Transmission 1.0 - System Protection Coordination

1.3 Terms and Definitions

  • Current Transformer (CT):
      - Instrument transformer that steps down large primary currents to a small secondary current suitable for relaying and metering.
      - Secondary current is proportional to the primary current.
  • Cycle:
      - One full sine wave (360 degrees).
      - In North America, both voltage and current waveforms complete 60 cycles per second (60 Hz).
  • Fault:
      - Any abnormal event on the electric system such as a short circuit, broken wire, or intermittent connection.
  • Misoperation:
      - Failure of a Composite Protection System to operate correctly, which includes:
        - Failure to trip during a fault.
        - Failure to trip when it should (non-fault).
        - Slow trip during a fault.
        - Slow trip when it should (non-fault).
        - Unnecessary trip during a fault.
        - Unnecessary trip when it should not.
  • Overlapping Zones:
      - Protective relay zones that overlap at a circuit or device to ensure no gaps in protection coverage.
  • Pilot Relaying:
      - High-speed relaying system that uses communication between relays at both ends of a line to provide fast and selective tripping for faults anywhere on the protected line.
  • Potential Transformer (PT):
      - Instrument transformer that steps down high primary voltage to a low secondary voltage (usually ~120 V) for metering and relaying.
      - Secondary voltage is proportional to primary voltage.
  • Power Line Carrier (PLC):
      - Technology that uses the transmission line itself to carry communication signals (e.g., between protective relays at both ends of a line).
  • Wave Trap:
      - Filter installed on the line to block the PLC communication signal and direct it to the protective relays while allowing power frequency (60 Hz) to pass through.
  • Protection System:
      - Complete set of equipment that includes:
        - Protective relays.
        - Communication systems.
        - Voltage and current sensing devices (CTs & PTs).
        - Station DC supply (batteries and chargers).
        - Control circuitry that trips circuit breakers.
  • Zones of Protection:
      - Specific areas of the power system that a protective relay is responsible for monitoring and clearing during abnormal conditions.
      - One-liner to remember: “CT steps down current, PT steps down voltage, PLC + Wave Trap enables fast relay communication, overlapping zones + pilot relaying = fast, selective protection with no gaps.”

1.4 Purpose of Protective Relays

  • Transmission and distribution systems are generally safe and reliable, delivering power to homes, industries, and businesses across North America.
  • However, they are vulnerable to:
      - Adverse weather.
      - Animals.
      - Human activity / the general public.
  • When these events damage the system, they can cause injury to people or damage to property.
  • Purpose of Protective Relaying:
      - Quickly detect faults.
      - Isolate (remove) the problem from the rest of the system.
      - Prevent further damage.
      - Keep the overall power system stable.
  • North American power systems operate at 60 Hz (60 cycles per second).
  • High-speed protective relays can detect a fault and initiate an interruption in just 2 to 3 cycles (about 33 to 50 milliseconds).
  • Interruption:
      - Refers to isolating the faulty equipment from the rest of the power system.
  • This quick isolation limits the spread of the fault and protects the larger Interconnection.
  • One-liner to remember: “Protective relays detect faults fast (in 2–3 cycles at 60 Hz) and isolate problems to prevent damage and keep the system stable.”

1.5 Protective and Auxiliary Relays

  • NERC Requirement:
      - Generator Operators, Transmission Operators, and Balancing Authorities must be familiar with the purpose and limitations of the protective equipment and schemes in their area.
  • Major Types of Protective Relays:
      - Overcurrent Relays:
        - Detect when current exceeds a preset value (protects against overloads and short circuits).
      - Ground Relays:
        - Detect ground faults (unbalanced current flowing to ground).
      - Differential Relays:
        - Compare current entering and leaving a piece of equipment (such as a transformer or generator).
        - Trips if there is a difference (indicating an internal fault).
      - Distance Relays:
        - Measure impedance (distance) to a fault, commonly used on transmission lines (also called impedance relays).
      - Monitoring Relays:
        - Watch equipment conditions.
        - Example: Temperature relays alert operators when equipment is getting too hot (approaching dangerous levels).

Auxiliary Relays

  • Tripping Relays:
      - Send the actual trip signal to circuit breakers.
  • Lockout Relays:
      - Prevent re-energizing equipment after a trip until the problem is fixed (manual reset required).
  • Timer Relays:
      - Introduce intentional time delays.
  • Re-closing Relays:
      - Automatically attempt to re-close a breaker after a temporary fault.
  • Regulating Relays:
      - Help maintain proper generator output and system voltage schedules.
  • One-liner to remember: “Protective: Overcurrent, Ground, Differential, Distance. Monitoring: Temperature. Auxiliary: Trip, Lockout, Timer, Reclose, Regulate.”

