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8/20/2019 Adv Trans Nuclear

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Advanced Transient Stability Analysis

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©Copyright EDSA Micro Corporation 2002

Application of EDSA’s Advanced Transient Stability in the Safety Related Studiesof the Nuclear Power Plant

1.0 Introduction

EDSA’s transient stability program can help power system engineers involved in Nuclear PowerPlant design and operation, meet NRC compliance standards and regulations. The United StatesNRC Regulations regarding Electric Power supply requirements are found in Title 10, of Code ofFederal Regulations, Appendix A, GDC 17. Similar requirements are found in IAEA Safety GuideNo.50-SG-D7 (Emergency Power Systems at Nuclear Power Plants).

The following sections will illustrate how to model and study the following nine important transientstability issues:

Study No.1 Bus Transfer AnalysisStudy No.2 Emergency Startup of Auxiliary Systems in Nuclear Power PlantsStudy No.3 Motor Sequencing in Nuclear Power PlantsStudy No.4 Failure of Automatic Voltage Regulators in Nuclear Power PlantsStudy No.5 Application of ULTC (Under Load Tap Changers)Study No.6 Modeling and Simulation of Over-Current RelaysStudy No.7 Advanced Dynamic Motor Starting of Induction Motors

Studies No.1 to No.4 are used to demonstrate the unique features of EDSA’s transient program inachieving analysis such as LOOP (Loss of Offsite Power) and LOCA (Loss of Coolant Accident).These studies are commonly carried out for NRC compliance and for the safe operation ofNuclear Power Plants.

2.0 The Transient Stability Program Interface

This Icon, displays the

Transient Stability Toolbar.

Transient Stability Toolbar.

Transient Stability Symbols Catalog.


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2.1 The Transient Stability Program Toolbar

Connectivity Check.

Run the Analysis.

View selected curves on

the Single Line Diagram.

View all curves.

Edit/View motor data.


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2.2 The Transient Stability Program Catalog

This symbol adds an Event to the Network.

This symbol adds a Generator Dynamic Modelto the Network.

This symbol adds a Motor Dynamic Model tothe Network.

To add Events, Generators and Motordynamic models to the network, simplyselect drag and connect the symbols overto the desired position on the single line

diagram. Once the element has beenattached to the diagram, the program willautomatically display its respectiveeditors. At this point proceed to enter the

data as required.


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2.3 The Transient Stability Calculation Control Interface

`

Important Note:

Ø Automatic Integration Adjustment Setting:

This setting is used to speed up the analysis in systems that are either very large and/or havenumerous events taking place during the course of the simulation. The program skips overthose sections that display steady-state behavior, and focus its simulation power on thosepoints where the events are occurring. If selected, the user must specify the minimum andmaximum steps to be used.

To initiate a Transient Stability Study,

select this icon.

Real Time Simulation Interface.

Type the Total Simulation Time,using the keypad.

Select whether or not to use Automatic Integration Step Adjustment.

Specify the Integration Step inseconds.

Set the Reporting Time Step as amultiple of the Integration TimeStep.

Specify the ConvergenceTolerance for the Analysis.

Specify the Maximum number ofiterations to be used.

This button Starts and Pauses thesimulation process.


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Important Notes:

Ø Select Components to display: This setting allow the user to select which componentswill have an output graphical report, once the simulation is completed.

Ø Set Zone Frequency: This setting allows the user to define the nominal frequencies onmechanically coupled systems (M-G Sets).

Ø Set Printing Selections: This setting allows the user to select which components will beincluded in the output text report.

ØView Text Results: This command produces an output text report. This option is onlyavailable after the simulation has been successfully completed.

Ø View Graphic Results: This button switches over to the graphical view interface, wherethe user can find all the elements that were selected in the “Select Components toDisplay” setting. This option is only available after the simulation has been successfullycompleted.

This button producesa text report of theanalysis.


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3.0 Preparing the Subject Files for a Transient Stability Analysis.

Step 1.

Once a viable system model has been created and checked for errors, proceed to run an Advanced Power Flow analysis in order to establish the initial conditions for the system. If faultsother than 3-phase balanced faults are to be simulated as transient disturbances, then a shortcircuit analysis must also be run. Before, running a transient analysis, one should examine thatthe power flow result and make sure system conditions are within acceptable steady stateoperation limits. I.e. voltages are for example within 95-105%, there are no severely overloadedlines/cables/transformers, the generators are not producing or absorbing reactive power outsidetheir reactive power capability, the active power generation is within the turbine capability, etc.

