A Comparative Study of Static VAR Systems for Improving Voltage Stability in Expansion of Mining Projects with Gearless Motor Drives

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Introduction
Troughout history, the extraction of precious materials from the earth has held a tremendous value. As we enter the 21st century, the mining industry has become increasingly globalized, with large multinational corporations at the forefront of this sector. Te industry's impact on the environment and concerns regarding overexploitation have prompted the implementation of regulations aimed at mitigating these issues. Mining operations are facing a competitiveness challenge as the world risks running out of copper due to a supply defcit and rising demand, with copper reserves currently at their lowest levels in the past 15 years following the pandemic. Furthermore, the demand for copper is expected to grow by 25% over the next decade. As a result, the copper price has sufered a revaluation of price higher than 28%, beginning 2021 at 7800 $/ton and reaching 10725 $/ton in May and 10652 $/ton in October (Figure 1). In the coming months, the price is expected to maintain its level around 9500 $/ton, and with this scenario, mine installations will be responsible for increasing their production to meet this exceptional demand, opening new mining exploitation or upgrading existing ones.
Te mining industry of today is predominantly powered by GMDs, which represent the most robust and highperformance mill drive systems available for SAG, ball, and AG mills. GMDs use the combination of synchronous motors and cycloconverters, reducing the number of components, as couplings, intermediate gearboxes, and bearings [1]. Consequently, this approach leads to a 4% increase in energy efciency and minimizes the need for spare parts. Latest trends in the mining industry are driving grinding mill sizes to greater levels than ever before. In the grinding step of the ore processing, GMDs have become a crucial component and their robust and proven design has been used for over 30 years, increasing reliability and efciency in grinding [2]. It is worth noting that approximately 60% of the electrical energy consumed by mining installations is attributed to motors, particularly when new gearless motor drives (GMDs) are installed in an existing mine. Tis increase in active and reactive power consumption can cause power quality issues, particularly related to voltage stability. Te primary objective of this study is to identify a FACTS capable of improving voltage stability in response to increased production demands and the installation of new motors in the mine's electrical grid. Specifcally, this research focuses on exploring the viability of an SVS as a solution for optimizing the existing grid performance and increasing mining production.
An SVS is defned as a combination of discretely and continuously switched VAR sources that are operated in a coordinated fashion by an automated control system. Tese VAR sources include SVCs and STATCOMs [3], members of FACTS controllers. SVSs can solve any power system dynamic performance problem (voltage stability, transient stability, voltage ficker, etc.) that can sufer in mining electric systems.
Te assessment of the voltage stability improvement performance will encompass the evaluation of various FACTS device confgurations and their compatibility with diferent power system structures. Te literature shows some comparative studies of dynamic performance of aforementioned FACTS devices; for example, the authors in reference [4] compared the dynamic response and stability margin of SVC and STATCOM in weak and strong systems, the authors in reference [5] studied the infuence in dynamic voltage stability during LVRT in wind farms, the authors in references [6][7][8] compared SVC and STATCOM in dynamic reactive power support in diferent conditions (wind speed variation and grid fault) in wind turbines, while the authors in references [9][10][11][12] showed the importance of