The CIGRE Session 2018

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The CIGRE Session 2018

The CIGRE Session is the international leading event for Power System industry, gathering worldwide experts in even-numbered years to discuss and learn about the future of the industry and to display the latest solutions for the whole value chain, from generation to distribution. The CIGRE Session is a comprehensive event that offers – during a whole week – an extensive range of activities to more than 3200 delegates and 6600 exhibition visitors.

On the occasion of the CIGRE Session 2018, from August 26-31 in Paris, SDCEM and RTE have written this following paper on “Disconnectors reliability on the French grid and means to reduce the consequences of their failures on the electrical system”.

Disconnectors reliability on the French grid and means to reduce the consequences of their failures on the electrical system

S.DOUILLARD, G. HENRY, S.BISIOU, F.GUILLON, F. MEES,

RTE, France, SDCEM, France

SUMMARY

The function of a disconnector is quite simple on a mechanical and electrotechnical level: to isolate or to connect part of a grid on various electrical nodes, to establish an electrical continuity and to allow the transfer of current between 2 isolated parts of a substation. This apparent simplicity has probably led to pay less attention to the reliability of this equipment by being satisfied, for example, with an indirect signaling of the position of its HV contacts, and the lack of information to apprehend if they are still able to fulfill their function over time. Indeed, the active part and the control cabinet of these devices are constantly subjected to external climatic conditions which generate, on one hand, more important aging phenomena than for other equipment, with 2/3 of the hotspots detected on the network, and on the other hand, makes its kinematics subject to mechanical seizures. However, any network operator knows that the consequences of a disconnector failure (bus bar fault, total loss of the substation) are very disproportionate with the anomaly causing this failure (loss of clamping in the linkage, erroneous information). After a summary of the behavior of the disconnectors on the French grid and the main consequences for the system operation, this article presents the ongoing developments, tested as demonstrators in some RTE substations to make these devices more reliable. These developments are based on communication and lean on :

– Torque measurements in the control cabinet that detect either an early seizure in the kinematic and require intervention for maintenance or a break in kinematics by the detection of sub-torque.
– Pressure and temperature measurements at the male and female contacts of the live parts in order to guarantee the closed position of the disconnector to the operator with a nominal transit capacity and to follow up this pressure and temperature evolution with time to detect any aging of the device.
– The measurement of the actual position of the active parts when opening to guarantee the minimum isolation distance, especially when the disconnector is operated remotely.

A presentation of the implementation of these different solutions and their benefits for a Transmission System Operator is carried out.

KEYWORDS
Disconnectors, failure modes, digital control cabinet, monitoring, torque measurement, contact pressure, wireless sensor

1. PERFORMANCE OF DISCONNECTORS ON THE FRENCH GRID

1.1 Main features

In 2018, more than 40518 disconnectors (all types included) with a rated voltage higher than 50 kV are installed on the RTE network. From a functional point of view, we shall consider the line and switch disconnectors (which will be designated by the letter S), the fast break disconnectors (SRB) and the earthing switches (ST).

Figure 1 below gives, by voltage level, the distribution of all disconnectors by functional categories defined above. Fast break disconnectors have the same function as line disconnectors but with an additional ability to break some capacitive current. Their number on the network is relatively low but this type of device is still interesting to de-energize a sub-part of a line with 3 or more extremities, without the need for circuit breakers. For disconnectors, there are mainly two types: pantographs (PAN) and center-break rotating ones (ROT).

Fig 1: Disconnectors distribution by functional category and type

In addition to this functional distribution, it is also necessary to consider the distribution of the disconnectors according to the substation technology (Figure 2) where the equipment is located considering that it influences the failure modes.

We shall distinguish:

  • Modular indoor substations (AIS/PIM) which are air insulated substations with reduced isolation distances. This type of substation only applies to 63 and 90 kV voltage substations located in urban areas to reduce the footprint.
  • Gas Insulated Substations (GIS) with SF6 gas.
  • The air-insulated external stations (AIS) which include 85% of RTE’s disconnectors.

Fig 2: Breakdown by the technology of substation

In the remainder of the paper, we will focus only on disconnectors in AIS substations where most of the failures encountered are due to permanent exposure to atmospheric conditions.

The figures below represent, by voltage level, the age pyramid of the disconnectors according to the pantograph or rotating type.

The typology of 63 and 90 kV substations is very diversified by the large period covered and because of their greater proximity to urban centers which puts them under strong environmental constraints. The diversity of the substations structures means that the pantograph and rotary disconnectors are both used, although the latter remains predominant.

