Network Coordination between High-Voltage DC and High-Voltage AC Transmission Systems Using Flexible AC Transmission System Controllers

: The strategic intent of the African Union is to develop a “Smart Integrated African Electric Power Super Grid” driven by modern tools and advances in high-voltage direct current (HVDC) engineering and ﬂexible alternating current technology systems (FACTS), which is central in support-ing Africa’s sustained economic growth and development. The southern African region, including South Africa, is beset by the critical challenges of perennial load-shedding, which impedes economic growth and aggravates unemployment. This has led to the insecurity of electricity supplies and degraded the quality of life. The parallel operation of high-voltage direct current (HVDC) and ﬂexible AC technology systems (FACTS) controllers is gaining traction as system conditions become more complex, such as weak power networks which requires increased stability requirements, resulting in load-shedding and power outages. These adversely affect business productivity and adversely affect GDP and economic growth. Thus, the application of innovative technologies such as HVDC links can stabilize weak power systems. It is established that HVDC delivery systems reduce losses in long transmission lines transporting bulk power compared with high-voltage alternating current (HVAC) transmission lines for power wheeling. This paper evaluates the parallel operation of the Cahora Bassa 1414 km bipolar HVDC link and a weak parallel 400/330 kV alternating current (AC) link. It demonstrates the use of FACTS controllers to enhance the technical performance of an existing network, such as voltage control, and technical loss reduction. It combines an HVDC line commutated converter (LCC) and HVAC transmission lines, in hybrid notation to increase the voltage stability of the system by controlling the reactive power with a Static Var Compensator (SVC). These modern tools can increase the transmission power controllability and stability of the power network. In this study, HVDC–LCC was used with a setpoint of 1000 MW in conjunction with the 850 MVAr SVC. The results show that the technical losses were reduced by 0.24% from 84.32 MW to 60.32 MW as Apollo 275 kV SVC was utilized for voltage control. The network analysis was performed using DIgSILENT PowerFactory software that is manufactured by DIgSILENT GmbH at Gomaringen, Germany