reactive power compensation to improve transient stability margin and power quality. When looking for applications of these FACTSs in electric power networks, the authors in reference [13] read up on the best location of a STATCOM in the Pakistan power system to improve voltage stability, the authors in reference [14] studied diferent voltage variations when the 100/145 MVAr FACTS is connected after the tripping of one overhead transmission line, the authors in reference [15] combined the D-STATCOM and a low THD SVC to enhance power quality, reducing voltage ficker and THD in a distribution grid, and the authors in reference [16] compared the dynamic performance of SVC and STAT-COM, showing reduction of the postdisturbance voltage recovery time after 3 phase faults. In reference [17], several series and shunt FACTS devices are combined in the Bangladesh power system to exploit the thermal limit but without the peculiarities of a mining installation.
If we refer to HPS, a comparison among STATCOM, SVC, and DVR can be found in reference [18], where it can be ascertained that reactive compensation devices increase voltage stability margin of the hybrid power systems, and in reference [19], a STATCOM is employed to meet the reactive power demand in an isolated hybrid microgrid, using the mine blast algorithm to tune the parameters of the FACT device. An isolated hybrid microgrid is a small-scale power system that combines two or more sources of energy to provide electricity to a local community or facility that is not connected to the main power grid. It typically consists of a combination of renewable energy sources, such as solar panels and wind turbines, and traditional fossil fuel generators, which work together to provide a reliable and sustainable power supply. Te hybrid microgrid is "isolated" in the sense that it is not connected to the main grid and operates independently to provide power to the local  community or facility. Te term "hybrid" refers to the fact that the system uses multiple sources of energy to generate power, allowing for greater fexibility and reliability. Te reactive power efect on frequency and the active power efect on system voltage are examined in reference [20], when a STATCOM is simulated in an IHPS. Mining power installations that utilize the FACTS have been documented in references [21,22] where an application study of an SVC for improving the power factor and THD in mining complexes is conducted. STATCOM applications in mining are reviewed in references [23,24], compensating quality problems (voltage sags, THD) in an underground mine, and in reference [25] where it is installed to support voltage during starting of an asynchronous machine in low voltage. Currently, there is a dearth of research on the application of FACTSs to expansion mining projects where the installation of new high-power motors, such as GMDs, poses a signifcant risk to the voltage stability of the electrical system.
Terefore, the main contributions of this paper are as follows: (i) Stability analysis of an actual mining expansion project in Chile, which involves a dynamic simulation of the expanded electrical mine system with synchronous GMD motors and SVSs based on SVC and STATCOM technologies. (ii) A methodology for selecting the most suitable SVS, which considers various factors afecting such projects, including performance, cost, and footprint.
Tis study adopts a methodical approach, consisting of a series of sequential steps, to achieve its objectives. Te introduction section commences with the purpose of the study, followed by a literature review and an overview of the contributions of this work. Section 2 presents the case study, while Section 3 outlines the methodology proposed to improve voltage stability through the SVS. Te modelling of the case study and the simulation procedure, including the main assumptions and model references, are provided in Section 4. Te results are discussed in Section 5, while the main conclusions of this work are presented in Section 6.

