ML12059A462
| ML12059A462 | |
| Person / Time | |
|---|---|
| Site: | Nuclear Energy Institute |
| Issue date: | 02/28/2012 |
| From: | Nuclear Energy Institute |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML120590853 | List: |
| References | |
| RIS 2011-12, Rev 1 | |
| Download: ML12059A462 (4) | |
Text
INDUSTRY REVIEW OF NRC PROPOSED DEGRADED VOLTAGE RELAY METHODOLOGY NEI Task Force on Degraded Voltage Analysis voltage at the DVR-monitored buses that dips during motor PURPOSE starting; however, for NPGS that utilize load sequencing, the pre-start bus voltage is readjusted between each load step to The IEEE Nuclear Power Engineering Committee (NPEC) the original level.
Working Group 4.7 1 and the Nuclear Energy Institute (NEI) have been working with the NRC staff to understand their In this evaluation, each of these methodologies is applied in current technical position related to degraded voltage relay actual power system analysis using the AC Auxiliary Power (DVR) protection requirements for motor starting voltage as System model of several existing NPGS designs that are described in Regulatory Issue Summary 2011-12, "Adequacy typical of the US nuclear industry. The impact on the DVR of Station Electrical Distribution System Voltages". setpoints, as predicted by both these methodologies, is Specifically, there is an industry concern that the guidance in compared to existing setpoints.
RIS 2011-12 is not being evaluated consistently at all Nuclear Power Generating Stations (NPGS) which could elevate the It should be noted that neither method is capable of potential of a nuclear safety event such as a delayed loss of demonstrating voltage recovery above DVR reset in order to offsite power (LOOP) with double load sequencing effects as prevent transfer to the emergency onsite power source. In discussed in NRC GSI-171 2. fact, voltage recovery must be shown to occur once loads have started and/or the transmission system has recovered using The purpose of this technical paper is to evaluate the technical further analysis using the methodology presented in the merits of analytical methodologies proposed by the NRC Offsite/Station Electrical Power System Design Calculation regarding the starting voltage requirements of safety related section of RIS 2011-12 and IEEE Std 741-2007, Annex A.
equipment at the DVR setpoint.
It is not the intent of this paper to infer that degraded voltage Scope protection is actually achieved during motor starting when the DVR minimum dropout voltage is set to correspond to the The scope of this evaluation is to determine the potential analytical value established by one or the other of these impact of two NRC proposed methodologies on current NPGS methodologies. In actuality, the very premise of using a DVR setpoints:
voltage relay to protect a power systems capability (capacity) to start motors is technically inadequate. Since a voltage relay Method 1: Referred to as the independent system method, only measures voltage (and not power system capacity), any determines the minimum voltage at the NPGS safety buses proposed analytical technique must assume some level of (i.e., safety division) required to support both safety related power system capacity, thereby violating the stated purpose of equipment starting and running voltage limits. Safety related the protective function.
buses are not connected to the transmission system and do not credit the effects of the non-Class 1E system: such as upstream impedance, voltage control, load changes, etc. This Background results in a fixed voltage (i.e., infinite source bus) at the DVR- The guidance for DVR protection has been developed through monitored buses during motor starting. three primary NRC documents (1) NRC issued Statement of Staff Positions Relative to Emergency Power Systems for Method 2: Referred to as the dependent system method, Operating Reactors - June 1977, (2) NUREG 0800 (Standard determines the minimum voltage that would be required at the Review Plan) Appendix A, BTP PSB-1 Revision 0, NPGS safety buses prior to motor starting to support both "Adequacy of Station Electric Distribution System Voltages,"
safety related equipment starting and running voltage limits; dated July 1981, and (3) Generic Letter 79-36, August 8, thus accounting for the effects of the non-Class 1E system 1979, Adequacy of Station Electric Distribution Systems while connected to the transmission network. This results in a Voltages. NPGS have implemented specific designs and 1
technical specifications to meet the intent of these NRC Responsible for IEEE Std 741, IEEE Standard Criteria for documents. In general, the NPGS licensing commitments are the Protection of Class 1E Power Systems and Equipment in specific to the plant distribution system design and time frame Nuclear Power Generating Stations in which the technical specifications were approved, based 2
NRC Generic Safety Issue 171, Engineered Safety Features upon one or more of the three NRC documents listed above.
