Analysis of Prerequisites for Connection of a Large-Scale
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IN DEGREE PROJECT ENERGY AND ENVIRONMENT, SECOND CYCLE, 30 CREDITS , STOCKHOLM SWEDEN 2021 Analysis of Prerequisites for Connection of a Large-Scale Photovoltaic System to the Electric Power Grid FANNY LILJA KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE
Analysis of Prerequisites for Connection of a Large-Scale
-authorIN DEGREE PROJECT ENERGY AND ENVIRONMENT, SECOND CYCLE, 30
CREDITS
, STOCKHOLM SWEDEN 2021
Analysis of Prerequisites for Connection of a Large-Scale
Photovoltaic System to the Electric Power Grid
FANNY LILJA
KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF ELECTRICAL ENGINEERING
AND COMPUTER SCIENCE
Analysis of Prerequisites for Connection of a Large-Scale
Photovoltaic System to the
Electric Power Grid
FANNY LILJA
Master’s Degree Project in Electric Power Systems School of
Electrical Engineering & Computer Science KTH Royal Institute
of Technology Stockholm, Sweden 2021
Supervisors: Mikaela Liss, Solkompaniet Anton Gronkvist, E.ON
Energidistribution Elis Nycander, KTH Royal Institute of
Technology
Examiner: Lennart Soder, KTH Royal Institute of Technology
Abstract
The deployment of large-scale photovoltaic (PV) systems is rising
in the Swedish power sys- tem, both in quantity and in system size.
However, the intermittent characteristics of the PV production
raises questions concerning the stability in the electric power
grid, and power output fluctuations from the PV systems can lead to
voltage quality issues. Hence, the dis- tribution system operator
E.ON Energidistribution and the solar energy developer company
Solkompaniet are interested in investigating potential challenges
and possibilities related to the integration of large-scale PV
systems in the electric power grid. This thesis studies fast
voltage variations in the electric power grid due to output
fluctuations from large-scale PV systems, and examines the
possibility to mitigate the voltage variations by reactive power
support strategies in the PV inverters.
Four studies are carried out to investigate the prerequisites for
establishing large-scale PV systems. Firstly, a worst-case study
considering eight existing substations in the electric power grid
as well as a new substation is carried out, to examine the impact
of different parameters on the voltage variations. Parameters such
as transformer operation mode, location of the point of connection,
switching mode and load capacity are compared in the study.
Further, time series calculations are done to investigate the
voltage variations over one year, and a study with an oversized PV
system is done to investigate the possibility for increasing the PV
capacity without grid reinforcements. Lastly, a study is performed
with reactive power compensation from the PV inverters to examine
the possibility to maintain a stabilized voltage level at the point
of connection. The studies are performed in E.ONs network model in
the power system simulator software PSS/E, with data for the
transmission grid, the regional grid, and parts of the distribution
grid included. PV systems with a rated capacity from 32 MWp and
upwards are connected to substations in the regional grid, where
fast voltage variations on nominal voltage levels of 20/10 kV are
studied and evaluated from the perspective of the power
producer.
From this thesis, it can be concluded that neither of the
implemented studies results in voltage variations that violate
E.ONs technical requirements on fast voltage variations in the
point of connection. Further, the results from the worst-case study
show the importance of analysing the specific system of interest
when connecting PV systems, since the properties of the existing
system have an impact on the voltage variations. The time series
calculations show that the voltage variations over a time period of
one year are highly influenced by the PV production and the load
capacity in the substation, and the study with an oversized PV
system shows the possibility for increasing the PV capacity without
curtailing large amounts of active power. Finally, the study with
reactive power compensation concludes that grid support strategies
in the PV inverters may be a key solution for making optimal use of
the existing electric power grid and enabling the continued
expansion of large-scale PV systems in the Swedish power
system.
Keywords: solar energy, large-scale photovoltaic system, PV,
Swedish power system, power output fluctuations, voltage quality,
voltage variations, reactive power compensation, PSS/E
Sammanfattning
Fyra studier genomfors for att undersoka forutsattningarna for att
etablera storskaliga sol- cellsanlaggningar. For det forsta
genomfors en varsta-fallstudie med beaktande av atta befint- liga
stationer i elnatet samt en ny station, for att undersoka olika
parametrars paverkan pa spanningsvariationerna. Parametrar som
transformatorns driftlage, plats for anslutningspunk- ten,
omkopplingslage och lastkapacitet jamfors i studien. Vidare gors
tidsserieberakningar for att undersoka spanningsvariationerna over
ett ar, och en studie med en overdimensionerad solcellsanlaggning
gors for att undersoka mojligheten att oka solcellskapaciteten utan
elnats- forstarkningar. Slutligen genomfors en studie med reaktiv
effektkompensation fran vaxelriktare for att undersoka mojligheten
att uppratthalla en stabiliserad spanningsniva i anslutningspunk-
ten. Studierna utfors i E.ONs natverksmodell i programvaran PSS/E
for kraftsystemsimule- ringar, med data for transmissionsnatet,
regionnatet och delar av distributionsnatet inkluderat.
Solcellsanlaggningar med en nominell kapacitet fran 32 MWp och
uppat ansluts till stationer i regionnatet, dar snabba
spanningsvariationer pa nominella spanningsnivaer om 20/10 kV
studeras och utvarderas ur kraftproducentens perspektiv.
Fran resultaten kan man dra slutsatsen att ingen av de genomforda
studierna resulterar i spanningsvariationer som overskrider E.ONs
tekniska krav pa snabba spanningsvariationer i anslutningspunkten.
Vidare visar resultaten fran varsta-fallstudien vikten av att
analysera det specifika systemet vid anslutning av
solcellsanlaggningar, eftersom egenskaperna hos det befintliga
systemet har en inverkan pa spanningsvarationerna.
Tidsserieberakningarna visar att spanningsvariationerna over en
tidsperiod av ett ar paverkas starkt av bade energiproduktionen och
lastkapaciteten i stationen, och studien med en overdimensionerad
solcellsanlaggning visar pa mojligheten att oka den nominella
kapaciteten utan att spilla stora mangder aktiv effekt. Slutligen
ger studien med reaktiv effektkompensation slutsatser om att
strategier i vaxelriktare kan vara en mojlig losning for att
utnyttja det befintliga elnat optimalt och mojliggora en fortsatt
expansion av storskaliga solcellsanlaggningar i det svenska
kraftsystemet.
Nyckelord: solenergi, storskalig solcellsanlaggning, PV, Sveriges
kraftsystem, effekforandringar, spanningskvalitet,
spanningsvariationer, reaktiv effektkompensering, PSS/E
Acknowledgements
I would like to thank all who have contributed and made this thesis
possible to conduct. Firstly, a warm thank you to Lars Hedstrom at
Solkompaniet and Andreas Svensson at E.ON Energidistribution for
believing in me and giving me the opportunity to implement this
thesis in cooperation with them. Next, I would like to address a
special thank you to my supervisors, Mikaela Liss at Solkompaniet,
Anton Gronkvist at E.ON Energidis- tribution and Elis Nycander at
KTH Royal Institute of Technology, for their guidance throughout
the entire thesis work. I am deeply grateful for all the support,
knowledge, and great ideas I received from them. Last but not
least, I would like to thank Lennart Soder, my examiner at KTH
Royal Institute of Technology, for his revision and feedback
throughout the thesis work.
Table of Contents
1 Introduction 8 1.1 Background . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 8 1.2 Motivation . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.3 Research
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 10 1.4 Scope . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 11 1.5 Report Outline . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 12
2 Theoretical Background 13 2.1 The Power System . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 13
2.1.1 Overview of the Electric Power Grid in Sweden . . . . . . . .
. . 13 2.1.2 Characteristics for Power Transmission . . . . . . . .
. . . . . . . 14
2.2 Technical Requirements for Voltage Quality . . . . . . . . . .
. . . . . . 16 2.2.1 Voltage Quality . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 17 2.2.2 The Network Operator . . . . . .
. . . . . . . . . . . . . . . . . . 17 2.2.3 The Power Producer . .
. . . . . . . . . . . . . . . . . . . . . . . 19
2.3 Solar Photovoltaics . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 21 2.3.1 Photovoltaic Systems . . . . . . . . . . .
. . . . . . . . . . . . . . 21 2.3.2 Power Output Fluctuations . .
. . . . . . . . . . . . . . . . . . . 22 2.3.3 Potential Challenges
with Photovoltaics . . . . . . . . . . . . . . 23
2.4 Photovoltaic Inverters for Voltage Regulation . . . . . . . . .
. . . . . . 25 2.4.1 Control Schemes . . . . . . . . . . . . . . .
. . . . . . . . . . . . 26 2.4.2 PQ Capability Chart . . . . . . .
. . . . . . . . . . . . . . . . . . 27 2.4.3 Implementation
Strategies . . . . . . . . . . . . . . . . . . . . . . 28
2.5 Alternative Solutions for Voltage Regulation . . . . . . . . .
. . . . . . . 29 2.5.1 Tap Changers . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 29 2.5.2 Shunt Compensators . . . . . . .
