Adaptive droop control for better current-sharing in VSC-based MVDC railway electrification system
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Abstract
The share of voltage source converter (VSC) technology is increasing in conventional power systems, and it is penetrating into specific transportation systems such as electric vehicles, railways, and ships. Researchers are identifying feasible methods to improve the performance of railway electrification systems (RESs) by utilizing VSC-based medium-voltage direct current (MVDC) railways. The continuous motion of electric trains makes the catenary resistance a variable quantity, as compared to the traction substation (TSS), and affects the current-sharing behavior of the system. A modified droop control technique is proposed in this paper for VSC-based MVDC RES to provide more effective current-sharing while maintaining catenary voltages above the minimum allowable limit. The droop coefficient is selected through an exponential function based on the ratio between the concerned TSS current and the system average current. This enables small adjustments of droop values in less concerning marginal current deviations, and provides higher droop adjustments for large current deviations. Meanwhile, the catenary voltages are regulated by considering the voltage data at the midpoint between two TSSs, which experiences the lowest voltages owing to the larger distance from the TSSs. The proposed techniques are validated via simulations and experiments.
Keywords
Railway electrification Voltage source converter (VSC) Medium-voltage direct current (MVDC) Current sharing Renewable energies in railways1 Introduction
In recent years, the railway transportation system has become a rapid and reliable means of traveling. Old fossil-fueled railway systems have been replaced with new highly sophisticated electric railways, owing to their higher performance, less energy costs, and environmentally friendly profile [1]. The AC electrification system is widely used in high power, high-speed corridors worldwide owing to its convenient transformation at high voltages (15–25 kV). Meanwhile, the DC electrification system is generally utilized in low power trains such as trams, metros, and suburban railways, with voltages ranging from 600 V to 3 kV, because of its complex voltage transformation system [2]. Notwithstanding the numerous advantages of the AC railway electrification system (RES), it exhibits certain challenges including voltage and current distortions owing to harmonics generated by the traction load of trains and the capacitive and inductive effects of catenary [3, 4]. However, advancements in the fields of power electronics and voltage source converter (VSC) technology have enabled engineers to consider DC transmission for long distances at high voltages in conventional power systems. The focus is mainly on the North Sea countries, where the plan is to transmit 100 GW of energy from offshore sites into the mainland [5]. Practical examples of such VSC-high voltage direct current (VSC-HVDC) transmission lines are the 320 kV HVDC system between Spain and France, and the 420 and 500 kV systems in China [6].
After the successful implementation of VSC-HVDC transmission systems for conventional power grids, there is increasing interest toward considering the application of this technology in the field of RES for high-speed railways. A few articles have considered the use of modular multi-level converters (MMCs) for catenary voltage stabilization [7, 8, 9, 10]. Certain fundamental concepts and the framework of the VSC-based medium-voltage direct current (MVDC) RES for high-speed railways are available in [2, 11, 12, 13]. Such MVDC systems consist of several VSC-based traction substations (TSSs) along the catenary line, providing energy to the system according to the requirement of the train loads. The catenary becomes a continuous network without neutral sections. Moreover, the VSC TSSs consume electricity from the main high voltage feeders as well as from renewable energy sources such as wind turbines, photovoltaic (PV) stations, and energy storage systems distributed along the traction line. Thereby, the whole RES becomes an MVDC grid in which each VSC-based substation contributes its share of power to the system.
Controlling such an MVDC railway grid is similar to controlling a conventional DC microgrid. In RES, the train load is continuously moving along the traction line and is changing its position; this makes the traction line resistance between the VSC TSS and train loads a variable quantity. Because of the variable traction line resistance, the VSC TSS near a train experiences less traction line resistance and contributes more power to the system as compared to the ones at a distance from the train. This behavior can overload renewable energy TSSs and take stiff grid connected TSSs to their maximum limits [11]. The conventional droop control technique improves the current-sharing behavior of the VSC TSS; however, it generates larger voltage deviations in the catenary network (its simulation is described in the next section). Different authors have devised improvements in the droop control technique for more effective current-sharing in conventional DC microgrids. A few of them will be discussed in the next section.
- 1)
Analysis of the effects of the dynamic traction line resistances caused by moving train loads, on the current-sharing behavior of VSC TSSs; analysis of the catenary voltage deviation caused by the conventional droop.
- 2)
An improved droop control scheme is proposed, which enhances the current-sharing behavior of VSC TSSs by selecting a droop coefficient using the TSSs current ratios and an exponential function. This is achieved without compromising on the maximum permitted deviation in the voltage at critical points along the catenary.
2 VSC-based MVDC RES
The general layout, advantages, and basic control of such a system is presented in [2, 11]. This paper is more related to the control technique for more effective current-sharing in similar MVDC-RES environments. For the convenience of readers, the system is described in the preceding subsections.
2.1 System topology
Renewable energies can conveniently be integrated to the RES with the aid of AC/DC and DC/DC converter modules. Energy storage facilities along the traction line can store the surplus energy, as well as energy from the braking system [14, 15, 16], which can be utilized by accelerating trains or can also be shared with metropolitan DC railways. Although [2] provides brief information about the basic architecture, advantages, and control of the system, in simulations, equally-spaced trains are considered on the traction line; this is the best-case scenario for a droop control scheme, as each VSC TSS encounters a similar traction line resistance and shares an equal amount of power to the system (simulated in Section 5).