Introduction
Modern power systems are extremely complex and they are required to meet the increasing demand for electricity with acceptable quality and pricing [1]. This study evaluated the application of power-electronics-based FACTS controllers in electric power systems [2], with reference to the Cahora Bassa bi-polar HVDC link between Mozambique and South Africa. Hingorani [2] pioneered the concept of FACTS in 1988 by using power electronics controllers to increase power transfer in existing AC transmission lines, voltage management, and system stability without adding additional lines [1]. In Africa, the first power exchange was between the Democratic Republic of the Congo (DRC), Zambia, and Zimbabwe, and was built in 1960 [3]. By 1970, an interconnection was established between voltage management, and system stability without adding additional lines [1]. In Africa, the first power exchange was between the Democratic Republic of the Congo (DRC), Zambia, and Zimbabwe, and was built in 1960 [3]. By 1970, an interconnection was established between Mozambique and South Africa, and between Botswana and South Africa in 1995 followed by a 400 kV link established between Zimbabwe and South Africa. Presently, only Tanzania, Angola, and Malawi do not have any interconnections with other countries; however, new links are planned [4]. In Africa, the first VSC-based (voltage source converter), the Caprivi HVDC interconnector between Namibia and Zambia, was commissioned in 2010 as shown in Figure 1 [5]. The overhead line is 950 km long and operates at 350 kV DC [6]. It has been proposed to use highly complex power system controllers to integrate African national power grids into one super-grid that can accept a large penetration of renewable powers, without compromising power quality, active and reactive power flow, and voltage and power system stability [7,8]. The proposed super-grid will be constructed using HVDC and flexible AC transmission systems along with dedicated AC/DC interconnectors and smart grids. DC interconnectors will be used to segment the entire continent's power systems into four or five large asynchronous segments (regions). Asynchronous segments will prevent AC fault propagation between segments while allowing power exchange between different parts of the super-grid, with minimum difficulty for grid code unification or the harmonization of regulatory regimes across the continent as each segment maintains its autonomy [7]. The Caprivi Link can be extended to a multi-terminal DC (MT-HVDC) system as part of the African super-grid, capable of harnessing gigawatts of hydro-power from the Congo River to energize the African continent, and for export. The innovative application of HVDC and FACTS can contribute to power system development and resolve some operational challenges posed by demand outstripping supply, as well as environmental regulation, and address the demand for changes in the way electricity is delivered [10]. The majority of transmission lines in an electrical power transmission system are 3-∅ AC transmission lines with changeable transmission voltages [11]. Transmission voltages continue to rise in response to a greater demand for power transmission capacity and/or longer transmission distances. Higher transmission voltages are directly associated with a reduction in technical losses [12]. The innovative application of HVDC and FACTS can contribute to power system development and resolve some operational challenges posed by demand outstripping supply, as well as environmental regulation, and address the demand for changes in the way electricity is delivered [10]. The majority of transmission lines in an electrical power transmission system are 3-∅ AC transmission lines with changeable transmission voltages [11]. Transmission voltages continue to rise in response to a greater demand for power transmission capacity and/or longer transmission distances. Higher transmission voltages are directly associated with a reduction in technical losses [12].
HVAC transmission lines are primarily constrained by their tendency to generate large amounts of reactive power [13]. In long-distance HVAC transmission systems, the system voltage varies continuously with load changes [14]. The reactive power also varies as the load changes, which affects the system voltage. Therefore, it is essential to carry Energies 2022, 15, 7402 3 of 15 out power system studies to determine the system parameters and their variation under different load conditions [15]. Due to the significance of power exchange, the necessity for power interconnections is expanding [16]. Utilizing HVDC schemes over long distances has various technological advantages over HVAC [17]. Advanced solutions such as HVDC and FACTS have the ability to deal with the modifications required for grid access [10].
Power pools and interconnections were built to enhance network efficiency across increasingly vast distances [18]. The power interconnection enables the export of electricity to other countries [19]. In this study, the Static Var Compensator was utilized as a part of the FACTS devices to control the system voltage by regulating reactive power [20], and the HVDC-LCC scheme was employed for the long transmission line; the results were compared with the HVAC base system. The advantages and characteristics of HVDC-LCC are available in [21]. By compensating, reactive power can be managed to enhance the performance of the AC power system [22,23]. Using DIgSILENT PowerFactory, the Songo-Apollo HVDC line with an SVC (FACTS device) was modeled and a power flow study was conducted. Numerous studies demonstrate that the concurrent functioning of HVDC and FACTS devices involves interaction between control channels [24].

HVDC and FACTS Device Coordination
During the initial years of HVDC technology development from mercury-arc to thyristor valves, the process simultaneously reduced costs while boosting robustness [25,26]. Thus, HVDC systems and FACTS controllers based on LCC technology have a long history [27,28]. Figure 2 shows how HVDC systems convert AC power to DC power at the rectifier terminal (sending end) and then back to AC power at the inverter terminal (receiving end) [29]. AC power is supplied to a rectifier-functioning converter. Since the rectifier outputs DC active power, it is unaffected by the frequency and phase of the AC supply [30]. Thyristor valve bridges and converter transformers comprise the thyristor-based converter topology. The conversion from alternating current to direct current is accomplished by arranging high-voltage valve bridges in a twelve-or six-pulse design, depending on the needed output voltage [29]. Due to its advantages, the HVDC-LCC line is predominantly utilized on long transmission lines transporting large amounts of power [21]. HVDC has many advantages over HVAC [31], such as asynchronous grid connections and lower transmission losses over long distances. Therefore, the vast network is HVDC with DC cables fitted and connected with DC−DC converters. To save costs and power loss, no fast-acting DC circuit breakers (DCCBs) are utilized in the DC network [32].  HVDC has many advantages over HVAC [31], such as asynchronous grid connections and lower transmission losses over long distances. Therefore, the vast network is HVDC with DC cables fitted and connected with DC-DC converters. To save costs and power loss, no fast-acting DC circuit breakers (DCCBs) are utilized in the DC network [32].

