Transcript Slide 1

Islanding detection for Low Power DPGS
Islanding detection for Low Power DPGS
Marco Liserre
[email protected]
Marco Liserre
[email protected]
Islanding detection for Low Power DPGS
Agenda
 International regulations
 What in the earth is anti-islanding?
 Anti-islanding requirements
 Anti-islanding methods (passive/active)
 Reference papers
Marco Liserre
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Islanding detection for Low Power DPGS
International regulations
 In Europe the standard IEC61727, in USA the recommendations IEEE929
and IEEE1574 (DG<10MVA), in Germany the standard VDE 0126-1-1
When the utility voltage is outside certain limits (typically around ± 15 %)
the inverter should cease to energize within 0.2 s to 2 s (depending on the
standards). More sever under/over voltages lead to shorter intervention times
When the utility frequency is outside certain limits (typically around ± 1 %)
the inverter should cease to energize the utility line within 0.2 seconds.
 Apart from voltage and frequency requirements these standards are
imposing requirements also on:
 harmonics (typically max 5% current THD)
 reactive power (typically min 0.9 power factor)
 dc current injection (especially for tranformerless structures)
 earth current
 anti-islanding
Marco Liserre
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Islanding detection for Low Power DPGS
International regulations
Conditions for
reconnection
after trip
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Islanding detection for Low Power DPGS
Understanding anti-islanding

Islanding for a distributed power generation system (DPGS) is defined when the
DPGS does not cease to operate when the grid is disconnected, i.e. is continuing
to operate with local load

Desired islanding is for example in power systems when a section containing
DPGS and load is running without connection to the grid

Undesired islanding is when it can pose danger for grid maintenance people

In PV, WT and other distributed power generation systems (DPGS) there are
anti-islanding features required by standards
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Islanding detection for Low Power DPGS
Understanding anti-islanding

DG grid-connected islanding may occur as a result of the following conditions:

a fault that is detected by the grid but which is not detected by the PV inverter
or protection devices;

accidental disconnection of the normal grid supply by equipment failure;

intentional disconnection of the line for servicing;

human error or malicious mischief;

an act of nature.
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Islanding detection for Low Power DPGS
Rationale for anti-islanding
requirements

The utility cannot control voltage and frequency in the island, creating
the possibility of damage to customer equipment

Utilities, along with the PV distributed resource owner, can be found
liable for electrical damage to customer equipment connected to their
lines that results from voltage/frequency excursions outside of the
acceptable ranges

Islanding may create a hazard for utility lineworkers by causing a line
to remain energized that may be assumed to be disconnected from all
energy sources
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Islanding detection for Low Power DPGS
Rationale for anti-islanding
requirements

Reclosing into an island may result in retripping the line or
damaging the distributed resource equipment, or other
connected equipment, because of out-of-phase closure

Islanding may interfere with the manual or automatic
restoration of normal service by the utility

Anti-islanding requirements can be defined in terms of
detecting the island condition (IEEE, IEC) or impedance (VDE
Germany)

Even if standards and recommendation will change allowing
island operation, the detection of this condition will remain
crucial for a safe operation
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Islanding detection for Low Power DPGS
IEEE1547/IEEE 929/ IEC 61727
 The inverter should cease to energize the utility in 10 cycles
(200ms for 50Hz) having an RLC local load with Q = 2.5 (Q =
2.5 P). For the new IEEE I547 the condition is Q=1!
 IEEE standard 1547-2005/929-2000 (detection time 2
seconds)

Balance of the power
in the system:
Pload  PDG  P
Qload  QDG  Q
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Islanding detection for Low Power DPGS
VDE0126-1-1 (ENS) requirements for PV
Having a perfect balanced local
loading (Igrid=0) the PV must
isolate from the grid within 5
seconds at a change of impedance
of
1
ohms
resistive.
The test should be repeated for
different grid impedance angles
The
automatic
disconnecting
facility consists in two independent
mains
monitoring
devices
(redundance)
As an option the requirements for
IEEE 1547 can ensure compliance
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Islanding detection for Low Power DPGS
Anti-islanding methods overview
Marco Liserre
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Islanding detection for Low Power DPGS
Under/Over Voltage and Under/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 is disconnected from the utility.
- Strengths: low cost, equivalent to utility protection, is used in conjunction
with other anti-islanding methods
- Weaknesses: Large NDZ, slow reaction times
- NDZ: dependent on impedances, power ratings, operating point
Vmax
V
f max
fmin
f
Vmin
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Islanding detection for Low Power DPGS
Under/Over Voltage and
Under/Over Frequency
Islanding voltage:

