Download results of the optinos project – deficits and unsureness in test

24th European Photovoltaic Solar Energy Conference and Exhibition, 21st to 25th September 2009, Hamburg, Germany
RESULTS OF THE OPTINOS PROJECT – DEFICITS AND UNCERTAINTIES IN PHOTOVOLTAIC INVERTER
TEST PROCEDURES
J. Kirchhof, G. Klein
Fraunhofer IWES Institute for Wind Energie and Energy System Technology
Koenigstor 59, 34119 Kassel, Germany
E-mail: [email protected], phone: +49 561 7294-254
ABSTRACT: Every PV inverter and electricity meter available on the market is tested according to the standards
which are valid for the individual product. Nevertheless, malfunctions of some electronic electricity meters have been
detected, when used in conjunction with special PV inverters. In one case, the meter displayed only 85% of the real
energy fed into the public grid by the PV inverter [1]. The meter was checked in a calibration laboratory without
negative results. Investigations by ISET have shown that the RF ripple current of the inverter influenced the meter. In
most inverters, the frequency of the ripple current lies between 3 kHz and 150 kHz. Unfortunately, no emission and
immunity requirements exist for inverters and meters within this frequency range [2] [3] [4]. ISET/IWES has
developed a special test setup, which can simulate electric ripple currents, similar to those generated by inverters.
Investigations on inverter EMC test setups with low differential mode impedance AC artificial mains networks
(AMN) are in progress.
disturbance limits within the frequency range of the
switch frequency (Figure 2).
1. INVERTER INFLUENCES POWER METER
On some PV plants, conflicting measurement results
of the energy meter reading and the PV monitoring data
records have occurred. This happened on PV plants
which were supplied with electronic energy meters. The
affected meters were calibrated and approved by the PTB
(national metrology institute of Germany). IWES carried
out in-situ energy measurements over one week in one of
the affected PV-plants. A comparison between meter
reading and IWES measurement has shown that the
energy meter displayed only about 80% of the correct
energy amount. Additional EMC measurements have
shown a high RF disturbance on the AC-lines of the
inverter with a large current magnitude at the switch
frequency of the inverter (Figure 1).
Figure 2: EMC disturbance voltage measurement of
the inverter according to EN55011
A laboratory test with both inverter and energy meter
has been carried out and under laboratory conditions the
meter failed again, as the mains impedance was low. The
displayed energy was 500Wh, while two calibrated
precision instruments measured an energy amount of
1230Wh. Disturbance currents in the range of 1A were
measured during this test.
2. DEFICIT OF THE STANDARDISATION
The safe and undisturbed operation of radio services
and equipment is a main goal of the EMC
standardisation. This is achieved by setting emission
limits and immunity requirements with a safety clearance
between both.
The generic standards for residential and industrial
areas are still in use for PV inverters because no productspecific standard for electromagnetic compatibility
(EMC) has been established so far. In the generic
Figure 1: In-situ measurement at a PV-plant with
disturbed energy meter. The AC disturbance current
at one inverter AC-line is displayed
Additional investigations have been carried out at the
IWES EMC laboratory using the same inverter and meter
with the result that the inverter passed the conducted
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24th European Photovoltaic Solar Energy Conference and Exhibition, 21st to 25th September 2009, Hamburg, Germany
standards, emission limits for the spectrum between 0Hz
and 3kHz in AC lines are defined, and also emission
limits in the frequency range between 150kHz and
30MHz are stated. However, for the frequency gap
between 3 kHz and 150 kHz, the behaviour of the
equipment is not defined because no emission and
immunity limits exist.
standard (EN50065, signalling on low-voltage electrical
installations in the frequency range 3 kHz to 148,5 kHz)
prescribes EMC measurements in the frequency range
between 3kHz and 150kHz in combination with a
modified CISPR AMN. Two different modifications are
suggested there. The first modification includes large
earth capacitors and an impedance of 50Ω parallel to
1.5Ω + 50µH to cover the EMC frequency extension
between 3kHz and 9kHz (Figure 5). An alternative
modification uses an additional network which is
connected in parallel with the classic AMN (Figure 6).
This network delivers more realistic low impedance
behaviour in comparison with typical grid impedances
and consists of an additional R-L-C series circuit of
33µF, 80µH and 1Ω, but this network is not applicable
for compliance tests. These networks extend the lower
frequency limit of the classic EMC LISN from 9kHz to
3kHz.
Figure 3: Disturbance current division between grid
impedance and X-capacitor of inverter
Figure 5: EN50065 AMN for frequencies between
3kHz and 9kHz
As a result, many inverters generate high disturbance
levels within the mentioned frequency range, mainly
because the switching frequency of the device lies within
this band (Figure 2). For special product types, e.g.
household inductive hobs, stoves or power line
communication equipment, emission limits within this
frequency range exist, but they are not applied to PV
inverters.
