A modern electrical power system catering to huge energy demands are spread over extensive areas. It contains several major components like generators, transformers, transmission and distribution lines, motors etc. It is evident that inspite of all the precautions taken in the design and installation of such systems, there are bound to arise abnormal conditions or faults. The fault like short circuits may prove extremely damaging to not only the faulty component but to the neighboring components and to the power system as a whole. It is of vital importance to limit the damage to a minimum by necessary precautionary measures and speedy isolation of the faulty section.
Major Reasons of fault in Electrical Substation
- A symmetrical 3 phase to ground fault can occur in case of switching ON of CB when earthing switch is inadvertently kept ON.
- Two phases can be bridged together in the transformers because of failure of insulation between phases.
- Two phase wires may get shorted together by birds, kite strings, tree limbs etc.
- In monsoon, two phase conductors may swing due to winds and storms, and a power-arc may occur between them causing line to line fault.
- Also the dielectric strength of air reduces in monsoon and 2 phase fault occurs.
- Because of flashover across the line insulators or failure of line insulators due to lightning or switching over voltage or defective insulator, the commonest line to ground fault occurs.
- Line to ground fault can occur in transformers too.
The standardization of preventive maintenance of equipment at switchyard and scrupulous adherence to the schedule is the basic need for healthy electrical system.
Standard Practices for Electrical Substation Wiring
- Permanent wiring
- Every lead should end at a terminal block.
- No junction by twisting.
- Copper stranded leads for CT/PT.
- ‘O’ lugs for CT, not ‘U’.
- Nut with washer is compulsory for CT termination.
- TBs shall be stud type and not screw type.
- Wherever two DC sources are available, Primary and Back-up protection DC should be from different sources.
- Main and Back-up protection should command TC1 and TC2 both for CB tripping on fault.
- DC/AC not through different cores of same cable.
- DC emergency lightning should be through separate cable.
- Negative of DC shall be extended directly, where as positive should be through command.
- Lead nos. should be standardized as follow.
- A→ CT secondary for main protection
- B→ CT secondary for Bus Bar protection
- C→ CT secondary for Back-Up protection
- D→ CT secondary for Metering
- E→ PT secondary
- H→ AC supply
- J→ Main DC
- K→ Control DC
- L→ Indication and Annunciation DC
- Proper ferruling (numbering).
- CT ratio available for the particular feeder should be indicated on panel with connected ratio underlined.
- CT core not used (spare) should be shorted by looping at CT secondary box.
- Use of appropriate size of glands.
- Avoid loose connection.
- Wires not used should be removed from panel.
Different type of panels used in Electrical Substation
Control & Relay panel
Bus Bar Protection panel
Synchronizing panel or trolley
Communication panel
Annunciation panel
DC Distribution panel
AC Distribution panel
Battery Charger
Marshalling Box
Periodic Testing in Electrical Substation
- The relay should be tested
- Annually
- Whenever mal-operation is suspected
- While revising the relay settings
- Breaker opening and closing time
- At the time of commissioning
- Annually during service
- Whenever trip or closing coils are changed
- Whenever major repairs to opening mechanism are done
- Whenever breaker contacts are changed
- Station Earth resistance → Annually
- Meggering (IR value) of Transformer, CT, PT & CB
- At the time of commissioning
- Annually during service
- If required after any fault.
- Other specific testing viz. tan ∂ measurement, LCM of LA, CB dynamic resistance, DUE factor etc.
- At the time commissioning
- Once in two years.
- At the time of overhauling/ major repairing
- Types of Routine Testing Carried out on Transformer
Station Battery and maintenance point in Electrical substation
Leakage indication lamps should be compulsorily connected.
Every DC takes off should be through fuses (H.R.C.) or MCB.
Leakage in DC should be attended on top priority.
Check electrolyte level in cell every morning. If low, top up with distilled water.
Check up the voltage & specific gravity of six cells in each shift and record.
Ensure battery in floating condition normally.
Never charge at a voltage higher than 2.4 V/Cell.
If any cell found with low volt/ sp. Gravity, bypass & charge separately and then replace.
Once in a day AC to charger should be made off, and measure DC
Ensure closing & tripping of CB (not in service) in AC off condition.
Restore AC immediately after testing.
Earthing in Electrical Substation
Safety of personnel
Prevent or at least minimize damage to equipment as a result of heavy fault current.
Improve reliability of Power Supply.
After weighing the merits & demerits of the common/ separate system earthing & safety earthing, the common practice of common and solid grounding system designed for effective earthing & safe potential gradients is being adopted.
EHT s/s → 1.0 Ω
33KV s/s → 2.0 Ω
D/P Structure → 5.0 Ω
Tower footing resistance → 10.0 Ω
This is standard value, It’s may be vary with standard document.
Maintenance Point in Electrical Substation
Check list for each equipment should be filled up in each shift
Any abnormality found should be brought to the notice of the maintenance engineer.
Any leakage in switch yard should be immediately addressed.
Any abnormal chattering sound in switch yard should be given attention.
Hot clamp or sparking is visible clearly at night. A round in switch yard at least twice in a night shift is must.
Ensure all indications on Panels/ Control rooms are in service.
Always insist for fuses of correct rating as per circuit requirement.
Drawing is the language of engineers. Keep all the drawings handy. A traceable or soft copy should be maintained.
Insist for all permissive and interlock for breaker/ isolator closing.
Keep the covers of the protective relay, meters and panel doors closed.
See that the panels in your sub station are rodent proof.
Change over of bus should be taken at least once in two months to ascertain isolator operation of each bus.
Confirm correct of operation of CT switching and PT selection relay on operation of isolators.
On tripping first acknowledge the annunciation and not reset before noting the detailed relay indication.
Judge the type of fault from relay indication.
If it seems to be a transient fault, take a test charge with the permission of Load Dispatch Center.
If both parallel lines have tripped and fault seems to be transient, never take test charge of a second line, if first has tripped on test charge.
Thorough inspection of transformer or patrolling of line is required for analysis of the fault.
If relay indication does not reveal to the proper fault condition, the protection scheme should be got tested through protection engineer.
The transformer tripped on Differential Protection should not be taken in service before testing.
Be ready for zero power in the sub station, take actions with cool mind and restore the system in a proper sequence of operation, isolating the faulty part if any.
Always report to and follow the instructions from Load Dispatch Center.
A shift engineer should take over all status of the system in the start of the shift.
The status of equipments out of service should be updated in every shift.
Over loading of the line/ transformer should be immediately brought to the notice of the Load Dispatch Center.
Always keep the ‘Under frequency Load Shedding panel in service.
Never switch ON the feeders tripped on U/F, without permission of LDC.
Update yourself with latest technology in the market.
Keep library of important technical documents/ books.
Discussion among the staff results in sound technical and HR environment.
Equip your sub station with proper testing instruments, tools & tackles.
Supervise your staff working on live system, follow safety precautions scrupulously.
Avoid delay, work with concentrated mind and goal to finish.
When you are on work write each and every activity, however small it is, in the register and before closing the job refer it for proper restoration.
If any electrical fault arise, try to solve self and if not solve then inform to higher authority.