Modern electrified railway systems face significant challenges in power quality, energy efficiency, and integration of renewable sources. The conventional railway energy router (RER) based on back-to-back converter topology requires duplicate transformers and inverters, leading to high costs and operational complexity. This paper introduces a novel single phase inverter based railway energy router (IBI-RER) structure that bridges the α-phase and β-phase traction buses while integrating photovoltaic and energy storage systems on the DC side. The proposed system achieves multi-port energy transfer with reduced hardware requirements and enhanced control flexibility.
The fundamental operating principle of the single phase inverter in IBI-RER involves creating three power ports: the DC port for PV and ESS integration, and two AC ports connected to the traction buses. The mathematical model begins with the voltage expressions of the traction buses relative to the three-phase grid reference:
$$ V_{\alpha}(t) = \sqrt{2}V_m \sin\left(2\pi f t – \frac{\pi}{6}\right) $$
$$ V_{\beta}(t) = \sqrt{2}V_m \sin\left(2\pi f t – \frac{\pi}{2}\right) $$
The voltage between α and β phases exhibits a stable sinusoidal form:
$$ V_{\alpha\beta}(t) = \sqrt{2}V_m \sin\left(2\pi f t + \frac{\pi}{6}\right) $$
The current injected by the single phase inverter into the traction buses is expressed as:
$$ i_{RER}(t) = \sqrt{2}I_{RER} \sin\left(2\pi f t – \frac{\pi}{6} – \phi\right) $$
where $I_{RER}$ represents the current magnitude and $\phi$ denotes the phase angle difference. The active and reactive power components injected into each traction bus are derived as:
$$ P_{c\alpha} = V_m I_{RER} \cos\phi $$
$$ P_{c\beta} = V_m I_{RER} \cos\left(\phi – \frac{\pi}{3}\right) $$
$$ Q_{c\alpha} = V_m I_{RER} \sin\phi $$
$$ Q_{c\beta} = V_m I_{RER} \sin\left(\phi – \frac{\pi}{3}\right) $$
The single phase inverter enables four-quadrant operation, facilitating bidirectional power flow among the three ports based on the phase angle $\phi$. The operational modes include peak shaving, regenerative braking recovery, power transfer, and valley filling, determined by the net power demand and storage state.
A critical challenge in the single phase inverter based IBI-RER is the inherent coupling between active and reactive power at the AC ports. To address this, a hardware decoupling method employing reactive power compensation devices is implemented. The total reactive power requiring compensation comprises components from the single phase inverter operation, V/V transformer characteristics, and traction loads:
$$ Q_{\alpha} = Q_{L\alpha} + Q_{V\alpha} – Q_{c\alpha} $$
$$ Q_{\beta} = Q_{L\beta} + Q_{V\beta} – Q_{c\beta} $$
The compensation system utilizes hybrid reactive power compensators combining thyristor-switched capacitors (TSC) and static var generators (SVG) to absorb the reactive components, enabling the single phase inverter to handle purely active power transfer.
The multi-layer optimal control strategy manages power distribution among the three ports while maintaining system constraints. The first layer implements comprehensive energy management based on load conditions and storage state-of-charge (SOC). The power references for the single phase inverter are calculated as:
$$ P_{c\alpha} = \frac{P_{L\alpha} – P_{L\beta} – P_{PV} – P_{ESS}}{2} $$
$$ P_{c\beta} = \frac{P_{L\beta} – P_{L\alpha} – P_{PV} – P_{ESS}}{2} $$
The current reference for the single phase inverter is then determined by:
$$ I_{RER} = \frac{\sqrt{P_{c\alpha}^2 + P_{c\beta}^2 – P_{c\alpha}P_{c\beta}}}{V_m} $$
$$ \phi = \arctan\left(\frac{\sqrt{3}P_{c\beta}}{2P_{c\alpha} – P_{c\beta}}\right) $$
When power constraints are violated, a particle swarm optimization algorithm solves the nonlinear optimization problem:
$$ \min f = |P_{RER\alpha} – P_{c\alpha}| + |P_{RER\beta} – P_{c\beta}| $$
$$ \text{subject to: } P_{RER\alpha}^2 + Q_{RER\alpha}^2 \leq S^2 $$
$$ P_{RER\beta}^2 + Q_{RER\beta}^2 \leq S^2 $$
The reactive compensation devices are sized using geometric progression for TSC units:
$$ Q_{TSC_{Lj}} = 2^{j-1}Q_{TSC_1}, \quad j=1,2,…,L $$
$$ Q_{SVG_j} = (2^L – 1)Q_{TSC_1} – \sum_{k=1}^{L} Q_{TSC_{Lk}} $$
For the power electronic converters, coordinated control strategies ensure precise reference tracking. The single phase inverter employs proportional-resonant control for current regulation, while the ESS maintains DC-link voltage stability using dual-loop control. The PV system operates in maximum power point tracking mode, and SVG units utilize hysteresis control for reactive power compensation.
