Download 1 Introduction

DISTRIBUTEDPOWERGENERATION UNITS
ANDTHEIRIMPACTTOTHE POWERNETWORK
CHRISTOPH HÄDERLI, ALEC STOTHERT, ALICE PIAZZESI, MARCO SUTER
ABB Switzerland Ltd., Im Segelhof 1, 5405 Baden-Daettwil, Switzerland
[email protected], Tel +41 58 586 83 18, Fax +41 58 586 73 12
UWE PRAUSE
ABB Calor Emag Schaltanlagen AG, Käfertaler Strasse 250, 68128 Mannheim, Germany
[email protected], Tel +49 621 386 27 96, Fax +49 621 386 27 85
ABSTRACT
Introducing high concentrations of Distributed Power
Generation units (DGs or DGs) into existing electrical
networks can compromise, amongst others, grid stability,
protection concepts, selectivity, and voltage profiles.
In this paper, case studies focusing on the introduction of
multiple DGs into a small European town are presented. Two
typical network configurations based on real data have been
chosen for this study; a meshed residential network and a
residential network in a station to station configuration. The
DGs have been virtually introduced into the network to
calculate the impact on network parameters. Different criteria
(e.g., voltage profile, equipment loading voltage rise, etc.) are
chosen to compare the two configurations and to support the
decision process for the placing of DG.
In addition, a detailed model of a power electronic interface
and techniques based on robust stability analysis of systems
with multiplicative model uncertainty are used to show that
both the frequency response of the installed units, in particular
resonant peaks, and the distance between MV/LV
transformers and the installed DGs are critical.
Based on the results of the studies, the feasibility of a “plug
and power” concept is evaluated.
Keywords: protection, modeling, network analysis, stability
1
INTRODUCTION
Interest in and application of Distributed Power Generation
(DG) has been growing in recent years for a number of
reasons. Emerging technologies have made small power
generation more flexible, scalable and economical. Combined
with the enhanced efficiency and environmental friendliness
(usage of heat and power, low emissions, low noise) DG is in
a very good position to provide power generation for the
future and large growth is expected through a wide range of
applications [1].
Small generators have been installed in large numbers for a
long time, but predominantly in emergency power and UPS
applications. Emergency systems have different requirements
(different operating hours, etc.) and do not have to comply
with the same regulations as grid connected systems. As they
are not permanently operating, their impact on the distribution
network under normal operating conditions is small. Installing
DG suitable for regular (and backup) power instead of pure
“stand by”- power asks for different schemes of
interconnection as the distribution network has not been
designed for large-scale installation of DG. In the past, the
advantages (economies of scale, available technologies,
geographic location) of large centralized power plants
outweighed the disadvantages (need for strong transmission
network, transmission losses, limited usage of waste heat) and
made them the optimal cost effective solution for generation
of electricity. Accordingly, the public grid has been designed
in a top down manner with a relatively small number of very
large generators feeding a very large number of relatively
small loads.
When applying DG in a large scale, the existing network will
be operated with different power flows. The network must not
be negatively disturbed; there will be constraints on the
placing and operation of DG and/or alterations in the network
may be necessary. The interconnection of DG units with the
grid has to be done without negatively impacting safety,
reliability and quality of supply. It has to be non-disruptive
and economical, particularly when applied to smaller units.
Furthermore, regulatory issues have to be considered (see also
[2]).
DG has many different faces and can be defined quite
differently [1]. Characteristically, a DG unit is a small source
of electric power generation or storage that is not a part of a
large central power source and is located close to the load.
These can be either grid connected or operate independently
of the grid. Those connected to the grid are typically
interfaced to the distribution system, and thus dispersed across
the utility’s electric network rather than concentrated in a
single location. Different definitions of DG refer to power,
proximity to load, mode of dispatching, mode of planning
(centrally or not) and other criteria. We can further distinguish
between the types of networks that are used for
interconnection (voltage level, industrial network, public
network, etc.). Larger DG (which are connected at MV levels)
and industrial DG usually allow for detailed planning,
whereas small residential units should be connected with a
scheme as simple as possible. No extensive planning (such as
an individual network study for each unit) is possible on this
level, as the costs for a single unit are too high.
For this reason, new local sources on the LV-level should be
introduced with a “plug and power” concept: all the possible
problems and restrictions should be checked on a broad level
during a planning phase and no further actions should be
required by the end-users when introducing individual units.
The focus of this paper is on the technical issues for
interconnecting DG units in the range below 1MW to the
public LV-grid.
2
Protection for DG depends largely on the topology of the
network and the existing protection schemes.
