Module 5: Power System Components

Circuit Breakers

• A switch with the capability of interrupting high current, even under fault conditions.
• Can interrupt normal load current and fault current to protect the system.
• Can be programmed to disconnect when current flow is too high for a specific period.
  • This feature prevents disconnection due to transient currents (e.g., inrush currents or oscillations).
  • Example settings: disconnect at 1000A in 0.1s, 500A in 0.5s, or 200A in 1s.
• Interrupts current with high values, even short-circuit currents.

Reclosers

• A type of circuit breaker with automatic closing capability.

• A regular circuit breaker opens and remains open until manually closed by an operator after fault clearance.

• Reclosers are programmed for a certain number of reclosures (e.g., four times).

• When a fault is detected:

  • The recloser opens for a specific period.
  • It recloses automatically.
  • If the current returns to normal, it remains closed.
  • If the fault persists (high current), it reopens.
  • This open and reclose process repeats for the programmed number of cycles.
  • If the fault is not cleared after all cycles, it remains open.

• Useful for restoring system operation with temporary faults.

  • Example: a tree branch touching a transmission line causes a temporary fault that might clear on its own or burn away.

• Can be used with sectionalizers to clear permanent faults.

Sectionalizers

• Used to isolate sections of a network with faults.

• Cannot interrupt fault current; it cannot open while high current is flowing.

• Used in conjunction with reclosers to isolate fault areas.

• Operation:

  • The sectionalizer detects fault current but does not open immediately.
  • It waits for the recloser to open first.
  • When the sectionalizer detects the current go to zero, it opens, isolating the fault area without interrupting high current.

• Programmable to wait for a certain number of recloser operations before opening.
  *Example

  *Grid connected to a transformer, then to a recloser (programmed to close three times).
  *Recloser output connected to a distribution network divided into two sections (Section 1 and Section 2).
  *Each section is connected through a sectionalizer.
  *Normal Operation:
  *Normal current flows from transformer through the recloser to each sectionalizer.
  *Fault in Section 1:
  *High current flows from the grid to the recloser to Section 1.
  *Sectionalizer 1 and recloser detect high current (first count).
  *Recloser opens and interrupts the current.
  *Current in sectionalizer goes to zero.
  *Sectionalizer is programmed not to open until two counts.
  *Recloser closes again to check if the fault was cleared.
  *High current is detected again (second count). Recloser opens again.
  *Current goes to zero at the recloser and sectionalizer 1.
  *After two open/close cycles, sectionalizer 1 opens (as it has counted two instances with zero current), isolating Section 1.
  *Recloser still has another cycle to try, so it closes again.
  *Since Section 1 is isolated, current flows normally from the grid to Section 2.

Isolators (Disconnecting Switches)

• Mechanical switches without current interrupting capability.
• Cannot be programmed to open after a certain number of faults, unlike sectionalizers.
• Can only be opened with very low or zero current flow.
• Used to isolate a circuit or equipment under maintenance from the live parts of the network.
• Example: Two generators (Generator 1 and Generator 2) are connected to a busbar.
  *Isolator switches are placed before and after the busbar.
  *A circuit breaker connects to a step-up transformer leading to a transmission line.
  *Another circuit breaker and isolator are on the other side, connected to Busbar 2.
  *Maintenance scenario:
  *To take out Transformer 1 and its circuit breakers for maintenance:
  *All isolators and circuit breakers are initially closed.
  *Open the two circuit breakers first.
  *Once the current is zero, open the isolators.
  *This disconnects the circuit breakers from the voltage side.
  *The isolator isolates the circuit breaker from Busbar 1, and another isolator isolates the breaker from Busbar 2 (high voltage side).
  *With the transformer and circuit breakers isolated, maintenance can proceed.
• Connection Sequence:
  *Close the isolator first (while the circuit breaker is open).
  *After ensuring no current will flow, close the circuit breaker.
  *Disconnection Sequence:
  *Open the circuit breaker first.
  *Then, open the isolator.
• Never open or close the isolator while current is flowing. Always operate it at zero current.

Air Brake Switch

• Similar to a circuit breaker but can only interrupt normal load current, not fault current.

• Can isolate circuits, but not designed to interrupt fault currents.

• Comparison:

  • Isolator: Cannot interrupt any current; it always opens and closes at zero current.
  • Air Breaker Switch: Can interrupt only normal current.
  • Circuit Breaker: Can interrupt normal and fault currents.
    *Arc interruption:
    *Air is used to interrupt the arc
    *As the switch opens, an arc forms in the air between the contacts

Lightning Bolts

• Simple metal rods installed to protect equipment from lightning strikes.
• Lightning tends to hit the highest point with the lowest impedance.
• The metal rod is connected to the ground to discharge the lightning strike energy.
• Prevents damage to transmission lines, transformers, and insulators.

