An Analysis of Open-Phase Fault in PowerGeneration Station

Saeed Mohajeryami, Milad Doostan, Zia SalamiEnergy Production and Infrastructure Center (EPIC)Electrical and Computer Engineering Department

University of North Carolina at CharlotteCharlotte, NC, USA

Email: {smohajer, mdoostan, zsalami}@uncc.edu

Abstract—This paper attempts to investigate and analyze theelectrical characteristics present on the power grid when anopen-phase event occurs at an off-site transformer. In orderto conduct such task, this paper uses transformer, load, andconnection data obtained from a real power plant to simu-late several different power system configurations and analyzesvarious model responses in EMTP-RV software. Several testingscenarios that encompass a variety of loading conditions that canbe encountered at the power plant are also defined. At the end,open phase simulation for these scenarios is carried out and allresults are presented and discussed in detail.

Index Terms—open phase fault; transformers; voltage study;sequence currents; EMTP-RV

I. INTRODUCTION

Several recent events in which protective relaying systemsdid not correctly detect open-phase conditions on three-phase,which led to safety related issues at many power plants,have necessitated the power plants’ operators to address thedifficulties inherent in open-phase detection [1].

A prime example of this situation is the loss-of-phase eventat the Byron power plant in Jan. 2012, in which a loss of thephase C conductor, which was feeding the off-site transformerwas not detected. This condition led to a voltage imbalancethat cascaded down to the station buses through the systemoff-site transformer. Since this open-phase condition was notdetected by any of the protective relays, the condition wasallowed to exist for an extended period of time [2]. Similaropen-phase events at other power plants went undetected forlong periods of time in one case for 21 days [3]. Moreover, ina recent case in which an Emergency Diesel Generator (EDG)failed to start after an undetected loss of two phases, the eventcaused damage to several motors [4].

Since the specific electrical quantities involved in the lossof one or two phases can vary greatly depending on systemconfiguration, equipment designs, level of loading, and typesand connections of loads [5], it has so far proven challengingfor the industry to design a protective relaying system whichreliably recognizes open-phase conditions and trips appropri-ately. As a matter of fact, the ability to detect open-phaseconditions allows plant operators to respond appropriatelywhen they occur. In addition, identifying open-phase eventscan reduce dangers to individuals in the vicinity of thecondition. A hanging, high-voltage cable that has not been

tripped by over-current relaying and has not been identified asan open phase presents a hazard to personnel in the area. Thishazard would be greatly reduced by the warning presented byopen-phase identification. As a result, various works have beencarried out for the purpose of detecting such events.

The authors in [6] intend to address different technicalissues associated with detecting an open-phase condition of apower plant auxiliary transformer by considering various loadlevels. To carry out such task, they apply different simula-tion techniques including frequency domain and time domainmethods. According to their results, the system response to anopen phase event depends on the transformer connection andcore configuration.

In [7], the authors simulate and examine the terminal behav-ior of a power plant auxiliary transformer under an open phasecondition on its primary side by employing electromagnetictransient simulations. The transformer is modeled in PSCADsoftware by using data gathered from manufacturer report.Their results provide insight into the per-phase voltage magni-tude and angles. Besides, positive, negative, and zero sequencevoltages and currents generated during the loss of one or twophases of the transformer’s primary side are evaluated. Inanother attempt, the authors in [8] analyze the open phase con-ditions in three phase transformers. Their work examines theimpact of transformer’s winding and core construction on thefault voltage and currents of the low-side of the transformer.According to their results, fault voltage and current on the low-side greatly depend on the winding configuration as well as thetransformer’s core construction. Nevertheless, the simplifiedtest system used by the paper, neglects many other parametersincluding the different loading scenarios. Moreover, Authorsin [9] analyzed these power transformers’ inrush current inthe presence of such internal faults. Furthermore, in [10],the authors explored how to detect such faults. Authors in[11] recommended the application of a positive temperaturecoefficient (PTC) devices, which can be connected in serieswith a circuit breaker and disconnect it. Moreover, authorsin [12] assessed the transient recovery voltages (TRV) inorder to improve the post-fault situation in the presence ofsuperconducting fault current limiters (SFCL). Besides, thereare some wide-area control approaches to mitigate the impactof disturbance propagations of such faults [13]- [16].