Description of the Electric Mining
System. Te mining industry plays a signifcant role in the Chilean economy, and Chile is the largest global producer of copper. In order to ensure the relevance and applicability of the study, a typical Chilean 3-line mine installation, as depicted in Figure 2, was selected as the source system for this research. It is composed by three level voltage buses, 220, 69, and 23 kV. Te original mining setup comprised three crushing lines, powered by one 20 MVA motor and two 14.2 MVA motors. Recently, two more gearless motor drives (GMDs) were installed, one rated at 20 MVA and the other at 14.2 MVA. Te aim of this study is to investigate the impact of these new GMDs on the power system and to explore how FACTS devices can enhance power quality.

Voltage Stability Problem.
Mine electrical systems normally have low short-circuit power with low ratio reactance resistance (typical values for X/R ratios of these installations are from 4.5 to 7). Furthermore, the geographic location of mine infrastructures makes it even more difcult to control their voltage levels, characterized by low shortcircuit power. Tese special characteristics make these facilities prone to short-term power quality problems, mainly voltage problems, which are due to the electrical distance between generation and loads and thus depend on the network structure [26]. Under these special conditions, a mine power system can exhibit a new type of unstable behavior characterized by slow voltage drops; these voltage drops will cause a signifcant decrease of the power drawn by other connected devices so that the total power consumed will decrease, and this could escalate to the form of a collapse [3]. Te concerns show the signifcance of reactive power compensation devices, injecting reactive power and consequently maintaining the voltages of the system within nominal values. Reactive power compensation is often the most efective approach to improve both voltage stability and power transfer capability.
Te capability of the combination of a FACTS (SVC or STATCOM) and MSCs to provide dynamic reactive power would prove to be useful in voltage stability (i.e., minimizing voltage drops) when a mine installation is optimized with new synchronous motors. However, the costs of these FACTSs increase in the same way as their capacitive rating. Te literature comprises several studies of the costs of these systems. For example, the authors in references [27,28] worked in the development of a methodology for fnding a cost-efective FACTS solution, while in reference [29], it is demonstrated that the optimally allocated FACTS devices could contribute considerably to savings in generation cost while the payback period of the investment is less than the life expectancy of device. For comparison studies, references [30][31][32] can be used for the calculation of SVSs' costs, including costs for equipment, project engineering, and installation. If we focus on the cost for each kVAr installed in Table 1 (operation and maintenance costs are not included), it can be observed that MSCs have the lowest cost level; so, the bigger the number of these components installed, the more economical solution will be obtained in the SVS.
When considering the environmental impact of increased surface occupation, the size of the SVS can vary depending on the type of FACTS device and its power rating [33], as outlined in Table 2. Table 2 shows that MSCs are the components which less surface occupy when a SVS is installed, so when lack of space is one constraint, MSCs would be a good solution.
With the addition of the two new motors to the mining system, voltage stability is compromised, with voltage drops and a higher risk of voltage collapse during overloads. In steady-state operation, one way to prevent voltage collapse is to either reduce the reactive power load or add more reactive power compensation. Te second option is calculated in Section 3.1, which determines the required reactive energy that the SVS must supply.
International Transactions on Electrical Energy Systems 3 Figure 3 illustrates the recommended design approach for selecting an SVS, which involves identifying the optimal solution while minimizing one of the primary cost drivers in power systems: either the cost of compensators or the footprint of the installation. To begin, it is essential to conduct a simulation of the new motors within the electrical mining system. Upon completion of the simulation, a comprehensive evaluation of the voltage levels across all buses is required. Should the voltage level on any bus fall below nominal values or exhibit any power quality issues, a determination must be made regarding the need for additional reactive power. In such cases, the suitability of two primary reactive compensator alternatives will be analyzed, depending on the driving factor, as follows:

Methodology of Improving Voltage Stability in a Mine System through SVSs
(i) For cost optimization of the installation, the combination of SVC and MSC is recommended (ii) If the primary concern is limited space and minimizing the footprint of the installation, the combination of STATCOM and MSC is a more suitable option to consider Once a suitable reactive energy compensator has been selected, the optimal location for the SVS installation must be determined. To achieve this, the analysis of the system voltage profle and PV curves is utilized to identify the best bus for SVS installation. Based on the selected bus, the requisite reactive energy necessary to compensate for voltage levels is calculated. Lastly, the control scheme for the SVS is designed, including the determination of the reactive energy contribution of each SVS component (Q stat , Q svc , and Q msc ).

3.1.
Sizing of the SVSs. Te lack of reactive power is one of the main causes of voltage instability [34]. Te necessary reactive power to inject at a bus can be calculated from the following equation: where P is active power, Q is reactive power to install, θ is the power factor, and θ′ is the power factor after reactive power compensation.   Reactive power capacity of the SVS to install will depend mainly on the robustness of the grid at the POI [35]. Te following equation could be used to determine the MVAr rating of the SVS: Q is the reactive power of the SVS to be installed, ΔU is the variation of the POI voltage with reference to bus nominal voltage, and S cc is the short-circuit power at the bus where the SVS is connected.
In Figure 4, various buses are shown where the SVS can be installed, including 220 kV, 69 kV, and 23 kV. To determine the required SVS capacity for each bus, equation (2) must be applied. Table 3 displays the necessary reactive power needed for voltage compensation at each bus. Te installation of new motors in the system results in varying voltage fuctuations across the buses (e.g., 2% voltage variation for the 23 kV bus versus 1.5% for the 220 kV and 69 kV buses).

