Design and Optimization of Flywheel Energy Storage System for Rail Transit

At present, the urban rail transit system has problems such as energy waste in the braking process and unstable grid voltage in the start-stop state. Aiming at the problems caused by the start-stop state of rail transit, considering the energy saving and voltage stability requirements of system energy management, a flywheel energy storage system (FESS) specially used for rail transit is designed. The energy system (FESS) can feed back the braking energy stored by the flywheel to the urban rail train power system when the rail train starts to cause the voltage and frequency of the traction microgrid to change. This paper proposes a flywheel energy management system based on a permanent magnet synchronous motor (PMSM), which can realize efficient energy management through efficient control of the flywheel side motor. The flywheel side permanent magnet synchronous motor adopts an improved flywheel speed expansion energy storage control strategy based on current feedforward control to improve the fast response capability of the flywheel, ensuring the output power of the flywheel at high speed. In addition, a specific multi-threshold voltage single FESS control strategy is suggested, which improves the energy-saving and voltage-stabilizing performance of the system.


Introduction
Since the 21st century, in order to implement the 'priority development of public transportation' strategy, urban rail transit in China has developed rapidly [1].At the same time, urban rail transit has the characteristics of large passenger volume and high density, so it is of great research significance in alleviating traffic pressure.However, the operation power of urban rail transit is high, and it consumes much electric energy.According to relevant data, when a tram runs normally, it is frequently braking.Under braking conditions, the electric energy consumed by braking accounts for about 40%-50% of the entire traction process [2].
The flywheel energy storage system is well-suited for applications requiring rapid charge and discharge, high energy efficiency, long service life, large charge and discharge cycles, and insensitivity to environmental conditions [3].It has been extensively applied in fields such as wind power generation and rail transportation.Currently, one solution for recovering regenerative braking energy is to install an energy storage system (ESS) next to the running rail.For example, sodium-sulfur batteries have been used on the Long Island Railroad, lithium-ion batteries have been used on the Philadelphia transit system [4], and stationary capacitors have been used in some European rail transportation systems.However, there is relatively little research on efficient control strategies for applying flywheel energy storage technology in the rail transportation industry, which this paper aims to address.This article focuses on the FESS technology for urban rail transit, considering the requirements of energy-saving, voltage regulation, and energy management of the FESS while taking into account the characteristics of urban rail transit operation and FESS.A control model has been designed for a single FESS for use in urban rail transit scenarios.An improved flywheel speed control strategy based on current feedforward control and an energy storage strategy with multiple threshold voltage regulations are proposed.These strategies can simultaneously solve the energy-saving, strong stability, and fast response issues of the flywheel energy storage system.The correctness of the proposed strategies is verified through simulation, experiments, and analysis.

Structure and principle of FESS based on rail transit
The FESS is a physical-based energy storage device, which is different from the traditional chemical battery energy storage method.It is a fast and efficient electromechanical energy storage and conversion device [5].The system consists of three main components: the motor, the power electronics device, and the flywheel itself [6].
Filter circuit

Train propulsion system
Dual-PWM inverter As can be seen from Figure 1, the FESS is mainly made up of a motor and a flywheel.Considering that the motor in the FESS can operate in both generator and motor states, the speed adaptation range should be large, the no-load loss should be small, the response speed should be fast, and it should have high torque density and easy maintenance.This article uses an internal permanent magnet synchronous motor (IPMSM) [7].In addition, the vehicle-side converter, grid-side inverter, and flywheel-side converter all use the controllable three-phase pulse width modulation method.This modulation method can achieve higher voltage utilization and lower electromagnetic noise while improving system operation efficiency and response speed.
The FESS mainly functions in three working states, namely mode of storing energy, maintaining energy, and discharging energy [8].Through coordinated control, the flywheel energy storage system realizes smooth switching under different working modes, realizes energy storage and feedback, and ensures the stability of load operation.

Charge and discharge control strategy for FESS
The control of the FESS is actually to control the motor coupled with the flywheel.The establishment of this control model is actually an additional inertial load model in which the flywheel is modeled on the PMSM rotor.When the flywheel is working in the state of energy storage, for the purpose of improving the robustness and fast response of the whole FESS, the excitation component is weakened as much as possible to achieve the demagnetization purpose and thus improve the energy storage capacity and response speed of the FESS.The current loop of the FESS uses a maximum torque current ratio control combined with the leading angle control phase lead based on the current feedback method to allocate the AC and DC axis currents.Combined with the current feedforward control to control the FESS, the robustness, and responsiveness of the entire system are improved.
When the flywheel operates below the base speed, the minimum current output maximum torque control method is adopted.To achieve maximum torque current ratio control, it is necessary for the motor to work in the normal state, and the magnitude of the current consumption vector is the smallest.Combining the torque equation of the motor with its stator vector equation, the Lagrange function equation group is obtained: Solving the equation yields the connection between the quadrature axis current q i and the direct 4( )

