ML19260D355

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Adequacy of Station Electrical Distribution Sys Voltages
ML19260D355
Person / Time
Site: Dresden 
Issue date: 01/29/1980
From:
SARGENT & LUNDY, INC.
To:
Shared Package
ML19260D354 List:
References
6030-31, NUDOCS 8002080538
Download: ML19260D355 (13)
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SARGENT & LUNDY ATTACHMENT A rNGlNEERS C HIC AGO Project No. 6030-31 January 29, 1980 ADEQUACY OF STATION ELECTRICAL DISTRIBUTION SYSTEMS VOLTAGES DRESDEN UNIT 1 I.

INTRODUCTION The Nuclear Regulatory Commission (NRC) has required the licensees of all power reactors, in a letter dated August 8, 1979, to show that each plant's station auxiliary system will provide an adequate supply of power to essential loads during the contingency which presents the largest load demand on the auxiliary system.

This request was a result of the incident at Arkansas Nuclear One Station described in detail in the NRC's IE Information Notice 79-04.

This report documents the results of the study conducted to determine the adequacy of the electrical auxiliary system at the Dresden Nuclear Station Unit 1 to provide a power source of sufficient capacity and capability to supply the engineered safety feature loads in the event of a con-tingency presenting the maximum load demand on the system.

II.

OFFSITE POWER SUPPLY The first offsite power at Dresden Unit 1 is provided from CECO 138 kV system (red bus) through transformer #12.

The voltage at this high voltage bus varies from 133 to 142 kV.

The short circuit level ranges from 1945 to 15000 MVA.

The second offsite power at Dresden Unit 1 is provided from CECO 34.5 kV line L7281 through transformer #13.

The voltage at this high voltage line varies from 34.8 to 36.3 kV.

The three phase symmetrical f ault current at this line is 4900A at 34.5 kV.

The switchyard arrangement at Dresden Nuclear Station is shown on Figure 1.

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SARGENT& LUNDY cuciucens January 29, 1980 C HIC AGO 6Q}Q. {Ggig 1) Page 2 The station auxiliaries for the unit are served from six, 4.16 kV buses, four of which are designated for safety-related loads. The electrical distribution system at Dresden Nuclear Station is shown on Figure 2. During normal operation, half the unit load is supplied from the unit auxiliary transformer #11 (UAT) and the other half from the reserve auxiliary transformer #12 (RAT). When the generator is not operation, as during start-up, shutdown, or unit trip, the loads fed from the UAT are transferred to the RAT. In the event voltage on both 4160V buses 11 and 12 drops to 25% of the rated voltage for 30 cycles, the feed breakers from transformers #11 and #12 to buses 11 and 12 ~ open and the feed breaker from transformer #13 closes to bus 11. This transformer constitutes a second independent offsite auxiliary power source which is available to all essential auxiliaries (except for HPCI pemps) during abnormal operations. III. LOADING ANALYSIS The auxiliary system of Dresden Unit 1 is connected directly through its RAT (transformer #12) to 138 kV bus 3 (red). The auxiliary system of Dresden Unit 2 is connected directly through its RAT (transformer #22) to 138 kV bus 1 (blue). The tie breaker (104) between these two buses is normally open. The RATS of the two units are, therefore, electrically independent. The loading on one unit's auxiliary system will have no effect on the performance of the other unit's auxiliary system.

Hence, the multi-unit analysis required by the guideline is not applicable in this case.

The case chosen for detailed voltage evaluation represents the loading of the auxiliary system which will result in the worst voltages at the auxiliary buses. The worst loading condition for transformer #12 operation exists when the unit experiences a reactor trip. The unit was assumed to be generating rated output a t the time of the event. All loads operating for the unit's 100% generation were, therefore, assumed to run from trannformer #12.

SARGENT& LUNDY January 29, 1980 E N GlN E E R S C H IC A GO 6030-31 (Unit 1) P Page 3 Upon completion of present modification work, the worst case loading on transformer #13 will occur when the transformer carries the loads connected to 480V substations 14, 16, 116, and 117. All other loads are tripped automatically upon a transfer to transformer #13. Administration procedures do not require any of the tripped loads to be reenergized. Bus loadings reflecting the above operating conditions are shown on Table 1. IV. CRITERIA FOR ACCEPTABLE VOLTAGE The criteria for the acceptable voltage range at motors, contactors, and control circuits is based on equipment ratings as defined by the National Electric Manufacturers Association. These standards require that the maximum voltage should be limited to 110% of equipment rated voltage and the minimum voltage to be limited to 90% of equipment rated voltage. In order to provide adequate torque for motor starting and to prevent contactors from dropping out at 480V motor cc ntrol centers, the starting voltage should be limited to an acceptable level. This level is 75% of otor rated volt age. V. SYSTEM PERFORMANCE Voltage analysis was made for transforn.ar #12 and transformer

  1. 13 carrying their worst case loading.

