The southwestern part of Pakistan is still not connected to the national grid, despite its abundance in renewable energy resources. However, this area becomes more important for energy projects due to the development of the deep-sea Gwadar port and the China Pakistan Economic Corridor (CPEC). In this paper, a voltage source converter (VSC) based high voltage DC (HVDC) transmission model is proposed to link this area to the national gird. A two-terminal VSC-HVDC model is used as a case study, in which a two-level converter with standard double-loop control is employed. The proposed model has a capacity of transferring bulk power of 3500 MW at 350 kV from Gwadar to Matiari. Furthermore, the discounted cash flow analysis of VSC-HVDC against the HVAC system shows that the proposed system is economically sustainable. The outcomes of this study reveal that the implementation of this project can bring economic stability and energy security in the southwestern region.
technical assessment; voltage source converter; power transmission; discounted cash flow; energy security
The demand for power supply in Pakistan is growing exponentially in domestic and industrial sectors , and thus, the country has faced severe energy crises in recent times. The two primary sources for electricity production in Pakistan, i.e., hydropower and fossil fuels, have been adopted for long, but cannot meet the increasing demands of the country. Currently, Pakistan has been taking initiatives to utilize renewable energy to bridge the gap between demand and supply effectively. However, the weak, old transmission system is still a significant hindrance to uphold the rapidity with growing energy production supply [2,3].
Pakistan has five provinces, namely, Gilgit Baltistan, Khyber Pakhtunkhwa (KPK), Sindh, Balochistan, and Punjab. Among these, Balochistan province with 347,190 km2 area is the largest one, consisting of approximately 45% land of the country. Additionally, the tremendous renewable energy resources, like wind and solar, are available to their vast extent and potential in the southwestern area of Balochistan province. Monthly mean solar radiations ranged from 153.61–281.94 W/m2 throughout the year, while the total wind power generation capacity in this area is about to 146 GW [4,5].
In this region, the population density is comparatively low due to the scarcity of natural resources, and only 6% of the population is residing in this region. Possibly, the one most haunting reason for neglecting this high potential energy-producing area is its unstable and uncertain bordered geographical location that is far away from the main grid. Keeping in view the power production potential of this area along with the rapidly increasing energy crisis in Pakistan, it is indispensable to discover cheap and high potential renewable energy sources like wind power. Whereas Punjab and Sindh provinces are the most populated areas of the country, approximately holding 76% of the total population .
Moreover, to date, there is no available transmission line from these areas to the main national grid. Figure 1 shows the existing transmission network in different regions of the country . Despite substantial available power potential, still, Pakistan is depended on importing the power from Iran to provide electricity in southwestern areas. Due to the lack of power generation projects and no available transmission network, these areas are still not connected with the main national gird.
On the other hand, China Pakistan Economic Corridor (CPEC) projects and Saudi Arabia investment in the oil sector this area becomes more important for the development of new power projects and the setup of a new transmission line. Exploiting these energy resources not only helps the CPEC and other future projects, but also fulfills the power demand of the country. Renewable energy department authority and Pakistan national transmission and dispatch company have evaluated that the transmission network of the country is overloaded. Therefore, new transmission lines are required to meet the power supply demand across the country.
Generally, power is transmitted from the generation side to the main grid. There are two possible ways to transmit the power to the national grid, i.e., high voltage alternating current (HVAC) and high voltage direct current (HVDC). HVDC is proposed to be suitable for transmitting bulk power over long distances in comparison to HVAC. HVDC transmission lines based on voltage source converters (VSC) or line commutated converters (LCC) both are considered as a viable alternative than HVAC [8,9,10]. Furthermore, due to the advancement of technologies and cost comparison over a long distance, the HVDC systems are considered economically sustainable. Detailed analysis includes the technical and economic comparison of HVDC and HVAC systems that are elaborated in .
