ANSI 40 Loss of Excitation Protection: Principles, Application & Relay Selection for Modern Synchronous Generators

Introduction

Loss of excitation protection (also termed loss of field excitation protection) is a core mandatory protection function for grid-connected synchronous generators. It is formally defined as loss of excitation protection in global power industry standards, widely deployed in thermal power plants, hydropower stations, and industrial self-generation systems worldwide.

For modern synchronous generators, sustained loss of excitation or underexcitation faults will trigger reactive power inversion, stator/rotor overheating, and even grid out-of-step collapse. A high-performance loss of excitation protection relay serves as the critical safety barrier to isolate faulty units rapidly, avoid unplanned outages, and secure overall power system stability.

This article delivers practical, standard-aligned technical insights for EPC contractors, power plant owners, field commissioning engineers, relay protection engineers, and procurement teams.

It covers working principles, fault causes, standard settings, field testing, relay protection testing, and professional relay selection criteria for loss of excitation protection of synchronous generator systems, fully complying with IEEE protection relay standard and ANSI industry specifications.

What Is Loss of Excitation Protection ?

Loss of excitation protection is a dedicated generator protection logic that detects complete or partial excitation failure of synchronous generators. Unlike general overcurrent or overvoltage protection, it targets abnormal operating states caused by excitation system anomalies, which are invisible to conventional protection functions.

A healthy synchronous generator relies on DC excitation current to build a stable rotor magnetic field, maintaining terminal voltage and controllable reactive power exchange with the grid. Once excitation is lost or severely attenuated:

• Generator terminal voltage drops sharply
• The generator transitions from reactive power output to reactive power absorption from the grid
• Rotor magnetic flux decays rapidly, triggering asynchronous operation
• Stator and rotor windings generate excessive eddy current heat

The loss of excitation protection 40 function continuously monitors generator impedance trajectory, terminal voltage, reactive power, and field current. It activates alarm or trip actions precisely when parameters breach standard protection thresholds, eliminating blind protection zones for loss of excitation protection for modern synchronous generators.

Core Risks of Unresolved Generator Loss of Excitation Faults

Field statistics show that delayed response to excitation loss faults is one of the top causes of synchronous generator permanent damage and regional grid voltage collapse. Reliable loss of field excitation protection effectively mitigates the following critical risks:

• Generator Permanent Damage: Long-term asynchronous operation causes rotor winding overheating, insulation aging, and mechanical vibration fatigue, leading to costly overhaul or unit replacement.
• Grid Stability Failure: Massive reactive power absorption by faulty generators pulls down system voltage, easily triggering cascaded tripping of transmission lines and transformers in weak grid systems.
• Project Economic Losses: Unplanned shutdowns disrupt EPC project commissioning progress and stable power supply for industrial and commercial loads, resulting in substantial operational losses.

Common Causes of Generator Loss of Excitation

Loss of excitation faults in synchronous generators mainly stem from four field-verified causes, covering equipment failure, control anomaly, and human operation errors:

Excitation System Equipment Failure

AVR (Automatic Voltage Regulator) malfunction, exciter burnout, excitation breaker accidental opening, and field main circuit open circuit are the most frequent root causes in operational power plants.

Rotor Circuit Abnormality

Rotor winding open/short circuit, field cable aging damage, and rotor insulation breakdown will directly interrupt normal excitation current transmission.

Control System Abnormality

Incorrect AVR parameter calibration, PLC control logic failure, and communication signal loss between upper computers and excitation devices will cause abnormal excitation attenuation.

Operation & Maintenance Errors

Improper wiring recovery after equipment maintenance and misoperation of excitation loop switches are common human-induced faults in new EPC project commissioning stages.

Working Principle of Loss of Excitation Protection

Modern numerical loss of excitation protection relay adopts the industry-standard offset mho impedance characteristic principle, which is the most reliable detection method specified in IEEE C37.102, compatible with all types of loss of excitation protection of synchronous generator scenarios.

Impedance Trajectory Detection (Core Logic)

When a generator loses excitation, the comprehensive impedance measured from the relay installation point changes regularly on the R-X impedance plane and falls into the predefined ANSI 40 protection zone. The relay calculates positive sequence voltage, positive sequence current, and apparent impedance in real time to judge fault validity and avoid maloperation caused by normal load fluctuation.

Multi-Criterion Auxiliary Verification

To improve protection reliability, mainstream relays integrate multi-dimensional auxiliary criteria: undervoltage locking, reactive power reverse monitoring, and field current zero-check. This dual verification mechanism effectively eliminates misjudgment during grid short-circuit faults and normal reactive power adjustment, optimizing the performance of loss of excitation protection for modern synchronous generators.