Figure 6: Modified CISPR AMN according to
EN50065 with realistic impedance
Between 3kHz and 150kHz the emissions of many
inverters are differential mode currents, which occur only
in low impedance networks because of the voltage source
characteristic of the internal disturbance source in
combination with the large X-capacitor at the inverter
terminals (Figure 3). The classic CISPR EMC AMN
(50Ω parallel to 5Ω + 50µH) delivers high differential
mode impedances which lies between 10 Ohm and 100
Ohm in the frequency range from 9kHz to 150kHz
(Figure 7). As a result of the voltage source
characteristic, the disturbance voltage measured with the
classic AMN can lay within the limits of inductive bobs
(Figure 2). In low impedance grids, the same inverter can
feed high disturbance currents into the grid impedance
because of a current division between the internal Xcapacitor of the inverter and the low grid impedance (see
Figure 3). Many inverters are connected to low
impedance points of the grid and PV-plants which exceed
the power limits of the low voltage grid (5kW per phase
in Germany) use individual medium voltage
transformers. In these cases, the mains impedance is
Figure 4: CISPR 11 AMN
2.1. Insufficient Emission Standard
Most PV-inverters apply switch frequencies between
3kHz and several 10kHz [5]. This switch frequency and
its harmonics can generate unwanted conducted
emissions in the public grid. The EMC emission standard
EN 61000-6-3 (Emission standard for residential,
commercial and light-industrial environments) does not
cover the frequency range of these disturbances.
Nevertheless, other EMC standards prescribe EMC
emission tests in the frequency range from 3kHz or 9kHz
up to 150kHz. In CISPR11/EN55011 an emission test
from 9kHz up to 150kHz is defined for inductive hobs,
which are using switched power electronic devices. The
classic EMC CISPR AMN with an impedance of 50Ω
parallel to 5Ω + 50µH (Figure 4) is applied here. Another
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24th European Photovoltaic Solar Energy Conference and Exhibition, 21st to 25th September 2009, Hamburg, Germany
much lower than the differential mode impedance of the
classic CISPR AMN and the discussed large emissions
can occur under these conditions.
disturbance currents with respect to earth. In this case, the
disturbance current flows through the electricity meter.
This leads to a high stress level for the current sensing
circuits in the meter. If a differential mode voltage occurs
in the instrument ports at the same time, an additional
influence on the voltage sensing circuits of the instrument
can occur.
3. NEW EMISSION TEST APPROACH
The classic EMC emission tests do not cover the
special emission characteristics of PV-inverters in the
frequency range between 9kHz and 150kHz. The classic
CISPR AMN delivers a too high differential mode
impedance. Voltage source behaviour of the internal
disturbance generator gives high disturbance currents on
low impedance networks even if the disturbance voltage
measured at the high impedance CISPR AMN is below
the limits of EN55011. Measurement of the disturbance
currents and voltages occurring in low impedance AMN
is necessary if a interference with power meters should be
avoided. EN50065 shows one way to reduce the
differential mode impedance by adding a special
modification network (see Figure 6). Simulations of the
differential mode impedance indicate that the combined
differential impedance of AMN and modification
network is still high and will not lead to disturbance
currents in the same magnitude as they occur in reality.
Figure 7: Differential mode impedances of different
AMN (simulated)
2.2. Shortcomings of Immunity Standardisation
The immunity requirements for electricity meters
according to EN 50470 and EN 62052.11 does not yet
prescribe test procedures for the frequency range up to
150kHz. Some basic standards include test setups for this
frequency range, but these standards cover only
immunity levels up to 200mA and 30V, which are too
low in comparison with real inverter ripple currents of up
to 1000mA (Figure 1). Another deficit is the wrong
symmetry. Immunity standards like EN 61000-4-16
prescribe a setup for asymmetric disturbance voltages and
currents. Common mode currents can simulate conducted
disturbances which are induced into cables by
electromagnetic fields.
Figure 9: IWES suggestion for a modified CISPR
AMN
IWES has simulated an alternative modification
network with better estimation of low impedance loads.
