evaluation of islanding detection methods for pv utility ...€¦ · pv power (p.u.), 1 converter 0...
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1/24/02 1SandiaSandiaNationalNationalLaboratoriesLaboratories
Evaluation of Islanding DetectionMethods for PV Utility-interactive
Power SystemsBy
Ward Bower: Sandia National LaboratoriesMichael Ropp: South Dakota University
For
The IEA PVPS Task V Workshop“Impacts of PV Penetration in Distribution Networks”
Arnhem, the NetherlandsJanuary 24-25, 2002
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Methods for Detecting an Island?
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Introduction
• Rationale for Anti-IslandingRequirements
• Standards and Code Activities• Overview of Anti-Islanding Detection
Methods:• Rationale for Test Methods• Test Methods and Standards
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Introduction
• Active and Passive Descriptions• Strengths & Weaknesses of Methods• Non-detection Zone (NDZ)
Descriptions• Testing Methods• Summary
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Rationale for Anti-islandingDetection
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Rationale for Anti-islandingRequirements
• 1. The Utility Cannot Control Voltage andFrequency in the Island, Creating thePossibility of Damage to CustomerEquipment in a Situation Over Which theUtility Has No Control.
• 2. Utilities, Along With the PV DistributedResource Owner, Can Be Found Liable forElectrical Damage to Customer EquipmentConnected to Their Lines That ResultsFrom Voltage or Frequency ExcursionsOutside of the Acceptable Ranges.
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Rationale for Anti-islandingRequirements
• 3. Islanding May Create a Hazard for Utility Line-workers by Causing a Line to Remain EnergizedThat May Be Assumed to Be Disconnected FromAll Energy Sources.
• 4. Reclosing Into an Island May Result in Re-tripping the Line or Damaging the DistributedResource Equipment, or Other ConnectedEquipment, Because of Out-of-phase Closure.
• 5. Islanding May Interfere With the Manual orAutomatic Restoration of Normal Service by theUtility.
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PV Inverters Must Not IslandWhen Connected to the Utility
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Anti-Islanding in ActionSample Voltage Surge TestVrms during surge is 163
-300
-200
-100
0
100
200
300
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
seconds
Vac
, Iac
0
1
2
3
4
5
6
Trig
ger
Vac Iac Vrms trigger
121 Vrms163 Vrms
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Methods forDetection of Islanding
• Passive Inverter Resident• Active Inverter Resident• Active Non-resident (Utility)• Passive Non-resident (Utility Control)
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Passive MethodsResident in the Inverter
• Under/over Voltage and Under/overFrequency
• Voltage Phase Jump Detection• Detection of Voltage Harmonics and
Detection of Harmonics
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Active Methods Resident in the Inverter
• Impedance Measurement• Detection of Impedance at Specific
Frequency• Detection of Voltage Harmonics and
Detection of Harmonics• Slip Mode Frequency Shift• Frequency Bias
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Active Methods Resident in the Inverter
• Sandia Frequency Shift• Sandia Voltage Shift• Frequency Jump• Mains Monitoring Units with
Allocated All-pole Switching DevicesConnected in Series (MSD). Also(ENS).
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Methods at the Utility Level
• ImpedanceInsertion (Active)
• ProtectionRelaying (Passive)
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Methods Using CommunicationsBetween the Utility and PV System
• Power Line Carrier Communications• Signal Produced by Disconnect• Supervisory Control and Data
Acquisition (SCADA)
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Rationale for Anti-islandTest Methods
• Verify Anti-island Detection Works– Tests Must be Low Cost– Number of Inverters Tested Minimized– Anti-Island For Multiple Inverters Must
be Verified– Tests Must be Repeatable
• Noise Levels and Test Circuit Specified• Utility, Simulated Utility Impedance Specified
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Multiple-inverter Tests
• Tests MustConsider ActiveAnti-islandSynchronization
• Tests MustConsider UtilityImpedance Values
• Noise May beRequired!
