Quality of erection and commissioning
Erection and commissioning of MV/HV switchgear are quite complex and full of tiny little details that can easily make your project slow down or even stop. Not to mention deadlines and penalties which are hanging over everybody’s heads involved in the project. This article is divided into two parts. This part deals with the inspection, installation, and assembly of MV/HV outdoor and indoor switchgear.
Often the switchgears cannot be shipped in completely assembled condition due to limitations of transportation and packing. The erection includes placing the equipment on its foundation, completion of assembly, i.e. assembly of items/sub-assemblies supplied loose, their mechanical and electrical inter-connection, fitting of any extra instruments/devices, and completion of control and power circuits.
Once the equipment is erected, it needs to be commissioned through generally pre-defined checks and tests before finally putting into service. Testing is necessary to prove that the installation meets the required specifications and safety. The work content for the installation of outdoor switchgear is different from that of indoor switchgear, but the principles followed are the same.
The availability of following documents should be ensured during erection and commissioning:
- Dispatch/delivery documents, i.e. Packing list;
- Instruction manual for operation and maintenance;
- Outline general arrangement, layout and floor plan drawing, Foundation drawing;
- Schematic and wiring diagrams; and
- Instruction manuals for relays, meters and other devices like switches, transducers, etc. fitted into the equipment.
- Inspection On Receipt at Site
- Storage And Handling
- Erection of switchgears
- Assembly of Switchgears
After manufacturing and completion of all routine tests and other quality and inspection checks, the switchgear is dismantled for packing and is made ready for dispatch. As many a time complete assembly cannot be dispatched due to transport limitations, these are made ready for dispatch after dismantling.
Following procedure and precaution should be followed while dispatching switchgears:
Outdoor switchgears are generally designed and dispatched so as to involve minimum amount of erection work. It is generally dismantled into four parts as follows:
- Pole unit with inter-phase mechanism
- Mechanism box with local control unit
- Structure for circuit breaker, and
- CTs and VTs.
An assembled outdoor vacuum circuit breaker (VCB) is shown in Figure 1 below.
The interconnecting drive rod with eyebolts (Figure 8) is de-linked and kept tied inside the inter-phase mechanism. The connecting hoods are kept inside the mechanism house. Structure parts are dismantled and tied together. These parts are marked suitably to facilitate easy assembly at site.
Openings on the top of the mechanism housing and bottom of the inter-phase house are covered with blanking plates in order to avoid ingress of dust, rain water and foreign bodies inside the units during transport and storage at site. These blanking plates are to be removed just before assembly.
Foundation bolts, loose hardware, terminal damps (if specified) and any other small items are packed in a small box which is fixed to the main packing box. The spring charging handle and slow closing handle are generally kept inside the mechanism house of each unit.
The structure assembly is packed separately with damps for easy transportation. The pole unit with inter-phase assembly is packed in a wooden crate, while the mechanism house is packed in a separate wooden box. In case of a breaker with outdoor CTs/VTs and associated mounting structure, the three single-phase CT/VT are packed in a separate packing case.
Suitable pictorial marks are made on the packages to ensure that the handling and transport are done in an upright position. While lifting by a crane or hoist, wire ropes should be attached to the bottom of the crate.
Indoor metal-clad switchgears up to 33 kV class should be dispatched by the following rules:
Rule #1 – For easy transport and handling, the switchboard is divided into dispatchable units, and generally two or three (Figure 2) panels are sent as one dispatchable unit depending upon the dimension of the panel, considering transport and shipping limitations.
Rule #2 – As far as possible, the bus-coupler panel with trunking panel and incomer panel with separate feeder VT panels are sent as one unit respectively so that minimum connections are required to be done at site.
Rule #3 – Panels are fully covered by polythene sheets to protect them from dust and moisture, and are packed in a wooden crate. Suitable felts are provided to protect the painted surface when it comes into contact with crates.
Rule #4 – Before packing, the withdrawable VCB truck is kept in service position inside the panel and the circuit breaker contacts are kept in open condition. The closing spring of the operating mechanism is kept in ‘free’ (discharged) condition.
Rule #5 – Suitable pictorial marks are made on the packages to ensure that the handling and transport are done in upright position. When lifting by a crane or hoist, wire ropes should be attached to the bottom of crate.
Rule #6 – For each consignment, a dispatch advice note and packing list are sent so that the units and corresponding accessories can be unpacked and identified. Each crate is marked with the Serial No. of the units kept within it.