V '  K V
K  PDG Pload

Islanding pulsation:
2
Q
4
Q 
 DG' 2   DG' 2  
CV
 CV  LC
' 
2
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Islanding detection for Low Power DPGS
The Non Detection Zone (NDZ)
 The NDZ depends on the characteristics of the distributed
generation (DG) system
 Grid disconnection:
* high ∆P ∆Q => voltage amplitude or/and frequency
variations
* small ∆P ∆Q => NDZ
 The probability that ∆P and
∆Q fall into the NDZ of
OUV/OUF can be significant
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Islanding detection for Low Power DPGS
Detection of voltage harmonics
- Description: detect the variation of THD in the PCC voltage that can occur
in islanding mode due to switching harmonics
– Strengths: should not be dependent on power factor, good for multiinverter applications
– Weaknesses: not always possible to select a threshold that guarantees
non-islanding without causing excessive false trips. Also some inductive load
could attenuate the switching harmonics below the threshold.
– NDZ: difficult to evaluate!
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Islanding detection for Low Power DPGS
Detection of voltage harmonics mismatch
 It is based on a modified sensorless single phase controller to estimate
the grid voltage harmonic distortion
 A Kalman filter allows to establish the energy mismatch between the
estimated and the measured harmonics
 The proposed method belongs to the family of passive detection methods
but it allows to minimize the Non Detection Zone (NDZ)
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Islanding detection for Low Power DPGS
Detection of power harmonics change
 A time-frequency detection algorithm based on monitoring the DPGS
output power considering the influence of the PWM, the output LCL filter
and of the employed current controller.
 Wavelet analysis is applied to obtain time localization of the islanding
condition.
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Islanding detection for Low Power DPGS
Voltage phase jump 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!
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Islanding detection for Low Power DPGS
Passive methods
experimental comparison
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Islanding detection for Low Power DPGS
OUV/OUF method
Grid Voltage variation with ∆P=25%
Grid frequency variation with ∆Q=-15%
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Islanding detection for Low Power DPGS
Voltage harmonic monitoring method
Grid Voltage harmonic distortion variation with ∆P=0 and ∆Q=0
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Islanding detection for Low Power DPGS
Phase monitoring method
Voltage phase change with ∆Q=0
Voltage phase variation with ∆Q=-15%
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Islanding detection for Low Power DPGS
Harmonic based methods
Grid-disconnection transient in case of
harmonically distorted grid voltage.
Channel 1 is the measured grid voltage (400
V/div). Channel 3 is the PCC voltage (400
V/div). Channel A shows the difference between
channel 1 and 3 (400 V/div). Channel 4 is the
inverter output current (5 A/div). Channel 2
shows voltage across the switch which
disconnects the isolation transformer.
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Test to verify
immunity of the
method (no false
trip) to frequency
variation
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Islanding detection for Low Power DPGS
Passive methods comparison
Marco Liserre
Method
NDZ
Trip time
OUV
-17 %≤ΔP≤24
%
Not
applicable
OUF
-5%≤ΔQ≤5%
Not
applicable
Phase
monitoring
-5%≤ΔQ≤5%
Not
applicable
Harmonic
monitoring
Absent
0,1 – 0,2 s
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Islanding detection for Low Power DPGS
Positive feedback methods
RLC load characteristics:
Two mechanisms
Voltage feedback – when inverter output voltage is increasing, the feedback will
command the inverter output power to increase. In order to balance the power, the
voltage will keep increase. As a result the voltage will be eventually out of nominal
ranges so the islanding can be detected! Similar but opposite destabilization
occurs when the sensed voltage is decreasing
Frequency feedback – when the inverter-sensed frequency is increasing, the
feedback will command the inverter reactive-power output to increase. Due to
load characteristic, the frequency will keep increasing in order to balance the
reactive power. The increased frequency will further drive the inverter reactive
power up, and as result, the frequency will increase more. Eventually the
frequency will be driven out of range and the inverter will trip. Similar results
occurs when the frequency is initially decreasing.
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Islanding detection for Low Power DPGS
Positive feedback methods
It is possible to vary the PV inverter active power to carry the amplitude and the
frequency of voltage out from the normal operation range if the grid is
disconnected

