Content uploaded by Gunnar Kaestle
Author content
All content in this area was uploaded by Gunnar Kaestle on Jul 14, 2014
Content may be subject to copyright.
1
Remote Control of Distributed Generation
in Low Voltage Networks
Gunnar Kaestle, Member, IEEE
Abstract--In this article a method will be described for remote
controlling of distributed power generating units connected to
low voltage grids. The main purpose is to minimize the effect of
excess power during extraordinary operating conditions which
may endanger grid assets or put system security at risk. A
similar method using the frequency has been applied in the past
during intended islanding to improve operational availability of
electrical supply. The disconnection switch of distributed
generating units is triggered by an overfrequency signal. Here,
the combination of sender (tap changer) and receiver (voltage
measurement at the generating unit) establishes a remote control.
Index Terms--Distributed Power Generation, Voltage Control,
Protection, Grid Connection Requirements, Active Power
Control
I. A NEED TO POWER DOWN
HE integration of distributed energy resources (DER) into
electric power systems sometimes needs the possibility to
de-energize distributed generation (DG). A remote control or
feed-in management as it is sometimes called is needed for
two apparent reasons. On the one hand it is necessary to
protect grid assets such as transformers and lines from
overloading. In case the day ahead grid simulations indicate
that n-1-security is lost during a situation with low load and
high renewable power production, feed-in management will
partially reduce the feed-in of distributed generators. On the
other hand a future scenario is that overall production is too
high and a power unbalance occurs driving the frequency up.
This has happened in the North-Eastern zone during the
European system split in November 2006. In Germany two
legal requirements are responsible for the intervention of the
relevant system operator into the business of power producers.
Renewable energy sources are addressed by the Renewable
Energy Sources Act (EEG §11 Feed-in management). All
other power generating units are covered more generally by
the Energy Industry Act (EnWG §13 System responsibilities
of transmission system operators). A measure according to
§11 EEG results in a financial compensation for the
production loss, whereas EnWG §13 measures implicate that
all delivery commitments are temporarily suspended.
This work was supported by the DIN e.V. managed TNS programme,
funded by the Federal Ministry of Economics and Technology [grant number
01FS10026].
G. Kaestle is with Clausthal University of Technology, 38678 Clausthal-
Zellerfeld, Germany (e-mail: kaestle@iee.tu-clausthal.de).
II. LEGAL AND TECHNICAL FRAMEWORK
The German Renewable Energy Act (EEG 2012) specifies
that all systems larger than 100 kW shall have a remote
control for active power output. Larger wind parks and solar
parks usually use a telecontrol installation, whereas smaller
units install ripple control receivers [1]. The effective
Operation Handbook - Policy 5: Emergency Operations of
ENTSO-E Regional Group Central Europe forbids automatic
resuming of power production after a tripping due to a
frequency deviation. “The TSO has to coordinate the
reconnection of generators tripped due to abnormal frequency
excursion. [..] For installation connected to DSOs grids the
local and remote reconnection has to be agreed in advance in
cooperation between the TSO and DSOs for the main units.
Automatic reconnection of all generators has to be forbidden
when in accordance with legislation.” ([2], clause C – S3.7,
page P5-14).
The current draft for comment of the ENTSO-E Network
Code “Requirements for Generators” specifies in article 7(1)d
as a general requirement for type A units: “In order to be able
to cease Active Power output, the Generating Unit shall be
equipped with a logic interface (I/O port) in order to be able
to disconnect it from the Network. The Relevant Network
Operator shall have the right to adopt a decision pursuant to
Article 4(3) determining the requirements for further
equipment to make this facility operable remotely.” ([3], p.16)
In Germany, about 80% of installed PV power is connected to
the low voltage grid. Therefore, the new EEG 2012 demands
that PV systems must have a remote control even for unit sizes
down to 30 kWp. Below 30 kWp the PV system operator can
choose between a general cap of feeding a maximum of 70 %
of the installed capacity (PV generator = PV panel size) and a
remote control equipment.
