Lappeenranta-Lahti University of Technology LUT LUT School of Energy Systems
Electrical Engineering Master’s Thesis 2020
Jooa Pursiainen
LVDC GRID IN MINE ENVIRONMENT
Examiners: D.Sc. Pasi Peltoniemi D.Sc. Jenni Rekola Supervisor: D.Sc. Pasi Peltoniemi
Jooa Pursiainen
LVDC grid in mine environment Master’s Thesis 2020
Lappeenranta-Lahti University of Technology LUT LUT School of Energy Systems
Electrical Engineering Lappeenranta
66 pages, 57 Figures, 19 Tables Examiners: D.Sc. Pasi Peltoniemi
D.Sc. Jenni Rekola Supervisor: D.Sc. Pasi Peltoniemi Keywords: LVDC, electrical grid, mine
Electrification of mine machines has several advantages. Compared to combustion en- gines they are friendlier to the environment, quieter, and require less ventilation in the mine. Usually, electrical grids in mines have been realised with AC distribution. DC distribution has however advantages over AC, such as greater power distribution capacity and smaller losses.
In this thesis, an LVDC grid for underground case mine is designed and analysed. Differ- ent components and realizations of DC grids are presented and the most suitable methods for mine environment are selected. For the case mine, the cable type and diameter are selected via voltage drop calculation and by analysing the cable’s current rating with the maximum loads of the mine. The chosen cable is used in simulation model which is used to examine the voltage quality under the dynamic loads of the mine. In addition, energy storages and their optimal location are studied. Without the energy storages the voltage drops too low for the mine loads, but with the support of energy storages the voltage stayed above the desired threshold. It was noticed that when energy storages were located at the back of the grid the maximum current in the grid was lower compared to situation where they were at the beginning of the grid. Finally, an estimate of the price for the LVDC grid for the case mine is given.
Tiivistelm¨a
Jooa Pursiainen
LVDC grid in mine environment Diplomity¨o 2020
Lappeenrannan-Lahden teknillinen yliopisto LUT LUT School of Energy Systems
S¨ahk¨otekniikka Lappeenranta
66 sivua, 57 kuvaa, 19 taulukkoa
Ty¨on tarkastajat: TkT Pasi Peltoniemi TkT Jenni Rekola Ty¨on ohjaaja: TkT Pasi Peltoniemi Avainsanat: LVDC, s¨ahk¨overkko, kaivos
Kaivoskoneiden s¨ahk¨oist¨aminen tuo useita etuja. Ymp¨arist¨oyst¨av¨allisyyden ohella s¨ah- k¨oiset koneet ovat hiljaisempia, energiatehokkaampia ja v¨ahent¨av¨at kaivoksen ilmastoin- nintarvetta verrattuna polttomoottoreihin. Toistaiseksi kaivosten s¨ahk¨overkot on toteutettu vaihtos¨ahk¨oll¨a. Tasas¨ahk¨onjakelussa on kuitenkin lukuisia etuja vaihtos¨ahk¨o¨on verrattu- na, kuten suurempi tehonsiirtokapasiteetti ja pienemm¨at h¨avi¨ot.
T¨ass¨a ty¨oss¨a suunnitellaan ja analysoidaan LVDC-verkon soveltuvuus maanalaiseen kai- vosymp¨arist¨o¨on. Ty¨oss¨a esitell¨a¨an DC-verkon eri komponentit sek¨a toteutustavat, ja va- litaan kaivosymp¨arist¨o¨on parhaiten soveltuva toteutus. Tutkimuskohteena olevaan kai- vokseen valitaan sopiva kaapelityyppi sek¨a poikkipinta-ala j¨annitteenalenemalaskennalla sek¨a tarkastelemalla kaapelien virrankestoa kaivoksen maksimikuormilla. Valittua kaape- lia k¨aytet¨a¨an kaivosverkon simulointimallissa, jonka avulla kaivosverkon j¨annitteen laatua tarkastellaan kaivoksen dynaamisilla kuormilla. Simulointimallilla tarkastellaan lis¨aksi energiavarastojen sek¨a niiden sijainnin vaikutusta j¨annitteen laatuun. Ilman energiava- rastoja j¨annitteen havaittiin putoavan liian alhaiseksi kaivoskuormille. Energiavarastojen avulla j¨annite saatiin pysym¨a¨an halutun rajan yl¨apuolella. Energiavarastojen sijoittamisen verkon per¨alle havaittiin pienent¨av¨an verkon maksimivirtaa verrattuna tilanteeseen jossa energiavarastot ovat verkon alussa. Lis¨aksi ty¨oss¨a arvioidaan LVDC-verkon kustannuksia.
This thesis was done in Lappeenranta-Lahti University of Technology during 2019 and 2020 in collaboration with Sandvik.
I would like to thank several people who have given me the essential help in getting this thesis done. At LUT University, D.Sc. Pasi Peltoniemi for guidance through this project, and M.Sc. Janne Karppanen for valuable tips for calculating the DC grid. At Sandvik, thanks to D.Sc. Jenni Rekola for lots of valuable comments on improving this thesis, and D.Sc. Raimo Juntunen for interesting ”twice in a lifetime” tours in Sandvik’s test mine.
And of course, heartfelt thanks to all the friends in Free State of Skinnarila who have made the years in university so great experience.
Jooa Pursiainen May 12, 2020 Lappeenranta
Contents
Abstract Tiivistelm¨a Preface Contents Nomenclature
1 Introduction 8
1.1 Research methods . . . 9
1.2 Research questions . . . 9
2 DC Grids 11 2.1 Voltage Level . . . 11
2.2 Topology . . . 11
2.2.1 Unipolar . . . 11
2.2.2 Bipolar . . . 13
2.3 Grounding . . . 13
2.3.1 TN system . . . 14
2.3.2 IT system . . . 14
2.4 Structure . . . 14
2.4.1 Radial Network . . . 14
2.4.2 Ring Network . . . 15
2.5 Rectifier . . . 15
2.5.1 Line commuted converters . . . 15
2.5.2 Voltage source converters . . . 17
2.6 Inverter . . . 18
2.6.1 Single-phase inverters . . . 18
2.6.2 Three-phase inverters . . . 19
2.7 Summary . . . 20
3 Case Study 21 3.1 Grid . . . 23
3.2 Steady state analysis . . . 26
3.2.1 Unipolar line . . . 26
3.2.2 Bipolar line . . . 27
3.3 Calculation results . . . 29
3.4 Conclusions . . . 33
4.1.1 Cable model . . . 34
4.1.2 Load model . . . 35
4.2 Comparison to calculation . . . 37
4.3 Dynamic loads . . . 38
4.4 Energy storage . . . 41
4.5 Chargers . . . 52
4.6 Conclusions . . . 58 5 Feasibility of the DC grid in mine environment 59
6 Results and Discussion 61
References 63
7
Nomenclature
Latin alphabet
I Current
l Length
P Power
r Resistivity
U Voltage
Subscripts
end End of the line
in Input of the line
n Nominal
neg Negative pole
neut Neutral pole
pos Positive pole
therm Thermal rating
TOT Total
Abbreviations
AC Alternating Current
DC Direct Current
IT Isol´e Terre
LCC Line Commuted Converter
LVDC Low Voltage Direct Current
NPC Neutral-Point-Clamped
MV Medium Voltage
pu per unit
PWM Pulse Width Modulation
RMS Root Mean Square
SOC State Of Charge
TN Terre Neutre
VAC Voltage, Alternating Current VDC Voltage, Direct Current VSC Voltage Source Converter
1 Introduction
Traditionally non-road vehicles used in mining industry have been diesel powered. Long history of combustion engines provides them the reliability and working range needed in mines. Recent years fight against the climate change has increased the interest of electric vehicles on the road as well as off the road, mining machinery being no exception. Elec- trification of mining machinery has also practical advantages other than being friendlier to the environment. As they do not produce exhaust gases, the need to ventilate the mines is greatly reduced (EY, 2019). Electric drives are also quieter and produce less heat than combustion engines (Paraszczak et al., 2014). One example of electrification of mine is Fekola gold mine in Mali, where W¨artsil¨a Energy storage system and solar plant is built to cut down fuel consumption and emissions of the mine (Gre, 2019). Electric motors are also more energy efficient than combustion engines and they can be very precisely controlled (Paraszczak et al., 2014).