Frequency
measured
and
voltage
are

Positive feedback

The control of the speed is
made by kv and kf
dP  K v  V  Vn 
dQ  K f   f n  f 
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Islanding detection for Low Power DPGS
Positive feedback methods
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Islanding detection for Low Power DPGS
Frequency positive feedback implementation
Concept
DQ implementation
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Islanding detection for Low Power DPGS
Frequency positive feedback
DQ pll using Inverse Park Transformation can be used in single-phase systems
to create the dq components!
BPF 1-10Hz needed to remove noise and dc-offset! Gain should be high enough
to make the system unstable in islanding but stable in non-islanding mode
For Voltage positive feedback, Vd (amplitude og Vgrid) is feedforwarded to
id_ref
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Islanding detection for Low Power DPGS
Impedance measurement - external device
Classical solution - using external device that produces a transient
For ex. A capacitor is switched in and the grid voltage zero-crossing
shift is measured. Thus the grid impedance is estimated
The disadvantage is that it requires extra hardware!
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Islanding detection for Low Power DPGS
Impedance measurement - harmonic injection

Special case of the harmonic monitoring method

Detection of islanding using only a monitoring PLL synchronized with the
specific harmonic
This method results in a
measure of impedance at a
specific frequency at inverter
terminals:
Z( h ) 
V( h )
I( h )
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Islanding detection for Low Power DPGS
Impedance measurement - harmonic injection
Ig
+
I g*
DFT
-
Zh 
*
VPWM
50 Hz
Uh
Ih
R  Re Z h 
+
Vg
Ih
-
L  Im Z h 
Zh
R
L
DFT
Vh
A non-characteristic harmonic current is injected in the grid by through the PWM. The
resulting harmonic voltage will depend on the grid impedance. The voltage and current
at this frequency are computed using DFT. The impedance is calculated by dividing the
voltage to the current. The real part is Rg and the imaginary part is Lg
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Islanding detection for Low Power DPGS
Simulation results
Impedance measurement - harmonic injection 75Hz
Comparison of spectra with and without harmonic injection.
a) Grid voltage. b) Grid current. The PV power is 3 kW.
A current transient in the current is introduced together with the injection
resulting in impedance estimation error!
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Islanding detection for Low Power DPGS
Impedance measurement – LCL-resonance

It is based on the controlled excitation of the LCL-filter resonance, for example
increasing the current control proportional gain the following systems resonate
0 mH
Root Locus Editor (C)
0.06
1
2.4e3
2e3
1.6e3
0.1
1.2e3
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
2.8e3
0.04
3.2e3
Imag Axis
0.5
3.6e3
3.6e3
-0.5
400
3.2e3
800
2.8e3
1.2e3
2.4e3
-1
0
1.5 mH
20
40
60
80
100
-1
-0.5
2e3
1.6e3
0
Real Axis
0.5
Root Locus Editor (C)
0.04
1
2.4e3
0.8
0.03
2e3
1.6e3
0.1
1.2e3
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
2.8e3
0.6
3.2e3
Imag Axis
0.4
0.02
3.6e3
0.2
0
800
400
4e3
4e3
-0.2
3.6e3
400
-0.4
0.01
-0.6
-0.8
3.2e3
800
2.8e3
0
20
40
60
80
100
-1
1.2e3
2.4e3
-1
0
Marco Liserre
400
4e3
0
4e3
0.02
0
800
-0.5
2e3
1.6e3
0
Real Axis
0.5
1
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Islanding detection for Low Power DPGS
Active frequency drift
- 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 to shift toward capacitive.
Ipv goes to zero before or
after the PCC voltage.
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Islanding detection for Low Power DPGS
Slip-mode Phase Shift (SMS)
- Description: the current-voltage phase is changed as function of
frequency so islanding causes a drift in frequency
– Strengths: very easy to implement with microprocessor based inverters,
works with multi-inverter applications
– Weaknesses: some RLC loads (high-Q loads with resonant frequencies
very near the line frequency) have phase response curves such that the
phase of the load increases faster than the phase of the PV inverter
– NDZ: relatively small.
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Islanding detection for Low Power DPGS
Active methods
experimental comparison
Marco Liserre
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Islanding detection for Low Power DPGS
Active and reactive power variation method
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Islanding detection for Low Power DPGS
Experimental results
Test setup
3 kW IGBT full-bridge inverter, TMS430F24xx DSP controller (40ms computation time) LCL
filter resonance freq = 4.25kHz, Ud = 370 V, Ug = 230 V rms, Kp=2, Ki=300, fs=8.5kHz
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Islanding detection for Low Power DPGS
Experimental results
3 kW PV inverter
Grid impedance
value
(1 V eqv. 1 W)
Non-continuous injection of the
harmonic into the grid.
Impedance estimation when the PV current is
changing from P=0.3 kW to P=3 kW.
The result is filtered every 1 s
Non-continuous injection of the harmonic. Estimation is checked for different
power levels
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Islanding detection for Low Power DPGS
Experimental results
3 kW PV inverter
Detection of both resistive and inductive impedance jump!
Grid impedance value
(1 V eqv. 1 W)
Grid
impedance
estimation
after
0.25 W
increase
due to Ltest
Detection time
Initial grid
impedance
estimation
value of 1.2 W
Voltage over Ltest
Dynamic response of the method detecting
Dynamic response of the method detecting 0.5 W
a 0.8 mH inductive increase of the grid impedance .
resistive increase.
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Islanding detection for Low Power DPGS
Active methods comparison
Method
NDZ
Trip time
Current
harmonic
injection
Absent
0,1 – 0,2 s in function
of set threshold
Active power
variation
Absent
0,45 s
0,3 s
kv=5
kv=10
Reactive power
variation
Absent
0,75 s
0,65 s
kf=50
kf=60
Capacitor
insertion
Absent
Marco Liserre
0,3 s with Qc=100
VAr
0,4 s with
Qc=50 VAr
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Islanding detection for Low Power DPGS
Islanding detection methods at the grid
level: impedance connection method