NETWORK ANALYSIS
A network analysis is needed to determine the impacts of DG
on the network in a given case: Changes in power flow,
changes of electrical parameters of the network (voltage
profiles, etc.), equipment loading, requirements for protection
etc. Different protection schemes at both low voltage and
medium voltage level will be required [6]. These will have to
allow for reverse power flow while assuring tripping in case
of a fault on the MV side. Stability of the network can be
compromised through the interaction of DG and the grid. A
network analysis is common practice on the MV level, where
wind turbines, industrial DG or larger CHP plants may be
connected. However, small DG units shall be interconnected
on the low voltage level on an almost random basis. A “plug
and power” scheme for small DG is highly desirable. Yet
practical experience with extensive deployment of DG on the
low voltage level is scarce.
2.1
Parts of a typical German city were chosen to perform the
studies (Area of ca. 2 km2). Real network data (transformers,
lines etc.) was used and hypothetical DGs were placed in
different locations. Different sizes and types of DG were used
(50 kW to 500 kW, synchronous generator and inverter type
DG). A small part of the chosen area can be seen in Figure 1.
The virtually introduced DG are marked with the
-Icon.
Figure 2: Meshed LV-network
There are a number of different network structures, each with
a different behavior [3],[4]. For comparative reasons, two
different network topologies were considered in this study. A
meshed low voltage grid with three redundant medium
voltage lines (Figure 2) and the same grid supplied on a
station to station basis (achieved by opening the connectors in
selected cable distribution cabinets between the different parts
of the grid, marked KD in Figure 1).
CASE STUDIES
Case studies with real data of an existing small town were
performed to study the effect of installing a large number of
DGs on the low voltage network. The simulations have been
carried out with ABB’s network analysis tool CALPOS [14].
Figure 3: LV-network in station to station configuration
Figure 1: Part of small town with DG (<-> ca. 250 m)
In the station to station topology (Figure 3), there is no
connection between different transformers on the low voltage
level. Every low voltage grid has its dedicated transformer. In
both cases, there were load profiles with a maximum power
consumption of 5MVA assumed (corresponds with the real
consumption of the considered grid). A DG generation profile
with a maximum power generation of 7 MVA was applied.
The effect of the DG on the electrical parameters of the
network was determined by short circuit and load flow
calculations. All calculations were carried out for a reference
case without DG and for the cases with the DG. No alterations
on the network to improve the performance were made at this
point (as to check the impact of random placing of DG on an
existing network). The following summary of results is based
on the application of load profiles for all loads in the network.
140
(according to the voltage rise constraint) in a given point.
Nodes closer to a substation have a higher maximum
allowable power generation than remote nodes on long lines,
as the short circuit power is much higher close to a
transformer.
120
number of
nodes out of
profile
100
35
80
30
60
25
40
20
Units
20
15
non meshed
meshed
0
DG off
1
0.9
10
5
0.8
Cos(phi)
0
20
50
100
Non meshed
Meshed
200
500
DG power (kW)
1000
Figure 4: Voltage range violation
 Voltage range constraints (-10/+6% for low voltage in
Germany) may be violated depending on power
generation, reactive power and grid configuration. It can
be seen from Figure 4, that regarding the voltage profile,
the meshed configuration can generally handle more DG.
The generation of reactive power has a negative impact on
the profile; many more node voltages are out of the
allowable range at cos()=0.8.
 The voltage rise caused by a single unit is a function
(Equation 1, [13]) of DG power (SDG) and short circuit
power of the grid in the point of common coupling
(S”kPCC). Larger single units may violate the voltage
profile or constraints of maximum voltage rise even if the
aggregated power in the grid is low.
Δu  1 
 S
j  ψ
  DG 
S DG
S
 e  kPCC
 1  1  2  "DG  cosψ kPCC   DG    "DG
"
S kPCC
S kPCC
 S kPCC
2

  1

Equation 1: Voltage rise in function of SDG and S”kPCC
du
[%]
10,00
9,00
8,00
7,00
6,00
5,00
4,00
3,00
2,00
1,00
0,00
0
DG
SDG = 0.5 MW
S”kPCC= 4.2 MVA
SDG = 0.1 MW
S”kPCC= 1.2 MVA
SDG = 0.5 MW
S”kPCC= 5.5 MVA
Figure 6: Units according to voltage rise constraint
Figure 6 shows the number of nodes where certain DG
(maximum admissible power according to the voltage rise
constraint) can be placed (for all main nodes in the grid). Note
that in the meshed configuration, the majority of nodes can
handle more power (200kW – 500kW) than in the station to
station configuration (~100kW), as the meshed configuration
has generally a higher short circuit power.
