Guard against over-fluxing: Ensure proper generator protection, maintenance

Although the duration of an over-fluxing condition may last only a few seconds, the consequences can be extremely damaging to your generator, warns Dr Isidor Kerszenbaum, PE, IzzyTech LLC—including expensive repairs, several months of forced outage, and loss of production. One of the most problematic aspects of an over-fluxing incident is the difficulty in assessing the condition of the generator after the event has occurred. Thus it is critical to avoid creating the conditions that could lead to such an incident.

The physics of over-fluxing. Backgrounder for the over-fluxing discussion: Generators convert mechanical energy to electrical energy. The generator rotor creates a magnetic flux that links the rotor to the stator through the air (gas) gap. This induces a voltage in the stator winding that drives the electrical output of the machine.

The magnetic flux has two sources: the first, internal to the generator, is the DC current carried by the rotor winding, also called the “field current” and “field winding.” The second is the current flowing in the stator winding. When the machine is not synchronized to the grid (generator breaker open), the only source of magnetizing flux is the rotor winding and the field current flowing in it. By changing the magnitude of the field current, the voltage in the terminals of the machine is controlled.

Recall the expression:

Izzy formula

 

Over-flux 1where B is the flux density, VT is the voltage at the terminals, and f is the frequency. The ratio is commonly known as “volts per hertz.”

In a typical turbogenerator, the flux density (B) is between 1.5 and 2 tesla, depending on the part of the core in question. The region of highest flux density is the tooth. Fig 1 shows the typical magnetic flux distribution inside a generator. A key characteristic of the flux, under normal operating conditions, is that in the stator, it is confined to the core material because of the high permeability of the constituent laminations.

However, increasing the flux density beyond a certain value (the “knee” region of the saturation curve of the core steel) reduces the permeability of the iron, allowing some of the flux to escape beyond the core boundaries. This gives rise to two serious conditions:

      • Large voltages and currents induced in the keybars and other structural members of the stator.

      • Degradation of the stator inter-laminar insulation with subsequent hotspots in the core. This condition is known as “over-fluxing” or “over-excitation.” But the latter, which happens when the field current is raised beyond its normal limits, does not necessarily lead to over-fluxing; hence, in this article over-fluxing is the term used.

Fig 2 shows the normal flux distribution inside a section of the core, and shows the flux distribution when the machine is over-fluxed.

Over-flux 2

What is interesting with over-fluxing events (and also a source of big headaches to those trying to ascertain the condition of the machine) is that once the over-fluxing ends, there is practically no way to tell if the core has been damaged and if it is advisable to resume operations. It can be very difficult to find signs that major rework is needed.

Perhaps the only way to effectively assess core condition is to remove the rotor and perform a loop flux test. There are serious doubts that an EL-CID test (with the rotor in or out) will be able to identify core insulation damage immediately after over-fluxing, and EL-CID certainly will not detect damage to the keybars, keybar insulation, or other structural members.

Hence, the best practice is to avoid—at all cost—experiencing an over-fluxing event. Damage to a core caused by over-fluxing can happen in a few seconds, if the flux density is high enough. This is more likely if the core inter-laminar insulation is aged or otherwise degraded.

Fig 3 shows the result of an over-fluxing event; portions of the core have melted. The unit was over-fluxed. Twelve hours later the machine tripped because of a winding ground fault and an examination revealed the damage.

Over-flux 3, 4

Missteps conducive to over-fluxing. Almost without exception, over-fluxing occurs when the field is applied, but before the machine is synchronized to the grid. Reason: Once the breaker is closed, increasing field current mainly increases VAr loading, while mildly increasing the terminal voltage.

There are two key protection functions that are supposed to detect an over-flux event, then alarm and trip the unit. One is the volts-per-hertz relay (also known by its device number, 24, as specified in ANSI/IEEE Standard C37.2). The second is the over-excitation limiter in the excitation system. Unfortunately in some cases, the over-excitation limiter is not set properly, and the No. 24 relay malfunctions, leading to a major failure—such as that shown in Fig 3.

