ONS-2015-014, Response to Request for Additional Information Associated with License Amendment Request (LAR) Regarding Keowee Hydro Unit (Khu) Steady State Frequency Requirements
| ML15055A168 | |
| Person / Time | |
|---|---|
| Site: | Oconee  (DPR-038, DPR-047, DPR-055) |
| Issue date: | 02/12/2015 |
| From: | Batson S Duke Energy Corp |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LAR 2013-01, Suppl. 1, ONS-2015-014 | |
| Download: ML15055A168 (13) | |
Text
DUKE ENERGY.
Sco. c Baots Vice President Oconee Nuclear Station Duke Energy ONOI VP I 7800 Rochester Hwy ONS-2015-014 10 CFR 50.90 Seneca, SC 29672 o: 864.873.3274 February 12, 2015
- f. 864.873. 4208 Scott.Batson@duke-energy.com Attn: Document Control Desk U. S. Nuclear Regulatory Commission 11555 Rockville Pike Rockville, MD 20852
Subject:
Duke Energy Carolinas, LLC Oconee Nuclear Station, Units 1, 2 and 3 Renewed Facility Operating License Numbers DPR-38, 47 and 55 Docket Numbers 50-269, 50-270 and 50-287 Response to Request for Additional Information Associated with License Amendment Request (LAR) Regarding Keowee Hydro Unit (KHU) Steady State Frequency Requirements LAR No. 2013-01, Supplement 1 On April 26, 2013, Duke Energy Carolinas, LLC (Duke Energy) submitted a LAR requesting Nuclear Regulatory Commission (NRC) approval to add steady state frequency requirements for the emergency power sources, KHUs, at the Oconee Nuclear Station (ONS). On January 16, 2015, the NRC issued a request for additional information (RAI). The NRC requested Duke Energy provide a response within 30 days. The enclosure to this letter responds to the RAI.
There are no Regulatory Commitments within this LAR supplement. Inquiries on this submittal should be directed to Boyd Shingleton, ONS Regulatory Affairs Group, at (864) 873-4716.
I declare under penalty of perjury that the foregoing is true and correct. Executed on February 12, 2015.
Sincerely, Scott L. Batson Vice President Oconee Nuclear Station
Enclosure:
Duke Energy Response to Request for Additional Information AooI www.duke-energy.com
U. S. Nuclear Regulatory Commission February 12, 2015 Page 2 cc w/
Enclosure:
Mr. Victor McCree, Regional Administrator U. S. Nuclear Regulatory Commission - Region II Marquis One Tower 245 Peachtree Center Ave., NE, Suite 1200 Atlanta, GA 30303-1257 Mr. James R. Hall, Senior Project Manager (by electronic mail only)
Office of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission 11555 Rockville Pike Mail Stop 0-8 G9A Rockville, MD 20852 Mr. Jeffery A. Whited, Project Manager (by electronic mail only)
Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission 11555 Rockville Pike Mail Stop 0-8 B1A Rockville, MD 20852 Mr. Eddy Crowe NRC Senior Resident Inspector Oconee Nuclear Site Ms. Susan Jenkins, Manager Radioactive & Infectious Waste Management Division of Waste Management South Carolina Department of Health and Environmental Control 2600 Bull St.
Columbia, SC 29201
License Amendment Request No. 2013-01, Supplement 1 Enclosure February 12, 2015 Page 1 of 11 Enclosure Duke Energy Response to Request for Additional Information (RAI)
NRC Background By letter dated April 26, 2013 (Agencywide Documents Access and Management System Accession No. ML13121A460), Duke Energy Carolinas, LLC, requested amendments to the Oconee Nuclear Station (ONS) Units 1, 2, and 3 Renewed Facility Operating License Nos.
DPR-38, DPR-47 and DPR-55. The license amendment request (LAR) proposes to incorporate steady state operating limits for the onsite emergency power sources, the Keowee Hydro Units (KHU), into the ONS Technical Specification (TS) Surveillance Requirements (SR). The changes would amend the SR to improve operation and testing of the KHU units by maintaining a more restrictive frequency band for operation when not connected in parallel with the offsite sources.
The NRC staff has reviewed the LAR and has determined that additional information is needed to complete its review. The staff's request for additional information (RAI) consists of the following questions regarding the proposed changes to TS 3.8.1, "AC Sources - Operating," SR 3.8. 1.9, related to KHU steady state frequency testing.
