Why does a generator experience loss of excitation?

The traditional criteria for loss-of-excitation protection mainly include four key elements: 1. Low-voltage criterion for the generator rotor, which encompasses both constant excitation voltage and variable excitation voltage criteria. 2. Stator impedance criterion at the generator terminal, covering the characteristics of the asynchronous boundary impedance circle and the static stability boundary impedance circle. 3. Two-phase simultaneous low-voltage criterion, comprising the terminal low-voltage criterion and the high-voltage side low-voltage criterion of the main transformer. 4. Reactive-power-inversion criterion, which identifies loss-of-excitation faults by detecting reverse reactive power along with stator overcurrent conditions. The primary differences among various loss-of-excitation protection schemes lie in their distinct protection trip conditions, time delays, or the specific combinations of criteria used.

The rotor of the generator loses its excitation current. After the generator loses excitation, it leads to loss of synchronism, inducing sub-synchronous currents in the rotor's damping windings, on the rotor surface, and within the rotor windings—resulting in additional temperature rise. This can cause localized overheating in the rotor, potentially leading to severe thermal stress and jeopardizing rotor integrity. Furthermore, the asynchronous motion of the synchronous generator induces pulsating currents in the stator windings, generating fluctuating mechanical torques that may trigger vibrations in the entire unit, compromising the generator's operational safety. At the same time, the increased stator current could elevate the temperature of the stator windings.

 

 

   1

How it works

 

 

During normal operation of the generator, if excitation suddenly disappears completely or partially, it is referred to as generator demagnetization.

During operation, a synchronous generator can lose excitation, leading to the disruption of its normal running state. Once a synchronous generator loses excitation, it will transition into Asynchronous Generator The operation shifts from generating reactive power (inductive) to absorbing reactive power. Large generator sets widely adopt static excitation, which has reduced the use of rotating DC machines. However, due to the complexity of the excitation system and potential quality issues with components, more than half of all failures in medium- and large-scale generator sets are caused by under-excitation (insufficient excitation) or loss of excitation.

For power systems with limited reactive power reserve capacity, a large generator losing excitation will first manifest as insufficient system reactive power and voltage drop. In severe cases, this can lead to a voltage collapse, causing the loss-of-excitation fault of a single generator to escalate into a widespread system-wide incident. Under such circumstances, it is crucial to promptly disconnect the affected generator from the system to maintain stable operation.

When the system has sufficient reactive power reserve, a loss-of-excitation fault in a steam turbine generator is allowed to temporarily reduce active power output—typically for 10 to 30 minutes—and transition into asynchronous generation mode. During this period, the fault must be promptly addressed to restore excitation; if restoration fails, the unit should be disconnected from the grid. In contrast, hydroelectric generator sets, due to their small asynchronous torque (and thus limited power capability) and ease of start-up and shutdown, usually do not operate asynchronously when experiencing a loss-of-excitation fault. Instead, the loss-of-excitation protection directly triggers a trip to shut down the unit.

For large generator units located far from load centers and with weak system connections, detection of loss-of-excitation faults tends to be delayed, which can easily lead to maloperations in backup protections on the opposite side due to reactive power backflow and excessive line currents. Therefore, careful attention should be paid to selecting an appropriate loss-of-excitation protection scheme and ensuring accurate setting calculations.

To thoroughly eliminate the severe consequences that generator loss-of-excitation faults may impose on the system, it is first essential to ensure that the individual capacity of each unit in the system remains below 5–7% of the total system capacity. If a single unit has an excessively large capacity, it could lead to an extremely challenging situation: isolating the lost-excitation unit would cause the system to collapse due to insufficient active power, while failing to isolate it would result in system instability caused by inadequate reactive power. Secondly, the excitation regulators of all generating units should never be deactivated arbitrarily. Additionally, operators must avoid reducing the excitation of non-lost-excitation units during a loss-of-excitation fault. After all, loss-of-excitation protection serves only as the last line of defense—designed both to prevent the fault from escalating and to identify the affected generator unit.

 

 

   2

Cause Analysis

 

 

 

Generator

The reasons for a generator losing excitation generally fall into two categories: open circuits or short circuits in the excitation circuit, which may include failures in the exciter, excitation transformer, or excitation circuit itself; accidental contact with the excitation switch; improper switching to backup excitation systems; loss of plant power supply to the excitation system; open circuits in the rotor winding or excitation circuit, or severe short circuits in the rotor winding; malfunctions in semiconductor-based excitation systems; or even rotor slip-ring fires or burnouts.

