ML20127J663
| ML20127J663 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 04/21/1978 |
| From: | Mayer L NORTHERN STATES POWER CO. |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 9211190392 | |
| Download: ML20127J663 (6) | |
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NORTNERN WTATES-POWER COMPANY MIN NE A PO A is. MINNE SOTA 95409 7:
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-April 21, 1978 4
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Director of Nuclear Reactor Regulation 5;
U S Nuclear Regulatory Commission y
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Washington, DC 20555 i
.1 i MONTICELLO NUCLEAR GENERATING PLANT
- l-Docket No. 50-263 License No. DPR-22 Modification to Provide Agematic Degaded Voltage Protection In a letter dated October-14,1977 to Mr D K Davis, Acting Chief, Operating'-
d Reactors Branch No. 2, Division of Operating Reactors,' Northern States Power Company informed the Commission Staff of our intention to install-automatic degraded voltage protection at the Monticelle Nuclear Generating.
Pl.nt, The purpose of this letter is ;to provide the Staf f-with a description.
of our proposed modification, Attached you will find-a description of our proposed modification to provide
. automatic degraded voltage protection. This modification conforms, where practical,. to the " Statement of Staff Positions Relative to Emergency Power Systems for Operating Reactors" provided to Northern States Power Company in a latter dated June 3,.1977 from Mr D K Davis..Our current plans call for installation and testing during the 1978 autumn refueling outage.
The modification will be made under the provisions of 10 CFR 50, Section
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50.59(b).
Please contact. us if you require further infomation concerning this modi-fication or if you wish to discuss this matter in further detail.
k L 0 Mayer, P Manager c^ Nuclear Support Services N
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-J G Keppler G Charnoff Enclosure-(1)l Automatic Degraded Voltage Protection Modification h
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9211190392 780421 PDR ADOOK 05000263 i
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', Enclosure (1) to letter dated April 21, 1978 from L 0 Mayar, NSP, to Director, NRR, USNRC AUTOMATIC DEGRADED VOLTAGE PROTECTION MODIFICATION Low voltage on either of the plut s two essential 4.16 IT buses will be sensed 8
by three new relays to be added to each bus. Sketch 1 shows the addition of these relays to the bus relaying scheme and Sketch 2 shows how they will be used. Actuation of any two of them relays passes a signal through the two out of three logic scheme fomed by catcs I through 4 At this point gate 5 ma; block the signal if the low voltage condition is a result of a complete loss of voltage. Le schene will operate as presently designed under such a loss of voltage condition.
If voltage does still exist on the bus, but this voltage is low, gate 5 will be satisfied and the signal will pass throuch gate 6, without time delay, to trip all offsite sources of power to the bus (breakers 152-308 and 152-511 for bus 15 examnle). Tripping of these breakers will produce a loss of voltage on the bus and gate 5 will no longer be satisfied.
However, since the icv voltage signal is still required to complete the desired load shedding, gate 6 is sea:ed in through gate 7 Gate 7 will be satisfied as long as the diesel generator breaker is open.
8Jpon the loss of voltage crected by the tripping of the offsite sources, the no voltage sensing relays (127-5,127-5X,127-5Y) shown in Sketch 1 also actuate. This systen will also be modified to a two out of three logic scheme, shown in Sketch 2, by the installation of one additional no voltage sensing relay per bus as shown in Sketch 1.
Actuation of these relays will begin the 5 and 10 second tine delays shown in Sketch 2.
These time delays normally initiate a transfer to the auxiliary reserve transfomer or the diesel generator through gates 8 or 9 respectively. Since all offsite sources are assumed to be degraded after a low voltage actuation, a signal from gate 6 is used to block gate 8 and prevent transfer to the auxiliary reserve transformer.