1.6 IEEE Relay Numbering

  • The North American power system is made up of many transmission systems owned by different companies.
  • These systems are interconnected through transmission tie-lines.
  • To standardize and simplify protection coordination across different owners, the Institute of Electrical and Electronics Engineers (IEEE) developed a standard numbering system for protective relays.
  • Each relay type is assigned a specific number (called ANSI/IEEE device numbers).
  • It is important for operators to recognize these relay numbers when reading single-line diagrams used by power companies.
  • One-liner to remember: “IEEE relay numbers standardize protection — learn the numbers so you can quickly understand single-line diagrams and relay schemes.”

1.7 IEEE Relay Numbering System

  • Common IEEE Protective Relay Numbers:
      - 21 – Distance Relay (measures impedance to detect faults at a distance, commonly used on transmission lines).
      - 25 – Synchronizing Relay (ensures generators or systems are in sync before closing a breaker).
      - 27 – Undervoltage Relay (trips or alarms when voltage drops below a set level).
      - 49 – Thermal Relay (monitors temperature/heat in equipment such as motors or transformers).
      - 50 – Instantaneous Overcurrent Relay (trips immediately when current exceeds a high setpoint – no intentional delay).
      - 51 – Time Overcurrent Relay (trips after a time delay that varies with how much current is above setpoint – inverse time characteristic).
      - 63 – Pressure Relay (detects abnormal pressure in equipment such as transformers or circuit breakers).
      - 67 – Directional Overcurrent Relay (trips only when fault current flows in a specific direction).
      - 79 – Re-closing Relay (automatically re-closes a breaker after a temporary fault).
      - 81 – Frequency Relay (trips or alarms for under-frequency or over-frequency conditions).
      - 86 – Lockout Relay (prevents re-energizing equipment after a trip until manually reset; usually activated by other protective relays such as 87 or 50/51).
      - 87 – Differential Relay (compares current going into and out of equipment; trips on internal faults).
      - 87T = Transformer Differential.
      - 87B = Bus Differential.
      - 87G = Generator Differential.
      - 94 – Auxiliary Relay (used for control, tripping, or other auxiliary functions).
  • One-liner to remember: “21 Distance, 50 Instant, 51 Time Overcurrent, 67 Directional, 79 Reclose, 86 Lockout, 87 Differential — these numbers are your quick reference on single-line diagrams.”

1.8 Primary and Secondary Current

  • Protective relays need accurate information about system conditions, but they operate on small amounts of current and voltage.
  • It is not practical (or safe) to connect relays directly to high-voltage equipment like a 500 kV transmission line.

Current Transformers (CTs)

  • CTs transform high primary current (on the transmission/distribution system) to a smaller secondary current.
  • The secondary current is proportional to the primary current.
  • This secondary current flows to the protective relays so they can monitor system conditions.
  • Example (CT ratio 1000:5):
      - If 1000 A flows in the primary → 5 A flows in the secondary.
      - If 200 A flows in the primary → 1 A flows in the secondary.
  • One-liner to remember: “CT steps down huge primary current to safe secondary current (proportional) — e.g., 1000:5 ratio means 1000 A primary = 5 A secondary for the relays.”

Potential Transformers (PTs)

  • Potential Transformers (PTs) supply voltage (potential) to protective relays.
  • PTs are installed on the substation bus or on the line (depending on the bus arrangement).
  • They step down the high primary voltage to a much smaller secondary voltage — typically around 120 volts.
  • The secondary voltage is proportional to the primary voltage.
  • What happens during a fault:
      - Primary voltage decreases.
      - Primary current increases.
  • Therefore:
      - Secondary voltage from the PT to the relays decreases.
      - Secondary current from the CT to the relays increases (proportionally).
  • These changing secondary values (lower voltage + higher current) allow protective relays to detect abnormal conditions and respond quickly.
  • One-liner to remember: “PT steps down voltage to ~120 V for relays. During a fault: PT voltage drops, CT current rises — relays see this and trip.”

1.13 Most Common Relays

  • On the transmission system, the most common protective relays are:
      - Ground Relays:
        - Detect ground faults — when current finds an unintended path to ground.
        - Common causes include:
          - High-voltage line contacting a tree or object.
          - Lightning strikes.
        - During a ground fault:
          - Voltage drops and current increases on the affected line.
        - Goal: Quickly isolate the fault to protect equipment and maintain system reliability.