Switchgear is often dismantled before being dispatched with the parts packed separately. All items of equipment should therefore be carefully inspected as soon as possible upon arrival at site to ascertain whether any items are missing or damaged. Hence, after unpacking, the contents should be checked against dispatch notes and the receipt of all items verified.
The recipient should check for damages, if any, which might have occurred during transport or handling.
In particular, he should carefully check for damages in the vacuum interrupter, inter-phase barriers, bushings, relay and epoxy insulators. If any damages are noticed, communication of the same may be sent to the transportation company, insurance company and a copy to the manufacturer.
Delivery of the switchgear should be made to the site at a time convenient for erection to proceed immediately thus obviating the need for any storage facility. The storage of switchgear is very important if it does not have to be erected immediately. It is always recommended that all the parts of the circuit breakers are stored indoors.
During handling, one must ensure that the slings are of adequate strength and secured properly, that no parts are subjected to undue strain, and that the surfaces are neither scratched nor damaged. For circuit breakers, wire ropes should never be attached on conductors and insulating parts.
These are detailed below.
The following precautions must be followed during storage of indoor switchgears:
- If the equipment is to be stored for more than three months or the weather is humid, switch on the anti-condensation heaters provided in the circuit breaker ensuring proper voltage supply to the appropriate terminal.
- The crates should be unpacked carefully after selecting a dust-free area. The dust sticking to the crates should be removed and the polythene cover taken off.
- All the hardware should be stored essentially in polythene bags and kept indoor.
- Circuit breakers should be stored in dean, dry airy place indoors and the site should be cool and free from dust, humidity and corrosive gases.
- When any work is done in the vicinity of the storage place, the panels should be protected with suitable covers.
- The packing case should be raised dear off the floor on bottoms so that air may circulate freely beneath it.
- Rats and other vermin are a source of considerable damage and periodic inspection is necessary to minimize the danger they pose.
The following precautions need to be followed while storing outdoor switchgears:
- In case of high voltage circuit breakers beyond 33 kV, all the insulators should be stacked properly and stored in a safe place to prevent damages to petty coats. The surface of sealing flanges should be protected adequately.
- Rubber parts like oil seals, gaskets, washers, etc. should be stored in such a way as to avoid mechanical damages and should be smeared with chalk powder before being stored. They should be stored away from heat and moisture to avoid condensation.
- All the sealing surfaces and surfaces of terminal plates should be protected from damages during handling and storage.
- Oil circuit breakers must be filled with oil.
- Packing material such as wood should be removed as it may absorb moisture.
- The packing case should be removed if it is not designed for outdoor storage.
- During handling, it must be ensured that the slings are of adequate strength and secured properly and that no parts are subjected to undue strains.
- In the case of circuit breakers, wire ropes should never be attached on conductors and insulating parts.
One must check that the unit serial number packed in all the cases received for installation of each breaker is the same. This will ensure that factory settings are achieved automatically and no elaborate checking/setting of various parameters such as travel, spring compression, etc. would be required, which otherwise may be necessary.
For outdoor switchgear the following installation checks should be done:
- Before unpacking the packing cases for installation, ensure that the civil work of the foundation blocks with pockets as per the foundation drawings is completed.
- The foundation bolt should project a minimum height as per the manufacturer’s foundation drawing above concrete level.
- Grout the foundation bolts and allow the cement to cure.
For indoor switchgear the following installation checks should be done:
- Refer Outline and General Arrangement drawing, floor plan layout and foundation layout, to ensure the completion/correctness of the civil work of the foundation.
- Check the serial number of panels and ascertain the position to be occupied by them.
- Before installation of the equipment, it is necessary for the civil work of the building to be completed.
- Do not hook or sling on support insulators for lifting the truck.
- Draw out the trucks from panels and store them separately in a dry and dust-free room. Cover them with polythene covers.
- Shift the panels to their specified location carefully so that no damage is caused to instruments and other parts.
- Ensure that all cable trenches and floor openings for main and auxiliary cables are present as per the foundation drawings.Switchgear panels fitted with foundation channels are to be laid as per the dimensions of layout drawings.
- Ensure that the foundation channel is levelled properly and that the surface to be covered by the panel base is at the main floor level. This is to ensure the free movement of the truck.
- On level floors, panels can be moved on rollers or pipes of adequate length kept underneath. The truck can be moved on its own wheels.
The following steps should be followed for erection of a typical outdoor circuit breaker:
Step #1 – Prepare foundation and place the support structure assembly with the mechanism house over the foundation blocks. Ensure that the vertical columns of the structure are perpendicular to the ground and use shims under the base angle wherever necessary. Tighten the nuts fully.