If the Transient Stability study requires motor starting analysis, you must first run an AdvancedPower Flow Motor Starting analysis, prior to engaging the Transient Stability program. This isdone so that motor characteristics (equivalent circuit, mechanical load, etc.) can be read by theTransient Stability program.

Reset the started motors back to the “Steady State” mode, and re-run the Advanced Power FlowProgram, so that the initial conditions of the network can be re-established.

Select Start Simulation toproceed with the analysis.

Once the simulation has been completed, theresults are shown in the 4 quadrants of this screen.

Select from the pick list the element tobe displayed in its respective quadrant.


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Step 2.

Enter the dynamic models for the generators, exciters, turbines, governors, induction machines,relays, etc. The preferred approach is to first enter the generator data by itself, and execute aTransient Analysis run without applying any disturbance. Under these circumstances one mustverify that all parameters (voltages, power flows, angles, etc.) remain constant as function of time.Next, add the data for controls (exciters, governors, etc.). It is wise not to use complex controlmodels when sufficient data for it is not available. It is better to use simpler model with good datathan having complex model and bad data. Garbage in, Garbage out!

Step 3.

Run the Transient Analysis once for a second time without applying any disturbance. Again, allmonitored quantities should remain constant over the time

Step 4.

Now that the data has been tested, the desired disturbance(s) can be entered and simulated.

Carefully examine the result of the transient program by plotting the monitored parameters.Paying close attention to the tabular report, check and correct modeling deficiencies as required.For example, if a fault has been applied, then, the generator field voltage should increase whenterminal voltage has fallen (provided an exciter has been defined for the machine). If machine hassped up, then, the mechanical power should decrease (provided a governor has been defined forthe machine).

4.0 Study No.1: Bus Transfer AnalysisEDSA File: Bus-Transfer.axd

The first study deals with the impact of a bus transfer operation on the stability of a user-definednetwork. This example is valid for both fast and slow bus transfers, all the user has to do in order to

accommodate the transfer speed, is to adjust the reconnection time of auxiliaries. The network inquestion is shown below in Figure 1.

Event 1.Loss of “UTIL” Generator.Supply line trip at 0.5 sec.

Event 2.Switch Closes 1.0 seconds later.

Figure 1.


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We will assume that initially, the auxiliaries are supplied from the UTIL generator (shown on the leftside of Figure 1). Half a second later we will simulate a loss of generation by means of a “line trip”command in the EDSA transient program. A second after the loss of supply to the auxiliaries, we willclose a switch between BUSA and BUSB.

4.1 Modeling the “Line Trip”

4.2 Modeling the Tie Breaker (Branch Addition)

Step 1.Double-click on the UTIL

Generator event.

Step 2. Add a Branch Tripping

event as indicated here.

Step 1.Double-click on the BUSA event.

Step 2. Add a “Branch Addition”event as indicated here.


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4.3 Summary of Generators in the System

4.4 Summary of Events in the System

4.5 Summary of Motors in the System


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4.6 Running the Analysis

Step 1.

Select “Start Simulation”.

Step 2.Review the response

of the Generator here.

Step 3.Review the responseof the Motors here.

Transient Response for BUSA.

Transient Response for BUSB.


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5.0 Study No.2: Emergency Startup of Auxiliaries in a Nuclear Power PlantEDSA File: EMERGENCYSTART.axd

The emergency startup of the auxiliary equipment in a nuclear power plant is of paramount

importance to its safe operation. In this example, the emergency supply is normally a Dieselgenerator. It is important to determine the capability of the diesel generator in successfullystarting all of the critical auxiliaries. Table 1, shows the auxiliaries that will be startedsimultaneously. It has been assumed that PUMPA is already running the rest of induction motorswill be energized at half a second intervals throughout the simulation. Figure 2. illustrates theplant configuration for this study.

Motor Motor Operation Mode Stop/Start

No. Bus ID Time(sec.)