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Standards and Codes Activities
• Photovoltaic Interconnect Standardsand Requirements are Being Written.Standards Organizations Include:– IEC– IEEE– Underwriters Laboratories
• IEA PVPS Member Countries
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USA (IEEE 929-2000) and(UL1741) Standards Methods
• Test Procedures to Verify IslandingDetection Works.– Required for– Interconnection
• Requirements forAnti-islanding and Interconnection Are
Spelled Out
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Anti-Island Test Circuit (929/1741)
Motor
DC
AC
V
I I
R
UTILITY GRID
LC
LEM
To Waveform Recorders
Recorder Trigger
V
Impedance
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Draft International Standard IEC 62116
DC power Supply(PV arraySimulatoror PV array)
Power conditioner under test
Load
AC power supply
Waveform recorder
SWCB
Trigger
WCNT, VrCNT
‚ u‚ o‚ b•CA‚ oC
VPC, APC, WPC, VrPC
VDC, ADC, WDC
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End Of Part 1
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Definitions of SystemConfiguration, Power
Flows and Terms
PV array Inverter
Utilitybreaker(recloser)
Grid
Node a(PCC)
RR LL CC
PPV + jQPV
Pload + jQload
∆P + j∆QTransf
ormer
PV System/Utility Feeder Configuration Showing Definitions of Power Flows andTerms.
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Passive Inverter Resident
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Under/over Voltage andUnder/over Frequency
Description• Inverter operation is only allowed within a
selected amplitude/frequency window.• If the amplitude or frequency of the PCC voltage
leaves the window, the PV system isdisconnected from the utility.
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Under/over Voltage andUnder/over Frequency
• Also Standard Protective Relays;Abnormal Voltage Detection– Strengths: Low Cost, Equivalent to Utility
Protection, Is Used in Conjunction with OtherAnti-islanding Methods
– Weaknesses: Large NDZ, Slow Reaction Times
– NDZ: Dependent on Impedances, PowerRatings, Operating Point
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U/O Voltage & U/O FrequencyNDZ Description
• NDZ Includes AllL and C AllowingConditions to FallWithin theCrosshatched Area
For ∆ P and ∆ Q in this region, islanding is not detected
∆ Q
∆ P UV
OF
OV
UF
Mapping of the NDZ within the PowerMismatch Space (?P versus ?Q forOver/under Voltage and Over/underFrequency.
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Voltage Phase Jump Detection
• Also Power Factor Detection;Transient Phase Detection– Description: Monitor the Phase Difference Between the
Inverter and the Utility for a Sudden Jump– Strengths: Easy to Implement, Does not Affect the
Output Power Quality or System Transient Response– Weaknesses: Difficult to Choose Thresholds that Detect
Islanding without False Trips– NDZ: Unity Power Factor Loads Produce No Phase
Error. If Inverter is Not Unity Power Factor Then It MustBe Bidirectional.
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Phase Jump Detection
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Time (sec)
Vol
tage
(V)
Phase error
PV current(dashed line)
Islandformed;voltage
(solid line)“jumps”
Diagram Showing the Operation of the Phase Jump Detection Method
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Detection of Voltage Harmonicsand Detection of Harmonics
PV array Inverter
Utilitybreaker(recloser)
Grid
Node a(PCC)
RR LL CC
PPV + jQPV
Pload + jQload
∆P + j∆QTransf
ormer
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Detection of Voltage Harmonics and PJD
Main Challenge!Threshold Selection Can be Very
Difficult—NDZ Size vs. Frequency of FalseTrips. Not Always Possible to Select aThreshold That Guarantees Non-islandingWithout Causing Excessive False Trips.
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Active Methods Resident in theInverter
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Impedance Measurement
• Also Power Shift; Current Notching,Output Variation; Used in ENS
Amplitude (usually), Frequency, Or Phase ofthe PV Output Current Is PeriodicallyVaried. In The Case of Islanding, UpsetsBalance. “Crazy Ivan”
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0 10 20 30 40 50 60 70 80 90 1000.8
0.85
0.9
0.95
1
Time (arbitrary units)
PV
po
wer
(p.u
.), 1
co
nve
rter
0 10 20 30 40 50 60 70 80 90 10048.5
49
49.5
50
Time (arbitrary units)PV
po
wer
(p
.u.)
, 50
con
vert
ers
Impedance Method Failure:Multiple Inverter
Demonstration of the Failure of the Impedance Measurement Method in the Multiple-inverter Case
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Detection of Impedance at Specific Frequency
• Also Harmonic Amplitude Jump– Description: Looks for an Amplitude Increase
of a Specific Harmonic (Typically Injected Intothe Utility)
– Strengths: Same as Harmonic Detection– Weaknesses: Thresholds Difficult to Choose,
The Utility is Not a Always Clean, LocalResonance or Noise Can Cause False Trips
– NDZ: Same as Harmonic Detection.Subharmonic Injection Can Eliminate NDZ butIs Problematic for the Utility
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Frequency Bias
• Also Active Frequency Drift,Frequency Shift Up/Down– Description: Output Waveform is Slightly Distorted So
Islanding Causes a Drift in Frequency– Strengths: Very Easy to Implement With Microprocessor
Based Inverters– Weaknesses: Small Degradation in Output Power
Quality,– NDZ: Relatively Large relative to Other Active Methods,
Depends on the Value of the Chopping Fraction Used,Small (<1% then Same as SMS), Larger Causes NDZ toShift Toward Capacitive.
