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What is AIS Substation and GIS Substation?
Answer:
AIS Substation (Air-Insulated Substation) and GIS Substation (Gas-Insulated Substation) are two types of substations used in electrical power systems to switch, transform, and control power. Here’s a clear comparison to help understand both:
1. AIS Substation (Air-Insulated Substation):
Definition:
An AIS uses air as the primary insulating medium for all components like circuit breakers, busbars, disconnectors, etc.
Characteristics:
- Insulation Medium: Air
- Size: Requires large space due to clearances needed between components
- Installation: Typically outdoor
- Maintenance: Easier and cheaper to maintain
- Cost: Lower initial cost
- Reliability: Less compact, more exposed to weather, dust, pollution
- Visual Accessibility: Easy to inspect visually and maintain
- Safety: More exposed components, so higher safety precautions needed
2. GIS Substation (Gas-Insulated Substation):
Definition:
A GIS uses SF₆ (sulfur hexafluoride) gas as the insulating medium. All equipment is housed in metal-enclosed gas chambers.
Characteristics:
- Insulation Medium: SF₆ gas
- Size: Very compact, ideal for urban or space-constrained areas
- Installation: Usually indoor or in underground spaces
- Maintenance: Less frequent but specialized and costly
- Cost: Higher initial cost
- Reliability: Very reliable; less affected by external factors
- Visual Accessibility: Internal components not visible
- Safety: High safety due to enclosed equipment
🔄 Key Differences Summary Table:
| Feature | AIS | GIS |
| Insulation Medium | Air | SF₆ Gas |
| Space Requirement | Large | Compact |
| Installation | Outdoor | Indoor/Underground |
| Cost | Lower | Higher |
| Maintenance | Easier and cheaper | Less frequent, more complex |
| Reliability | Affected by environment | Highly reliable |
| Safety | Moderate | Very High |
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What Kinds of Cables Are Used for Power Transmissions?
Answer: Cables used for power transmission vary based on voltage level, distance, environment (underground, overhead, or submarine), and power capacity. Here’s a breakdown of the main types of power transmission cables:
1. Overhead Power Transmission Cables
These are bare conductors, typically used for long-distance high-voltage transmission.
Common Types:
- ACSR (Aluminum Conductor Steel Reinforced):
- Most common for overhead lines
- Aluminum for conductivity, steel for strength
- AAAC (All Aluminum Alloy Conductor):
- Lighter and corrosion-resistant
- AAC (All Aluminum Conductor):
- High conductivity but lower tensile strength
- ACAR (Aluminum Conductor Alloy Reinforced):
- Improved strength and conductivity balance
2. Underground Power Transmission Cables
Used where overhead lines are not feasible (urban areas, safety concerns, aesthetics).
Common Types:
- XLPE Cables (Cross-Linked Polyethylene):
- Most widely used for medium and high-voltage underground systems
- Excellent thermal and electrical properties
- EPR Cables (Ethylene Propylene Rubber):
- Flexible and moisture-resistant
- PILC (Paper-Insulated Lead-Covered Cables):
- Traditional type, now largely replaced by XLPE
- MI Cables (Mineral Insulated):
- Fire-resistant, used in special applications
3. Submarine Power Cables
Used for underwater transmission, such as between islands or offshore wind farms.
Types:
- HVAC (High Voltage Alternating Current) Submarine Cables
- HVDC (High Voltage Direct Current) Submarine Cables
- Often preferred for long distances (e.g., > 50 km) due to lower losses
Materials: Usually XLPE or oil-filled with copper or aluminum conductors, armored for protection.
🔌 Key Cable Components:
Regardless of type, power cables generally have:
- Conductor: Copper or aluminum
- Insulation: XLPE, EPR, or oil-impregnated paper
- Shielding: To prevent interference and ensure safety
- Sheath/Jacket: For mechanical protection (PVC, lead, etc.)
- Armoring (for underground or submarine cables): Steel tape or wire
🧭 Selection Depends On:
- Voltage level (LV, MV, HV, EHV)
- Distance of transmission
- Environment (air, ground, sea)
- Cost and efficiency
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How can power factor be improved?
