In order to highlight malfunctions of its substation equipment (HVDC Circuit Breaker (DCCB)) , Current Transformers, Voltage Transformers, surge arresters and transformers), before these can cause an outage of the substation, a new concept of on-line diagnostic system was developed with the goal of better handling maintenance intervention. This system, called EDS (Expert Diagnostic System), is based on the utilization of simple sensors acquiring the most significant decision-making parameters from various substation equipment, in combination with the MBI system.
In order to automatically schedule the maintenance activities, the system transmits signals trends and logic signals to a central computer. In addition, any further controls with dedicated instruments could be programmed depending on the information received. Such asset management tool allows the technicians to switch from periodic maintenance policy to a condition based-maintenance and therefore to standardize their behaviour.
The definition of the monitoring system comes from the experience reached by Terna since 2003 through preliminary monitoring devices and data transfer both to MBI and to its platforms for remote data transmission and processing.
Finally in 2007 Terna concluded that, in order to have a reliable monitoring system, applicable to all types of equipment (regardless of manufacturer and model) it was necessary to design and implement an "ad hoc" system based on the following principles:
applicability to both new and already on service equipment,
primary importance of the parameters to be monitored in relation to cost and ease of data interpretation, and
integration into the already available maintenance management systems (MBI system and the Operation & Maintenance Support Platform PSE).
In addition, the monitoring system should be able to provide the following benefits:
early detection of quick degradation phenomena,
optimization of “on-condition” maintenance actions,
additional information on the causes of failure, and
improvements in the knowledge of phenomena and behaviours by a collection of data from a large amount of HV equipment and transformers produced by different manufacturers.
On the basis of the above considerations, the following specifications have been defined:
HV equipment that should be considered for continuous monitoring,
equipment parameters to be monitored,
diagnostic criteria that should be applied, and
The following are the power equipment considered, the parameters to be monitored for each of them and the measurement sensor/methodology used for the equipment of the same category:
- Circuit breakers:
- operating times (via auxiliary devices already present in Terna switches),
- sum of the interrupted fault currents (via the current transformers in Terna facilities),
- time-varying density of SF6 gas (by inferring the incoming alarm by the presence of gas losses),
- number of operations (via auxiliary contacts devices already present in Terna switches);
- SF6 insulated current transformers (CTs):
- time-varying density of SF6 gas;
- Capacitive voltage transformers (CVTs):
- measurement of the secondary voltages in order to identify on-line any failure of the VTs (by comparing the same secondary voltages by means of a dedicated algorithm);
- Surge arresters:
- peak value of the leakage current,
- rms of the third harmonic component of the leakage current,
- number of interventions.
For both CVTs and surge arresters, periodical control procedures, usually carried out by operators yearly, were automated. Monitoring of these two types of equipment is of particular importance for security purposes (damages caused by the material thrown outwards as a result of internal faults; functional deficiencies resulting in lack of system protection).
Concerning the disconnectors, their on-line asset monitoring was not considered essential, given their low number of operations and the difficulty for correctly interpreting the information measured for diagnostic purposes.
gasses dissolved in oil,
oil temperatures (of the transformers and in and out of the coolers),
load currents (via the current transformers present in Terna substation).
As for this kind of gears, another phenomenon that should be properly checked in the future is on-service partial discharge, in order to early detect any deterioration of the insulation conditions, thus preventing major failures. Such measurements, once validated, could be adopted for the on-line monitoring of gears that are considered to be very important for the transmission grid.
According to the system project criteria, components of high reliability and minimum invasiveness towards equipment, monitored device and control system were employed.
The system architecture is as follows: each (new or old) component to be monitored is equipped with a "monitoring unit"; each remote monitoring unit allows to get the most important information of the relative apparatus. In the case of the AIS system the various units are connected via fiber optics to a "bay concentrator" installed in the kiosk that also houses the Protection and Control System (SPCC), which also receives the information (as shown in the figure). In the case of GIS each bay is equipped with a "bay monitoring unit" which is connected to a "substation concentrator". From the concentrator the measurements and reports are transmitted by the Electrical Grid Monitoring “MRE” peripheral system, if any. Then some warning and degradation indicators are transmitted to the Operation & Maintenance Support (PSE) and MBI platforms (by using the IEC 60870-5-101 or IEC60870-5-104 protocols).
The described on-line monitoring systems were installed in different cases, which are now in service:
AIS system: HV equipment monitoring system in 380 kV and 132kV substation section (e.g. Lacchiarella substation, near Milan)
GIS system: monitoring of 245 kV substation section
Transformers: monitoring systems installed in substations.