Practically, trains are not always equally spaced, and the train timetables are mostly managed according to the density of the passengers. Unequally-spaced trains with respect to stationary TSSs causes unequal traction line resistance between the VSC TSSs and train loads. This creates a current-sharing unbalance, and the system drags more current from the VSC TSS located near to the loads than from those at a distance from the loads. The effects of catenary resistance on the load-sharing behavior are explained in the next sub-section.
2.2 Effects of variable traction line resistance under conventional droop control scheme
Resistances of wires and rails
Parameter | Type | Value (Ω/km) |
---|---|---|
Resistance of contact wire (\(R_{c}\)) | CTMH150 | 0.2420 |
Resistance of messenger wire (\(R_{m}\)) | JTMH120 | 0.1840 |
Resistance of rail (\(R_{r}\)) | P60 | 0.0273 |
Traction line resistance (\(R_{T}\)) | – | 0.1318 |
The condition of equal current-sharing can be satisfied if the train reaches the midpoint between the two TSSs, i.e., point A. At this point, \(R_{\text{avg}}\) = 6.59 Ω. If the train continues to move and reaches point B, \(R_{ 1}\) becomes 8.18 Ω, whereas \(R_{ 2}\) becomes 5 Ω. At point B, the train draws more current from TSS#2 as compared to TSS#1.
Hence, changes in traction line resistance due to continuous motion of the train disrupt the ideal current-sharing property of the system. It is also noteworthy that the maximum voltage deviation owing to the cumulative effect of the traction line resistance and increase in the droop value occurs at point A, as it is the midpoint between the two TSSs. In this paper, we will use the term critical point voltage (CPV) for the voltages at these midpoints between TSSs.
In Fig. 5, the voltage profile exhibits a larger sag at the CPV locations as compared to the terminal positions of the TSSs. This is mainly because of the cumulative effect of the higher droop value and traction line resistance. This behavior of loading the TSSs near the load density is not suitable for renewable energy sources or for stiff, grid connected VSC TSSs. Although a further increase in the droop value of the conventional droop scheme improves the current sharing, it further degrades the catenary voltage at the TSSs terminals; moreover, its effect is more adverse at CPV. Better current-sharing can also be achieved by increasing the number of TSSs; however, it will incur higher expenses.
Several articles that are aimed at improving the droop control method for better operation of the conventional DC microgrids are available in the literature; these articles are discussed in the following subsection.
2.3 Droop control schemes
3 Proposed adaptive droop scheme
3.1 Average current
3.2 Exponential droop function
3.3 CPV regulator
3.4 Voltage and current control
In that case, the concerned TSS will use a constant droop value of R_{c} [25]. The remaining TSSs will continue their operation according to the proposed scheme.
4 Stability analysis
Specifications of system used in simulations
Category | Specifications |
---|---|
Transformer | 220 kV/13.8 kV |
TSSs ratings | 24 kV, 30 MW |
VSC topology | 3-level |
Line inductance, filter capacitance | 10 mH, 4400 μF |
Traction line resistance (\(R_{T}\)) | 0.1318 Ω/km |
Resistance between two TSS | 11.33 Ω |
Train load (constant power load) | 8 MW |
Conventional scheme droop value | 4 |
r and x for proposed droop control | r = 4, x = 1 |
CPV_{ref} for proposed control method | 21 kV |
5 Simulations for verifications
Simulations are performed for equally spaced trains, unequally spaced trains, and a single moving train under both the conventional droop method and proposed improved droop method.
5.1 Equally spaced trains
5.2 Unequally spaced trains
5.3 Moving train case
This is mainly because when the train approaches the midpoint between the two substations, the difference between the currents decreases. Hence, the proposed method reduces the value of the droops accordingly.
6 Experiments for verification
Specifications of system used in experiments
Category | Specifications |
---|---|
Transformer | 380 V/52 V, 1500 VA |
VSC, TSSs ratings | 100 V, 500 W |
VSC topology | 2-level |
Line inductance, filter capacitance | 10 mH, 2200 μF |
Total traction line resistance (\(R_{T}\)) | 7 Ω |
Train load (constant power load) | 200 W/each |
Conventional droop control | 4 |
r and x for proposed droop control | r = 4, x = 1 |
CPV_{ref} for proposed control method | 90 V |
6.1 Unequally spaced trains
The results reveal a better current-sharing under the proposed droop control method when the trains are unequally spaced, in conjunction with an increased efficiency owing to the higher catenary voltages.
6.2 Moving train
7 Conclusion
This paper provides a modified droop control scheme for the VSC-based MVDC RES, with an objective of obtaining better current-sharing among the TSSs in conjunction with acceptable catenary voltages. Better current-sharing has been achieved with lower droop values through the proposed method, as compared to the conventional droop. Moreover, the catenary voltages are maintained above the minimum permissible limits through a CPV regulator. The proposed techniques are verified through both simulations and experiments. A rapid and reliable communication link is one of the necessary requirements for the operation of such systems; this will be addressed in future, together with an energy management system for MVDC RES.
Notes
Acknowledgements
This work was partly supported by “the Open Project of National Rail Transit Electrification and Automation Engineering Technique Research Center” (No. NEEC-2017-A03), and partly supported by “the Fundamental Research Funds for the Central Universities” (No. 2682017CX041).
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