Simulation of Ungrounded Shipboard Power Systems in PSpice

Haibo ZhangIEEE Student Member

Karen L.ButlerIEEE Member

N.D.R.SarmaIEEE Member

Power System Automation LabElectrical Engineering Department

Texas A&M UniversityCollege Station, TX 77843-3128

AbstractNavy shipboard power systems(SPSs) have differentcharacteristics when compared to utility power systems.To conduct system studies on SPSs, an effectivesimulation tool is required. PSpice is a very robustanalog/digital circuit simulator, but it is rarely used inpower system studies. This paper presents a PSpicemethodology for modeling SPSs. Transient simulationresults are also presented.

I. Introduction

Ungrounded delta type system configuration is widelyused in U.S. Navy shipboard electric power distributionsystems with the objective of providing continuity ofpower supply. To conduct system analysis, such as faultanalysis, system reconfiguration/restoration, etc. forShipboard Power Systems (SPSs), an effective simulationtool is needed. In the literature, there is no informationdiscussing the appropriate simulation tool for simulatingSPSs. PSpice[1,2,3] is a popular simulation tools used foranalog/digital circuits and power electronics. In thispaper, modeling and simulation of SPSs using PSpice arediscussed.

In section II, SPSs are briefly described. Section IIIexplains modeling of various components of SPS usingPSpice. The details of the test system and the case studiesconducted using PSpice are presented in section IV.Concluding remarks are given in section V.

II. Shipboard power systems (SPSs)

Navy ships use three phase power generated anddistributed in an ungrounded delta configuration, whichis used to ensure continuous operation of the powersystem in the presence of a single phase to hull fault [4,5].The voltages are generated at levels of 450 volts a.c. at 60

hertz. The most popular topology used in SPSs is a ringconfiguration of the generators which allows anygenerator to provide power to any load. This feature is ofgreat importance in order to ensure supply of power tovital loads if an operating generating unit fails.Generators are connected to switchboards called asgenerator switchboards which are composed of one ormore switchgear units and breakers for various deviceslike generators, loads, and load centers. These generatorswitchboards are located close to their associatedgenerators. Bus tie circuits that interconnect the generatorswitchboards allow for the transfer of power from oneswitchboard to another.

The other components of SPSs are load centers, powerdistribution panels, bus transfer units, transformers, andcable used for delivering power to the loads. Load centerdistribution, which is a modification of radial distribution,is used below the generator switchboard level. One ormore load centers are connected to each generatorswitchboard to supply power to load concentrations invarious areas of the ship. The load centers supply powerto power panels or individual loads, either directly or viaautomatic bus transfers (ABTs) or manual bus transfers(MBTs). For vital loads, two separate sources of power(normal and alternate) are provided via ABTs and MBTs.Loads are fed directly from generator switchboards, fromload centers, or from power distribution panels. The loadsrequire power at 440, 115, and 4,160 volts at 60 hertz,and 440 and 115 volts at 400 hertz. Transformers areused to convert supply voltage from one level to another.The interfaces used between 60 hertz and 400 hertzsystems are either motor-generator sets or solid-statefrequency converters (SSFCs).

Although SPSs appear simpler than their commercialterrestrial counterparts, they have unique characteristicsthat demand special attention during designing and

performing other kinds of investigations, such as faultstudies, reconfiguration, etc.

To conduct system studies, it is required to modelvarious components of the SPSs and conduct simulationstudies. In the present study, PSpice has been used tosimulate SPS. Initially various components of SPS aremodeled using the basic elements available in PSpice.Later these modules are used to build a simple test systemto conduct simulation studies. In the current studies,protective devices are not modeled.

III. Modeling of SPSs using PSpice

PSpice, a computer software developed by MicrosimCorporation, is considered to be one of the most powerfuland popular analog circuit simulators[1]. PSpice useslinear, discrete, passive and active electrical elements fornetwork simulation. Here PSpice is used to simulate asimple test system which includes components commonto SPSs. Following sections will show how thesecomponents are modeled in PSpice.

Since PSpice does not allow the floating condition,very large resistors (107 ohms) are used to avoid thefloating condition whenever required.

A. Generator model

In PSpice, the voltage sources can be connected indelta which are used directly to build the generator sets.The generator set is modeled in PSpice as shown in Fig.1 with the shunt resistors (to avoid floating condition)and source impedance. The parameters of the generatormodel include: voltage magnitude, frequency, sourceimpedance of each phase, shunt impedance of eachphase.

Fig.1 Generator model

B. Transformer model

Three-phase delta-connected transformer banks aremodeled as three single-phase transformers connected indelta. PSpice provides modeling of a single-phasetransformer with only primary and secondary inductance.Hence the primary and secondary resistance are added tothe basic transformer model provided in PSpice. The

transformer model is shown in Fig.2. The parameters fortransformer model include: primary/secondaryinductance/resistance, coupling coefficient.