With no other changes to the existing scheme a transfer to the diesel generators would occur after a 10 second delay. To shorten this time, for transfers initiated by low voltage, the output of gate 6 is also used as one input to gate 10 whose other input will be satisfied after a 5 second time delay. A signal is then passed through gate 1] to initiate a transfer to the diesel generator. The closing of the diesel generator breaker will block gate 7 and cancel the seal-in of gate 6.
The relay used to implenent this gate will drop out after a short time delay, his delay is used to allow time for reset of the 5 second time delay which feeds gate 8 before the blocking signal to this gate, from gate 6, is removed.,
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C10$ute (1) Continued
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In addition to the preceding description the following is given in specific response to_your Statement of Staff Positions Relative to the Emergency Power Systems for Operating Reactors. The listing of responses given below corresponds to the structure used in that statement.
2 A.
POSITION 1 l
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- 1) Plant modifications will be made to provide a second level of 1
voltage protection for our onsite power system.
1 a) An analysis of all safety related loads indicates that'a voltage setpoint corresponding to 3 90 kV on our 4.16 kV l
safety _ related buses will satisfy the voltage requirements of these loads while allowing the plant to operate within the l
presently allowed range of generator voltage and the historical i
range of system voltage.
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It is apparent after preliminary analysis that the best method of selecting a low limit for the time setpoint will be through i
actual testing to determine the voltage recovery characteristics l
of the plant buses under various loading transients.
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b) A 2 out of 3 type of coincidence logic will be used for degraded voltage sensing.
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c) Basis of time delay selection l
(1) The time delay between sensing of a true degraded voltage condition (i.e., one which persists for a time in excess of that allowed for motor starting i
transients) and the time when the onsite power source will be closed onto the bus and ready to accept. load will i
not.be greater than the time delay which now exists'under a loss of voltage condition-at Monticello.
(2) The selection of the time delay, with respect to 3
its ability to minimize the effect of short duration i
disturbances,-will be made based upon test results.
t (3) An analysis of the length of time that safety related loads can be expected to endure a low voltage condition 3
will be made prior to selection of a time setpoint.
g d) The voltage monitors will. initiate the. disconnection of all offsite power sources to the essential plant buses, w
e)_ The relays used as low voltage monitors will be designed to satisfy the requirements of IEEE Std. 279-1971.
f) Accomplishment of this design change does not require a change to the Technical Specifications. Following installation and j
testing, however, the additional limiting conditions for operation and surveillance requirements contained in the "model technical i
specifications"_which are appitcable-to the Monticello plant will be included in a License Amendment RAquest (see also item C below).
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' Enclosure (1) continued s.
B.
POSITION '
l The current system design meets all of the stated requirements.
Load shedding of the emergency buses, once the onsite sources are supplying l
power, is prevented through interlocks which sense the closed position of the onsite source supply breakers.
If these breakers are tripped, load shedding will be automatically reinstated and will function when an emergency power source becomes available.
C.
POSITION 3 During each refueling outage the requirements of simulating a loss of offsite power inconjunctionwith a safety injection actuation signal are currently being met. Proper operation is determined by the methods described in Position 3, items a) and b).
Bus de-energi:ation, load shed, bus re-energi:ation and load sequencing are all verified. The operation of the system, with emergency loads connected, for five minutes is not currently specified but will be included.
During times when the diesel generator is supplying loads an automatic
" interruption" may be caused by any one of the following:
1.
Essential bus lockout relay' actuation 2.
Anti-rotoring relay actuation 3.
Differential overcurrent relay actuation 4.
Phase overcurrent relay actuation 5.
Generator loss of field 6.
Diesel overspeed No interlocks with the undervoltage load shedding or load sequencing schemes exist in the trip logic. To simulate an interruption of the onsite power sources as suggested in Position 3 would therefore be equivalent to simulat-ing a diesel failure of the type which is not likely to be self-correcting.
In fact, each of the breaker trips in items 1 through 6 must be manually reset. Postulating a subsequent automatic.re-loading of the generator is therefore unrealistic.
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