Swedish based, international mining equipment company Sandvik has a range of electric loaders (San, n.d.). Electric machines however can’t operate without suitable infrastruc- ture to power them, so in order to utilize them in the mining environment, suitable elec- trical grid needs to be designed for mines. Nowadays, the mines are usually AC powered.
Typically, mines have one voltage level for distribution and one for utilization. Distribu- tion level is in most mines 7200 V or 13200 V and it is dropped to 480 V or 660 V for loads (Darling, 2011). In this thesis, LVDC (Low Voltage Direct Current) grid is studied for the underground mine environment. LVDC (1500 V DC) is suitable as the typical voltage levels for the loads are low voltage.
For over a century, power distribution has been based on AC technology as at the time it proved to be easier way to shift between different voltage levels and transmit the power for long distances. As the converter technology and power electronics developed, DC started to gain ground in datacenters and in ships (Kim et al., 2018; Elsayed et al., 2015).
In marine use, DC systems provide a flexible way to connect variable speed generators batteries and fuel cells. Higher voltage level can be used with DC than with AC which saves costs in cables. DC provides practical advantage as with ships can be connected to shore at different ports without need to change the network frequency. (ABB, n.d.) Recently, interest of research in the area of DC grids has been, among the marine use and datacenters, in residential distribution. Salonen (2006) studied the possibilities to replace the 400 V AC distribution link with LVDC, Partanen et al. (2007) studied safety aspects and fault situations of LVDC grid, Naakka (2011) studied the reliability and feasibility of LVDC distribution to mention some. The mine environment sets different demands for the distribution grid, as the load profile of the mine differs from the residential areas as the individual loads can vary greatly and be quite large. Also, in residential grid, renew- able energy sources such as solar panels can be distributed at various points of the grid, near the loads. Solar panels and wind turbines can be installed on mine site, but not under-
1.1 Research methods 9
ground to the proximity of the loads, so all the energy sources need to be outside the mine.
DC grid offers several advantages over AC grid. In Europe for example, European Com- mission low voltage directive (LVD, 2014) sets the maximum voltage levels for low volt- age grids to 1500 V DC and 1000 V AC. Higher voltage means that more power can be transmitted. Also, even if the AC peak voltage was the same than DC, less power would be transmitted as the RMS voltage of the AC is lower. Higher voltage means lower cur- rent when delivering the same power. Therefore with DC, smaller cables can be used compared to AC thus saving costs. As the inductance of the cable doesn’t affect DC like AC, and there is no skin effect in the cables, the power losses with DC are smaller than with AC. Other advantages of DC are that the frequency doesn’t need to be controlled, and there is no reactive power. (Salonen, 2006; Saeedifard et al., 2010)
DC distribution also simplifies the grid compared to AC. With AC, every load would need their own AC/DC/AC conversion to get the correct operating voltage (Prenc et al., 2016).
Centralized DC distribution reduces the amount of conversions simplifying the grid and lowering the costs.
DC grids offer also easy connectivity of energy storages as the batteries store energy in DC form. Energy storages can bring stability to the grid and they can be used as power back ups during blackouts (Gre, 2019).
1.1 Research methods
In this thesis, literary research is done on DC grid technology and different alternatives to realise the LVDC grid. Suitability of the LVDC for the mine environment is then studied with the case mine. Matlab-script to calculate the steady-state behaviour of the grid is created to evaluate the grids capability to withstand the maximum load of mine. After that, Simulink-model of the designed grid is created to test the grid with dynamic loads.
Once the grid components and cables are determined, the cost of the DC grid for the case mine is estimated.
1.2 Research questions
The following aspects of the LVDC grid are studied for the mine environment:
• Voltage level. What limits there are for the voltage level and what requirements mine sets for it?
• Topology. What is the difference between unipolar and bipolar grid and which one is better for mine?
• Grounding. How the grounding is executed in DC grids and what method is used for mine grids?
• Structure. Which structure is more suitable for mine environment, radial or ring structure?
• Converters. What type of AC/DC converters there are and what demands the mine grid sets for them?
• Energy storages. How the energy storages affect the DC voltage quality and how they should be located?
• Chargers. What effect the chargers of battery operated vehicles have on DC volt- age?
Last two items are studied through simulation model, so in order to create the model, the following things need to be examined:
• Loads. How the constant and dynamic power loads are modelled?
• Cables. How the cable parameters are modelled?
• Grid components. How the medium voltage (MV) grid, transformer and rectifiers are modelled?
11
2 DC Grids
There are several aspects that need to be taken into consideration when designing a DC grid. They include decision of the voltage level, the line topology, grounding method, the grid structure, grid converter topology and inverter topology.
2.1 Voltage Level
Low voltage directive (LVD, 2014) sets the maximum voltage level of LVDC grid to 1500 V in Europe and the same level is used also internationally. Higher voltage level brings several advantages like higher transmission capacity and lower transmission losses (Karppanen et al., 2015). However, the highest possible level cannot automatically be chosen as the voltage level also sets requirements for grounding and selection of other grid components (Kaipia et al., 2013). It is also important to take into account the devices that will be connected to the grid and their requirements for voltage level. For example, telecommunications use 48 V, datacenters 380 V, and trams 750 V (Rodriguez-Diaz et al., 2016). In mines, the typical voltage levels for machines are 480 VAC and 660 VAC (RMS) (Darling, 2011). If full-bridge inverters are used, the DC voltage needs to be the peak value of desired AC voltage for the inverter to produce it (Rekola, 2009). For the mentioned AC voltages, DC voltage would then need to be 678 V and 933 V respectively.
Therefore, in mine environment it would be beneficial to use the full 1500 V.
2.2 Topology
The possible configurations to distribute power in DC grid are unipolar configuration and bipolar configuration. The topology decision dictates the number of voltage levels avail- able, affects the grounding methods as well as the number of rectifiers needed. (Salonen, 2006)
2.2.1 Unipolar
In unipolar system, there are two lines, negative and positive. All the loads are connected to those two lines and therefore there is only one voltage level available. Figure 2.1 shows the wiring of unipolar network (Salonen et al., 2008a).
Figure 2.1: Unipolar network with loads connected through DC/AC inverters. Modified from (Salonen et al., 2008a).
