of implementing a microgrid at Vaksala-Eke

Mars 2021

A feasibility investigation

of implementing a microgrid at Vaksala-Eke

Lovisa Wallenbert

Teknisk- naturvetenskaplig fakultet UTH-enheten

Besöksadress:

Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0

Postadress:

Box 536 751 21 Uppsala

Telefon:

018 – 471 30 03

Telefax:

018 – 471 30 00

Hemsida:

http://www.teknat.uu.se/student

Abstract

A feasibility investigation of implementing a microgrid at Vaksala-Eke

Lovisa Wallenbert

As the global energy demand increases each year and the need for alternative energy solutions is highly sought after, microgrids are a concept that has been increasingly interesting as a solution to meet these demands. This study presents a solution to implement a microgrid at the business park Vaksala-Eke in Uppsala. Since Vaksala- Eke already has on-site renewable generation sources in combination with an energy storage system, there are possibilities of creating a microgrid where these sources are included. Information and data regarding the consumers and generation sources on-site was collected in order to compare the production and consumption. A microgrid model was designed and two different simulation cases were tested to

investigate the potential of a microgrid. The two cases were chosen to simulate the different load variations at Vaksala-Eke and how a difference between a high and a low load would influence the microgrid design. An examination was also performed to explore the possibility of integrating a larger energy storage system and if it would be beneficial for Vaksala-Eke. Results showed that integrating a microgrid at Vaksala-Eke would increase the self-consumption and reduce both the amount of energy imported from the main grid as well as the environmental footprint.

ISSN: 1654-7616, UPTEC E 21002

Examinator: Mikael Bergkvist

Ämnesgranskare: Markus Gabrysch

Handledare: Maria Säfström

I am thankful to AB Uppsala kommuns Industrihus (Ihus) for the chance to do this project. I especially would like to thank Maria Säfström for the opportunity and support to realize this project. I would also like to thank my subject reader

Markus Gabrysch for advising me during this thesis. I cannot express enough thanks

for his commitment and support throughout the entire process.

Svensk Sammanfattning

Energibehovet ökar för varje år som går och efterfrågan av alternativa energilös-

ningar blir allt större. Mikronät är ett koncept som blir en allt mer intressant lösning

för att möta dessa behov. Den här studien undersöker och presenterar en lösning för

att implementera ett mikronät vid företagsparken Vaksala-Eke i Uppsala. Eftersom

Vaksala-Eke redan har förnybara energikällor installerade på området i kombination

med ett energilagringssystem finns det möjligheter att konstruera ett mikronät där

de befintliga källorna ingår. Information och data beträffande konsumenterna och

energikällorna i området samlades in med syftet att jämföra produktion och kon-

sumtion. En modell av mikronätet designades där två olika simuleringsfall testades

för att undersöka potentialen av att implementera ett mikronät. De två fallen valdes

för att simulera hur skillnaden mellan en hög och en låg last skulle påverka mikro-

nätet. En utredning gjordes även för att utforska möjligheterna av att integrera ett

större energilagringssystem och om det skulle vara fördelaktigt för Vaksala-Eke. Re-

sultaten visade att ett mikronät vid Vaksala-Eke skulle öka egenförbrukningen samt

reducera den importerade mängden el från elnätet så väl som miljöavtrycket.