| Parameter | Conventional RER | Proposed IBI-RER |
|---|---|---|
| Inverter Units | 2 sets | 1 set |
| Transformer Units | 2 sets | 1 set |
| Total Capacity (MVA) | 26 | 15 |
| Voltage Unbalance (%) | 2.52 | 1.58 |
| Average Power Factor | 0.688 | 0.966 |
| Regenerative Braking Recovery (%) | 62.22 | 61.65 |
| PV Utilization (%) | 94.57 | 94.16 |
| System Cost Reduction (%) | 0 | 37.23 |
Simulation results under typical operating conditions demonstrate the effectiveness of the single phase inverter based system. During peak shaving mode, the single phase inverter transfers power from the DC side to both traction buses, reducing grid power consumption by 9.53%. In regenerative braking mode, the single phase inverter redirects braking energy from one traction bus to another or to the ESS, achieving 61.65% recovery efficiency. The power transfer mode enables balanced power distribution between traction buses while utilizing PV generation.
The hardware decoupling via reactive compensation ensures that the single phase inverter handles only active power, significantly improving power factor from 0.688 to 0.966. The three-phase voltage unbalance decreases from 2.52% to 1.58%, meeting power quality standards. The DC-link voltage maintains stability during mode transitions with minimal fluctuation.
| Component | Specification | Value |
|---|---|---|
| Single Phase Inverter | Transformer Ratio | 27.5/2 kV |
| Single Phase Inverter | Capacity | 6.4 MVA |
| Single Phase Inverter | DC-link Voltage | 4000 V |
| Energy Storage | Technology | LTO Battery |
| Energy Storage | Capacity | 500 kWh |
| Photovoltaic | Installed Capacity | 2 MW |
| TSC (α-side) | Stages/Capacity | 2 stages/6.9 Mvar |
| SVG (α-side) | Capacity | 1 Mvar |
| Peak/Valley Thresholds | Power Levels | 14 MW/4 MW |
Experimental validation using 24-hour measured data from a traction substation confirms the practical viability of the single phase inverter based approach. The system reduces daily energy consumption from 328 MWh to 296.75 MWh while maintaining stable operation under fluctuating traction loads and PV generation. The single phase inverter structure demonstrates robustness across all operational modes, with seamless transitions between power transfer states.
The economic analysis reveals that the single phase inverter based IBI-RER reduces main equipment capacity by 42.31% compared to conventional solutions. The elimination of one transformer and inverter set, combined with optimized reactive compensation, lowers overall system cost by 37.23% while maintaining equivalent functionality. This significant cost reduction enhances the economic feasibility of railway energy routing systems, particularly for existing infrastructure upgrades.
Future work will focus on optimizing the single phase inverter control for harmonic suppression and extending the multi-port concept to three-phase railway systems. The scalability of the single phase inverter architecture allows for modular expansion, supporting higher power applications through parallel converter arrangements. The successful implementation of this single phase inverter based system paves the way for more economical and efficient railway electrification solutions.
In conclusion, the single phase inverter based railway energy router represents a transformative approach to traction power management. By leveraging the unique phase relationship between traction buses, the single phase inverter enables three-port energy exchange with minimal hardware requirements. The multi-layer decoupling control ensures optimal power distribution while maintaining power quality standards. The substantial cost reduction and performance improvements demonstrate the significant potential of single phase inverter technology in modern railway electrification systems.