Failure from Loss-of-Offsite-Power subsequent to a Loss-of-Coolant Accident NEI Letter Concerns associated with NRC release of RIS 2011-12, Rev 1, February 2012
2 The recent Component Design Basis Inspections (CDBI) have the worst case design basis event (e.g., LOCA, Safety resulted in NRC inspection findings that are requiring plants to Injection, Containment Isolation, Load Rejection).
perform DVR analyses that are not consistent with the plants Other components, such as static loads and MCC licensing basis. The evaluations result in scenarios that may contactors, were assumed to be bounded by the potentially lead to double sequencing of the NPGS current calculation of record.
engineering safety system loads.
Method 1, independent system, determines the DVR setpoint NPGS have implemented the NRC DVR requirements based minimum voltage (analytical limit) without crediting the on the steady state equipment ratings and not on the starting effects of the non-Class 1E system: upstream impedance of voltage requirements of equipment. The current NRC staff the non-Class1E plant or offsite electrical network, voltage position that degraded voltage protection includes motor control, load changes, etc. The intent of the independent starting at the DVR analytical minimum dropout is system method is to bound any postulated transient condition inconsistent with current industry practice in two respects: and assure required starting voltages. To accomplish this, voltage is fixed at the safety division measurement point prior
- First, many NPGS have addressed the transient to the start of any motor start sequence (sequenced loading or period of motor starting by demonstrating that the block loading). This represents the voltage seen at the safety DVR time delay is long enough to prevent spurious division at the moment of a motor start sequence, regardless of tripping during the voltage transients. the upstream voltage response. Utilizing the existing methodology and loading for each NPGS, the constant voltage
- Second, NPGS that account for starting voltage source was adjusted and a motor starting analysis was transients typically use the DVR relay maximum performed until the most limiting component received reset voltage value as an analytical ending condition sufficient starting voltage (terminal voltage). This method and establish an initial condition that will ensure that provides the base minimum voltage required at the DVR to this criterion will be met. This initial condition is support starting equipment limits. This method cannot predict often the subject of positive administrative controls, the required voltage at the DVR prior to the motor start alarms, and Technical Specification LCO action sequence, it can only determine voltage adequacy during the statements. The reset value is used because any sequence.
transient that causes a voltage dip below the dropout setpoint must recover to the reset value in order for Key assumptions of Method 1 include:
the electrical distribution system to stay connected to the grid. All transients that start below the reset 1. The non-Class 1E system will maintain the value initiate the DVR, and if they exceed the relay voltage throughout the sequence (i.e., infinite source time delay, will result in separation from the offsite capacity).
source of power. Hence, analyses below the reset value are moot unless they also address the delayed 2. Nominal load sequencing times are used, as LOOP and double sequencing effects as discussed in applicable.
NRC GSI-171.
Method 2, dependent system, determines the DVR setpoint Approach minimum voltage by accounting for the effects of the non-Class 1E system (e.g. non-Class 1E distribution elements, As previously identified, there are two NRC proposed offsite power grid, etc.) and non-Class 1E load change methods that will be evaluated to determine the effect on the (addition or removal). The intent of the dependent system NPGS DVR setpoint. These methodologies are being method is to present the least conservative transient condition considered to determine the impact to the current NPGS DVR (using operable grid capacity) which would assure required setpoints when considering starting voltage adequacy, not to motor starting voltages, given that this method is dependent on suggest changes to DVR protection. upstream conditions. Other upstream conditions (e.g.,
degraded grid capacity, negative operation of system voltage The following items were applicable for both methods: compensating equipment, spurious operation of any non-Class 1E component) would produce an even higher analytical limit.