1 Introduction 1

1.1 Background . . . . 3

1.2 Purpose of the thesis . . . . 3

1.3 Goal . . . . 3

Milestones . . . . 3

2 Theory 4 2.1 Microgrid components . . . . 4

2.1.1 Distributed generation . . . . 4

2.1.2 Energy storage system . . . . 4

2.1.3 Energy management system . . . . 5

2.1.4 Converters . . . . 5

Rectifier . . . . 5

Inverter . . . . 5

Boost converter . . . . 6

Buck converter . . . . 6

Bidirectional DC-DC converter . . . . 6

2.2 Microgrid control . . . . 7

2.2.1 Control functions . . . . 7

2.2.2 Phase-locked loop . . . . 7

2.2.3 Pulse-width modulation . . . . 8

2.2.4 Clarke and Park transformation . . . . 8

2.2.5 Instantaneous power in the dq reference frame . . . 10

Current-mode control . . . 11

2.3 Microgrid protection . . . 11

2.4 Solar energy . . . 11

2.4.1 Electrical power output of PV systems . . . 11

2.4.2 Different types of modules . . . 12

Mono-crystalline . . . 12

Poly-crystalline . . . 12

2.4.3 I-V and P-V curve of a solar module . . . 12

2.4.4 Maximum power point tracking . . . 13

2.4.5 Incremental conductance method . . . 13

2.5 Wind energy . . . 14

2.5.1 Wind turbines . . . 14

2.5.2 Power output of a Wind turbine . . . 15

2.6 Economics . . . 15

2.6.1 Life cycle cost . . . 15

2.6.2 Levelized cost of electricity . . . 16

3 Case study: Vaksala-Eke 17

3.1 Vaksala-Eke . . . 17

3.1.1 Buildings . . . 17

3.1.2 PV generation . . . 17

3.1.3 Wind turbine . . . 18

3.1.4 Energy storage system . . . 19

3.2 Uppsala solar energy goals . . . 20

3.3 Incoming solar radiation . . . 20

3.4 Grid standards . . . 20

4 Methodology 21 Delimitations . . . 22

Assumptions . . . 22

4.1 Load profile . . . 23

4.2 Solar power production . . . 23

4.3 Simulink model . . . 23

4.3.1 Subsystems . . . 24

4.3.2 Main grid . . . 24

4.3.3 Voltage source converter system . . . 25

4.3.4 PV models . . . 28

4.3.5 Energy storage system . . . 30

Buck mode . . . 31

Boost mode . . . 31

4.3.6 Wind model . . . 31

4.4 System costs . . . 33

Levelized cost of electricity . . . 33

5 Results 34 5.1 Simulation results . . . 34

5.1.1 Case 1 . . . 34

5.1.2 Case 2 . . . 38

5.2 Load sharing between the ESS and the main grid . . . 41

5.3 Levelized cost of electricity . . . 42

6 Discussion 44 6.1 Simulation results . . . 44

6.2 Life cycle cost and load sharing . . . 45

6.3 Levelized cost of electricity . . . 45

7 Conclusion 46 7.1 Future Work . . . 46

Bibliography 47 A Appendix 50 A.1 Input data to simulations . . . 50

A.1.1 Load profiles . . . 50

A.1.2 Solar power production . . . 51

2.1 Schematic of the boost converter . . . . 6

2.2 Schematic of the buck converter . . . . 6

2.3 Phase locked loop diagram . . . . 8

2.4 The three reference frames . . . . 9

2.5 I-V curve and P-V curve from a solar module [22] . . . 13

2.6 Flowchart of the incremental conductance method [24] . . . 14

3.1 Vaksala-Eke business park (2016) . . . 18

4.1 Block overview of the model . . . 21

4.2 Microgrid model in Simulink . . . 24

4.3 VSC system connected to the main grid . . . 25

4.4 Phase locked loop . . . 26

4.5 Conversion of the grid voltages and currents from abc quantities to dq0-frame . . . 26

4.6 Current control loop . . . 28

4.7 PV model in Simulink . . . 29

4.8 ESS model in Simulink . . . 30

4.9 ESS control system in Simulink . . . 30

4.10 Wind model in Simulink . . . 32

4.11 Wind turbine model in Simulink . . . 32

5.1 Active power balance showing the main grid, the load, the production sources and the battery power . . . 35

5.2 DC-link voltage compared to a reference voltage signal . . . 36

5.3 Main grid voltage and current waveforms . . . 36

5.4 Battery measurements of the SOC, current and voltage . . . 37

5.5 Main grid current THD . . . 37

5.6 Active power balance showing the main grid, the load, the production sources and the battery power . . . 38

5.7 DC-link voltage compared to a reference signal . . . 39

5.8 Main grid voltage and current waveforms . . . 40

5.9 Battery measurements showing the SOC, current and voltage . . . 40

5.10 Main grid current THD . . . 41

5.11 Power provided by the grid vs. power provided by the grid together with power provided by the battery. . . 42

A.1 Load profile for 2015 . . . 50

A.2 Load profile for 2016 . . . 51

A.3 Total consumption during 2015 and 2016 . . . 51

A.4 Power production from the solar tracker 2015 and 2016 . . . 52

A.5 Power production from the façade mounted solar panels 2015 and

2016 . . . 52

List of Tables

3.1 Specifications of the VAWT and generator . . . 19

3.2 Information displayed on the EMS screen . . . 19

3.3 Battery specifications . . . 20

4.1 The amount of consumed energy during 2015 and 2016 for the sub- scribers at Vaksala-Eke . . . 23

4.2 The solar power production from the solar installations during 2015 and 2016 . . . 23

4.3 Main grid parameters, RL-branch parameters and DC capacitor value in Simulink . . . 25

4.4 PV array parameters used for the Tracker in Simulink . . . 29

4.5 PV array parameters used for the Facade-mounted installation in Simulink . . . 29

4.6 PV model parameters used in Simulink . . . 29

4.7 Battery parameters used in Simulink . . . 31

4.8 Wind model parameters used in Simulink . . . 32

5.1 Load and battery settings for case 1 and 2 . . . 34

5.2 Case 1 . . . 35

5.3 Case 2 . . . 38

5.4 Solar tracker cost parameters . . . 42

5.5 Façade-installation cost parameters . . . 42

AC Alternating Current DC Direct Current

Ei Energimarknadsinspektionen EMS Energy Management System ESS Energy Storage System

HAWT Horizontal Axis Wind Turbine

IEEE Institute of Electrical and Electronics Engineering IGBT Insulated-Gate Bipolar Transistor

Ihus AB Uppsala kommuns industrihus LCOE Levelized Cost Of Electricity

MOSFET Metal-Oxide Semiconductor Field-Effect Transistor MPP Maximum Power Point

MPPT Maximum Power Point Tracking PCC Point of Common Coupling PD Phase Detector

PI Proportional Integrator PLL Phase-Locked Loop

PMG Permanent Magnet Generator PV Photovoltaics

PWM Pulse-Width Modulation

SMHI Swedish Meteorological and Hydrological Institute SOC State Of Charge

TDD Total Demand Distortion

THD Total Harmonic Distortion

VAWT Vertical Axis Wind Turbine

VCO Voltage Controlled Oscillator

VSC Voltage Source Converter

1 Introduction

The fast exhaustion of fossil fuels along with their environmental impact has moti- vated the research and expansion of renewable energy sources, as well as distributed generation. The energy demand increases each year, and the projected trend is that the demand will be 60% higher in 2030 in comparison to 2002 [1]. With this in mind and the addition that non-renewable sources, such as oil and coal, still hold the majority of our energy generation, calls for developments in renewable sources [1]. Solar and wind energy are considered two of the sources with most potential and progressed research. As energy companies compete in the market to offer high- quality clean electricity to their customers, new energy generation methods are in high demand. One solution to meet these increasing demands, which has started to attract more attention over the years, is the microgrid systems.

As the electrical power generated by distributed generation has increased throughout the years, the microgrid concept has been developing during the same period. Microgrids are important for future smart-grid distribution systems due to their many possibilities and advantages. A microgrid is a small local electrical energy grid which often has the ability to disconnect from the traditional centralized power grid and operate autonomously, enabling flexible integration of variable energy sources with high efficiency [2]. Microgrids also enable energy storage systems and can reduce greenhouse gas emissions. The energy systems will see large improvements in forms of power quality and increased reliability due to the microgrids control capabilities [2].

The most important feature separating a microgrid from a conventional distribution system is its controllability. By using proper control, microgrids can increase the electrical reliability in a system. This by decreasing the events of outages as well as their duration. Some microgrids can be seen as controllable loads as they can perform peak-load shifting functions during times of high demand by reduc- ing their own consumption and distributing extra energy to the main power grid [3].

The constellation of the traditional centralized power grid includes generation,

transmission and distribution. Typically the majority of the energy generation is

located far away from the main consumption. For example, in Sweden the majority

of generation takes place in the northern parts of the country while the biggest con-

sumption can be seen in the central and southern parts of the country. Therefore,

energy must be transported long distances to reach its final destination. As a result,

much effort has been focused towards developing transmission of energy in order to

minimize transmission losses [4]. One of the positive aspects of a microgrid is that

it decreases the losses within the transmission and distribution system. This is done

by implementing the generation sources on-site, and as a result the energy does not

have to be transmitted long distances, thus reducing the transmission losses. Addi-

tionally, as the microgrid has the generation sources on-site it will not be affected by

outages as heavily in relation to the rest of the grid.

In today’s electrical power grids, integrating distributed generation such as renew- able sources can be difficult due to their intermittency and contribution to fluctu- ations. Microgrids can help facilitate the integration of distributed generation to an electrical power grid, and most importantly they facilitate the integration of re- newable sources. A microgrid often includes some type of energy storage which is responsible for keeping the energy balance in the system. Having an energy storage system in the microgrid therefore contributes to stabilising the system and makes it easier to integrate alternative energy sources. By making it easier to integrate alter- native energy sources it is possible to reduce the environmental footprint caused by transmission losses and fossil fuels [3].