1) Excitation transformer failure trip causes generator demagnetization.

Due to an insulation manufacturing defect in the transformer, or because insulation defects gradually worsened during operation and triggered discharge phenomena, the excitation transformer protection system tripped, causing the generator to shut down as a result of the loss-of-excitation protection action. It is essential to strictly adhere to procedures and standards, conduct regular tests, verify implementation status, and thoroughly investigate any identified issues.

Carefully carry out the implementation of regular insulation-related tests according to relevant procedures and standards.

2) Demagnetization of the generator caused by the tripping of the demagnetizing switch

Causes for the demagnetization switch tripping include:

Misfiring of the field suppression switch trip command on the DCS

The output relay fault triggers the demagnetization switch to trip.

The electrical control room's excitation suppression switch trip button contacts close, sending a trip command.

Manually trip the field suppression switch on the excitation room's local control panel;

The insulation of the demagnetization switch control circuit cable has deteriorated;

The switch body mechanically trips the field-disconnection switch;

A transient ground fault in the DC system caused the demagnetization switch to trip.

3) Excitation slip ring arcing causes generator demagnetization

The cause of the accident was the carbon brush. Compression spring Uneven pressure leads to an uneven distribution of current among the carbon brushes, causing some brushes to carry excessive current and subsequently generating heat. Additionally, the carbon brushes are contaminated with dirt, which fouls the contact surfaces between the brushes and slip rings, increasing the contact resistance in certain areas and leading to arcing. Furthermore, the wear rates of the positive and negative carbon brushes are inconsistent, with the negative brush wearing significantly more than the positive one. This severe wear exacerbates the unevenness of the slip ring surface, and because the issue isn’t addressed promptly, it eventually results in a fire on the slip ring.

4) DC system grounding causes generator demagnetization

After a positive-ground fault occurs in the DC system, due to the presence of long cables Distributed capacitance However, the voltage across the capacitor cannot change abruptly, which triggers the external trip circuit of the generator’s field-discharge switch to conduct capacitive current through its external trip outlet and the intermediate relay. As a result, the relay operates, tripping the generator’s field-discharge switch and causing Generator Loss-of-Excitation Protection Action trip.

5) Excitation regulator system failure causes generator loss of excitation

Generator Excitation System Regulator EGC A board failure caused the rotor overvoltage protection of the generator exciter regulator to activate, triggering the loss-of-excitation protection and resulting in a trip.

6) Complete shutdown of the rectifier cabinet leads to loss of generator excitation.

During the startup of the electric pump, the system voltage dropped, triggering an auxiliary power failure alarm from the excitation system. Due to the excessively high resistance in the switching circuit relay's auxiliary contacts, the power transfer failed, causing the rectifier cabinet fans to stop functioning properly. This, in turn, led to overheating in the rectifier cabinet, resulting in a trip due to overtemperature protection and initiating demagnetization protection, ultimately shutting down the unit. Additionally, the silver plating on the AC-side power switch contacts of the rectifier cabinet was either too thin or of poor quality. As the equipment operated, copper exposed to air formed an oxide layer, significantly increasing contact resistance. With rising current levels and further temperature elevation, the contacts overheated, eventually activating the demagnetization protection and causing the unit to trip.

 

 

   3

Harm

 

 

3.1 The Hazards of Generator Loss of Excitation to the Power System

When a generator experiences loss of excitation, the low-excitation or de-excited generator will start absorbing reactive power from the system, causing the voltage in the power system to drop. If the power system has limited capacity or insufficient reactive power reserves, the generator terminal voltage, the bus voltage on the high-voltage side of the step-up transformer, or the voltage at other nearby points may fall below acceptable levels. This could disrupt the stable operation between the load and the power source, potentially even leading to a voltage collapse in the entire power system.

When a generator experiences low excitation or loss of excitation, the voltage drops. In response, other generators in the system, aided by their automatic excitation control systems, will increase their reactive power output. This, however, can lead to overcurrent conditions in certain electrical components within the system—such as transformers or transmission lines—triggering backup protection devices to isolate the overloaded elements and, unfortunately, causing the fault to spread further.

