to be supplied and installed are Telegyr 5700s, which have an open architecture to permit the addition of new processing modules each linked by a fully protected high speed local area network. A built-in maintenance panel provides a man-to-machine interface with the capability to troubleshoot RTU malfunctions down to board level and to retrieve I/O data, such as analogue and accumulator status values from the database.

The contract includes all the necessary modifications to substation facilities such as installation of air conditioning, smoke detection and renovation to accommodate the RTU equipment, power supplies, communications equipment and associated items. All field instrumentation, including transducers and auxiliary relays required for the field wiring of all analogue equipment, accumulator monitoring, status, control and sequence of events points, is also included.

Software

Control by computer software offers not just the obvious `housekeeping` advantages, but more importantly it bestows greater systems flexibility and insures to a large degree against medium-term obsolescence: programmes are easy to up-date, conventional hardware is not. The operational flexibility of all Sinaut systems is based on this concept.

With the CRCC project, the total system software comprises millions of lines of high-level language code, organized as three levels: data, programmes and inter-task communication. The data level is realised by a real-time relational database and the necessary access routines.

For rapid access and fast data updating, the database contents are distributed among the various servers and workstations. Responsibility for supporting the workstation displays is shared by the respective servers, which hold the stored data. The real-time topology-oriented systems data is held in the communication servers. The related archives and working schedules are located in the information management and support servers.

The static display data and the continuously refreshed graphic displays are within the man-to-machine interfaces of the workstations. To facilitate administration, the complete master database and its system parameters are located in the information management and support servers.

The way forward

The CRCC system will be sufficiently flexible to allow it to be readily adapted and upgraded to meet most future requirements. This is an important consideration in view of the expansion plans of the EEA. For example, the proposed power interconnection that will run from eastern Turkey across Asia Minor and into north Africa is seen as a vital part of Egypt`s energy expansion programme. A key part of this project is a 500 kV feeder between Egypt and Jordan – a link connecting Asia to Africa in which Siemens is also involved. This power line runs across the southern part of the Sinai Peninsula and has to traverse the Suez Canal for connection to the Egyptian NUPS.

With the CRCC system, reliability and availability are largely the result of redundant system configurations built around fault-tolerant sub-systems and resilient computer software. Servicing and maintenance schedules were designed with maximum economy in mind without reducing operational safety standards.

Considerable effort has been put into developing a powerful but user friendly data management system. The computer database has been designed to be easily modified by operational staff (who need not be computer specialists) at any time to accommodate changes in the power system configuration without disturbing the functioning of the main system.

Click here to enlarge image

Figure 1. The EEA ordered a state-of-the-art regional control centre for Cairo in 1998

Click here to enlarge image

Figure 2. The CRCC multi-ring fibre network

Click here to enlarge image

Click here to enlarge image

Figure 3. A reliable energy network is essential for Egypt

Click here to enlarge image

Figure 4. The turnkey contract includes the construction of a new three-storey control centre building