FROM ISLANDING PROTECTION TO ISLANDING ... - UfE GmbH

FROM ISLANDING PROTECTION TO ISLANDING CONTROL
10 Years of Experience with ENS Safety Interface
Klaus Köln, Axel Grabitz
UfE GmbH
18059 Rostock, Joachim-Jungius-Str. 9
klaus.koeln@ufegmbh.de
Purpose of the Work:
The purpose is to show the progress in Islanding Control Interfaces based on Grid Impedance Step Monitoring
Background:
The last generation of UfE ENS (automatic disconnection device) is reaching an excellent level in safety and performance.
The devices are meeting the new release of the DIN VDE 0126 prestandard. With the approved functional safety defined in
the DIN VDE 0126 there is no need of repeated testing procedures.The UfE ENS is used as a separate device or integrated
in inverters including Fronius, Aixcon, Mastervolt, Pairan, Philips and Xantrex.
Innovation:
The use of impedance step monitoring allows a lower grid frequency limit and a resulting support to grid stability without
the problems of a larger Non Detection Zone (NDZ). The improvements are based on a lot of new methods like:
new methods of mesurement
self optimizing switching level
automatic calibration and self test
automatic syncronization and signal limiting in parallel connections
automatic optimization of signal patterns.
Results:
Performance of ENS has been testet by the ISET in Kassel using 15 ENS26 Devices in parallel connection with up to 16
different inverters and 15 BISI units (The BISI is deveoped in the SIDENA Project by ISET as another safety interface
using Impedance Step Monitoring). The devices showed best performance even under these critical conditions.
The ENS is not only used in PV-Systems but also in any other DG (Distributed Generation) systems connected to the low
voltage grid. ( small wind and hydro systems, motor- , sterling- and fuell-cell CHP). The multiple use of different methods
and – if neccessary different country settings- allows to meet any requirements. A new function is the automatic passive
synchronization of controlled islanding inverters or synchronous machines to support back-up supply without interruption.
The latest development is a three-phase ENS with integrated relais. These type of relays use permanent magnets for contact
pressure. No power is needed to keep a switching state. This saves installation and wiring of external contactors and their
energy consumtion.
Conclusions:
Many years of field experience in Germany ,Austria, Belgium, France and the ISET research shows that improved
Impedance Step Monitoring is a secure method to detect uncontrolled islanding even under balanced load conditions where
passive methods could fail.
Background:
Impedance step monitoring as a method to detect
uncontrolled islanding of grid connected inverters was first
used in the UfE NEG 1500 Inverter in 1992. In islandig
tests this Inverter could not be forced into islanding
conditions by matched load and generation.
So this method was used to establish the new ENS -
standard of grid monitoring for PV-inverters in Germany.
Also new was a level of approved functional safety,
defined in the drafts of the new standard. The typical Non
Detection Zone (NDZ) known from the passive methods
monitoring frequency and voltage does not accur with the
new method. But there had been some problems with
overreaction. Some inverters using the ENS reacted too
sensible under special grid conditions like remote houses
conneted with long overhead wires. There were also
problems with multiple use of inverters, the ENS systems
could disturbe each other. But about 98% of the
installations had no problems with overreaction of the
ENS. There is also in other coutries the concern of grid
interference with the use of many ENS systems adding
their test signals. But the main background of this concern
was not the UfE ENS using a low reactive current of 50
Hz but another producer of inverters using high current
sharp pulses as test signals. Another problem was related
to high levels of remote control signals in the grid
disturbing the impedance mesurement.
These problems are solved.
UfE specialized in the ENS technology and developed
seperate devices that could be used with any inverters.
Two patents were issued on special methods of impedance
measurment and ENS devices and the technology was also
used in licence by other inverter producing companies.
There were Fronius, Sunpower, Sunways, Sputnik
Engineering, Wuseltronik, Aixcon and later Mastervolt,
Pairan, Philips and Xantrex. There must be more than
100,000 of Inverters running with UfE-ENS until now.
The ENS was not used only in PV Systems but also in
small CHP units.
UfE did a lot of research and developement to solve the
problems with overreaction and paralled operation. The
new generation of ENS Systems (ENS26 and ENS31) is
used since 4 years and is demonstrating excellent
performance.
Innovation:
Improved methods of impedance mesurement were used:
Different signal patterns, synchronisation of parallel
operating ENS forming a kind of decentral pilot tone and
regulation of the level of this signal according to the
quantity of ENS devices and grid conditions. A new
method is used to adept the switching level of impedance
step monitoring to the noise of the grid without loosing
safety.