III. SHUTDOWN BY EXTERNAL SIGNAL
From island grids it is already known how to communicate
via frequency. Please refer to the More Microgrids project
(www.microgrids.eu) on Kythnos for details. Even parts of
regular distribution grids are sometimes running in islanded
mode e.g. if there is a scheduled maintenance. In this case
emergency gensets will provide power to parts of the distri-
bution grid downstream to the de-energized section where the
DSO’s field service personnel work on grid equipment.
T
2
A. Frequency Dependent Feeding
As these back-up gensets are not able to absorb power
which is fed-back in the same way as transformers feed back
into the next voltage level, distributed generators have been
told to disconnect by increased frequency. This rule has led to
the so called 50.2 Hz issue [4]. The threshold for over-
frequency protection chosen in 2005 during a review cycle of
DIN V VDE V 0126-1-1 (automatic disconnection device
between a generator and the public low-voltage grid) is very
close to nominal frequency. New grid connection standards
[5] being more robust are currently deployed within Europe
such as the German VDE-AR-N 4105 or the Italian CEI 0-21
and the amended version of EN 50438.
B. Voltage Dependent Feeding
In the following section, a similar method for grid
structures with hierarchical cell types will be proposed based
on voltage. Figure 1 displays the structure of a distribution
grid feeder. On the upper right side there is the HV/eHV grid.
A substation serves a MV feeder (the MV plant). A MV/LV
substation delivers 400 V along a LV line (the LV plant). The
controllable DER reacts on frequency as well as on voltage.
Fig. 1. Schematic view of a branch in a distribution grid with a power
generating unit reacting on frequency and voltage (protection functions or
advanced ancillary services)
Irrespective of the detailed behaviour DERs may develop
in the future, they do already react on grid parameters today.
At least there is a discrete switching implemented by the
protection functions f>, f<, U>>, U>, U<. Recently, other
functions have been developed such as the over-frequency
droop P(f) or as it is called in ENTSO-E language Limited
Frequency Sensitive Mode – Overfrequency (LFSM-O). On
the reactive power side, Q(U)-droops have been transferred
from high voltage to medium voltage and are integrated in low
voltage applications. Even a P(U) function has been integrated
in grid connection standards e.g. the Italian CEI 0-21,
defusing the sharp U> protection function and smoothing the
shifting of the operating point u(i) at the point of common
coupling.
Equation (1) shows the voltage level at node S in relation
to incoming and outgoing currents i’n of the neighbouring
nodes alongside a feeder of length L (figure 2).
()
1L}{1,...,Snode,)(',)(
1
0∈=+⋅= ∑∑ ==
LS
S
mn
n
n
m
m
m
muiiuiZu
The equation is linear. Instead of P= k1 (constant) which
equals i(u)=k1/u* a proportional relation i(u)= k2•u should be
examined further. With k2=1/R this resembles the behaviour
of an ohmic resistor (P is quadratic). By this approach, a new
problem (power electronics in low voltage grids) has been
reduced to a known situation (grid operation with resistors).
Fig. 2. Simplified scheme of a low voltage feeder with a length of L=3
sections
IV. MANIPULATING THE VOLTAGE
The idea is not to use frequency in order to push a cluster
of DERs over the protection threshold as it has been done
during intended islanding, but to do this with voltage. Voltage
can be regulated via the on-line tap changer (OLTC) at the
substation. Current research is moving towards equipping
distribution transformers with on-load tap-changers as well.
These voltage regulating actuators may not only be used for
improving voltage quality with a high share of fluctuating
renewable energy sources. Moreover, they serve as a backup
element to guarantee system security. In case information and
communication technologies (ICT) have a failure and Smart
Grid solutions for remote control do not work anymore, this
simple and robust way of transmitting data via the three phase
system is still available. The method will also work if there is
no data connection at all, e.g. to power down even the smallest
power generating units which have not been obliged to be
equipped with a remote control such as a radio ripple receiver.