      - Distance Relays:
        - Primarily used to detect and clear phase-to-phase faults.
        - These faults cause very high current flow and also create imbalances in the three-phase system, which can stress generators.
        - Distance relays measure impedance (apparent distance to the fault) and trip accordingly.
  • One-liner to remember: “Ground relay = most common (tree/lightning faults); Distance relay = phase-to-phase faults with high current and imbalance.”

1.14 Overlapping Zones

  • Modern protective relay systems use overlapping zones to ensure no gaps in protection coverage for all energized equipment.
  • Overlapping:
      - Means two or more relay zones cover the same equipment (e.g., circuit breakers, transformers, buses).
  • Example:
      - The transformer differential scheme overlaps with the bus differential scheme, providing complete coverage for circuit breakers C and D.

Two Main Types of Protective Zones

  • Open Zones:
      - Typically use Distance Relays.
      - Protect a specific section of line or equipment by measuring the “distance” (impedance) to a fault.
  • Closed Zones:
      - Typically use Differential Relays.
      - Protect a clearly defined piece of equipment (transformer, bus, generator) by comparing current entering vs. leaving the zone.
  • One-liner to remember: “Overlapping zones = no protection gaps. Open = Distance relays; Closed = Differential relays.”

1.15 Principle of Differential Protection

  • Differential Protection is one of the most effective methods for protecting:
      - Station buses.
      - Transformer banks.
      - Generating units.
  • Basic Principle:
      - Under normal conditions, power flowing into a protected zone (bus, transformer, or generator) equals power flowing out.
      - CT secondaries are connected so their currents cancel each other → no current flows through the differential relay coil.
  • During a Fault:
      - Current balance is disturbed (more current flows into the fault than out).
      - Current now flows through the relay coil.
      - Relay contacts close → trips the circuit breakers.
      - This quickly disconnects the faulty equipment from the system.
  • One-liner to remember: “Differential = normal: in = out (relay sees zero). Fault: in ≠ out → relay trips fast and isolates the problem.”

1.16 Differential Protection

  • Bus Differential Protection:
      - Current transformers (CTs) with the same ratio are installed on all circuits connected to the bus.
      - All CT secondaries are connected together so their vector sum flows through the differential relay.
      - Under normal conditions: Power into the bus equals power out → vector sum current = zero → relay does not trip.
      - During a bus fault:
        - Current flows into the fault → currents are no longer balanced → differential relay operates and trips all breakers connected to the bus.

  • Generator Differential Protection:
      - Works the same way:
        - CTs measure current at both ends of each phase winding in the generator.
        - Under normal conditions: Current entering equals current leaving each winding → differential relay sees zero difference.
        - If a fault occurs inside the generator winding: Current imbalance is detected → differential relays operate and separate (trip) the generator from the system.

  • One-liner to remember: “Bus & Generator Differential: Normal = currents balance (sum = 0). Fault = imbalance → relay trips everything connected.”

1.17 Differential Zones

  • Example:
  • 87B (Bus Differential) and 87T (Transformer Differential) both cover circuit breaker B; circuit breaker C is protected by:
      - 21 (Distance Relay) and 67 (Directional Overcurrent Relay) on the line, plus the 87B bus differential relay.
  • If a fault occurs inside circuit breaker C, the operator would see targets from 87B + either 21 or 67.
  • Transformer Differential Zone (87T):
      - Determined by the location of the current transformers (CTs).
      - In this case: CT on bus side of breaker A and CT on bus side of breaker B.
      - The protected zone extends from the bus side of breaker A to the bus side of breaker B.
      - If a fault occurs between A and B, the 87T relay operates and trips both breaker A and breaker B, isolating that section.

Key Concept of Differential Relaying

  • It is a closed protective zone.
  • The relay continuously compares current entering the zone with current leaving the zone.
  • Under normal conditions: Current in = Current out → relay sees zero difference and does not operate.
  • During an internal fault: Current in ≠ Current out → relay operates and trips the breakers.
  • One-liner to remember: “Differential (87) = closed zone: in = out (normal). Fault inside zone → imbalance → trips breakers. Breaker C protected by 87B + line relays (21/67).”