 The short circuit power in the grid rises because of the DG
short circuit current contribution. This may result in
unacceptable short circuit levels. One cable distribution
cabinet in the meshed configuration exceeded the
admissible value (50kA in this particular case). In the
station to station configuration, short circuit currents are
much lower. No point in the grid exceeded the admissible
value. Synchronous generators were assumed for these
calculations. With inverter based system, the short circuit
current contribution is negligible.
 Cables in the LV network may overload due to changes in
power flow. It can be seen from Figure 7 that this is
mainly a problem in the station to station configuration
where some cables are overloaded even in the case of
cos()=1 for all the DG. In the meshed configuration, no
cables are overloaded. Additional simulation show, that
even if one of the three MV strands fails (meshed
configuration), the loading remains within the admissible
limits for all cables.
SDG = 0.25 MW
S”kPCC = 5.5 MVA
14
12
0,02
0,04
0,06
0,08
S DG /S"kPC C
0,1
0,12
Figure 5: Voltage rise in station to station network
Figure 5 shows the analysis of the grid (station to station
topology) with the inserted DG. It can be seen, that a number
of the randomly placed generators violate the voltage rise
constraint (3% voltage rise in Germany [5]). When calculating
the short circuit power in all the nodes of the network, a map
can be drawn with the maximum allowable power generation
10
number of
overloaded
cables
8
6
4
2
Non meshed
Meshed
0
DG off
1
Cos(phi)
0.9
0.8
Figure 7: Overloaded cables
 Losses in the distribution network can be reduced with the
implementation of DG. Figure 8 shows, that for both
configurations, losses are lowest for cos()=1 for all DG.
This value could be further optimized by determining the
reactive power individually for each DG. The avoided
losses of around 40kW (through the usage of DG)
correspond with approximately 1% of the power
consumption in the considered case.
250
200
150
Losses
100
(kW)
50
Non meshed
Meshed
0
DG off
1
0.9
Cos(phi)
0.8
Figure 8: Losses
The protection of the grid (MV and LV) and DG is affected in
several ways.
 If distance relays (impedance measurement) are used
(instead of DTO or ITO), parameter settings are affected
by the infeed effect [6]. The determined impedance is
changed due to current input along the protected line. This
may shift the selective tripping schedule and selectivity
may not be achieved. A re-setting of the distance relay
parameters is necessary whenever a significant power is
introduced into the MV-network.
 The selectivity of short circuit current indicators may be
compromised. In case of a fault, the indicators may not
only trip from the feeder side, but also from the dead end
side of the line. This may happen if there is a significant
short circuit current contribution by DG (possible with
synchronous generators, but not usually with inverter
based systems).
 The fusing scheme on the LV level may have to be
adjusted to prevent overloading of lines. If a DG is
introduced along a line, the line may be overloaded
between the DG and a load on the same line without
blowing the fuses at the end of the line. In this case
additional fuses are needed.
 Reverse power relays (these relays are located at each
transformer in a meshed LV grid and trip in case of a fault
in the corresponding MV line) in meshed systems may
have to be replaced or readjusted, as they may trip under
normal working conditions when DG on the low voltage
level are delivering power to the medium voltage level.
An individual analysis has to be made to determine, if the
nominal current differs sufficiently from the fault current
to achieve selectivity.
 Additional protection devices are needed for DG
interconnection, as required from regulations (e.g. loss of
mains detection).
It can be concluded that selectivity can be compromised both
on LV and MV level. Accordingly, measures have to be taken
(adjustment of protection devices, changes in protection
schemes). The two cases considered behave quite differently.
Both have their advantages for the placing of DG.
 Meshed distribution networks have a high short circuit
power. Their advantage is a relatively balanced voltage
profile and high reliability through redundancy. These
networks can usually handle more aggregated DG power,
if the short circuit power stays within allowable limits.
 Station to station based networks have a lower short
circuit power, are relatively simple to design, but the
voltage profile is more vulnerable to load steps. The
introduction of DG usually has a higher impact on the
voltage profile in these configurations (Depending on the
location in the network). Station to station based networks
do not need reverse power relays and therefore do not
need any alternative solution for these relays when
introducing DG.
A meshed grid can usually be converted to a station to station
configuration by opening the disconnectors in the according
cable distribution cabinets. This could be necessary, when the
short circuit current in a meshed system increases above
allowable values (according to the installed equipment) by
introducing DG. The splitting of a meshed network reduces
the short circuit power in any given point in the grid.
