ML18227D324

Response to Request for Additional Information Grid Voltage Conditions

ML18227D324
Person / Time
Site:	Turkey Point
Issue date:	09/22/1976
From:	Robert E. Uhrig Florida Power & Light Co
To:	Lear G Office of Nuclear Reactor Regulation
References
Download: ML18227D324 (10)

Text

~

~

I

~

~

I

~ ~

~

~

~

~ ~

~

~

~

~

~ '

~

~

~

~ ~

I

~

~

~

~

I

~

~

f

~

~

~

~

~

~

l ~

%" BWXK~

g Q0Q Wg Or II W

I I

~ ~ I L

)

~ ~

/If j II II J" t

Regula~os

~ BOX 013100. MIAMI FL 33101 iih Ii~

a 8 FLORIDAPOWER 8 LIGHTCOMPANY September 22, 1976 L-76-339 Office of Nuclear Reactor Regulation Attention:

Mr. George Lear, Chief Operating Reactors Branch e3 9 Division of Operating React U.

S. Nuclear Regulatory Commission Washington, D. C.

20555 Ogh01D

~LG

~@+V'I

Dear Mr. Lear:

Re:

Turkey Point Units 3 and 4

Docket Nos.

50-250 and 50-251 Grid Volta e Conditions Your 1'etter of August ll, 1976 contained a Request for Xnformation concerning grid voltage conditions at Turkey Point Units 3 and 4.

Our response to your request is attached.

Very truly yours, W5A Robert E. Uhrig Vice President REU/MAS/cpc Attachment cc:

Mr. Norman C. Moseley Jack R.

Newman, Esquire ef jItg' 3EPgrFi'g 9704 PEOPLE... SERVING PEOPLE

t i

8 f

4, 1 I'

ATTACHMENT ANALYSIS OF THE CONSEQUENCES OF UNDERVOLTAGE CONDITIONS AT THE TURKEY POINT AND ST.

LUCIE PLANTS Two sets of limiting parameters can be defined for the minimum and maximum voltage conditions of the auxiliary power system.

The first condition is defined by the main generator terminal voltage excursions when the auxiliary power system is fed from the unit auxiliary transformer.

Design limitations of the generator permit a maximum variation of + 5%

of the nominal generator terminal voltage of 22,000 volts.

Since the primary of the auxiliary transformer is connected directly to the generator terminals, it can be seen that the primary, voltage can vary only from 20,900 volts to 23,100 volts.

Operation outside these limits is avoided in order not to damage the main generator.

The second condition that determines the auxiliary power system voltage is the expected range of the 240 KV switchyard voltage when the auxili.ary power system is fed through the start-up transformer.

Normal operating practice is to maintain 240 KV at the station switchyard.

Normally

.this voltage varies between 238 and 242 KV.

A review of operating experience has proven that the switchyard voltage may vary between 228 and 242 KV due to various system p'arameters such as reactive load on the generators, instantaneous power demand,

outages, etc.

At Turkey Point the auxiliary power system is estimated to be on the startup transformer less than 1% of the time during normal operating conditions.

Even though there has been less than one year of operating experience at St. Lucie, the time on the startup transformer at St. Lucie is expected to be approximately the same as that of Turkey Point.

In accordance with operating procedures, the plant auxiliary systems at both plants are supplied with offsite power via the startup transformer until the operating power level is above approximately 170 Mw at St. Lucie and 70 Mw at Turkey Point.

The minimum auxiliary bus voltages occur when the generator terminal voltage i.s 20.9 KV (auxiliary transformer) or if the auxiliary bus is being supplied by the grid (start-up transformer) when the start-up trans-former primary voltage is 228 KV.

The voltage on the auxiliary busses corresponding to the minimum expected voltage conditions are presented herein.

The listed busses are those which have the lowest voltage under maximum credible load.

Voltage at all other busses are greater than the given values.

ST.

LUCIE PLANT Auxiliary bus voltages with loads being supplied by the auxiliary transformer with generator terminal voltage at 20.9 KV:

NOMINAL VOLTAGE ACTUAL VOLTAGE

% OF NOMINAL 4 KV bus lA2-1A3 4160 4002 96%

480 V bus 1A2 MCC bus lA5-lA8 480 480 441 436 92%

91%

Auxiliary bus voltages with loads being supplied by the start-up transformer with grid voltage at 228 KV:

4 KV bus 1A2-1A3 480 V bus lA2 MCC bus 1A5-1A8 480 90%

480 429 89%

NOMINAL VOLTAGE ACTUAL VOLTAGE

% OF NOMINAL 4160 3935 95%

TURKEY POINT PLANT Auxiliary bus voltages with loads being supplied by the auxiliary transformer with generator terminal voltage at 20.9 KV:

NOMINAL VOLTAGE ACTUAL VOLTAGE

% OF NOMINAL 4 KV bus 3B 4160 3959 95%

480 V bus 3D 480 434 90%

Auxiliary bus voltages with loads being supplied by the start-up transformer with grid voltage at 228 KV:

NOMINAL VOLTAGE ACTUAL VOLTAGE

% OF NOMINAL 4 KV bus 3B 4160 3854 93%

480 V bus 3D 480 422 88%

The minimum acceptable bus voltages are established by the design parameters of the various connected devices and equipment.

