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WHITEPAPER ON OFFGRID AND BACKUP SYSTEMS Fronius Microgrid Solution Fronius PV-Genset Solution
Transcript of WHITEPAPER ON OFFGRID AND BACKUP SYSTEMS/downloads/Solar Energy... · Whitepaper on Offgrid and...
WHITEPAPER ON OFFGRID AND BACKUP SYSTEMS
Fronius Microgrid Solution
Fronius PV-Genset Solution
.............................................................................................................................................................................
Whitepaper on Offgrid and Backup Systems 2/17
© Fronius International GmbH
Version 1.0/2014
Solar Energy
Fronius reserves all rights, in particular rights of reproduction, distribution and translation.
No part of this work may be reproduced in any way without the written consent of Fronius. It must not be saved, edited, reproduced or
distributed using any electrical or electronic system.
You are hereby reminded that the information published in this document, despite exercising the greatest of care in its preparation, is
subject to change and that neither the author nor Fronius can accept any legal liability.
Gender-specific wording refers equally to female and male form.
.
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Whitepaper on Offgrid and Backup Systems 3/17
TABLE OF CONTENTS
1 Introduction ..................................................................................................................................... 4
2 Inverter-Charger driven microgrids .............................................................................................. 5
2.1 AC-coupling ...................................................................................................................................... 6
2.1.1 Basic Inverter-Charger AC-coupling ................................................................................................. 7
2.1.2 AC-coupling as grid back-up ............................................................................................................. 7
2.1.3 Optimising Microgids / Extended Inverter-Charger AC-coupling ...................................................... 7
2.2 Fronius Microgrid Solution ................................................................................................................ 8
3 Operation of PV-Genset Systems ............................................................................................... 10
3.1 Fronius PV-Genset Solution ........................................................................................................... 12
3.1.1 Protecting and optimizing the diesel genset ................................................................................... 13
3.1.2 Optimising the PV-Genset system .................................................................................................. 13
3.1.3 Online system monitoring ............................................................................................................... 14
3.1.4 Products for the Fronius PV-Genset Solution ................................................................................. 14
4 Appendix: Inverter features supporting microgrids ................................................................. 15
4.1 Gradual Frequency dependent Power Reduction (GFdPR) ........................................................... 15
4.2 Gradual Voltage dependent Power Reduction (GVdPR) ................................................................ 16
4.3 Reactive Power as a function of Voltage (Volt-VAr-curve) ............................................................. 16
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Whitepaper on Offgrid and Backup Systems 4/17
1 INTRODUCTION
Photovoltaics (PV) are continually gaining the focus of microgrid (MG) operators. In the fast growing sector of
MGs, PV and wind power are currently providing the best and cost effective solutions, in comparison to
traditional fossil fuel resources like diesel, oil, coal, etc. Annual costs of PV systems have been decreasing,
and when compared to the increasing costs of traditional fossil fuels and their transportation aspects, the
very positive market for solar business becomes subtle.
A MG is an energy system comprising of distributed generators, consumers and storage facilities. These
distributed generators can not only be fuelled by fossil energy resources (diesel, gas, etc.), but also by
renewables (biogas, PV, fuel cells, etc.). MGs can be established in areas with weak security of energy
supply and in remote rural communities or industrial areas. Generally, MGs are able to work in parallel to an
existing power grid (“on-grid-mode”), in addition to the islanding mode (“off-grid-mode”). The main focus of
this document lies on the “off-grid” behavior, in case of such MGs. Diesel generator sets in MGs are able to
build a stable grid (voltage, frequency) within limited areas offering a unique advantage that the site of power
generation is located in the vicinity of loads. MGs act like little self-sustaining cells where the demand and
generation are in equilibrium. The lack of long distance distribution networks makes the energy supply
through these grids even more valuable, since the distribution losses become minimal.
We classified the MGs into two types:
/ Inverter-Charger driving small and medium scale MGs (up to 300 kVA) with or without a backup AC
source (diesel genset or AC backup grid)
Fronius Microgrid Solution
/ Diesel Genset driven medium and large scale MGs (100 kVA up to MVA)
Fronius PV-Genset Solution
MGs formed by Inverter/Chargers are usually smaller than dieselgrids1 (<300kVA), and are used to power
commercial buildings or smaller settlements in remote locations.
“Dieselgrids” are rather large, and are located in areas, where it is economically or technically difficult to build
the transmission lines. They are popular in countries with variations in its geographical demography
(resulting in a number of remote locations, for e.g. Australia), and in countries with a weak security of energy
supply, for e.g. regions of India and the Asia/Pacific, in general. The size of such MGs can be of several
100s kVA.
1 MGs mainly powered by diesel gensets
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Whitepaper
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....................
5/17
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....................