A microgrid can be designed to have the ability to operate in two different modes; grid-connected mode and island mode. In grid-connected mode the micro- grid is connected to the main power grid and imports or exports energy and control services from the main grid. In island operating mode the microgrid is isolated from the main power grid, and is not able to import control functions. Therefore the microgrid has to change from power control to frequency control. Changing to frequency control has to be done in order to maintain the stability in the microgrid, as frequency control is used to balance the production and consumption. When the microgrid operates in grid-connected mode the main power grid will be responsible for maintaining the frequency of the microgrid [3].

There are ongoing studies and work of implementing microgrids in developing

countries. For these countries microgrids has the potential to bring clean, small-scale

and self-contained electricity supply to remote areas where connection to the cen-

tralized power grid would be difficult or expensive [5]. Currently there are projects

investigating the performance of solar microgrids implemented in rural areas in de-

veloping countries [5, 6].

1.1. Background 3

1.1 Background

In this master thesis a feasibility investigation of implementing a real-life microgrid system, located at the business park Vaksala-Eke, has been conducted. Vaksala-Eke is located in Uppsala and currently has two solar installations, a battery storage bank and a wind generator. The solar installations consist of a solar tracker and façade-mounted panels. The plan for the sources is to connect them and form a small microgrid. During this project a microgrid model was created and simulations were performed as to gather data and establish whether a real-life system is feasible to implement at Vaksala-Eke. The model developed in this project includes the two solar installations, the wind turbine and the energy storage system.

1.2 Purpose of the thesis

The purpose of this thesis project is to investigate the potential of implementing a microgrid at the businesses park. This is achieved by comparing the consumption at Vaksala-Eke with the production from the generation sources on-site. Today’s appli- cation area of use of the battery storage is to store the excess energy from the solar tracker to be able to use later. The battery storage is used to cut power consump- tion peaks; as these can influence the electricity cost for the buildings on-site. The solar tracker is the only source connected to the battery at present time. This thesis includes an investigation regarding the possibilities of installing a larger battery- storage and if Vaksala-Eke would benefit from this. In addition, to answer what size the storage should be when connecting all sources to the battery.

1.3 Goal

The aim of the project is to design a small microgrid for Vaksala-Eke with distributed generation and battery storage possibilities. One of the goals is to obtain a high proportion of self-consumption, and to define what components are needed for the design. By investigating the order of magnitude regarding the investment of such a system, a model for how the microgrid could be designed in relation to sizing and optimization is proposed.

Milestones

• Define the essential system components of the microgrid.

• Define and describe how the existing generation sources at Vaksala-Eke could be integrated in a microgrid design.

• Define the possibilities for developing a microgrid at Vaksala-Eke, and what

the difficulties/challenges may be.

In this section a theoretical overview of what a microgrid consists of will be pre- sented. The two focus areas are the physical components of a microgrid as well as how they can be controlled. This section covers predominantly the theory that is connected to the microgrid intended for Vaksala-Eke. The later stages of this section includes the economics which have to be considered when determining a microgrids cost over its lifespan.

2.1 Microgrid components

A common definition of a microgrid is that different types of energy sources and several consumers are connected at a common point to the main power grid. A microgrid can be seen as a controllable unit, which can be connected to or discon- nected from the main grid. The relationship between microgrid and main grid is highly dependent on the situation [2]. Microgrids can benefit from some sort of en- ergy storage system as they can improve the stability in the system. Batteries are a common technology for storage of energy due to their fast response, easy integration and possibility to stabilize the grid [3].

2.1.1 Distributed generation

Distributed generation are different technologies generating electricity either on site or close to where it will be consumed. Distributed generation can be defined as a variety of grid connected devices generating electrical energy, often referred to as distributed energy resources [7].

A microgrid always consists of distributed generation sources, which can be seen as one of the main components of a microgrid, together with storage systems and loads. Typical generation sources are renewable sources such as solar energy and wind energy.

2.1.2 Energy storage system

Distributed energy storage systems are of importance for improving microgrid sta- bility. As mentioned, a high-level penetration of renewable sources based on photo- voltaics (PV) and wind power can make it difficult to maintain stability, as they can contribute to intermittency and fluctuation issues. An energy storage system (ESS) can therefore play an important role in improving stability and reliability in a micro- grid. It can reduce fluctuations from the distributed generation sources, contributing to a more stable operation.

The ESS has the advantage of being able to operate as a load in charging mode and as a generator in discharging mode. Therefore, it can balance the power and reduce the impact of load fluctuations in the microgrid.

Depending on its purpose the ESS can be integrated in various places in the mi-

crogrid. The ESS can perform load levelling functions to flatten the load variations

2.1. Microgrid components 5

and peak-load shifting functions to support peak demand, when acting as a load.

The ESS also has the ability to work as an uninterruptible power system, providing emergency power when the main power source fails during a failure on the grid [3].

2.1.3 Energy management system

An energy management system (EMS) is essential for an ESS in a microgrid. It han- dles the operation process of the ESS, and most importantly it handles the ESS’s charging and discharging process. By keeping track of energy produced by the generation sources in the microgrid, the EMS can decide where to distribute the produced energy. The EMS can determine if the energy will be provided to the consumers in the microgrid, to the energy storage system or if the energy will be dispatched to the connected grid [3].

2.1.4 Converters

Electrical power converters are devices converting electrical energy from one volt- age or current level to another using semiconductor electronic switching devices.

Other than converting the voltage or current level, electrical power converters can convert alternating current (AC) to direct current (DC) and vice versa. Converters can consist of either passive or active components. An example of a passive com- ponent is a diode. By replacing diodes with controllable switches one or more pa- rameters can be controlled. In an active converter the conversion is controlled using controllable switches, often transistors. A transistor usually has to be switched on in order to conduct. The transistor is switched on and off using a control system that generates pulses to the gate of the transistor. The types of transistors commonly used for switching applications are IGBTs and Power MOSFETs. The type of transis- tor chosen depends on the required characteristics such as the switching frequency and power level in the system [8]. For microgrids including distributed generation sources, energy storage systems and loads a very important aspect to consider in the microgrid design is the control of the power converters [3].