The LAR states that KHU frequency is allowed to vary as follows:
On an actual or simulated emergency actuation signal, each KHU auto starts and:
a) Achieves frequency _ 57 Hz and < 63 Hz and voltage _ 13.5 kV and < 14.49 kV in < 23 seconds; b) Achieves steady state frequency > 59.4 Hz and < 61.8 Hz; and c) Supplies the equivalent of one Unit's Loss of Coolant Accident (LOCA) loads plus two Unit's Loss of Offsite Power (LOOP) loads when synchronized to system grid and loaded at maximum practical rate The LAR also states that small perturbations outside of the steady state criteria due to expected load additions or removals are permitted. However, the transient peak values shall be within
+/-5% frequency limits and of short time duration being no more than 10 seconds.
The staff notes that changes in KHU frequency will affect the performance of the components, pumps and motor operated valves (MOVs).
For induction motors, motor speed is directly proportional to power supply frequency. As power supply frequency varies, pump speed varies, causing variations in pump flow and discharge pressure. MOV stroke time may be negatively affected by lower emergency diesel generator frequency, causing the MOV to operate more slowly. The staff also notes that the frequency and voltage recovery time for most onsite power systems is less than 5 seconds during load sequencing, in accordance with guidance provided in Regulatory Guide 1.9 (Revisions 1-4). The KHU frequency is expected to recover within 10 seconds.
License Amendment Request No. 2013-01, Supplement 1 Enclosure February 12, 2015 Page 2 of 11 Based on the above observations, please provide the following information:
- a.
Emergency Safety Features (ESF) Pump Performance: For induction motors, motor speed is directly proportional to power supply frequency. Please provide a summary of the evaluations or analyses performed to validate that flow rates and system pressures for ESF pumps are within the parameters assumed in accident analyses and plant design.
Please provide a summary of the minimum margin of motor torque allowed over the pump load torque during the motor accelerating period.
- b.
MOV Performance: Operation of the KHUs at the high end of the frequency range may cause a higher differential pressure across MOVs as compared to nominal conditions.
Operation of MOVs at lower end of the allowable frequency may negatively affect the MOV stroke time.
- i.
Please provide a listing of critical valves, the required stroke times as considered in accident analyses, and the measured stroke times during the last surveillance.
ii.
Please provide a summary of the analyses performed to demonstrate that sufficient margin exists between actual stroke times and maximum allowed stroke times to account for the minimum expected KHU frequency.
iii. Please provide a summary of the analyses performed to demonstrate that MOVs will operate satisfactorily with maximum differential pressures.
iv. Please explain if the frequency recovery time of 10 seconds was factored into the above evaluations and the impact of voltage variations on available torque was considered.
- c.
The KHU frequency and voltage, immediately upon connecting to the plant safety busses, can be > 57 Hz and < 63 Hz and voltage > 13.5 kV and < 14.49 kV The accident signal is actuated at time zero and is typically present in the control logic of ESF pumps and MO Vs when the onsite power systems is connected to the safety busses. Please confirm whether the wider range of allowable frequency and voltage coupled with a 10 second delay in voltage and frequency was considered in the performance capabilities of pumps and MOVs.
- d. Please provide a summary of the changes in KHU loading as a consequence of operation at the extremes of the proposed frequency and voltage range.
Duke Energy Response During the conference call on January 15, 2015, the NRC requested Duke Energy provide a summary of the emergency power system design and the testing as they relate to startup and steady state frequency requirements for the KHUs. This information is provided below and is referenced in response to specific NRC RAI questions as appropriate.
Emergency Power System Design The Oconee Nuclear Station's Emergency Power is supplied by the Keowee Hydro Station which consists of two units rated 87.5 MVA each. This electric power source is much larger than
License Amendment Request No. 2013-01, Supplement 1 Enclosure February 12, 2015 Page 3 of 11 the ONS emergency power requirements as seen when compared to the size of the CT4 underground transformer below.
The Keowee Hydro Station is connected to the Oconee Nuclear Station by two separate and independent routes. One route is a 4000 ft. underground 13.8 kV cable feeder to 12/16/20 MVA Transformer CT4. The second route is a 230 kV transmission line to the 230 kV switching station at ONS which supplies each unit's startup transformer.