The 4 KV motors are designed to operate at 86% of 4160 volts and the 460 volt motors have also been designed for operation at 86% of 480 volts.

Relays, motor control center contactors, and other auxiliary devices are designed for a minimum of 85%

of their nominal voltage and as such are not'the limiting items.

Therefor'e, it is demonstrated that the minimum voltages on the auxiliary power'ystem are well above the minimum necessary for proper operation of the various safety and non-safety related equipment.

In order to adequately protect against voltage excursions below the minimum required value for safe and reliable operation, it is necessary to consider many different parameters.

Among the.primary considerations are 1) to alarm on voltages below a predetermined

setpoint,

2) to initiate necessary pro-tective action below a second setpoint, and 3) to avoid unnecessary or false trips.

To satisfy the criteria described

above, the St. Lucie Plant incorporates a Westinghouse type CV-2 undervoltage relay on each of the safety related 4.16 KV busses.

These relays are used to initiate their respective diesel generator loading sequence.

They are induction disc type units which inversely relate voltage to time, that is, the greater the drop in voltage below the setpoint the faster the response.

In order to achieve the design criteria the relay tap was selected to be 105 volts.

This corresponds,to 88.34% of the '4.16 KV bus voltage.

This setting was selected in order to provide a high level of, protection and, at the same time, avoid spurious and undesirable false trips due to transients from such causes as inrush currents from the starting of large motors, faults, etc.

Since Turkey Point does not currently have a similar type of undervoltage circuitry, plans are underway to develop a scheme similar to the one used at St. Lucie.

When the auxiliary loading is maximum and supplied through the station start-up transformer, the grid voltage which corresponds to the undervoltage relay setpoint is 224.6 KV at St. Lucie. It should be noted that this voltage is below the grid voltage range.

When the auxiliary system is supplied through the unit auxiliary transformer, the range of voltages are determined exclusively by the generator terminal voltage.

This cannot be below 20.9 KV without: risking damage to the main generator.

However, the undervoltage relay setpoint corresponds,to 19.3 KV at St. Lucie on the generator terminals under maximum auxiliary load conditions.

The setpoint is also below the operating range of the generator terminal voltage.

Thus, in both cases, spurious trips, due to voltage supply conditions, can be avoided since the undervoltage setpoints are out of the range of operation.

At St. Lucie, with maximum load on the 480 volt.transformer, the minimum 480 volt motor control center bus voltage corresponding to the undervoltage relay setpoint is 83%, of 480 volts.

A similar setpoint is being considered for Turkey Point.

Such a setpoint will avoid spurious trips under the conditions previously described.

It is improbable that the 480 volt bus voltage will remain between 83% and 86% for an extended period of time because only transients could cause the voltage to pass through this region.

In the unlikely event that'the voltage should pass through this region, voltage monitoring and alarm/annunciator equipment is provided at both St. Lucie and Turkey Point to alert the operators of.the condition.

,N

Diesel Generators Turke Point and St. Lucie The loss of offsite power undervoltage relays remain functional through the loading sequence and operation of the diesel generators.

Proper design philosophy requires this in order to establish maximum reliability of the diesel generators to assume load.

That is, it is considered more advantageous to strip the diesel generator loads and resequence the load than to risk stalling the diesel engine, possibly damaging equipment, and losing all capability of picking up load again on the given engine-generator.

r Grid Stabilit Limits The results of the stability study referenced in the St. Lucie Unit l FSAR imposed no limits on plant operation.

'The Turkey Point FSAR did not address a stability analysis.

Subsequent analyses and responses to bulk system disturbanc'es have resulted in an extension of the Florida Power 6 Light Company Emergency Load Management Program and the addition of underfrequency tripping logic for generator protection.

The Emergency Load Management Program and preventive maintenance programs for relay hardware assure that the facility is being operated within appropriate grid stability limits.

Pro osed Followu Action The St. Lucie bus voltage alarm setpoints will be revised to correspond to approximately 88% of the 480 volt bus voltage under full load conditions.

The advantages of the St. Lucie design are recognized, and a plan is being considered for utilizing the same principles at Turkey Point.

0

~,

r,e

( ~

~

'E-