Fig 3: Distribution of 63 and 90 kV disconnectors according to the year of installation

The arrangement adopted in the 225 kV substations since the 1950s is of the low-busbars mixed-phase type [1]. This provision is still in force as adapted to the rated short-circuit current Icc (31.5 kA) for this voltage level and remains economical. In this arrangement, the bus selector disconnector is rotating and serves as a busbar support.

Fig 4: Distribution of 225 kV disconnectors according to the year of installation

In 400 kV substations, the majority of disconnectors are pantographs. Indeed it was decided in the late 1970s (development of the nuclear program in France) to adopt a provision with high busbars, which was adapted to the short-circuit current (Icc) of 63 kA [2]. In addition, this arrangement optimizes the size of the station, the selector disconnector is positioned under the busbar.

Fig 5: Distribution of 400 kV disconnectors according to year of installation

1.2 Contribution of disconnectors to the performance report of HV equipment

In a macroscopic way, RTE usually uses the following 5 indicators to characterize the performance of its substations with regard to customers, system safety and maintenance:

  • Number of Busbars faults
  • END = Energy Not Distributed in MWh: This is the power cut x cutoff time minus the energy recovered by a backup
  • TCE = Equivalent Breakdown Time: This is the ratio of undistributed energy (END) to the average annual distributed power.
  • ESS = Significant Event for the Safety of the electrical system.
  • The number of anomalies that require curative maintenance procedures

Figure 6 puts into perspective the part of the disconnectors in relation to all the HV equipment for each of these indicators. The important part of the ESS taken by disconnectors comes from the loss of the “open or closed” position during operations, mainly due to anomalies on certain types of well identified control cabinets. These ESS are ranked at level 1 of importance on a scale of 7 because they restrict topology changes, weaken the system and require on-site intervention. A replacement policy for these control cabinets over the 2014-2021 period, as part of the implementation of the remote operation of 400kV line feeder disconnectors, will significantly reduce the number of related ESSs at disconnectors.

Fig 6: Disconnectors contribution to electrical system performance indicators

Disconnectors have moving parts, which are subject to the following failure modes:

  • a position of the equipment which is neither “open” nor “closed” following a mechanical blockage. The position information of a disconnector does not usually come directly from the active part of the disconnector but from auxiliary contacts located in the control cabinet. But the poles of the disconnector are connected to the control cabinet by a linkage of length and complexity varies according to the facilities. In the case of an anomaly of the mechanical transmission between the poles of the disconnector and the control cabinet, the position signal transmitted to the driving system will be wrong.
  • mechanical wear on the main contact fingers or a position of the apparatus which is not completely mechanically “closed” (a low finger contact pressure) and which may lead to excessive heating of the contacts and subsequently to a reduction of the current that it can carry.

The main anomalies encountered on disconnectors are grouped into 4 major families:

  1. Electrical faults in disconnector control cabinets, including faults affecting position feedback contacts, the main cause of ESSs.
  2. Breakdowns of the kinematic chain. This rupture can be due either to a disengagement of the mechanical transmission or to a rupture of insulating columns, particularly by porosity. These 2 types of damage are not remotely detectable and almost always lead to a busbar fault.
  3. Hot spots. Regular infrared thermography campaigns are organized in our substations (every year in 400 kV) to try to detect them.
  4. Seizures or blockage (hard spots): Mainly found on equipment subject to salt pollution.

Preventive maintenance on the disconnectors includes periodic opening and closing performed every 6 months remotely from dispatching and on-site operation with visual control every year. The issues relating to disconnectors for RTE are, therefore:

  • improve the reliability of “open / closed” position feedback information to reduce the number of ESSs and busbar faults
  • to be able to anticipate the maintenance actions before the occurrence of a failure and for that reason to be able to monitor this equipment with measurable criteria. Currently, the operating time is the only information available to the maintenance crew, but it is not representative of the status of the disconnector.

2. SOLUTIONS UNDER DEVELOPMENT
To answer these challenges this document retraces three experimentations carried out or in progress in RTE substations with 3 different manufacturers’ equipment. Here is the presentation of the Experimentation at the Champagnier substation on a brand rotating disconnector SDCEM.