Placement of the SVSs.
Finding the best location for providing reactive power support is important for the enhancement of voltage stability in electrical power systems [36]. Placing FACTS controllers at the proper place increases the loadability margin and hence the stability of the system. Te best location for reactive power compensation for improving steady-state voltage stability margin is the weakest bus in the system [37]. Estimating the ideal bus for installing a reactive power compensator can be achieved through diferent techniques as follows: (i) Power-Voltage (P-V) curves: Te power-voltage (P-V) curves illustrate the relationship between load and bus voltage [38] by plotting the real power transfer at the constant power factor while monitoring bus voltages, highlighting the most critical buses. Figure 4 shows the voltage variation at different buses of the electrical mining system. It is evident that when power is increased, the voltage at the 69 kV bus decreases. (ii) Figure 5 presents a comprehensive simulation model that analyzes the voltage profle of various buses in the system upon switching on the GMD motors. Figure 5 reveals that the bus at 23 kV is the most vulnerable when GMDs are incorporated, as it exhibits greater voltage drops compared to the 69 and 220 kV buses. Te selection of the 23 kV bus as the optimal location for the reactive power compensation has led to the decision to place a 30 MVAr SVS at this bus, which is expected to enhance the overall stability of the system.
After identifying the 23 kV bus as the optimal location for the SVS, two options for a 30 MVAr SVS are evaluated based on cost and footprint, as shown in Tables 1 and 2. (i) Te frst SVS confguration comprises a 15 MVAr SVC and a 15 MVAr MSC, representing a more costefective solution driven by cost considerations (ii) Te second SVS confguration consists of a 15 MVAr STATCOM and a 15 MVAr MSC, which is a more expensive option that is typically preferred when space constraints are a primary driver 3.3. Strategy of the SVS. A static VAR system is, per CIGRE/ IEEE defnition, a combination of static compensators and mechanically switched capacitors and reactors whose operation is coordinated. Te emphasis in a static VARsystem is on coordination [39]. Te strategy of the coordination between the SVS components will be essential to provide with the necessary reactive power after each VAR demand change in the system. In this case, the fast compensation will be provided by the FACTS (SVC/STATCOM). If the maximum capacity of the FACTS is reached, the compensator control would activate, in a predetermined sequence, the mechanically switched capacitor banks until the output of the compensator is reduced below that level, when it will switch of the capacitors [40]. Among the pros and cons of this control strategy, we can highlight the optimization of the compensation of reactive energy due to the FACTSs and, at the same time, the cost of the installation is minimized because of the presence of MSC components. Simultaneously, enhancement of system loadability is achieved [41].
On the other hand, MSCs introduce harmonics in the power system due to inherent operation and thus can amplify existing power system harmonic distortion [42]. Terefore, a power quality study has to be performed to detect possible resonances and design the proper flter solution.

Dynamic Models of the System Components.
Detailed dynamic models for diferent components of the mine are presented in this section.

Synchronous Motors.
GMDs are low speed units, operating in 9-12 r/min speed range and have an internal diameter between 10 and 15 meters, see Figure 6. With 60-72 poles, these motors are fed by a cycloconverter [29] which is a device that converts AC to a lower frequency AC with no intermediate DC link.
Synchronous motors are simulated as the standard model of synchronous machines with salient pole rotor [44][45][46]. Te input parameters for these synchronous motors are the short-circuit parameters, listed in Table 4. Te cycloconverters are not included in this simulation.            considered appropriately in the positive-and zero-sequence systems [47]. Data of the transformers and loads are shown in Table 5. Figure 7 accurately represents the behavior of the SVS-SVC system. Its objective is to calculate the necessary susceptance (B svc ) that the SVC must control to inject or absorb reactive power from the system. Te model measures the voltage and reactive power in the bus to control.