The impedances of cable between the following buses were included in the evaluation: 1. 4160V Buses 11 and 112 2. 4160V Buses 12 and 110 The minimum calculated running voltages on the various buses for operation from transformers #12 and #13 are shown in Figures 3 and 4, respectively.

SARGENT& LUNDY January 29, 1980 E H GlN E E R s 6030-31 (Unit 1) C HIC A GO Page 4 Transformer #12 Operation The results of voltage drop calculations shown in Figure 3 indicate that the voltages at the auxiliary buses are satisfactory when the present tap setting of 138.0 kV is reduced to 134.55 kV to provide a boost of 2 %. The 2% voltage boost through the transformer #12 is acceptable as the no-load voltage at the motor terminals is limited to 110% of its ruting with 142 kV voltage (maximum) at the 138 kV system (red). The no-load voltage on the auxiliary system buses are shown on Table 2. The no-load voltages at different levels are calculated by using the transformation ratio with the maximum voltage assumed at the switchyard source. The minimum running voltage at the 4.16 kV buses is 3866V which is 92.9% of 4.16 kV or 96.6% of 4 kV. This allows enough margin to account for voltage drop in the motor feeder. A h% drop in voltage is a typical value for a motor feeder cable at 4 kV level. In making the calculations, the minimum value of switchyard voltage and the maximum value of system impedance were assumed. The starting bus voltage is also shown in Figure 3. The minimum starting voltage is 3577V (89.4% of 4000V) when a 1750 horsepower primary feed pump is started on bus 11 or 12. This value is well above the acceptable level. Figure 3 also indicates that voltage at the 480V level is acceptable with unit substations set at the 4160V tap. The minimum running voltage at the 480V level is 426V (92.6% of 460V) at 480V motor control center 117 and allows for a 2.6% voltage drop in the feeder cable. A maxinium voltage drop of 1% of 460V has been calculated in the cable between 480V motor control center and 460V motor terminal, The runniry voltage at other 480V switchgear buses is 94.3% of 460V and has ample cargin to account for the worst voltage drop in the feeder between a 480V switchgear and a 460V motor. The running voltages at selected loads are shown in Table 3. These values do not include voltage drop in the cables from the bus to the load.

SARGENTa LUNDY January 29, 1980 cucancrns c mc.co 6030-31 (Unit 1) Page 5 Transformer #13 Operation The results of voltage drop calculation shown in Figure 4 indicate that the voltages at the auxiliary buses are acceptable with the present high voltage tap setting of 34.4 kV. The maximum no-load voltage at the motor terminal is limited to 110% of its rating with this tap setting and with the maximum bus voltage of 36.3 kV. The no-load voltages on the auxiliary buses are shown on Table 2. The minimum running voltage at 4.16 kV bus 11 is 4030V which is 96.9% of 4.16 kV or 100.75% of 4 kV. The minimum running voltage at the 480V level is 445V (96.7% ot 460V or 92.7% of 480V). These minimum voltages are well above the minimum design criteria limits and have ample margin to account for the worst voltage drop in the motor feeder. VI. UNDERVOLTAGE RELAYS Undervoltage relays are provided for each ESP bus to transfer the ESF load to the onsite diesel generator in case offsite power is lost or degraded. The minimum voltage to transfer load to the ESP buses is 2929V or 73.0% of 4000V. Since the minimum expected vol'tage during normal motor starting operation (3577V) is well above the relay setting, transfer to the onsite power supply should not occur. The undervoltage relays incorporate sufficient time delay so that short circuits can be cleared without undervoltage relay operation. H. Ashrafi /ss

SARGENT & LUNDY ENGINEERS January 11, 1980 c"'ca " 6030-31 (Unit 1) TABLE 1 BUS LOADINGS ASSUMED FOR OFFSITE POWER SUPPLY ANALYSIS A. For Transformer #12 Operation Bus Load 4160V Bus 11 + 12 11.3 MVA 4160V ESF Bus 112 + 113 0.6 MVA 4160V r:SF Bus 110 + 111 0.6 MVA 12.5 MVA B. For Transformer #13 Operation Bus Load 4160V Bus 11 1.5 MVA 4160V ESF Bus 112 + 113 0.6 MVA 2.1 MVA

SARGENT& LUNDY ENGENEERs January 29, 1980 '"'C " 6030-31 (Unit 1) TABLE 2 NO-LOAD VOLTAGES-TRANSFORMER #12 OPERATION Percent of Bus No-Load Volts Equipment Rating Switchyard 142,000 4.16 kV Buses 4,390 109.7 Unit Substations Fed 506.6 110.1 From 4.16 kV NO-LOAD VOLTAGES-TRANSFORMER #13 OPERATION Percent of Bus No-Load Volts Equipment Rating 34.5 kV Line 36,300 4.16 kV Buses 4,390 109.7 Unit Substations Fed 506.6 110.1 From 4.16 kV

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