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Settings for generator sync-check relays

Posted byHameedullah EkhlasFri at 8:00 AM - 250 views - Filed inPower Systems

Another relatively easy way to damage a turbine/generator is to synchronize or parallel out of

phase with the electrical system. Out-of-phase synchronizing operations can damage or reduce

the remaining life of turbine/generator rotors and stationary components. Angular differences aslittle as 12 degrees can instantly apply 1.5 per unit or 150% of full load torque on the

turbine/generator shaft system. The 1.5 per-unit value was measured by a shaft torsional moni-

toring data acquisition system (EPRI project) at a large coal plant that was paralled to the 500 kV

bulk power electrical system with a 12-degree angular difference during synchronization. Plantoperations acknowledged that the turbine deck really shook. Although turbines are generally

built to with-stand angular differences above 10 degrees, most manufacturers recommend

limiting out-of-phase synchronizing operations to no more than 10 degrees maximum. Generator

sync-check relays should supervise both manual and automatic modes of operation to preventturbine/generator damage from operator errors or from malfunctioning automatic synchronizing

relays. For this reason, it is normal practice to have the sync-check relay function pro-vided in a

different package than the automatic synchronizing relay to avoid failure modes that can impactboth functions.

A clockwise rotation of the synchroscope in the majority of designs indi-cates that theturbine/generator has a higher speed or frequency than the electrical system. This condition is

desirable to reduce the possibility that the unit will be in a motoring mode of operation and trip

on reverse power protection when the breaker is closed. The voltages should be matched duringsynchronizing with a slightly higher generator voltage to ensure var flow into the system insteadof the generator.

Figure provides suggested settings for generator sync-check relays. The proposed default anglesare 5 degrees advance and 5 degrees late. The calculations consider circuit breaker closing times

and maximum allow-able slip rates and determine the minimum seconds per scope revolution

and the worst-case angle. The minimum seconds per scope revolutions is provided as a guide for

operations or for setting auto synchronizing relays; the scope revolutions cannot go faster and be

within the operating range of the sync-check relay. The worst-case angle assumes the breakerclose signal is dispatched at the maximum late angle and that the breaker XY control scheme

seals in. It then looks at the maximum allowable slip frequency (proposed setting of .05 Hz) and

calculates the worst-case out-of-phase angle when the breaker contacts actually close. In thiscase, the worst-case angle of 8.6 degrees complies with manufacturers recommendations not to

par-allel if the angle exceeds 10 degrees. Experience has shown that the pro-

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7/27/2019 Settings for Generator Sync-check Relays

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Synchronizing Check Relay Settings

Notes:

Many manufactures recommend limiting out-of-phase conditions to 10 degrees. A plus andminus 5- degree setting should provide enough margin for operations depending on the response

of the particular governor control system.

posed settings are practical and within the operating capability of most turbine-governor control

systems.

Other available settings in the newer digital relays might include ratio correction factors for

GSUT taps because generator potentials are normally compared to switchyard high voltage

potentials and allowable percent volt-age mismatches. Although this writer is less concernedabout var flows from voltage differences because they do not represent real power and the shaft

torques are minimal, plant operators should limit voltage mismatches to less than 5%.

Some of the newer digital sync-check relay functions also include slow breaker protection. Oncethe breaker control signal is dispatched, it seals in, and there is no way to abort the close

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operation because there is a 52a con-tact in series with the trip coil that prevents the coil from

being energized until the breaker is actually closed. The slow breaker function could be set to

operate breaker failure relaying to clear the adjacent breakers if the angular differences reach 10degrees or more, indicating that the breaker is slow to close for mechanical reasons.

The maximum amount of symmetrical AC current that flows during synchronizing at rated

frequency can be approximated by the expression in Figure 4.22. Generator side voltage andohms from Figures 4.1 and 4.2 were used in the calculation and reflected to the 765 kV side. The

system three-phase 765 kV short circuit ohms were transferred from Figure

Maximum Symmetrical Synchronizing Current

Figure shows that the 765 kV current would be approximately 983 amps and the generator amps

would be approximately 32,348 at 30 degrees. At 60, 90, and 180 degrees, the approximated 765kV currents for the parameters presented in the figure would be 1897, 2682, and 3793 amps,respectively. The generator side current at 180 degrees out would be around 124,767 amps. This

does not include the DC component or peak asymmetrical current, which will also be present.

Obviously, the generator and transformer wind-ings need to be able to handle the peakelectromechanical forces. The event is transitory in nature as the asymmetrical current decays

and the generator pulls into step with the system and the power angle becomes congruent with

the prime mover. The air gap torque is difficult to calculate and depends on electromechanical

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forces, circuit resistance, and the amount of power transfer from angular differences. Possible

damage assessment is particu-larly complicated and associated with the peak torques and the

natural fre-quencies of the shaft and other mechanical components as the event decays. Theassociated apparatus may have reduced life from other events or excur-sions, startup/shutdown

cycles, or design or repair oversights, and major equipment damage may occur if the incident is

severe enough

Reference: Power Plant Design Books

Article Tags:#Synchroscope#generator sync-check relays#clockwise rotation#Settings for

generator sync-check relays#sync check

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