. . . . . . . . . . . . . . . . . . 30 2.5.3 Battery Energy Storage
Systems . . . . . . . . . . . . . . . . . . . 30 2.5.4 Solar-Wind
Complementation . . . . . . . . . . . . . . . . . . . . 31
3 Methodology 32 3.1 Power System Modelling Software . . . . . . .
. . . . . . . . . . . . . . . 32 3.2 Network Model Description . .
. . . . . . . . . . . . . . . . . . . . . . . 32
3.2.1 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 32
1
3.2.2 Switching Mode . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 33 3.2.3 Load- and Generator Units . . . . . . . . . . .
. . . . . . . . . . . 33 3.2.4 Operation Mode . . . . . . . . . . .
. . . . . . . . . . . . . . . . 34 3.2.5 Point of Connection . . .
. . . . . . . . . . . . . . . . . . . . . . . 35
3.3 Power Flow Simulations . . . . . . . . . . . . . . . . . . . .
. . . . . . . 37 3.3.1 Power Output Fluctuations . . . . . . . . .
. . . . . . . . . . . . 37 3.3.2 Load Flow Calculations . . . . . .
. . . . . . . . . . . . . . . . . 38 3.3.3 Voltage Quality
Assessment . . . . . . . . . . . . . . . . . . . . . 39
3.4 Worst-Case Study: Existing Substations . . . . . . . . . . . .
. . . . . . 39 3.4.1 Study Overview . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 39 3.4.2 Substations . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 40 3.4.3 Operation Mode &
Point of Connection . . . . . . . . . . . . . . . 42 3.4.4 Load
Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44 3.4.5 Short Circuit Capacity & Switching Mode . . . . . . .
. . . . . . 45 3.4.6 Power Flow Simulations . . . . . . . . . . . .
. . . . . . . . . . . 46
3.5 Worst-Case Study: New Substation . . . . . . . . . . . . . . .
. . . . . . 47 3.6 Time Series Calculations . . . . . . . . . . . .
. . . . . . . . . . . . . . . 49
3.6.1 Hourly Production Data . . . . . . . . . . . . . . . . . . .
. . . . 49 3.6.2 Hourly Active- and Reactive Load Data . . . . . .
. . . . . . . . 50 3.6.3 Power Flow Simulations . . . . . . . . . .
. . . . . . . . . . . . . 51 3.6.4 Constant Load Data . . . . . . .
. . . . . . . . . . . . . . . . . . 52
3.7 Oversized Photovoltaic System . . . . . . . . . . . . . . . . .
. . . . . . . 53 3.8 Reactive Power Compensation . . . . . . . . .
. . . . . . . . . . . . . . . 56
4 Results & Discussion 60 4.1 Worst-Case Study: Existing
Substations . . . . . . . . . . . . . . . . . . 60
4.1.1 Parallel Operation Mode . . . . . . . . . . . . . . . . . . .
. . . . 60 4.1.2 Sectioned Operation Mode . . . . . . . . . . . . .
. . . . . . . . . 64 4.1.3 Summary . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 69
4.2 Worst-Case Study: New Substation . . . . . . . . . . . . . . .
. . . . . . 75 4.3 Time Series Calculations . . . . . . . . . . . .
. . . . . . . . . . . . . . . 78
4.3.1 Time Series Calculations with Hourly Load Profiles . . . . .
. . . 78 4.3.2 Time Series Calculations with Constant Loads . . . .
. . . . . . . 86 4.3.3 Summary . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 88
4.4 Oversized Photovoltaic System . . . . . . . . . . . . . . . . .
. . . . . . . 91 4.5 Reactive Power Compensation . . . . . . . . .
. . . . . . . . . . . . . . . 96
5 Conclusion 101
List of Figures
2.1 PQ capability chart for the Sungrow inverter model SG250HX [1]
. . . . 28 2.2 Monthly electricity production from PV- and wind
power systems during
the period 2014-2017 [2] . . . . . . . . . . . . . . . . . . . . .
. . . . . . 31
3.1 Parallel transformer operation mode with a direct PV-connection
. . . . 35 3.2 Sectioned transformer operation mode with a direct
PV-connection . . . 35 3.3 Parallel transformer operation mode with
a separate PV-connection . . . 36 3.4 Sectioned transformer
operation mode with a separate PV-connection . . 36 3.5
Investigated parameters in the worst-case study . . . . . . . . . .
. . . . 40 3.6 Simulation chart for the power flow simulations in
the worst-case study . 46 3.7 The standardized and up-scaled hourly
PV production profile . . . . . . 50 3.8 Simulation chart for the
time series calculations . . . . . . . . . . . . . . 51 3.9
Simulation chart for the study with an oversized PV system . . . .
. . . 55 3.10 Reactive power compensation by the Q(U) curve in the
Volt-Var function 57 3.11 Simulation chart for the study with
reactive power compensation . . . . . 59
4.1 Voltage variation at POC in a parallel operation mode with a
direct PV- connection . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 62
4.2 Voltage variation in a parallel operation mode with a direct
PV-connection 63 4.3 Voltage variation in a sectioned operation
mode with a direct PV-connection,
both sections affected by the power output fluctuation . . . . . .
. . . . 65 4.4 Voltage variation in a sectioned operation mode with
a direct PV-connection,
one section affected by the power output fluctuation . . . . . . .
. . . . . 66 4.5 Voltage variation in a sectioned operation mode
with a separate PV-
connection, both sections affected by the power output fluctuation
. . . . 68 4.6 Voltage variation in a sectioned operation mode with
a separate PV-
connection, both sections affected by the power output fluctuation
. . . . 69 4.7 Voltage variation on each bus for each studied case
for the new substation 77 4.8 Voltage variation each hour of the
year for substation 1 . . . . . . . . . . 79 4.9 Voltage variation
each hour of the year for substation 2 . . . . . . . . . . 80 4.10
Voltage variation each hour of the year for substation 3 . . . . .
. . . . . 81 4.11 Voltage variation each hour of the year for
substation 4 . . . . . . . . . . 82 4.12 Voltage variation each
hour of the year for substation 5 . . . . . . . . . . 83 4.13
Voltage variation each hour of the year for substation 6 . . . . .
. . . . . 84
3
4.14 Voltage variation each hour of the year for substation 7 . . .
. . . . . . . 85 4.15 Voltage variation each hour of the year for
substation 8 . . . . . . . . . . 86 4.16 Voltage variation each
hour of the year at POC 1 of substation 2 . . . . . 87 4.17 Voltage
variation each hour of the year at POC 1 of substation 2 . . . . .
88 4.18 Correlation between the oversizing rate and the curtailed
active power for
different load sizing rates . . . . . . . . . . . . . . . . . . . .
. . . . . . . 91 4.19 Produced respective curtailed active power
each hour of the year for sub-
station 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 93 4.20 Produced respective curtailed active power
each hour of the year for the
new substation . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 94 4.21 Voltage variation each hour of the year at POC
1 with an oversizing rate
of 40% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 95 4.22 Voltage level and reactive power compensation
in the new substation every
hour of the year . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 97 4.23 Voltage level and reactive power compensation
in substation 6 every hour
of the year . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 98 4.24 Voltage level and reactive power compensation
in substation 6 every hour
of the year . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 99
List of Tables
2.1 Limitations on short-term voltage drops for Un ≤ 45 kV . . . .
. . . . . . 18 2.2 Limitations on short-term voltage drops for Un
> 45 kV . . . . . . . . . . 18 2.3 Limitations on short-term
voltage rises for Un ≤ 1 kV . . . . . . . . . . . 19 2.4
Limitations per day on the sum of rapid voltage changes and
short-term
voltage drops present in area A in Table 2.1 and Table 2.2
presented previously . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 19
2.5 Compilation of the ramp rate for different amounts of installed
PV capacity 23
3.1 Substation characteristics . . . . . . . . . . . . . . . . . .
. . . . . . . . 41 3.2 Investigated parameters for each substation
. . . . . . . . . . . . . . . . 42 3.3 Cable characteristics . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.4
Characteristics for the two cases with a new substation . . . . . .
. . . . 48 3.5 Constant load capacity for each substation in each
scenario . . . . . . . . 53 3.6 Load sizing rates and corresponding
dimensioning load factors for substa-
tion 1 in the correlation study . . . . . . . . . . . . . . . . . .
. . . . . . 54 3.7 Oversizing rates and corresponding rated PV
capacity for substation 1 in
the correlation study . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 54 3.8 Input data for the case study with an oversized
PV system . . . . . . . . 54 3.9 Specified voltage limits in the
Volt-Var function for each substation . . . 58
4.1 The short circuit capacity on the primary- and secondary side
of each substation in a parallel operation mode with a direct
PV-connection, for normal- and reserve switching mode respectively
. . . . . . . . . . . . . . 61
4.2 The short circuit capacity on the PV-bus at each substation in
a parallel operation mode with a separate PV-connection, for
normal- and reserve switching mode respectively . . . . . . . . . .
. . . . . . . . . . . . . . . 63
4.3 The short circuit capacity on the buses at each substation in a
sectioned operation mode with a direct PV-connection, for normal-
and reserve switching mode respectively . . . . . . . . . . . . . .