- 1. Determined an analytical limit for DVR setpoint Utilizing the existing methodology and loading for each (dropout) at the safety division measurement point. NPGS, the Transmission voltage source was adjusted and a motor starting analysis was performed until the most limiting
- 2. Utilized the existing electrical model (i.e., ETAP component received sufficient starting voltage (terminal model) used in current DVR analysis at each NPGS. voltage). To accomplish this, the transmission system voltage was adjusted until the voltage at the DVR (prior to the motor
- 3. Determined the most limiting motor (defined as the starting sequence) was at the analytical limit. This method one which received the lowest required percent of starting voltage, down to the NPGS 480V MCC) for NEI Letter Concerns associated with NRC release of RIS 2011-12, Rev 1, February 2012
3 attempts to predict the minimum required voltage prior to the setpoint (dropout) will be slightly higher to account design basis event. for tolerances.
- 2) The maximum DVR reset value, including Key assumptions of Method 2 include: tolerances.
- 3) The change in existing DVR dropout setpoint
- 1. Automatic On-Load Tap Changers did not actuate required to satisfy these criteria.
during the event (i.e. were locked in place).
The independent system method shows an increase of the
- 2. The non-Class 1E systems were modeled per NPGS existing DVR setpoint from, 0.4% to 1.0% for five of the units current analytical basis. Examples may include: the while one plant showed no increase. Some of these NPGS non-class 1E load shed schemes, bus transfers, load may be successful in demonstrating reset of the relay, required application, and all balance of plant automatic for GDC-17 conformance (as historically interpreted);
functions (including unit trip delay). however, others may not have margin in the Offsite/Station Electrical Power System Design Calculations as discussed in
- 3. Typical transmission system impedance was not the RIS. As a minimum, five of the NPGS would require degraded below the operable impedance for the licensing amendments to change the Technical Specifications, offsite transmission system. (Note: This is a non- unless a more refined analysis can demonstrate lower motor conservative assumption with respect to capacity, starting voltage.
since operable transmission system impedance doesnt represent a degraded grid.) Impact on DVR Setpoints Method 2 (Dependent System)
Utilizing the existing methodology and loading of each NPGS, the transmission system voltage source was adjusted and a Plant NPGS 1 NPGS 2 NPGS 3 NPGS 4 NPGS 5 NPGS 6 motor starting analysis was performed until the most limiting Block Load component, down to the 480V MCC, received sufficient ECCS Load Block Block Start Load Sequence Loading Type Sequence Start Start w/ Bus Sequence w/ Bus starting voltage (terminal voltage). Once sufficient starting Xfer Xfer voltage was achieved, the corresponding pre-event switchyard DVR Dropout 1 91.4% 92.8% 91.9% 91.7% 88.7% 91.0%
voltage was recorded. Once the analytical limits were (minimum) established for each method, the required DVR settings were DVR Reset 2 94.7% 95.0% 96.6% 94.2% 91.5% 93.6%
(maximum) determined using tolerances and minimum ranges (between DVR Dropout 1 dropout and reset) applicable to the existing hardware for each (minimum) 98.7% 101.3% 102.5% 96.7% 92.7% 99.8%
NPGS. The resulting DVR dropout and reset settings for each DVR Reset 2 101.8% 103.9% 107.8% 101.4 95.5% 102.4%
NPGS are included in the results section for comparison to (maximum) existing DVR settings. DVR Setpoint 3
+7.3% +8.5% +10.6% +5.0% +4.0% +8.8%
Change (+/-)
Switchyard 4 104.0% 108.9% 104.9% 103.9% 104.8% 112.2%
Results Voltage Impact on DVR Setpoints Notes:
Method 1 (Independent System) 1) The DVR setpoint analytical limit that produces required voltage to all required loads. Actual DVR Plant NPGS 1 NPGS 2 NPGS 3 NPGS 4 NPGS 5 NPGS 6 setpoint (dropout) will be slightly higher to account Block Load ECCS Load Block Block Start Load Sequence for tolerances.
Loading Type Sequence Start Start w/ Bus Sequence w/ Bus 2) The maximum DVR reset value, including Xfer Xfer tolerances.
DVR Dropout 1 3) The change in existing DVR dropout setpoint 91.4% 92.8% 91.9% 91.7% 88.7% 91.0%
(minimum)
DVR Reset 2 required to satisfy these criteria.
94.7% 95.0% 96.6% 94.2% 91.5% 93.6% 4) The switchyard voltage required to produce new (maximum)
DVR Dropout 1 DVR Reset (maximum).
