Here’s a list of commonly asked 110kV electrical substation interview questions with sample answers. These questions cover technical knowledge, operations, safety, and maintenance practices expected from engineers or technicians working in high-voltage substations.
1. What is a 110kV substation and its purpose?
Answer:
A 110kV substation is a high-voltage electrical facility that steps down voltage from transmission levels (typically 110kV) to lower voltages suitable for distribution. It acts as a node for switching, protection, and controlling electrical power, ensuring stable and reliable supply to downstream networks.
2. What are the main components of a 110kV substation?
Answer:
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Power transformers
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Circuit breakers
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Isolators/disconnectors
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Current transformers (CTs) and Potential transformers (PTs)
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Busbars
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Surge arresters
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Control and protection panels
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SCADA and communication systems
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Earthing system
3. What type of circuit breaker is used in 110kV substations?
Answer:
Typically, SF₆ (sulfur hexafluoride) circuit breakers are used in 110kV substations due to their high dielectric strength and arc-quenching capabilities. They provide reliable operation under high-voltage and high-current interruption conditions.
4. How do you ensure safety while working in a 110kV substation?
Answer:
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Follow permit-to-work (PTW) and isolation procedures
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Wear proper PPE (insulated gloves, arc flash gear, etc.)
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Ensure equipment is de-energized and earthed
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Use lock-out/tag-out (LOTO) practices
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Maintain minimum approach distances
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Perform regular safety audits and risk assessments
5. What are the types of grounding used in substations?
Answer:
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Main earthing grid: Interconnected copper or galvanized steel mesh buried in the ground.
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Equipment earthing: Connects non-current carrying metal parts to the ground.
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Neutral grounding: Usually through a resistor, reactor, or solidly grounded depending on the system design.
6. Explain the operation of a CT and PT in a 110kV substation.
Answer:
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Current Transformer (CT): Steps down high line current to a lower value for protection relays and metering.
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Potential Transformer (PT): Reduces high line voltage to a safer, measurable level for monitoring and protection. Both are vital for accurate and safe relay operation.
7. What is the difference between busbar and feeder protection?
Answer:
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Busbar protection ensures that faults within the busbar zone are isolated quickly to maintain system stability.
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Feeder protection safeguards individual outgoing/incoming lines or cables from faults using overcurrent, distance, or differential relays.
8. What is a line differential protection scheme?
Answer:
It’s a protection method that compares current at both ends of a transmission line using communication links. If there is a difference (indicative of fault current), the relay isolates the line. It is fast and highly selective.
9. How do you test and maintain a power transformer in a substation?
Answer:
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Insulation resistance test (IR)
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Transformer turns ratio (TTR) test
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Oil dielectric breakdown test
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DGA (Dissolved Gas Analysis)
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Winding resistance and capacitance testing
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Thermography for hotspot detection
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Regular oil level and breather condition checks
10. What are SCADA systems and their role in substations?
Answer:
SCADA (Supervisory Control and Data Acquisition) systems enable real-time monitoring, control, and data acquisition of substation equipment remotely. It improves response time during faults, enhances operational efficiency, and provides historical data for analysis.
Here’s a set of common 110kV substation protection interview questions with answers that you can review as part of your preparation:
🔌 110kV Substation Protection: Interview Questions & Answers
1. What are the main protection systems used in a 110kV substation?
Answer:
Common protection systems include:
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Differential protection (for transformers and buses)
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Distance protection (for transmission lines)
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Overcurrent and Earth fault protection
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Backup protection relays
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Buchholz relay (for oil-filled transformers)
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Restricted Earth Fault (REF) protection
2. Explain the working principle of distance protection.
Answer:
Distance protection operates by measuring the impedance (Z = V/I) between the relay location and the fault point. If the measured impedance falls below a preset value (indicating a fault within the zone), the relay trips. It is a common method for protecting long transmission lines due to its speed and selectivity.
3. What is differential protection, and where is it used?
Answer:
Differential protection compares the current entering and leaving an equipment zone (like a transformer or busbar). If the difference exceeds a set threshold (indicating internal faults), the relay trips. It is fast and selective, suitable for transformers, generators, and busbars.
4. What is the function of CTs and VTs in a substation?
Answer:
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CTs (Current Transformers) reduce high currents to a measurable and safe level for protection and metering devices.
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VTs (Voltage Transformers or PTs) step down high voltages for monitoring, protection, and control.