. . . . . . . . . . . 64
4.4 The short circuit capacity on the two PV-buses at each
substation in a sectioned operation mode with a separate
PV-connection, for normal- and reserve switching mode respectively
. . . . . . . . . . . . . . . . . . . . . 67
4.5 Compilation of the highest achieved voltage variation at POC 1
for each substation, in a sectioned operation mode with a separate
PV-connection 70
5
4.6 Compilation of the highest achieved voltage variation at POC 2
for each substation, in a sectioned operation mode with a separate
PV-connection 70
4.7 The short circuit capacity for each studied case for the new
substation . . 76 4.8 Compilation of the results from the case
study with oversized PV systems 92
6
List of Abbreviations
AC Alternating Current DC Direct Current DSO Distribution System
Operator GW Gigawatt HV High-Voltage I Current kV kilo-Volt LV
Low-Voltage MPPT Maximum Power Point Tracker MV Medium-Voltage MW
Megawatt MWh Megawatt-hour MWp Megawatt-peak P Active Power POC
Point of Connection PV Photovoltaic Q Reactive Power R Resistance S
Apparent Power Ssc Short Circuit Capacity STATCOM Static
Synchronous Compensator SVC Static VAR Compensator TSO Transmission
System Operator U Voltage VA Volt-Ampere VAR Volt-Ampere reactive
VSI Voltage Source Inverter W Watt X Reactance Z Impedance
7
Introduction
This chapter gives the background and the motivation for the
thesis. Further, the research objectives and the scope are stated,
followed by an outline of the remainder of this report.
1.1 Background
To mitigate climate change and reduce the carbon dioxide emissions
in the atmosphere, the transition from fossil fuels to renewable
forms of energy is becoming increasingly significant for the global
energy system. The integration of renewable energy technolo- gies
in the power generation sector is crucial for the overall
transformation, and the use of variable renewable energy resources
such as wind and solar are rising. A trend for both wind- and solar
power technologies is constantly declining costs, where the costs
are forecasted to further decline in the coming years [3]. Out of
solar power technologies, solar photovoltaic (PV) systems make an
essential part of the transition, where tech- nology improvements
and policy frameworks constitute the main driving forces for the
continuing development and growth of the PV market [4]. Data from
the International Renewable Energy Agency (IRENA) show that the
global weighted average levelized cost of electricity (LCOE) for PV
systems decreased by 77% during the period 2012-2018 [3]. That
declining costs is one of the drivers for the expansion of higher
penetrations of PV systems across the globe, is showing in the
recent increase in installed PV capacity. At the end of 2019, the
global installed capacity of PV exceeded 623 GWp whereof nearly 72%
have been installed over the last five years. Globally, centralized
large-scale PV systems dominate over distributed PV systems,
standing for approximately 63% and 37% of the cumulative installed
capacity respectively [4].
In the northern hemisphere, the deployment of PV systems in Sweden
is rising, and by the end of 2020 there were almost 66 000 PV
facilities connected to the electric power grid. Statistics from
the Swedish Energy Agency shows that, with a total installed
capacity of 1090 MWp, the development of PV systems exceeded the 1
GWp boundary. Compared to a total installed capacity of 698 MWp the
year before, that represented an
8
increase in capacity of about 56%. However, in contrast to the
global trend, the statistics show that the share of distributed PV
systems outreaches large-scale PV systems in the country. By the
end of 2020, the average system size corresponded to 17 kWp, and
large-scale PV systems greater than 1 MWp constituted about 5% of
the total installed PV capacity, an increase from 2.7% the previous
year [5]. In spite of the relatively small share of large-scale PV
systems in Sweden, the segment is constantly growing and expected
to increase both in quantity and in system size in the near future
[6].
However, even if variable renewable energy resources are considered
clean with plenty of benefits, the unique topologies of the
technologies raise concerns in the operation of the overall power
system. For solar power technologies, the intermittent and weather
dependent characteristics of the solar resource directly affect the
electric power genera- tion from grid-connected systems, where
variations in the solar irradiance proportionally result in power
output fluctuations. The effects of power output fluctuations
affects the stability of the electric power grid, leading to
challenges regarding the flexibility of the power system [7]. For
now, the electric power generation in Sweden substantially re- lies
on hydro- and nuclear power which have important functions in the
power system, contributing to voltage stabilization solutions.
Voltage stabilization is a crucial system service for a reliable
power system and is achieved by reactive power support strategies,
where supply and absorption of reactive power enables to regulate
the voltage level [4].
Conversion-based generation such as wind- and solar do not directly
contribute to volt- age stabilization in the same way as power
plants with synchronous rotating generators, and with more variable
renewable energy sources in the power system, it becomes in-
creasingly important how this is technically solved. However, there
are possibilities. Inverters with advanced functionalities are
being developed, where the inverters have the ability to contribute
with reactive power support strategies and other system ser- vices
for grid support [4]. To adapt for the ongoing development and
expansion of centralized large-scale PV systems, the flexibility in
the overall power system needs to be secured. Therefore, the
impacts of power output fluctuations on the operation and
management of the electric power grids need to be fully understood
and addressed for each unique situation. At the same time, possible
mitigating solutions for voltage stabi- lization purposes need to
be investigated, where inverters with advanced functionalities can
be a key solution [7].
1.2 Motivation
The deployment of large-scale PV systems is growing in the Swedish
power system, both in quantity and in system size. In recent years,
the increase in requests to connect large-scale PV systems to the
electric power grid has been noticed by the distribution system
operator E.ON Energidistribution, who operates at the regional- and
local grid in Sweden. The size of the requested systems varies from
single megawatts to hundreds of megawatts, and a common trend for
developers is to search for locations close to the existing
electric power grid to get an as efficient grid-connection as
possible, as the largest
9
economic profitability is achieved when as much as possible of the
existing infrastructure is used. Connection of PV systems to the
distribution grid on the medium-voltage level and transformation to
the regional grid is efficient, but leads to questions about
whether the existing customers will be negatively affected. For
E.ON, the increase in requests of large-scale PV systems have
raised questions concerning the negative effects that the
connections can have on the existing customers in the electric
power grid, where large challenges are voltage quality impacts such
as voltage variations as a result of power output fluctuations from
the systems.
A pioneer within the solar energy sector is the solar energy
developer company Solkom- paniet, who works towards speeding up the
overall development process by establishing large-scale PV systems
to the Swedish power system. Today, the possibility to connect
larger PV systems to the local grid is limited, as new
infrastructures and voltage reg- ulation devices need to be
implemented to ensure optimal operation with good voltage quality.
The need for mitigation solutions for voltage stabilization
purposes is rising, and one possible solution is the use of
advanced inverter functionalities. With functions such as active-
and reactive power support strategies in the inverters, a PV system
has the ability to contribute with beneficial system services to
the power system, enabling grid support and voltage
stability.
On behalf of E.ON Energidistribution and Solkompaniet, there is a
significant interest in understanding and addressing the above
stated challenges and possibilities regard- ing connection of
large-scale PV systems to the distribution grid on medium-voltage
level. Therefore, this master thesis investigates the prerequisites
for ensuring an optimal operation of the existing electric power
grid when establishing large-scale PV systems. The high-level goal
of the thesis is to gain valuable knowledge of the impact that
large- scale PV systems have on the electric power grid in terms of
voltage variations, and come up with connection requirements that
can work as prerequisites for the continuing deployment of
large-scale PV systems.
1.3 Research Objectives
The aim of the thesis is to investigate the prerequisites for
connection of a large-scale photovoltaic system to the electric
power grid. The thesis aims to study the impact of voltage
variations in the electric power grid in situations of power output
fluctua- tions from large-scale photovoltaic systems, and examine
the possibility to mitigate any negative effects by reactive power
support strategies in the photovoltaic inverters.
To fulfill the aim, a number of objectives have been identified.
Firstly, explore and identify important characteristics regarding
the power system, technical requirements related to voltage
quality, power output fluctuations from photovoltaic systems, and
adequate mitigating solutions for voltage regulation through a
literature study. Secondly, study the event of voltage variations
in situations of power output fluctuations from large-scale
photovoltaic systems through four studies performed in the power
system
10
simulator software PSS/E. The implemented studies include the
following:
• A worst-case study considering eight existing substations in the
electric power grid as well as a new substation, to examine the
impact of different parameters on the voltage variations.
Parameters such as transformer operation mode, location of the
point of connection, load capacity and switching mode are compared
in the study.
• Time series calculations to investigate the voltage variations
over a time period of one year.
• A study with an oversized photovoltaic system to investigate the
possibility for increasing the capacity of photovoltaics without
grid reinforcements.
• A study with reactive power compensation from the photovoltaic
inverters to exam- ine the possibility to maintain a stabilized
voltage level at the point of connection.
1.4 Scope
Geographical boundaries: The study is geographically limited to
study the electric power grid in electricity area SE4 located in
the south of Sweden. The network model and the obtained data
resources originate from the area.
Voltage quality assessment: The thesis studies fast voltage
variations in the electric power grid in situations of power output
fluctuations from large-scale PV systems, and no other voltage
quality parameters influenced by output fluctuations are
investigated. The fast voltage variations are evaluated from the
perspective of the power producer, based on the technical
requirements stated in the document “Technical requirements for
grid-connection in the regional grid and at the 20/10 kV
substations” [8] by E.ON regarding demands on fast voltage
variations on the power producer.