In case a yo-yo effect occurs during a 50.2 Hz incident with
synchronous disconnection and almost parallel reconnection
after a 30s cycle, it can be used as well to power down
existing installations [5].
Figure 3 shows how communication via voltage works -
simply by applying Ohm’s Law on lines with high R/X ratio.
The DSO may raise the reference value for the voltage
controller at the substation which may trigger the overvoltage
protection of some DER units at the far end of the feeder.
3
Overvoltage protection settings of VDE-AR-N 4105
(Power generation systems connected to the low-voltage
distribution network) complying with EN 50160 (Voltage
characteristics of electricity supplied by public distribution
networks) have two stages, which will keep the voltage within
the usual range of 0.9 – 1.1 pu during the first minutes and
also avoid high voltage spikes by fast action of the
disconnection switch if hitting the U>> protection level.
Fig. 3. Schematic voltage profile along a LV feeder: the rising voltage (dotted
green) will enlarge the share of the line where DERs will hit the U> threshold
If the average of the last 10 minutes is larger than 1.1 pu,
overvoltage protection U> will disconnect the device. An
immediate disconnection will be triggered at 1.15 pu.
Consequently, a dynamic equilibrium around 1.1 pu emerges
along the line, depending of the load characteristics. The
higher the reference voltage will be raised at the transformer,
the larger is the share of the line with voltages next to 1.1 pu.
In these segments, DERs will only reconnect if a large load
next to the point of common coupling draws the mains voltage
down. A 2% margin is usually calculated along the MV line.
Thus it is recommended to slowly approach to the value of
1.08 pu at the OLTC’s voltage regulator within minutes.
V. PROSPECT – SAVING PRIVATE AND PUBLIC EUROS
Overvoltage protection and variable ratio HV/MV-
transformers are existing equipment. Therefore, the cost for
this kind of basic communication link is quite low compared
to an interface with ripple control receiver plus
communication box to the inverter. If all of Germany’s
700.000 small scale PV systems up to 10 kW had to be
retrofitted with a ripple control receiver costing 700 Euro per
system, the overall costs would be about half a billion Euros.
Future LV grid codes to come, such as the Italian CEI 0-21,
may extend the simple disconnection triggered by U> and
U>> protection with a P(U) droop similar to the P(f) droop
which has been introduced in addition to the over-frequency
protection. In order to avoid any problems caused by
measuring noise, a low-pass filter with customizable time
constant should smooth the input measurements. For further
details compare with IEC TR 61850-90-7:2011-06, p. 30.
Fig. 4. Voltage-watt management according to IEC TR 61850-90-7 (object
models for photovoltaic, storage, and other DER inverter functions), p. 62
VI. MODEL DESCRIPTION
A simple line model according to figure 2 has been imple-
mented in Matlab Simulink, calculating complex currents and
the corresponding voltage drops instead of working with
active and reactive power. Step size for the voltage and
current calculation cycle is 1 second.
Ten line elements have been aggregated radially together
with a MV/LV-distribution substation. Impedance values have
been chosen according to [6], p. 39ff. In particular for an
400 kVA transformer Z = (0.05+0.15i) Ω and for a 150 mm²
Al cable Z’=(0.206+0.067i) Ω/km are used for the simulation.
The section’s length is 50 m between two house connection
lines. All feed-in nodes have the same profile, as solar
inverters are assumed to show a high simultaneity factor
within a streets length. Voltage profiles along the line from
the transformer as first node to the last one are the result.
Two alternatives have been programmed for the behaviour
of the generating units. The first one mirrors the overvoltage
protection requirements of VDE-AR-N 4105 (Generators
connected to the low-voltage distribution network [7]). In
detail, this means an immediate disconnection if the measured
voltage is higer than 1.15 pu or the 10 min moving average of
the voltage is higher than 1.10 pu. Automatic reconnection is
allowed if permissable voltage up to 1.1 pu has been contin-
uously measured for at least 60 s. Thresholds have been
slightly randomized in avoidance of perfect synchronism. The
other option resembles figure 4. A low pass filter for input
filtering with a time constant of 5 min is used together with a
P(U) droop, lowering active current according to a character-
istic curve. Power reduction begins at 1.09 pu and is finished
at 1.10 pu resulting in a stop of feeding at the upper threshold.