Rectifier

A rectifier converts AC to DC and the rectification is usually performed using semi- conductor devices. A rectifier can be used to produce an output that is dc, or to produce a voltage or current waveform that has a specified dc component. A recti- fier can consist of either passive or active components. A passive rectifier is usually a diode-bridge with a smoothing capacitor, while an active rectifier usually consists of thyristors or transistors. An advantage of using a active rectifier is that it is possible to improve the efficiency by controlling the output voltage. A disadvantage of using active rectifiers is that they require a control circuit to ensure that the devices turn on at the right time [8]. In a microgrid a rectifier can be used for example to connect to a common DC-bus in the microgrid [3].

Inverter

Inverters convert DC to AC and are used in different applications such as drives

for electrical machines and active power filtering. An inverter is a type of converter

which is made of several electronic switches such as transistors. Inverters typically

require an input power which is relatively stable in order to be able to supply a stable

power output [8]. In a microgrid inverters are often used to operate as an interface

between the generation and consumption points. So in addition to conversion from DC to AC inverters are used to control the power flow [3].

Boost converter

A boost converter is a DC-DC step-up converter. The boost converter is used to step up the voltage and step down the current from its source to its output. Boost converters are commonly used together with photovoltaic systems to increase their output voltage. The output voltage is increased in order to match the voltage-level of the connected grid (in cases where there is no transformer connected in between). By using a boost converter this can be done without having to use a generation source with more capacity. As the voltage output from a PV source is dependent on the solar irradiation, a boost converter can be used to compensate when the output of the PV source is changing to make sure maximum power is extracted from the PV cells [8]. Figure 2.1 shows a schematic of the boost converter.

V

L

C R

FIGURE2.1: Schematic of the boost converter

Buck converter

A buck converter is a DC-DC step-down converter. A buck converter steps down the voltage from its supply to its output, while stepping up the current. Buck con- verters are used for example when charging a battery from a supply which has a higher voltage level than the battery. [8]. Figure 2.2 shows a schematic of the buck converter.

V

L

C R

FIGURE2.2: Schematic of the buck converter

Bidirectional DC-DC converter

Bidirectional DC-DC converters are used in applications where bidirectional power

flow is needed, as for energy storage systems such as batteries. In a bidirectional

converter current can flow both ways, and the converter has different modes of op-

eration, depending on the desired current flow. A simple bidirectional DC-DC con-

verter works as either a buck or a boost converter depending on its operating mode.

2.2. Microgrid control 7

When the bidirectional converter operates to charge the battery it works as a buck converter and when discharging the battery it operates as a boost converter [9].

2.2 Microgrid control

An important feature that separates a microgrid from a conventional distribution system is its control capabilities. The purpose is that a microgrid should perform as a coordinated and controlled unit when connected to the main power grid [2]. As previously mentioned, a microgrid can often operate in two different modes; grid- connected mode or island mode. In grid-connected mode the microgrid is connected to the main power grid and imports or exports energy and ancillary services, such as frequency and voltage control services. When it is in island mode the micro- grid operates on its own, isolated from the main power grid. In island mode the microgrid changes from power control to frequency control and can drop loads if necessary. Changing to frequency control is necessary to maintain stability in the microgrid. Frequency control is used to balance the production and consumption by slightly changing the frequency within certain limits if there is a difference between load and supplied power. When the microgrid operates in grid-connected mode the main power grid is responsible for maintaining the frequency and voltage of the microgrid [11].

2.2.1 Control functions

The control functionalities of a microgrid can be divided into three groups. These groups are: the upstream network interface, internal microgrid control and local control. The upstream network interface determines if the microgrid operates in grid-connected mode or island operation mode. It also coordinates with the main grid and makes decisions for the import or export of energy. The internal microgrid control includes secondary voltage and frequency control, secondary active and re- active power control, load management and forecasting, security monitoring and black start support. Local control includes the functionalities primary voltage and frequency control, primary active and reactive power control, protection and battery management [2].

2.2.2 Phase-locked loop

The phase-locked loop (PLL) detects the phase difference between two periodic

(usually sinusoidal) voltage signals, and produces a signal proportional to the dif-

ference. PLL-circuits are commonly used in various modern systems, such as com-

munication and IT [12]. For our purpose, the PLL is used to synchronise the invert-

ers output voltage with the main grid. The PLL is a control system composed of a

phase detector (PD), a loop filter and a voltage controlled oscillator (VCO). Figure 2.3

shows a diagram of the phase locked loop control system. The VCO’s purpose is to

track the frequency of a periodic input voltage signal. It is responsible for generating

an AC signal with a frequency depending on the input voltage signal. The PD cre-

ates an output signal proportional to the phase difference between the input signal

and the signal which is generated from the VCO. The loop filter is a low-pass filter

which is responsible for eliminating high frequency components in the error voltage

signal [12].

Reference signal

Phase detector

(PD)

Voltage controlled oscillator (VCO)

Loop filter

+ _

error signal

FIGURE2.3: Phase locked loop diagram

2.2.3 Pulse-width modulation

Pulse-width modulation (PWM) is a modulating technique commonly used to get the desired power output of the electrical power converter [13]. Duty cycle and switching frequency are the two main components that define the PWM signal. A PWM signal controls a power switch output by varying its on and off times, where the duty cycle is the ratio of the switching period when the switch is closed. The duty cycle is adjusted to regulate the output voltage to the load. The switching frequency determines how fast a cycle is completed [8].

2.2.4 Clarke and Park transformation

Mathematical transformations are frequently used in the complex plane for electrical machine analysis in order to solve equations of quantities which are time varying, by setting the variables to the same voltage or current reference frame. In a three- phase system two commonly used methods are Clarke and Park transformation. The transformations are used to rotate the reference frame of AC waveforms in a way so they become DC signals. The transformation version used preserves the amplitude of the electrical components (the voltages and currents). In Clarke transformation the three-phase quantities abc are converted into αβ0 components. In Park trans- formation vectors in an orthogonal stationary reference frame, αβ0 are converted into an orthogonal rotating reference frame, dq0. In this thesis a balanced system is considered, meaning that the zero-component is assumed to be absent and therefore ignored in the following discussion. This means that the three-phase quantities abc can be mapped to only two components: αβ or dq.

Figure 2.4 shows the three reference frames abc, αβ and dq of the balanced system.

2.2. Microgrid control 9

α = a β

d b q

c

θ

FIGURE2.4: The three reference frames

The (magnitude-preserving) Clarke transformation (abc → αβ) is expressed using Equation 2.1.

i

= ² 3 i

− ¹

3 ( i

− i

) (2.1)

i

= √ ²

3 ( i

− i

)

where i

, i

and i

are the three-phase quantities and i

and i

the orthogonal stationary reference frame components in the complex plane.

