Out of Step Protection: Working Principle, ANSI 78 Relay, Settings and Applications

What Is Out of Step Protection?

In simple engineering terms, out of step protection is a critical stability protection function for AC power systems. It detects loss of synchronism between synchronous generators, motors, or interconnected grid areas and executes targeted tripping or islanding actions to avoid large-scale system collapse and equipment damage.

Unlike conventional fault protection that targets short circuits or overloads, out of step relay protection focuses on rotor angle stability — the core operating foundation of all synchronous power equipment. It applies to both rotating equipment and power transmission networks, covering out of step protection of line and generator out of step protection scenarios.

Core basic concepts for field engineers:

• Synchronism: The consistent phase angle and frequency operation between generators and the power grid
• Rotor angle stability: The ability of synchronous units to maintain stable power output during system disturbances
• Loss of synchronism: Irreversible phase angle deviation between rotating equipment and the grid, also defined as pole slipping
• Pole slipping: Severe out-of-step failure that causes periodic power reversal and mechanical impact on generators

Core engineering purposes of 78 protection relay:

• Prevent mechanical burnout and shaft damage of synchronous generators and motors
• Block the spread of system oscillation and avoid regional power grid instability
• Split unstable grid sections to retain stable power supply areas
• Guarantee overall power system operation security

What Causes an Out of Step Condition?

Out-of-step failures do not occur randomly. They are triggered by specific system disturbances that break rotor angle balance. For EPC contractors and plant operation teams, mastering these causes is the premise of accurate out of step protection calculation and setting optimization.

Severe System Faults

High-impact grid faults are the most common inducement. Transmission line short circuits, transformer faults, and busbar faults cause instantaneous system voltage depression, reduce power transmission capacity, and lead to unbalanced generator rotor torque. The rotor accelerates rapidly, deviates from the synchronous state, and eventually causes loss of synchronism.

Large Power Transfer and Weak Grid Conditions

Long-distance power transmission, ultra-high power transfer levels, and low short-circuit capacity grids are highly susceptible to power oscillation. This scenario is frequent in new energy gathering areas and remote power plants, requiring customized out of step protection parameter configuration.

Problems with motor excitation

Abnormal excitation systems directly damage generator synchronization stability, including AVR failure, full or partial loss of field, and improper excitation control. It is the primary internal fault cause for out of step protection for synchronous generator and out of step protection for synchronous motor.

Sudden Load Changes

Large-scale motor startup, sudden load rejection, and adjacent generator tripping will instantly change system power balance, induce rotor angle oscillation, and trigger potential out-of-step risks.

Difference Between Power Swing and Out of Step Condition

Most on-site engineers confuse stable power swing and unstable out-of-step faults, which easily leads to protection misoperation or refusal. The following table clearly distinguishes the two core scenarios, guiding how to set out of step protection accurately.

Item	Power Swing	Out of Step
System condition	Stable, recoverable disturbance	Unstable, irreversible system state
Rotor angle variation	Fluctuates within a safe range and recovers automatically	Continues to increase without self-recovery
Protection action	Blocking protection to avoid false tripping	Active tripping and grid splitting
Core purpose	Ensure protection selectivity during normal oscillation	Eliminate unstable links and protect system safety

A qualified out of step protection relay must accurately identify the two states, block actions for stable swings, and trip reliably for true out-of-step faults.

Working Principle of Out of Step Protection

Out-of-step protection fundamentals and advancements show that modern out-of-step protection mainly adopts two mature detection mechanisms, which are widely used in generator out of step protection relays and line stability protection devices.

Impedance-Based Detection Principle

This is the most mainstream technical principle of ANSI 78 relay. The relay monitors the real-time system impedance trajectory on the R-X plane. Under normal operating conditions, the system impedance stays in a stable zone. When disturbance occurs, the impedance point moves continuously with power oscillation.

