Voltage Vector Shift Protection

ANSI 78 refers to the “Voltage Vector Shift Protection” (VVSP) function, which is designed to detect an out-of-step condition between interconnected power systems or machines. This condition, also known as asynchronous operation, can arise during events like mains failures when a generator is running in parallel with the grid, causing a “vector shift.” The protection function ensures system stability by identifying and responding to these potentially damaging operational discrepancies. It’s a crucial element in robust power system protection schemes.

Vector shifts can occur when a mains failure happens while a generator is running in parallel with the grid.

A vector shift happens because the stator magnetic field lags behind the rotor magnetic field. When mains power is lost, the phase angle between the stator and rotor magnetic fields changes, which is the vector shift.

The alarm responds to this change in phase angle caused by the mains failure. The trigger can be based on the change in a single phase or on the change across all phases.

In grids with fast automatic reconnection attempts, this protection may open the breaker to prevent damaging faults.

Fast frequency changes can also trigger this alarm. If the configuration is too sensitive, it can lead to too many unwarranted vector-shift detections.

Vector shift protection (VVSP) and out-of-step protection are both aimed at detecting abnormal conditions when connected rotating machines (like generators) are running in parallel with a network, but they focus on different aspects of the phenomenon and have different operating principles.

Key differences:

• What they monitor
  • Vector shift protection (VVSP): Monitors the relative angular displacement (vector difference) between interconnected sources or machines. It detects a sudden shift in the voltage/angle vector—i.e., when the internal angle of a machine drifts relative to the bus or another source, indicating asynchronous operation or loss of synchronism risk.
  • Out-of-step protection (OOS, or out-of-step/anti-synchronism protection): Detects loss of synchronism between two or more systems or machines, usually by monitoring rotor angle relative to the system, speed deviation, or torque swings. It aims to trip when machines can no longer stay in step with the network due to a large power-angle swing, often after a disturbance.
• Physical meaning
  • VVSP: A fast-acting protection that responds to a vector (angle and magnitude) discrepancy between sources, typically during transient events like faults or mains disturbances. It’s about preventing an imminent vector shift that could destabilize the connection.
  • OOS: Indicates a sustained inability to remain in synchronism, often due to power transfer overload, low system stiffness, or large disturbances. It signals outright loss of synchronism and prevents damaging islanding or severe circulating powers.
• Protection objective
  • VVSP: Prevents a dangerous vector shift from propagating by tripping or reconfiguring before the shift fully develops. It acts on a short timescale to maintain stable parallel operation.
  • OOS: Prevents severe destabilization by islanding or fighting a losing battle against the grid. It triggers when the system cannot maintain synchronism, usually after a disturbance, and may operate as a protective disconnection to avoid damage.
• Typical implementation cues
  • VVSP: Relies on comparing instantaneous voltage/phase angle vectors between sources or between a generator and the bus, with a fast pick-up around a few electrical degrees of shift.
  • OOS: Often uses synchronism checks, rate of change of angle (dδ/dt), or power-angle relationships, with thresholds that reflect stable vs unstable operating regions.
• Response characteristics
  • VVSP: Very fast, aimed at preventing a transient vector shift from causing protection or control misoperation.
  • OOS: May involve a more deliberate trip to prevent sustained loss of synchronism, often coordinated with system-wide stability schemes.

In practice, both protections complement each other in a robust scheme:

• VVSP provides a quick guard against imminent vector mismatch during transients.
• OOS monitors the broader stability of parallel operation and trips if true loss of synchronism occurs.

ANSI 78 (Voltage Vector Shift Protection, VVSP) settings can vary by manufacturer, system voltage, generator size, and network strength. However, there are common guidance ranges and concepts that are often used as starting points. Always confirm with the relay vendor’s documentation and coordinate with the overall protection philosophy for your plant.