Additionally, the recent development in DC circuit breaker technologies and reliable protection schemes makes the VSC-HVDC transmission links more promising [11,12]. The concept of a hybrid DC circuit breaker with fault limiting capacity for VSC-HVDC transmission network is proposed in , which can rapidly isolate the fault within permissible limits of fault current. Likewise, a novel multi-terminal HVDC protection scheme based on artificial neural network (ANN) and high-frequency components from fault current signals is utilized to detect, classify, and locate overhead line faults accurately. This includes high impedance faults without compromising the accuracy of fault location [13,14]. Based on the consecutive data window method (CDWM), another protection scheme is introduced for the VSC-HVDC transmission system, which is capable of prompt fault detection and isolation [15,16]. Furthermore, a new theory for locating line fault in the transmission system is presented in , which has thoroughly presented the theoretical aspects, and simulations for the line faults. With ongoing improvements in protection technology, the authors believe this will be sufficiently supportive of the proposed transmission line.
There are various factors, such as technical considerations, economic aspects, and legal requirements, that are critical in the planning of new transmission lines. A technical consideration transmission line based on VSC-HVDC and economic aspects are presented in this paper. It is hoped that the outcomes of this study can help to understand the importance of the new proposed VSC-HVDC based transmission line in this area and provide a base for the further academic research in this field.
2. Overview of HVDC Systems
AC-based transmission networks were commonly used in the earlier era of power system development, while DC networks were rarely practiced due to lack of technologies at that time. Nevertheless, after 1945, the solid-state power electronics components penetrated into the power sector, and HVDC got researchers’ attention and made a comeback. HVDC transmission network consists of two types of technologies, depending upon the control of converters, i.e., voltage source converter (VSCs) or line commutated converters (LCCs). In the initial stages, LCC-HVDC networks were commonly used, but with time, HVDC transmission-related technologies became more efficient and mature, which also provided a window for the development and implementation of VSC-HVDC based transmission networks [10,18,19,20]. Some HVDC transmission networks based on both technologies were installed in different regions of the world and are summarized in Table 1, and other projects are discussed in [21,22,23,24].
After many years of research and development process, the VSC based HVDC transmission systems became an attractive option for bulk power transmitting over long distance. Recently, VSC-HVDC is being preferred to LCC-HVDC, because VSCs offer the independent control of both reactive and active power. Also, VSCs do not need external sources to push off the switches, while LCCs needed the external mechanical switches. VSC-HVDC based networks offer many advantages over LCC-HVDC, and some of them are summarized in Table 2 [25,26].
3. Deployment of Proposed Transmission Line
The proposed transmission line is based on VSC-HVDC, which can stretch approximately 815 km from Gwadar, Balochistan, to Matiari, Sindh. A major part of the line will be in Balochistan province for up to about 600 km, and the remaining portion will be in Sindh province. The Pakistan first-ever LCC-HVDC transmission line project (under the vision of CPEC) from Matiari to Lahore is under construction and can be operational at the start of 2021. This HVDC line is 878 km long, will be capable of bulk power transmission of about 4000 MW at 660 kV, generated from various power projects in the south part to the most populated area of Punjab province .
Gwadar is one of the busiest sea routes in between Southwest and South Asia on the Arabian Sea shore due to its geographical, commercial, and strategically significant deep-sea port. It covers the 35% of global sea-trade, including 45% of China’s oil trade. It is being bordered with Iran, which has broadened the opportunities to reach the overland business activities. With this recent development of Gwadar Sea Port under CPEC projects, it has become the hub of global economic activities. Recently, the memorandum of agreement [MOU] between Saudi Arabia and Pakistan has been signed to set up a joint working group for the petrochemical complex and the state-of-the-art oil refinery in Gwadar city. Moreover, Iran has made an agreement with Pakistan to build an oil refinery, whereas Qatar also has shown its interest in establishing a food storage system at Gwadar port. This initiative would help boost the economic and energy development of the region [28,29,30].
Owing to substantial renewable energy resources, development of port and oil refinery, and other economic activities, the southwestern region should be connected to the national grid, which can provide the power security and demolish the power shortages of the country. The proposed VSC-HVDC transmission line is capable of dispatching bulk power approximately 3500 MW at 350 kV from Gwadar to Matiari. The location of the proposed bipolar HVDC transmission line is shown in Figure 2. The whole area is easily accessible for all types of vehicles through Makran coastal high way in Balochistan and the national high way in the Sindh part. Most of the line will pass through the rural areas and have a negligible effect on the social instability.