Lightning Arrestors

• Protects against high voltage surges from lightning strikes or sudden load disconnections.
• Even with lightning rods, strikes can still hit transmission lines.
• Formula: $V = L \frac{di}{dt}$, where a sudden disconnection of a heavy load can cause a high voltage spike.
• Operation is similar to Zener diodes.
  • Under normal conditions, it has very high impedance, acting as an open circuit.
  • If the voltage reaches a certain threshold, the impedance drops significantly, creating a path to discharge the surge to the ground.
• Also called Metal Oxide Varistor (MOV).
  • A "varistor" is a combination of "variable" and "resistor."
  • Acts as a variable resistance inversely proportional to voltage.

Potential Transformer

• An instrument transformer used for measurement, not power delivery.
• Measures voltage levels for protection and control devices.
• Types:
  • Electromagnetic Type: A normal transformer with primary and secondary windings. Voltage is stepped down by the ratio $N<em>1/N</em>2$.
  • Capacitive Voltage Transformer: Used with very high voltage transmission lines.
    • Uses a voltage divider consisting of two capacitors ($C<em>1$ and $C</em>2$).
    • If $C<em>2$ is much higher than $C</em>1$, its impedance is much smaller.
    • Voltage over $C<em>2$ is much smaller compared to $C</em>1$, dividing the voltage for measurement.
    • An isolated transformer is connected to the secondary side for measurement.
• Formula: $Z = \frac{1}{\omega c}$

Current Transformer

• A special transformer to measure current.
• An instrument transformer.
• Primary winding is the conductor carrying the current to be measured (often one turn).
• Secondary winding consists of an iron core and a high number of turns ($N_2$).
• Relationship: $I<em>1 N</em>1 = I<em>2 N</em>2$.
• With $N<em>1 = 1$, the secondary current $I</em>2 = \frac{I<em>1}{N</em>2}$.
  • Example: If $I<em>1 = 1000$ A in the main conductor and $N</em>2 = 500$ turns, then $I_2 = \frac{1000}{500} = 2$ A.
• Allows for easy measurement of high currents by scaling them down.

Overhead Transmission Line

• Transmits bulk generated power from generation plants to load centers over long distances.
• Uses high voltage to reduce transmission current, losses, and voltage drop.
• High voltage levels range from 200 to 1200 kV.
• Overhead lines are preferred due to the high cost of underground cables.
• Conductors are mounted on tall towers, usually 25 to 45 meters tall.
• Suspended by insulators to prevent connection to the ground.
  *Insulators
  *Insulators must withstand static and dynamic forces.
  *Static force: Weight of the conductor.
  *Dynamic forces: Wind, heavy rain, freezing, snow accumulation.
  *Conductors
  *Conductors should be of good conducting material (copper or aluminum).
  *Conductors should be flexible and strong.
  *Stranded wire:
  *Stranded wire is used for flexibility.
  *Construction: Small diameter conductor strands.
  *Reinforcement: Steel inner strands are used for increased strength.
  *Skin Effect:
  *AC current tends to flow in the outer diameter of the conductor at high frequencies.

Corona Discharge

• Occurs with high voltage conductors due to ionization of the air around the conductor.

• Formula: $E = \frac{V}{d}$, where E is the electric field intensity, V is the voltage, and d is the diameter of the conductor.

• High E ionizes the gas around the conductor, leading to electrical discharge.

• Visible as a purple light around high voltage transmission lines at night.
  *Problems
  *Leak current produces losses
  *Electromagnetic interference.
  *Damage to the conductor surface.
  *Reduction

  *Use bundled conductors
  *Bundled Conductor

  *Increasing Wire Diameter to Reduce Electric Field
  *Each conductor is still stranded with a steel reinforce middle.
  *Use of Spacers in bundled Ductors for Voltage Equalization

Static Shield Wire (Overhead Ground Wire)

• Same function as a lightning rod for transmission lines.
• Installed at the highest point of the transmission tower.
• Connects to ground to discharge lightning strike energy through the tower.

Distribution Line

• Smaller towers and lower voltage levels compared to transmission lines.

• Surface Transformer: Steps down voltage from the distribution level to consumer voltage (100-240 V).

• Commercial and Industrial loads often connect directly to the distribution network.
  *Voltage/Frequency Standards:

  *United States and Canada: 120 V, 60 Hz.
  *Europe, North Africa, China: 220 V, 50 Hz.
  *Japan: Both 50 Hz and 60 Hz.

Grid Control Center

• Monitors and controls power system components in real time.
  • Generation, transmission, and distribution networks are monitored.
• Data collected from substations, transformers, transmission lines, protection devices, and circuit breakers.
• Control signals are sent to disconnect or reclose circuit breakers, change transformer tap settings, etc.
• Algorithms and applications are used to predict future demand.
• Energy Trading
  • Predict load patterns and establish energy trades between utility companies.
• Control Center Mimic Board
  • Displays system conditions, such as power flow and transmission line status.