The inverse Clarke transformation (αβ → abc) is expressed using Equation 2.2. Us- ing the inverse Clarke transformation the orthogonal two-axis reference frame is transformed to three-phase stationary frame.

i

= i

(2.2)

i

= − i

+ √ 3i

2 i

= − i

− √

3i

2

Equation 2.3 is the (magnitude-preserving) Park transformation (αβ → dq). Park

transformation maps the stationary αβ-frame to the rotating dq-frame.

i

= i

· cos ( θ ( t )) + i

· sin ( θ ( t )) (2.3)

i

= i

· cos ( θ ( t )) − i

· sin ( θ ( t ))

where i

and i

are the rotating reference frame components. i

is called the direct component and i

the quadrature component. i

and i

are the orthogonal stationary reference frame components. θ is the angle between the two coordinate systems, which can be further explained by i

in the orthogonal rotating reference frame being at an angle θ to the α-axis. i

is perpendicular to i

along the q-axis. In this application the dq-frame coordinate system is rotating with the same frequency as the grid we are synchronized to, in order to get DC waveforms that are easier to control with a simple PI controller.

Equation 2.4 is the inverse Park transformation (dq → αβ). Applying the inverse Park transformation the components in the rotating reference frame can be transformed back to stationary reference frame.

i

= i

· cos ( θ ( t )) − i

· sin ( θ ( t )) (2.4)

i

= i

· cos ( θ ( t )) + i

· sin ( θ ( t )) 2.2.5 Instantaneous power in the dq reference frame

Equations 2.5 and 2.6 can be used to control the power flowing through an inverter.

The power is controlled by the currents in the rotating reference frame (dq-frame).

When the dq-frame is chosen so that v

is zero, Equations 2.7 and 2.8 can be used. As can be seen in Equations 2.7 and 2.8 the current which controls the active power is the direct component i

and the current controlling the reactive power is the quadrature component i

. If the wanted power levels are known, the equations can be rewritten as Equations 2.9 and 2.10 to obtain the reference current values [10].

p ( t ) = ³

2 ( v

( t ) i

( t ) + v

( t ) i

( t )) (2.5) q ( t ) = ³

2 (− v

( t ) i

( t ) + v

( t ) i

( t )) (2.6) p ( t ) = ³

2 v

( t ) i

( t ) (2.7)

q ( t ) = ³

2 (− v

( t ) i

( t )) (2.8) i

( t ) = ²

3v

( t ) ^p ( t ) _(2.9)

i

( t ) = − ²

3v

( t ) ^q ( t ) (2.10)

2.3. Microgrid protection 11

Current-mode control

An approach to controlling the active and reactive power in a voltage source con- verter (VSC) system, like the one used in this project, is current-mode control. Using this approach, the VSC AC-side current is controlled by a control scheme, through the VSC terminal voltage. In this case both the active and reactive power are con- trolled by the amplitude and phase-angle of the VSC line current, with respect to the voltage at the point of common coupling (PCC). PCC is the point where the local microgrid connects to the main grid. Using a current regulation scheme makes it possible to protect the VSC against overload conditions [10].

2.3 Microgrid protection

The protection of a power system is important as the protection and quality of the protective devices decides system reliability, stability and controllability [3]. The increase and expansion of distributed energy resources is providing a way for distri- bution systems to work as microgrids. Microgrids could help avoid disruptions and serve as resources for fast recovery during main grid disturbances [15].

A major challenge with microgrids is protection. A microgrid should have the control of completely disconnecting from the main grid when service is performed on the main grid, for safety of the service workers as well as the main grid compo- nents. In addition, it is also important to protect the microgrid components in the event of faults occurring inside the microgrid itself. If a fault occurs in one part of the microgrid it should have the capability of isolating that part of the grid from the other components [15].

A coordinated protection system is important to guarantee that a distribution network system can operate according to the safety requirements. The use of au- tomatic operation is needed to ensure that faults are isolated as quickly as possible in order to minimize system damage. Disconnector and switchgear devices require special sensing devices for activation, also referred to as protective devices [16].

2.4 Solar energy

Solar energy is the technology used to capture the sun’s energy and converting it to electrical energy. Solar panels, or photovoltaic cells are used to absorb sunlight.

When sunlight hits the cells, electrons get excited from the valence to the conduction band and as they flow through the cell electricity is generated [17].

2.4.1 Electrical power output of PV systems

The potential electrical power output generated from solar panels can be calculated using Equation 2.11. Where P is the potential power production in W. A higher solar irradiance (G) and a larger solar panel area (A) will generate more power. Other than using a large area where the incoming solar irradiance is high, it is also beneficial to use solar modules with a large efficiency (η) to increase the electrical power output [18].

P = A · G · η (2.11)

2.4.2 Different types of modules

There are two main types of solar cells which are briefly described and compared below.

Mono-crystalline

Mono-crystalline solar cells are made of silicon wafers. The cells are easily recog- nized by their even colouring and uniform look. A higher efficiency and longer life- time are two benefits using these cells. One disadvantage is that the cells are more expensive compared to the poly-crystalline cells due to their production process [19, 20].

Poly-crystalline

Poly-crystalline solar cells are made of several silicon crystals which are melted to- gether to form the wafers for the panels. Poly-crystalline cells are the most occurring type of solar cell due to their easy and cheap manufacturing process. The disadvan- tages using poly-crystalline cells compared to mono-crystalline cells are their lower efficiency and lower heat tolerance which leads to a lower temperature coefficient, meaning that their efficiency drops at higher temperatures [19, 20].

2.4.3 I-V and P-V curve of a solar module

A current-voltage curve, referred to as an I-V curve, shows different combinations of current and voltage output from a solar module. A solar module operates at a particular voltage and current value, called the operating point. The power at the operating point is the power produced and delivered to the load. The ambient conditions of a solar module, such as solar irradiance and cell temperature affect the shape of the I-V curve. A higher solar irradiance gives a better I-V curve in the sense that the operating point reaches a higher power level. A higher cell temperature will on the other hand generate an I-V curve with an operating point which has a lower power level.

A solar module also has a power-voltage curve, referred to as a P-V curve or

power curve, displaying the power from the solar cell. The power curve has an

operating point corresponding to a maximum power point (MPP) at the I-V curve,

see Figure 2.5. At MPP the maximum power from the solar cell is obtained. In order

to obtain this point on the I-V curve the module is forced to operate at a voltage

corresponding to the voltage at MPP, and the current has to be adjusted to have the

same value as the current at MPP. After forcing the module to operate at MPP, the

ambient conditions could change. Changes in irradiance or cell temperature might

cause the I-V curve and power curve to change as well. The old MPP value is no

longer valid under the new conditions. So in order to stay continuously at the MPP

any changes in the I-V curve need to be traced to find a new MPP. This process is

called maximum power point tracking [21].