Advantages in unipolar system are that its structure is simpler compared to bipolar and there are no load asymmetry. Also, as it needs only two lines, existing cables with four conductors can be used efficiently. Cons of the configuration are higher voltage level that increases safety risks and lower reliability. As there is only one powerline, single fault affects the whole system. (Karppanen et al., 2015)
Compared to AC grid unipolar DC grid provides higher distribution capacity for power.
1500 V unipolar grid can distribute same power 16.5 times further than 400 V AC grid with the same voltage drop The power transmission capacities of unipolar, bipolar 1000 V AC and 400 V AC are shown in Figure 2.2. (Partanen et al., 2007; Kaipia, 2014)
Figure 2.2: Power transmission capacities of unipolar and bipolar DC line and AC lines at different voltage level (Kaipia, 2014).
2.3 Grounding 13
2.2.2 Bipolar
Bipolar system consists three lines, positive pole, negative pole and neutral. Loads can therefore be connected in several ways; between positive pole and neutral, between neg- ative pole and neutral or between positive and negative pole. Figure 2.3 shows the wiring and different connections of bipolar grid (Salonen et al., 2008a).
Figure 2.3: Bipolar network with loads connected to pole and neutral (1 and 2), between two poles (3) and to both poles and neutral (4). Modified from (Salonen et al., 2008a).
There are several advantages in bipolar grid over unipolar. It provides better reliability as the the other line can still be used in case of fault in the other. The neutral line provides a natural grounding point, and safety is improved as the voltage levels are lower than in unipolar. On a downside, unbalanced loading between the poles cause additional losses.
(Karppanen et al., 2015)
Challenge with bipolar grid is that asymmetrical loads between the poles can cause volt- age instability (Gu et al., 2016). Bipolar grid isn’t capable to distribute power as far as unipolar, but still further than AC grid as shown in Figure 2.2. With the same voltage drop,±750 V bipolar grid can distribute same power 7 times further than 400 V AC grid (Partanen et al., 2007). For the mine environment, bipolar±750 V would be suitable as it provides two voltage levels, 750 V and 1500 V, for the mine machines.
2.3 Grounding
DC grids can be either grounded TN systems or isolated IT systems. The purpose of the grounding is to ensure safe operation for people using the equipment connected to the grid.
2.3.1 TN system
In bipolar grid, the neutral line is grounded in TN system. In unipolar grid, either of the the two lines is connected to the ground (Salonen et al., 2008b). The main safety benefit of TN system is the reduced risk of floating potentials (Karppanen et al., 2015). The neg- ative aspect of grounding is, that the gounding resistance needs to be very low to keep the touch voltages at safe level in case of ground fault (Karppanen et al., 2015).
TN system requires that on the AC side, the starpoint point of the transformer is un- grounded. Otherwise grounding the DC grid causes shortcircuit through ground (Partanen et al., 2007). Typical application where TN systems are used are for example households.
2.3.2 IT system
The main aspect of safety in earth isolated IT systems is the absence of current path in the case of earth fault. This keeps the touch voltages at safe level in case of earth fault. This makes the IT system suitable for challenging environments where the earth resistances are high. However, the earth fault turns the isolated system into grounded system, mak- ing double faults dangerous. Insulation monitoring devices are suggested to detect the ground faults. Other safety concern is the possibility of floating potentials. (Karppanen et al., 2015)
In IT system, the grounding of the AC transformer does not matter in normal circum- stances, but the earth fault in the DC grid leads to shortcircuit through ground (Partanen et al., 2007), so it is recommended to not ground the transformers star point (Salonen et al., 2008b). Typically, moving mining machines are isolated IT systems with insulation monitoring devices, making the IT natural grounding method for mine environment.
2.4 Structure
The possible grid structures are radial network and ring network. The main differences are the cost of the network and the reliability of the network.
2.4.1 Radial Network
In radial network, DC grid is connected to the AC grid from one end and the power has one path from AC grid to the load. It is simple structure, but does not provide alternative paths for power in case of faults. Typically, power grids in mines have radial structure.
As the mines normally expand over time, radial networks are simpler and less expensive to expand with them. (Kumar et al., 2017; Morley, 1990)
2.5 Rectifier 15
2.4.2 Ring Network
The ring network is circular as the name suggests, therefore there are two or more routes for power from AC grid to the load. This improves reliability as one fault in grid does not cut the power from all the loads down the grid. The faulty part of the network can be isolated from the rest of the network, so it does not affect the rest of the loads. Downside is that it requires more cable than radial network, being therefore more expensive. (Kumar et al., 2017)
2.5 Rectifier
In order to convert AC power to DC, rectifier is needed. Rectifiers can be divided into two category, line commuted converters (LCC) and voltage source converters (VSC). LCCs consists passive components while VSCs have also active parts. Differences between the rectifier topologies are the desired amount of voltage levels, the capability of bidirectional power flow, complexity and cost.
2.5.1 Line commuted converters
Simplest type of LCC is a 6-pulse diode bridge. Its efficiency is high, and it is very robust design. With diode bridge, the output DC voltage is uncontrollable and it is proportional to AC voltage. Output voltage of 6-pulse diode bridge isn’t pure DC, but it has 300 Hz component due the diodes switching cycles. Because of the uncontrollability, additional circuit is needed to limit the current spikes when powering up the grid. Diode bridge allows power flow only to one direction, from AC to DC. 6-pulse diode bridge with ca- pacitors creating the zero voltage between the poles is presented in Figure 2.4. (Rekola, 2009)
Figure 2.4: 6-pulse diode bridge with three voltage levels (Rekola, 2009).
More complex rectifier type is thyristor bridge that can be implemented half controlled or fully controlled. In fully controlled bridge, all the components are thyristors, whereas in half controlled half of them are diodes. Half controlled thyristor bridge does not require
current limiting circuit like diode rectifier do. With changing the firing angle of thyristors, DC voltage can be adjusted. This however adds distortion to current. Like diode bridge, thyristor bridge can suppy power only to one direction. (Partanen et al., 2010)
6-pulse, half controlled thyristor bridge suits for the unipolar grid. The problem with the bipolar grid however is that it is a two level rectifier and bipolar grid has three voltage lev- els. Possible implementation for bipolar grid is connecting two 6-pulse bridges in series and creating a 12-pulse bridge. 12-pulse bridge requires a transformer with two secondary windings. The other winding is wye-connected, and the other is delta-connected. There- fore, output voltages have 30◦phase shift. In Figure 2.5, 12-pulse half controlled thyristor bridge with zero voltage between two 6-pulse bridge is presented. (Partanen et al., 2010;
Rekola, 2009)
Figure 2.5: 12-pulse half controlled thyristor bridge with three voltage levels (Rekola, 2009).
LCCs aren’t capable of producing the full 1500 V DC if fed from 1000 V AC line. The mean output voltage of the 6-pulse rectifier is 1350 V and 12-pulse rectifier is 1398 V.
The parallel capacitance of the DC side smoothens the fluctuations of the voltage and raises the mean DC voltage up to the peak voltage of the AC line, which in case of 1000 V AC would be 1414 V DC (Salonen, 2006). For the mine environment, the diode- or thyristor bridges are not usable, as the DC-voltage need to be controllable to keep the desired voltage level under heavily varying loads.
