1). What are the required clearances for 33 kV systems?
The clearances for 33 kV systems ensure safety and proper functioning. Typical clearances include:
- Phase-to-ground clearance: Minimum 320 mm
- Phase-to-phase clearance: Minimum 350 mm
- Vertical clearance from the ground: At least 4.57 m in open-air substations.
- Horizontal clearance between live parts and buildings: Around 1.2 m.
2). What transformer protection systems are used in power plants?
Common transformer protection systems include:
- Differential Protection: For detecting internal faults.
- Buchholz Relay: To detect gas accumulation or oil pressure fluctuations.
- Overcurrent and Earth Fault Protection: To safeguard against overloads and ground faults.
- Temperature Protection: Monitors winding and oil temperatures.
- Restricted Earth Fault Protection: For enhanced ground fault detection near the neutral point.
3). What are CTs and PTs? Where and why are they used?
Current Transformers (CTs)
Measure current and provide scaled-down values for protection and metering devices. Used in high-voltage lines for safe current measurement.
Potential Transformers (PTs)
Step down high voltage to a lower, safer level for measurement and protection. Used to monitor line voltages in substations and switchgear.
4). What generator protection systems are implemented in power plants?
Common generator protection systems include:
- Differential Protection: Protects against internal faults.
- Over-speed Protection: Prevents generator damage from over-speeding.
- Over/Under Voltage and Frequency Protection: Maintains stable generator operation.
- Stator Earth Fault Protection: Detects ground faults in the stator windings.
5). What equipment is present in the power plant?
Typical equipment includes:
- Transformers (distribution and power)
- Circuit Breakers (SF6, VCB)
- Isolators
- CTs and PTs
- Lightning Arresters
- Busbars and Switchgear
- Control Panels
- Protective Relays
6). What are the protections for transformers and generators?
Transformer Protections
Differential protection, Buchholz relay, overcurrent, and earth fault.
Generator Protections
Differential protection, over/under voltage protection, stator earth fault protection, and over-speed protection.
7). Explain Restricted Earth Fault (REF) protection for a transformer.
REF protection detects earth faults close to the neutral point by comparing current from CTs on the phase side and the neutral side of the transformer. If the current differs, it indicates a fault near the neutral.
8). How do you operate and maintain a switchyard?
Regular maintenance includes:
- Visual Inspection: For signs of wear, corrosion, (or) damage.
- Routine Testing: For breakers, CTs, PTs, & transformers.
- Cleaning & Lubrication: Of isolators & other moving parts.
- Earth Resistance Testing: To ensure safe grounding.
- Functional Testing: Of protection relays & control systems.
9). What are the required clearances in a switchyard?
Clearances in a switchyard depend on the voltage level:
For 33 kV: Minimum Phase-to-Phase: 350 mm, Phase-to-Ground: 320 mm.
Vertical clearance from ground: Around 4.5 meters.
10). What is the significance of earthing?
Earthing provides a low-resistance path to the ground for fault currents, protecting equipment and personnel from electrical shocks and ensuring stability in power systems.
11). What are the specifications of an earth pit?
An earth pit typically consists of:
- Electrode Material
- Size
- Backfill
- Earth Resistance
Electrode Material: Copper or galvanized steel.
Size: Minimum diameter of 40 mm and length of 2.5 m for rods.
Backfill: Charcoal and salt to reduce resistance.
Earth Resistance: Should be below 1 ohm for power systems.
12). What is the permissible earth resistance for the switchyard?
For substations, the earth resistance should ideally be below 1 ohm. For smaller switchyards, it may go up to 2-5 ohms.
13). How would attend a defect on an isolator?
- Isolate the circuit before working on the isolator.
- Inspect visually for any signs of damage.
- Test the contacts and moving parts.
- Lubricate where necessary and tighten connections.
- Replace parts if damaged or worn out.
14). What is the diameter of the corona ring for the switchyard?
The typical diameter of a corona ring for high-voltage equipment like isolators and arresters in a 33 kV system ranges from 500 to 1000 mm, depending on the equipment.
15). What is bay width and what is the typical width for a 400 kV substation?
Bay width refers to the horizontal space allotted for equipment in a substation.
For a 400 kV substation, bay width is typically around 22 to 28 meters.
16). What is the clearance for a 400 kV line-to-line setup?
The minimum line-to-line clearance for a 400 kV system is around 4.0 meters.
17). What is a soak pit, and why is it required?
A soak pit is a structure that helps with the drainage of water from earth pits (or) areas near transformers. It prevents waterlogging, which could affect the earth resistance of grounding systems.
18). What are the size requirements for earth flat and its ratings?
Size: Common sizes are 50 x 6 mm or 75 x 10 mm.
Material: Copper or galvanized iron, rated for 10 kA for 1-second fault current.
19). Why is a Neutral Grounding Resistor (NGR) necessary?
NGR limits fault currents during a ground fault, preventing damage to equipment and ensuring system stability by controlling the fault current to a safe level.
20). Why is power plant earthing necessary?
Plant earthing ensures
- Personnel safety,
- Protects equipment from lightning strikes & fault currents, and
- Stabilizes voltage during transient conditions.
21). What are the starting methods for a slip ring motor?
Common methods include:
- Rotor resistance starting
- Auto transformer starting
- Soft starters or VFDs (Variable Frequency Drives) for controlled acceleration.
22). How to select a starting method for a motor above 100 kW?
- Auto transformer starts for reduced starting current.
- Star-delta starter for motors without heavy starting loads.
- Soft starters or VFDs for precise control and reduced mechanical stress.
23). What is a Single-Line Diagram?
A Single-Line Diagram is a simplified graphical representation of an electrical system showing the major components like transformers, breakers, and busbars in a single line to aid in design and fault analysis.
24). What is the horn gap for a 33 kV HG fuse?
The typical horn gap setting for a 33 kV fuse is around 250 mm.
25). What is the permissible earth resistance value in a 33 kV switchyard?
The permissible earth resistance value in a 33 kV switchyard should typically be below 5 ohms (5Ω).