2.4. Solar energy 13

FIGURE2.5: I-V curve and P-V curve from a solar module [22]

2.4.4 Maximum power point tracking

A drawback with PV systems is that the efficiency varies based on the solar irradia- tion level. Maximum power point tracking (MPPT) is a method used to improve the energy extraction of a solar system. An MPPT-controller is used to control the input current to the DC-DC converter. Based on the algorithm used, the controller will ad- just the duty cycle of the gate pulse of the DC-DC converter to track the MPP of the photovoltaic array. Various MPPT-techniques are used, one technique commonly used is the incremental conductance method [23].

2.4.5 Incremental conductance method

Incremental conductance method predicts the maximum power point (MPP) which is obtained when

= 0. This by deciding if the PV system is proceeding to the right or to the left of MPP. The expression

is negative when the MPPT is to the right of the MPP and positive when to the left of MPP. Based on whether the

is negative or positive the incremental conductance algorithm will guide the system towards MPP [23]. A flowchart of the incremental conductance method is shown in Figure 2.6.

The incremental conductance method can determine when the MPP is attained and stop perturbing the operating point. If the MPP is not reached, the direction in which the MPPT operating point has to be perturbed can be calculated using the relation between

and −

. At MPP

= −

, and the relation between them is derived from the fact that

is negative when the MPPT is to the right of the MPP and positive when it is to the left of the MPP [24]. The relation between

and −

is explained by Equation 2.12.

0 = ^dP

dV = ^d ( V · I )

dV = I dV

dV + V dI

dV = I + V dI

dV ⇒ ^dI

dV = − ^I

V (2.12)

dI

dV < − ^I

V right of MPP dI

dV > − ^I

V left of MPP

Start

Return dV=V(k)-V(k-1)

dI=I(k)-I(k-1)

dV=0

dI=0 dI/dV=-I/V

dI/dV>-I/V dI>0

No Change

No Change

Decrease Duty Cycle

Decrease Duty Cycle Increase

Duty Cycle

Increase Duty Cycle

Update V(k-1)=V(k)

I(k-1)=I(k) NO

NO

NO

NO

NO YES

YES YES

YES YES

FIGURE2.6: Flowchart of the incremental conductance method [24]

2.5 Wind energy

Wind power is one of the fastest growing technologies of renewable energy. The installed wind power has increased globally from 7.5 GW in 1997 to 622 GW in 2019 [25]. Wind is used to produce electricity by using the kinetic energy created from air in motion, which is transformed to electrical energy using wind energy conversion systems or wind turbine generators [25].

The kinetic wind power flowing through a surface with cross-section A can be defined according to Equation 2.13.

P

= ¹

2 · ρ · A · U

(2.13)

where ρ is the air density, and U is the wind speed [26].

2.5.1 Wind turbines

A wind turbine extracts the wind energy and converts it to mechanical power. In short, a wind turbine has rotating shafts that rotates by utilizing the wind, this pro- duces mechanical power. The mechanical power is then transformed into electricity using a generator. Wind turbines can be placed into one of two groups by the orien- tation of their axis: horizontal axis and vertical axis wind turbines. The horizontal axis wind turbine (HAWT) is the most common design. In a HAWT the axis of rota- tion is parallel to the ground [26].

Vertical axis wind turbines (VAWTs) have their axis of orientation perpendicular

to the ground. VAWTs can have their main components such as generator and gear-

box located at the base of the turbine. Having the main components at ground level

will make it easier to perform maintenance. A VAWT also has the advantage that

2.6. Economics 15

it can accept wind from any direction, not having to be faced in the direction of the wind, thus, not requiring orientation or wind-sensing mechanisms in comparison to a HAWT. However, compared to a HAWT, the VAWT has a lower efficiency as not all of the blades contribute to energy production as in a HAWT. The up-scaling of the size of a VAWT is also limited due to the design, as the design is not as sturdy as the design of a HAWT. VAWTs are still popular for small-scale installations where space is limited, as for residential installations [26 - 28].

2.5.2 Power output of a Wind turbine

The output power of a wind turbine varies with the wind speed and different wind turbines have a power performance curve showing the power output prediction of the specified turbine. The characteristics of the power performance curve can often be received from the manufacturer. There are three main points on the scale of the performance curve that can predict the performance of a given wind turbine gen- erator: cut-in speed, rated wind speed and cut-out speed. The cut-in speed is the minimum wind speed at which the machine will deliver usable power. The rated wind speed is the speed at which the rated power, which often is defined as the max- imum power output of the electrical generator, is reached. The cut-off wind speed is defined as the maximum wind speed where the turbine is allowed to deliver power.

The cut-off wind speed is usually limited by the design of the components and its safety restrictions [26].

The power output can be calculated using Equation 2.14.

P = C

· P

(2.14)

where P is the rotor power, C

is the power coefficient and P

is the power in the wind as described in Equation 2.13 [26].

2.6 Economics

In order to maximize the economic profit of the microgrid the goal is to minimize the total system costs over its lifetime. The objective is to reduce the microgrid costs without harming the reliability of the system or increasing the environmental impact of the microgrid [29].

When determining the lifetime microgrid cost, the capital cost, maintenance and operating costs of the distributed generators and the operating costs of electricity imported from the main grid should be taken into account. Some units might have to be replaced during the microgrid lifetime, the sum of this cost is added to the cap- ital cost. The maintenance cost is a function of the installed systems capacity. The operating cost depends on the generator size and the power generated at a specific time. The sum of these three costs is the total generator cost. The energy storage sys- tems capital cost and maintenance cost can be formulated in a similar way without including the operating cost. A battery storage is a generating system without any operating costs. When determining the microgrid lifetime cost only the operational cost is involved [30].

2.6.1 Life cycle cost

Life cycle cost consists of the initial capital cost, installation, replacement, mainte-

nance and operating costs over the system lifetime. Included in the operating costs

is the electricity bought from the main grid. The goal of the life cycle cost is to assess all applicable expenses of the microgrid during its total service life period. Should the lifetime of the microgrid exceed the lifetime of the individual units, a replace- ment cost and a recurring capital cost is added to the life cycle cost [31].

2.6.2 Levelized cost of electricity

A tool which can be used in order to compare the viability of different generation sources is the levelized cost of electricity (LCOE). The LCOE denotes the net present value of the total unit cost over an assumed lifetime.