Flexible AC Transmission System
FACTS devices use static equipment to transmit AC electrical power, by boosting the network's controllability and power transmission capacity; they are based on power electronics [33]. Figure 3 illustrates several types of FACTS controllers which are classified as series-connected controllers, shunt-connected controllers, combined series-shuntconnected controllers, and combined series-series-connected controllers [34]. Padiyar [1] categorized FACTS controllers depending on the power electronics equipment employed for control.  Based on their switching method, the FACTS devices can be split into three groups: mechanically switched (which includes phase-shifting transformers), thyristor switched, and rapidly switched utilizing an insulated-gate bipolar transistor (IGBTs) [35]. Figure 4 illustrates the SVC family. The SVC can be used in both voltage regulation and VAR control modes [36].
Some forms of FACTS, such as the phase-shifting transformer (PST) and the SVC [37], are already well-known and widely employed in the power network. Recent advances in power electronics and control have expanded the application of FACTS [38]. SVCs are typically employed in power systems for voltage regulation, or as a tool to achieve system stabilization [39,40]. For adjusting the node voltage, parallel FACTS such as SVC are utilized [24]. Dr. Laszlo Gyugyi suggested UPFC as the most flexible FACTS controller for regulating voltage and power flow in a transmission line. It is made up of two voltage source converters (VSC), one shunt, and one series-connected. The two converters' DC capacitors are linked in parallel. The IPFC may be used to solve the challenge of compen- Based on their switching method, the FACTS devices can be split into three groups: mechanically switched (which includes phase-shifting transformers), thyristor switched, and rapidly switched utilizing an insulated-gate bipolar transistor (IGBTs) [35]. Figure 4 illustrates the SVC family. The SVC can be used in both voltage regulation and VAR control modes [36].

Songo-Apollo HVDC-LCC Network
The Cahora Bassa HVDC inverter station is located at Apollo in South Africa [41], while the Songo Station rectifier is 1414 km north of Mozambique. The rating of the HVDC link is 1920 MW, 533 kV [42]. In 2006, Eskom awarded ABB a contract to replace eight sixpulse converters and two AC filters while preserving the existing transformers and DC sector infrastructure, incorporating the smoothing reactors [43]. Thus far, South Africa's Eskom has only one HVDC scheme between the hydroelectric power plant at the Cahora Bassa Dam in Mozambique and Johannesburg [44]. The second HVDC installation in Africa is the VSC-based Caprivi Link interconnector between Namibia and Zambia.
The 5 × 480 MVA generators are shown in Figure 5 as G1 to G5. Cahora Bassa is connected to the double 220 kV busbar substation in Songo, six km away [45]. Busbars are assigned to the primary AC and DC loads. The HVDC bus is referred to as the "DC bus" while the Bindura AC line that supplies Zimbabwe is typically linked to the "AC bus" [46]. Figure 5 shows the ZESA and Eskom electricity public utilities for Zimbabwe and South Africa, respectively. Some forms of FACTS, such as the phase-shifting transformer (PST) and the SVC [37], are already well-known and widely employed in the power network. Recent advances in power electronics and control have expanded the application of FACTS [38]. SVCs are typically employed in power systems for voltage regulation, or as a tool to achieve system stabilization [39,40]. For adjusting the node voltage, parallel FACTS such as SVC are utilized [24]. Dr. Laszlo Gyugyi suggested UPFC as the most flexible FACTS controller for regulating voltage and power flow in a transmission line. It is made up of two voltage source converters (VSC), one shunt, and one series-connected. The two converters' DC capacitors are linked in parallel. The IPFC may be used to solve the challenge of compensating for several transmission lines at a substation. While pure series reactive (controllable) compensation (in the form of a TCSC or SSSC) can be used to control or regulate active power flow in a line, controlling reactive power is impossible unless active (actual) voltage in phase with the line current is injected [1].

Songo-Apollo HVDC-LCC Network
The Cahora Bassa HVDC inverter station is located at Apollo in South Africa [41], while the Songo Station rectifier is 1414 km north of Mozambique. The rating of the HVDC link is 1920 MW, 533 kV [42]. In 2006, Eskom awarded ABB a contract to replace eight six-pulse converters and two AC filters while preserving the existing transformers and DC sector infrastructure, incorporating the smoothing reactors [43]. Thus far, South Africa's Eskom has only one HVDC scheme between the hydroelectric power plant at the Cahora Bassa Dam in Mozambique and Johannesburg [44]. The second HVDC installation in Africa is the VSC-based Caprivi Link interconnector between Namibia and Zambia.
The 5 × 480 MVA generators are shown in Figure 5 as G1 to G5. Cahora Bassa is connected to the double 220 kV busbar substation in Songo, six km away [45]. Busbars are assigned to the primary AC and DC loads. The HVDC bus is referred to as the "DC bus" while the Bindura AC line that supplies Zimbabwe is typically linked to the "AC bus" [46]. Figure 5 shows the ZESA and Eskom electricity public utilities for Zimbabwe and South Africa, respectively. The hydroelectric power station at Cahora Bassa (CB) is 6 km from the Songo rectifier station. It was designed to transfer 1920 MW from the generated 2075 MW to the South African Apollo inverter station [47]. The Songo−Apollo HVDC−LCC link is shown in Figure 6. It is located in South Africa, but the rectifier is 1414 km to the north in Mozambique. The present HVDC transmission line rating is 1920 MW with 533 kV [48].