It is worth mentioning here that this kind of research is hardly carried out before for this area. It is hoped that the implementation of this project can be a game-changer in the region, provides employment opportunities, infrastructure development, increases land revenue, and uplifts the socio-economic. Besides, the access amount of electricity that can be available in the national grid ensures the energy security, which can also result in industrial development and expansion of industry in the country.
4. VSC-HVDC System Theory and Simulation
Different types of voltage source converters are used for HVDC transmission; the most commonly used in this regard are a two-level converter, three-level neutral-point clamped (NPC) converter, and modular multilevel converter (MMC) . In this paper, a case study with results is incorporated using MATLAB simulation to understand the significate of the VSC-HVDC system. In this context, a two-terminal VSC-HVDC model is used, in which a two-level converter with standard double-loop control is employed. Where terminal-I is controlling DC-voltage and active power, while terminal-II is controlling active and reactive power.
4.1. Converter Control
There are several approaches to control the converter; out of many, vector control and direct power control are commonly used ones. In vector control, voltage and current vectors in reference (rotating) frame are transformed into a two-dimensional stationary frame using Clark’s transformation . The AC side voltage vectors are represented by Equations (1)–(3), whereas the application of Clark’s transformations is given in Equation (4).
The details of variables attached to Equations (1), (2), and (3) are as follows. , , and are the respective phase voltages in the balanced AC grid, represents the amplitude of phase voltage, represents the phase angle measured in radians. The design of any control method in the original/natural three-dimensional frame (or ABC frame) is complicated because of three independent vectors concerning different state variables, e.g., voltage or current, etc. Therefore, the variables in the original/natural three-dimensional frame are converted to a two-dimensional frame, either static frame (or αβ-frame) using Clark’s Transformation, or a rotating frame (or dq-frame) using Park’s Transformation, depending on the design of the control. In this way, control schemes can be designed based on the control of two independent vectors, simpler in design and easier to implement.
In Equation (4), “T” is the transformation matrix, represents the voltage vector in the αβ-frame, while denotes the voltage vector in ABC-frame. Using similar approaches, current in the two-dimensional stationary frame can also be obtained. Once the values in the two-dimensional stationary frame are obtained, then these transformed into two-dimensional rotating frame or dq-frame, by the application of Park’s t transformations, which is given in Equation (5).
In Equation (5), represents the voltage vector in dq-frame, “To” is the transformation matrix, and indicates the voltage vector in αβ-frame. It should be noted that if the d-axis of the rotating frame (dq-frame) is aligned to the AC voltage vectors, such that , consequently, the instantaneous active and reactive power is given in Equations (6) and (7), respectively, which are based on the instantaneous power theory of Akagi .
From the mathematical relationships, it is evident that the control of active and reactive power is directly linked with controlling current and voltage in the dq-frame. Figure 3 explains the working of conventional double-loop control for two-level voltage source inverter (VSI) . An outer control loop produces the reference currents and , and these reference values serve as input for the inner control loop. The inner control loop controls the actual values of and to produce desired reference current, and ultimately and are transformed into frame and then sent to the pulse width modular PWM controller to directly control the firing angle of IBGTs to achieve the desired results. Where is the line inductance and , , are the reference values of DC voltage, active power, and reactive power, respectively.
4.2. Two-Terminal Model Simulation
The schematic layout used for simulation is shown in Figure 4, whereas the double-loop control structure explaining the control aspects of VSI is given in Figure 3. For simulation, a two-terminal system is developed with conventional 2-level VSI and double loop control.
The terminal-I used to regulate the DC-voltage and reactive power, whereas for terminal-II active and reactive power is regulated. Between two terminals, an 800 km long transmission line model is also used. The essential parameters for the system are given in the Appendix Table A1; the simulation model is also built under these parameters.
The Figure 5a contains the AC grid voltage concerning terminal-I, likewise, Figure 5b shows the terminal DC-voltage at VSC-I, and it can be seen that the regulation of terminal DC voltage is achieved convincingly, and terminal DC voltage is kept at the desired level of 400 kV.