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Frequency Bias
tz
TIpv
2
TVutil
2
iPV Goes toZero
Before orAfter the
PCCVoltage.
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Frequency Jump
• Usually Involves a “Dithered” Freq Bias
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Cycle number
Bia
s co
effic
ien
t
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Positive Feedback Methods
• Slip Mode Frequency Shift (SMS):Positive Feedback on Phase of Ipv
• Sandia Frequency Shift (SFS):Positive Feedback on Frequency ofIpv
• Sandia Voltage Shift (SVS): PositiveFeedback on Amplitude of Ipv
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Slip Mode Frequency Shift
• Also Slide Mode Frequency Shift;Phase-Lock-Loop Slip; “Follow-the-Herd”.– Note That There Are Also Similarities to the
SVS and SFS Except the Acceleration (Gain inThis Case) Is Nearly a Constant Value.
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Slip Mode Frequency Shift
0 10 20 30 40 50 60 70 80 90 100-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Frequency (Hz)
Ph
ase
resp
on
se (
deg
)
SMS charac-teristic
R=14.4, L=5e-1,C=1.4072e-5
R=14.4,L=1e-1,C=7.036e-5
R=14.4, L=1e-2,C=7.036e-4
R=14.4, L=1e-3,C=7.036e-3
R=14.4, L=1e-4,C=7.036e-2
0 10 20 30 40 50 60 70 80 90 100 -40
-20
0
20
40
60
80
100
Frequency (Hz)
B
A
C
Pha
se r
espo
nse
(deg
)
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Sandia Frequency Shift
• Extension of Frequency Bias:
where F Is a Gain or Function (NeedNot Be Constant—Acceleration).
( )line0 ffFcfcf a −+=
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• Similar to SFS Except Applied toAmplitude:
Where F Is a Gain or Function.
Sandia Voltage Shift
( )00, PCC,PCCPVPV VVFII −+=
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þ Very Small NDZs—High Q Loadsþ Relatively Easy to Implementþ Retains Effectiveness With Multiple
Inverters, esp. With ACCELERATIONý Require a Reduction in Power
Quality (but Usually Manageable)ý Can Lead to Problems on Weak Grids
Summary ofPositive Feedback Methods
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Mains Monitoring Units with Allocated All-pole Switching
Devices Connected in Series (MSD)• Also ENS
– Description: Looks for a Sudden Change in Impedance withAdditional Over/Under Voltage and Frequency Circuits
– Strengths: Redundant Methods, Self Check for ReducingNeed for Periodic Retesting.
– Weaknesses: Interference with Other Units with MultipleInverters, May Result in Nuisance Trips, Multiple UnitsDilute the Effectiveness. Impedance Detection Range WillChange with Higher Rating of Inverter or the Utility GridCharacteristics.
– NDZ: All Voltages, Frequencies and Impedances Within theNDZ. NDZ Increases With Multiple Inverters.
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Methods at the Utility Level
• Typically for LargeSystem Interconnects
• May be The Only Anti-islanding Protection
• Set Points Controlledby the Utility
• InteractiveCommunicationsOften Involved
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Impedance Insertion
• Also Reactance Insertion, ResistanceInsertion
PV array PV PCS
Load(R,L,C)
Grid
Pload + jQload
PPV + jQPV ∆P + j∆Qa
b
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Methods Using CommunicationsBetween the Utility and PV System
• Power Line Carrier Communications• Signal Produced by Disconnect• Supervisory Control and Data
Acquisition (SCADA)
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Summary• The Rationale For Anti-Islanding Shows
There Is a Need to Include Detection• Rationale For Testing and Test Methods
Shows a Need For Accuracy & Consistency• Standards and Codes Are Being Drafted
and Implemented• Inverter Resident and Non-Resident
Methods Presented• Passive and Active Detection Methods
Described with Strengths & Weaknesses
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Summary
• Task V Has Positively Impacted theAnti-islanding Understanding andProgress Through Workshops andCollaborative R&D