1.18 Differential Relay Schemes

Typical Bus Differential Relay Scheme (87B)

  • Current transformers (CTs) are located on circuit breakers C, CC, and CCC.
  • They measure current entering and leaving the bus.
  • Normal Operation Example:
      - 200 A enters the bus through breaker C.
      - 100 A leaves through breaker CC.
      - 100 A leaves through breaker CCC.
      - Current in = Current out → vector sum is zero.
      - No current flows through the bus differential relay → no trip.
  • During a Bus Fault:
      - Fault current flows toward the faulted bus from all connected lines.
      - Current increases significantly through all three breakers toward the bus.
      - Current entering the zone no longer equals current leaving.
      - This creates an imbalance in the CT secondary circuits.
      - Current now flows through the bus differential relay (87B).
      - The relay operates and trips all connected breakers (C, CC, and CCC).
      - This quickly isolates the faulty bus from all sources of power.
  • One-liner to remember: “Normal: Current in = Current out → relay sees zero. Bus fault: Big imbalance → 87B trips all breakers to isolate the bus.”

1.19 Transformer Differential Protection

  • Transformer Differential Protection (87T) works on the same principle as bus differential:
  • Under Normal Conditions:
      - Power (MW) entering the transformer equals power leaving it.
      - CT secondaries are connected so currents flow through restraint coils but not through the operating coil.
      - Normal Operation Example:
        - 50 MW flows into the transformer through breaker 914.
        - 50 MW flows out through breakers 624 and 634 (30 MW + 20 MW).
        - Currents balance → all secondary current circulates through restraint coils.
        - No current flows through the operating coil → relay does not trip.
  • During a Transformer Fault (e.g., blown lightning arrestor):
      - Current into the transformer through breaker 914 increases to 200 MW.
      - 50 MW is normal load.
      - 150 MW feeds the internal fault.
      - Current leaving through breakers 624 and 634 stays at 50 MW total (radial circuits, not tied to the Interconnection).
      - This creates a large imbalance between incoming and outgoing current.
      - Extra secondary current from CT 914 cannot return through the restraint coils.
      - It flows through the operating coil of the 87T differential relay instead.
      - The relay operates and trips all three breakers (914, 624, and 634), isolating the faulty transformer.
  • One-liner to remember: “Transformer Differential: Normal = in MW equals out MW (restraint only). Internal fault = big imbalance → extra current goes through operating coil → trips all breakers.”

1.23 Distance Relays

  • Distance Relaying is an open zone of protection.
  • It protects a specific length (section) of a transmission line.
  • The relay works by measuring the impedance (apparent resistance) of the line to detect faults within its protected zone.

How Distance Relays Work:

  • Under normal conditions:
      - The line has a certain impedance based on normal operating voltage and load current.
      - Impedance = Voltage ÷ Current (Ohm’s Law).
  • When a fault occurs on the line:
      - Voltage at the relay location decreases.
      - Current increases dramatically.
      - Therefore, the measured impedance drops significantly.
      - The relay has a preset impedance limit (also called reach setting).
      - When the measured impedance falls below this set-point, the distance relay operates and trips the breaker to clear the fault.
  • One-liner to remember: “Distance relay (open zone): Normal = high impedance. Fault = voltage drops + current rises → impedance falls below setpoint → relay trips.”

1.24 Distance Relay Zones

  • Reach of a distance relay = the maximum impedance (or distance along the line) at which the relay is set to detect and trip for a fault.
  • Example:
      - Line section between breakers A and B has a total impedance of 100 Ohms.
      - The distance relay is set with a reach of 90 Ohms.
      - This means the relay will operate for any fault whose impedance is less than 90 Ohms.
      - In practice, this covers approximately the first 90% of the protected line section.

Important Points:

  • Distance relays are typically set to protect Zone 1 = the first 90% of the line.
  • During normal operation, voltage and current constantly vary, so the actual reach of the relay can be slightly more or less than the set value.
  • If the relay is set too high (e.g., 97%), it may detect faults on the next line section (adjacent line) and trip incorrectly.
  • Unnecessary tripping puts extra stress on the Interconnection.
  • One-liner to remember: “Reach = how far the distance relay can ‘see’ a fault. Set to 90% for Zone 1 to avoid overreaching into the next line section.”

1.25 Zone Protection Schemes

  • Most transmission systems use Zone Protection Schemes with distance relays.
  • The exact reach settings can vary slightly between utilities, but the general scheme is very similar.

Zone Protection Overview:

  • Zone 1 (Primary Protection):
      - Set to cover 90% of the protected transmission line.
      - Trips instantaneously (no intentional time delay) when a fault is detected within its reach.
      - Provides fast, high-speed clearing for most of the line.