The LCOE of renewable energy sources varies by technology, country, capital, operating costs and performance of the technology.

A formula which can be used to calculate the LCOE of the renewable energy technologies is given in Equation 2.15 [32].

LCOE =

∑

Ii+Mi+Fi

(1+r)ⁱ

∑

Ei

(1+r)ⁱ

(2.15)

Where the variables in the equation are defined as:

LCOE = average lifetime levelized cost of electricity generation n = economic life of the system (in years)

I

= investment expenses in year i

M

= operation and maintenance expenditures in year i

F

= fuel expenses in year i (which are assumed as equal to zero for many renewable generation sources)

E

= electricity generation in year i

r = discount rate, which is the financing costs where the future value of a generation

source is discounted against today’s value

3 Case study: Vaksala-Eke

This section includes a description of the existing technology at Vaksala-Eke. In addition, Uppsalas future solar energy goals are included as well as information re- garding the study of obtaining areas with high incoming solar radiation in Sweden.

The section also includes some information regarding grid standards which has to be met in order to ensure a good quality in the electricity grid.

3.1 Vaksala-Eke

Vaksala-Eke is a business park located in Uppsala with storerooms and industrial premises. Several companies are currently (2021) renting storerooms in the park which is owned by the company AB Uppsala kommuns Industrihus (Ihus). As pre- viously mentioned, a wind turbine, two solar installations and an ESS are located in the park. The two solar installations are a dual axis solar tracker and façade- mounted solar panels, which both were installed in 2014. The intention of installing the façade-mounted solar panels was to investigate how well these cells absorb re- flected light from the surrounding fields in winter time. In 2016 the ESS was installed and connected to the solar tracker. With the purpose of storing excess energy from the tracker during the day to be able to use it later. The ESS also has the purpose to cut power peaks as it can influence the electricity cost for the industrial premises.

In 2017 the wind turbine (a VAWT) was installed, in order to investigate its power production in a Swedish landscape.

3.1.1 Buildings

As of today (2021) Vaksala-Eke consists of nine buildings. Some of the buildings can be seen in Figure 3.1. Three more has been built since the photo was taken.

Electricity meters that register the amount of consumed energy are connected to every building.

3.1.2 PV generation

Figure 3.1 shows the solar tracker, marked with a C, which is coupled to building A.

The façade panels are mounted on building B. The wind turbine is not pictured as

the photo was taken before it was installed.

FIGURE3.1: Vaksala-Eke business park (2016)

The solar tracker consists of 36 poly-crystalline panels of 300 W each from EC So- lar and has a rated total power of 10.8 kW. Connected to the solar tracker is a module for manually controlling the tracker, for example if snow needs to be cleared from the module area, and a sensor for measuring the wind speed. If the wind speed exceeds a certain value the tracker will change to horizontal position to prevent damaging.

The control used to orient the dual-axis solar tracker to the position with highest energy production not only takes into account the incoming solar radiation, but also reflected light and diffuse irradiation. Light is often reflected from snow, water or rocks and diffuse irradiation comes from sunbeams piercing through clouds. The tracker mechanism will move the panel surface to the position with highest light intensity so that the best direction is attained in any weather condition throughout the entire year. Two sensors mounted on the tracker will determine this position.

One of the sensors measures elevation and the other sensor directs the panel in the direction east-west.

Advantages using a solar tracker compared to a roof or façade-mounted solar installation of the same size:

• Better cooling of the panels as they are not directly mounted on a roof or façade

• Captures almost 40 % more energy as it adjusts position to constantly achieve the highest possible level of incoming solar irradiation

Disadvantages using a solar tracker compared to a roof or facade-mounted solar installation of the same size:

• The tracking system is more expensive compared to a fix-mounted installation of the same surface area due to its additional foundation parts and control mechanisms

• More sensitive to high wind speeds

The façade-mounted solar system consists of 44 black 250 W mono-crystalline panels by EC Solar and has a total rated power of 11 kW. Both solar systems use power optimizers performing MPPT in order to increase the total harvested energy from the solar panels.

3.1.3 Wind turbine

Energy generation from wind power at Vaksala-Eke is generated by a vertical axis

wind turbine with a rated power of 10 kW, produced by the company Sawt Energy.

3.1. Vaksala-Eke 19

The VAWT has five blades, a mill height of about 6 meters and is mounted at ground level. The generator is a permanent magnet generator (PMG). Some of the specifica- tions of the VAWT and generator are listed in Table 3.1.

TABLE3.1: Specifications of the VAWT and generator

Generator parameters

Type PMG 3-phase

Rated voltage 250 V AC Rated current 23 A AC Performance of the VAWT Rated power 10 kW

Max power 12 kW

Rated wind speed 12 m/s Cut-in speed 2 m/s Cut-out speed 25 m/s

3.1.4 Energy storage system

The energy storage system used at Vaksala-Eke was developed by the company Al- phaESS. The system is called Storion T5 and is specifically developed for power grid applications. Some components included in the system are inverter, battery and energy management system. Connected to the ESS is a PV meter which mea- sures the output power from the solar tracker. A grid meter measuring the power exchanged with the grid is also coupled to the ESS. Table 3.2 shows information dis- played on the EMS screen. The screen shows the present values of the PV output power, the battery state of charge (SOC), as well as the self-consumption rate and self-sufficiency rate. Self-consumption rate is the ratio of energy produced by the solar panels which is consumed at Vaksala-Eke at the same time as it is produced.

The self-sufficiency rate is the ratio between energy output and consumption.

TABLE3.2: Information displayed on the EMS screen

PV size 10 kW

PV output power varies Battery capacity 15 kWh Battery Inverter size 5 kW

Battery SOC 20 – 100% (varies) Self-consumption rate 0 – 100% (varies) Self-sufficient rate 0 – 100% (varies)

The ESS contains five battery modules, which specifications are listed in Table 3.3.

Each module has a LED display that displays battery SOC and state of battery. The SOC is a percentage of the maximum possible charge inside a battery, where 100 % indicates that the battery is fully charged. State of battery is a measure of the general condition of a rechargeable battery and how much it can deliver in comparison to a new battery.

The battery model and type of the modules are also listed in Table 3.3. The bat-

tery type used is LiFePO

, which is a promising cathode material to use in large

scale lithium ion batteries due to its low cost, thermal stability and relatively large

capacity [33].

