With the fast growing needs & expectations of industries in India and the world over, customer/utilities are looking for:
1. Continuous uninterrupted power
2. Reliable and maintenance free equipment
3. Maximum performance and no shortfall in safety, across all respects.
When switchgear malfunctions the consequences are often catastrophic. Damage to the switchgear itself can be extremely expensive. As it is reliable, robust and hermetically sealed, Medium voltage Gas Insulated Switchgear provide a convincing answer even under volatile conditions.
However, when it comes to execution, the end user has to take care of one very important part of switchgear installation, which is: Termination of HV cables during the commissioning of equipment. It has been observed that the majority of fault/failure/flashover occurring outside the sealed tank are due to improper cable termination. Some do’s and dont’s are discussed in this article with measures necessary to prevent the failures, gas leakage & power outage in switchgear caused due to errors in cable termination.
Generally, the life of GIS (declared by all manufacturers) is 30 years and it is easily possible to achieve this if termination work is properly carried out. This article provides some guidelines for the same.
1. Cable Termination
Once a GIS is installed in place, the very next activity is cable termination, which needs to feed power to switchgear & from switchgear to transformer, or other switchgear for forming a ring network in distribution system. It should be the aim of any GIS manufacturer to support and give assistance for carrying out this activity successfully (under manufacturer’s supervision or through written guidelines). Special cable termination kits are required for termination of cable in GIS, these kits can be procured based on cable data. The cable termination task can be supervised by a manufacturer’s representative.
Before starting the cable termination activity, carefully check the below conditions:
- The cable terminator must be a skilled and certified person
- Ensure the unit is de-energized
- Ensure availability of suitable/required tools
- Instruction manual supplied with termination kit must be followed
- The local electricity board norms, if any, should be followed
- Ensure earthing switch is closed before opening the cable compartment door (usually ensured through interlock)
2. Trench Requirement for GIS
Cables can be routed from different directions via a trench and the size of trench is determined by the type of unit being installed (I.e. Indoor/Outdoor), type of gas exhaust during arc fault, Extensible/Non Extensible, type of cable and its cross section area. Since different customers use cables with different cross section area, ranging from 50Sq.mm to 400Sq.mm/more and also different types of cables (i.e. Single core/Three core), the trench height required for suitable accommodation of the radius of cables should be ensured. The general guideline for height of trench should be >15D (where D is the outer diameter of cable to be terminated).
3. Cable Routing
The Unit should be installed as per the recommended fixing guideline. Ensure the clamping of cable to ensure no stress is exerted on bushing. The cable termination must not pull on the bushing else it may lead to gas leakage; it is necessary that the cable terminal is aligned correctly on bushing.
Use cable grommets as per outer diameter of cable in order to prevent entry of foreign particles from bottom of cable box followed by assembly of base sheet/gland plate.
4. Cable Clamping
Ensure the cable clamping takes the weight of the cable and no mechanical load is exerted on bushing.
Also, ensure the tightening of mounting hardware as recommended.
5. Assembly of cable lug with bushing
Ensure there is no gap between bushing surface and cable lug. It must be in full contact with bushing surface. Apply recommended torque on hardware (Ensure assembly fitments), else it may lead to a higher temperature rise.
6. Cable CT Position & Assembly
Position of CT should be straight, and must be assembled with given mounting. The earth shield of cable must be passing through the CT window. The placement of CT should be after the anti-tracking tube in case of touch proof termination.
7. Cable Boot Assembly
Cable boots should be fully inserted and cover complete length of bushing, failing this may lead to higher electrical stresses.
Also, ensure the assembly of touch proof cable boots as recommended in the instruction manual of the cable boot manufacturer.
The Cable boot should not interfere with each other and/or be twisted. There should be visible clearance between cable boots. Insert the end terminal cap on cable boot after tightening of lugs with bushing.
Ensure the earthing of cable boots with main earthing bus bar.
8. Key check-points
- The connection of cable on phases should be from same source of supply
- Use of termination kit according to height of cable bushing to trench and cable size
- Ensure application of recommended torque on all the applicable hardware used for termination
- Ensure no materials/spanners/foreign bodies are left inside the cable compartment after completion of termination work
- Ensure connection of cable braid with earth bar of panel
- Rain water/Moisture should not be in contact with cable joints/boots
- Recommended to use touch proof cable boots
- The Routing of LV cable should be properly separated from power cable & mounted through cable tie
- Ensure there is no gap at entry of cable, fill it with sealing if required to avoid foreign particle/vermin from entering
- Cables, boots, joints should be cleaned regularly to prevent partial discharge failure
- The Online Partial Discharge monitoring sensors may be used for checking the healthiness of bushing/cable/boots
The desired performance and full benefits of innovative and efficient switchgear would not be achieved without healthy cable termination. Above recommendations would be helpful for getting expected performance of switchgear over its life and also prevent unplanned outages ensuring uninterrupted power.