6/17
-time energy
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batteries can
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Whitepaper on Offgrid and Backup Systems 7/17
The main advantages of AC coupling include the following:
/ High system efficiency because of efficient components and less conversion losses in case direct power
supply to the loads
/ Economical cost of components
/ Possibility of easy up-gradation of existing systems
2.1.1 Basic Inverter-Charger AC-coupling
In remote regions that are deprived of access to electrical energy, battery inverters offer a simple solution to
build MGs. Battery inverters when replaced by Inverter-Charger – renders other sources of energy to
interact within the system. Additionally, diesel gensets can be used as a back-up option – in case of battery’s
State of Charge SoC) drops, because of the lack of solar irradiation.
/ The basic rule valid for every Inverter-Charger driven MG application mentions that an installed PV
power (i.e. max. AC Power) must not exceed the installed Inverter-Charger power.
This ensures that the Inverter-Charger is not overloaded at any time.A strategy to increase the PV
penetration in such systems is described in 2.1.3.
/ All Inverter-Charger units in parallel or three phase systems must be connected to the same battery
2.1.2 AC-coupling as grid back-up
It is not necessary to connect the entire electrical installation to the Inverter-Charger while employing it as an
AC-back-up. For such a case, it should be sufficient to define some critical loads that are required in an
event of outage. In such cases, the installed capacity of Inverter-Charger can even be lower than that of a
stand-alone case. Nevertheless, if the grid-tie inverter should also be on-line in a back-up case, it can be
installed on the AC-out terminals of the Inverter-Charger.
2.1.3 Optimising microgrids / Extended Inverter-Charger AC-coupling
As a matter of fact, batteries are the known and obvious cost drivers in an Inverter-Charger MG. To
guarantee an un-interrupted supply of power, it becomes essential to have sufficient back-up of power
capacity, and therefore, battery banks must not be undersized. By adding a quantum of intelligence into the
system – it is possible to oversize the AC-coupled PV inverters with respect to the installed Inverter-Charger
capacity (by monitoring the power fed into the battery). Therefore it needs a control that measures the load
and controls PV power in a way that the maximum allowed power of charging the batteries trough the
Inverter-Charger is not exceeded at any time.
If most power demand is during the day-time (certain base load) it renders feasibility to cover most of the
demand, supplied by PV (to reduce the battery charging losses), and also to ensure simultaneous charging
of the batteries. This could lead to the decrease of the battery bank sizing, and in turn results the system to
be cost effective. In case of reduced solar irradiation the Inverter-Charger nevertheless must be able to
cover the peak load or small backup generators help out during these rare events. Also in case of poor
irradiation that last for several days, a backup generator can prevent form the system from “run dry”.
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Whitepaper
2.2 Fro
The Fronius
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....................
8/17
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..................
Whitepaper
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....................
9/17
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..................
Whitepaper
3 OP
Diesel gen
technologie
systems to
economical
In remote a
diesel gens
changeable
costs can va
(€0.05 – 2.5
The price tr
from PV sys
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every diese
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to keep the
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system in ba
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photovoltaic
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s now bec
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absolute mus
g as it does
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ystem. Every
pecially in ca
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PV-GE
technology
demand by
come extrem
he electricity
st. The cost
on fuel and
country, tra
sector is muc
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ase of MGs
grids, surplu
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proportion i
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....................
10/17
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Whitepaper on Offgrid and Backup Systems 11/17
In a MG system, conditions are mostly similar to the ones that are as mentioned above. A stable operation of
a MG is of high significance. PV inverters are intended to not negatively impact the operation of genset
systems. Power quality and reliability are therefore intended to improve, and if not, are at least intended to
remain at the same level.
Various control algorithms that can be supported by PV inverters are further explained in this paper.
Gensets can be operated in isochronous or droop mode. Isochronous case of operation refers to the
operation of generator at one constant frequency (similar to the inter-connected grids). Droop mode
operation refers to the variation of frequency with respect to the load (i.e. open circuit frequency is higher
than frequency at full load).
Firstly, active and reactive power generation and demand are required to be in balance. A mismatch in active
power mainly leads to frequency deviation, and a mismatch in reactive power to a voltage deviation. In case,
when two or more synchronous generators feed one load - the load sharing is typically implemented via
either a droop load sharing or an isochronous load sharing.
Droop load sharing refers to the alteration of frequency with respect to the load such that the actual load
remains supplied by all generators in the system, approximately at the same level.
Isochronous load sharing requires communication between gensets - either by means of an analog signal or
digital communication (CAN, RS485 or Ethernet).
Using grid-tie inverters without special functionality3 in MGs is - provided that the island detection is disabled
– not a big issue, as long as inverter’s power is very low (<10%) compared to the generator power and the
load. In this case, the inverter’s power is always consumed by the loads and that the actual grid source is
slightly unburdened.