Design, Modeling, Results, and Discussion of the Songo−Apollo HVDC−LCC line
In designing and simulating the Songo−Apollo link network model, DIgSILENT PowerFactory software was utilized. The model was constructed using previously acquired data for the Songo−Apollo transmission line. To assure the system's reliability, an HVAC and HVDC load flow study was performed utilizing Newton Raphson's method. The hydroelectric power station at Cahora Bassa (CB) is 6 km from the Songo rectifier station. It was designed to transfer 1920 MW from the generated 2075 MW to the South African Apollo inverter station [47]. The Songo-Apollo HVDC-LCC link is shown in Figure 6. It is located in South Africa, but the rectifier is 1414 km to the north in Mozambique. The present HVDC transmission line rating is 1920 MW with 533 kV [48]. The hydroelectric power station at Cahora Bassa (CB) is 6 km from the Songo rectifier station. It was designed to transfer 1920 MW from the generated 2075 MW to the South African Apollo inverter station [47]. The Songo−Apollo HVDC−LCC link is shown in Figure 6. It is located in South Africa, but the rectifier is 1414 km to the north in Mozambique. The present HVDC transmission line rating is 1920 MW with 533 kV [48].

Design, Modeling, Results, and Discussion of the Songo−Apollo HVDC−LCC line
In designing and simulating the Songo−Apollo link network model, DIgSILENT PowerFactory software was utilized. The model was constructed using previously acquired data for the Songo−Apollo transmission line. To assure the system's reliability, an HVAC and HVDC load flow study was performed utilizing Newton Raphson's method.