When a loss of offsite power (LOOP) occurs, electrical equipment required to mitigate any concurrent event receives a signal to operate as required by the event. Valves are ready to stroke and pumps ready to start but power is not yet available. All nonessential equipment is separated from the safety bus. Both KHUs receive an emergency start signal and either begin accelerating to rated speed (if unit was in standby) or perform a load rejection and decelerate to rated speed (if unit was generating to the grid). Breakers then close on each of the emergency power paths when their respective KHU reaches 90% of rated voltage and frequency increasing (for an emergency start from stand still) or 110% of rated voltage and frequency decreasing following a load rejection (when emergency start occurs while supplying to the grid) within 23 seconds of receipt of emergency start signal.
After the start transient is complete, the KHUs operate at steady state. This is a frequency band of -1% to +3% of rated frequency.
Once the event is mitigated and ONS is in a stable condition, recovery can be initiated and systems configuration may be manipulated. Adding and removing large loads during recovery may cause the frequency to drift outside the steady state band but within +/-5% of rated frequency and return to within the steady state band within 10 seconds. Oconee Nuclear Station will be returned to the grid as soon as practical after offsite power becomes available.
Emergency Power System Testing The U.S. Nuclear Regulatory Commission (NRC) offices of Nuclear Reactor Regulation (NRR) and Analysis and Evaluation of Operational Data (AEOD) has previously performed reviews of the design and operational characteristics of the ONS emergency power system. Draft reports of these reviews dated July 8, 1996 were provided to ONS. An issue noted in these reports was the lack of a fully integrated functional test involving actual mitigating equipment equivalent in magnitude to the design basis required levels. Pre-operational testing of the ONS emergency power system included functional, integrated tests of each individual ONS unit with the onsite power sources. However, no integrated functional test simultaneously involving all three units' emergency power systems had ever been performed.
Duke Energy performed an integrated emergency power electrical system test designed to demonstrate the adequacy of the emergency power system from January 2 - 5, 1997, and subsequently transmitted a report (Emergency Power and Engineered Safeguards Functional Test Report) to the NRC by letter dated April 30, 1997. This test was a one-time integrated test of the ONS emergency power system that was performed during a three ONS unit shutdown.
Because of the ONS emergency power system design, integrated testing of one shut down ONS unit that connects to the onsite emergency power source cannot be performed without some impact on the reliability of emergency power to the other two operating units. The test documented the response of the emergency AC power system during several test scenarios.
The response from the power source through the auxiliary power system down to the safety
License Amendment Request No. 2013-01, Supplement 1 Enclosure February 12, 2015 Page 4 of 11 related 208V buses was monitored during this test. Various selected pieces of equipment including 4160V and 600V motors, a battery charger input, and motor operated valves (MOVs) were monitored.
The test exercised and challenged the emergency power and ESF systems. The scenarios selected for this test were worst case or bounding loading scenarios. The scenarios included both three-unit LOOP and Loss of Coolant Accident (LOCA) /LOOP scenarios. Both of the Keowee emergency power paths (i.e. overhead and underground paths), from the KHU unit through the respective path to each ONS unit's main feeder buses, were included in the test scope. Likewise, both modes of KHU operation (standby and grid generation), were included.
The emergency power system, engineered safeguards system and all other systems tested performed satisfactorily for this test. The test acceptance criteria (TAC) for each of the test scenarios were all satisfied. The ultimate function of the emergency power system, to deliver power to the required load such that cooling water can be delivered to the core to mitigate an accident, was demonstrated for the simulated design basis accidents. The loads used in these test scenarios were the actual ONS mitigating accident loads of the magnitude expected for a design basis accident. All safety related 4160V pumps challenged in an accident [i.e. high pressure injection (HPI), low pressure injection (LPI), low pressure service water (LPSW), motor driven emergency feed water (MDEFW), and reactor building spray (RBS)] were started during these tests. For the LOOP units, additional non-LOOP loads (one LPI pump for Units 1 & 2) were also added to the other hot shutdown loads for each case. Each safety related motor started and accelerated to rated speed well before the motor over current relay actuation setpoints. For the tests where the ESF MOVs were challenged, all MOVs stroked to their proper ESF position with no problems. The response of the system from the source down to the 208V buses was monitored for voltage, current, and power for each test. The data collected during these tests supports the conclusion that the emergency power system is able to perform its intended design functions.
