Ministry of Defence? Perhaps a worthwhile security measure in substations around Middlesbrough: a pair of Challenger tanks might deter the cable thieves... ;-)
More seriously, it's best if TLA's are explained, especially in a multi-national forum where devices have different names in the NEMA/FM world compared to the IEC world.
In my neck of the woods, they are used all the time as 345 kV line disconnects. Controls are both in the control house and SCADA. In our region, I have not seen MOD's utilized for any other purpose in Air Insulated stations. I have seen MOD's applied extensively in Gas Insulated Switchgear around breakers and lines.
I believe the advantages are #1 safety (at higher voltages), #2 faster connecting / disconnecting and #3 System Operations can isolate lines, particularly for remote stations.
At 115 kV, MODs can be used to automatically/remotely isolate a section of bus or a transformer after a fault. An MOD is lower in both capital cost and ongoing maintenance than a circuit switcher. My state requires visible air gaps to isolate equipment, so we end up need both a circuit switcher and a separate regular disconnect if we go the circuit switcher route. As the switches get physically bigger to accommodate higher voltages, they become unrealistic to manually operate at a reasonable speed.
Most SCADA systems I've worked with utilize SBO (Select before operate), giving the system operator that extra chance to verify the intended operation. As Davidbeach said, there are switching orders developed, which generally have a review process between system operations and the operations crew in the field.
Other than in GIS (Gas insulated Switchgear), I have not seen electrical interlocks, however there are things that go on in EMS (Energy Management System) / SCADA that the system operators see that I can not in the field. The MOD's I've worked with have a control switch (we call them 101 switches) and a local / remote switch (we call it a 43 switch) in the control house. The MOD controls are wired such that the 43 Switch, which is normally in the remote position, electrically blocks operation of the 101 switch, thus preventing an accidental bumping of the 101 switch.
Many MOD's have Open/Close switches and a Local/Remote switch located in the motor operator box. This is provided for maintenance / testing. I've seen switching orders that call for verifying the proper status of the Local/Remote switch in the box. I have also seen utilities disable this function in the motor operator box.
My utility does not use interlocks to prevent opening MODs via SCADA. If the MOD is used as a clearance point during maintenance, it will be disabled both via the 43 switch DTR mentioned, and by removing the mechanical linkage between the motor and the shaft going up the tower. When MODs are operable via SCADA, we usually bring back position indication. We also have some MODs on transmission lines that operate based on locally sensed loss of voltage where we do not have any communication to the site.
Most MOD's have multiple auxiliary switches for local and remote indication. Some are internal and reflect the motor travel, while others are connected to the vertical operating shaft. For SCADA purposes, I have seen the external shaft coupled auxiliary switches used most.
Some utilities I've worked for are very specific about how the indication should come in, relative to the travel of the switch. All MOD's I've worked on have a means of decoupling / locking out operation. I've seen these same coupling mechanisms used to lock the switches closed (Wind Farms rather than utilities).
I believe system operations utilizes line side VT's and or bus VT's to confirm operation if field operations personnel are not on site.
Is it heard of, or common to have a switch in a position other than what the indicators indicate? My concern is under loop splitting a wrongful position indicator can do a lot of damage.
Other states were declared as illegal and locked out operation, and the two paired switches had to operate more-or-less together with a defined time-out.
Thats what I was thinking. Blocking any switching unless the breaker is opened or the line is in parallel with the transfer buss. Any scenario that puts full load across the departing disconnect contacts.