Answer:
Power factor can be improved using the following methods:
- Capacitor banks: Adding capacitors to the system compensates for the inductive effects of the load, reducing the phase difference between current and voltage.
- Synchronous condensers: These are synchronous motors that can adjust their reactive power to improve power factor.
- Power factor correction equipment: Using modern equipment like power factor controllers and automatic capacitor switching devices can help maintain an optimal power factor.
- Load balancing: Ensuring that loads are evenly distributed across phases can also help improve power factor.
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What is the power factor in a Star-Delta connected motor?
Answer:
In the Star connection, the motor draws less current, so the power factor is generally lower due to the reduced voltage. When the motor is switched to Delta, the power factor improves as the motor runs at full voltage and capacity, leading to a higher current draw and better overall efficiency.
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What are the disadvantages of using Star-Delta starting method?
Answer:
- Reduced torque during startup: In Star, the starting torque is significantly lower, which may not be suitable for applications requiring high torque at startup.
- Not suitable for all motors: Some motors, especially those designed for high torque applications, may not function optimally with a Star-Delta starter.
- Complexity in switching: The Star-Delta starter requires additional control circuits to switch between Star and Delta, which can add complexity to the system.
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What is the significance of grounding in electrical systems?
Answer: Grounding (also known as earthing) is a critical safety and operational practice in electrical systems. Its significance lies in protecting people, equipment, and maintaining system stability. Here’s a breakdown of why grounding is essential:
⚡ 1. Safety of Human Life
- Main Purpose: Prevent electric shock.
- When a fault occurs (e.g. a live wire touches a metal casing), grounding ensures the excess current is diverted safely into the earth, rather than passing through a person who touches the equipment.
- Keeps touch and step voltages at safe levels.
⚙️ 2. Equipment Protection
- Protects electrical equipment from damage due to faults, such as short circuits, lightning strikes, or insulation failures.
- Prevents over voltages by giving high voltages a path to dissipate safely.
⚖️ 3. Voltage Stabilization
- Provides a reference point for system voltages (zero potential).
- Helps balance unbalanced loads and maintains system voltage within safe and predictable limits.
- Especially important in three-phase systems to stabilize neutral points.
⚡ 4. Facilitates Protection System Operation
- Grounding enables protective devices like circuit breakers and relays to detect fault currents and operate quickly.
- For example, when a fault occurs and current flows to ground, the protective system can sense this change and isolate the faulted section.
🌩️ 5. Lightning and Surge Protection
- Provides a low-resistance path for lightning surges or switching surges to dissipate safely.
- Protects substations, transformers, and sensitive electronics.
🏗️ 6. Prevents Static Build-Up
- In industrial and high-voltage environments, grounding helps discharge static electricity, avoiding sparks that could cause fires or explosions.
🔌 Types of Grounding:
- System Grounding: Grounding of one point of the electrical system (e.g., transformer neutral).
- Equipment Grounding: Connecting the metal parts of equipment to ground.
- Functional Grounding: For signal references or noise reduction (e.g., in electronics).
🧲 Common Grounding Systems:
| Grounding Type | Description |
| Solid Grounding | Direct connection to earth – fast fault clearing |
| Resistance Grounding | Uses resistors to limit fault current |
| Isolated (Ungrounded) | No intentional ground – used in some critical systems but harder to detect faults |
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Why HVDC Transmission is used in power system?
Answer: High Voltage Direct Current (HVDC) transmission is used in power systems for several key technical and economic reasons. Here’s a clear breakdown of why HVDC transmission is used:
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Lower Transmission Losses Over Long Distances
- AC transmission suffers from higher losses due to capacitive, inductive, and skin effects, especially over long distances.
- HVDC reduces these losses because:
- There’s no reactive power.
- It eliminates charging currents in cables.
- It avoids the skin effect, allowing full conductor utilization.
Example: For distances over ~600 km (overhead lines) or ~50 km (submarine cables), HVDC becomes more efficient than HVAC.
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Economical for Long-Distance Transmission
- HVDC requires expensive converter stations, but the line cost per km is lower than AC.