Increasing the installed PV power closer to the generator’s power poises challenging aspects in terms of
control. If PV power installed lies between 10 and 30% of generator’s nominal power, an external device for
controlling the inverter’s output can be necessary. For increased PV power capacity (above 30 %) of
generator’s nominal power, an external control is necessary and also relevant information of consumption
becomes essential. Otherwise the synchronous generator could get into operation modes that are
disadvantageous for permanent operation or even dangerous in case of reverse power.
To participate in MGs with diesel gensets and significant PV penetration, it is important to control the
inverter’s output reliable and fast by means of a superior control device.
3 ”Normal“ grid-tie inverter with unity power factor (PF = 1) and no additional characteristic curves
..................
Whitepaper
3.1 Fro
If the powe
controlled to
This is whe
together wi
system can
use, then th
controllable
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onius PV-G
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(in terms of
....................
and Backup
Genset So
hotovoltaic s
best possible
ius PV-Syste
asurements
ed. This allow
ve to be con
time) loads
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olution
system is pro
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em Controlle
(as well as
ws the Froni
ntrolled at th
can also be
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oportionally
ngs.
er comes into
diesel gens
ius inverters
e same time
taken into ac
....................
quite large,
o play. By u
set measure
to be contro
e. Further po
ccount.
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ements were
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otential for o
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optimisation e
....................
12/17
be optimally
ius inverters
), the entire
ensets are in
exists where
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..................
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3.1.1 Pro
The diesel g
the generat
The Fronius
/ Ensurin
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/ Guaran
fluctuat
3.1.2 Opt
It is very e
represent a
simply be id
The differen
normally qu
that tempor
from photov
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otecting and
genset is a f
or would cau
s PV-System
ng that the m
ng that the d
vice life, even
ntees an ex
tions are com
timising the
easy for a sm
cost-effectiv
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nce between
uite large. Th
rary power r
voltaics. In o
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d optimizing
fundamental
use the entire
m Controller a
minimum num
diesel genset
n at low load
xtremely qui
mpensated im
e PV-Genset
mall PV sys
ve form of op
m the annual
the costs of
he saving is t
restrictions a
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...................
Systems
the diesel g
part of the s
e power supp
assumes the
mber of diese
ts are always
s.
ck and res
mmediately a
t system
stem in relat
ptimisation. A
energy calcu
f producing P
therefore mu
are the norm
ify the most
....................
genset
system. Prote
ply to fail.
following fun
el gensets ne
s being used
ponsive con
and the strain
tion to the lo
As the PV o
ulated in the
PV energy an
uch higher if
m. This signif
cost-effectiv
....................
ecting it has
nctions:
eeded is in us
d in a manne
ntrol over th
n on the dies
oad to pay
utput never
respective r
nd the costs
the size of th
ficantly incre
ve PV system
...................
to take the h
se during any
r that has the
he PV outp
sel genset is
for itself fina
has to be res
region.
of producing
he PV system
ases the am
m size for eac
....................
highest prior
y given situa
e lowest pos
put. As a re
reduced dra
ancially, but
stricted, the
g electricity fr
m has been
mount of pow
ch project, it
....................
13/17
ity, as losing
ation.
ssible impact
esult, power
amatically.
t it does not
savings can
rom diesel is
designed so
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is important
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.............................................................................................................................................................................
Whitepaper on Offgrid and Backup Systems 14/17
to examine the insolation and load profile alongside the variable diesel power costs and the cost of the PV
system.
Simulations are then used to calculate the highest possible savings that could be achieved.
Fronius can provide you with an unbeatable support in the planning of your PV-Genset system.
3.1.3 Online system monitoring
Every PV-Genset system can be designed, monitored, analysed and visualised at any time using the free
Fronius Solar.web online portal. Up-to-date system data can be accessed at any time and is clearly
presented: the portal is very user-friendly and easy to use, and a comprehensive range of analysis functions
is included.
Fronius Solar.web is also available as an app to allow you to check your PV system data from your
smartphone at your own convenience.
3.1.4 Products for the Fronius PV-Genset Solution
All Fronius inverters can be used in a PV-Genset system. The Fronius PV-System Controller assumes all
control functions over the system. Depending on the configuration, the required measuring accessories will
also be offered.
Due to the open nature of Fronius communication solutions, it is also possible to implement individual
solutions containing third-party components. With the Fronius Datamanager 2.0 or Fronius Datamanager
Box 2.0, it is possible to incorporate all components into a control system using Modbus RTU or TCP with
the SunSpec Inverter Control Model.
..................
Whitepaper
4 AP
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Regardless
application,
of the past.
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Key feature
reactive pow
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15/17
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..................
Whitepaper
To top thin
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....................
16/17
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..................
Whitepaper
The mentio
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Figure 5
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5: Over-exc
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....................
17/17
to a certain
7
n