A 2-year experimentation was conducted at CHAMPAGNIER’s RTE substation on a 72.5kV disconnector fitted with a universal smart cabinet MR41E, equipped with S-TORQUE, a torque monitoring function during operation. This control cabinet differs from conventional ones in that the geared motor unit is controlled by an electronic module. In addition to the conventional functions of disconnector control, universal smart cabinets offer new functions [3]. The main parameters of the electronic control and disconnectors (commands, operating time, motor current, operating torque, number of operations, temperature …) are monitored and processed by the electronic module. In the case of this experiment, this information had to be consulted locally, but it can be relayed or carried directly on the communication network of a substation having a digital control command, via Ethernet cable or an optical fiber with the secured protocol IEC 61850.

Principle: The electronic control cabinet MR41E includes an intelligent electronic solution (S-TORQUE) which allows the analysis and the real-time monitoring of the operating torque of the disconnector which it activates, with alarm emission in case of the exit of the torque envelope, set and saved during installation. The torque is measured during each opening and closing operation of the disconnector, in relation to the operating angle of the electronic control, and is compared instantly with the values of the maximum and minimum reference torque curves [4]. Thus, S-TORQUE device confirms the right opening and the right closing of the disconnector and allows a redundancy of the position indication, confirming or disconfirming the position information from the auxiliary contacts.

Two cases can occur:

  • over-torque: for example blockage of the linkage or the mechanism of the disconnector, jamming.
  • under-torque: for example breakage or disconnection of the linkage which constitutes an urgent defect.

Fig 7: Example of disconnector torque with minimum and maximum template curves.

Implementation: Once the disconnector is installed, connected to the high voltage and set, a calibration is carried out in a few minutes on site using a laptop connected to the electronic control cabinet. The operator performs an open-closed cycle to record the opening and closing torque curves that are distinct. The maxi and mini templates proposed by the system can then be adjusted by translating them upwards or downwards and are then recorded in the electronic module for real-time condition monitoring during switchgear operations.

For the adjustment of the minimum and maximum templates, it is necessary to find the right balance between the monitoring accuracy and the risk of irrelevant alarms: very tight curves (for example +/- 10%) allow a fine monitoring of the drift of torque but may cause false alarms in the case of seasonal variations of torque (temperature variation, deposition of frost or ice, rain …) Indeed, a marked drop in ambient temperature significantly increases the operating torque and some defaults that one seeks to detect may be masked by a difference in ambient temperature. On some disconnectors, torque deviations of 30% over a measuring range were measured for temperature differences of 13 °C. In the same way, an operation in severe ice conditions temporarily increases the operating torque. A correction algorithm is, therefore, necessary to be able to compare the curves recorded under various conditions.

Fig 8: Recordings of operating torque curves as a function of temperature

The alarms transmitted by the torque monitoring system are relayed to inform the dispatch center, and grouped together on the first relay for urgent faults (sub-torque which may be an indication of mechanical transmission anomaly) and on the second relay for non-urgent faults (over-torque which may be an indication of maintenance to be performed).

Fig 9: Disconnector with electronic control cabinet MR41E on Champagnier substation

Use for predictive maintenance: For each operation, the torque curves and data (date, ambient temperature, humidity level, etc.) are stored in the data logger module (S-MEMORY) dimensioned for the entire lifetime of the electronic control. These data can then be retrieved and remotely processed by a post-processing algorithm (S-PREDICT) which compares the torque curves (taking into account the variation of the torque with the recorded data), and performs the analysis of the possible drift of the torque during the closing and opening. This analysis then makes it possible to establish a diagnosis of operation of the disconnector and to recommend a maintenance task if necessary (cleaning and lubrication of the main contacts, lubrication of the bearings and the joints).

Results: The disconnector performed more than 100 operating cycles and no inappropriate alarm was observed during this period: the torque curves of the disconnector have changed in the specified torque envelope and climatic variations have not generated torque value variation that may cause wrong alarms.

3. CONCLUSION
This paper has demonstrated the interest of the new digital control cabinets that allow, in particular, to monitor the torque in real time during the movement. These innovations are likely to meet the challenges of a Transmission System Operator that are improving the reliability of the network and the efficiency of maintenance. These developments will be widely used as part of the digitization program for half of the RTE substations by 2030.

BIBLIOGRAPHY

[1] RGE 1971 “Evolution de la construction des postes” J. Parizy et H. Fournier
[2] RGE 1981 “The new design of 400 kV substations in the Franch system” P. Delétang
[3] MATPOST 2011 “A new generation compact and intelligent disconnector electrical operating mechanism, with high energy efficiency and limited environmental impact » – F. Mees
[4] MATPOST 2015 “Benefits of the intelligent disconnector motor operating mechanism for digital substations” – F. Mees