  • Zone 2 (Backup Protection):
      - Covers the entire protected line plus part of the adjacent line(s).
      - Typically set for 120% to 150% of the line’s impedance.
      - Operates with a time delay.
      - Acts as backup if Zone 1 fails to clear the fault.

  • Zone 3 (Remote Backup Protection):
      - Covers from 150% up to 300% of the line’s impedance.
      - Looks beyond the next line section and can even reach into a third line section.
      - Provides remote backup protection.
      - Has a longer time delay (typically 60 to 90 cycles or more) — slower than Zone 2.

  • One-liner to remember: “Zone 1 = 90% instant primary; Zone 2 = 120-150% delayed backup; Zone 3 = 150-300% remote backup with longest delay.”

1.26 Overcurrent Relays

  • Overcurrent Relays are widely used as backup protection to primary relays (distance and differential relays).

Example Setup:

  • A 230 kV line feeds a 230 kV / 138 kV transformer bank.
  • Primary protection for the transformer = Differential Relay (87T).
  • Primary protection for the 138 kV circuits (breakers 724 and 734) = Distance Relays.

How Overcurrent Relay Acts as Backup:

  • If a fault occurs on the 138 kV line and breaker 724 fails to open, very high through-fault current flows from the 230 kV system through the transformer into the fault.
  • The overcurrent relay on the transformer measures this high current.
  • When the current exceeds the relay’s setpoint, it closes its contacts.
  • This activates the trip circuit, which opens all three breakers:
      - Breaker 1024 (230 kV side).
      - Breaker 724.
      - Breaker 734 (138 kV side).
  • This isolates the transformer and stops the fault current.
  • One-liner to remember: “Overcurrent relay = backup: If primary (distance/differential) fails, high fault current through transformer makes overcurrent trip all breakers to isolate the equipment.”

1.27 Purpose and Ratings for Overcurrent Relays

  • Overcurrent Relay:
      - Operates when the current flowing through it exceeds a predetermined setpoint.
  • Purpose of Overcurrent Protection:
      - Protects power system equipment against excessive currents.
  • Main causes:
      - Short circuits, ground faults, and other abnormal conditions.
  • Can be used on:
      - Transmission lines, transformers, generators, motors, feeders, etc.
  • Feeder Protection:
      - Multiple overcurrent relays are installed at different points along a feeder.
      - Relays are coordinated so the relay closest to the fault operates first (selective coordination).
  • Three Ways to Coordinate Adjacent Overcurrent Relays:
      - Time (time-delay settings).
      - Current (different pickup current settings).
      - Combination of time and current (inverse-time characteristics).
  • What Overcurrent Relays Protect Against:
      - Phase faults (phase-to-phase or three-phase).
      - Ground faults.
  • Short-Circuit Currents:
      - Typically 5 to 20 times larger than normal full-load current.
      - Fast fault clearance is critical to minimize damage.
  • One-liner to remember: “Overcurrent relay trips on excessive current. Coordinated by time/current so nearest relay to fault acts first — protects against phase & ground faults.”

1.28 Usage of Overcurrent Protection

Important Uses & Benefits of Overcurrent Relays:

  • Detect abnormal conditions — Quickly sense excessive current caused by faults, overloads, or short circuits.
  • Isolate the faulty part of the system — Trip breakers to remove only the problem area.
  • Fast operating speed — Minimize equipment damage, fire risk, and danger to personnel.
  • Discrimination / Selectivity:
      - Coordinate with other relays so only the faulted section is isolated (nearest relay trips first).
  • Dependability & Reliability — Consistently detect and clear faults when they occur.
  • Security & Stability — Avoid unnecessary trips that could destabilize the Interconnection.
  • Cost-effective protection — Balance the cost of installing protection against the potential cost of equipment damage, outages, or hazards.
  • One-liner to remember: “Overcurrent relays: Detect → Isolate fast & selectively → Protect reliability while balancing cost vs. hazard.”

1.29 Types of Overcurrent Relays

  • Instantaneous Overcurrent Relay:
      - Trips immediately (no intentional time delay) as soon as current reaches the pickup (operating) value.
      - Used for very fast fault clearing.
  • Timed Overcurrent Relay (Time-Delay Overcurrent):
      - Has a built-in timer.
      - Example: Set for a 30-cycle delay.
      - The timer must complete (timeout) before the relay trips the equipment.
      - Allows coordination with other relays.
  • Non-Directional Overcurrent Relay:
      - Operates regardless of the direction of current flow.
      - Trips whenever current exceeds the setpoint in either direction.
  • Directional Overcurrent Relay:
      - Contains a directional element that determines the direction of current flow.
      - Trips only when fault current flows in the pre-set (correct) direction.
      - Requires a polarizing quantity (usually voltage or current from another source) to determine direction.
  • One-liner to remember: “Instantaneous = trip right away. Timed = wait (e.g. 30 cycles). Non-directional = trips any direction. Directional = trips only in one direction (needs polarizing).”

