Directional Relays: Power System Protection

• Relaying is an area within electrical power engineering
• Relaying involves the design and operation of equipment, relays or protective relays
• Relays identify abnormal power system conditions
• Relays initiate corrective action to restore the power system to its normal state

Directional Relays

• Electrical engineering uses directional relays as a protective device in power systems
• Directional relays identify the direction of fault currents
• Directional relays isolate damaged segments of the electrical system while maintaining network functionality

Directional Overcurrent Relays

• Directional overcurrent relays respond to excessive current flow in a specific direction in the power system
• These consist of two elements including a directional relay
• A directional relay determines the direction of current flow relative to a voltage reference

Feeder Protection

• Directional relays protect radial and parallel feeders from short circuits, ground faults, and overloads

Directional Relay Sections

• These relays contain 3 main sections
• Potential Transformer (PT) Section includes a potential transformer -This part reduces the voltage of the power system to a manageable processing level
• Current Transformer (CT) Section has current transformers that reduce the current for the relay to manage
• To Trip Circuit Section describes the mechanism for tripping the circuit breaker when a fault is identified
• The directional relay identifies whether the fault is in the forward or backward direction
• The relay compares the voltage and current signals
• If the fault is in the forward direction, it isolates the faulty component by sending a trip signal to the circuit breaker

Relay Coils

• Directional Relays have potential coils which are also known as voltage coils
• Potential coils receive voltage signals from the potential transformer
• Current coils, directional coils, receive current signals from the current transformers

Transformer & Generator Protection

• Transformer Protection: Directional relays can work with other protective measures like differential relays to guard against internal and external faults on connected transmission lines

• Generator Protection: Directional relays protect generators from faults that may occur within the generator or connected transmission lines

Distance Relays

• Distance relays offer protection and perform based on the distance of the fault point on the transmission line and calculate distance from the generating unit
• Impedance is calculated by evaluating fault voltage and fault current

Distance Relay Working Principle

• The distance relay operation is based on the ratio of voltage and current, namely impedance
• The relay has a potential transformer and a current transformer for the current element
• The current transformer is connected in series with the entire circuit
• The secondary current creates the deflecting torque
• The potential transformer creates restoring torque
• The relay, or impedance relay, operates depending on the ratio of the impedance value,
• Distance relays can protect transmission lines, transformers, and busbars in modern power systems
• Useful for finding faults on long-distance lines due to estimating the physical distance between the relay's sensing transformers and the fault location

Relay Construction

• A time distance impedance relay has a current-driven element, such as a double-winding type induction overcurrent relay
• The mechanism involves a spindle with a disc and a spiral spring to manage the relay contacts
• An electromagnet energized by the circuit's voltage holds these contacts open under normal conditions

Impedance Relay Operation

• The voltage transformer measures voltage being used by equipment
• The current transformer measures current flow
• The sensing unit calculates the impedance based on those measurements, and compares it to thresholds
• If impedance exceeds thresholds, the relay trips, which sends a signal to the circuit breaker, isolating the issue
• The relay will also measure impedance of a fault. If impedance is lower than relay settings, a command will be sent. The relay constantly monitors the current and voltage to ensure the voltage and current ratio is not exceeded.