3
STABILITY ANALYSIS
Most of the DG sources are connected to the distribution grid
via power electronic converters. Various authors have noted
[7],[8] that mass introduction of power electronic devices to a
distribution network could result in stability, protection, power
flow, and harmonics difficulties. A well-documented case is
the Swiss national railway shut down [9] caused by
interaction of newly introduced converter driven locomotives.
This section investigates the effect of connecting
concentrations of DG-inverters to a low voltage bus-bar (A
specific 100kW inverter is chosen). In this study, the unit is
locally controlled and no coordinating control is allowed.
Similar models can be developed for different DGs, with the
appropriate Power Electronics interface.
Prime Mover
Rectifier
Converter
~
=
Grid
EMC filter
~
=
Generator
~
DC link
Figure 9: Inverter interconnection scheme
The rectifier, the generator, and the prime mover are modeled
as constant DC Thevenin equivalent, i.e. a constant voltage
source and a resistance. With the exception of the converter,
all elements shown in Figure 9 are modeled as linear time
invariant systems in a rotating dq0 coordinate system [10].
The converter is however inherently non-linear and for
analysis purposes a suitable linear model around a specific
operating point must be identified. By assuming ideal
switching, and no energy loss or storage in the converter a
linearized model can be derived [11].
By combining this model with models of the other
components illustrated in [Figure 9] a transfer function for the
DG can be computed. In particular for grid stability analysis a
voltage controlled current source model is required:
I l,dq   H  Vl,dq 
Equation 2
The model gives an indication of the inverter currents
produced when grid voltage fluctuations occur. Equation 2 is
a general description of various DGs while values of the
matrix [H] depend on the specific system.
3.1
ANALYSIS METHOD
Figure 10 illustrates a low voltage network with a connected
DG. Note that the figure represents a single bus (node Vl)
connected via a transmission line (Zl) to a strong grid. By
adjusting the transmission line characteristics a weak grid
condition can be simulated.
Vs
Transmission line
Vl
result from [12] which gives an upper bound on the size of
delta that guarantees loop stability.
3.2
RESULTS
The models and analysis techniques presented above were
tested on the scenario of the small town in Germany presented
in paragraph 2.1. The permissible concentrations of DG units
at various nodes in the low voltage network were calculated. It
must be noted that:
 Only the DG system model described above is considered,
the matrix [H], parameters of the chosen DG inverter
system (100kW).
 All the distributed generators are connected to the same
grid node and they feed energy into the grid.
 There are no disturbance loads connected to that node.
 For analysis, the transmission lines (Zl in Figure 10) are
modeled as series L, R impedances with equivalent grid
values calculated at specific nodes using the data from the
small town case study.
5
4
DG-Inverter 1
Il
I l  f Vl 
3
Units
2
Zl
1
Stiff generator
DG-Inverter 2
Il
I l  f Vl 
Figure 10: LV equivalent network with DGs (considered as load)
The question under investigation is how many DGs can be
connected to Vl without causing stability problems. For our
purposes, this implies that if there is a bounded disturbance in
Vs the current through the transmission line (or alternatively
the voltage at Vl) remains bounded. This can be addressed by
considering Figure 10 in a control theoretic framework as
shown in
Figure 11.
Vs

Vl
+
-
+
H
e
 Il
+
Zl
Figure 11: Negative feedback of the network impedance
In this case, delta is a scalar (representing the no. of additional
DGs) and the stability question reduces to identifying the
bounds within which delta can vary without resulting in an
unstable system. One approach is to use the robust stability
0
0
75
150
285
385
475
Distance from MV/LV transformer (m)
Figure 12: Number of units in safe operation at increased
distance from the MV node.
Figure 12 shows the effect of the line inductance on the
number of DG in safe operation. Connecting the units directly
on the MV transformer (considering the inductance of the
cables negligible) the number of possible parallel units is 5,
while at 500m away from the transformer, this number is
reduced to two units. It is evident that an increased inductance
(both transformer and cables) leads to a reduced stability of
the system. These numbers depend strongly on the DG
control parameters and the filter values (L, R and C). They
apply only for this specific inverter.
Additional DG via transmission lines can be added to the
node of interest (Figure 13) in order to make more realistic the
simple configuration of Figure 10. The results obtained are
similar to Figure 12 in case of short lines (<1km), while for
lines longer than 1km the interaction of the new unit can be
neglected.