STATCOM-Based SVS Dynamic Model.
Te STAT-COM serves the purpose of regulating the bus terminal voltage by means of a PI control, which adjusts the AC side voltage to a predetermined reference value. Te implementation of the SVS-STATCOM system is realized by means of a current injection model, depicted in Figure 8, where the STATCOM current is maintained in quadrature with respect to the bus voltage. In this case, the output of the model is the STATCOM current (I q_stat ) which can be positive or negative if the STATCOM behaves as a reactor or a capacitor, respectively. Te parameters of the dynamic models from Figures 6 to 7 are listed in Table 6:

Procedure and Scenarios of the Dynamic Simulations.
An evaluation is conducted to assess the dynamic performance of the SVS based on SVC and STATCOM in their ability to enhance the voltage stability of a power system when new GMDs are integrated. Tis comparative analysis is carried out across the following scenarios: (i) Scenario 0: simulation without SVSs (ii) Scenario 1: the STATCOM-based SVS is simulated in the most insecure bus (iii) Scenario 2: the SVC-based SVS is simulated in the most insecure bus A comparative analysis will be presented to examine the diverse voltage profles and the behavior of each component.

Voltage Regulation after Motors Start-Up.
To enhance the overall performance of the mining system, a strategic approach was implemented whereby the two new motors were activated at diferent time intervals during the simulation. Specifcally, the 20 MVA motor was initiated at t � 1240 s, followed by the activation of the 14.2 MVA motor at t � 2800 s. Figure 9 illustrates the voltage profle at the 23 kV bus in various scenarios, specifcally under two conditions: without any connected SVS (scenario 0) and with the integration of SVSs (scenarios 1 and 2). It is worth noting that upon activation of the new motors, voltage stability issues arise in the bus to which they are connected.
In scenario 0, it is evident that upon the activation of the motors, the voltage level in the 23 kV bus experiences a decline, dropping to 0.99 p.u. upon the initiation of the frst motor and further decreasing to 0.98 p.u. once the second motor is operational. Under scenarios 1 and 2, the SVS control system detects any voltage discrepancies and addresses them by introducing reactive power to counteract the voltage drop, ultimately bringing the voltage back to its initial level of 1 p.u. In both of these scenarios, the FACT system responds by injecting reactive power up to its  International Transactions on Electrical Energy Systems Te behavior of the FACT system and its response to the connection of the MSC can be observed in Figure 10. Upon the activation of the capacitors, a micro peak is produced at t � 3398 s due to the connection of the MSC. A comparison of the STATCOM-based SVS and SVC-based SVS reveals that the former exhibits faster response times, with the MSC being switched on a few seconds later, resulting in slightly better performance: If we focus on the reactive power delivered to the 23 kV bus, in Figure 11, reactive power delivered by the STAT-COM/SVC is shown. When the frst GMD is started, the STATCOM/SVC injects 9 MVAr to restore the voltage to 1 p.u. However, when the second GMD is connected, the required reactive power exceeds 15 MVAr. Tus, when the FACTS reaches its maximum reactive power compensation capacity of 15 MVAr, the MSC is activated and the reactive power injection by the STATCOM/SVC decreases to zero. Tis is illustrated in Figure 11, where a sudden change from 15 MVAr to 0 is observed. Te MSC then provides the remaining 15 MVAr required for optimal system stability.
Zooming on the peak, it can be stated that STATCOM control is faster than the SVS and reaches before the limit of the FACTS, switching the MSC some seconds before (i) t � 3398 s in the STATCOM-SVS case (ii) t � 3400 s in the SVC-SVS case Simulation results indicate that both STATCOM and SVC are signifcant compensation devices in mining electrical networks. Nonetheless, the STATCOM exhibits a faster response time in delivering reactive power than the SVC.