The NRC concluded in their final report (dated January 19, 1999) to ONS that the January 1997 integrated tests were successful as one time tests that demonstrated the capability of the ONS emergency power system to perform its intended functions. The report acknowledges the design of the ONS emergency AC electrical power system is unique among U.S. nuclear power plants. That uniqueness is not only due to the use of hydroelectric generators as emergency power sources, but also extends to the site switchyard that doubles as an emergency power path from the emergency power sources, and to the serial nature of the design that seeks one capable emergency power source after another to power the entire complement of emergency loads. The size of the emergency power sources makes the emergency AC power system capable of powering a large complement of non-safety equipment (including RCPs) depending upon the scenario, the emergency power source, and the emergency power path available.
Subsequent to the January 1997 integrated testing, in 2004 and 2005, ONS replaced the mechanical governor with a digital governor on each KHU to address a frequency overshoot problem that was occurring during a KHU emergency start. Load testing performed after the modifications confirmed each KHU can achieve and maintain the required frequency within 23 seconds and be within steady state frequency requirements within 60 seconds. Technical Specification (TS) Surveillance Requirement (SR) 3.8.1.9, which is required to be performed every 12 months, verifies the ability to achieve and maintain the startup frequency range
(> 57 Hz and < 63 Hz) in 23 seconds. Selected Licensee Commitment (SLC) SR 16.8.5, which is required to be performed every 12 months, verifies the ability to achieve and maintain the
License Amendment Request No. 2013-01, Supplement 1 Enclosure February 12, 2015 Page 5 of 11 steady state frequency range (2> 59.4 Hz and <- 61.8 Hz) in 60 seconds. The proposed LAR incorporates the SLC SR into TS SR 3.8.1.9.
NRC RAI a Emergency Safety Features (ESF) Pump Performance: For induction motors, motor speed is directly proportional to power supply frequency. Please provide a summary of the evaluations or analyses performed to validate that flow rates and system pressures for ESF pumps are within the parameters assumed in accident analyses and plant design. Please provide a summary of the minimum margin of motor torque allowed over the pump load torque during the motor accelerating period.
Duke Energy Response to NRC RAI a Summary of the evaluations or analyses performed to validate that flow rates and system pressures for ESF pumps are within the parameters assumed in accident analyses and plant design.
The ESF pumps at ONS are the ECCS pumps (HPI and LPI), and containment heat removal pumps (RBS). Note that the Emergency Feedwater (Auxiliary Feedwater) system is not an ESF system at ONS. A calculation for each system has been performed to evaluate the consequences of a KHU supplying power with frequency within the limits. For each of the ECCS system analysis, the hydraulic analyses performed are based on the weakest pumps (of the nine pumps for both the HPI and LPI systems and additional allowed in-service testing degradation) and low KHU frequency to reduce the system capacity. The results of the analyses are used in the proprietary Chapter 15 accident analysis. RBS is not needed for immediate containment response. The same hydraulic conservatism was employed in analyzing the system. The result of the analysis have been provided to Duke Safety Analysis group to ascertain the containment response. Test Acceptance Criteria (TAC) have been developed that ensure the pumps are tested and operate above the analyzed condition. High KHU frequency is used to evaluate pump NPSH and establishing system pressure for input to other analyses.
Summary of the minimum margin of motor torque allowed over the pump load torque during the motor accelerating period.
When the KHUs are supplying power to the ONS safety system loads, they are not synchronized to the grid. Instead they are designed to operate in a single generator isolated mode. The maximum possible rate of loading which occur by design in two large blocks: a single unit's LOCA loads followed approximately 10 seconds later by two units' LOOP units concurrently. The maximum LOCA/LOOP loads are identified in UFSAR Table 8-1. No ESF pump motor stalls are predicted by electrical analyses, nor were any stalls seen during the 1997 Integrated ESF test. Since periodic performance of a test simulating the LOCA/three Unit LOOP event is only possible when all three Units are shut down, data was collected during the 1997 test and used to validate the ONS models for use in future design change evaluations.