Step #2 – Open the covers of the mechanism and Pole Unit base frame.
Step #3 – Place the drive rod in position (Figure 8) and connect to the operating shaft by pin and circlip. Do not alter the length of the drive rod, as any change will render the breaker inoperative.
Ensure that the serial numbers of the structure, inter-phase/three-phase assembly, mechanism-house and drive rod are same. Place the pole assembly and base frame over the support structure, aligning the fixing holes of both assemblies. Ensure that the gasket over the mechanism house is in position.
Do not hook or sling insulators while lifting the pole assembly for the sling position.
Step #4 – Insert all the fining hardware. Now gradually tighten the top fixing hardware to compress the sealing gasket between the base frame and the mechanism housings. Finally tighten the bottom sheet fixing hardware.
Use plain washers and spring washers with all the hardware. Also check the tightness of all hardware on poles, structures, etc.
For indoor switchgears, refer the floor plan layout drawing, erection and commissioning instructions furnished in the instruction manual for erection to ensure the completion/correctness of erection. Spirit level and plumb line should be used to ensure that the equipment is erected vertically.
Move the panel to the appropriate position and locate on foundation channels. Ensure that the correct sequence of panels is maintained as per the layout drawings. Remove the back and top covers of the bus bar chamber of every panel. Connect the busbars between panels and align them.
After the busbars have been aligned and tightened, weld the foundations on embedments and lighten the inter-panel bolts to complete the formation of the switchboard.
Lay HV, LV cables and seal all cable entry holes using cable sealing plate and cable glands. Ensure that the fronts of all panels are in one line and that the front face is at a right angle to the floor.
Earthing bars mounted on the rear of the breaker chamber should be inter-connected to the earth bar of the adjacent panel using the connecting strips provided and this earth bar should, in turn, be connected with the station earth terminal.
After busbar connections have been made, assemble busbar shrouds. Smear petroleum jelly after cleaning on the mating surfaces of conductors during the jointing process. On completion of the erection work of panels, trials should be carried out with the respective trucks by inserting and withdrawal inside the panel to ensure that smooth movement of the breaker truck is achieved.
Procedure/precaution to be followed for assembly of switchgear are described below.
Refer to instructions furnished in the instruction manual. Switchgear/VCB sub-assemblies and devices supplied loose have to be assembled, and the pole unit assembly should be connected properly with the operating mechanism.
It is advisable to slow operate the breaker a few times to check the alignment of the moving parts of the mechanism and verify contact stroke (travel), contact pressure spring compression, before operating the breaker/interrupter through the spring mechanism. The spring mechanism should not be operated until it is connected to pole assembly as otherwise it is liable to damage the mechanism parts and hurt the operator.
The assembly of the cable gland should be done in the following manner:
Guideline #1 – The undrilled cable gland plate is provided at the bottom of the operating mechanism. Remove this plate and drill holes to suit the cable glands supplied loose with each breaker. The number of holes for the cable glands should be based on the actual site requirement.
Guideline #2 – The switchgear should be connected properly by the jumper connection to overhead lines through an isolator.
Guideline #3 – The mechanism is normally supplied in an open position with springs discharged. The contacts of the vacuum bottle, when not connected to the mechanism and/or pulled out, will be in a dosed condition but without any external contact pressure acting on them.
Therefore, do not try to check continuity between the terminal pads of poles not connected to the mechanism and closed under force of the dosing spring, otherwise the observations will be misleading and erroneous.
A loose busbar is sent along with the hardware with each packing case. The manufacturer’s instructions and drawings should be referred to for making busbar joints. Connections between two units should be made and tightened properly. Necessary air clearance should be ensured.
In case of busbars, due regard should be given to expansion, which is normally taken care of by the manufacturer.
Before making the joints, the faces of the connectors must be cleaned by a scratch brush/ emery paper. Petroleum jelly should be applied and wiped clean with a lint-free cloth. The assembly of the busbar joints should then be completed and the hardware of the busbar joints tightened properly at the specified torque with a torque spanner.
Depending upon the design, busbar joints are insulated/shrouded. The preparation of power cable for connection to the switchgear is a specialized function and an experienced cable jointer should be employed for this purpose. The cable has to be carefully laid to avoid sharp bends, and the length to the cable lug accurately measured to avoid stresses on the cable or on the terminal to which it is attached.
Checks should be carried out to ensure proper phase sequence.
Will be continued in the next part…
- Switchgears book by BHEL – Bharat Heavy Electricals Limited