1 PUMPA Running 999.00

2 PUMPB Starting 0.50

3 BCFG Starting 0.50

4 HPC2 Starting 0.50

5 PUMEM Starting 0.50

6 WTR Starting 0.507 HPC Starting 0.50

8 M100 Starting 0.50






Table 1. Induction Motor Startup Time Table

Diesel Generator.

PUMPA is in the running mode.

Figure 2.


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As it can be seen from above screen-capture, a severe voltage and frequency drop would beexperienced if an emergency startup, as the one described in this example, were to take place.This simulation would indicate to the user that a simultaneous startup is not a viable option.

6.0 Study No.3: Motor Sequencing in a Nuclear Power PlantEDSA File: MOTORSEQUENCE.axd

In this example, the same loads that were energized at once in Study No.2 will be sequentiallypowered in order to minimize the impact on the Diesel generator. Since this source is consideredto be weak, (relative to a utility supply bus), an optimum sequence must be determined in order toensure that all the safety related loads can be powered first, followed by the non-critical ones.The loads will be energized according to the schedule shown in Table 2. It is also assumed inthis case that PUMPA is running prior to the sequencing of the other loads. The single linediagram is identical to the one shown in Figure 2 of section 5.0.

Generator Response.

Starting response for PUMPB.


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No. Motor Bus ID Motor Operation Mode Start Time

1 PUMPA Running N/A

2 PUMPB Starting 5.50

3 BCFG Starting 2.504 HPC2 Starting 2.50

5 PUMEM Starting 2.50

6 WTR Starting 2.50

7 HPC Starting 2.50







Table No.2 Example of a sequential startup schedule




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Generator Response.

Equipment Response.


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7.0 Study No.4: Failure of AVR in a Nuclear Power PlantEDSA File: AVRFAILURE.axd

In some cases Nuclear Power Plants may require studies involving the failure of Automatic

Voltage Regulators, following a LOOP - LOCA simulation. EDSA’s transient stability programmakes this kind of study possible with minimum effort. The network to be studied is identical tothe one shown in Figure 2, section 5.0. Essentially we still have a plant being supplied solely bythe Diesel generator. Figure 3, below indicates how to access the AVR that corresponds to theDiesel generator under study.

As shown in Figure 3, the exciter control for this particular example is capable of controlling thefield voltage by means of a step function. The user can take advantage of this feature, and definevalues that can simulate a failure of the device. In this particular case, the following data enablesthe system to simulate an AVR control failure at 3.5 seconds into the simulation. This isaccomplished by setting the device as follows:

Time 1: 3.5 secondsTime 2: 100 seconds (The total simulation period will be 20 seconds)EFD1: 2.2

This first step indicates that the AVR output will remain constant throughout the simulation period,which will be set to a value of 20 seconds. The remainder of the steps are set at 100 secondsand EFD=0 so that they are simply ignored.

In this example, it will be assumed that the motors listed in 7.2 will start simultaneously 1 secondinto the simulation.

Step 1.Double click on theGenerator Dynamicsymbol.

Step 2.Invoke the AVR editor andcomplete the data as shownhere.

Figure 3.


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Generator Response

Voltage drop caused by simultaneousmotor startup 1 second into the simulation.

After 3.5 seconds, EFD remains constantat 2.2 per unit as specified in Figure 3.


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8.0 Study No.5: Application of Under Load Tap ChangersEDSA File: testultc.axd

In this example, we will illustrate how to model an Under Load Tap Changing Transformer

controlled by a servomotor. The basic idea is illustrated in Figure 4, below. This figure assumesthat the tap changer is located on the secondary windings of the transformer.

In order to designate a transformer as a ULTC branch, first it must be defined in the databaseeditor as a voltage control device. Once this is done, a ULTC device can be assigned to thecontrolled bus, from the Transient Stability program interface. The network under study is shownin Figure 5 below.

Servo Motor

Transformer Primary

Transformer Secondary

(Variable Tap)

Servo Motor is fedfrom a different bus.

Figure 4.

ULTC Controlled Bus(Event)

ULTC Servo is fed fromhere (Bus “SERVO”)

Figure 5.

ULTC Transformer


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8.1 Preparing the ULTC Control Transformer(s)

Notice from the above screen-capture that the taps have been placed on the primary windings ofthe transformer.