Figure 13: Additional connection of sources via transmission lines
4
CONCLUSIONS
The presented case studies based on real data have shown that
distributed power generation in the same power range as the
load in the given network is feasible, specifically:

When the generated power in the grid is significantly
smaller than the grid power (e.g. several units in the low
kW-range from PV adding up to 25% of the load), a “plug
and power” scheme without detailed network analysis can
be applied.

At intermediate levels of power generation (up to 100% of
the load), care has to be taken with the placing of the DG
units. Depending on the network configuration, this may
still be possible without alterations in the grid.

Elevated levels of power generation (above 100% of the
load) may still be possible. Alterations or reinforcement of
the grid may be necessary depending on the existing
configuration.
A random placing of units at an elevated power level as in the
presented case studies cannot be done (generated power
equaled 125% of load). Several constraints were violated, yet
they could all be met with some planning effort and
optimization of the network.
A large number of aggregated DG power on the LV network
will affect all three areas that were discussed in this paper
(network parameters, protection, stability). The impact on the
distribution network depends largely on the aggregated power
level, size and type of DG (synchronous generator/ inverter
based), configuration of network, modes of operation (local
supply/ selling power) and distribution of DG units. If the
aggregated generated power is restricted to below 100% of the
load in a low voltage – “microgrid”, requirements are
simplified, as there is no reverse power flow to medium
voltage networks.
Based on a specific location, a network analysis may lead to
general constraints that allow a “plug and power” scenario
within certain limits (limitation of the aggregated power as
well as a limitation of the DG size, requirements for cos(),
restriction for reverse power flow and constraints on the
proximity of DG). If any of these constraints cannot be met,
detailed planning is needed. In such a case, some modern
planning tools provide specific functions [14] to optimize
distribution networks with DG (e.g., optimization of
transformer tap settings, MV-separation points, reactive
power, etc.). Modern DG units with power electronics can
provide flexible cos() and can have a positive impact on
voltage profile (ev. including local U/Q control) and losses.
Regarding stability, the study has demonstrated that the
number of units that can be connected on the same node
without causing instabilities is strongly dependent on the
network impedance at the point of common coupling as well
as on the filter design and the control of the unit. This may
limit a “plug and power” scheme. DG suppliers usually do not
guarantee parallel operation with other inverter-based
equipment, as stability can be compromised.
If the DG do not feed back power to the utility but only serve
as local supply support, there is no negative impact on the
operation of the grid (But care has to be taken with short
circuit current contribution, protection and stability). The
customer can supply a large part of his own load without
feeding back to the grid; the loading of the network is
decreased. In this way, a significantly large portion of the
power generation can be provided by DG.
5
REFERENCES
[1] Peter Dondi et al., Network integration of distributed
power generation, Journal of Power Sources, Special
Issue Grove Fuel Cell Symposium, 2002
[2] Isabel Alvrez Ortega, Marco Suter , Interconnection of
distributed Power Generation Resources to the European
Low Voltage Electrical Grid, IASTED International
Conference on Power and Energy Systems, 2001
[3] E. Lakervi and E.J. Holmes, Electricity distribution
network design (London, Peregrinus Ltd.,2nd ed, 1995)
[4]
Planung
und
Betrieb
städtischer
Niederspannungsnetze, VDEW, Frankfurt, 1984
[5] Parallelbetrieb mit dem Niederspannungsnetz,
VDEW, Frankfurt, 3. Edition 1991/ Reprint 1996
[6] Walter A. Elmore, Protective Relaying, Theory and
Applications (New York, ABB Power T&D Inc, 1994)
[7] N. Hadjsaid, J. Canard, F. Dumas, Dispersed
Generation Impact on Distribution Networks, IEEE
Computer Applications in Power, April 1999, 23–28
[8] CIGRE Working Group 37.23, Impact of increasing
contribution of dispersed generation on the power system,
Cigre report no. 137, February 1999
[9] M. Meyer and Jürg Schöning, Netzstabilität in grossen
Bahnnetzen., Eisenbahn Revue No 7-8, 1999, 312–317
[10] P. Kundur, Power system stability and Control
(McGraw Hill, 1994)
[11] A. Stothert, E. Möllerstedt, A Model of a MicroTurbine Line Side Converter, IEEE-PES / CSEE
International Conference on Power System Technology,
Perth Australia, December 2000
[12] M. Green, D. Limebeer, Linear Robust Control
(Prentice-Hall, 1995)
[13] Audring, Netzbeeinflussung von BrennstoffzellenHeizkraftwerken
zur
Energieversorgung
von
Wohngebäuden, 2001
[14] Optimal Distribution Network in CALPOS User
guide v4 (Mannheim, ABB Calor Emag, 1999), 7-1 to 7-5