Key settings and concepts to consider:

• Pick-up (threshold) angle or vector shift
  • Most VVSP schemes use a small angle threshold to detect a rapid vector shift between sources or between a generator and the bus.
  • Typical pick-up is on the order of 2° to 5° (sometimes up to 6°–8° in very weak networks), depending on desired sensitivity and nuisance tripping concerns.
• Time delay (operating time)
  • VVSP is a fast transient protection, so the operating time is usually quite short.
  • Typical time delays range from a few cycles to a fraction of a second (e.g., 10 ms to 200 ms) after the condition is met, often with a fixed or very short delay to avoid misoperation during switching transients.
• Vector difference reference
  • The scheme compares the instantaneous voltage vector (or angle) of the local source with a reference vector (bus bar voltage, another source, or a remote terminal).
  • Some implementations use a two-channel comparison (local vs. remote) and may incorporate a deadband to prevent chatter around small, noise-like variations.
• Deadband and slope
  • Deadband: a small zone around zero vector difference where no protection action occurs to reduce nuisance trips.
  • Slope: some schemes use a gradual increase in sensitivity with larger vector differences to improve selectivity.
• Directionality (if applicable)
  • Depending on the relay, the VVSP may be configured to trip only when the vector shift is in a particular direction or when a mismatch exceeds a bidirectional threshold.
• Coordination with other protections
  • VVSP settings should be coordinated with:
    • Out-of-step protection (OOS) to prevent unnecessary trips during large transients.
    • Other protection schemes on the same generator or bus (PSS, distance protections, over/under voltage, etc.) to avoid miscoordination.
  • Ensure compatibility with system stability criteria and reclosing practices.
• Stability considerations
  • In very strong grids, you may select tighter (smaller) angle thresholds to catch fast transient shifts.
  • In weak networks or with high source impedance, you may allow slightly larger thresholds to avoid nuisance trips but risk slower response to actual issues.
• Health checks and test parameters
  • Include alarm vs trip thresholds for maintenance and testing.
  • Plan for functional testing under simulated vector shifts to validate correct operation.

Example starting point (typical for a medium-sized generator on a strong connected grid):

• Pick-up angle: 3° (with deadband 0.5°)
• Operating time: 20 ms to 100 ms
• Reference: bus voltage vector (local vs. bus)
• Deadband: 0.5°
• Coordination: set after OOS and other protections with a priority that VVSP trips before any slower stability-based actions

Example starting point (weaker network or higher impedance):

• Pick-up angle: 4° to 6°
• Operating time: 20 ms to 60 ms
• Deadband: 0.5° to 1°
• Consider adding a slight slope or adaptive element to reduce nuisance trips during normal oscillations

Important: ANSI/IEEE standards and vendor implementations vary. Here’s how I can help next:

• If you share the specific relay model or vendor, I can pull or synthesize the exact recommended settings from their manuals.
• I can provide a step-by-step method to set, test, and validate VVSP in a commissioning plan.
• I can draft a safe default set for a hypothetical system (voltage level, generator size, grid strength) and show how to tune it.

Would you like me to tailor starting values to a particular generator rating, network strength, or relay vendor? If you have a schematic or system parameters (bus voltage, generator impedance, known disturbances), share them and I’ll tailor the guidance.

• Purpose
  • Detects rapid misalignment between voltage vectors of interconnected sources or a generator and the bus to prevent asynchronous/unstable parallel operation.
• What it monitors
  • Instantaneous voltage angle and magnitude differences between two references (e.g., local generator vs. bus or between two sources).
• Key concepts
  • Pick-up (threshold): Small angle difference that triggers protection (typical ~2°–5°, may be higher in weak networks).
  • Time delay: Very fast operation (milliseconds to a few hundred milliseconds) to catch transient shifts.
  • Reference vector: The vector against which the comparison is made (bus voltage, another source, or remote terminal).
  • Deadband: A small range around zero to avoid nuisance tripping.
  • Directionality: Optional setting to trip based on the direction of vector shift.
• Objectives
  • Prevent imminent vector shift from propagating into instability.
  • Complement other protections (e.g., Out-of-Step) to maintain stable parallel operation.
• Typical implementation
  • Compare instantaneous voltage angle vectors, often using two channels with a fast pick-up.
  • Use deadband and a slope or adaptive element to improve selectivity.
  • Coordinate with OOS, distance protections, and control schemes to avoid misoperation.
• Applications
  • Interconnected generation in transmission or large distribution networks.
  • Systems prone to transient disturbances where loss of synchronism risk exists.
  • Scenarios requiring fast separation to protect equipment and maintain stability.
• Practical considerations
  • Settings depend on network strength, generator size, and protection philosophy.
  • Vendor-specific terminology and UI paths vary; look for “Vector Shift,” “Voltage Vector Shift Protection,” or “Synchronism/Angle” in protection blocks.
  • Validate via commissioning tests simulating vector shifts and ensuring proper coordination with other protections.