2.5 Rectifier 17
2.5.2 Voltage source converters
In voltage source converters semiconductor switches are used. Advantages over LCC:s are fully controlled DC voltage and possibility for bidirectional power flow. In the sim- plest form, VSC produce two voltage levels, but multilevel topologies also exist. For example, Neutral-Point-Clamped converter (NPC) or Vienna rectifier that is not however commercially available. (Rekola, 2015)
The most common rectifier type is the 2-level, 3-phase rectifier. It has six switches with antiparallel diodes, presented in Figure 2.6. Two voltage levels are produced, and neutral connector can be added from the transformer. With neutral conductor, there are how- ever the third harmonic of the AC fundamental frequency present in the DC current and voltage. (Rekola, 2009)
Figure 2.6: Two-level, Three-phase rectifier (Rekola, 2015).
Three-level, bidirectional NPC converter has twelve active switches with antiparallel diodes and six clamping diodes as shown in Figure 2.7. Advantage of NPC is its low distortion levels in current and voltage. (Rekola, 2009)
Figure 2.7: Three-level NPC converter with neutral connector (Rekola, 2015).
For the mine environment’s varying loads, active, voltage source converters are required.
Two- or three-level converter can be chosen depending on the availability.
2.6 Inverter
Inverter is needed to change the DC back to AC for the loads. For different applications, there are different inverter topologies. They can be three- or single-phase, two- or three- level and have halfbridge or fullbridge construction.
2.6.1 Single-phase inverters
Single-phase inverters can be two- or three-level, and half- or fullbridge. Fullbridge in- verter has twice the amount of active switches compared to halfbridge, and its output voltage is twice the halfbridge’s voltage (Nuutinen, 2006).
Figures 2.8 and 2.9 shows the single-phase, two-level halfbridge and fullbridge inverters respectively. Inverters are connected to both halves of the bipolar grid.
Figure 2.8: Two single-phase, two-level halfbridge inverters (Rekola, 2009).
Figure 2.9: Two single-phase, two-level fullbridge inverters (Rekola, 2009).
2.6 Inverter 19
In the halfbridge configuration, the output is between±UDC/4 and in fullbridge config- uration between±UDC/2.
Figures 2.10 and 2.11 show the single-phase, three-level halfbridge and fullbridge invert- ers respectively. Inverters are connected to the full DC voltage.
Figure 2.10: Single-phase, three-level halfbridge inverter (Rekola, 2009).
Figure 2.11: Single-phase, three-level fullbridge inverter (Rekola, 2009).
The AC voltage in halfbridge inverter can have three voltage levels, +UDC/2, 0 and - UDC/2. Fullbridge inverter can produce 5 levels, +UDC, +UDC/2, 0, -UDC/2 and -UDC. The advantage of three-level inverters over two-level inverters is lower distortion in AC voltage due to higher number of voltage level. Disadvantage is the higher number of required switches. (Rekola, 2009)
2.6.2 Three-phase inverters
Commonly used inverter, three-phase, two-level inverter is presented in Figure 2.12. It is capable of bidirectional power flow and it was presented in converter use in Figure 2.6.
In Figure 2.12, inverter is connected to the full DC voltage. In the bipolar grid, inverters can be connected between both poles and neutral, and the AC voltage level is then half compared to situation where the full DC voltage is used. (Rekola, 2009)
Figure 2.12: Three-phase two-level inverter (Rekola, 2009).
As being two-level inverter, it can produce two voltage level for each phase. For example, when switch S1is on and S2 off, phase c is connected to UDC/2 and when S1is off and S2 on, phase c is connected to -UDC/2. (Anaya-Lara et al., 2014)
Almost all loads in mine are three-phase motors, lighting being an exception. There- fore two- or three-level, three-phase inverters are the most suitable inverters for the mine environment.
2.7 Summary
In this chapter the various aspects of DC-grids were presented. For the voltage level, 1500 V is studied further for the mine environment as it maximises the power distribution.
Bipolar, radial grid is chosen for the structure. Bipolar grid offers two voltage levels, 1500 V and 750 V for the loads, and chance to divide the loads between two poles. Radial grid is more economical, and is typically used in mines. As the DC voltage needs to be controlled, line commuted converters are not applicable and active, voltage source converters are needed. The grounding method is not considered further in this study, it can be noted that the mining machines usually are isolated from the ground making the mine an IT system. Inverter topologies were presented as the loads in mine are typically AC powered motors. It can be noted that majority of the loads in mine are three-phase loads, so the inverters need to be three-phase as well. They are not however studied further and not included in simulation model as the focus of this thesis is on the quality of the DC voltage. DC voltage can vary as long as its above the minimum level and the wanted AC voltage can still be produced. The ripple in DC line can however produce harmonics in load-side AC that might need to be looked at.
21
3 Case Study
The case mine for which the LDC grid is designed is so called underground block cave.
In block caving, a network of tunnels is created underneath the excavation. At the junc- tions of tunnel network are vertical funnels to the ore body. The ore is then collapsed in a controlled manner to the funnels where the crumbled ore is collected with loaders and transported to the crusher. The layout of the mine is presented in Figure 3.1. In block caving, unlike other mining methods, the production site stays constant for tens of years.
This means that the electric grid doesn’t need to be expanded, making the block cave suitable for testing electrical loaders and DC power distribution.
Figure 3.1: Layout of the mine.
The case mine is located at Northparkes mines in New South Wales, Australia. Therefore the European standards do not apply. However, the Australian limit for the low voltage is the same 1500 V for DC and 1000 V for AC as stated in the AS/NZS 3000, ”Wiring Rules” (LVD, 2018).
The loads of the mine are presented in Table 3.1. The biggest single power consumer is the gyratory crusher, which power is 315 kW with duty cycle of around 80 %. The ventilation is constant 220 kW. Dewatering pump is estimated to use constant 22 kW. Loader’s peak power is 150 kW during accelerations. Average power during the work cycle is around 56 kW. The total power consumption would then be 1587 kW in a situation where all the
loaders are working at their peak power. This however is not normal operating situation, instead the loaders starts and stops are synchronized to even the power consumption. At normal working condition the total power consumption of the analyzed mine production area is around 1000 kW. Figure 3.2 shows the cyclical nature of the loaders at normal use. Cycles are 30 second averages so they do not show the peak powers. The loaders are automated and can therefore be in use around the clock. The cable loaders in the mine have previously been powered solely by AC grid.
Table 3.1: Loads of the mine.
Description Power [kW] Number of pcs Total power [kW]
Crusher 315 1 315
Pump 22 1 22
Fan 22 10 220
Loader 150peak 8 1200peak
56avg 448avg
Other 10 1 10
0 100 200 300 400 500 600 700 800 900 1000
Time [s]
0 10 20 30 40 50 60 70
Power[kW]
Loaders
Loader1 Loader2 Loader3 Loader4 Loader5 Loader6 Loader7 Loader8
Figure 3.2: Average cycles of the loaders.
Figure 3.3 shows the measured power of one loader. Power is measured with 0.1 s inter- vals, and shows the occasional peaks of 150 kW. Negative power comes from downhill breaking as the loaders are capable of harvesting the energy and feeding it back to the grid.
3.1 Grid 23
0 50 100 150 200
Time [s]
-100 -50 0 50 100 150 200
Power [kW]
Figure 3.3: Measured cycle of the loader.
The minimum DC voltage that the loaders’ inverters require to work properly is 700 V.