- The break-even distance (where HVDC becomes cheaper than HVAC) is around:
- 500–600 km for overhead lines.
- 50 km or more for submarine/underground cables.
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Asynchronous Grid Interconnection
- HVDC allows interconnection of two different AC grids that are not synchronized.
- This is valuable for:
- Stabilizing weak grids.
- Interconnecting countries or regions with different grid frequencies or phases.
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Better Control of Power Flow
- HVDC systems provide precise control of power flow using power electronics.
- Enables:
- Rapid power adjustments.
- Prevention of cascading failures.
- Improved grid stability.
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Easier Underground or Underwater Transmission
- AC cables have high capacitive losses over long distances, making them impractical for underground/subsea use.
- HVDC does not generate reactive power, making it ideal for:
- Submarine cables (e.g., intercontinental links).
- Urban underground cables.
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Enhanced System Stability
- HVDC can act as a firewall to disturbances (like blackouts) in one part of the grid.
- It can also help dampen oscillations and enhance voltage stability in weak or remote systems.
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Renewable Energy Integration
- HVDC is ideal for integrating remote renewable energy sources like:
- Offshore wind farms.
- Solar plants in deserts or remote regions.
- Allows efficient transmission from remote areas to demand centers.
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What is Transformer Humming?
Answer: Transformer humming is a low-frequency sound (typically around 50 or 60 Hz) that comes from a transformer during operation. This humming is mainly caused by magnetostriction.
What is Magnetostriction?
- When the transformer core is magnetized by the alternating current (AC), the iron core slightly changes its shape (expands and contracts) due to magnetic forces.
- This mechanical deformation occurs twice every cycle of AC, creating a continuous vibrating sound—which we hear as humming.
Other contributors to humming:
- Loose core laminations.
- Vibration of windings, enclosures, or mounting structures.
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Why Does a Transformer Get Hot?
Answer: Transformers get hot during operation due to energy losses. The main reasons are:
1. Copper Loss (I²R Loss)
- Caused by current flowing through the windings (resistance).
- Heat is produced due to electrical resistance of the winding conductors.
2. Iron Loss (Core Loss)
These are losses in the magnetic core:
- Hysteresis Loss: Due to repeated magnetization and demagnetization of the core.
- Eddy Current Loss: Circulating currents induced in the core material.
3. Overloading
- When the transformer operates beyond its rated capacity, it draws more current, increasing both copper and core losses.
- Leads to excessive heating.
4. Poor Ventilation or Cooling
- Transformers rely on air, oil, or water cooling systems.
- If these systems fail or are insufficient, heat cannot dissipate properly, causing overheating.
5. Harmonics in Supply
- Non-linear loads (like computers, inverters) introduce harmonics.
- Harmonics increase eddy current losses and cause additional heating.
6. Short Circuits or Internal Faults
- If there’s a fault inside the transformer, it can draw excessive current, producing heat quickly.
7. Loose Connections
- Loose or corroded electrical connections can create localized heating due to arcing or resistance.
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Why open circuit test of a single phase transformer is done on Low Voltage Side?
Answer: The open circuit (OC) test of a single-phase transformer is typically done on the low voltage (LV) side, and here’s why:
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Safety Considerations
- The high voltage (HV) side operates at dangerous voltage levels (e.g., 11 kV, 33 kV).
- Performing the test on the LV side (e.g., 230 V, 400 V) is much safer for operators and test equipment.
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Easier Instrumentation
- During the open circuit test, you need to measure:
- Voltage (V)
- Current (I)
- Power (W)
- These measurements are much easier and more accurate at low voltages using standard meters.
- Current in OC test is small, so using LV side simplifies current measurement without needing expensive CTs (Current Transformers).
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Practical Power Supply
- It’s much easier to supply a controlled low voltage AC (like 230 V) from a standard source in a lab or field environment.
- Supplying high voltage for the test would require special equipment, insulation, and safety protocols.
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No Effect on Core Losses
- The OC test is used to measure core (iron) losses, which depend on:
- Voltage applied to the core (NOT current).
- Since the test is done with rated voltage on either side, it doesn’t matter which side is energized for the sake of core loss measurement—but LV is more practical.