The receiving voltage at DC-link capacitors of terminal-II is depicted in Figure 6a. It can be observed that DC-voltage is slightly less than the rated value; this is obvious because line parameters can influence the value of DC-voltage at the receiving station. The remaining results concerning terminal-II includes AC voltage and current in Figure 6b.
Lastly, Figure 7 shows the active power transmitted to terminal-II, where it is apparent that the active power is regulated successfully at terminal one, which is kept at the level of 400 MW following the designed criteria.
5. Cost Assessment
Economic comparisons can usually be made by various methods, as depicted in Figure 8. However, the discounted cash flow method (DCF) is the commonly used method due to its time value of money. In this context, all imminent cash flows are computed and discounted to get the net present values (NPV) .
The discount cash flow (DCF) analysis method is applied for the economic comparison of HVAC and VSC-HVDC transmission systems. This method uses the difference in the investment costs and annual costs of the systems for economic comparison. The maintenance costs and costs of transmission losses are accounted for the annual cost; here, reliability and insurance factors are neglected. DCF analysis result is the difference of NPVs of mentioned transmission line-technologies, which also comprises the discounted annual cost (Cda) and initial investment cost (Ci) for each transmission system. The mathematical expressions for the DCF method are given as [8,35];
where, , , , and are the cost of substation, land, overhead transmission line, and cost of reactive power compensations, respectively. The annual cost (), NPV, Annuity factor (), are expressed as;
where, stands for maintenance cost, denotes the cost of losses, is the discount factor, t shows system lifetime, is the present cost for , represents annual costs, and are taxation rate and depreciation value, respectively. To get a more representative reflection, the lifetime (t) of the transmission system is assumed to be 30 years, whereas the discount factor (Ð) and the tax rate of 13.25% and 17%, respectively, are set for this study [36,37].
5.2. Investment Cost Analysis
VSC-HVDC system is based on a relatively new technology, which makes it better than other DC or AC options. However, this choice may also depend on the economic aspects of the two systems. In this regard, a cost analysis is drawn to compare the HVDC and HVAC systems to determine the better option for a 3500 MW generation over 815 km overhead transmission line. The DC system considered in this work is comprised of bipolar ground-return transmission lines at 350 kV having the cable cross-section of . A substation with voltage-rating 350 kV is to be installed at each of the proposed locations, i.e., Gwadar and Matiari.
Nevertheless, the HVAC systems are proposed to be 500 kV grid at both ends. The cost is analyzed based on the transmission system components. These components are the substations, transmission lines conductors, installation, and land expenses, as illustrated in Table 3 (cost/component is calculated in a million US dollars “$M”). STATCOM expenses are also included in the case of HVAC. It can be seen that despite the higher price of HVDC substations, the total price is kept low due to the cheaper overhead transmission line equipment and absence of STATCOM. The total cost required for an 815 km AC transmission line is 1201 $M, with a cost of 1.47 $M/km, which is 1.35 times of HVDC line cost.
5.3. Annual Cost
The system maintenance costs and losses each year will account for the annual cost of the system. The annual maintenance cost is considered to be 0.5% of the total components cost. These components are the substations costs, cable installment costs, and STATCOM costs in case of the HVAC system. The annual maintenance costs of the HVDC and HVAC systems are found to be 4.45 $M and 3.37 $M, respectively. The VSC based HVDC systems have comparatively higher losses, with a percentage of 1.8% for each station. In the case of two stations, 3.6% of the total power flow is considered, which is found to be 52.02 $M. In the case of HVAC systems, 5% of total power flow is considered to be the loss. Hence, 89.92 $M is computed to be the loss’s cost for HVAC systems. As a result, the annual costs for HVDC and HVAC systems are calculated to be 56.48 $M and 94.3 $M, respectively.
5.4. Discounted Cash Flow (DCF) Analysis
From the perspective of transmission systems, the DCF analysis results presented in Figure 9, are of great significance. It shall be noted that on a lifetime basis, the HVDC system is cheaper than the HVAC system. As compared to the investment costs, the higher difference in NPV is due to the significant difference in the annual costs. It must be stated that the VSC based HVDC systems are still evolving and are expected to be more feasible in the future.