1.30 Circuit Breaker Failures

  • Circuit Breakers are strategically placed throughout the power system to connect and disconnect circuits and equipment.
  • They are controlled (opened or closed) by protection and control systems that monitor power system conditions.

Role of Protective Relays:

  • Detect abnormal conditions, especially faults and short circuits.
  • Send trip signals to one or more circuit breakers to isolate the faulty section or equipment.

Coordination:

  • Protection systems are designed so the circuit breaker(s) closest to the fault operate first.
  • This clears the fault with the least impact on the rest of the power system.

Critical Function:

  • Circuit breakers must successfully interrupt (clear) the high fault current.
  • Circuit Breaker Failures (though infrequent):
      - Fail to trip when commanded.
      - Fail to clear the fault current.
      - Misoperate (trip when they shouldn’t).
  • When a breaker fails, other breakers (farther away) must then trip to isolate all sources feeding the fault.
  • One-liner to remember: “Circuit breakers isolate faults on command from relays. Coordinated so nearest breaker trips first — if it fails, others must clear the fault.”

1.31 Breaker Failure

Typical Causes of Circuit Breaker Failure:

  • Loss of DC supply — No power to the trip circuit (most common cause).
  • Open trip coil — The coil that initiates the trip is broken or disconnected.
  • Short-circuited trip coil — The trip coil is shorted and cannot generate enough force to open the breaker.
  • Mechanical failure of the tripping mechanism — Linkages, springs, or latches fail to operate properly.
  • Failure of main contacts to interrupt — The breaker contacts separate but cannot extinguish the arc, so fault current continues to flow.
  • One-liner to remember: “Breaker failure causes: No DC, bad trip coil (open or short), mechanical issues, or contacts that won’t interrupt the arc.”

1.32 Breaker Failure Relaying

  • Breaker Failure Relaying (BF):
      - Provides fast backup protection when a circuit breaker fails to open after receiving a trip signal.

How Breaker Failure Relaying Works:

  • Detects that a trip signal has been sent to the breaker (isolating device).
  • Starts a timer.
  • Monitors the breaker’s open/close contact (auxiliary switch) and checks for non-directional fault current.
  • If the breaker does not open (contact state doesn’t change) and fault current is still flowing after the timer expires, the BF relay:
      - Sends trip signals to all other breakers that can feed the fault.
      - Operates faster than remote backup distance relaying (Zone 2 or Zone 3).
      - Requires no communication between substations.
  • One-liner to remember: “Breaker Failure: Trip sent → timer starts → if breaker doesn’t open and fault current remains → trip all surrounding breakers fast (no comms needed).”

1.35 Pilot Relays

  • Pilot Relaying:
      - A protective system in which relays at both ends of a transmission line communicate with each other.
      - Goal:
        - Provide faster and more accurate fault clearing for all faults on the protected line section (including end-zone faults).
      - Communication methods used:
        - Microwave signals.
        - Telephone systems.
        - Power Line Carrier (PLC).
        - Fiber optics.

Power Line Carrier (PLC) Pilot Relaying:

  • A high-frequency signal is impressed (superimposed) onto the transmission line conductor itself.
  • The relays at both ends use this signal to coordinate tripping decisions.
  • Provides fast, selective protection without needing separate communication links.
  • Wave Traps (also called line traps):
      - Installed at each end of the line section.
      - Act as filters that block the carrier signal from leaving the protected line section.
      - Allow the 60 Hz power current to pass freely while keeping the PLC signal contained between the two relays.
  • One-liner to remember: “Pilot relaying = relays talk to each other for fast, accurate tripping. PLC uses the line itself + wave traps to keep the signal inside the protected section.”

1.36 Wave Trap

  • Wave Trap (also called Line Trap):
      - Installed on each phase of the transmission line (example shows B phase).
      - Connected in series with the line conductor.
      - Works with a coupling capacitor (potential device) to interface with the carrier signal.
      - Function:
        - Allows normal 60 Hz power frequency current to pass through with almost no impedance.
        - Blocks (traps) the high-frequency carrier signal used for pilot relaying or Power Line Carrier (PLC) communication.
      - Purpose:
        - Keeps the PLC communication signal confined to the specific line section between breakers A and B.
        - Prevents the carrier signal from spreading to other lines or affecting other relaying schemes on the system.
  • One-liner to remember: “Wave trap = lets 60 Hz power through, blocks high-frequency PLC signal so it stays only on the protected line section.”