8.2 Summary of Disturbances in the System

The following table summarizes the disturbances that will be dealt with in this example. Theobjective is to determine whether the ULTC can appropriately compensate for the voltageexcursions caused by these events.

Step 1.

Double click on the controltransformer. In this case

branch 7 – 10.

Step 2.Select the Load Flow tab, andactivate the “Automatic Tap

Adjustment” feature by selecting

“On”. Also specify the tap location.

Step 3.Select the “Controlled Bus”

command.

Step 4.Specify the required control

parameters and press “OK”.


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8.3 Defining the ULTC characteristics at Bus 7

Enter the characteristics of the

ULTC as indicated here.


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Results for ULTCControlled Bus 7.

Notice the effect of the motorizedtap changer as it brings the voltageback up to 1.02 Pu following thedisturbances listed in 7.2.


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9.0 Study No.6: Modeling and Simulation of Over-Current RelaysEDSA File: OCRELAY.axd

This example is based on the network shown in Figure 6, below.

Active and Reactive Power Flowacross the ULTC Transformer

(Branch 02 – 7).

Three-Phase to Ground fault@ 1.0 sec. lasting 0.1 sec.

Over-current Relay actingupon line CCC138 – BBB138.

Figure 6.


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In this network, a three-phase to ground fault has been placed on bus CCC138. The fault willtake place 1 second into the simulation, and will last for 0.1 seconds. An over-current relaydesigned to trip line CCC138 – BBB138 has been placed on bus BBB138. The objective is tohave the relay trip the faulted line, and study the effect of this event on the overall stability of thenetwork.


9.2 Defining the Fault at Bus CCC138

Step 1.Double click on the event

shown on bus CCC138.

Step 2.Define the fault as indicated

here and press “OK”.


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9.3 Defining the Over-Current Relay

9.4 Summary of Events in the System

Step 1.Double click on the event

shown on bus BBB138.

Step 2.Define the branch’s “To Bus ID”, “CircuitNumber”, and “Breaker Operating Time”

as indicated here.

Step 3.Finally, enter the tripping curve of therelay, by either selecting one from thedatabase (From Relay DB), or selecting a

relay associated to a pre-existing PDCstudy performed on this branch (FromPDC). In this example, the option “From

PDC” was selected. Select “OK”.


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Transient response for all

generators in the system.

Active & Reactive power flows across

tripped branch CCC138-BBB138.


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10.0 Study No.7: Advanced Dynamic Motor Starting of Induction MotorsEDSA File: MOTORSEQUENCE.axd

The EDSA Advanced Transient Stability Program and Advanced Power Flow programs allow the

user to model multiple motors using various starting methods. The available starting methods are:

1. Full Voltage

2. Full Voltage Square with three types of controls. These controls are function of Time, %Motor Voltage, %Motor Speed.

3. Wye-Delta with three types of controls. This controller switches the motor from starconfiguration to delta configuration. These controls are function of Time, % MotorVoltage, %Motor Speed.

4. Autotransformer with three types of controls. This controller modifies the tap settingwithin two steps. These steps are controlled as function of Time, % Motor Voltage,%Motor Speed.

5. Part winding with three types of controls. This controller modifies the tap setting withinfive steps. These steps are controlled as function of Time, % Motor Voltage, %MotorSpeed.

6. Series Resistance with three types of controls. This controller reduces the amount ofseries impedance within five steps. These steps are controlled as function of Time, %Motor Voltage, %Motor Speed.

7. Series Reactance with three types of controls. This controller reduces the amount ofseries reactance within five steps. These steps are controlled as function of Time, %Motor Voltage, %Motor Speed.

8. Shunt Capacitance with three types of controls. This controller reduces the amount ofsupplied reactive power within five steps. These steps are controlled as function of Time,

% Motor Voltage, %Motor Speed.

9. Solid State Voltage Control with three types of controls. This controller reduces the tapsetting within five steps. These steps are controlled as function of Time, % Motor Voltage,%Motor Speed.

10. Solid State Current Limit – this controller set’s the motor’s current limit to the current Puvalue specified.

11. Solid State Current Ramp- this controller increases the current gradually from the firstvalue to the end value over the Tramp (Time 2 – Time 1).