TABLE3.3: Battery specifications

Energy Capacity (per module) 3 kWh

Nominal Voltage 153.6 V

Operation Voltage Range 134.4 – 175.2 V Capacity (5 modules) 15 kWh

Model M15020

Type LiFePO

(LFP)

3.2 Uppsala solar energy goals

Uppsala municipality had the goal that by year 2020 to have 30 MW installed solar power. By the year 2030 the goal is set to 100 MW. In spring 2018 the municipality had around 7.5 MW installed solar power [34]. Some companies that currently have solar installations in Uppsala are AB Uppsala kommuns industrihus, Skolfastiheter, Uppsalahem and UKFAB.

3.3 Incoming solar radiation

The incoming solar radiation is studied to obtain suitable locations for PV installa- tions. Solkartan by Framidens solel [35] and the Swedish Meteorological and Hy- drological institute (SMHI) [36] are two organizations that gather and present so- lar irradiation data for Sweden. Using Solkartan the incoming solar radiation on rooftops can be estimated. By examining the incoming solar radiation the most suit- able panel position can be decided, which is the area with highest solar irradiance in kWh/m

/year.

3.4 Grid standards

When designing a microgrid it is important to meet the requirements concerning grid standards, as a deviation from the standards could result in grid disturbances.

Energimarknadsinspektionen (Ei) is a Swedish state administrative authority that makes sure the electricity companies in Sweden follow the requirements con- cerning the electricity grid. One of Ei’s constitutions (EIFS 2013:1) states the general advice and requirements that must be met for the transmission of electricity to be of good quality [37]. Chapter 7 in EIFS 2013:1 describes the requirements regarding voltage quality when transmitting energy.

The institute of electrical and electronics engineering (IEEE) uses IEEE-519 as a

system guideline to set limits on voltage and current distortion. The most resent

standard is IEEE-519-2014. The standard includes two requirements on harmonic

distortion. The absolute maximum voltage total harmonic distortion (THD) and a

variable maximum total demand distortion (TDD) level. TDD is the ratio of the root

mean square of the harmonic content expressed as a percentage of the maximum

demand current. The limits are applied to the point of common coupling [38].

4 Methodology

In this section the process of designing a model of the microgrid in a simulation envi- ronment is presented. The model is designed in MATLAB/Simulink which is a sim- ulation software developed by MathWorks. The purpose of designing this model is to reach a suggestion of how a real-life microgrid could be implemented at Vaksala- Eke. As the generation sources are already installed the focus when designing the model was the configuration of the microgrid and its components. An examina- tion is performed regarding the contribution of energy from the battery storage and whether Vaksala-Eke would benefit financially of having a larger energy storage, when the main purpose is to charge the battery using the renewable sources.

As Vaksala-Eke aims for a high proportion of self-consumption and optimizing the energy storage to use as a balancing source during peak demand, a simulation is performed comparing the amount of energy imported from the grid with the energy imported from the grid when the battery also acts as a source.

PV array

PV array

ESS

External grid

Load

DC

DC

DC

DC DC

DC

DC

DC AC

AC DC-bus

Wind turbine

FIGURE4.1: Block overview of the model

Figure 4.1 shows a block overview of the microgrid system. The designed micro-

grid is operated in grid-tied mode, meaning that the microgrid will always be con-

nected to the main grid. The microgrid contains an ESS which handles the power

flow between the battery and the DC bus. The power exchange with the main grid

is predominantly extraction of power from the main grid to the microgrid. Feeding

power to the grid is rare for this microgrid, in theory this could occur when both

the wind generator and PV installations are close to producing maximum power, in

combination with a battery at full capacity and a low consumption.

In steady state, the power flow in the microgrid can be stated as:

P

= P

+ P

+ P

− P

The definition of each variable is:

P

= the power extracted from or delivered to the main grid. A positive value indi- cates that power is delivered to the grid and a negative value that power is extracted from the grid

P

= the power produced by the two PV arrays

P

= the power generated from or distributed to the ESS P

= the power produced by the wind turbine

P

= the power consumed by the customers

Different topics and aspects that are central to this study are presented below.

• Delimitations and assumptions in the Simulink model

• Data collection from the subscribers at Vaksala-Eke

• Data collection from the solar installations at Vaksala-Eke

• An overview of the Simulink model design and description of the different parts in the model

• Values of system components and how they are chosen

• The type of converters chosen and how their components were chosen Delimitations

In this microgrid model a passive rectification system using a diode-bridge was cho- sen for the wind power conversion instead of using an active rectification system. A passive rectification system was chosen to limit the model design, with knowledge that it does not have the same controllability compared to an active rectification sys- tem. This can result in a system which does not perform as well in comparison with a system using an active rectifier.

Another choice made in the model was that the different subscribers were com- bined into one three-phase load. The load power is set to fixed values for certain time periods during the simulation.

Assumptions

Lithium batteries usually have a management system that ensures safe operation.

As they limit for instance the reference current, an assumption was made in the mi-

crogrid model that the battery has a main controller which makes sure the battery

can not have a current that exceeds the chosen value. Instead of including a main

controller in the ESS control system, saturation blocks were used to limit the refer-

ence current to make sure the current stays within the allowed limits in the model.

4.1. Load profile 23

4.1 Load profile

In order to estimate the desired amount of produced energy the total amount of consumed energy must be obtained. Currently there are 10 subscribers at Vaksala- Eke and their consumption for 2015 and 2016 is listed in Table 4.1.

The load profiles have been studied with the aim of providing an overview of how much electricity the subscribers at Vaksala-Eke consume on a daily and yearly basis. The total amount of consumed energy during the years 2015 and 2016 have been extracted from the load profiles for each subscriber. Both the total amount of consumed energy for the whole system, as well as the load profiles for each subscriber has been obtained.

TABLE 4.1: The amount of consumed energy during 2015 and 2016 for the subscribers at Vaksala-Eke

Yearly energy consumption [kWh]

Subscriber 2015 2016

Total 631 100 776 100

A1 108 600 121 800

A3 14 900 41 000

A4 10 900 14 500

Lighting 29 100 25 500

B1 95 600 129 800

C 60 200 65 200

D 106 100 126 600

E 97 100 107 100

F 108 600 124 700

G - 19 900

4.2 Solar power production

The total amount of solar power produced from the solar tracker and the façade- mounted solar installation was collected from Ihus solar portal and the total pro- duction on a yearly basis is shown in Table 4.2.

TABLE4.2: The solar power production from the solar installations during 2015 and 2016

Solar power production [kWh]

Installation 2015 2016 Solar Tracker 14 700 15 100 Façade Panels 8200 7700

4.3 Simulink model

The model proposition of a possible microgrid system at Vaksala-Eke is explained

in this section. Figure 4.2 shows the microgrid in Simulink. The microgrid model

includes four sources. These are a PV-array representing the solar tracker system, a