When a generator experiences low excitation or loss of excitation, the resulting fluctuations in active power and the drop in system voltage may lead to loss of synchronism between adjacent normally operating generators and the system—or even between different parts of the power system—causing system oscillations and triggering large-scale load shedding.

3.2 The Harm of Generator Loss of Excitation to the Generator Itself

After a generator loses excitation, it not only poses significant risks to the power system but also causes certain damage to the generator itself:

When loss of excitation occurs, a slip difference arises, causing sub-synchronous currents to appear in the generator rotor. If the losses generated by these sub-synchronous currents in the rotor circuit exceed the allowable limit, the rotor may overheat. Moreover, the sub-synchronous currents flowing through the rotor's surface can lead to severe localized overheating—or even burning—on the contact surfaces between the rotor body and the slot wedges and retaining rings.

After a generator enters asynchronous operation due to low excitation or loss of excitation, its equivalent reactance decreases, and the reactive power drawn from the system continues to increase. In the event of loss of excitation under heavy load, the resulting overcurrent can cause excessive heating in the generator stator.

For directly cooled, high-utilization large-scale steam turbine generators, after a loss of excitation under heavy load, the generator's torque and active power will experience severe, periodic oscillations. At this point, significant electromagnetic torque—often exceeding the rated value—will periodically act upon the generator shaft system and, via the stator, be transmitted to the machine base. Simultaneously, the slip will also undergo periodic variations, leading to severe, cyclical overspeeding of the generator.

When the generator operates with low excitation or loss of excitation, the enhanced leakage flux at the stator end will cause overheating in the end components and edge core sections.

 

 

   4

Loss-of-Magnetization Protection

 

 

4.1 Traditional Generator Loss-of-Excitation Protection Criteria

The traditional criteria for loss-of-field protection mainly include four:

Generator rotor low-voltage criteria, including the constant excitation voltage criterion and the variable excitation voltage criterion.

Generator terminal stator impedance criteria, including the characteristics of the asynchronous boundary impedance circle and the static stability boundary impedance circle.

Two-phase simultaneous low-voltage criterion, including the generator terminal low-voltage criterion and the main transformer high-voltage side low-voltage criterion.

Reverse reactive power criterion—this involves detecting demagnetization faults by analyzing reverse reactive power along with stator overcurrent. The key differences among various demagnetization protection schemes lie in their distinct protection trip conditions, time delays, or the specific combinations of criteria used.

4.2 Auxiliary Criteria for Generator Loss-of-Excitation Protection

When various types of short-circuit faults occur at the generator terminal or on the high-voltage side of the main transformer, as well as in cases of system oscillations, the primary criteria used in失磁 protection may all potentially trigger false operations. In practical applications, generator失磁 protection typically employs a combined operating mode that integrates both primary and auxiliary criteria. The traditional auxiliary criteria for generator失磁 protection include:

The excitation voltage is decreasing.

No negative sequence component appears.

Use delay to avoid oscillation.

Reactive power changes direction.

4.3 A New Principle for Generator Loss-of-Excitation Protection

Combining knowledge of neural networks, a two-layer neural network is used to distinguish between stable and demagnetization phenomena in synchronous generators. By selecting a set of feature vectors that represent the operating state and then applying the Fourier transform to them as input to the neural network, the network leverages its pattern recognition capabilities to accurately differentiate between stable conditions and demagnetization events. Furthermore, through a study of the fundamental principles underlying generator demagnetization, the literature has derived three new criteria for demagnetization protection.

During the time interval from when reactive power equals 0 to when the power angle reaches 90 degrees, the absolute value of the active power's time derivative is less than a certain fixed value when the generator experiences loss of excitation. However, during system oscillations, the absolute value of the active power's time derivative exceeds this same fixed value.

During the time when the power angle is less than 90 degrees, the time derivative of the generator's electromotive force will be negative during loss of excitation, whereas when the system experiences oscillations, the absolute value of the generator's time derivative in electromotive force will fall below a very small threshold.

Take the system's maximum oscillation period T; if during this time the reactive power changes from negative to positive, it is determined that the system is experiencing oscillations. However, if the reactive power becomes negative and remains consistently negative, it can be concluded that the generator has suffered a loss of excitation fault.