5.5. HVDC Systems’ Economics Comparison
It is difficult to determine the exact cost of an HVDC system because each project has different features and circumstances, such as rated power, line distance, terrain, and used technology. However, an overall estimate can be obtained based on the knowledge from previous projects. In particular, the Chongqing-Hubei back-to-back DC grid interconnection project in China was the first distinct VSC-HVDC line with the highest voltage capacity of 420 kV and the largest power transmission capacity of 5000 MW, covering the total distance of 1711 km [39,40]. The project cost was 1498.36 $M, which is roughly 0.9 $M/km. Accordingly, the cost of 1.3 $M/km was calculated for South Australia to Queensland 400 kV HVDC line in Australia with a length of 1450 km . The Southern Hami-Zhengzhou (SHZ) HVDC line is one of China’s large-scale HVDC projects (800 kV, 8000 MW), which is stretched from Xinjiang province in the northwest down to Henan province in central China. The project cost reached 3500 $M and about 1.8 $M/km. Whereas a slight decrease in the cost of 1.7 $M/km was incurred for the Lingzhou-Shaoxing HVDC line (with the same ratings of SHZ line) due to its shorter line coverage of 1722 km as compared to SHZ line distance, which is 2200 km [42,43].
Furthermore, Pakistan’s first-ever HVDC Line Matiari to Lahore with a rated power of 4000 MW and voltage of 660 kV, spanning 878 km, is in progress by the cooperation of State Grid Corporation of China (SGCC) and Ministry of Water and Power Pakistan under the vision of CPEC. The project estimated cost is 1658.34 $M (approximately 1.9 $M/km) . Similarly, this study provides an estimated cost of 1.7 $M/km for the proposed HVDC line, which is reasonably comparable to the cost of previously selected HVDC projects. Hence, the proposed HVDC model is economically sustainable and a better choice for the future transmission projects in Pakistan.
The southwestern region of the country is expected to become a hub for economic and industrial development due to CPEC projects. Conversely, power shortage and energy security are critical factors in this area and should be solved promptly to enhance the CPEC projects and other economic activities. In this context, a study of technical and economic aspects of a proposed two-terminal VSC-HVDC transmission line is carried out. The simulation results for the power, voltage, and current waveforms are found stable and reliable at the receiving end. The DCF analysis shows that the net present value of the HVDC system is lower as compared to the HVAC system. The NPV for HVAC is found to be 3725 $M, while the HVDC NPV was 2599 $M. Moreover, the cost/km of the proposed model is relatively analogous to selected existing HVDC projects. Therefore, HVDC proves to be a realistic and feasible transmission system.
Further research should be carried out by considering the socio-economic impact of such projects. Similarly, comprehensive studies comprising of further technical considerations and legal requirements of HVDC systems can be carried out for the real-time implementation of the project.
Core idea, data collection, formal analysis, writing the original draft, software, and simulations, M.G.; resources, supervision, N.T.; methodology, review & editing, validation, W.H.; data collection, software, validation, M.H.N., M.A., and M.Y.
This work was supported in part by 2019 major scientific research and innovation project of Shanghai Education Commission (Grant number: 2019-01-07-00-02-E00044).
Conflicts of Interest
The authors declare no conflict of interest.
China Pakistan Economic Corridor
Voltage source converter
Modular multilevel converter
High voltage direct current
High voltage alternate current
Line commutated converter
Discounted cash flow method
Net present value
Discounted annual cost
Initial investment cost
Cost of substation
Cost of land
Cost of overhead transmission line
Cost of reactive power compensations
The following parameters are used to build the simulation model.
System’s parameters used for simulation.
System’s parameters used for simulation.