1.37 Relay Communications

Example of Carrier (Pilot) Relaying:

  • Fault occurs at 95% of the line length from breaker A (very close to breaker B).
  • Without Pilot Relaying (Non-Pilot Distance Relaying):
      - Breaker B:
        - Zone 1 relay sees the fault (within its 90% reach).
        - Trips instantaneously.
      - Breaker A:
        - Zone 1 relay does not see the fault (95% is outside its 90% zone).
        - Zone 2 relay sees the fault (120–150% reach) but has a time delay (example: 25 cycles).
        - Breaker A trips 25 cycles later.
  • Result:
      - Fault current continues flowing for a long time → risk of line damage and system instability.
  • Key Problem:
      - Non-pilot relaying causes delayed clearing from one end of the line when the fault is near the remote end.
  • One-liner to remember: “Fault at 95% from A: B clears instantly (Zone 1), but A waits 25 cycles (Zone 2) — lots of fault energy flows until A finally trips.”

1.38 High Speed Tripping

  • Pilot Relaying (such as Power Line Carrier) is used to trip both ends of a transmission line section simultaneously at high speed.
  • Goal:
      - Reduce the time that high fault current flows into the faulted area, minimizing damage and system stress.
  • High-Speed / Fast Tripping:
      - Most transmission operators consider tripping within 3 cycles or less (50 milliseconds or less at 60 Hz) as high-speed clearing.

Two Main Types of Pilot Relaying:

  1. Power Line Carrier (PLC) Pilot Relaying:
       - Relays at both ends communicate over the transmission line itself using high-frequency signals.
       - Provides fast, selective tripping for faults anywhere on the line.
  2. Transfer Tripping:
       - A signal is sent from one location to another via a communication channel (microwave, fiber, etc.).
       - Can directly trip a circuit breaker at the remote end or trip an auxiliary relay that then trips multiple breakers.
       - Also used for selective tripping: sends a signal to a remote distance relay to trip if that relay has detected a fault.
  • One-liner to remember: “Pilot relaying = fast simultaneous tripping (≤ 3 cycles). Transfer trip = send trip signal remotely to clear faults faster or selectively.”

1.39 Synch-check Relays

  • Synch-Check Relays (25):
      - Used to safely synchronize and connect two parts of the power system (or two systems) by closing a breaker only when conditions are acceptable.
  • The relay continuously monitors three critical parameters across the open breaker:
      - Frequency difference.
      - Voltage difference (magnitude).
      - Phase angle difference.
  • If any of these values is too large, the relay blocks the breaker from closing to prevent damage or system instability.
  • To allow closing, the system operator may need to:
      - Re-dispatch generation or adjust load on one or both sides to reduce frequency and phase angle differences.
      - Adjust voltage (raise or lower) on one or both sides using generators, capacitors, or reactors.
  • Some synch-check relays latch in (stay active) and keep monitoring until all three parameters fall within acceptable limits.
      - They can then automatically permit and execute the breaker close command — sometimes hours after the initial close request.
  • One-liner to remember: “Synch-check (25) blocks breaker close until frequency, voltage, and phase angle differences are all acceptable — may take hours of adjustments.”

1.43 Communication for Reliability

  • Protective devices are critical to the reliable operation of the Bulk Electric System (BES).
  • NERC Requirement:
      - When a protective relay or equipment fails or misoperates and this reduces system reliability, the Transmission Operator must immediately notify:
        - Its Reliability Coordinator.
        - All affected Transmission Operators.
        - All affected Balancing Authorities.
      - The Transmission Operator must then take corrective action as soon as possible.
  • Each entity (Transmission Operators, Balancing Authorities, etc.) must keep others informed of:
      - Any occurring changes, or
      - Any planned changes that could jeopardize the reliability of the Interconnection.
  • Modern relays and protection schemes work together across company boundaries — therefore, good communication and coordination between companies is essential for overall Interconnection reliability.
  • One-liner to remember: “Protective relay fails or misoperates? Notify RC and all affected TOs/BAs immediately and fix it fast — communication keeps the Interconnection reliable.”