The analytic method used for ONS analyses calculates the motor current and torque at each time step, based on calculated motor terminal voltage and frequency using frequency dependent motor and system models. Motor torque is compared to the required load torque curves to determine how quickly the motor is accelerated, based on the difference between the two. The following examples for one HPI, LPI and RBS motor illustrate the available margin to
License Amendment Request No. 2013-01, Supplement 1 Enclosure February 12, 2015 Page 6 of 11 overcurrent protection settings for safety motors. These were developed consistent with the 1991 IEEE Buff Book, "Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems."
Summary of Available Safety Motor Margin ESF Pump Motor Start Time Motor Stall Overcurrent Margin in (Load 90% Case)
Predicted During Relay Seconds LOCA or LOOP Setting Unit Start?
HPI 3.41 Sec No 5 Sec 1.59 Sec LPI 1.25 Sec No 8 Sec 6.75 Sec RBS 2.25 Sec No 5.4 Sec 3.15 Sec Motor Start Coordination Curve 10 o
-CO-5 Mme%ý Start (HK WM)
-MaDdrnum Starby CWW 4
I 0
Curran (snips) 1000 1200 HPI Motor Starting Coordination
License Amendment Request No. 2013-01, Supplement 1 Enclosure February 12, 2015 Page 7 of 11 Motor Start Coordination Curve I
ý=
toStad (LPt, WN)I 0
100 200 300 400 500 Current (aMIN) 000 700 800 900 LPI Motor Starting Coordination
License Amendment Request No. 2013-01, Supplement 1 Enclosure February 12, 2015 Page 8 of 11 Motor Start Coordination Curve 10 9
8 6
IS S-au(RBS, M)I 0
0 100 20D 3001 CWrut mnps) 400 50D 600 RBS Motor Starting Coordination All loads that will be started subsequent to the LOOP unit transient will be manually started by operations as needed to maintain the plant in a safe shutdown condition and are relatively small (Condenser Cooling Water (CCW) pump discussed below is -1 750HP), -1/10 of the accident block load requirements. The impact to frequency by these small transients will be minimal.
NRC RAI b.i MOV Performance: Operation of the KHUs at the high end of the frequency range may cause a higher differential pressure across MOVs as compared to nominal conditions. Operation of MOVs at lower end of the allowable frequency may negatively affect the MOV stroke time.
- i. Please provide a listing of critical valves, the required stroke times as considered in accident analyses, and the measured stroke times during the last surveillance.
License Amendment Request No. 2013-01, Supplement 1 Enclosure February 12, 2015 Page 9 of 11 Duke Energy Response to NRC RAI b.i MOV Critical Valves on an ES Actuation that supports the ESF Valve Unit-1 Unit-1 Unit-2 Unit-2 Unit-3 Unit-3 Close to Max limit Close to Max limit Close to Max limit open Close to open Close to open Close to open open open BS-1 13.95 19 16.42 19 15.53 19 BS-2 13.37 19 16.16 19 15.03 19 HP-24 13.21 20 12.64 20 13.03 20 HP-25 13.41 20 12.57 20 12.86 20 HP-26 12.88 16 13.59 16 13.31 16 HP-27 13.27 16 13.34 16 13.54 16 LP-17 29.21 36 29.32 36 28.78 36 LP-18 28.43 36 30.82 36 28.76 36 NRC RAI b.ii Please provide a summary of the analyses performed to demonstrate that sufficient margin exists between actual stroke times and maximum allowed stroke times to account for the minimum expected KHU frequency.
Duke Energy Response to NRC RAI b.ii Analyses to demonstrate that sufficient margin exists between actual stroke times and maximum allowed stroke times to account for the minimum expected KHU frequency have not specifically been performed. The system calculations and Generic Letter 89-10 program are based on degraded electrical conditions which bound the minimum expected KHU frequency. It should be noted that the valves will not be exposed to steady state frequency fluctuation since they would have already fulfilled the design function position prior to a KHU reaching steady state conditions. By inspection of the tabulated results presented above for the critical valves that have been identified the ratio of maximum limit to actual stroke time is greater than 115%. It can therefore be concluded that sufficient margin is available that a reduction in allowed frequency is not significant.
License Amendment Request No. 2013-01, Supplement 1 Enclosure February 12, 2015 Page 10 of 11 NRC RAI b.iii Please provide a summary of the analyses performed to demonstrate that MOVs will operate satisfactorily with maximum differential pressures.