System boundaries: The study is carried out in E.ONs network model
in the power system simulator software PSS/E, with the entire
transmission grid, regional grid, and parts of the distribution
grid present in the model. The voltage variations at the sec-
ondary side of the substations are examined, i.e. on nominal
voltage levels of 10 or 20 kV. The study is performed in a low load
network model which represents a typical summer day with low loads
in the power system.
Data resolution: For the time series calculations, the resolution
of the collected data for production- and consumption are based on
hourly values.
Photovoltaic system size: The rated capacity of the modelled PV
systems varies depending on the studied substation, ranging from 32
MWp to 160 MWp in the worst- case study. In the study with an
oversized PV system, rated capacities up to 224 MWp are
examined.
11
1.5 Report Outline
The remainder of this report is organized as follows: Chapter 2
presents a theoretical background for the thesis, Chapter 3
addresses the methodology, Chapters 4 presents and discusses the
results, Chapter 5 highlights the conclusions, and Chapter 6 gives
the suggestions for future work.
12
Chapter 2
Theoretical Background
This chapter presents a theoretical background for the thesis,
including areas of research that are of relevance for the
continuing thesis work.
2.1 The Power System
This section presents an introduction to the power system,
including an overview of the electric power grid in Sweden and
characteristics for power transmission.
2.1.1 Overview of the Electric Power Grid in Sweden
The electric power grid is fundamental for providing electric
energy services to the society, as the infrastructure enables
transmission of electric power across the country. The main
functionality is to transfer electricity from producers to
consumers, while maintaining balance in the overall system [9]. In
Sweden, the majority of the electric power generation is located in
the northern parts while the demand for electricity is higher in
the south. This results in large transfers of electric power from
the north to the south. For a functioning electricity market, the
power system is divided into four electricity areas, from north to
south: SE1 (Lulea), SE2 (Sundsvall), SE3 (Stockholm), SE4 (Malmo).
The borders between the electricity areas are located where there
are physical restrictions on the amount of power that can be
transferred. The electricity market needs to some extent take into
account the limitations on transmission capacity that exist in the
power system in order for the outcome from the electricity market
to result in a distribution of production and consumption of
electricity that is technically possible [10].
In Sweden, the electric power grid is divided into the transmission
grid (220-400 kV), the sub-transmission grid (40-130 kV) and the
distribution grid (≤20 kV), operating at dif- ferent voltage levels
[11]. The voltage levels of the electric power grid is
characterized by the nominal voltage (Un), but the real voltage
level that the power system is designated to operate at is called
the normal operating voltage (for example 140 kV, 42.5 kV,
21.8
13
kV etc). The power lines at the different voltage levels are
connected at substations, where transformers adapt the voltage to
the desired level on the next power line in order to transfer the
electric power in the correct direction [12]. The transmission grid
that operates at high-voltage (HV) level is also called the
national grid, and is owned by the state and managed by the
Transmission System Operator (TSO) Svenska Kraftnat who is system
responsible for the entire power system in Sweden. The high voltage
of the transmission grid enables the transfer of electric power
over long distances with relatively small losses, and the main
function for the TSO is to maintain the short-term energy balance
in the power system by monitoring so that the stability and
necessary safety margins are upheld. The transmission grid connects
to large power producers, and transfers the generated electric
power via the sub-transmission grid and the distribution grid to
the final consumers. From the transmission grid, there are also
connections over the national border to other European countries
[9].
The sub-transmission grid which operates at a slightly lower
voltage level is commonly called the regional grid, and is the
connection link from the transmission grid to power producers,
larger consumers such as industries and urban areas, and the
distribution grids. Further, the distribution grid connects to
small-scale power producers, and dis- tributes the electric power
to the smaller consumers such as households, public buildings, and
offices [9]. The distribution grid is also called the local grid,
and is normally divided into medium-voltage (MV) grid at 10-20 kV
and low-voltage (LV) grid at 230/400 V [11]. Both the
sub-transmission- and the distribution grids are owned by
Distribution Grid Operators (DSOs). There are approximately 170
different DSOs in Sweden, where the three largest (who together
provide electricity to more than half of the electricity users in
Sweden) are E.ON Energidistribution, Ellevio and Vattenfall
[9].
2.1.2 Characteristics for Power Transmission
To understand the process of power transmission in the power
system, there are a number of concepts which are of particular
importance. In the power system, the locations where input
respective output of electric power occur are referred to as nodes
or buses. In each node, there is a constant voltage level, while
the current changes according to Kirchhoff’s law which states that
the current cannot disappear in a node. Due to the relationship
between the voltage and the current, a power balance must occur in
each node of the power system, meaning that the net production of
power is equal to net transmission of power to the other nodes. At
the same time as the power balance must be maintained, there are
requirements on the current and voltage levels in different parts
of the power system. In Sweden, most of the power transmission
occurs in three-phase AC power, where the ideal state is symmetric
three-phase which characterises that both the voltage and the
current in respective phase are sinusoidal with the same amplitude
and phase from each other with 120 degrees. The cycle repeats with
the frequency of the power system, which is 50 Hz in Sweden
[13].
Electric power is normally divided into active power (P) measured
in Watts (W) and
14
reactive power (Q) measured in Volt-Ampere reactive (VAR), related
to each other by the phase angle (φ) which is the phase shift
between the current and the voltage. The combination of active- and
reactive power is called the apparent power (S) measured in
Volt-Ampere (VA), and the relationship can be expressed with the
Pythagorean theorem according to:
S = √ P 2 +Q2 (2.1)
The phase angle defines the power factor (PF) which is a
dimensionless quantity that describes the relationship between
active- and reactive power, and is expressed as:
PF = cosφ = P
( arctan
Q
P
) (2.2)
As observed from equations 2.1 and 2.2, the power factor is related
to the amount of active- and reactive power. Commonly, a power
factor close to 1 is desirable as it indicates a phase angle of
zero, meaning that the current and voltage is totally in phase with
each other (resistive loads). In this situation, the momentan power
over a time period is constantly positive as the current and
voltage are either both positive or both negative at the same time.
A power factor of 1 indicates that the active power is at maximum
and that all of the supplied power contributes to useful work i.e.
transfer of electric energy to the load. However, at the same time
active power is preferred in the power system, reactive power is
hard to avoid due to magnetic fields (inductive loads) and electric
fields (capacitive loads). The reactive power is referred to as
non-useful work as it creates a flow of current in the power lines
without contributing to useful work, i.e. reducing the capacity for
active power transmission. Due to the increased current flow,
reactive power has a large impact on the power losses in the power
lines, where an increased flow of reactive power leads to increased
losses. The transmission of reactive power indicates a phase shift
between the current and voltage, meaning that the reactive power
oscillates back-and-forth in the power line. When the phase angle
increases, the power factor decreases according to equation 2.2
presented above. Depending on the character of the phase shift, the
power factor is referred to as lagging/inductive when the current
lags the voltage, and leading/capacitive when the current leads the
voltage [14].
Due to the characteristics of power transmission, a power line can
have a net production or net consumption of reactive power
depending on the situation. To mitigate, it is possible to
compensate by connecting devices with inductive or capacitive
character that locally absorb or supply reactive power
respectively. As the reactive power is strongly correlated with the
voltage, the devices have the ability to both control the voltage
level in the node and reduce the power losses in the line.
Absorption of reactive power decreases the voltage level in the
node, while supply of reactive power results in an increased
voltage level [13]. As a change in reactive power has the ability
to regulate the voltage level in the node, these types of
strategies are commonly used
15
to mitigate deviations in the voltage. The strategies are used by
several devices for different purposes in the power system, which
will be further presented in section 2.4 (Photovoltaic Inverters
for Voltage Regulation) and section 2.5 (Alternative Solutions for
Voltage Regulation). Below, equation 2.3 presents the relationship
between a voltage variation and a change in active- and reactive
power in a node [15]. In the equation, (U) is referred to as
voltage variation, (P ) is active power change, (Q) is reactive
power change, (R) is accumulated resistance, (X) is accumulated
reactance, and (U) is the voltage magnitude.
U = P ·R +Q ·X
U (2.3)
Further on, rewriting equation 2.3 and setting (U) as 0, i.e. no
voltage variation, it can be observed that the amount of reactive
power that needs to be injected to the node in order to cancel the
voltage variation is dependent on the active power change and the
accumulated X/R ratio, according to the expression below.
Q = −P ·R X
R
)−1
(2.4)
As observed from equation 2.4, the accumulated X/R ratio has an
impact on the need for reactive power compensation. Depending on
the type of power line, i.e. overhead line or cable, the
characteristics of R and X vary strongly. For example, overhead
lines which are more common in the transmission grid, have a low
resistance, which results in a high X/R ratio. The high X/R ratio
results in the reactive power having a large influence on the
voltage variation. Contrary, the distribution grid, which often
consists of a mix of overhead lines and cables, usually has a lower
X/R ratio, which means that the reactive power has a lower
influence on the voltage variations.