92.4% 93.8% 92.3% 91.7% 89.7% 91.7%
(minimum)
DVR Reset 2 All six NPGS show significant setpoint impact under the 95.7% 96.0% 97.0% 94.2% 92.5% 94.2%
(maximum)
DVR Setpoint 3 dependent system method (4 to 10.6% increases). All would
+1.0% +1.0% +0.4% 0.0% +1.0% +0.7% require a minimum switchyard voltage greater than currently Change (+/-)
predicted by the transmission system provider and, in two Notes: cases, greater than the maximum voltage typically allowed by
- 1) The DVR setpoint analytical limit that produces a transmission system (105%). The Offsite/Station Electrical required voltage to all required loads. Actual DVR Power System Design Calculations would not show acceptable results. Setpoints raised to these values would NEI Letter Concerns associated with NRC release of RIS 2011-12, Rev 1, February 2012
4 require operating the plant distribution at higher voltage than
- Since the DVRs provide no protective function while presently allowed by operating procedures. As an example they are timing out, their voltage setting is not a under light loading conditions, there would be a significant factor in the voltages to which the plant equipment concern of overvoltage conditions. This could result in may be exposed during that period and increasing overexcitation in motors and transformers in addition to other their voltage setpoint would have no benefit during potential concerns such as circuit breaker interrupting ratings this period.
as well as bus withstand (close and latch).
- Each evaluated methodology analyzes a scenario that Conclusion would be incredible at most plants (significant voltage degradation at the same instant as an accident A review of the results demonstrates that setting the DVR on signal) and fails to consider the credible scenario of a the basis of providing motor starting protection will likely voltage change later into the event when the main result in increased DVR setpoints. Either method is shown to generator trips.
potentially raise the DVR analytical limit, forcing an increase in the dropout setting and therefore the reset setting. This will
- NRC GSI-171 concluded that a LOCA with delayed reduce if not eliminate the margin between required LOOP was an improbable event. Raising the DVR switchyard operating voltage and anticipated post-accident setpoint will negate this conclusion, leading to voltage, increasing the probability of a LOOP from DVR greater possibility of a delayed LOOP with double timeout.
sequencing effects.
In the case of Method 2, the results demonstrate a significantly A more realistic scenario is to set the DVR to protect accident higher DVR setting will be required along with potential mitigating equipment from damage during steady state elimination of switchyard operating voltage margin.
conditions (not motor starting), which is the current licensing Operating in such a manner would be unreliable and would basis for most plants. NPGS typically use the DVR maximum not reflect actual system conditions expected during a reset voltage value or minimum anticipated voltage as response to a design basis event. The resulting minimum described in IEEE Std 741, as an ending condition following switchyard voltages required for DVR reset are unrealistic if automatic load sequencing or block loading of safety related not unattainable and would be counter to Transmission loads and establish an initial switchyard voltage to avoid DVR Operator criteria.
actuation. This ensures the grid provides sufficient capacity for starting required loads and for resetting the DVR; thus, Although each evaluated methodology is purported to ensuring continuity of a qualified offsite power source.
demonstrate that DVRs would provide protection against all eventualities regarding the voltage response of the non-Class Setting the DVR dropout to protect accident mitigating 1E electrical system, it fails to do so. To the contrary, each equipment from damage during steady state conditions will method makes many assumptions about the non-Class 1E avoid the risk of early offsite power separation associated with system voltage response in an attempt to quantify the higher DVR settings based solely upon motor starting voltage magnitude of voltage dip on the Class 1E system during motor demands.
starting. Making these assumptions violates the intended purpose of the DVR to provide a Class 1E protective function against all conditions.
Resetting DVRs in an attempt to provide perceived motor starting protection has no practical benefit and would have significant negative consequences for the following reasons:
- Increased DVR settings would significantly increase the probability of spurious LOOP and delayed LOOP with double sequencing effects as discussed in NRC GSI-171.
- Increased DVR settings would reduce or eliminate switchyard voltage operating bands for NPGS. If based on Method 2, this may result in a minimum switchyard voltage so high that it would result in overvoltages to plant equipment during light loading conditions.
NEI Letter Concerns associated with NRC release of RIS 2011-12, Rev 1, February 2012