Fig. 1: Schematic of the power system model, modeled in EMTP-RV

This paper attempts to investigate and analyze the electricalcharacteristics present on the power grid when an open-phaseevent occurs at an off-site transformer. Off-site transformerstypically provide operating power to a power plant from thegrid when the plant is unable to provide its own power, suchas upon startup or during accident scenarios. As such, loss ofa phase at the transformer can result in severe unbalances andadverse operating conditions for critical plant machinery andinfrastructure, including safety related equipment. However,since different power plants have a variety of configurations inthe way they are connected to the power grid, a loss of phaseon one configuration might not produce the same observedconditions as loss of phase on another configuration. The goalof this paper, therefore, is to simulate a specific model ina variety of configurations to gather data about the systemresponse to open phase conditions. In order to overcomethe problems associated with transient phenomenon, severalapproaches are suggested in the literature.

The organization of this paper is as follows. In section II,the test system is presented and its components are describedin detail. Then, in section III, several testing scenarios thatencompass a variety of loading conditions that can be en-countered at the power plant are defined, and various openphase types are described. Also, in this section the testingprocedure is explained. Afterwards, open phase simulation forthese scenarios is carried out and all results are presented anddiscussed in section IV. Finally, section V closes the paper bydrawing conclusions.

II. SYSTEM MODELING

The power system model is determined based on the genericlayout of the typical offsite power grid connection at power

plants, which allows flexibility in loading levels and types, aswell as the potential for various transformer types or othersimilar changes. The overview of the model can be seen inFig. 1.

All components that are used in the model are discussed indetail as follows.

• Power Supply: The power supply to the system is a gridconnection modeled using a voltage source set at 345kV,line-to-line.

• Transformer: A 3x1 phase, 3 winding, 345/6.9/4.16kV,Yg-Yg-Yg connected (solid ground on primary, resistiveground on secondary and tertiary), 40/35/5MVA nominaltransformer. For the transformer core, a 3-legged core isselected. The secondary and tertiary grounding resistorsare calculated to limit the ground current in the eventof a ground fault to 600A. For the secondary winding,the resistor calculation produces 6.64 ohms, and for thetertiary winding, 4.00 ohms.

• Buses: Three types of electrical buses are modeled: Theswitchyard high-side bus, an Engineered Safeguard (ES)bus, and a Balance of Plant (BOP) bus. The switchyardhigh-side bus is simply modeled as the connectionbetween the transformer and the grid interconnect. Itis modeled as an ideal (no impedance), 3-phase bus inEMTP-RV. An ES bus runs safety related equipment,such as safety injection pumps and critical equipmentfor accident situations, and as such, would normallybe lightly loaded during plant normal power operation.The BOP bus provides power for non-safety loads

such as steam-side pumps (e.g. feedwater, condensate,circulating water), lighting, HVAC, etc. The ES andBOP buses are modeled at 4.16 and 6.9kV, respectively,as these are the conditions at Byron power plant. Bothbuses are modeled as ideal, 3-phase buses.

• Load: Two types of electrical loads are modeled on eachbus: asynchronous inductive motor loads, and static PQloads.A. Motor Loads: An asynchronous, inductive motor loadis modeled on each low-side bus according to the infor-mation provided in Table I.B. Static Loads: Static loads are modeled according tothe information presented in Table II.

• Switches: Open-phase conditions are simulated using anideal, three-phase switch placed at the high-side of thetransformer. The switches are also placed between thebuses and motor loads to allow the motors to be switchedon after the occurrence of the open phase. These motorcontrol switches will be operated as three phase devices,all closing simultaneously.