VII. SIMULATION RESULTS
During the simulations shown below, the feed-in profile at
the line nodes has been held constant. The effect of fluctuating
electrical power will not be analysed here. Only the impact of
a voltage controller either at the HV/MV substation or at the
distribution transformer in future layouts shall be shown. Tap
changing activities (2% up) occur at 0 s and in four further
steps each one 600 s later.
4
A. Generating Units Switching Off
Fig. 5. Actual Voltage of 10+1 nodes along the line. The dotted blue line at
the bottom is the transformer node whereas the upper, dotted magenta line
shows the last node at the end of the line. Between 600 s and 1200 s cyclic
disconnection and resynchronisation can be observed.
Fig. 6. Moving Average versus Actual Voltage (thin black line from the figure
above, to be compared with the upper dotted line in magenta). Slightly over-
stepping of the 440 V line occurs (1.5 V) with a time lag of 10 min after the
third and fifth step change.
Fig. 7. Cumulative current in dotted blue line and currents fed into the line
being switched off as exogenous voltage rises (set of curves at the bottom of
the graph). Caused by the nonlinear behaviour of the voltage protection, a
reconnection of single units is possible.
B. Generating Units Gradually Reducing Power
Fig. 8. Actual Voltage of 10+1 nodes along the line. The dotted blue line at
the bottom is the transformer node whereas the upper, dotted magenta line
shows the last node at the end of the line. Voltage spikes decay after a step
due to the input filter’s falling exponential behavior in the time domain.
Fig. 9. Moving Average versus Actual Voltage (thin black line from the figure
above, to be compared with the upper dotted line in magenta). The over-
stepping of the 440 V threshold is shrinking (1V) and only visible between
2400 s and 3000 s.
Fig. 10. Cumulative current in dotted blue line and currents fed into the line
being curtailed as exogenous voltage rises (set of curves at the bottom of the
graph). Here, nonlinear effects can also be seen, as the characteristic curve is
continuous, but not differentiable at the breakpoints (Fig. 4: P2 & P3).
5
C. Discussion of Results
The method of reducing power within a low voltage grid
by raising the voltage seems to be effective. The existing fast
overvoltage protection U>> at 460 V is not triggered which is
a sign for only modest overvoltages. Nevertheless, a lagging
warning signal can be observed when the 10 min moving
average reaches the threshold of 440 V due to past voltage
readings. Furthermore, the path dependency of the current rule
regarding disconnection and reconnection cause in certain
circumstances on-off-switching between 600 and 1200 s, as
shown in fig. 5. A more thorough investigation is needed to
evaluate whether from a technical view this grid condition can
be accepted by both grid operator and appliance owner, and if
the legal aspects allow a deliberate triggering in case of an
emergency. Both, the relevant EN 50160 for the assessment of
voltage quality and any grid connection requirement, which
defines also the dynamic behaviour of distributed energy
resources, should be doublechecked if the use of moving
averages in combination with disconnection triggers is still
appropriate.
The improved version with a droop controlled power
reduction shows smoother reactions at elevated voltage level,
which may also naturally emerge by a high density of low
voltage feed-ins. Further analysis may help to optimize the
interplay between step size, stepping frequency and the time
constant of the generating unit’s PT1 input filter. To sum up,
the main advancement is the exchange of discrete switching
impulses against continuously acting elements. This results in
a smooth curve of the curtailed cumulative current and conse-
quently reduced active power as well (compare figure 7 with
figure 10).