Results:
The ISET Institute in Kassel did some research on parallel
operation of Inverters and separate islanding protection
devices. Different inverters, 15 ENS26 and 15 BISI units
were tested. (the BISI Bidirectional Safety Interface) is
another system using impedance step monitoring. It was
developed by ISET in the SIDENA Project. ISET tested
up to 15 ENS or BISI devices in parallel operation
connected to a single phase together with up to 16
Inverters. Even under this extremly difficult conditions the
ENS and also the BISI demonstrated excellent functional
safety.
Tests were also performed with a resonant cicuit. The
results show that impedance step monitoring is easily able
to pass these kind of islanding test procedures. This is the
chance to discriminate between unsufficient generation in
the grid and uncontrolled islanding and to support grid
stability.
Results from ENS Dataloggers:
Each of the new ENS devices has a memory chip for
datalogging integrated. The datalogger is recording one
second samples of 16 classes of impedance step maxima.
The sums are recorded every 8 hours and in any case of
fault. Fault reasons are recorded. Because of this, there is
an undervoltage record every time the power supply (grid)
is switched off.Impedance step data recorded from two
ENS26.
Data recorded showing very low levels of impedance
steps. Two ENS26 devices were recorded for 85,5 and
101 days. The blue record shows data of an ENS26
operated in parallel with an older ENS25. There is no
synchronisation with these different devices. This might
be the reason of a few recorded impedance steps of more
than 0,3 Ohms. The other device was running in the UfE
buero.
Impedance step data recorded from two ENS26
Data recorded showing very low levels of impedance
steps. Two ENS26 devices were recorded for 85,5 and
101 days. The blue record shows data of an ENS26
operated in parallel with an older ENS25. There is no
synchronisation with these different devices. This might
be the reason of a few recorded impedance steps of more
than 0,3 Ohms.
The other device was running in the UfE buero.

6250000
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4250000
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3750000
3500000
3250000
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2750000
2500000
2250000
2000000
1750000
1500000
1250000
1000000
7500 00
5000 00
2500 00
0
0
Impedance
Zeile 2 -0,3 -0,15 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 1,1 1,2 1,3 1,4 1,6 2 2,4 2,8
Impedance step
DR Ohms -1,6 -1,3 -1 -0,7 -0,5 -0,3 -0,2 -0,1 0 0,1 0,2 0,3 0,5 0,7 1 1,3 1,6
Solarverlag
Büro
Solarverlag85,5d
Büro 101,5d
Conclusions:
There are more than 10 years of field experience with
ENS-Systems in PV-Inverters and also with seperate
devices not only in Germay, but also in Austria, Belgium
and France.
There was also a lot of research and testing done with the
new Islanding protection Systems. There is very good
performance demonstated by the latest generation of
divices.
The early problems with overreaction are solved. There
are no grid interferences even with massive use of ENS
systems.A wider range of frequency limits is possible with
the use of impedance step monitoring as a secure method
to detect uncontrolled islanding.
Secure methods to discriminate between uncontrolled
islanding and an unsufficient balance between load and
generation give the chance to support grid stability.The
existing guidlines and standards for integrating
Distrebuted Energy Resources (DER) into the low voltage
grid are still based on the assumption of a few percent
added to the conventional generated amount of Power.
This is not enough to meet the requirements of the
future.This future will be powered by renewable Energy.
The PV-Systems of the future will be able to operate grid
connected as well as in stand lone or mini grid conditions.
Inverters will controll and stabilize frequency and voltage
of future energy networks. Grid control technologies and
standards will have to meet these challenges.
THREE-PHASE ENS ISLANDING CONTROL
DEVICE WITH INTEGRATED RELAYS
The latest ENS development is new three-phase device
with integrated relays. These are polarized trip relays that
need only energy for the moments of switching (about
20ms) . Contact pressure is per- formed by permanent
magnets. The switching capability is up to 100A..
Simplified grid connection of three phased systems:
Until now a three phased ENS had to be installed together
with two external contactors. Auxilary contacts had to be
wired for monitoring the switching status of each of the
contactors.
Contactors saved:
Costs for contactors and wiring are saved, also some space
in the electrical cabinet. The new housing with seperated
terminal box could also be installed without an electrical
cabinet.
Energy saved:
The new ENS is saving the whole energy that would be
necessary to drive two conventionell contators.With PVsystems
and their EEG based high energy prices 20-60
Euros per year could be saved. This will sum up to 400-
1200 Euros in 20 years!
Safety:
The use of polarized trip relays is a novum in safety
related electric applications because energy is needed to
switch to a safe condition. The new ENS is storing this
energy in capacitors. The capacity and charge of these
capacitors are permanently monitored by microprocessors.
With any fault in the system all relays are switched off
instantly using this energy. For there are two sets of relays
connected in series switching off is guaranteed even if a
relay itself is failing.
EINSPEISER
1
3
5
-K10.1
R1R2R3R4
RELAIS
21
-K10.1
22
UfE - ENS31
2
4
6
A1 A2
21
-K10.2
22
1
3
5
K1 K2
N L1 L2 L3
KONTAKTE NETZ
2
4
6
A1 A2
-K10.2
N
L1
L2
L3
NETZ
* * * min. 6 A
max. 25 A

CONCLUSIONS
There are more than 10 years of field experience with
ENS-Systems in PV-Inverters and also with seperate
devices not only in Germay, but also in Austria, Belgium
and France.
There was also a lot of research and testing done with the
new Islanding protection Systems. There is very good
performance demonstated by the latest generation of
divices.
The early problems with overreaction are solved. There
are no grid interferences even with massive use of ENS
systems.A wider range of frequency limits is possible with
the use of impedance step monitoring as a secure method
to detect uncontrolled islanding.
Secure methods to discriminate between uncontrolled
islanding and an unsufficient balance between load and
generation give the chance to support grid stability.The
existing guidlines and standards for integrating
Distrebuted Energy Resources (DER) into the low voltage
grid are still based on the assumption of a few percent
added to the conventional generated amount of Power.
This is not enough to meet the requirements of the future.
This future will be powered by renewable Energy. The
PV-Systems of the future will be able to operate grid
connected as well as in stand lone or mini grid conditions.
Inverters will controll and stabilize frequency and voltage
of future energy networks. Grid control technologies and
standards will have to meet these challenges.