ANSI 40 Protection vs. Overexcitation Protection: Key Differences

Power plant engineers often confuse loss of field excitation protection with overexcitation protection. The following standardized comparison table clarifies core differences for accurate configuration and maintenance:

Comparison Item	Loss of Excitation Protection	Overexcitation Protection
ANSI Standard Code	ANSI 40	ANSI 24
Core Fault Condition	Partial/complete loss of field current	Excessive rotor magnetic flux and over-excitation
Main System Hazard	Reactive power inversion, grid voltage collapse, generator asynchronous operation	Stator core overheating, insulation aging, magnetic saturation
Detection Principle	R-X plane impedance trajectory + reactive power monitoring	V/Hz ratio over-limit detection
Main Application Scenario	Synchronous generator grid-connected operation	Generator-transformer unit overall protection

Key Setting & Commissioning Guidelines for Field Engineers

Reasonable parameter setting and standardized commissioning are the keys to ensuring stable operation of loss of excitation protection 40. All settings comply with IEEE C37.102 industry guidelines:

Protection Zone Calibration

Set offset mho impedance zones based on generator synchronous reactance (Xd, Xd’) and system short-circuit impedance. Stage 1 protection is for light load excitation loss faults, and Stage 2 covers full-load extreme working conditions to achieve full-condition coverage for loss of excitation protection of synchronous generator.

Time Delay Matching

Adopt graded time delay logic: short-delay alarm for minor underexcitation deviation and long-delay trip for complete excitation loss faults. This avoids unnecessary tripping caused by transient grid disturbances and normal reactive power regulation.

Standard Commissioning Steps

1. Verify CT/PT wiring accuracy and parameter consistency
2. Upload customized generator parameters and ANSI 40 protection logic
3. Complete secondary injection testing to verify impedance zone action accuracy
4. Check trip loop linkage and remote communication signal output
5. Record test data and generate standardized commissioning reports

How to Select a Qualified Loss of Excitation Protection Relay

For EPC projects and power plant owners, a high-quality loss of excitation protection relay is not only a hardware device but also a complete set of standardized technical solutions. The core selection criteria for overseas projects are as follows:

Standardized Protection Function Integration

Integrate full-range generator protection logic including ANSI 40 loss of excitation protection, 32 reverse power protection, 50/51 overcurrent protection, 87G differential protection, and 27/59 over/undervoltage protection to meet full-working-condition protection requirements of modern synchronous generators.

International Communication Protocol Compatibility

Support IEC 61850, Modbus TCP/IP, and IEC 60870-5-103 mainstream protocols, compatible with overseas substation intelligent monitoring systems, realizing seamless data transmission and remote monitoring.

Professional Full-Lifecycle Technical Support

Reliable suppliers provide one-stop services including project technical requirement review, relay customization, factory full-function FAT testing, anti-vibration export packaging, international logistics, and on-site commissioning guidance, solving core pain points of overseas EPC project technical docking and after-sales service.

FAQ

Q1: What is the standard industry name for generator excitation loss protection?

The official standard terms are loss of excitation protection and loss of field excitation protection, uniformly corresponding to ANSI 40 protection function in global power engineering.

Q2: What failures will occur if a generator lacks ANSI 40 protection?

Without valid loss of excitation protection for modern synchronous generators, excitation loss faults will cause rotor burnout, grid voltage drop, out-of-step splitting, and even large-scale power outages.

Q3: Is impedance-based detection the only working principle for ANSI 40 protection?

Impedance trajectory detection is the core mainstream principle. High-end relays adopt impedance + reactive power + voltage multi-criterion fusion logic to improve protection accuracy and anti-interference ability.

Q4: What is generator distance relay (ANSI 21G)?

Generator distance relay, designated ANSI 21G, is also known as generator under-impedance protection. It continuously calculates apparent impedance at generator terminals. When the measured impedance falls within the predefined operating characteristic, the relay identifies short-circuit faults on generator leads, bus duct and step-up transformer, acting as main or backup phase fault protection for generator-transformer unit.

Conclusion

Loss of excitation protection is an indispensable core protection for loss of excitation protection of synchronous generator systems. It effectively avoids generator permanent damage and grid stability risks caused by excitation system faults, ensuring safe and efficient operation of hydropower, thermal power, and industrial self-generation projects.

For overseas EPC contractors and power plant owners, selecting a high-precision, standard-compliant loss of excitation protection relay with professional full-lifecycle engineering support is the key to improving project delivery quality and long-term operational reliability.

Our products and services fully comply with IEEE protective relay standards and ANSI international standards, delivering customized configuration, standardized testing, and global on-site technical services for loss of excitation protection projects of modern synchronous generators.

Reference & Technical Sources (EEAT Compliance)

• IEEE C37.102-2006, IEEE Guide for AC Generator Protection (Chapter 10: Loss of Excitation Protection)
• ANSI Standard Device Numbers for Power System Protection (ANSI 40: Loss of Excitation Relay)
• IEEE Std 115-1995, Test Procedures for Synchronous Machines
• GE Multilin UR Series Relay Official Manual: ANSI 40 Loss of Excitation Element Setting Guidelines
• Siemens SIPROTEC 5 Underexcitation & Loss of Excitation Protection Technical Specification