1.44 Communication Example

  • Scenario:
      - Transmission Operators X and Y are connected by a transmission tie-line.
      - A relay failure/misoperation occurs at Company X’s end (circuit breaker C).
      - The carrier relay set (part of the pilot relaying scheme) for the line distance relays at breaker C is no longer functioning.
  • Required Action:
      - Transmission Operator X must immediately notify:
        - Its Reliability Coordinator.
        - Transmission Operator Y.
      - Notification allows both companies to take preventative steps (e.g., disabling automatic tripping or switching to alternate protection) to avoid problems.

Consequence if No Notification:

  • Without the carrier blocking signal from C to D:
      - A fault on the line between breakers A and B (in Company Y’s territory) could cause an unnecessary trip of breaker D.
      - Unnecessary tripping weakens the Interconnection by removing healthy equipment from service.
  • One-liner to remember: “Failed carrier set at C? Notify RC and adjacent TO (Y) immediately — otherwise a fault elsewhere can cause unnecessary trip of D and weaken the grid.”

1.45 Generator Operator Communications

  • Generator Voltage Regulator:
      - Keeps the generator operating at the correct voltage schedule.
      - Also helps maintain proper voltage on the connected transmission system.
  • Generators are dynamic sources of reactive power:
      - They can instantly boost or lower system voltage as needed.
  • If the voltage regulator fails:
      - The generator can no longer provide reactive power support.
      - During a serious voltage problem (low or high voltage), the generator would be unable to help stabilize the system.
  • Required Action (per NERC):
      - The Generator Operator must immediately notify:
        - Its Transmission Operator.
        - Its Host Balancing Authority.
      - The Generator Operator must take corrective action (repair the regulator) as soon as possible.
      - NERC Statement:
        - “If a protective relay or equipment failure reduces system reliability, the Generator Operator shall notify its Transmission Operator and Host Balancing Authority. The Generator Operator shall take corrective action as soon as possible.”
  • One-liner to remember: “Failed generator voltage regulator? Notify TO and Host BA immediately — it can’t provide dynamic reactive support when the system needs it most.”

1.46 New Equipment — Generator

  • When new equipment (especially large generators) is installed, good communication and coordination between neighboring utilities is required.
  • Example:
      - A 700 MW generator built by Utility D near or inside the borders of Utility C and Utility E.
      - Why Coordination is Necessary:
        - The new generator significantly increases available fault current in the surrounding area.
        - Studies must be performed by all affected companies (D, C, and E) to evaluate:
          - Substation buses.
          - Transmission lines.
          - Circuit breakers.
          - Other equipment.
        - If existing equipment owned by Utility C or E is not rated for the higher fault current, it must be replaced with higher-rated equipment.
  • Relay Scheme Coordination:
      - All three utilities must work together to ensure matching relay schemes on both ends of interconnecting transmission lines.
  • NERC Requirement:
      - “A Generator Operator or Transmission Operator shall coordinate changes in generation, transmission, load, or operating conditions that could require changes in the protection systems of others.”
      - Generator Operators must coordinate all new protective systems and all changes to existing protection with their:
        - Transmission Operator.
        - Host Balancing Authority.
      - Transmission Operators have the same coordination obligation.
  • One-liner to remember: “New big generator? Coordinate studies & relay schemes with neighbors — increased fault current may require equipment upgrades and matching protection.”

1.47 New Equipment - Transmission

  • NERC Requirement:
      - Each Transmission Operator must coordinate all new protective systems and all changes to existing protective systems with neighboring Transmission Operators and Balancing Authorities.
  • Even a well-intentioned project like building a new high-voltage transmission line can cause problems if not properly coordinated.
  • Example:
      - Transmission Operator H plans to build a new 345 kV line from its northern border to its southern border.
      - The new line will increase power transfer capability across H’s system.
  • However, the line’s installation and potential loss (due to a fault) will affect parallel flows on neighboring systems.
  • Required Coordination:
      - Neighboring Transmission Operators, Generator Operators, and Balancing Authorities must study the impact of the new line, especially:
        - How power flows will change under normal conditions.
        - What happens if the new line trips due to a fault.
  • Protective relay schemes of all involved companies must be coordinated to ensure correct and selective tripping during faults.
  • Special Protection Systems (SPS):
      - Each Transmission Operator and Balancing Authority must monitor the status of every SPS in their area.
      - They must notify affected Transmission Operators and Balancing Authorities of any change in SPS status.
  • One-liner to remember: “New 345 kV line? Coordinate studies, relay settings, and parallel flow impacts with all neighbors — plus monitor and notify on Special Protection System status changes.