Another important parameter for power transmission is the short
circuit capacity which characterises the strength of the grid and
shows the ability to resist disturbances. The short circuit
capacity directly depends on the configuration of the grid and the
impedance of components such as power lines, transformers, and the
grid-connected loads and production facilities. A high short
circuit capacity and a low impedance characterises a strong grid,
while a weak grid is characterised by a low short circuit capacity
and a high impedance. Commonly, the size of the short circuit
capacity defines the limit of the allowed grid-connected loads and
production facilities in a specific part of the electric power grid
[13].
2.2 Technical Requirements for Voltage Quality
This section presents the concept of voltage quality together with
technical requirements regarding voltage variations and reactive
power exchange that applies for the network operator and the
grid-connected power producer.
16
2.2.1 Voltage Quality
In situations when the voltage deviates from the ideal state may
result in disturbances which affect the reliability and stability
of the power transmission. The presence of disturbances have a
negative impact on the operation of the whole power system, as it
can cause power outages, and harm or break electronic equipment
[16]. Due to this, a number of measurable parameters have been
defined, for which requirements have been set that indicate if the
power transmission is of good quality. In regard to voltage
quality, voltage variations are commonly measured. Voltage
variations occur when the voltage level deviates from the desirable
voltage level, either by an increase or decrease in voltage, and is
divided into slow and fast variations handling different time
scales [17].
At the different voltage levels of the electric power grid, there
are specific standards and regulation frameworks that state
specific requirements for good voltage quality. Among others, the
configuration of the grid and the characteristics of the
grid-connected production facilities have a large impact. Both the
network operator and the grid- connected power producer have
responsibilities for maintaining good voltage quality, where each
part has different restrictions to adapt to. In the event of
disturbances, both parties exchange information about the
disturbances in order to ensure normal operation as soon as
possible and eliminate the risk of recurrence [8].
2.2.2 The Network Operator
At the Point of Connection (POC), which is defined as the physical
point in the distribu- tion grid at which the power producer
connects the production facility, the network oper- ator holds main
responsibility for maintaining good voltage quality. The Swedish
Energy Markets Inspectorate has decided on the Swedish regulation
EIFS 2013:1 “Requirements that must be met for the electricity
transfer to be of good quality” [18] which needs to be fulfilled
together with the applicable European Standard SS-EN 50160 “Voltage
charac- teristics of electricity supplied by public distribution
systems” [19] which specifies voltage parameters and permissible
deviations.
Voltage variations are measured from the supply voltage at the POC,
defined as the Root- Mean-Square (RMS) value of the voltage at a
given moment (measured over a given time interval). The
requirements on voltage variations are based on the continuous
operating voltage (Uc) which is the voltage that the network
operator strives to maintain in the node. Commonly, the continuous
operating voltage is the same as the normal operating voltage by
which the power system is designated to operate at, but depending
on internal agreements these voltages may differ. In regard to slow
voltage variations, the network operator is responsible for
ensuring that all the 10-minute values of the supply voltage at the
POC during one week maintains between 90% and 110% of the
continuous operating voltage. The 10-minute value is calculated as
the RMS value of the voltage over a time period of 10
minutes.
Fast voltage variations are restricted by the number of short-term
voltage drops, short-
17
term voltage rises, and rapid voltage changes. A short-term voltage
drop is defined as a temporary decrease in the RMS value of the
voltage below 90% of the continuous operating voltage. The
restrictions on short-term voltage drops at time scales from 10 ms
to 1 minute are characterised in Table 2.1 and Table 2.2 for Un ≤
45 kV and Un > 45 kV respectively. The different areas (A, B and
C) refer to the permission of disturbance. Voltage intervals
considered acceptable (area A), voltage intervals where the network
operator is obliged to take measures to improve the quality to the
extent that the cost is reasonable in relation to the
inconveniences from the disturbance (area B), and voltage intervals
not permitted (area C).
Table 2.1: Limitations on short-term voltage drops for Un ≤ 45
kV
U [%] Duration, t [ms]
10 ≤ t ≤ 200 200 < t ≤ 500 500 < t ≤ 1000 1000 < t ≤ 5000
5000 < t ≤ 60000
90 > u ≥ 80 A
40 > u ≥ 5
5 > u C
Table 2.2: Limitations on short-term voltage drops for Un > 45
kV
U [%] Duration, t [ms]
10 ≤ t ≤ 100 100 < t ≤ 150 150 < t ≤ 600 600 < t ≤ 5000
5000 < t ≤ 60000
90 > u ≥ 80 A
40 > u ≥ 5
5 > u C
A short-term voltage rise is defined as a temporary increase in the
RMS value of the voltage equal to or above 110% of the continuous
operating voltage. The limitations on short-term voltage rises at
time scales from 10 ms to 1 minute are characterised in Table 2.3
below and refer to nominal voltages equal to or below 1 kV. The
same requirements apply for area A, area B and area C as stated
previously.
18
Table 2.3: Limitations on short-term voltage rises for Un ≤ 1
kV
U [%] Duration, t [ms]
u ≥ 135 C
111 > u ≥ 110 A
A rapid voltage change is a deviation in the RMS value of the
voltage that is faster than 0.5% per second, and where the RMS
value before, during, and after the change is between 90% and 110%
of the continuous operating voltage. Rapid voltage changes are
defined by the stationary voltage change (Ustat), which is the
difference between the RMS voltage value before and after the
voltage change, and the maximum voltage change (Umax), which is is
the maximum voltage change under a voltage change process.
There are restrictions on the sum of the number of rapid voltage
changes added with the number of short-term voltage drops present
in area A in Table 2.1 and Table 2.2 presented previously. The
obtained sum per day must not exceed the limits specified in Table
2.4 below. If there is no rapid voltage change, the limitations
still apply for the number of short-term voltage drops, and vice
versa if there is no short-term voltage drops but only rapid
voltage changes.
Table 2.4: Limitations per day on the sum of rapid voltage changes
and short-term voltage drops present in area A in Table 2.1 and
Table 2.2 presented previously
Un ≤ 45 kV Un > 45 kV
Ustat ≥ 3% 24 12
Umax ≥ 5% 24 12
2.2.3 The Power Producer
As mentioned previously, the network operator holds the main
responsibility for main- taining good voltage quality at the POC.
However, as the operating current (Id) fed to/from the electric
power grid by several production facilities affects the voltage
level in the node, as well as the voltage characteristics in the
rest of the grid, the network op- erator must enforce connection
requirements on grid-connected power producers in order to maintain
the requirements stated in EIFS 2013:1. The requirements on the
power producer are determined by the network operator and because
all the disturbances from several power producers are stored
together, the requirements on the power producer are commonly
stricter than the requirements set for the network operator. The
network operator E.ON has specified demands on the grid-connected
power producers regard- ing voltage quality in the document
“Technical requirements for grid-connection in the regional grid
and at the 20/10 kV substations” [8].
19
Slow voltage variations are affected by slow variations in the
operating current from the production facility, which in turn occur
due to variations in the active- and reactive power exchange at the
POC. Commonly, there is an agreement between the network operator
and the power producer with restrictions on the exchange of
reactive power at the POC. If there is no such agreement, the
operating current at the POC is restricted as follows:
Id,max =
1.1 · Iref , for the 10-minutes mean value of Id (2.5)
Where the reference current (Iref ) is dependent on the subscribed
active power produc- tion (Pab) and the continuous operating
voltage (Uc) according to the following equation:
Iref = Pab√ 3 · Uc
(2.6)
In regard to fast voltage variations, the power producer must
operate the production facility so that the voltage change at the
POC does not exceed ±3% of the continu- ous operating voltage. In
situations when the operating current causes two or more rapid
voltage changes in the same direction within a time period of 10
seconds, the voltage changes are not allowed to exceed the limit on
the maximum operating current (mentioned above for slow voltage
variations) alone or stored together.
In addition to the requirements stated regarding voltage
variations, the power pro- ducer that connects their facility to
the grid must meet all the requirements of Com- mission Regulation
(EU) 2016/631 “Network code on requirements for grid-connection of
generators” [20] and the related Swedish regulation EIFS 2018:2
“Generally applica- ble requirements for grid-connection of
generators” [21] by the Swedish Energy Markets Inspectorate. In the
regulations, there are requirements regarding the reactive power
capability. The requirements vary depending on the nominal voltage
level at the point of connection and/or the maximum continuous
power delivery of a production facility, divided into four
categories: type A (Un <110 kV and Pmax > 800 W), type B (Un
< 110 kV and Pmax ≥ 1.5 MWp), type C (Un < 110 kV and Pmax ≥
10 MWp), type D (Un < 110 kV and Pmax ≥ 30 MWp, or Un ≥ 110
kV).
If the network operator does not announce other requirements for
the power producer, power-generating facilities (power park modules
such as solar- and wind power) of type C and type D need a
capability for generation and absorption of reactive power corre-
sponding to 1/3 of the momentan supplied active power. Due to the
relationship between the active- and reactive power and the power
factor, extraction of 1/3 corresponds to a power factor of
approximately 0.95. Commonly, the reactive power is automatically
supported by control methods for reactive power, power factor or
voltage.
