short circuit contribution from pv power plants.pdf
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Short Circuit Contribution from PV Power Plants
DOE/NREL/SNL Distribution System Modeling Workshop
La Jolla, California, July 27, 2012
Michael Behnke, Principal Engineer
BEW Engineering, a DNV Company
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Presentation Outline
Importance of Short Circuit Analysis in DG
Interconnection and Integration
Synchronous Generator Short Circuit
Characteristics
PV Inverter Short Circuit Characteristics
Status of Commercial Analysis Tools
Conclusions
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Purpose of Short Circuit Analysis
Power system faults (short circuit, ground faults) cannot be
eliminated
Utility protection systems must be designed to clear faultsthrough interruption of the source(s) and post-clearing
restoration of service to as many customers as possible
Short circuit analysis aids in achieving these objectives by:1. Quantifying the magnitude of fault current through interrupting devices
(circuit breaker, fuses, reclosers) to ensure that interrupting capacities
are adequate for fault clearance
2. Providing a basis for protection coordination so that the device(s) thatinterrupt the fault are the ones that minimize loss of load (selectivity)
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Synchronous Generator Short Circuit Characteristics
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Source: Chapman, Electric Machinery
and Power System Fundamentals, 2002
Decay of
damper
winding
transient
Decay of
field
winding
transient
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Synchronous Generator Short Circuit Characteristics
Time-variant response represented by three separate
time-invariant models
Allows for Thevenin equivalent representation of
generator with constant voltage behind appropriate
impedance for time period of interest (Xd, Xd, Xd) for
balanced faults
Apply X2 and X0 for unbalanced faults
Short circuit calculation methods well documented inANSI and IEC standards
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DG Inverter Short Circuit Characteristics
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IMPORTANT CAVEAT:
We will be talking about a certain class of inverters:
- 3-phase
- Voltage source topology
- High-frequency pulse width modulated (PWM)
-AC current regulated
This describes nearly all inverter models currently being
applied in utility and commercial scale DR
Some inverters (particularly some PV micro-inverters) donot fall into this class
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DG Inverter Short Circuit Characteristics
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AD
dcv
oi R
+
-
L eA
B
AD
AT
AT
BD
BD
BT
BT
di
C
CD
CD
CT
CT
n
Three-Phase, Two-Level Voltage Source Converter Topology
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DG Inverter Short Circuit Characteristics
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Sinusoidal Pulse Width Modulation
2
dcV
2
dcV
2C
1C
dcV v t
P
Carrier and Modulating Signals
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DG Inverter Short Circuit Characteristics
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Example of
AC Current
Regulation
Modulating signals
are generated from
AC Current Regulator id* and iq* are real
and reactive current
commands
High PI gains cause
inverter to behave as
stiff current source
ip
KK
s
13
ga a gb b gc c
gab a gbc b gca c
P v i v i v i
Q v i v i v i
dcv
dcV
P
Q
i
v
P
Q
di
qi
Q controller
P controller
dcV controller
PQ controller
gv gv
gv
gV
e gdv
gqv
*v
abc
j
e
i
i
di
qi
di
di
gdv
qi Current
controller
Currentcontroller
gqv
e
v
v
ip
KK
s
ip
KK
s
ip
KK
s
ip
KK
s
L
L
i
abc
abc
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DG Inverter Short Circuit Characteristics
AC current regulator continues to regulate (and limit)
current during fault
Xd, Xd, Xd, X2 are only meaningful for a singleinverter operating point and one single fault location!
Danger! : Underestimation of fault current contribution
is possible with Thevenin representation whenimpedance is not changed to adapt to fault location
Line synchronization technique (e.g., PLL
implementation) has great impact on unbalanced fault
response
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DG Inverter Short Circuit Characteristics
Response to unbalanced faults: Manufacturer #1
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DG Inverter Short Circuit Characteristics
Response to unbalanced faults: Manufacturer #2
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Status of Commercial Analysis Tools
Disclaimer: these comments are general in nature are not directed at any
particular software vendors products
Approaches to modeling current-regulated inverters:
1. None (far most common)
- No option for user but Thevenin generator model
- What to do?
a. For balanced faults, use X = 1/Isc (Careful !!!
), orb. Manually iterate Thevenin impedance, voltage or both to achieve Isc
c. For unbalanced faults?
2. Iterative solution
- Automates manual process above, can work well for balanced faults- Provide user means to scale negative sequence current relative to positive
sequence current (but without user guidance) for unbalanced faults
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OK for one generator,
but for dozens?
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Conclusions
Inability to properly model inverter-based generation in utility industry tools will drive
penetration limits as utilities feel increasingly exposed on system protection issues
No relevant industry standards (IEEE, ANSI, IEC) currently exist
Commercial software packages are based on inapt Thevenin generator models
(though some vendors are making initial attempts at more suitable representation in
an uncoordinated way)
New computational methods are necessary
Generalizations about unbalanced fault response are difficult due to manufacturer
specific controls implementation
Consensus standard is badly needed, could be led by third party (DOE with National
Labs) and include key stakeholders:
- Utility Protection Engineers
- DG Inverter Manufacturers
- Software Vendors
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