5. How do you ensure selectivity in protection schemes?
Answer:
Selectivity ensures that only the faulty section is isolated. This is achieved by:
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Proper coordination of relay settings (time, current, zone)
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Using zone-based protection (like distance and differential)
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Grading time settings in backup protection relays
6. What is Zone 1, Zone 2, and Zone 3 in distance protection?
Answer:
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Zone 1: Covers 80–90% of the line and operates instantaneously.
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Zone 2: Extends beyond Zone 1 (typically 120–150% of the protected line), has a time delay.
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Zone 3: Provides remote backup with a longer time delay, extending into the adjacent line section.
7. Describe the function of a Busbar Protection Scheme.
Answer:
Busbar protection (typically differential) detects faults within the busbar zone. It isolates the bus quickly to prevent damage and system instability. Modern schemes use high-speed relays with breaker failure backup.
8. What is the role of the breaker failure protection?
Answer:
If a circuit breaker fails to trip after a protection signal, the breaker failure relay initiates tripping of the upstream breakers to isolate the faulted section. This prevents fault escalation.
9. Why is relay coordination important in a substation?
Answer:
Relay coordination ensures correct relay operation in sequence and minimizes service disruption. Proper coordination prevents unnecessary tripping of healthy sections and enables selective fault clearance.
10. What is SCADA and how does it interface with protection systems?
Answer:
SCADA (Supervisory Control and Data Acquisition) enables remote monitoring and control of substations. It interfaces with protection relays for real-time data, alarm logging, and fault diagnosis.
Here are some commonly asked interview questions and answers related to the earthing system in a 110kV electrical substation. These are especially relevant for roles in design, commissioning, operations, or maintenance in power transmission or heavy industrial settings.
1. What is the purpose of earthing in a 110kV substation?
Answer:
Earthing (grounding) ensures safety by providing a low-impedance path for fault currents to flow into the earth. It protects equipment and personnel from electric shock and maintains the voltage at known levels under both normal and fault conditions. It also facilitates the operation of protection relays.
2. What are the types of earthing used in substations?
Answer:
Main types include:
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Equipment Earthing: Connecting metallic parts of equipment to the ground.
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System Earthing: Connecting the neutral point of transformers or generators to the ground.
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Lightning Protection Earthing: Dispersing lightning strike energy safely into the earth.
3. What standards govern substation earthing design?
Answer:
Commonly followed standards include:
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IEEE 80: Guide for Safety in AC Substation Grounding
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IEC 61936: Power installations exceeding 1 kV AC
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IS 3043: Indian Standard Code of Practice for Earthing (for India-specific installations)
4. How do you calculate the size of the earthing conductor?
Answer:
The conductor size is typically based on the magnitude and duration of the fault current using the formula:
Where:
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A = Cross-sectional area of the conductor (mm²)
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I = Fault current (kA)
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t = Duration of fault (seconds)
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k = Material constant (depends on material and temperature rise)
5. What is a ground grid and how is it laid out?
Answer:
A ground grid is a network of horizontal and vertical conductors (usually copper or GI) laid out under the substation yard, forming a mesh pattern. It is connected to all metallic equipment, fences, and structures. The grid spacing and depth are designed to keep step and touch voltages within safe limits.
6. What are step and touch voltages?
Answer:
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Step Voltage: Voltage difference a person experiences between their feet while walking near a fault location.
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Touch Voltage: Voltage a person experiences between a grounded object and the feet when touching that object during a fault.
Both must be limited as per safety standards (e.g., IEEE 80) to prevent hazardous shocks.
7. What is the role of soil resistivity in substation earthing?
Answer:
Soil resistivity affects how effectively the earth can dissipate fault current. Lower resistivity means better dissipation and safer voltage levels. Soil resistivity is measured during site surveys using methods like the Wenner or Schlumberger technique.
8. How is earth resistance tested in a substation?
Answer:
Earth resistance is typically tested using the fall-of-potential method or clamp-on earth resistance testers. The goal is to ensure the total resistance is within permissible limits (typically <1 Ohm for substations, though values may vary by standard).
9. What is a Ground Potential Rise (GPR)?
Answer:
GPR is the maximum electrical potential that the earthing system may reach during a fault condition relative to remote earth. It’s critical for evaluating safety of personnel and equipment, especially communication interfaces.
10. What materials are commonly used for earthing in 110kV substations?
Answer:
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Copper (solid or stranded): Highly conductive and corrosion-resistant.
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Galvanized Iron (GI): More economical but prone to corrosion.
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Stainless Steel: Used in corrosive environments.