The relay judges system stability by analyzing the moving speed, direction, and residence time of the impedance trajectory. It distinguishes stable power swing from unstable out-of-step faults and outputs corresponding blocking or tripping commands. This principle is the core basis for out of step protection calculation cross compound for complex units.

Voltage and Current Angle Measurement

The protection device collects real-time voltage and current signals, calculates phase angle differences between system sides and rotating equipment, and tracks power flow reversal trends. When the phase angle exceeds the preset stable threshold and continues to diverge, the system is judged to be out of step, realizing stable detection for out of step protection for generators and transmission lines.

ANSI 78 Out of Step Protection Relay

Out of step protection ansi code is defined as ANSI 78, the industry-standard device number for out-of-step and pole slip protection relays in global power systems.

What Is ANSI 78 Protection?

ANSI 78 relay is a dedicated stability protection device, independently responsible for system loss of synchronism detection and tripping control. It complements conventional fault protection and is widely deployed in thermal power, hydropower, new energy stations, and high-voltage transmission networks.

Main Functions of Out of Step Relay

Function	Engineering Description
Power swing detection	Real-time monitoring of system power oscillation amplitude and frequency
Out-of-step tripping	Trip circuit breakers to split unstable grid sections
Pole slip detection	Identify generator pole slipping faults to protect unit mechanical safety
Blocking logic	Prevent misoperation during stable power swing
Event recording	Record fault waveforms and action logs for post-analysis

Out of Step Protection Schemes

According to application scenarios, out of step protection is divided into generator-side and transmission system-side protection schemes, covering conventional units and special cross-compound units.

Generator Out of Step Protection

Out of step protection of generator is mandatory configuration for all grid-connected synchronous units, including out of step protection for cross compound generators. Cross-compound generators have complex operating characteristics, requiring targeted out of step protection calculation cross compound to adapt to dual-unit synchronous oscillation features.

Standard protection architecture: Generator → CT/PT sampling → ANSI 78 relay → Circuit breaker trip output. It is universally applied in thermal, hydropower, and nuclear power plants.

Transmission System Out of Step Protection

Out of step protection of line is used for tie-line stability control of regional power grids. When inter-regional oscillation loses stability, the protection executes controlled islanding to isolate fault areas and ensure the normal operation of the main grid.

Out of Step Protection Setting Principles

Reasonable parameter setting is the key to reliable protection operation. How to set out of step protection follows unified industry principles, covering impedance zones, action timers, and pole slip thresholds.

Impedance Zone Setting

Calculate based on system equivalent impedance, generator transient reactance, and transformer impedance. Ensure the protection zone covers all possible out-of-step oscillation trajectories while avoiding overlapping with fault protection zones.

Out-of-Step Timer Setting

Coordinate with system fault clearing time and stable swing duration. Reserve sufficient time for the system to recover spontaneously, and trigger tripping only when oscillation continues to deteriorate.

Number of Pole Slip Setting

Determine according to generator mechanical tolerance, grid stability requirements, and system simulation results. Different types of out of step protection for synchronous generator adopt differentiated pole slip protection thresholds.

Testing and Commissioning of Out of Step Protection

Strict testing and commissioning eliminate hidden dangers of protection refusal and misoperation, which is a key link for EPC project delivery and plant daily maintenance.

Protection Relay Testing

Test core indicators including relay pickup characteristics, action time accuracy, logic validity, and trip output reliability to verify the performance of generator out of step protection relays.

Dynamic Simulation Test

Adopt real-time power system simulators to simulate stable swing, unstable oscillation, and pole slipping conditions, verifying the relay’s identification and action capability under actual working conditions.

Commissioning Checklist

Test Item	Test Purpose
CT polarity test	Ensure accurate current sampling and avoid trajectory judgment deviation
PT wiring check	Verify voltage signal authenticity and stability
Relay setting verification	Confirm all calculation parameters match design requirements
Trip circuit test	Guarantee reliable breaker tripping output

Out of Step Protection vs Other Power System Protection Functions

Out-of-step protection belongs to stability protection, which is essentially different from conventional fault protection. The comparison below helps engineers quickly distinguish protection types in scheme design.