Duke Energy Response to NRC RAI b.iii Duke Energy developed and implemented a program to provide for testing, inspection, and maintenance of motor operated valves (MOVs) to provide the necessary assurance that they will function when subjected to design basis conditions. This program was developed in accordance with the guidelines of Generic Letter (GL) 89-10.
ONS unit specific analyses were performed to document the minimum required thrust values and the maximum allowed thrust and torque values as well as other important force values required for proper field set-up for all GL 89-10 gate and globe motor operated valves, using the methodology defined in OSC-7184, "Generic Letter 89-10 MOV Calculation for Units 1, 2, and 3 Gate and Globe Valves at Oconee." In the analyses the maximum available thrust is taking the lesser of the force available at undervoltage condition, and spring pack limit and a comparison to the structural limit of valve and actuator. The maximum available torque is determined by taking the minimum of the torque available at undervoltage conditions, actuator, valve, and spring pack limit. The maximum allowable thrust is determined by taking the minimum of the valve structural limit, actuator structural limit and compensator limit.
ONS unit-specific analyses were performed to document the minimum and maximum allowable torque values for the proper operation of Unit 1, 2, 3 butterfly valves using the analytical results of the calculation methodology defined in OSC-5883, "MOV Calculation Methodology for Butterfly Valves at Oconee." In the analyses, the minimum (required opening and closing) torques are calculated by taking the maximum of the Seating/Unseating torque and the fluid dynamic torque. The available torque is determined by taking the minimum of the torque available at under voltage conditions, valve structural limit, actuator structural limit, gear box structural limit and spring pack limit. The maximum allowable torque is determined by taking the minimum of the valve structural limit, actuator structural limit and the gear box structural limit.
NRC RAI b.iv Please explain if the frequency recovery time of 10 seconds was factored into the above evaluations and the impact of voltage variations on available torque was considered.
Duke Energy Response to NRC RAI b.iv As described in the Emergency Power System Design discussion above, required mitigation equipment receives an actuation signal at the initiation of the event. When voltage and frequency requirements are met, power breakers are closed energizing all buses and all selected equipment begins to operate. Motor operated valves stroke during the initial power transient with their strokes completed before or shortly after power reaches its steady state conditions. The 10 second recovery time in the LAR is for transients during the steady state phase when frequency may drift outside the steady state band as a result of changing major system load. An example of such major system load change would be starting of a CCW pump to restore main condenser heat removal capability (a desirable action that, while not required for
License Amendment Request No. 2013-01, Supplement 1 Enclosure February 12, 2015 Page 11 of 11 event mitigation, does improve event mitigation capabilities). Since MOVs are not operating at this time no evaluation is required.
NRC RAI c The KHU frequency and voltage, immediately upon connecting to the plant safety busses, can be >_ 57 Hz and < 63 Hz and voltage >- 13.5 kV and < 14.49 kV. The accident signal is actuated at time zero and is typically present in the control logic of ESF pumps and MOVs when the onsite power systems is connected to the safety busses. Please confirm whether the wider range of allowable frequency and voltage coupled with a 10 second delay in voltage and frequency was considered in the performance capabilities of pumps and MOVs.
Duke Energy Response to NRC RAI c Plant safety buses are energized at 90% of rated voltage and frequency increasing for a KHU start from stand still or 110% of rated voltage and frequency decreasing for a KHU generating to the grid prior to receipt of emergency start. Within 23 seconds the frequency will be between 57 Hz and 63 Hz. This is consistent with ONS current licensing basis and is not changed by the LAR. The 10 second recovery time is for transients during the steady state phase when frequency drifts outside the steady state band as a result of changing system load.
The MOVs have already performed their design function by this time. Engineering judgment concludes that the mechanical system hydraulic response will not be significantly affected by a 10 second frequency transient.
NRC RAI d Please provide a summary of the changes in KHU loading as a consequence of operation at the extremes of the proposed frequency and voltage range.
Duke Energy Response to NRC RAI d There is no change in loading ONS equipment for this LAR. The emergency power system will operate as described above in the Electrical Power System Design discussion. After the start transient is complete, the KHUs operate at steady state. This is a frequency band of -1% to +3%
of rated frequency. Transients outside the steady state band would only occur when adding loads during the recovery phase of an event. Load variations due to power frequency variation within the specified steady-state band are insignificant compared to the available capacity of each of the KHUs.