9.3 Protection and Control of Electrical Power System

9.3 Protection and Control of Electrical Power System

1. Protection Principles & Devices

1.1 Basic Protection Principles

1. Selectivity (Discrimination): The ability to isolate only the faulty part of the system. The protection device closest to the fault should operate first.

2. Reliability: The protection system must operate correctly when required (dependability) and must not operate for faults outside its zone (security).

3. Speed: Faults must be cleared as fast as possible to minimize damage and maintain system stability.

4. Sensitivity: The ability to detect even minor faults.

5. Simplicity & Economy: The system should be as simple as possible while meeting requirements.

1.2 Primary Protection Devices

(a) Fuses

• Principle: A short length of wire designed to melt and break the circuit when current exceeds a predetermined value for a sufficient time.

• Operating Characteristic: Time-Current Characteristic (TCC) curve: inverse time – higher the current, faster the operation.

• Types:

  • Rewirable (Semi-enclosed): Simple, cheap, but less accurate. Used in domestic wiring.

  • Cartridge (HRC - High Rupturing Capacity): Sealed ceramic tube with silica sand filler. Can safely interrupt high fault currents.

• Application: Overcurrent protection for transformers, motors, and low-voltage circuits.

(b) Miniature Circuit Breakers (MCBs)

• Principle: Thermal-magnetic device. Thermal element (bimetallic strip) provides overload protection (inverse time). Magnetic element (solenoid) provides instantaneous short-circuit protection.

• Curve Types: B (3-5 x I_n), C (5-10 x I_n), D (10-20 x I_n), for different load characteristics (resistive, motor starting, etc.).

• Application: Standard protection for final sub-circuits in domestic, commercial, and industrial installations. Can be manually reset.

(c) Power Circuit Breakers (CBs)

• Function: To make, carry, and break currents under normal conditions, and to break currents under fault conditions.

• Operating Principle: Uses a mechanism to open contacts within an insulating/arc-quenching medium to extinguish the arc that forms when interrupting current.

• Types (based on arc quenching medium):

  1. Air Circuit Breaker (ACB): Uses air. For low voltage (up to 1 kV) and high current applications.

  2. Oil Circuit Breaker (OCB): Uses oil as insulating and arc-quenching medium. Arc decomposes oil, creating hydrogen gas bubble that extinguishes arc. Being phased out.

  3. SF6 Circuit Breaker: Uses Sulphur Hexafluoride (SF6) gas, an excellent insulator and arc-quencher. Standard for HV and EHV systems (66 kV and above).

  4. Vacuum Circuit Breaker (VCB): Contacts open in a high vacuum chamber. Arc extinguishes at first current zero due to lack of ionizable medium. Common for MV (11-33 kV) systems.

• Key Ratings: Rated voltage, rated current, short-circuit breaking capacity (in kA), making capacity.

1.3 Relays

• Definition: A sensing device that detects abnormal conditions (faults) and initiates the opening of a circuit breaker.

• Operation: Monitors parameters (current, voltage, frequency). When a preset threshold is exceeded, it sends a trip signal to the circuit breaker's trip coil.

Types of Relays

1. Electromechanical Relays:

   • Use magnetic forces produced by input quantities to move physical contacts.

   • Examples:

     • Attracted Armature: For instantaneous overcurrent.

     • Induction Disc: For time-delayed overcurrent (inverse time characteristic). The disc rotates by eddy currents induced by input coils.

     • Buchholz Relay: Gas-actuated relay for oil-immersed transformers. Detects incipient internal faults (gas accumulation) and major faults (oil surge).

2. Static (Solid-State) Relays:

   • Use analog electronic components (transistors, op-amps) without moving parts.

   • More accurate and faster than electromechanical.

3. Digital/Numerical Relays:

   • Use a microprocessor/DSP. Input signals are sampled, converted to digital, and processed by algorithms.

   • Advantages: Highly flexible (programmable), multi-functional, self-monitoring, communication capability (IEC 61850), event recording.

1.4 Protection Schemes for Key Equipment

(a) Generator Protection

• Stator Faults: Differential (87G) protection – compares current at both ends of each phase winding. Any imbalance indicates an internal fault.

• Rotor Faults: Field failure (loss of excitation - 40) protection.

• Abnormal Conditions: Overcurrent (51), Overvoltage (59), Reverse Power (32) – protects against motoring, Under/Over Frequency (81).

(b) Transformer Protection

• Main Protection: Differential Protection (87T). Compares HV and LV side currents (corrected for transformer ratio and phase shift). Operates for internal winding faults.

• Buchholz Relay: For oil-immersed transformers.

• Backup Protection: Restricted Earth Fault (REF) protection, Overcurrent (51), Overload (thermal).

• Oil & Winding Temperature alarms and trips.

(c) Transmission Line Protection

1. Overcurrent (50/51) & Earth Fault (50N/51N): Simple, used as backup for radial feeders.

2. Distance Protection (21):

   • Principle: Measures the impedance (Z = V/I) seen from the relay location. A fault reduces the measured impedance proportionally to the distance to the fault.

   • Uses characteristic zones (Mho circle, quadrilateral):

     • Zone 1: Instantaneous trip for ~80-90% of the line length (to avoid overreach).