Rehman, A.; Deyuan, Z. Pakistan’s energy scenario: A forecast of commercial energy consumption and supply from different sources through 2030. Energ. Sustain. Soc.2018, 8, 26. [Google Scholar] [CrossRef]
Gul, M.; Tai, N.; Huang, W.; Nadeem, M.H.; Yu, M. Assessment of Wind Power Potential and Economic Analysis at Hyderabad in Pakistan: Powering to Local Communities Using Wind Power. Sustainability2019, 11, 1391. [Google Scholar] [CrossRef]
Kamran, M. Current status and future success of renewable energy in Pakistan. Renew. Sustain. Energy Rev.2018, 82, 609–617. [Google Scholar] [CrossRef]
Adnan, S.; Khan, A.H.; Haider, S.; Mahmood, R. Solar energy potential in Pakistan. J. Renew. Sustain. Energy2012, 4, 032701. [Google Scholar] [CrossRef]
Hur, D. Economic considerations underlying the adoption of HVDC and HVAC for the connection of an offshore wind farm in Korea. J. Electr. Eng. Technol.2012, 7, 157–162. [Google Scholar] [CrossRef]
Meah, K.; Sadrul, U. Comparative evaluation of HVDC and HVAC transmission systems. In Proceedings of the 2007 IEEE Power Engineering Society General Meeting, Tampa, FL, USA, 24–28 June 2007. [Google Scholar]
Rudervall, R.; Charpentier, J.P.; Sharma, R. High voltage direct current (HVDC) transmission systems technology review paper. Energy Week2000, 2000, 1–19. [Google Scholar]
Candelaria, J.; Park, J.D. VSC-HVDC system protection: A review of current methods. In Proceedings of the 2011 IEEE/PES Power Systems Conference and Exposition, Phoenix, AZ, USA, 20–23 March 2011; pp. 1–7. [Google Scholar]
Ahmad, M.; Wang, Z. A Hybrid DC Circuit Breaker with Fault-Current-Limiting Capability for VSC-HVDC Transmission System. Energies2019, 12, 2388. [Google Scholar] [CrossRef]
Yang, Q.; Blond, S.-L.; Aggarwal, R.; Wang, Y.; Li, J. New ANN method for multi-terminal HVDC protection relaying. Electr. Power Syst. Res.2017, 148, 192–201. [Google Scholar] [CrossRef]
Li, J.; Yang, Q.; Mu, H.; Blond, S.-L.; He, H. A new fault detection and fault location method for multi-terminal high voltage direct current of offshore wind farm. Appl. Energy2018, 220, 13–20. [Google Scholar] [CrossRef]
Nadeem, M.; Zheng, X.; Tai, N.; Gul, M. Identification and Isolation of Faults in Multi-terminal High Voltage DC Networks with Hybrid Circuit Breakers. Energies2018, 5, 1086. [Google Scholar] [CrossRef]
Nadeem, M.; Zheng, X.; Tai, N.; Gul, M.; Tahir, S. Analysis of propagation delay for multi-terminal high voltage direct current networks interconnecting the large-scale off-shore renewable energy. Energies2018, 8, 2115. [Google Scholar] [CrossRef]
Zhang, X.; Tai, N.; Wu, P.; Fan, C.; Zheng, X.; Huang, W. A New Theory for Locating Line Fault in Power System: Theoretical Part. IEEE Access2019, 7, 37–46. [Google Scholar] [CrossRef]
Bahrman, M.-P.; Johnson, B.-K. The ABCs of HVDC transmission technologies. IEEE Power Energy Mag.2007, 5, 32–44. [Google Scholar] [CrossRef]
Agelidis, V.G.; Demetriades, G.D.; Flourentzou, N. Recent advances in high-voltage direct-current power transmission systems. In Proceedings of the 2006 IEEE International Conference on Industrial Technology (ICIT), Mumbai, India, 15–17 December 2006; pp. 206–213. [Google Scholar]
Flourentzou, N.; Agelidis, V.-G.; Demetriades, G.-D. VSC-based HVDC power transmission systems: An overview. IEEE Trans. Power Electron.2009, 24, 592–602. [Google Scholar] [CrossRef]
Raza, A.; Xu, D.-G.; Su, X.-W.; Li, W.-X. Invasion of high voltage direct current till 2014. In Proceedings of the 2014 IEEE International Conference on Control Science and Systems Engineering, Yantai, China, 29–30 December 2014. [Google Scholar]
Cai, D.; Zhou, K.; Wang, W.; Liu, H.-G.; Cao, K.; Wang, Y. Influence of back-to-back VSC-HVDC project on the operation characteristics of Hubei power grid. J. Eng.2017, 13, 801–805. [Google Scholar] [CrossRef]