20
For power producers connected to the electric power grid owned by
E.ON, demands on active- and reactive power exchange is stated in
the document “Technical requirements for grid-connection in the
regional grid and at the 20/10 kV substations” [8]. The de- mands
on active- and reactive power exchange may occur in situations when
the grid does not have enough capacity for maximum active power
production, in case of ab- normal connections in the grid, in case
of disturbed operation, or in situations when the network operator
lacks resources for reactive power control. Moreover, active power
curtailment can occur in situations of both normal- and reserve
switching mode in order to avoid thermal overload on the power
lines and substations.
2.3 Solar Photovoltaics
This section introduces solar photovoltaics, including a
description of photovoltaic sys- tems, the concept of power output
fluctuations, and potential challenges with the tech- nology.
2.3.1 Photovoltaic Systems
Solar photovoltaic (PV) cells use the global horizontal irradiance
(GHI) from the sun and convert it directly to electricity via the
photovoltaic effect. PV cells take advantage of both the direct
normal irradiance (DNI) which is received directly from the sun,
and the diffuse horizontal irradiance (DHI) which is received from
the sun after which the direction has been changed due to
scattering from clouds and particles in the atmosphere. The
photovoltaic effect occurs when the irradiance reaches a
semiconductor material formed as a p-n junction with a positive and
a negative charged side, connected to an external circuit. When the
light reaches the semiconductor, a flow of electrons excites across
the junction, which creates a DC current. To increase the electric
output, the PV cells are connected into modules, where the cells
within the module are connected in series and parallel to increase
the power output from the module. The individual cells are commonly
made from mono-or poly-crystalline silicon (first-generation solar
cells) or thin film (second-generation solar cells). A junction box
is placed at the rear of each module and terminates the connections
from the individual cells to further connect the modules into
arrays and form a PV system [14]. As the PV modules create DC
current, a PV system uses an inverter to convert from DC to AC. For
a grid-connected PV system, the inverter uses the frequency and
voltage of the electric power grid as reference, which is
constantly monitored by the inverter for a normal operation.
Further, there are requirements and regulations set for the
electric power grid which defines the conditions at which the
inverter operates. There are different types of inverters, where
the selection mainly depends on the layout and topology of the PV
system. The most commonly used are central inverter which connects
to parallel connected strings of modules, string inverter which
connects to strings of modules, and micro-inverter which connects
to individual modules [14].
21
The ambient conditions that mainly affect the electric power output
from a PV module are the irradiance and the cell temperature. To
achieve maximum active power from the module, a Maximum Power Point
Tracker (MPPT) is normally integrated to the inverter [14]. The
relationship between the current and the voltage for a module is
explained by the IV-curve, which also defines the point at which
the maximum power is achieved. Via the MPPT, the electric power
production from the PV module is maximized by searching for a point
in the IV-curve that contributes to the highest power output [22].
The MPPT is a DC-DC converter which adjusts the voltage by varying
the resistance presented to the modules, where there are MPPT
methods of different complexity. The resistance is continuously
adjusted so the product of the current and the voltage is at
maximum, enabling maximum power. Further, the DC power from the
MPPT is inverted to AC power in the inverter, which is achieved by
switching transistors in the inverter. The action synthesizes a
sinusoidal waveform that is synchronized with the frequency and
voltage of the electric power grid [14].
2.3.2 Power Output Fluctuations
As mentioned, the electric power output from a PV system is
proportionally dependent on the global horizontal irradiance from
the sun. The irradiance intensity varies on both daily- and yearly
timescales, mainly depending on the sun’s path across the sky, and
weather patterns such as cloud coverage. On a yearly timescale, the
irradiance varies seasonally, with longer days and higher
irradiance in the summer, in contrast to shorter days and lower
irradiance in the winter. The days of the year when the sun stands
at its highest respective lowest position are called summer
solstice and winter solstice, and in 2020 these days occurred on
June 20 and December 21 respectively. Furthermore, at a daily
timescale the irradiance varies from sunrise to sunset, usually
peaking at noon. As the irradiance transfers with the speed of
light, rapid variations in irradiance can occur on short
timescales. For example, at the event of cloud movements, the
direct normal irradiance can change in intensity between 900 W/m2
and 0 W/m2 in only a few seconds. However, despite the change in
direct normal irradiance, the diffuse horizontal irradiance
remains, which is why the power output from a PV system does not
reduce directly from 100% to 0% due to cloud movements [23].
As the electric power output from a PV system varies according to
the irradiance, the variability in the solar resource has a large
impact on the power output from the PV systems. Variations on
different timescales result in power output fluctuations. For
grid-connected PV systems, the effects of power fluctuations is
considered one of the main challenges for an increased penetration
of PV in the electric power system. The variable character of the
power fluctuations considerably affects the quality and safety of
the power supply, leading to disturbances in the electric power
grid [17]. Smoothing approaches by geographical dispersion is an
interesting trend which can contribute to reduced effects of power
fluctuations from the PV generation. Due to the fact that the
majority of the grid-connected PV systems are widely distributed,
geographical dispersion of the generation can be beneficial for
balancing the power output fluctuations
22
in the whole power system. When irradiance variations cause a ramp
in the PV power output, the overall impact for a large PV system or
for several PV systems occupying a large geographical area can be
aggregated due to the smoothing effect [24].
The effects of power output fluctuations due to variability in
irradiance have been investi- gated by several researchers. In the
study [25] by Marcos et al, power output fluctuations from PV
systems for time intervals shorter than 10 minutes were
characterized with data of one second resolution. The study
investigated the performance of seven PV systems located in Spain,
with installed capacities ranging from 1 MWp to 9.5 MWp and with a
total combined installed capacity of 20 MWp. It was concluded that
the smoothing effect and the magnitude of the power output
fluctuations were influenced by the size of the PV system, where
larger PV systems indicated a larger smoothing effect and thereby
lower fluctuations. Further, the sampling time had an impact on the
results, where shorter sampling times gave larger smoothing
effects. A sample period of 1 second resulted in ramp rates between
5-55% of the installed capacity for the investigated PV systems.
For a time interval of 10 minutes, no smoothing effect was observed
and power output fluctuations of 90% of the installed capacity were
reached for all the investigated PV systems. Further on, the study
showed evidence of larger smoothing effects due to geographical
dispersion of several individual PV systems.
A similar study [26] was performed by van Haaren et al, where the
fluctuations in power output from six PV systems in USA and Canada,
with a total installed capacity of 195 MWp were studied. The data
used for the characterization was based on minute- averaged data
from each PV system and the power output from 390 inverters. It was
concluded that the ramp rate of power fluctuations decreased with
an increased size of the PV system, where the maximum ramp rates
for PV systems of 5, 21, 48 and 80 MWp were observed as 70, 58, 53
and 43% per minute of the installed capacity respectively. Below,
Table 2.5 compiles maximum ramp rates and the corresponding
installed capacity of the PV system from a few studies.
Table 2.5: Compilation of the ramp rate for different amounts of
installed PV capacity
Reference Installed capacity [MWp] Ramp rate [% of installed
capacity/min]
[25] 2.6 85
2.3.3 Potential Challenges with Photovoltaics
Even if the solar resource is considered infinite, the intermittent
character of the so- lar irradiance can cause problems when
integrating larger capacities of solar power to
23
the power system. There are several studies indicating potential
challenges associated with power output fluctuations from
grid-connected PV systems. In the study [27] by Shivashankar et al,
technical aspects involving the voltage quality, generation
dispatch control, protection, and reliability are investigated as
potential problems for the power system due to the power output
fluctuations. For example, there is evidence that the fluctuations
influence disturbances that affect the quality of the power supply
and lead to problems in the operation of the electric power grid.
Depending on the situation, the disturbances can be of different
characteristics, where voltage variations are explained as the most
common problem. Fluctuating power output from grid-connected PV
sys- tems may result in voltage variations. The voltage is directly
linked to the PV power output, where an increased output will
increase the voltage and thereby risk influencing the restrictions
set on the voltage level. Also, as a result of short irradiance
variations, flicker induced by fast voltage variations risks to
occur during shorter time periods.
Potential impacts from grid-connected PV systems are further
presented by Kraiczy et al in IEA TCP Task 14 [28], where voltage
variations, flicker and harmonics are highlighted together with the
risk of overloaded power lines and transformers. In the report [24]
by the Swedish Energy Agency, voltage variations and power flows in
the power lines are presented as common problems in the power
system due to a higher penetration of PV. Other voltage quality
issues explained are harmonics, which mainly occurs when the PV
system is connected to asymmetric three-phase with one-phase
inverters. This is however not considered to be a common problem in
the future as three-phase inverters are more common nowadays.
Similar potential impacts were presented in the report [24] by
Persson et al, where the main findings from earlier studies
investigating how the quality of the power transmission is affected
by grid-connected PV systems were compiled. The studies compiled in
the report were performed in Sweden, and followed two main research
paths. Either modelling and simulations by scenario analysis that
examined how an up- scaled capacity of PV in the low-voltage grid
affected the impacts of voltage quality and power outages, or
measurements of how the voltage quality in the existing electric
power grid was affected by PV. Voltage variations in the weak parts
of the distribution grid, and asymmetrics between phases were
considered as the main challenges with the grid-connected PV
systems.
In the study [29] by Bagge et al, the impact on the voltage quality
from the first PV system of MW-size in Sweden were analysed. The PV
system was connected to the medium-voltage grid on 10 kV, and the
analysis was done on a timescale of 6 seconds for the voltage and
current, and on a timescale of minutes for the PV power output.