So this is the requirement for the DC grid to be able to supply the loads.
Additionally, there are chargers for machines that can be operated by battery power. Load- ers can be powered by their 400 m cable, by an on board battery, or by combination of the two. Maximum power of one charger is 350 kVA, and they can be paired to produce 750 kVA of power.
3.1 Grid
The proposed grid for the mine is presented in Figure 3.4. The connection points and the line for the loaders are marked red at the start of each tunnel. The ventilation power needed for the mine this type and size is 220 - 230 kW. This is achieved with ten ven- tilation fans, 22 kW each. Ventilation fans and their feeding line are marked blue. One dewatering pump is marked green and the lines for lighting are yellow. The crusher is located at the left side of the figure.
Figure 3.4: Proposed grid.
After the loads have been positioned in the mine and the possible routes for cables have been found, the loads need to be divided between the two poles of the bipolar±750 V grid. The aim is to balance the loads as equally between the poles as possible to keep the pole voltages even.
Loaders and ventilation fans are divided into two groups because of the bipolar main line is located in the middle of the mine area. The upper group of loaders and crusher are connected to the positive pole. To the negative pole are connected the two ventilation groups, lower group of loaders, dewatering pump and lighting. When four loaders are located to each group, the total, maximum load for positive pole is then 915 kW, and for the negative pole 852 kW. It is estimated that the connection point for crusher is 600 m from the rectifier and the connections point for the other loads are 848 m from the rectifier.
Figure 3.5 presents the loads and their distances from the rectifier in each pole.
3.1 Grid 25
Figure 3.5: Loads and distances of bipolar line.
The crusher is estimated to be 37.5 m of the bipolar line, and the dewatering pump 19 m. Figures 3.6 and 3.7 show the distances of the upper half and the lower half of the ventilation group in Figure 3.4.
Figure 3.6: Loads of and distances of ventilation group of upper tunnels.
Figure 3.7: Loads of and distances of ventilation group of lower tunnels.
Figure 3.8 shows the group for loaders located in upper tunnels and Figure 3.9 shows the loaders in lower tunnels in Figure 3.4. Four loaders are located at furthest points of the line and their power is the loaders peak power.
Figure 3.8: Loads of and distances of loader group of upper tunnels.
Figure 3.9: Loads of and distances of loader group of lower tunnels.
With the loads distributed as above, the load difference between the poles was 63 kW with the maximum power for the loaders. As the loader’s real power consumption is not constant, stepping the start times of their work cycles gives freedom to adjust the difference between the poles if it comes necessary.
3.2 Steady state analysis
The proposed grid is analyzed through voltage drop and thermal current ratings of the cables. The unipolar calculation is needed for the loads connected to the main grid and bipolar calculation for the main line from rectifier to the back of the mine.
3.2.1 Unipolar line
Current in unipolar line can be calculated as
I =P/U (3.1)
whereP is the power of the load connected to the line, and U is the voltage to which the load is connected.
The voltage drop of the line can be calculated as
∆U =I·(rpos+rneg)·l (3.2)
whererposandrneg are the resistivities of the positive and negative pole respectively, and lis the length of the line (Vornanen, 2009).
3.2 Steady state analysis 27
The voltage at the end of the line is then
Uend =Uin−∆U (3.3)
whereUin is the voltage at the beginning of the line. When calculating the voltage drop in line for the first time, input voltageUin is used in (3.1). As this is not the actual volt- age the load is connected due to the voltage drop, the output voltage from (3.3) is then used again in (3.1). With couple of iterations, the actual output voltage of the line is found.
3.2.2 Bipolar line
In bipolar line, currents in positive and negative poles flow opposite directions and can be depicted as
Ipos =Ppos/Upos (3.4)
Ineg =−Pneg/Uneg (3.5)
WherePpos andPneg are load powers connected to each pole, and Upos andUneg are the pole voltages which the loads are connected. Both voltages are considered positive here.
The voltage drops in both poles can be calculated as
∆Upos=Ipos·rpos·l+ (Ipos+Ineg)·rneut·l (3.6)
∆Uneg =−Ineg·rneg·l−(Ipos+Ineg)·rneut·l (3.7) whererpos,rnegandrneutare the resistivities of the positive pole, negative pole and neutral pole respectively, andlis the length of the line (Jung et al., 2018).
Pole voltages at the end of the line are then
Uend,pos=Uin,pos−∆Upos (3.8)
Uend,neg =Uin,neg−∆Uneg (3.9)
As with the unipolar line, the end voltages are obtained by iterating (3.4) to (3.9) couple of times.
The resistivities of the poles in equations 3.2, 3.6 and 3.7 depend on used cable and the number of parallel connected conductors per pole,
rpole= 1 P
i=n 1 ri
(3.10)
wherenis the number parallel conductors and ri is the resistivity of the individual con- ductor.
The cable needs to withstand the current going through it, so its thermal current rating has to be determined. If all the parallel connected conductors have equal resistivity, then the total thermal rating is the amount of the parallel conductors times the thermal rating of the individual conductor,
ITOT, therm=n·Itherm. (3.11)
That is because the current through the cable gets divided equally between each conductor.
If the conductors have different resistivities, the total thermal rating can be calculated through Kirchoff’s laws. The voltage over each parallel conductor is equal
U1 =U2 =...=Un. (3.12)
In the case of two parallel conductors, the relationship of the thermal ratings can be calculated as follows:
U1 =U2
r1·l·I1, therm =r2·l·I2, therm (3.13)
I1, therm = r2
r1·I2, therm
where r1 and r2 are resistivities of the conductors 1 and 2, l is the length of the both conductors, and I1, therm and I2, therm are the thermal ratings of conductors 1 and 2. The total thermal rating of the two parallel conductors is the sum of the individual thermal ratings:
ITOT, therm=I1, therm+I2, therm= r2
r1·I2, therm+I2, therm= (r2
r1 + 1)·I2, therm, (3.14) in the condition that
r2 ≤r1. (3.15)
(Salonen, 2006)
For this thesis, a Matlab-script for voltage drops was created. User inputs for the script are the input DC voltage, number of parallel cables, vector of load powers at different points of the line and vector of distances of loads. Data for different cables is read from Excel sheet where new cables can be added. Script then calculates resistivity for each line with the equation 3.10 based on the cable data and number of parallel cables. For unipolar connection, the four conductors in each cable are connected in a way that 2 parallel phase conductors make the positive line, and phase and neutral conductors make the negative line. Figure 3.10 shows how the four conductors are connected (Salonen et al., 2008a).
3.3 Calculation results 29
Figure 3.10: Connection of four conductors in unipolar system (Salonen et al., 2008a).
If more parallel cables are needed, they are added similar way. Bipolar grid is more com- plex as there are four conductors and three lines. If only one cable is used, positive and negative line consist phase conductors, neutral line the neutral or ground conductor and one phase conductor is unused. If more cables are needed, they are added so that positive and negative line have equal amount of phase conductors and neutral line has all the neu- tral conductors. Except if three cables are used, then positive, negative and neutral line have each one cable of four conductor. The total thermal rating is calculated with 3.11 if the parallel conductors are identical, and with 3.14 if not.