III. TESTING PLAN AND PROCESS

The testing plan refers to the types of open-phase events thatwill be placed on the power system, and to the configurationof the power system during each open phase. The individualsections of the plan were determined based on various sourcesand will be discussed individually as follows.

A. Power System Configurations

This subsection outlines the steady state and motor startconditions. This refers to the conditions on the power systemwhen the open phase occurs. Motor start means that an openphase will occur on the system and a motor will then beswitched into service. Tables III and IV show the steady stateand motor start scenarios that will be tested, respectively.

According to Table III, normal power operation is simulatedby setting the ES and BOP static loads to 20% and 100%,respectively, of their nominal operating powers. That is tosay, load level (%) only refers to the level at which thestatic loads are set. Motor loads are always run at full power.With all components at steady state, the specific open phase

TABLE I: MOTOR LOADS INFORMATION

Bus Type Power(hp)

Apparent Power(MVA)

Efficiency(%)

BOP 7000 6.184 99ES 1000 1.285 99

TABLE II: STATIC LOADS INFORMATION

Bus Type Real Power(MW)

Reactive Power(Mvar)

Voltage(kVLL)

BOP 25.934 12.56 6.2ES 3.344 1.619 4

is applied and all relevant measurements are taken once thesystem reached its new steady state. Similar procedures arerepeated for ”refueling outage”, ”accident”, and ”lowest loadconditions”, with the only change being the respective levelsat which the static loads are set.

According to Table IV, accident conditions with motor starton ES are simulated by setting the ES and BOP static loadsto 80% of their nominal operating powers. The BOP motorload begins the simulation already turned on at 100% power;however, the ES motor load is switched on and run at 100%power after the open phase is applied to the system. Allrelevant measurements are taken before and after the ES motorload is switched on. Similar procedures are repeated for lowestload with motor start on ES, and lowest load with motor starton BOP with the only change being the load levels and thebus on which the motor starts.

All conditions that are outlined in Tables III and IV arerepeated for each type of open phase.

B. Open Phase Types

This subsection outlines the exact manner in which the openphase events are applied to the system. All open phase faultsare applied in simple open phase (case 1), open phase withline-to-ground fault on the grid side (case 2), open phase withline-to-ground fault on the transformer high-side (case 3), ordouble open phase (case 4).

A further subdivision of cases 2 and 3 is the insertion ofresistances in the ground faults in addition to testing solidground faults.

The resistances used for the ground faults (0, 100, and400 ohms) are decided based on [20]. They represent high,middle, and low estimates of ground resistance, depending onthe type of material in contact with the downed line and/orhow solid the contact between the line and ground is. Figure2 demonstrates the four basic open phase types.

C. Testing Procedure

All faults are placed exactly as described earlier, includingload values and system configurations for each case. Thetiming of open phase fault placement and motor start variesdepending on the exact scenario and case. Before data arecollected for each test, a sample simulation is performedto determine how long the system requires to reach steadystate, before and after fault or motor start initiation. The openphase fault or motor start is then applied to the system atthe appropriate time and the simulation is run long enough toensure that no normal system transients are included as fault

Fig. 2: Four basic open phase types

TABLE III: STEADY STATE TESTING CONFIGURATIONS

Condition ES Load Level BOP Load Level

Scenario 1: Normal Power Operation 20% (P=0.6688MW, Q=0.3238MVar) 100% (P=25.934MW, Q= 12.56MVar)Scenario 2: Refueling Outage 20% (P=0.6688MW, Q=0.3238MVar) 20% (P=5.1868MW, Q= 2.512MVar)Scenario 3: Accident 100%(P=3.344MW, Q=1.619MVar) 80%(P=20.7472MW, Q= 10.048MVar)Scenario 4: Lowest Load 10%(P=0.3344MW, Q=0.1619MVar 10%(P=2.5934MW, Q= 1.256MVar)

TABLE IV: MOTOR START TESTING CONFIGURATIONS

Conditions ES Load Level BOP Load Level

Scenario 5: Accident Conditions withMotor Start on ES 80%(P=2.6752MW, Q=1.2952MVar) 80%(P=20.7472MW, Q=10.048MVar)

Scenario 6: Lowest Load withMotor Start on ES 10%(P=0.3344MW, Q=0.1619MVar) 10%(P=2.5934MW, Q=1.256MVar)

Scenario 7: Lowest Load withMotor Start on BOP 10%(P=0.3344MW, Q=0.1619MVar 10%(P=2.5934MW, Q=1.256MVar)

or motor start effects. Because the normal transients change inlength depending on the exact scenario being tested, the datasets for each scenario can vary from 2 seconds to 40 seconds.