Conclusions made from the study were that the PV system met the set
requirements in the regulation framework EIFS 2013:1 “Regulations
and general advice on requirements that must be met for the
electric power transfer to be of good quality”, but that the impact
on the grid was not negligible as impacts of slow voltage
variations exceeded the recommendations at some occasions. Also, it
was identified that the slow voltage varia- tions were lower at
days with low production, and higher at days with high production.
The study concluded that the main reason for variations was that,
when compared to
24
the size of the installed capacity of the PV system, it was
connected to a relatively weak part of the grid with a long
distance to the transformer. Therefore, important lessons from the
study was to check voltage variations in the grid when connecting
new PV facilities.
Furthermore, in the study [15], by Trindade et al, it is stated
that the increased pene- tration of PV systems, both in size and in
quantity, connected to the low- and medium- voltage grids arise
concerns regarding problems associated with cloud transients and
voltage regulation. Voltage variations at different timescales,
slow (steady-state) and fast (transient). According to the study,
slow voltage variations were observed on cloud- less days at the
maximum power output as a result of voltage rises, whereas fast
voltage variations were observed on partly cloudy days as a result
of cloud transients. PV sys- tems of MW-size range connected to the
distribution grid on the medium-voltage level were considered to
cause main challenges in the operation of the electric power grid.
As PV generators have no mechanical inertia, the variability in
irradiance during cloud tran- sients can cause rapid fluctuations
in the power output. In turn, the power fluctuations result in
imbalances from load-generators in the power system, which
instantaneously is absorbed by the substations, resulting in the
voltage variations in different time scales (duration times from a
few seconds to minutes). Further, the study shows that the power
output from a PV system directly affects the voltage magnitude. It
is however stated that on partly cloudy days, the voltage magnitude
depends on several factors such as the short circuit capacity, the
rated power of the PV system and the geographical dispersion of the
system. Simulations show correlation between the magnitude of
voltage variations and active power variations in terms of the
percentage of the rated installed PV power. At different buses of
the grid, higher values of voltage variations are observed when a
PV system is connected at buses with low short circuit capacity,
i.e. when there is long distance between the PV system and the
substation.
2.4 Photovoltaic Inverters for Voltage Regulation
Power output fluctuations that influence the voltage quality is a
limiting factor for the expansion of large-scale PV systems. Due to
disturbances such as voltage variations, there is a limit for how
large capacity can be connected to the electric power grid before
the requirements set for the voltage quality are exceeded. This can
be addressed by reinforcement and strengthening of the electric
power grid by new overhead lines, cables and transformers. To
construct new infrastructures is however an expensive solution, and
there are other possible methods that can be implemented to make
optimal use of the existing grid and mitigate the problems from
power output fluctuations [17]. The following section reviews
advanced PV inverter functionalities that may be used to enable the
integration of a higher penetration of PV to the power system.
Different control schemes for grid support will be described
together with possible implementation strategies.
25
2.4.1 Control Schemes
Advanced functionalities in the PV inverters enable them to
regulate the voltage and re- duce the impact of voltage variations
by strategies for active- and reactive power control. By
controlling the active- and reactive power output from the PV
system, the inverter has the ability to assist with grid support
services in addition to the normal inverter functions [30]. The
advanced functionalities include both direct control strategies
that are changed manually by the operator, and autonomous control
strategies that allow the inverter to make decisions automatically
depending on different parameters. The control strategies can be
achieved by incorporating functions that respond to variations in
several parameters set in the inverter, for example the operating
power factor in the inverter or the voltage level at the POC
[31].
Active power control methods have the main purpose to prevent
overvoltages in the electric power grid by curtailing the active
power production from the PV system. The method is commonly used at
high voltage levels in order to decrease the voltage level at the
POC. For example, voltage regulation can be achieved by the
autonomous Volt-Watt function where the active power output
generated from the PV system is adjusted in response to the voltage
level at the POC by a P(U) curve [31].
Reactive power control methods regulate the voltage level at the
POC by the capability of reactive power support. At overvoltages,
the inverter absorbs reactive power from the electric power grid in
order to decrease the voltage, while it in situations of
undervoltage delivers reactive power to increase the voltage level.
Due to the relationship between the active- and reactive power,
reactive power support reduces the ability of active power
generation from the PV system. As the maximum apparent power of the
inverter cannot be exceeded, the method of reactive power
compensation is possible when there is excess capacity in the
inverter that is not being used for generation of active power.
Commonly, the inverter is sized according to the maximum capacity
of the PV system. Since the peak power output of the PV system is
rarely achieved, the inverter works below the maximum inverter
rating over approximately 95% of the time. In these situations, the
excess inverter capacity can be used to provide reactive power
support. However, at times of peak power, the inverter has no
excess capacity and the reactive power compensation must be
achieved by curtailment of some active power production. A possible
solution to ensure that there is always excess inverter capacity
available for reactive power support is to oversize the inverter in
relation to the capacity of the PV system [30].
There are several control schemes for reactive power control, where
the two main strate- gies are power factor- or reactive power
output control. Power factor- control is achieved by adjustment of
the operating power factor, which is done by tracking of the active
power in order to maintain an active-reactive power ratio according
to the prevailing power factor command and within the maximum
apparent power limit. For example, voltage regulation by power
factor- control can be achieved by a direct control strategy using
a fixed power factor function that controls the reactive power
absorption/supply by specifying the power factor of the inverter to
a fixed value. The power factor is
26
specified as lagging/inductive or leading/capacitive depending on
absorption or supply of reactive power respectively. As standard,
the power factor in the inverter is designed to be as close to 1 as
possible, as it generates more active power, but for voltage
regula- tion purposes a lower power factor is desirable as it
enables reactive power support [32]. Another strategy for power
factor control is the autonomous Watt-cosφ function, where the
power factor is varied in response to the active power output from
the PV system by a cosφ(P) curve. Furthermore, reactive power
output control is achieved in several ways. There are two main
methods. The Watt-VAR function which uses a P(Q) curve to define
the amount of reactive power that can be supported as a function of
the ac- tive power output from the inverter, and the Volt-Var
function which directly varies the reactive power output depending
the voltage level at the POC by a Q(U) curve. Either of these
functions apply for reactive power control strategies, and the
response curves operate similarly but in response to different
parameters [31].
In addition to active- and reactive power control methods, the PV
inverters have ad- vanced abilities that enable the PV system to
stay connected to the grid as a dynamic support at minor
disturbances. The need for disconnection at times when the
frequency or voltage exceeds the predetermined requirements set is
flexible, and the inverters stay connected unless severe
disturbances occur. The dynamic support makes it possible for the
PV system to deliver power to the electric power grid and
contribute to stabil- ity at minor under- or overvoltages. The main
functionalities which enable the control method are low/high
voltage ride-through (LVRT/HVRT) functions where the inverter
continuously checks the voltage level of the grid [30].
2.4.2 PQ Capability Chart
As previously mentioned in section 2.2 (Technical Requirements for
Voltage Quality), there are requirements set on the capability of
reactive power support in the grid codes. Normally, the power
factor in a PV inverter can be adjusted between 0.8 lagging and 0.8
leading. A PQ capability chart illustrates how the active- and
reactive power is limited by the apparent power. Below, Figure 2.1
presents the PQ capability chart for the string inverter model
SG150HX, developed by the PV power company Sungrow [1]. The
inverter model has a nominal apparent AC power (Sn) of 250 KVA,
with a power factor adjustment range from 0.8 lagging to 0.8
leading.
27
Figure 2.1: PQ capability chart for the Sungrow inverter model
SG250HX [1]
As observed from Figure 2.1, a power factor close to 1 enables an
active power output from the inverter equal to the nominal apparent
power of the inverter, i.e. 250 kW active power and 0 kVAR reactive
power. Moreover, the maximum reactive power output from the
inverter is achieved when the power factor is 0.8, corresponding to
a reactive power output of 150 kVAR (reaching 60% of the nominal
apparent power of the inverter).
2.4.3 Implementation Strategies
As presented previously in this section, different control schemes
in inverters can func- tion as important system services for the
power system. However, the service of reac- tive power support for
voltage stabilization is the main interest for the grid operators,
whereas the owner of the PV system aims to maximize the generation
of active power. This dilemma leads to trade-offs in how the
integration of grid support functions should be made, where one
implementation challenge is the matter of profitability. As men-
tioned, reactive power support reduces the ability to generate
active power, as it is only possible to provide reactive power when
there is excess inverter capacity. Curtailment of active power
enables reactive power support but leads to a loss of income for
the active power that would otherwise be sold. To oversize the
inverter according to the capacity of the PV system is a possible
solution to ensure the presence of excess inverter capacity to meet
the needs of reactive power support, but to invest in a larger
inverter than nec- essary leads to increased investment costs [30].
There are discussions whether the owner of the PV system should be
compensated for the service of reactive power compensation by
incentives from the utilities or the network operators. In the
literature, there are two main compensation models discussed.