12. Solid State Voltage Ramp – this controller increases the voltage gradually from the firstvalue to the end value over the Tramp (Time 2 – Time 1).

13. Solid State Torque Ramp – this controller increases the torque gradually from the firstvalue to the end value over the Tramp (Time2 – Time 1).

14. Variable Frequency Drive – this controller provides higher starting torque. This meansthat the motor starting current is higher but the design of the VF is trade off between therequirement of high starting torque and motor withstand capability of carrying highercurrent.


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Some examples of starting methods are listed here:

Shunt Capacitor

Shunt Capacitor controllers are used toreduce the amount of supplied reactivepower in up to five steps.

The time is related to the instance thecapacitor is switched on line. If thecapacitor to be removed off line at a

particular time, then that time must beenter with KVAR equal to zero.

Current Limit

This controller is designed to limit theLocked rotor current to a value definedby the user.


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Current Ramp

This controller gradually increasesthe current from Point 1 to Point 2 byinterpolating a linear Ramp functionbetween the two. For the exampleshown here the current will increasegradually from 3.0 to 5.0 pu within atime frame ranging from 0 to 4seconds.

Voltage Ramp

This controller gradually increasesthe voltage from Point 1 to Point 2 byinterpolating a linear Ramp functionbetween the two. For the exampleshown here the voltage will increasegradually from 0.7 to 0.9 pu within atime frame ranging from 0 to 4seconds.


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Variable Frequency Drives

Variable frequency (VF) starters canbe used to assist motor starting in anumber of different scenarios. Onecommon use is where a highstarting torque is required, withouthaving to replace the existing motordesign.

In this example, a frequency of 0.8pu (48 Hz) is applied for up to 3

seconds. Then, the frequency ischanged to 0.9 pu (54 Hz) for anadditional 2 seconds and finallythe frequency is set to nominalvalue at 5 seconds.

The following graphs show motor starting performance differences between some of thestarting methods used in EDSA

Chart 2 - Torque

-458

2365

5187

8009

10832

13654

l b f -

f t

Time in Seconds

0 1 2 4 5 6 7 9 10 11 12

Motor

Load

Chart 2 - Torque

-459

2372

5203

8034

10865

13696

l b f -

f t

Time in Seconds0 1 2 3 5 6 7 8 9 10 11

Motor Load

Part Winding Motor Starting Series Resistance Motor StartingTime (Sec) %Torque (lb-f t ) Time (Sec) %Torque (lb-f t )

0 110.25 0 110.25

0.1 44.45 0.1 60.96


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Chart 2 - Torque

-463

2390

5242

8094

10947

13799

l b f -

f t

Time in Seconds0 2 5 7 9 12 14 16 19 21 24

Motor Load

Chart 2 - Torque

-459

2371

5201

8031

10861

13691

l b f -

f t

Time in Seconds0.00.9 1.7 2.6 3.5 4.35.26.06.97.88.6

Motor

Load

Solid State Current Limit Motor Starting VFD Motor Starting TimeTime (Sec) %Torque (lb-f t ) Time (Sec) %Torque (lb-f t )

0 110.25 0 110.25

0.1 34.15 0.1 103.04

For this example, we will resort to the same file used to illustrate the sequential startup of Auxiliary equipment in a Nuclear Power Plant (refer to Study No.3, section 6.0). This time focuswill be placed on how to set selected motor(s) in the starting position, and how to evaluate theresults. As you can recall, in this example PUMPA is running, while the rest of the motors arebeing sequenced-in as indicated in Table No.2 and section 6.2. In order to illustrate how motorsare treated in Transient Stability Starting Analysis, we will focus our attention on PUMPB,understanding that al the rest of the motors have been treated in a similar manner.

PUMPA is in the running mode.

Figure 7.

PUMPB will be usedto illustrate the study.


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10.1 Setting a Motor in the Starting Position

Step 1.Double click on the Motor

Dynamic Symbol.

Step 2.Modify the data as required.

Remember that when the AdvancedPower Flow Motor Starting isexecuted, this information isautomatically passed-on to the

Transient Stability Motor editor.

Step 3.Ensure the “To Be Started”

has been selected here.

Step 5.

Select “OK” when finished.

Step 4.Remember to specify the

Starting time here.


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Complete Results for PUMPB.

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