A low-value impedance, usually a capacitor bank is installed
on the grid system inside the potential island
Qload  Q DG  Q
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Islanding detection for Low Power DPGS
Methods based on the
communication between the
grid and photovoltaic inverter

A transmitter (T) is installed near the line protection switch and a
receiver (R) is positioned in the PCC in proximity of the inverter.

There are two kind of communication:
 PLCC (power line carrier communication)
 SPD (Signal produced by disconnected)
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Islanding detection for Low Power DPGS
Conclusions
 The anti-islanding requirement prevents islanding for safety
reasons and are very different across the world.
 Software Anti-islanding methods are possible and the algorithm
can be easily implemented in the ”existing” DSP control platform
and is not requiring extra hardware
 Especially in Germany as required by VDE0126 leaded to complex
and expensive solutions. The new VDE0126-1-1 bringsd good
news.
 In US UL1471 and IEE1547 (2005) have been consolidated
 In Germany VDE0126-1-1 (2006) has ”softened” the ENS
requirements by increasing the detectable griud impedance
detection threshold from 0.5 ohms to 1 ohms and by introducing
the option of a requirement similar to IEEE1547. Redundancy
requirements still remians!
Marco Liserre
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Islanding detection for Low Power DPGS
Conclusions
 IEC61727 tries to get worldwide ”unified” standard (based on
IEEE1547). It has been recently approved and will be published
soon (exp. 2008).
 Thus the trend is to use active anti-islanding methods based on
drifting the frequency/voltage in combination with passive
methods to obtain minimum NDZ.
 The harmonic injection was used a lot for the old ENS but now is
possible to comply with “softer” methods like AFD
 Islaning detection will be useful even if anti-islanding wil not be
required or standards will change
 Islanding detection helps in managing connection/disconnection
to the main grid in a soft way
Marco Liserre
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Islanding detection for Low Power DPGS
References
[1] Dugan, R.C.; Key, T.S.; Ball, G.J., "Distributed resources standards," Industry Applications
Magazine, IEEE , vol.12, no.1, pp. 27-34, Jan.-Feb. 2006
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Systems”, Underwriters Laboratories Inc. US, 2001
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December, 2004
[7] IEC 62116 CDV Ed. 1 – “Test procedure of islanding prevention measures for utilityinterconnected photovoltaic inverters”, IEC 82/402/CD:2005
[8] VDE V 0126-1-1 “Automatic disconnection device between a generator and the public low-
voltage grid”,VDE Verlag, Doc nr. 0126003, 2006
Marco Liserre
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Islanding detection for Low Power DPGS
References
[9] IEC 61000-3-2, Ed. 3.0 – “Electromagnetic compatibility (EMC) –Part 3-2: Limits –Limits for
harmonic current emissions (equipment input current ≤16 A per phase)”, ISBN 2-8318-8353-9,
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[10]
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[11]
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[13]
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Marco Liserre
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Islanding detection for Low Power DPGS
References
[14]
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Marco Liserre
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Islanding detection for Low Power DPGS
References
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Marco Liserre
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Islanding detection for Low Power DPGS
References
[27]
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Marco Liserre
[email protected]
Islanding detection for Low Power DPGS
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Marco Liserre
[email protected]