For unipolar line, equations 3.1 to 3.3 are iterated 5 times to get the output voltage for the line. If the line consists more than one point where loads are connected, the line is calculated in parts where the input voltage for the next part is the output voltage of the previous part. Same applies to bipolar line with equations 3.4 to 3.9. Output of the script is then the output voltages of the line, highest current of the line, thermal ratings and voltage drop percentages of the line with different cables.
3.3 Calculation results
To determine the suitable cable for the bipolar line, the voltage drop with the loads pre- sented in chapter 3.1 were calculated using different cables. Calculations were done with six AMKA cables, eleven AXMK cables and eight MCMK cables. Figure 3.11 shows the voltages of the two cables of each type. These cables had the lowest voltage drops, the rest are not presented for the clarity of the figure. The AMKA cables show the largest voltage drop. The voltage between the poles at the end of the grid is less than 1400 V, meaning that the voltage in at least one pole is less than 700 V where the loaders are connected.
Also their thermal rating wasn’t enough for the currents in the grid. MCMK cables show bit lower voltage drop than AXMK cables, but they are much more expensive. The chosen cable for the further study is therefore the AXMK 4x300.
0 100 200 300 400 500 600 700 800 900 Distance [m]
1250 1300 1350 1400 1450 1500
Voltage [V]
AMKA 3x70+95 AMKA 3x120+95 AXMK 4x240 AXMK 4x300 MCMK 4x185 MCMK 4x240
Figure 3.11: Voltages of the bipolar line with different cables.
The AXMK 4x300 cable has the resistivity,rof 0,1Ω/km, and the thermal rating,Ithermof 396 A (Prysmian, 2019b). To get the thermal rating high enough for the loads, 3 parallel cables were needed in a way that each pole consists one cable of four conductors, as is recommended by ABB (2000). As the resistivities of each conductor are equal, the total thermal rating can be calculated with equation 3.11. Total thermal rating for both poles and neutral is 1584 A. With the loads presented in Figure 3.5, highest current in positive pole was calculated to be 1254 A, and for the negative pole 1176 A.
The voltage drop of each pole were calculated at the load connection points presented in Figure 3.5, first after 600 m and then 248 m further, at 848 m. Voltages where the load groups are connected are presented in Table 3.2, calculation is done with the assumption that all loaders are using their peak power. Even though the poles are called positive and negative, both of their voltages are presented positive here.
Table 3.2: Loads and voltages of the bipolar line with AXMK 4x300.
Pole pos 750.0 V Crusher 315 kW 729.8 V 4 loaders 600 kW 726.9 V
Pole neg 750.0 V 734.0 V 4 loaders 600 kW 724.5 V
dewatering pump 22 kW ventilation fans 220 kW lighting/other 10 kW
Voltages of each pole is illustrated in Figure 3.12 as a function of distance from the rec- tifier. The voltage in positive pole drops faster than in negative pole up to 600 m and the crushers connection point as the load seen by the cable is greater. After 600 m, the loads connected to the negative pole are greater than loads in positive pole, making the negative pole voltage drop faster. The total voltage between the poles at the end of the line is 1451.4 V.
3.3 Calculation results 31
0 100 200 300 400 500 600 700 800 900
Distance [m]
720 725 730 735 740 745 750
Voltage [V]
AXMK 4x300
Pole pos Pole neg
Figure 3.12: Pole voltages of the bipolar line with AXMK 4x300.
With AXMK 4x300, voltages in the bipolar line stayed above 700 V limit in points where the loads are connected. Cables by which the loads are connected to the bipolar line, need to be selected so that voltage doesn’t further drop below the 700 V. Voltage drops for the unipolar loads are calculated based on the voltages presented in Table 3.2. As the loads in each individual group are identical, the smallest voltage is found at the load furthest from the bipolar line. For the crusher and dewatering pump it is enough to calculate the voltage drop in one point as both loads are located at the end of their feeding lines. For the groups for the loaders and ventilation fans, voltage needs to be calculated at several points before the furthest point, as the load seen by the cable changes along the way. For the group for ventilation, the furthest load is the fan located at the fourth tunnel from the top in Figure 3.4. For the two groups for the loaders, the furthest load is located at the topmost tunnel in Figure 3.4. In the calculation it is considered that the four loaders for the group are located at the furthest four tunnels. The distances are estimations based on the layout of the mine and known tunnel lengths, and are presented in chapter 3.1.
Unipolar lines were calculated with 5 different AXMK-cables and 1 AMKA-cable for reference. The cables and their resistivities and thermal ratings are presented in Table 3.3 (Prysmian, 2019a,b).
Table 3.3: Cables used for unipolar loads.
Cable rphase [Ω/km] rneutral[Ω/km] Itherm[A]
AXMK 4x300 0.100 0.100 396
AXMK 4x240 0.125 0.125 343
AXMK 4x185 0.164 0.164 291
AXMK 4x150 0.206 0.236 255
AXMK 4x120 0.253 0.253 220
AMKA 3x120+95 0.253 0.363 250
Table 3.4 shows the end voltage and highest current in each group. The leftmost column presents the cable and its thermal limit when one cable is used and when two parallel ca- bles are used as described in section 3.2. Loaders needed two parallel cables to withstand the current, for other groups one cable was enough.
Table 3.4: Group voltages and currents with different cables.
Cable Pump Fans Crusher Loaders
AXMK 4x300 724.5 V 722.6 V 728.2 V 719.4 V
Itherm,1= 792 A 30.4 A 151.9 A 432.6 A 831.4 A
Itherm,2= 1584 A
AXMK 4x240 724.5 V 722.1 V 727.8 V 717.6 V
Itherm,1= 686 A 30.4 A 152.0 A 432.8 A 832.9 A
Itherm,2= 1372 A
AXMK 4x185 724.4 V 721.3 V 727.1 V 714.6 V
Itherm,1= 582 A 30.4 A 152.0 A 433.2 A 835.2 A
Itherm,2= 1164 A
AXMK 4x150 724.4 V 720.5 V 726.5 V 711.5 V
Itherm,1= 510 A 30.4 A 152.0 A 433.6 A 837.7 A
Itherm,2= 1020 A
AXMK 4x120 724.4 V 719.6 V 725.7 V 707.9 V
Itherm,1= 440 A 30.4 A 152.1 A 434.1 A 840.6 A
Itherm,2= 880 A
AMKA 3x120+95 724.4 V 719.2 V 725.3 V 706.2 V
Itherm,1= 500 A 30.4 A 152.1 A 434.3 A 842.0 A
Itherm,2= 848.48 A
Figures 3.13 and 3.14 shows the voltages of the upper ventilation group and upper loader group, presented in Figures 3.6 and 3.8 respectively. Voltages are calculated with AXMK 4x300 cable.
Figure 3.13: Voltages of the ventilation group with AXMK 4x300.
3.4 Conclusions 33
Figure 3.14: Voltages of the loader group with AXMK 4x300.
The installation temperature and method sets some limits to the maximum current that can go through the cables. The maximum current in bipolar line was calculated to be 1254 A and the thermal limit for the line was 1584 A. If the temperature in the mine is assumed to be 25◦C, and the three cables are installed on ventilated shelf, we get the correction factors of 1.04 and 0.82 for temperature and installation respectively, according to SFS 6000 -standard (SFS, 2017). The maximum allowed current would then be
1.04·0.82·1584A= 1350.80A,
which is higher than the calculated current with the maximum loads.