Fig.2 Transformer model

C. Cable model

Using PSpice, cables of SPSs are modeled as a lumpedπ-model whose component values are calculated usingthe distributed resistance r, self-inductance ls, self-capacitance cs, mutual inductance lm and mutualcapacitance cm. The values of these parameters depend onthe type of the cable. The parameters of the cable modelinclude: distributed r, ls ,lm, cs, cm, and the cable length.The cable model is shown in Fig 3.

Fig.3 Cable model

D. Three phase switches

PSpice provides two types of single-phase time-controlled switches, one initially closed and anotherinitially open, which are used to model circuit breakersand switches. In SPSs modeling, two types of three phaseswitches are modeled. One is normally closed andanother is normally open. The parameters of the threephase switches include the open / close time for eachphase, transient time, the resistance across the switchwhen it is open or closed. This switch model is also usedto simulate a fault.

E. ABT/MBT model

Automatic Bus Transfer switch (ABT) or Manual BusTransfer switch (MBT) provide the normal and alternatepath to vital loads. The bus transfer switch model isshown in Fig.4. It is composed of one 3-phase normallyclosed switch 1 (connecting node 1 and 2 as normal path)

Lab

Lbc

Lac

and one 3-phase normally open switch 2 (connectingnode 1 and 3 as alternate path). When the path throughnode 2 is not available to node 1, switch 1 and 2 willexchange their status at the same time. The parameters ofthe ABT/MBT model include the switch on/off time,transient time, the resistance values in open and closedstates.

BusTransferSwitch(ABT orMBT)

Load

normal path alternate path

(a) ABT/MBT function (b) ABT/MBT model

Fig. 4. ABT/MBT model

F. Switchboard, load center and power panel

Generator switchboards are composed of one or moreswitchgear units and breakers for various devices likegenerators, loads, and load centers. Bus tie switches thatinterconnect the generator switchboards allow for thetransfer of power from one switchboard to another. Thegenerator switchboard is modeled as shown in Fig. 5.The parameters of the switchboard include theparameters of each switch. For load center and powerpanel, the model is similar to that of generatorswitchboard but without the bus tie switches.

Fig 5. Switchboard model

G. load model

Loads in SPSs are either rotating or non-rotating. Thenon-rotating loads are constant, passive auxiliary loadssuch as lighting, heaters, etc. These kinds of loads aremodeled as combinations of resistor and inductor,depending on their respective power factors, voltage andpower ratings.

In SPSs, induction motor is the most common rotatingtype load that is used for water and fire pumps,compressors, and other applications that may consist ofeither continuous or intermittent operation.

Based on the circuit equations, a double cageinduction motor model [6] is implemented usingamplifier, difference junction and integrator, etc.

Accordingly, the configuration of induction motor modelis shown in Fig.6.

3 phase->dq axis

statorflux

rotorflux

stator vs.rotor flux

vavbvc

vqs

vds

torque loadtorque

dq axis->3 phase

iqs

ids

iqr

idr

T l

T

ω r

iaibic

iqs ids

Fig 6. Configuration of Induction Motor

The parameters needed for the induction motor modelare: stator resistance, stator leakage inductance,magnetizing inductance , number of pole pairs,synchronous speed, bigger/smaller cage rotor resistance,bigger/smaller cage rotor leakage inductance.

IV Test system and case studies

A. Test system

The radial test system shown in Fig. 7 is simulatedusing components models developed in the previoussection. The component specifications are as follows:

• Generator G: 12 KVA, 450 V line-to-line, 60 Hz• Transformer T: 5 KVA, 450/115V line-to-line, ∆-∆connected

• Load L1: 2 kW, PF = 1.0, 450 V line-to-line,balanced, ∆-connected

• Load L2: 5 KVA, PF = 0.8, 115V line-to-line,balanced, ∆-connected

• Induction motor M1: 5.63 HP, 450 V line-to-line,synchronous speed = 3600 rpm

• Cable C1: 3-φ, 350 MCM, length = 12.2 meters• Cable C2: 3-φ, 6 AWG, length = 20 meters• Cable C3: 3-φ, 6 AWG, length = 12.2 meters• Cable C4: 3-φ, 6 AWG, length = 12 meters.

M

450 V line-to-line, 60 Hz12 KVA delta-connected generator G

generator switchboard GEN

switchboard SB

cable C1

FAULT POINT F

cable C2

cable C3

450/115 V5 KVA delta-delta

transformer T

load L2:5 kVA, PF = 0.8

load L1:2 kW, PF = 1.0

induction motor M1:5.63 HP, PF = 0.883

cable C4

Fig.7 The test system

… …

2 3

1 Load

From generator

Bus-tiebreaker

Bus-tiebreaker

To loadcenters/powerpanels/loads

Three different scenarios are developed to study thegiven test system under fault conditions.

∗ Case I: Normal system operation (no fault)∗ Case II: Single-Line-to-Hull(SLH) fault between

phase A and hull in the middle of cable C1∗ Case III: Line-to-Line (2L) fault between phase B

and C in the middle of cable C1

B. Simulation results

In this section, the results of the simulations will bediscussed. Each case is illustrated with waveforms.PSpice adjusts the time step for simulation automatically,but one can define the maximum time step called as thestep ceiling. For all the simulations in current studies, thestep ceiling is chosen as 100µs.