IV. RESULTS AND DISCUSSION

After processing and analyzing the data, the authors havedrawn some conclusions that can serve as indicators of thedifferent types of open phases studied. These conclusionsare not intended to be definitive, but rather serve as startingpoints for anyone seeking to look for signs of an open phasecondition.

A. Significant negative sequence current for all open phasesconditions

All open phase cases produce significant negative sequencecurrent (I2 ) on the low-side buses, even for lightly loaded(10%) scenarios. The exact level of I2 with respect to positivesequence current (I1 ) varies depending on the loading scenarioand case; however, I2 is always significant and detectable.Examples of I2 behavior for scenario 1 are illustrated inFigures 3 and 4.

From Figure 3, it is understood that the negative sequencecurrent on the ES bus (scenario 1, case 1) increases from 0 toapproximately 1100 A upon open phase initiation at t=0.5s.The negative sequence current is actually greater than theoriginal positive sequence current in this simulation.

From Figure 4, the negative sequence current on the BOPbus (scenario 1, case 3) increases from 0 to approximately1800A upon open phase initiation at t=0.5s. The negativesequence current is less than the positive sequence current inthis simulation; however, it is still a significant and detectablequantity. The general trend of the I1 to I2 ratio is that negativesequence current is greater than the positive sequence currentfor the ES bus and the opposite for the BOP bus. Thisdifference in behavior is likely due to loading levels, since100% loading on the ES bus is 5MVA, and 35MVA on theBOP. Indeed, even light loading on the BOP is larger than themajority of scenarios on the ES bus.

With respect to the ES bus having higher negative sequencecurrent than the pre-fault positive sequence, the authors believethat this is due to the three winding transformer; that is to say,

it is caused by the interaction between the BOP and ES busesthrough the transformer. If the BOP has much heavier load,it has a greater effect on sequence currents, which are thenpartially forced onto the ES side.

All other cases and scenarios produce similar results. Forthe specific transformer windings, grounding configuration,and lack of core, tested in this paper, tripping the circuit ondetection of I2 would appear to be a legitimate response toisolate the open phase fault. However, other types of faultsproduce I2; hence, this is not a guaranteed method to positivelyidentify an open phase condition. It is possible that using I2detection in conjunction with some of the other conclusionsdiscussed in what follows would be sufficient to make apositive open phase identification.

B. Zero sequence current is Split Equally Between Buses

For all Cases and Scenarios, the zero sequence current (I0 )flowing through the BOP bus is equal to the I0 flowing throughthe ES bus, always to within 5A, but usually with less than1A difference. However, the authors believe this behavior isnot specifically related to the open phase condition, but ratherto the selection of the low-side transformer grounding resistor.

C. Phase voltage depends on exact open phase application

The phase voltages for each bus vary substantially depend-ing on the specific case being tested. For case 1 and case 2, thethree phase voltages settle at three different levels. The basicpattern is: the phase B stays close to the pre-fault voltage,the phase C (open) drops to the lowest, and the phase Aapproximately falls between B and C. The authors believethat this behavior is related to phase rotation; that is, if thephase rotation was reversed (ACB instead of ABC), the phaseA would stay at pre-fault voltage, the phase B would fall, andthe phase C (open) would remain the same as the previoustest. Figures 5 and 6 illustrate this behavior for scenario 1,cases 1 and 2.