3.4 Conclusions
In this chapter the bipolar ± 750 V DC grid for the block cave case environment was presented, the loads were divided between the poles, and the cable type and diameter was determined for the grid and the loads. The most suitable cable for the grid is AXMK 4x300. Of the presented cables, it provides the lowest voltage drops for the loads, without being as expensive as MCMK cables. For the bipolar grid, three parallel cables, one for each pole and neutral, was needed to achieve high enough thermal rating. The most critical loads, loaders, were connected to the bipolar grid with two parallel cables to keep the voltage above 700 V and the current below the thermal limit of the cable. For other loads, one cable was enough. AXMK 4x300 withstands the currents also if installed on ventilated shelf and the installation factors for current are taken into account. This was calculated for the bipolar line as the current is highest in it. It is also reasonable to use the same cable for all the mine as then only one type of cable is needed to be stocked in case some part of the grid needs to be replaced.
4 Dynamic Modelling
In this chapter the simulation model of the grid is described. The model is needed to study the dynamic loads that can not be analyzed through steady-state calculation. Also, the energy storage systems and battery chargers are studied.
4.1 Simulink-model
Grid proposed in chapter 3.1 was modelled using Simulink and is presented in Figure 4.1.
Figure 4.1: Grid modelled in Simulink.
On the left side there is the MV grid with the phase-to-phase voltage of 20500VRM Sand frequency of 50 Hz. Transformers nominal power is 2000 kVA, to accommodate the the- oretical maximum power of over 1700 kW. Two capacitors after the PWM rectifier blocks were determined by trial and error to be 25 mF each to achieve the 750 V in both poles.
With smaller capacitors, the poles couldn’t reach the 750 V measured after the rectifiers.
4.1.1 Cable model
Three blocks in the middle in the Figure 4.1 are masks for the cable model shown in Figure 4.2. Cable was modelled asΠ-equivalent with the resistance, inductance and capacitance per kilometer and length of the cable part given to the mask.
4.1 Simulink-model 35
Figure 4.2: Π-equivalent of the bipolar line.
The cable chosen in chapter 3.3 was AXMK 4x300. Its parameters that are used in model are presented in Table 4.1.
Table 4.1: Parameters of AXMK 4x300 cable.
Resistance [Ω/km] Inductance [mH/km] Capacitance [µF/km]
0.1 0.25 0.26
The bipolar line was calculated to be constructed by three cables, one for each pole, so the resistance and inductance of four parallel wires were given to the mask. The capacitance was kept at 0.26µF/km, as the calculation of the correct value is more complicated, and testing with capacitances in a range of nF to mF didn’t affect the behaviour of the model, as the distances are so short. Shunt capacitance of the cable isn’t meaningful for cables shorter than 80 km (Kundur, 1994).
4.1.2 Load model
Figure 4.3 shows the load model of the crusher found in topright in Figure 4.1. As the loads in mine are fed by inverter, they were modelled as constant power loads where the current drawn from the grid is adjusted based on the measured voltage to achieve the wanted power. If the voltage of the grid drops, the higher current is drawn. The load power, given as step function, goes through a first order filter with time constant of 10 ms to simulate the limited rise of the current in loads.
Figure 4.3: Load model of the crusher.
The capacitors at the loads were chosen to be 71 µF/kW as suggested by Lana (2014).
The efficiency takes into account the losses in the DC/AC inverter to which the load is connected. Figure 4.4 shows the efficiency of the inverter components by Lana (2014).
Total efficiency of the inverter with the load of 1 pu, is around 90 %.
Figure 4.4: Component efficiencies of the inverter (Lana, 2014).
90 % is a bit low for todays inverters. For example, Silicon carbide inverters can reach the efficiency of 99 % (Rabkowski, 2014) and amorphous cores can improve the transformers
4.2 Comparison to calculation 37
efficiency. In this thesis, efficiency of 96 % is used.
4.2 Comparison to calculation
The voltage drops in Simulink-model were compared to steady-state calculation methods presented in chapter 3.2. Purpose of this is to verify the model, so it can be used with dynamic loads. The used loads represented the normal working condition at one moment presented in Figure 3.2. Lighting and other loads are omitted from the simulation. The inverter efficiency in the model was set to 1 as it wasn’t included in the steady-state calculation. Loads are presented in Table 4.2
Table 4.2: Loads of the simulation.
Description Power [kW] pcs
Crusher 315 1
Pump 22 1
Fan 22 10
Loader 1 67 1
Loader 2 67 1
Loader 3 56 1
Loader 4 56 1
Loader 5 56 1
Loader 6 56 1
Loader 7 67 1
Loader 8 56 1
Table 4.3 presents the calculated and simulated voltages and currents of the bipolar line.
Voltages are measured at the connection points for each unipolar load groups and at the rectifier.
Table 4.3: Voltages and currents of the bipolar line.
Calculation Pole pos 750.0 V, 761 A Crusher 736.8 V Loaders 1-4 736.7 V
Simulation 750.0 V, 764 A 736.0 V 736.8 V
Calculation Pole neg 750.0 V, 648 A 742.1 V Loaders 5-8 736.2 V Pump
Fans
Simulation 750.0 V, 650 A 743.0 V 735.9 V
Table 4.4 presents the calculated and simulated voltages and currents of the unipolar load groups. Voltages are measured the furthest load of the groups of many loads as the voltage drop is highest there. Currents are measured at the beginning of the groups as they are highest there.
Table 4.4: Voltages and currents of unipolar loads.
Pump Fans Crusher Loaders
U [V] I [A] U [V] I [A] U [V] I [A] U [V] I [A]
Calculation 736.1 29.9 734.3 149.5 735.2 428.5 733.8 334.9 Simulation 735.8 29.9 734.0 149.8 734.4 429.0 733.9 335.0
Obtained results show that for the loads and distances in question, the simulation model matches very well the voltage drop calculations in steady-state situation.
Steady state, maximum loads that were used to calculate the voltage drops in chapter 3.3 were then simulated. With eight loaders of constant 150 kW, total load for positive pole is 915 kW and for negative pole 842 kW. The grid however couldn’t stay up with these loads. Once the fourth loader connected to the pole started, the pole voltage started to drop and couldn’t recover. This happened for both poles. Grid stayed up if only three loaders per pole were in use, with total load for positive pole being then 765 kW and for negative pole 692 kW.
4.3 Dynamic loads
As the models accuracy was verified with steady state loads, dynamic load cycles of the loaders were simulated. Each of the eight loaders were simulated to drive the cycle presented in Figure 3.3. One 217 s cycle were set to last 2.17 seconds in simulation.
Start times were stepped so that first loader started at the simulation time 0.27 s and last one at 2.17 s. Loads were stepped so that if the grid fails, the load that causes it can be determined. Loads were created using Simulinks signal builder. Figure 4.5 shows the total load of the grid and the loads of each pole. Graphs show the start ups of the loads and all loads are running after around 2.2 second mark. The X-axis shows the simulation time and the Y-axis loader power in kW. Eight loaders were distributed half and half to the two loader groups and placed to the furthest points in grid leaving five empty tunnels.
4.3 Dynamic loads 39
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Time [s]
0 500 1000 1500
Power [kW]
Total load
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Time [s]
0 500 1000
Power [kW]
Pole loads
Load + Load -
Figure 4.5: Loads in simulation. Total load in upper graph and loads in positive and negative pole separated in lower graph.
