Remote control of natural gas flow
Envestra Limited's natural gas distribution system in Victoria comprises over 8300 km of natural gas mains and supplies over 400,000 consumers in metropolitan and regional areas. Under a long-term O&M Agreement with Envestra, Origin Energy Asset Management (OEAM) is the network operator. The gas distribution system is fed from the main pipeline from the Longford processing plant in Sale, Victoria and also from the Moomba to Sydney pipeline inter-connector.
Within the Victorian distribution system, there are a number of operating pressures used to maintain supply. At the system's entry points, known as 'custody transfer' points, the operating pressure is in the range of 200-4200 kPa, although usually it is at high pressure, typically 450 kPa. The gas is then regulated down at various points in the system to lower pressures, which ultimately supply households and businesses.
At a number of these 'regulated' points, and at selected 'fringe' points in the metropolitan high pressure reticulation system, information is collected using field remote telemetry units (RTUs) and sent to a control centre where the information is used to monitor the system's integrity.
In the past, the entire Victorian network was monitored and controlled via a single gas control centre; however, the privatisation of the network resulted in three distribution companies operating defined parts of the network.
The Gas Telemetry Control project was necessary due to the transfer of responsibility for Envestra's gas system to OEAM.
The project is managed and operated from OEAM's Victorian operations, based in Melbourne's northern suburb of Thomastown.
The Gas Control Telemetry project comprises SCADA (supervisory control and data acquisition) redundant servers and telemetry master stations located at Thomastown, a telemetry communications system incorporating radio and PSTN lines, and 118 RTUs which monitor and control the gas assets.
In addition, a backup server and master station with full monitoring and control capability is located at another site.
As well as monitoring asset status and performance, a number of RTUs are included in 13 established regulator control groups. A typical group will comprise one or more fringe pressure monitoring sites and one or more gas regulator sites. The regulator sites operate a regulating valve that reduces high pressure to distribution pressure, and opens or closes under computer control to maintain the distribution pressure at a pre-set amount.
The Control Centre server has a control algorithm that provides either full automatic, partial automatic or manual control of the gas pressures in any of the 13 areas located around Melbourne. Maintenance of these networks is assisted by the SCADA software, which provides the operators with continuous and historic trend graphs of the pressures as well as custom reports of network performance.
The SCADA system
The SCADA servers comprise a pair of Dell Pentium 3 1.13 MHz computers connected to a SCADA LAN. A separate SCADA UPS provides mains power autonomy to the servers, telemetry master station and peripherals.
Operator workstations are used by operations, planning and management and connect either via LAN or from outside through a secure VPN connection.
As for the actual SCADA application, it is CITECT version 5.40 and the gas algorithm was designed by OEAM and implemented in CICODE by Serck Controls.
The telemetry master stations are Kingfisher and comprise redundant BA6 backplane, CP-21 processor, MC-11 communications and IO-2 modules - all of which are powered by a separate 240 VAC power supply and battery. A modem provides a paging message in the event of a UPS failure.
The SCADA system provides the operators with asset alarming, site status including pressures and temperatures, valve position, heater status and site security information and trending graphs for all of the analog points are available on demand.
Paging and short message services (SMS) provide alarm escalation, while remote monitoring of sites is available by dialling in with a laptop computer or by telephone through an interactive voice response facility.
The telemetry system comprises both radio and PSTN communications. Two elevated locations at Collins Street and Mt Dandenong operate a hot-keyed base station providing radio coverage for 88 remote telemetry units. These units are located adjacent to regulator pits, kiosks or enclosures. All legacy RTU hardware has been replaced with new Kingfisher RTUs comprising BA4 backplane, PC-1 CPU, AI-1 8 channel analog input module and IO-2 combination 8 digital in/8 digital out module.
The radio sites are polled sequentially every two minutes starting with the control group sites. The control algorithm then outputs any control adjustments necessary. The polling sequence completes after polling the monitor and district regulator sites.
The 30 PSTN sites are regional monitoring sites that mostly monitor the gateways between the bulk transport supplier and the Envestra distribution network. These sites are polled at six hourly intervals. Alarms are reported on exception, and the site also logs events that are regularly uploaded by the RTU and processed by the SCADA server for trend information.
The SCADA system has been installed and commissioned and is now in use by OEAM to monitor and control the gas network.
Over the next 12 months, OEAM's Operations group will be optimising the control algorithm as seasonal gas usage patterns become available and as their network requirements change.
According to OEAM, "The new SCADA system will enable OEAM to meet its obligations to the Office of Gas Safety as well as to our principal Envestra, and will provide a better level of service to gas consumers."
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