VIII. FURTHER RESEARCH
The idea of having generating units to react on grid
parameters is not new at all. During the last years, voltage
stabilisation by reactive power has been introduced in medium
[8] and low voltage grid [9] connection standards. Frequency
response is mandatory for even the smallest generator [3], and
as presented here, a P(U)-droop is more useful than voltage
triggered disconnection. The combination of all these droops
with their time response characterized by time constants needs
careful analysis. Within the multivariable control system for
active and reactive power different process variables influence
each other depending on the line impedance angle.
Additionally, linear control structures on linear control
paths allow analytical studies. The simplification for low
voltage grids in equation 1 consists of a fixed reference
system for the currents to feed in. In reality, the voltage
phasor at node 0 (transformer) has a different angle than the
voltage phasor at node n where the current is fed in. In low
voltage grids this can be neglected as lines are relatively short
and the R/X ratio is high. Using complex current instead of
active and reactive power and modelling with linear current
density for the feed-in instead of discrete currents and discrete
line elements, integral transforms may help to find a closed
solution.
IX. SUMMARY
Within this article an heterodox method for remotely
powering down microgenerators has been proposed. Similar to
the state of the art using elevated frequency levels in islanding
grids, a voltage rise shall trigger the overvoltage protection.
Proposals for a much better system response of the generating
unit compared to current standards have been discussed. With
the assistance of a simulation model, first improvements could
be shown and should be taken into consideration during the
next review cycle of VDE-AR-N 4105. Similar standards such
as the CLC/TS 50549 are in development. This shall be the
basis for a future European Low and Medium Voltage
Guideline under the roof of ENTSO-E’s Network Code
Requirements for Generators. The general technical feasibility
has been shown. It is now up to the jurisprudence to eliminate
obstacles concerning the lawfulness and questions of liability.
The next step from the engineering point of view will be to
test this simple method in real life.
X. REFERENCES
[1] D. Quadflieg, “Empfehlungen zur Umsetzung des neuen EEG § 6,“
Forum Network technology / Network operation in the VDE (FNN),
Berlin, 14.12.2011.
[2] ENTSO-E, “Operating Handbook RG CE, 2nd release - Policy 5:
Emergency Operations,” European Network for Transmission System
Operators for Electricity, Brussels, August 2010.
[3] ENTSO-E, “Draft Network Code for Requirements for Grid Connection
applicable to all Generators,” European Network for Transmission
System Operators for Electricity, Brussels, 26.06.2012.
[4] J. Bömer, K. Burges, P. Zolotarev, J. Lehner, “Impact of Large-scale
Distributed Generation on Network Stability During Over-Frequency
Events & Development of Mitigation Measures,” Ecofys & IFK,
Berlin/Stuttgart, September 2011.
[5] G. Kaestle, T. K. Vrana, “Improved Requirements for the Connection to
the Low Voltage Grid,” Paper 1275, CIRED 2011, Frankfurt, 6-9 June
2011.
[6] G. Bartak, H. Holenstein, J. Meyer, „Technische Regeln zur Beurteilung
von Netzrückwirkungen,“ VEÖ, VSE, CSRES, VDN (Eds.), Wien et al.,
2007.
[7] VDE, “Generators connected to the low-voltage distribution network,”
Application Rule VDE-AR-N 4105:2011-08, FNN, Berlin, 2011.
[8] BDEW, “Generating Plants Connected to the Medium-Voltage
Network,” Technical Guideline, BDEW, Berlin, 2008.
[9] CEI, “Reference technical rules for the connection of active and passive
users to the LV electrical Utilities,” CEI 0-21:2011-12, Comitato
Elettrotechnico Italiano, Milano, 2011.
XI. BIOGRAPHY
Gunnar Kaestle holds a diploma in industrial
engineering from Karlsruhe University. He is now
with Clausthal University of Technology and works
at the Institute of Electrical Power Engineering. His
main research interests are virtual power plants,
micro cogeneration, distributed energy systems, and
standardisation issues. He has been involved in
solving the so called 50.2 Hz issue. Currently, he is
working on a PhD project using grid parameters as
communication channel for micro generation.