Figure 4.6 shows the voltages measured at the rectifier end and Figure 4.7 shows the currents with the loads presented above. Measurements are taken from 0.5 s onwards, to exclude the transients at the start up of the simulation.
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Time [s]
580 600 620 640 660 680 700 720 740 760 780 800
Voltage [V]
Bipolar line voltage
Pole + Pole -
Figure 4.6: Voltages measured from the bipolar line at the rectifier.
0.5 1 1.5 2 2.5 3 3.5 4 4.5 Time [s]
-1500 -1000 -500 0 500 1000 1500 2000
Current [A]
Bipolar line current
Pole + Neutral Pole -
Figure 4.7: Currents measured from the bipolar line at the rectifier.
It can be seen from the Figure 4.6 that the voltage of the positive pole which has larger load drops to 600 V when the fourth and final loader of the pole is started. Once all the loads were running, the voltage of the positive pole takes periodically dip to about 660 V.
Current rises to about 1390 A and then settles to 890 - 1260 A. Current in negative pole fluctuates between 725 A and 1060 A. Grid was tested with inverter efficiencies of 94 - 95 % making the loads seem larger to the grid, and in those cases the grid did not recover from the voltage drop in the positive pole. This indicates that either the grid is on the very limit of its stability with the loads in question, or the modelling of rectifiers or MV grid causes instability.
Table 4.5 shows the range of voltage fluctuation at the connection points of unipolar loads.
Measurements were done between the simulation time 2.5 s - 5 s, when all the loads were running their cycles.
Table 4.5: Voltage ranges of the bipolar line.
Pole Pos 662 V−772 V Crusher 636 V−758 V Loaders 1-4 633 V−759 V Pole neg 731 V−771 V 719 V−762 V Loaders 5-8 709 V−756 V
Pump Fans
Voltages and currents were measured for unipolar loads and they are presented in Table 4.6. Table shows the voltage range between the simulation time 2.5 s - 5 s as well as the highest current of that time range. Voltages are measured from the furthest load of the group for highest voltage drop. Currents are measured at the beginning of the group for the highest current. The momentary voltage drop or increase percentage is compared to 750 V.
4.4 Energy storage 41
Table 4.6: Voltage ranges of the unipolar loads.
U [V] ∆U [%] I [A]
Crusher 634−757 -15−+1 518 Loader 626−755 -17−+1 783 Pump 709−756 -5−+1 32
Fan 707−756 -6−+1 233
Loaders cycle time in simulation was 2.17 s, so it can be seen from the Figure 4.6 that the voltage drops to 660 V 4 times during one working cycle of the loader. As the loaders required 700 V, it is too low to them to work properly.
4.4 Energy storage
The simulated grid couldn’t stay up with the maximum loads of 1757 kW on its own, and the voltage under the dynamic loads dropped too low, so the effect of energy storages on a grid were tested. Figure 4.8 shows the modelled energy storage. The model is based on a energy storage model by Mathworks. One energy storage was connected to each pole.
Figure 4.8: Battery energy storage system.
The battery unit, shown at the left in figure 4.8, was set to be a lithium-ion battery with nominal voltage of 1100 V and capacity of 900 Ah. As the maximum load for the positive
pole when all loaders are running at their peak power is 915 kW, 990 kWh batteries could support the loads for a bit over one hour in case of total blackout for the grid given the suitable control. The droop control function gives the reference to the converter-block whether charge or discharge the battery as depicted in Figure 4.9. The maximum current drawn from the battery was set to 1 C, 900 A.
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
PBESS,ref [pu]
0.96 0.98 1 1.02 1.04 1.06
UDC [pu]
Droop Control
charge discharge
Figure 4.9: Droop control for the battery with nominal DC voltage of 750 V.
The nominal voltage, Un, is 750 V, the battery is charged when the voltage is higher than 1,01Un(757.5 V) and discharged when the voltage is below 0.99Un(742.5 V). These are
±1 % of the nominal voltage to test the effect of the energy storages and can be optimized for better results. If the measured voltage is higher than the threshold to charge (1.01Un), the function subtracts the threshold from the measured voltage and divides the result by the slope of the curve (-0.05). The obtained reference is then limited to be -1 at lowest.
The discharge side of the droop control works in a similar way. The reference from the droop control is given to the DC/DC -converter block shown in Figure 4.10.
4.4 Energy storage 43
Figure 4.10: DC/DC Controller -block for the energy storage system.
In DC-DC converter, the reference from droop control is converted to current that is either fed to or drawn from the battery. The battery current is multiplied by battery voltage to get power of the battery. This power is taken from or fed to the grid, so by dividing it by the grid voltage the current that goes to the grid is obtained.
The energy storages were tested on a two locations, at the beginning of the grid (Figure 4.11) and at the end of the grid (Figure 4.12).
Figure 4.11: Energy storages located at the beginning of the grid.
Figure 4.12: Energy storages located at the end of the grid.
Effect of the energy storages on the grid stability was tested with the steady state maxi- mum loads. Figure 4.13 shows the loads of the both poles and the total load. Positive pole has the load of 915 kW and its reached at simulation time 0.55 s. Negative pole has the load of 842 kW (lighting wasn’t modelled) and its reached at 0.5 s. The total load is 1757 kW.
0 0.2 0.4 0.6 0.8 1 1.2
Time [s]
0 500 1000 1500 2000
Power [kW]
Total load
0 0.2 0.4 0.6 0.8 1 1.2
Time [s]
0 500 1000
Power [kW]
Pole loads
Load + Load -
Figure 4.13: Load profiles of the poles and total load of the grid.
Figure 4.14 shows the voltages of the bipolar grid when the energy storages are connected at the beginning of the grid (as in Figure 4.11). The biggest voltage drops happened when the final loads of each pole were started up. After that, the voltages settled at around 736 V for positive pole and 739 V for negative pole.
4.4 Energy storage 45
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1
Time [s]
700 710 720 730 740 750 760 770 780
Voltage [V]
Bipolar line voltage
Pole + Pole -
Figure 4.14: Voltages of the bipolar line when energy storages are at the beginning of the grid.
The power drawn from the energy storage connected to the positive pole was 317 kW at the peak when the final load started up and settled for 182 kW. The highest current in bipolar line was 1506 A in positive pole when the last load started up and it settled to 1350 A. Settled current is 8 % higher than the maximum current calculated in section 3.3, but in the steady state calculation the inverter efficiency wasn’t taken into account. Table 4.7 shows the settled voltages and currents of the loads. Voltages are measured from the furthest load of the group for highest voltage drop. Currents are measured at the beginning of the group for the highest current. The voltage drop or increase percentage is compared to 750 V.
Table 4.7: Voltages and currents of the unipolar loads.
U [V] ∆U [%] I [A]
Crusher 712 -5 461
Loader 703 -6 890
Pump 712 -5 32
Fan 710 -5 161
Figure 4.15 shows the voltages when the energy storages are located at the back of the grid (as in Figure 4.12). The voltage at positive pole settles to oscillate between 710 V and 771 V and at negative pole between 747 V and 753 V. Large oscillations in positive pole are because the crusher causes a voltage drop in the pole, demanding more from the energy storage. The droop controller then overshoots the voltage above the 750 V, stopping the discharge and causing the voltage to drop back. With better optimization of the droop control, oscillations could be reduced.
