Download Mark SMITH - SIECI 2004 - Politechnika Wrocławska

Elektroenergetyczne sieci rozdzielcze – SIECI 2004
V Konferencja Naukowo-Techniczna
Politechnika Wrocławska
Instytut Energoelektryki
Emil DVORSKÝ
Pavla HEJTMÁNKOVÁ
Miloslava TESAŘOVÁ
University of West Bohemia in Pilsen, Faculty of Electrical Engineering
Department of Electrical Power Engineering and Ecology, Univerzitní 8, 304 16 Plzeň, Czech Republic
e-mail: [email protected], [email protected], [email protected]
UTILIZATION OF MULTIPLE PHOTOVOLTAIC POWER
GENERATION SYSTEM AT WBU IN PILSEN
Photovoltaic cells (PV) are one of the perspective technologies of distributed energy sources (DER).
PV grid interconnection must satisfied the technical requirements from both the utility power system
grid side and the PV system side in order to ensure the safety of the PV installer and the reliability of
the utility grid. Clarifying the technical requirements for grid interconnection and solving the
problems are therefore very important issues for widespread application of PV systems. PV grid
connection is determinate by many questions. Test results shows that grid interconnection of multiple
photovoltaic power generation systems has little effect on distribution line short circuit, AC-DC
mixing fault, and output variation, but does affect harmonics and islanding. Further consideration of
these effects, especially the islanding conditions, is required. The PV system of the University of
West Bohemia in Pilsen has possibility to investigate impact on PV penetration in distribution
network and other aspects.
1. INTRODUCTION
Small, distributed generation (DG) technologies such as micro-turbines, photovoltaic and
fuel cells are gaining wide interest because of rapid advances in technologies. The deployment of
these generation units on distribution networks could potentially lower the cost of power
delivery by placing energy sources nearer to the demand centres. The capacity of the devices
ranges from 1 kW to 2 MW in power level. The instantaneous power production from PV in the
near future will often exceed the instantaneous power consumption in residential areas with a
high concentration of PV systems. The power flow backwards through the MV/LV transformers,
i.e. the power flows from the LV network to the medium voltage (MV) network will increase.
There is need to know the impact of the backward power flow to the Electric power system
(EPS) and to set the upper limits to of PV amount that can be fed into a power system without
causing problems to the power systems and find the possibilities of stretching the limits.
The PV system of the University of West Bohemia in Pilsen has possibility to investigate
impact on PV penetration in distribution network and other aspects regarding the future PV cells
utilisation and interconnection to EPS. This system is one of the greatest systems of Central
Europe. Further, the other DER are concentrate in new campus of Faculty of power engineering,
such as the wind power plant, heat pumps, combined heat and power unit (CHP). These sources
together with storage possibilities and interconnection to EPS and central systems system of
municipal power and heat station allow to investigate cooperation integrate power systems. The
operation and regulation of the micro grid will be investigated.
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First the distributed power systems will be step by step equipped by the measure and
regulation equipments in dependence on the operation results in order to the backward impact on
the interconnected power systems should be minimised and maximal cooperation efficiency
achieved.
2. PV LOAD PLACEING
Regarding to the load place and the interconnection to the power supply system, the load
or DG systems can be divided to:
• Stand alone (SAPV systems) – grid off
• Grid on (IPV) – interconnected to EPS
In SAPV systems (Fig. 1), special attention must be paid to the used appliances and loads.
In appropriate loads is very often the origin of PV system malfunction or failure. Start-up power
peaks, or reactive power and harmonic distortion can cause system signal instability and
protective devices will close the system down.
Available
solar energy
ηP
PV modules
Regulator
Load
Loadfactor
Variation in
insolation
Battery
Load profile
Back-up energy system?
Figure 1. SAPV system
A well-matched load together with a carefully selected choice of appliances can lead to
significant savings in terms of reduced need for PV and electricity storage capacity. Conversely,
inefficient appliances and processes, standby loads and inappropriate loads will increase the
requirement for expensive PV and storage capacity.
In IPV systems, the power flow from PV cells can partly covers the power consumption
and the rest is covered by the EPS (Fig. 2). If the PV output is higher then consumption, a part of
backward power flow to the EPS. In any case PV cells have influences on the electric power
parameters changes. The major issue related to interconnection of distributed resources onto the
power grid is the potential impacts on the quality of power provided to other customers
connected to the grid. Attributes which define power quality include:
• Voltage regulation - The maintenance of the voltage at the point of delivery to each
customer within an acceptable range.
• Flicker - The repetitive and rapid changes of voltage, which has the effect of causing
unacceptable variations in light output and other effects on power consumers and their
equipment.
• Voltage imbalance - The grid voltage does not have identical voltage magnitude on each
phase, and a 120° phase separation between each pair of phases.
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• Harmonic distortion - The injection of currents having frequency components which are
multiples of the fundamental frequency.
• Direct current injection - A situation which can cause saturation and heating of
transformers and motors, and can also cause these passive devices to produce unacceptable
harmonic.