Case I: The waveform of induction motor phase acurrent is shown in Fig. 8. The speed and torquewaveforms for the motor are shown in figs. 9 and 10respectively. The steady state values are obtained asexpected (torque = 11.17 N-m, Ia = 8.17 A )

Case II: In this case the single line to hull fault is stagedat 2.060s in the middle of cable C1 at point F. The line-to-line and line-to-hull (Vah ) voltage waveforms for thiscase are as shown in Fig. 11. It can be seen that there is alittle spike in the waveforms of line-to-line voltages Vbc

and Vca at the fault time. The steady state values of line-to-line voltages are the same as in case I. This illustratesthat an ungrounded delta configuration ensures normalvoltage supply even in the presence of a single line tohull fault. This behavior also explains why navy shipdesigners adopted the delta configuration.

The waveform of induction motor phase a current isthe same as in case I. There is no change for theinduction motor phase a current even the single line tohull fault occurred in phase a.

Fig.12 shows the waveforms of the induction motortorque. At the fault time, there is a little transient.

Fig.13 gives the waveforms of transformer primaryand secondary line to line voltages Vab and Va’b’. The lineto line voltages remain same with an exception that at thefault time, there is a small spike on both waveforms.

Case III. In this case, the line to line fault is staged at2.060s between phase b and c in the middle of cable C1.

Fig.14 gives the waveforms of line-to-line voltagesVab, Vbc, Vca at the fault point F.

Fig.15 gives the waveform of induction motor speed.When there is a L-L fault, the speed decreases andbecomes zero(standstill state) at about 1.6 seconds afterthe fault. From this moment, the induction motor

changes its direction of rotation and operates in brakingmode, which leads to overheating of the motor. Normallythe stator is disconnected before the braking mode. Sincewe haven’t modeled the protective devices here, it’s notshown in the simulation result. From Fig. 16, it can beseen that the torque begins to oscillate at fault time. Thephase a current of the motor is as shown in Fig. 17 whichindicates that it increases after the fault.

From the results in case III, it can be seen that L-Lfault is a serious fault for SPSs, it influences the systemvoltages, currents and also the load characteristics.

Fig.8 Induction motor phase a current (caseI)

Fig.9 Induction motor speed (case I)

Fig.10 Induction motor Torque(case I )

Fig. 11 Line-to-line voltages and line to hull voltage Vah

at point F ( Case II)

Vab Vbc Vca

Vah

Fig. 12 Induction motor torque(case II)

Fig. 13 Transformer primary/secondaryvoltages Vab and Va’b’(case II)

Fig. 14 Line-to-line voltages at point F (case III)

Fig.15 Induction motor speed (case III)

Fig. 16 Induction motor torque (case III)

Fig. 17 Induction motor phase a current (caseIII)

V. Conclusion

SPSs components are modeled in PSpice. A testsystem containing typical components of ShipboardPower Systems (SPSs) has been simulated using PSpice.Different case studies have been discussed. PSpice has anexcellent graphical interface for building complex SPSsand flexibility in establishing monitoring points andprocessing output data. However, PSpice must be givenbasic circuit parameters for each component model,which can be obtained from other power systemsimulation tools or can be directly calculated. Inconclusion, PSpice is a promising tool for simulatingSPSs.

VI. Acknowledgments

The authors acknowledge the Office of NavalResearch for the support of this project through grantN00014-96-1-0523.

VII. References

[1] PSpice A/D Simulator, MicroSim Corporation, ver 8.0, July1997.

[2] K.J.Tseng, C.F. Foo and P.R.Palmer, “ImplementingPower Doide Models in SPICE and Saber”, 25th IEEEAnnual Power Electronics Specialists ConferenceProceedings, PESC’94, June 20-24,1994, pp 59-63.

[3] Zhixin Yan,M.J. Deen, “New RTD large-signal DC modelsuitabl for PSPICE”, IEEE-Transactions on ComputerAided Design of Integrated Circuits and Systems. v. 14Feb. '95 p. 167-72

[4] Naval Sea Systems Command, Engineering Directorate,Electrical Engineering Group, NAVSEA Design Practiceand Criteria Manual for Electrical Systems for SurfaceShips, Chapter 300.

[5] K.L.Butler, N.D.R.Sarma, C.A. Whitcomb, H.J.DoCarmoand H.Zhang, “Shipboard Power Systems DeployAutomated Protections”, IEEE Computer Applications inPower, Volume 11, No.2, April, 1998, pp 31-36

[6] Junichi Arai, Yasuhiro Noro, “An Induction MachineModel for an Analog Power System Simulator”, IEEETransactions on Power Systems, Vol.8, No.4, November1994, pp1478-1482

Vab Va’b’

Vab Vbc Vca