According to Figures 5 and 6, for scenario 1, cases 1 and 2,on the ES bus, the phase B (green) remains at pre-fault voltage,the phase C (blue, open) drops by approximately 50%, and thephase A (red) is roughly the average of B and C.

However, for case 3, phase A and B voltages drop roughly15-20% from pre-fault voltage, while phase C (open) voltagedrops to approximately half of the A and B levels as illustratedin Figure 7.

For case 4 (double open phase), all three phase voltagesdrop to 15% or less of the pre-fault voltage. While all threephases are low, the two open phases (B and C) decrease furtherthan the unopened phase. Figure 8 shows an example of thissystem behavior.

From Figure 8, the phase A (red, unopened) has droppedto 13% of the pre-fault voltage, while the B and C (green andblue, open phases) have dropped to around 2% of the pre-faultvoltage.

It is worth mentioning that although the exact levels andpercent changes of the phase voltages can vary depending onthe loading and motor conditions, the general pattern of threedifferent phase voltages holds for all cases.

D. Long motor acceleration times result from double openphase

Long electrical transient times are observed in the case 4motor start scenarios. The authors believe that these transienttimes are likely caused by long motor acceleration times,which are in turn caused by low voltage and insufficient,unbalanced current. Figures 9 to 12, show a scenario 7 motorstart on the BOP bus. Figures 9 and 10 show the scenariowith no open phases, while Figures 11 and 12 show the samescenario with a double open phase on B and C.

Comparing Figures 9 and 11 shows that with no open phase,the BOP motor draws a peak current of around 4000 A uponstarting at t=2s . However, with a double open phase applied,the BOP motor has to wait 30 seconds before it can start whilethe ES motor is reaching steady state in Figure 11. After theBOP motor starts at t=30 seconds, the motor is only able todraw about 220 A of positive sequence current, with severeunbalances shown by the other sequences.

The voltage graphs tell the same story. Comparing Figures10 and 12 shows that with no open phase, the BOP bus hasthe full pre-fault voltage available after a short startup sag.However, with the double open phase applied, the ES motorhas unbalanced, low voltages available which drop even furtheronce the BOP motor switches on at t=30s.

The graphs show long, slow changes in the electrical valueswhile initial motor builds up to speed. Once the initial motorhas reached steady state and the second motor is started, thesevalues drop even further.

V. CONCLUSION AND RECOMMENDATION

This paper investigated the electrical response to open-phaseconditions on a power plant. EMTP-RV software was chosenfor simulation of the relevant conditions. Then, a testing planthat included system layout, desired configurations, and equip-ment to be modeled was developed. The testing plan, also,covered a range of realistic alignment and loading conditionsat power plant, as well as encompassing the known ways inwhich an open phase can develop. All identified alignments,

Fig. 3: I2 Behavior (Red Line)on ES bus, scenario 1, case 1

Fig. 4: I2 Behavior (Red Line)on BOP bus, scenario 1, case 3

Fig. 5: ES Phase Voltages forscenario 1,case1

Fig. 6: ES Phase Voltages forscenario 1, case 2

Fig. 7: ES Phase Voltages forscenario 1, case 3

Fig. 8: ES Phase Voltages forscenario 1, case 4

Fig. 9: Sequence Currents forscenario 7, No open phase

Fig. 10: Phase Voltages forscenario 7, No open phase

Fig. 11: Sequence Currents forscenario 7, case 4

Fig. 12: Phase Voltages forscenario 7, case 4

loading conditions, and open phase types were simulated andthe results were presented. At the end, by analyzing the results,the discussions were provided about system behavior duringopen phase conditions. The authors believe that the model,data, and conclusions resulting from this study could giveinsight into open phase conditions, as well as provide a solidfoundation for researchers who want to extend the study.As a future study, the authors plan to investigate the impactof various transformer winding types, load connections, andtransformer cores on the open-phase condition.

ACKNOWLEDGMENT

The authors would like to thank Mr. Joel Mathewson,Mario Poujol, Volodymyr Habovda, and Lee Easter for theirinvaluable contributions to this work.

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