One of the functions (“modules”) of the MBI system consists in proposing maintenance activities (i.e. maintenance and repair actions, inspection visits and technical investigations) to be carried out on the grid components.
This module implements the Condition-based maintenance (CBM) criteria that are the core of the maintenance policy endorsed by the company. The module includes a set of maintenance models implementing the technical/economic rules leading to the maintenance decisions. A maintenance model has been developed for each grid component type. The component types considered by the MBI-substation system are the following: HVDC Circuit Breaker (DCCB), Phase-shifting Transformer (PST), disconnector, surge arrester, CT (Current Transformer), VT (Voltage Transformer), condenser, protection devices, auxiliary systems, SF6 bay. The model output consists of a list of suggested maintenance and/or monitoring actions to be carried out on the grid component. Each action is associated with the proposed deadline.
The figure below represents in graphic form the information flows involving the maintenance models.
In the central part of the picture the different pieces of input information considered by the diagnostic model are shown, namely:
The grid component status
(see the left hand side boxes), i.e. all the maintenance related data for each specific component collected during its lifetime. The following data are part of the component status:
the component age and the life spent by the component during its operation (the latter information is particularly significant for some component types, e.g., the transformers);
the findings collected during surveillance and technical controls (e.g., the value of circuit breaker contact resistance) or by means of on-line condition monitoring systems, when available;
number and date of the overhauls carried out on the component;
The component status is continuously updated with the input data considered by MBI. The component status is the principal source of information for the diagnostic models; nevertheless the models can be executed also when the component status is incomplete: in this case the maintenance model results will be less accurate but still available.
The relation between the component and the grid functionality. This information accounts for the impact of a component failure on the grid service. The more critical is the part of the grid (e.g. the line) where the component is installed, the more severe is the effect of the component outage on the grid customers in term of energy not supplied.
The environmental conditions
of the area where the component operates. The influence of pollution
and the salt from the sea can significantly reduce the insulation in the substation components as well as in the aerial lines. In addition the salt may cause corrosion in the wires. Those phenomena have to be taken into account by both inspection and maintenance criteria.
The maintenance model produces its results by comparing the component status with a set of reference (target) values that depends on the component characteristics (see the box in the lowest part of the figure). The reference values are defined taking also into account the information given by the component manufacturer. Thus, all the specimens of a given component model share the same set of target values, according to the concept that maintenance criteria must be as far possible customized on the technical characteristics of the component.
Each maintenance model is implemented as a set of rules that represent the relations between component malfunctions and their underlying causes. The rules also define the maintenance/repair actions to overcome a given malfunction and/or to fully restore the component functionality. As an example, in Figure 2 some rules taken from the maintenance model of the circuit breaker are shown.
In the left hand side of the figure some degradation findings are reported. Some of them are detected during visual inspections (e.g.” seriously oxidized points in the control cabinet”, “insulator degradation: surface discharges”), while others can be sensed by an on-line condition monitoring system (e.g. “abnormal compressor loading time”). Each arrow represents a rule that relates the findings to the maintenance /repair/replacement actions (right hand side). For instance, rule n° 1 states that the breaker control cabinet has to be overhauled when some seriously oxidized parts in the cabinet are detected. Rule n° 4 suggests to carry out an aggregated maintenance action (“breaker major overhaul”) instead of three separated maintenance actions (“control cabinet revision”, “air compressor revision”, “breaker pole replacement”) resulting from the previous rules.
The diagnostic model for a specific component is executed in the following situations:
whenever a visual inspection or a diagnostic test make available new findings about the component;
whenever an on-line condition monitoring system detects a specific event (e.g. high temperature values of the transformer oil) that has to be analyzed by the diagnostic model rules;
at a fixed time (e.g. monthly) in order to periodically check some cumulated indicators (e.g. number of maneuvers for the breaker, high load for the transformer) accounting for component degradation or possible incipient failures. Should the indicator value exceeds a predefined threshold; MBI schedules the relevant condition-based maintenance actions.
Finally Figure 1 also includes the inspection planner, i.e. the module in charge of planning the inspection visits. It has to be noticed that the planning rules implemented in this module refer to the same pieces of information about the component (e.g., the component status) taken also into account by the maintenance models.