PV array
System
User load
Invertor
Parray
Uarray
Grid
Pouver
Pout,inv
P
=
Pused
Pback-up
N
~
~
load
Figure 2. IPV system
3. PHOTOVOLTAIC POWER GENERATION SYSTEM AT WBU IN PILSEN
The PV system of WBU is the part of the new renewable mini-park sources, which was
created in the new campus of Faculty of electrical engineering. The park includes PV systems
(IPV, SAPV), solar systems, heat pumps and wind power station. That means, that together with
other power means as batteries, heat pumps, thermal load, it can be possible to simulate the DER
work and their influences on power quality in the interconnection place. The interconnections of
the PV array to EPS and measurements possibilities are shown in Fig. 3. The PV trecker is used
for the research activity connected with influence of air pollution on PV cells. In the near future,
the system will be equipped with others measurement and regulation means for the purpose of
the micro-grid utilization and control.
4. PRESENT INVESTIGATION RESULT
Voltage in electric power systems with DER sources
One of the problems which are solved at present is setting which PV operation state and
interconnection to EPS is suitable and maintenance of the voltage in interconnection place in
definite limits. It is the main reason which limits penetration of PV systems in electric power
networks. The results of the IEA Photovoltaic Power Systems Programme (PVPS) Task V was
applied one the WBU PV system interconnection conditions.
The fundamental condition for the computation is that the supply voltage at the delivery
point (I J, Fig. 4) must lie within ±10% of the nominal voltage for European LV networks
according to EN 50160. Stricter limits may apply nationally, both in Europe and elsewhere.
Accepted voltages typically lie between 90% and 106% of the nominal voltage. Stricter limits
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may apply nationally, both in Europe and elsewhere. Accepted voltages typically lie between
90% and 106% of the nominal voltage.
Figure 3. WBU PV system
VHV/VHV
A
B
C
VHV/HV
D
E
HV/HV
F
HV/LV
G
110
UPPER LIMIT
105
100
95
90
LOWER LIMIT
85
Figure 4. Voltage drops in the central supply EPS
H
I
J
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The PV penetration can be simplified and limited to the MV and LV due to automatic
voltage regulation and therefore approximately 100 % voltage values in point A (Fig. 5). The
results are in Fig. 6. As the computation data, the typical values for the EPS were assumed - five
LV lines per MV/LV transformer, 10 MV/LV transformers per ring, average values for line
impedances, line lengths and transformer sizes. The minimum load is set to 25% of the
maximum load.
HV EQVIVALENT
C2
B2
HV RING
D2
HV/LV
A
0,4 kV
B1
C1
D1
Figure 5. Schema for PV penetration computation
Three cases of high PV penetrations are investigated:
• high PV penetration from a single LV line (in the order of 30-80 kWp)
• high PV penetration from all the LV lines connected to a single MV/LV
transformer (in the order of 200-400 kWp)
• high PV penetration from all the MV/LV transformers connected to a 10 kV ring
(in the order of 1-2 MWp)
MAXIMUM PV
PENETRATION
(% of MAX. LOAD)
MINIMUM LOAD
160
140
120
A
100
B
80
C
60
40
20
0
0
10
20
30
40
50
60
70
80
90
LOAD (% OF MAXIMUM LOAD)
Figure 6. Overall limits of PV penetration
100
132
In principle, no PV penetration is acceptable at minimum load. However, the excess
voltages are rather limited for PV penetrations up to the minimum load. In addition, only a small
increase in the load from the minimum load opens up for a considerable amount of PV,
especially if PV power only penetrates from a single LV line.
Economy of SAPV application
4
3,5
Power needed (kWp)
3
Decreasing PV module cost
Grid connection is cheaper
2,5
2
1,5
1
PV is cheaper
0,5
0
0
0,5
1
1,5
2
2,5
Distance from existing grid (km)
The SAPV are usually applied
in the place with the higher distance
from EPS. For the investors and
operators, in this case, the economy
is important. They are trying to find
the best economy solution for the
load requests. It mainly depends on
costs. The computation can be solved
by the objective function creation
and the minimum function value
finding. It is clear that this function
depends on the load place distance
from the EPS and the PV cells
installation cost (Fig. 7). The present
situation will be change by the next
PV cells costs decreasing.
Figure 7. SAPV economy
5. CONCLUSION
In the longer term, it is expected that more flexible and accommodated consumption will
remove the barriers of limits to PV penetration. It is essential that PV, together with other
elements of distributed generation must be considered in the future network planning. For the
fulfillment of the renewable power source increasing in Europe, as was announced by European
committee, it is necessary to find the more suitable technology and economy solutions in DER
enlargement. Therefore, it is need to do new investigation in the DER utilization and their
interconnection to the present power systems. The PV system of WBU in Pilsen has the
possibilities to improve PV utilization efficiency in West Bohemia region and make the research
and education work on the field cooperation with other European institutions.
This paper was written as a part of the GAČR project (GAČR 102/02/0949).
REFERENCES
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[4]
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distribution power networks. 13 th International expert meeting “Power Engineering”, Maribor,
Slovenia, 2004, ISBN 86-435-0617-6
Noháčová, L. Mühlbacher, J. Noháč, K.: The Problematic of Distributed Power Systems, Second
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