Example 1: 500-MVA coal-fired unit. A maintenance error during a routine outage allowed the potential transformer circuit, feeding both the automatic voltage regulator (AVR) and the volts/hertz relays, to remain open. When the unit was energized, the AVR didn’t receive a signal indicating the terminal voltage was going up and the field current had increased beyond normal conditions. The over-excitation function was not set, or not set properly, and the No. 24 volts/hertz relay also had no voltage input.

The voltage increased to about 135% of nominal, and the condition lasted for about 16 seconds, until the unit was tripped. A cursory inspection of the machine did not reveal any obvious problems, so it was returned to service. The result was a partially melted core requiring a complete stator rewind, major core re-staking, the replacement of one retaining ring, and several months of lost production.

Example 2: A 180-MVA GT generator. The unit was returned to operation after some work was done on the isophase bus. During the work, the leads to the potential transformers feeding the AVR and other protection systems were left unterminated.

As with the previous case, this oversight put the generator in jeopardy, with the potential for a catastrophic failure and long forced outage. But unlike the previous case, the machine was tripped in a few seconds by a timer in the excitation system, without discernible damage to the core. Nevertheless, degradation to the inter-laminar insulation might have occurred, reducing the expected core life.

Example 3: 1300-MVA, 4-pole nuclear unit. While being returned to service after an outage, the AVR malfunctioned, “going ceiling” and increasing the excitation current well beyond normal limits—thereby creating an over-fluxing situation. The volts/hertz relays were arranged many years ago with a very permissive setting, and there was little doubt that if the unit was not tripped by serendipity (an auxiliary transformer protective relay), it might have led to serious core failure.

In the case of large units, the main transformer oftentimes is at the generator side of the main breaker, meaning that a large over-fluxing event of the generator may also result in over-fluxing of the main and auxiliary transformers.

Fig 4 shows how the over-fluxing incident happened. The field voltage is routinely applied to this machine at about 1450 rpm, as it is accelerated to its rated 1800 rpm. When all goes as it should, the field current follows the green line and settles on the open-circuit field current value (OCFC). When the AVR malfunctioned, the field current followed the red trace, creating both an over-flux and overvoltage condition—that is, the stator was inadvertently “high-potted.”

The common thread through the foregoing examples is an absence of diligence in designing the protection scheme (be it hardware configurations or relay settings) coupled with easily avoidable maintenance blunders.

Takeaways:

      • Over-fluxing events are very serious, and in some cases can lead to major equipment damage and loss of production.

      • Over-fluxing events overwhelmingly tend to happen during energization of the generator and prior to synchronizing to the grid.

      • Damage to the core and/or other stator components is very difficult to identify, and may require a full loop test to uncover.

      • Degradation leading to catastrophic failure may occur in over-fluxing events even though they only last several seconds. The actual failure of a component and trip (typically a stator-winding ground fault) may happen several hours, days, or months after the over-fluxing event.

      • Routine electrical tests such as megger and hipot or other dielectric tests will yield no information about damage to the inter-laminar insulation or other structural damage caused by over-fluxing.

      • It is extremely important to properly set the volts/hertz protection and over-excitation limits.

      • If possible, the potential transformer feeding the AVR should be different from the one feeding the volts/hertz relay. This can be accomplished in most large units, but may not be possible in the case of small machines.

      • After a serious over-fluxing accident, it is strongly recommended plant operators ascertain, as judiciously as possible, whether the generator is healthy enough for a return to reliable it to operation. This may require consultation with the OEM and/or other informed parties.

Generator training

Registration is now open for “Operation and Maintenance of Large Turbogenerators,” to be held in Irvine, Calif, September 28 and October 2. For details, please visit www.izzytech.com. The company, founded by IEEE Fellow Izzy Kerszenbaum, PE, specializes in supporting powerplant owner/operators in the operation, maintenance, monitoring, testing, troubleshooting, and failure analysis of electric generators, motors, and transformers.

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