Protection Function	ANSI Code	Core Purpose
Differential Protection	87	Internal fault protection for generators and transformers
Distance Protection	21	Line short-circuit fault protection
Overcurrent Protection	50/51	Overcurrent and short-circuit backup protection
Out of Step Protection	78	Power system stability and loss of synchronism protection
Loss of Field Protection	40	Generator excitation failure protection

Applications of Out of Step Protection

Power Plants

All grid-connected generator sets are equipped with generator out of step protection, including conventional synchronous generators, synchronous motors, and special cross compound generators, to prevent unit damage caused by pole slipping.

High Voltage Transmission Networks

Widely used in 110kV, 220kV, and 500kV high-voltage grids, out of step protection of line realizes regional grid stability control and fault isolation.

Renewable Energy and Modern Grids

For weak grids with high renewable energy penetration, out-of-step protection compensates for insufficient system inertia, effectively improving overall grid stability and anti-disturbance capability.

How to Select a Reliable Out of Step Protection Relay Supplier?

For EPC projects and power plant owners, relay reliability determines long-term grid operation safety. Professional suppliers provide standardized products and full-cycle technical services.

Core Supplier Evaluation Factors

• Standard compliance: Fully compliant with IEC 60255 international relay standards and ANSI device specifications
• Algorithm performance: Mature impedance trajectory identification algorithm, accurate distinction between swing and out-of-step faults
• Testing capability: Complete FAT dynamic simulation testing process to verify out of step protection calculation accuracy
• Engineering experience: Rich project cases in generator and transmission line stability protection
• Technical support: Provide on-site commissioning, parameter setting, and after-sales technical guidance

Standard Relay Supply Process

Technical Requirement Review → Protection Scheme Confirmation → Relay Configuration Customization → Full Function FAT Testing → Export Anti-vibration Packaging → Logistics & Shipment → On-site Commissioning Support

FAQ About Out of Step Protection

Q1: What is the purpose of out of step protection?

A1: It detects power system loss of synchronism, distinguishes stable power swing from unstable out-of-step faults, executes targeted tripping or blocking actions, protects generator equipment, and maintains grid stability.

Q2: What is ANSI 78 relay used for?

A2: ANSI 78 is the standard device code for out-of-step and pole slip relays, applied to generator and transmission line stability protection to solve system loss of synchronism faults.

Q3: What is the difference between power swing and out of step?

A3: Power swing is a stable, recoverable system oscillation requiring protection blocking; out-of-step is an unstable, irreversible fault requiring immediate tripping and grid splitting.

Q4: How does an out-of-step relay detect instability?

A4: It tracks system impedance trajectory and voltage-current phase angle changes, judges oscillation stability through moving speed and duration, and identifies out-of-step faults accurately.

Q5: Where is out of step protection applied?

A5: It covers synchronous generators, synchronous motors, cross-compound units, and high-voltage transmission lines, applicable to thermal, hydropower, new energy power plants and regional power grids.

Conclusion

Out of step protection is an indispensable core link of modern power system stability control. With the continuous upgrade of grid structures, out-of-step protection fundamentals and advancements further improve the accuracy and adaptability of stability protection. Standardized ANSI 78 relay configuration, scientific how to set out of step protection schemes, and strict testing and commissioning ensure the safe and stable operation of generator out of step protection and transmission line protection systems, providing reliable technical guarantees for global power EPC projects and power plant operation.

Technical References & Data Sources

• IEC 60255-121: Measuring relays and protection equipment – Stability protection for power systems
• ANSI/IEEE C37.100: Standard Device Numbers for Power System Protection (ANSI 78 Definition)
• IEEE Guide for Power System Stability Control and Out-of-Step Protection Application
• GB/T 14285-2006: Technical Specifications for Power System Relay Protection
• International Journal of Electrical Power & Energy Systems: Out-of-step protection fundamentals and advancements