     • Zone 2: Time-delayed trip for ~120% of the line (covers entire line + some of next).

     • Zone 3: Longer time delay, backup for the next line section.

3. Pilot Wire / Communication-Assisted Schemes:

   • Directional Comparison (DCB): Relays at both ends communicate to determine fault direction.

   • Current Differential (87L): Compares current phasors at both ends via a communication channel (e.g., fiber optic). Highly sensitive and selective.

1.5 System Earthing (Grounding)

• Purpose: Safety, voltage stabilization, and facilitating protection.

• Types:

  1. Solidly Earthed (Effective Earthing): Neutral directly connected to earth. Common in distribution (<33 kV). High fault current, easy protection.

  2. Resistance Earthing: Neutral connected through a resistor. Limits earth fault current, reducing damage and shock hazard. Used in industrial MV systems and generator neutrals.

  3. Reactance Earthing: Neutral connected through a reactor. Similar to resistance but affects system reactance.

  4. Unearthed (Isolated): No intentional connection. Used in some small systems. A single line-to-ground fault doesn't cause a trip, but overvoltages can occur.

1.6 Surge Protection (Lightning Arresters)

• Principle: Divert high-voltage surges (from lightning or switching) safely to ground, thereby protecting equipment insulation.

• Operation: Acts as a voltage-dependent switch. Under normal voltage, it has high impedance. When a surge exceeds its breakdown voltage, its impedance drops drastically, providing a low-resistance path to earth. Returns to high impedance state after the surge passes.

• Types: ZnO (Metal Oxide) Arresters are now standard. They have a highly non-linear V-I characteristic without series gaps.

2. System Control & Automation

2.1 Blackout Prevention and Load Shedding

• Blackout: Complete loss of power in a large area, often caused by a cascading failure.

• Causes: Generation-load imbalance, loss of a critical line, voltage collapse, frequency collapse.

• Prevention Strategies:

  1. Under Frequency Load Shedding (UFLS):

     • Principle: When generation is less than load, frequency falls. To restore balance, pre-defined blocks of non-critical load are automatically disconnected in stages based on frequency thresholds (e.g., 49.5 Hz, 49.2 Hz, 48.8 Hz).

  2. Under Voltage Load Shedding (UVLS): Similar logic based on voltage levels to prevent voltage collapse.

  3. System Separation (Islanding): Intentional splitting of the grid into stable islands to prevent total collapse.

  4. Automatic Generation Control (AGC): Adjusts generator output to match load and maintain frequency.

2.2 SCADA (Supervisory Control and Data Acquisition)

• Definition: A central computer system for monitoring and controlling a geographically dispersed power system in real-time.

• Components:

  1. Remote Terminal Units (RTUs): Located in substations, gather data (current, voltage, breaker status) and execute control commands from the master.

  2. Communication System: Links RTUs to the master station (PLCC, fiber optic, microwave).

  3. Master Station: Central computer with HMI (Human-Machine Interface) displaying the system one-line diagram, alarms, and data. Allows operators to issue commands.

• Functions:

  • Data Acquisition & Monitoring.

  • Supervisory Control (remote switching).

  • Alarm Processing.

  • Event & Disturbance Recording.

  • Data Logging & Reporting.

2.3 Energy Management Systems (EMS)

• Definition: An advanced software suite that works on top of SCADA to provide analytical functions for optimal, secure, and economic operation of the power system.

• Core Functions:

  1. State Estimation: Uses redundant measurements from SCADA to calculate the most probable state of the system (bus voltages, angles). Filters bad data.

  2. Contingency Analysis: Simulates the outage of key components (N-1 criterion) to check if the system can withstand the loss without violations.

  3. Optimal Power Flow (OPF): Adjusts generator outputs, transformer taps, and other controls to minimize total generation cost while satisfying all operational constraints.

  4. Automatic Generation Control (AGC): Real-time function to maintain system frequency and tie-line power schedules.

  5. Load Forecasting: Predicts future load (short-term: hourly, day-ahead).

2.4 Smart Grid Concepts

• Definition: The modernization of the electricity delivery system using two-way digital communication, advanced sensors, and automation to improve reliability, security, efficiency, and integration of renewables.

• Key Features/Technologies:

  1. Advanced Metering Infrastructure (AMI) / Smart Meters: Two-way communication meters providing real-time data, enabling demand response, time-of-use pricing, and remote connect/disconnect.

  2. Demand Response (DR): Incentivizing consumers to reduce or shift their electricity use during peak periods.

  3. Distributed Energy Resources (DER) Integration: Seamless integration of rooftop solar, wind, and energy storage at the distribution level.

  4. Distribution Automation (DA): Use of sensors and automated switches (reclosers, sectionalizers) on feeders for self-healing – automatically isolating faults and restoring power to healthy sections.

  5. Wide Area Monitoring, Protection, and Control (WAMPAC): Uses PMUs (Phasor Measurement Units) synchronized by GPS to provide real-time, time-synchronized grid measurements for enhanced visibility and control.

  6. Cyber Security: Critical for protecting the grid from digital attacks.

• Benefits: Improved reliability and power quality, reduced operational costs, increased integration of renewables, empowered consumers.