This network delivers an impedance with almost
inductive behaviour and a very low start value of 0.2Ω at
9kHz. In this short circuit situation the internal
disturbance source of the inverter can deliver its short
circuit current which can be measured alternatively with
a current probe on a single inverter line or as voltage drop
above the network impedance using the RF output port of
the AMN. The network has quite low impedances even at
50Hz or 60Hz because of the large capacity of 100µF
between the line and neutral conductors. A reduction of
the resulting capacitive grid current is possible if the
capacitor is connected to the 40mH inductor together
with a series resistor of 1Ω to build a parallel resonance
circuit. In this case, the load current falls from 20A to
values below 5A. The high parallel resonance currents at
the inductor have to be respected. For this network, new
emission limits must be stated to take into account that
the disturbance current shall be limited to values that are
acceptable for the electronic energy meter which is
actually connected to the inverter lines. A modification of
the resistive part of the network is possible when
evaluating the disturbance behaviour of voltage sources
at low impedance grids as well as current sources on high
Figure 8: IWES energy meter immunity test setup
Common mode disturbances with respect to earth
have only a weak influence on units without ground
conduction, because only the ground impedance of the
other ports (grounded via 150Ω networks during the test)
and the stray capacity of the device under test can
generate an RF current into the device which can
generate any EMC influence. In contrast to common
immunity standards, an inverter generates symmetric
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24th European Photovoltaic Solar Energy Conference and Exhibition, 21st to 25th September 2009, Hamburg, Germany
impedance grids. Unfortunately the modification network
changes the impedance behaviour of the classic AMN not
only in the frequency range below 150kHz but also in the
frequency range from 150kHz up to 4MHz. This
disadvantage makes it impossible to use the modification
network in combination with an AMN above 150kHz. If
it is necessary to use the test setup for both frequency
ranges, a switch can be used to disconnect the
modification network if an emission above 150kHz
should be measured. In future, IWES will examine the
behaviour of inverters at different AC AMN and
modification networks. New limits will be calculated and
the results will be published.
currents may lead to a significantly wrong electricity
meter reading. This has been examined on a plant and in
laboratory tests. The main cause for this effect is an
incomplete standard for EMI immunity and emission.
Faults can be prevented only when the EMC frequency
gap is closed. IWES is working on this topic and feeds
the results into national and international standardisation
organisations.
The investigation have taken place within the
research project OPTINOS (Testing and Optimizing of
Test and Certification Procedures for Quality Assurance
and Harmonisation of Norms at PV Inverters). The
project is supported by BMU (Federal Ministry for the
Environment Nature Conservation and Nuclear Safety)
and industrial partners. Project No. 0327576. The author
is solely responsible for the content of this publication.
4. NEW IMMUNITY TEST APPROACH
6. REFERENCES
[1] Kirchhof, Klein: „EMV - Grenzwertlücke –
Wechselrichter stört Zähler“, 24. Symposium
Photovoltaische Solarenergie, Bad Staffelstein,
2009
[2] Kirchhof et. al.: “Ergebnisse aus dem Projekt
OPTINOS – Defizite und Unsicherheiten bei
Prüfprozeduren
von
PhotovoltaikStromrichtern”, 23. Symposium Photovoltaische
Solarenergie, Bad Staffelstein, 2008
[3] Degner et. al.:”EMC and Safety Design for
Photovoltaic Systems - ESDEPS - Publishable
Final Report”, ISET, 2002
[4] Kirchhof: “ EMV-Grenzwerte, -Messverfahren
und -Messergebnisse für PV“, EMV und Blitzund Brandschutz für Solaranlagen, Fachseminar
Technologie-Kolleg OTTI, 04.-05.12.2007
[5] Schmidt, Burger: “EMV – gerechtes Geräteund Anlagendesign”, EMV und Blitz- und
Brandschutz für Solaranlagen, Fachseminar
Technologie-Kolleg OTTI, 05.-06.11.2008
Figure 10: Immunity levels used during the IWES
test. The symbol “ ” indicates the actual test
frequency
IWES has designed a new test setup which can
simulate differential mode disturbance currents in the
frequency range between 9kHz and 1MHz (Figure 8).
This setup consists of a RF broadband power amplifier, a
RF signal source and a special coupling unit with a high
power RF matching transformer. This signal source feeds
an artificial disturbance current into the meter. At the
same time, a precision power analyzer or precision
energy meter acts as reference instrument and is
connected to this test circuit. Meter and reference
instrument are supplied by a precision AC source. A
resistive load leads to an energy flow from the AC source
over the meter and reference instrument to the load.
Source and load resistor are equipped with bypass
capacitors for small disturbance path impedances. The
test is controlled by a computer programme and
stimulates a frequency sweep over the complete
frequency range of the test setup. Optional modulation of
the RF signal is also possible if needed.
A previously executed signal calibration guarantees a
reproducible immunity level. PV inverters generate
disturbance levels which sink with rising frequency. The
immunity test also uses different frequency ranges with
different signal amplitudes (Figure 10). The reading of
the energy meter during the test can be captured by
counting the number of flashes at the signal lamp of the
meter or by counting the pulses at the S0 interface of the
meter. An electronic counter can be used for this task.
5. SUMMARY AND PERSPECTIVES
Investigations performed by IWES have shown that
PV inverters can influence electronic electricity meters
under special circumstances. Exceedingly high ripple
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