Design, Modeling, Results, and Discussion of the Songo-Apollo HVDC-LCC Line
In designing and simulating the Songo-Apollo link network model, DIgSILENT PowerFactory software was utilized. The model was constructed using previously acquired data for the Songo-Apollo transmission line. To assure the system's reliability, an HVAC and HVDC load flow study was performed utilizing Newton Raphson's method.
As indicated in Figure 7, the Songo-Apollo HVDC link has multiple voltage levels, with 5 × 480 MVA generators at Cahora Bassa rated at 220 kV. The 220-400 kV step-up transformer is used to increase the voltage to 400 kV for the 1414 km Songo-Apollo transmission line. The 275-400 kV step-down transformer is used at Apollo to reduce voltage.
Energies 2022, 15, x FOR PEER REVIEW 8 of 17 As indicated in Figure 7, the Songo−Apollo HVDC link has multiple voltage levels, with 5 × 480 MVA generators at Cahora Bassa rated at 220 kV. The 220-400 kV step-up transformer is used to increase the voltage to 400 kV for the 1414 km Songo−Apollo transmission line. The 275-400 kV step-down transformer is used at Apollo to reduce voltage. The voltage at every busbar must be maintained within a 5% tolerance of its nominal value. In emergencies involving a failed element, the temperature rating of any equipment must not be exceeded, and the voltage must be maintained between 95% and 105% of its nominal value. However, a 15% overvoltage is permissible for five seconds, while a 20% overvoltage is acceptable for one and two seconds. Consequently, transformers, generators, and line loads must fall within the range of 80-100%, and all busbar voltages must be maintained within a tolerance of 5% of their nominal value. Figure 8 shows the Cahora Bassa generators running at full capacity, as well as the 6 km CB−Songo transmission lines. The Cahora Bassa substation is operating at 220 kV voltage (1 p.u) as shown in Figure 8.  The voltage at every busbar must be maintained within a 5% tolerance of its nominal value. In emergencies involving a failed element, the temperature rating of any equipment must not be exceeded, and the voltage must be maintained between 95% and 105% of its nominal value. However, a 15% overvoltage is permissible for five seconds, while a 20% overvoltage is acceptable for one and two seconds. Consequently, transformers, generators, and line loads must fall within the range of 80-100%, and all busbar voltages must be maintained within a tolerance of 5% of their nominal value. Figure 8 shows the Cahora Bassa generators running at full capacity, as well as the 6 km CB-Songo transmission lines. The Cahora Bassa substation is operating at 220 kV voltage (1 p.u) as shown in Figure 8. As indicated in Figure 7, the Songo−Apollo HVDC link has multiple voltage levels, with 5 × 480 MVA generators at Cahora Bassa rated at 220 kV. The 220-400 kV step-up transformer is used to increase the voltage to 400 kV for the 1414 km Songo−Apollo transmission line. The 275-400 kV step-down transformer is used at Apollo to reduce voltage. The voltage at every busbar must be maintained within a 5% tolerance of its nominal value. In emergencies involving a failed element, the temperature rating of any equipment must not be exceeded, and the voltage must be maintained between 95% and 105% of its nominal value. However, a 15% overvoltage is permissible for five seconds, while a 20% overvoltage is acceptable for one and two seconds. Consequently, transformers, generators, and line loads must fall within the range of 80-100%, and all busbar voltages must be maintained within a tolerance of 5% of their nominal value. Figure 8 shows the Cahora Bassa generators running at full capacity, as well as the 6 km CB−Songo transmission lines. The Cahora Bassa substation is operating at 220 kV voltage (1 p.u) as shown in Figure 8.     Figure 9; thus, the voltage instability is slightly reduced (see Appendix B). The DC current for an HVDC−LCC link is as follows:  Figure 10 shows the Songo-Apollo HVDC link, which is parallel to the HVAC 1414 km transmission line. The goal of the HVDC link is to minimize line losses. For the Eskom load to receive 1920 MW, Cahora Bassa produces 1983.1 MW. The losses in the line are determined to be 63.08 MW. Consequently, losses are reduced by 21.24 MW when compared with Figure 9; thus, the voltage instability is slightly reduced (see Appendix B).   Figure 10 shows the Songo−Apollo HVDC link, which is parallel to the HVAC 1414 km transmission line. The goal of the HVDC link is to minimize line losses. For the Eskom load to receive 1920 MW, Cahora Bassa produces 1983.1 MW. The losses in the line are determined to be 63.08 MW. Consequently, losses are reduced by 21.24 MW when compared with Figure 9; thus, the voltage instability is slightly reduced (see Appendix B). The DC current for an HVDC−LCC link is as follows: The DC current for an HVDC-LCC link is as follows:

Songo−Apollo HVDC Link
where V ac is the AC voltage, V dc is the DC voltage, I dc is the dc current, α is the firing angle, δ is the extinction delay angles, R l is the resistor from the loop, R r is the resistor from the rectifier, and R i is the resistor from the inverter The voltage current is: 3.3. Songo-Apollo HVDC Link with SVC Figure 11 shows the Songo-Apollo HVDC link with SVC, which is utilized to control the system's voltage stability. To produce a 1920 MW Eskom load from the Apollo busbar, 1980.3 MW is required, which means 60.32 MW is system technical losses. It is observed that all the voltages are now between 0.99 p.u and 1.00 p.u, indicating that the SVC did regulate the voltages by managing a portion of the reactive power, given that all the voltages are now between 0.99 p.u and 1.00 p.u. The power losses for the Songo-Apollo transmission line when it was operating as an HVAC transmission line, when the HVdc link was connected along the HVAC line, and when the SVC was utilized to enhance the voltage are shown in Table 1.
where is the AC voltage, is the DC voltage, is the dc current, is the firing angle, is the extinction delay angles, is the resistor from the loop, is the resistor from the rectifier, and is the resistor from the inverter The voltage current is: Figure 11 shows the Songo−Apollo HVDC link with SVC, which is utilized to control the system's voltage stability. To produce a 1920 MW Eskom load from the Apollo busbar, 1980.3 MW is required, which means 60.32 MW is system technical losses. It is observed that all the voltages are now between 0.99 p.u and 1.00 p.u, indicating that the SVC did regulate the voltages by managing a portion of the reactive power, given that all the voltages are now between 0.99 p.u and 1.00 p.u. The power losses for the Songo-Apollo transmission line when it was operating as an HVAC transmission line, when the HVdc link was connected along the HVAC line, and when the SVC was utilized to enhance the voltage are shown in Table 1.  The SVC is then applied to the Apollo busbar to inject reactive power and regulate the voltage, as shown in Figure 10. The system represented in Figure 11 shows the Songo-Apollo link with minimum losses and enhanced voltage stability (see Appendix C).  The SVC is then applied to the Apollo busbar to inject reactive power and regulate the voltage, as shown in Figure 10. The system represented in Figure 11 shows the Songo-Apollo link with minimum losses and enhanced voltage stability (see Appendix C). Table 2  SVC. The results were generated by DIgSILENT PowerFactory based on the performance of the Songo-Apollo network under various conditions. According to Table 2 below, the use of the SVC in the Apollo 275 kV busbar improves voltage stability because all voltages connected to the busbar with the SVC increased to approximately 1 p.u.  Table 3 shows transmission line loading for several different scenarios: (a) when examining the load flow between Songo and Apollo with all HVAC lines; (b) when the load flow study was performed on Songo-Apollo HVDC-LCC connections; and (c) when a load flow study was performed on the Songo-Apollo network with LCC-HVDC and SVC in the Apollo 275 kV busbar.  Table 4 shows transmission line losses under three distinct conditions: (a) when the Songo-Apollo network load flow analysis is performed on all HVAC lines; (b) when the load flow analysis is conducted using HVDC-LCC links, for which losses dropped to 63 MW; and (c) when analysis is conducted using both SVC and HVDC-LCC systems, for which there were 60.4 MW total technical losses.

