Transformer on-line oil-gas ratio (green) was increasing and the low-voltage current (red) was excessive. After disconnecting tertiary connected reactors, the oil-gas ratio started to decrease, localizing the defect into tertiary windings.
Fingrid Oyj is responsible for the main transmission system in Finland. The system comprises approximately 14,000 km (8,700 miles) of transmission lines and 113 substations, operating at voltages from 110 kV to 400 kV, which are mainly equipped with air-insulated switchgear.
The maintenance concept at Fingrid is based on outsourcing while retaining technical expertise in the form of centralized equipment specialists and regional specialists for main asset classes. Maintenance work is purchased from suppliers through competitive bidding, with contracts divided into long-term for basic maintenance and short-term for specialized maintenance.
Fingrid collects a variety of information about the transmission grid from several databases that is used by grid-management specialists. The root task is to verify that components are in good condition and that condition- or time-based maintenance activities proceed as planned. The long-term target is to discover the weak-performing assets and hidden defects early enough to allow repairs or replacement investments to be undertaken at an optimal time. To fulfill this objective, a tool was developed to process the large volume of data in a way that improved visualization and automatic early-stage notifications.
The target for the condition-monitoring system (CMS) was to give easy access to on-line historical data for maintenance purposes and to process the off-line maintenance data. The objective was a common tool for maintenance specialists and control room personnel to monitor assets. The function of the system required was to provide a smart overview of the status of equipment, generate alarms, display relevant trends and analyze data on user demand. The project was based on existing OSIsoft PI system, resulting in a ready-to-use CMS within eight months of startup.
The CMS comprises several components. Grid power flows, voltages, currents, SF6 pressures and transformer gas development come from supervisory control and data acquisition (SCADA) and on-line condition-monitoring devices. Nominal values, manufacturing date, device defects, defect-severity categorizing and measurements from device service actions come from the off-line asset-management system.
Redundant PI servers store incoming on-line data, enabling the use of data either directly or indirectly. The PI Server processes both on-line and off-line measurements, which are combined with the asset framework so that measurement tags are linked with a corresponding asset element. The asset framework is accessible from all PI software, which speeds up searching for tags related to a specific asset.
The asset framework also provides displays for each element, whereby only one display is needed for each substation asset (for example, one transformer display, one circuit breaker and so forth). When users change from transformer 1 to transformer 2, the display stays same but the values are updated.
SF6 On-Line Monitoring
European Union regulations have become more stringent concerning the use of fluorinated gases and monitoring of their emissions. The implementation of SF6 on-line monitoring supports the gas management and improves the monitoring of the gas levels in the switchgear. The system uses a milliamp output sensor in the gas compartment’s back-pressure valve. The signal is scaled and transferred to the SCADA system. The milliamp output is proportional to the SF6 density in the gas compartment. Density changes are recorded by the sensor output and noted immediately in the monitoring system. To have more user-friendly measures, density values are converted to temperature-compensated pressures.
The system is now a common practice for all the new SF6 installations and proves to be a powerful method for analyzing the gas compartments tightness levels in the Nordic environment with high temperature variations.