Transient Stability Study after 3 Phase Short Circuit.
Large motor's transient stability is defned as the ability to return to its normal state when a fault occurs, depending mainly on the initial state and the severity of the disturbance [48]. During a fault, electrical power is reduced suddenly while mechanical power remains constant, thereby accelerating the rotor. With the aim of studying transient stability, a 3 phase short circuit in the weakest bus (23 kV bus) is simulated. According to Technical Annex of the Chilean National Standard [49] (where this installation is situated), the duration of the short circuit must be 1 second.
Te voltage profle at the 23 kV bus is illustrated in Figure 12, following a 3-phase short-circuit simulation lasting 1 second, which occurred at t � 20 seconds.
Following the disturbance, the synchronous motors experience transient voltage oscillations, which are prolonged due to their high moment of inertia. As illustrated in Figure 12, the following are the occurrence of the shortcircuit event: (i) In scenario 0, where the SVS is absent, the initial oscillation reaches 0.93 p.u., while the main oscillations hover around 0.98 p.u.
(ii) In scenarios 1 and 2, the SVS systems inject reactive power, reducing the magnitude of the frst oscillation and keeping the voltage oscillations above 1 p.u.
In this type of phenomena, reactive power delivered during this short circuit can be seen in Figure 13; after the occurrence of the short circuit, the SVS delivers its maximum reactive power to restore the voltage level. Once the disturbance is cleared, both systems gradually reduce the reactive power injected into the system.

Sudden Load
Change. Mine electrical networks are often weak networks, and their sensitivity to load changes is very high. A sudden change in the active load can cause voltage disturbances, and the SVS can help mitigate the efects. Te study involves analyzing the efects of a sudden 50% reduction in the load at t � 20 s, which leads to an overvoltage of 0.8% and voltage oscillations. Figure 14 illustrates that, in the absence of the SVS, the system experiences sustained overvoltage upon the sudden load reduction at t � 20 seconds. Upon the introduction of the SVS, voltage oscillations that arise immediately following the load reduction are mitigated and the SVS subsequently absorbs reactive power to stabilize voltage at 1 p.u.

Conclusion and Future Research Directions
Tis paper presents a comparative analysis of two static VAR systems (SVSs), namely, SVC-and STATCOM-based SVSs, as applied to a mining electrical system, within the context of FACTS technology. Te paper explores the potential of SVSs in enhancing the stability of mining electrical systems, with a focus on comparing the performance of SVC-and STATCOM-based systems. Te results indicate that both systems are suitable for strengthening grid stability. However, the STATCOM exhibits slightly better performance, delivering the maximum of its reactive power a few seconds earlier than the SVC. Te study demonstrates the potential benefts of implementing SVSs in mining projects and highlights the importance of choosing the appropriate technology for the specifc application. Table 7 summarizes the results of the comparison between the two SVS confgurations. It can be observed that when cost is the primary driver, the SVC + MSC confguration proves to be the most economical choice, providing a cost reduction of 30% compared to a 30 MVAr SVC and 26% less expensive than STATCOM + MSC. On the other hand, when space is the primary concern, the combination of STATCOM + MSC presents as the optimal solution, occupying signifcantly less space than the SVC + MSC option by 60% and even more compact than a 30 MVAr SVC by 75%.
In addition, both confgurations exhibit satisfactory performance in transient stability scenarios, reducing the frst oscillation voltage peak value by 3.5% in a 3-phase short circuit and minimizing overvoltage by 0.75% in load changes. Te table shows that each confguration has its International Transactions on Electrical Energy Systems 13 advantages and disadvantages, and the fnal decision should be based on the specifc requirements of the mining electrical system. It can be fgured out that SVSs ensure grid stability and improve the system behavior against transient disturbances. By taking advantage of SVSs, new components as GMDs can be integrated in a mining power grid.
Te results of this study should be considered in the context of certain limitations, such as the dearth of prior research on the use of FACTSs to enhance mining operations. Nonetheless, the potential of FACTSs to enhance mining productivity and their suitability for such applications represent promising avenues for future investigation.
As a future direction, exploring the synergistic efects of combining shunt and series FACTS devices is likely to yield more efective results than a single controller, making it a powerful approach for future studies. However, the complexity of control and coordination presents a challenge, and various control techniques such as bang-bang control, GA optimization, and moth-fame optimization should be evaluated and assessed to achieve better performance and explore new applications of the FACTS in mining installations.
An imperative task for future work is the verifcation of the SVS on a real-time platform prior to its implementation in a power system, as it is deemed indispensable for the triumph of the project.