Conclusions
The primary goal of this study is to conduct load flow studies on power exchanges between countries and communities within Southern Africa, particularly those without access to electricity, with minimal losses while ensuring grid reliability. This study models the interplay between the existing and established FACTS device and HVDC (LCC) on the Songo-Apollo HVDC link.
As new generations of FACTS devices are developed and deployed for industry testing at the transmission (UPFC) and distribution level (D-FACTS) to mature technology, further studies will be conducted at those levels to ascertain their inherent impact on the technical performance of these HVDC schemes and their impact on the broader, smart distribution networks and regional power grid. These aspects were not part of the current study.
DIgSILENT PowerFactory was utilized to study the transmission line performance when utilizing the HVAC transmission line to transmit 1920 MW of power from Songo to Apollo. Large distances carrying massive amounts of power resulted in 84.32 MW of line losses. The HVDC-LCC was added to the existing HVAC transmission line to reduce losses, resulting in 63.08 MW of transmission line losses. Figure 10 shows the voltage instability on the Apollo receiving side, where the Apollo 400 kV and 275 kV busbars and the Eskom busbar have 0.96 p.u.
By controlling a portion of the reactive power, 850 MVAr SVC is utilized to decrease voltage instability in all busbars connected to the SVC busbar. Figure 11 represents an entire model with both HVDC-LCC and SVC, where it is determined that the transmission line losses are 60.34 MW (See Table 1). The voltage instability is controlled as seen in Figure 11, with the Apollo 400 kV busbar at 0.99 and the Apollo 275 kV and Eskom busbars at 1 p.u. In Figure 11, losses are minimized, and voltage instability is under control, making the network more resilient. Tables 2-4 show the respective voltage busbar profile, the transmission line loading profile, and the transmission line losses that occur in the system to compare the results of the three distinct networks that are shown in Figures 9-11. This study can be applied in the future, including the most recent FACTS controllers, such as the UPFC, which is the most adaptable FACTS controller for regulating voltage and power flow in a transmission line, with the addition of IPFC to address the issue of compensating for multiple transmission lines connected at a substation.
HVDC links and FACTS controllers have been utilized as innovative methods for increasing the entire power system performance. Through this study, it is noted that HVDC and FACTS devices offer a superior solution for the long-distance transmission of bulk power; that they improve grid resilience against unforeseen events; that the voltage is regulated and the power is optimized through the HVDC-LCC and SVC systems; and that HVDC-LCC and SVC are used to control and optimize voltage and power, respectively. With further interconnections planned across Africa, this study provides preliminary insight into how large power networks with several power interconnections will enhance electricity trading and power exchanges throughout the African region or among African power pools.