Firstly, as the owner of the PV system is only compensated for the
amount of active power fed to the electric power grid, curtailment
of active power for voltage stabilization purposes is an economic
loss for the owner of the PV system. In these situations, one
suggestion is that the owner of the PV system should be compensated
for the income lost through curtailment of active power. Secondly,
the solution of oversized inverters that enables it to always
support reactive power without curtailment of active power
generation result in additional investment costs for the
over-
28
all PV system. Suggestions are that the owner of the PV system
should be compensated by promotions for the investment in oversized
inverters [33]. Further on, National Re- newable Energy Laboratory
(NREL) discusses whether alternative ownership options of the
inverters would be a viable solution. The inverter is part of the
PV system and is typically owned by the owner of the PV system. To
consider alternative ownership to coordinate and provide the grid
support functions, such as utility ownership or customer ownership
of the inverters could address cost barriers [30].
In the report [34] by Vlahinic et al, it is mentioned that one
viable integration strategy for enabling grid support functions
from inverters could be to compensate the owner of the PV system
for the service of reactive support. It is discussed that different
approaches are used across the globe for charging consumers for
consumption of reactive power, and that a similar solution could be
used to provide incentives for PV systems with grid support
services. Spain is mentioned as an example, where the owner of the
PV system receives an incentive (given as a percentage of the price
for active energy in kWh) for providing reactive power.
2.5 Alternative Solutions for Voltage Regulation
As a continuation of the previous section, this section presents
alternative solutions for voltage stabilization, including tap
changers on the transformers, installation of shunt compensating
device, battery energy storage systems, and the concept of
solar-wind complementation.
2.5.1 Tap Changers
A common technique for voltage regulation purposes is the use of
tap changers at the transformers at the substations. A tap changer
is a switching mechanism installed on the transformer which alters
the connections on one of the transformer windings to vary the
turns ratio on either the primary- or secondary side in order to
achieve the desired voltage level [14]. The tap changer is normally
placed on the primary side of the transformer (the high-voltage
side), where it controls the voltage level on the secondary side
(the low-voltage side) by increasing or decreasing the turns ratio
on the transformer windings [17]. There are two main types of tap
changers, the on-load tap changer (OLTC) and the no-load tap
changer (NLTC). The tap changing mechanism for the on-load tap
changer proceeds automatically while the transformer is in
operation, and is more common for higher voltage levels of the
grid. For this mechanism, the tap changer steps the turns ratio
up/down in order to maintain an even voltage level on the secondary
side of the transformer. The stepping occurs when the voltage level
exceeds specified limits set on the tap changer, called the
dead-band limits. The other type of tap changer, the no-load tap
changer, is more used in the low-voltage grid. The changing
mechanism is done manually, and the transformer must be taken out
of service for manual adjustment of the turns ratio of the windings
[14].
29
2.5.2 Shunt Compensators
In addition to tap changers on the transformers, voltage regulation
can be achieved by reactive power control strategies. As previously
introduced in section 2.1.2 (Charac- teristics for Power
Transmission), capacitive and inductive elements have the ability
to compensate for the transmission of reactive power on a line and
regulate the voltage in the node. In practice, this is achieved by
connection of shunt compensators to the power line, which include
shunt capacitors and shunt reactors that have different functions.
Shunt capacitors are capacitive elements that produce reactive
power and result in an increased voltage level in the node, while
shunt reactors are inductive elements that consume reactive power
and decrease the voltage level in the node [13].
Two shunt compensating power electronic devices commonly used for
this voltage regula- tion technique are Static VAR Compensator
(SVC) and Static synchronous compensator (STATCOM), which are both
members in the Flexible Alternating Current Transmis- sion System
(FACTS) devices family. FACTS devices have the ability to control
the power flow on a power line by supporting reactive power
compensation. In SVC de- vices, which commonly includes a
thyristor-controlled reactor (TCR) in parallel with a
thyristor-switched capacitor (TSC), the voltage level at the Point
of Common Coupling (PCC) is adjusted depending on the situation. If
the voltage level at the PCC is higher than the reference value,
the SVC is predominantly reactive and absorbs reactive power,
resulting in a decreased voltage level. Conversely, if the voltage
level at the PCC is lower than the reference, there is a capacitive
character of the SVC, and supply of reactive power occurs which
increases the voltage level. The control strategy is optimized by
the rating of the SVC, where the rating can be both symmetric or
asymmetric with respect to capacitive and inductive reactive power.
Further on, a STATCOM device is a more advanced technology based on
the principle of Voltage-Source Converter (VSC). If the voltage of
the VSC is higher than the voltage level at the PCC, the STATCOM
device supplies reactive power, and the reverse occurs in the event
of a lower voltage at the VSC in comparison to the PCC [35].
However, in contrast to inverters that have the ability to both
generate active power and support with reactive power, shunt
compensating devices such as VSC and STATCOM only support with
reactive power [32].
2.5.3 Battery Energy Storage Systems
As mentioned previously, active power control by inverters is a
possible method for voltage regulation purposes, where the active
power output from the PV system is limited in order to regulate the
voltage level at the POC. A complementary solution in situations of
active power curtailment is the integration of battery energy
storage systems. The implementation of storage is widely used for
capturing the excess electricity generation and solving imbalances
regarding the power supply in the power system. At peak times of
high production conditions, the excess electricity is stored to
later be discharged and utilized at times of high peak demands and
low production conditions. The technique can solve problems of
overvoltages and high currents in the power lines,
30
and further lead to increased flexibility and reduced peak demands
in the power system. Additionally, similarly as a battery energy
storage system can be incorporated for voltage regulation purposes,
electric vehicle charging at peak times is a possible strategy for
taking advantage of the excess active power output from the PV
system [17].
2.5.4 Solar-Wind Complementation
A strategy that may contribute to mitigation of the negative
impacts from power fluctu- ations on a longer time scale are the
concept of solar-wind complementation. Previous research presented
by Fraunhofer Institute for Solar Energy Systems in the report [2]
shows the importance in scaling up the share of both of the
variable renewable energy technologies simultaneously. Synergies
between the electric power production from PV- and wind power
systems show that a balanced expansion of both technologies can be
beneficial for the power system. Weather patterns for the solar
irradiance and for the wind speed indicate a non-correlation,
meaning that at high solar irradiance, the wind speed is low and
vice versa. The negative correlation results in that the power
production from the two sources can complement each other and
contribute to great advantages. The research by Fraunhofer
Institute for Solar Energy Systems shows that the stabiliza- tion
of the power fluctuations apply on time scales from hours to
months. Below, Figure 2.2 presents the monthly values of the
electricity production together with the respective linear trend
lines during the period 2014-2017, for combined and separate PV-
and wind power systems.
Figure 2.2: Monthly electricity production from PV- and wind power
systems during the period 2014-2017 [2]
As observed from Figure 2.2, the relative deviations from the
linear trend lines are significantly lower for the combination of
both technologies compared to the separate systems. With respect to
voltage stabilization purposes, it can be concluded that a balanced
integration of both PV- and wind power systems to the electric
power grid can reduce the resulting effects of power output
fluctuations on longer time scales.
31
Methodology
This chapter addresses the methodology, starting with the power
system modelling soft- ware used for the study as well as a
description of the network model and important assumptions for the
power flow simulations. Moreover, the four implemented studies are
presented, which includes a worst-case study, time series
calculations, a study with an oversized photovoltaic system, and a
study with reactive power compensation.
3.1 Power System Modelling Software
The research methodology followed for the study was a quantitative
approach, using the modelling software tool Power System Simulator
for Engineering (PSS/E) combined with Python programming. PSS/E is
a power system simulator that performs steady-state or transient
calculations for power systems and is commonly used by network
operators when integrating new components to the electric power
grid. The software includes functions such as power flow
calculations, dynamic simulations, building network equiv- alents,
and fault- and contingency analysis. The application program
interface (API) of PSS/E is compatible with the programming
language Python, where Python is used to control the PSS/E
environment.
3.2 Network Model Description
This section describes the network model used for the study,
together with the principle of important components included in the
model.
3.2.1 Model
The study was performed in E.ONs network model in PSS/E,
representing the entire Swedish power system with data for the
transmission grid, the regional grid, and parts of the distribution
grid included. The network model is built as a bus-branch model
which means that buses and branches respectively represent nodes
and power lines,
32
each characterized by a nominal voltage level. The network model
includes substations with transformers that constitutes the
connection between two or three voltage levels of the electric
power grid, connecting several buses at different voltages to each
other. Two-winding transformers operate at two voltage levels, and
three-winding transformers operate at three voltage levels. Each
transformer has a tap changer that regulates the voltage at a
controlled bus to a specified voltage. Commonly, the tap changer
controls the voltage at the secondary side of the transformer (i.e.
the low-voltage side), which is the bus where components such as
load- and generator units are connected to. Load- and generator
units represent consumption and production of active power
respectively, and both units can supply and/ or absorb reactive
power. Since a power balance must be maintained in the power system
at each moment, the network model includes swing-bus components
that regulate their power to maintain the balance of active- and
reactive power in the system during the power flow
simulations.
3.2.2 Switching Mode
The electric power grid consists of meshed- and radial networks.
The transmission- and regional grid are meshed networks, meaning
that different parts of the electric power grid are interconnected
and there are several paths between two points. Radial networks are
the opposi