2( )
When the Speed loop output value is used as the input variable of MTPA, this maximum torque current ratio control can be achieved according to the equation above.
When the flywheel-coupled motor is operating above the base speed, to improve performance such as smoothness of current transition and stability of speed response, an improved voltage feedback-based weak field control strategy with current lead angle has been proposed in this paper.It is specifically designed for flywheel energy storage, which can achieve efficient energy storage for the flywheel.According to the information you provided, the vector diagram of lead angle control is presented in Figure 2, where θ represents the angle of the current vector, and θ Δ represents the lead weak magnetic angle.To achieve flywheel acceleration, it is necessary to adjust d i and q i in real time.The improved lead angle weak magnetic regulation used in this paper is built on the utilization of the motor terminal voltage to judge the level of DC bus voltage utilization.When the flywheel speed is greater than the base speed, the voltage of the current loop reaches saturation.At this time, the direct-axis current is increased in the reverse direction, causing the speed to continue to increase.
In Equation ( 3), M represents the utilization rate.When 1 M > , it indicates that the voltage of the inverter has reached its limit, and it is necessary to increase the lead angle to achieve flywheel acceleration. .The moment when the flywheel enters the weak magnetic acceleration phase can be accurately determined, achieving the purpose of flywheel acceleration and energy storage.When the real-time utilization rate is lower than the reference utilization rate * M , the PI regulator output is 0, and the flywheel coupled PMSM operates below the base speed.When this bus voltage of the DC power supply real-time utilization rate exceeds the reference utilization rate * M , a negative angle is calculated and output through the PI regulator.The current phase angle is obtained by integrating the obtained current angle, thereby controlling the change of the two-phase current of the stator to achieve the purpose of flywheel acceleration.This paper proposes an improved current lead angle acceleration strategy designed for the FESS, which avoids the current challenge facing us stator current oscillation caused by high-speed operation of the flywheel and improves system stability.

Single FESS control strategy based on multiple thresholds
It is usually required to the monitoring of the bus voltage of the DC power supply status in order to carry out charging and discharging operations when FESS is applied to urban underground trains.In this article, an energy storage system built on single flywheel multi-voltage threshold judgment is designed for different working conditions of urban underground railways.Through uninterrupted collecting of data on the bus voltage of the DC power supply, the speed of the train, and the energy consumed by the power system, the FESS proposed in this paper controls the flywheel to enter different working states.In this article, a FESS is used to achieve energy saving while also considering voltage regulation.Specifically, during train braking, it absorbs regenerated braking energy and suppresses contact wire voltage rise.In addition, the paper aims to absorb as much of the regenerated braking energy as possible through adjacent trains on the traction network, so the charging threshold can be set slightly higher.To achieve these goals, this paper uses multiple voltage thresholds to set three thresholds, char u , dc u + and , the energy storage system ought to disconnect in time to avoid overvoltage damage to the flywheel energy storage system or damage to the energy storage system resulting from false discharge during maintenance of traction network.

Simulation and analysis
For the purpose of verifying the feasibility of the control method proposed in this paper, as well as the correctness and effectiveness of the charge and discharge control strategy, a simulation platform was established using MATLAB development environment.Related experiments and data analysis were carried out.The parameters of PMSM on the flywheel side are shown in Table 1 The flywheel energy storage system adopts the control strategy of using a current loop, speed loop, and voltage loop during the charging phase, and a multi-threshold current and voltage dual closed-loop control during this energy release phase, which can ensure the good operation of the flywheel energy storage system.The specific parameters of the simulation system are as follows.The bus voltage of the DC power supply Udc is 600 V, a three-phase 380 V, 50 Hz micro-grid is used to simulate the generation of braking energy of a rail transit vehicle, and the corresponding simulation and control block diagram is shown in Figure 5. Simulation is carried out based on the above development Simulink.At 0~0.6 seconds (energy storage charging stage), the train braking state is simulated, the control DC bus voltage is 600 V, and the flywheel speed is increased at the same time; at 0.6~1 s, during the flywheel speed maintenance stage, the voltage is maintained at 600 V, and this flywheel speed is maintained at the speed value set in the energy storage stage; at 1~1.3 s, during the feedback discharge stage, the simulated train is working in the starting state, the power side converter of the rail transit is working in the inverter mode, and the DC bus voltage is controlled to be constant to prevent the impact of the train starting on the traction net.It can be seen from the bus voltage waveform in Figure 6 that the existence of the flywheel is a major component of the stability of the bus voltage.The simulation waveforms from Figures 6 to 9 make it clear that the improved control pattern recommended has a significantly faster speed waveform response, faster torque response, and smaller pulsation compared to the traditional control method.Through the analysis of the FESS, the control strategy proposed in the article can maintain a high-power factor of 0.989 on the track-side converter while reducing the bus voltage fluctuation coefficient δ to only 2.1% and increasing the system response speed by 13.3%.

Conclusion
The study presented in this paper aims to use this flywheel energy storage system to attain the goal of energy management and take into account the voltage stabilization function.In addition, the current feed-forward negative I d control strategy based on multi-threshold voltage energy management proposed within the framework of the paper for FESS can reasonably switch the corresponding working mode under different working conditions.It can reduce torque fluctuation in switching operations and recover this regenerative system.The catenary voltage is stabilized, and the flywheel response speed and energy storage system efficiency are improved.
operation, 0 θ = .When the flywheel reaches the base speed and needs to accelerate, θ is increased appropriately, i.e., θ θ + Δ .It can increase the d-axis component, weaken the air gap magnetic field, and attain the goal of increasing the speed of the flywheel.

Figure 3 .
Figure 3. Flywheel side current lead angle weak magnetic control block diagram.The control block diagram is in Figure3.

Figure 4 .
Figure 4. Working mode diagram of a single FESS.When the value range of the bus voltage of the DC power supply dc u is max char dc dc u u u ≤ ≤ , the FES system operates in the mode of storing energy, the state suppresses the bus voltage of the DC power supply rise by recovering regenerative braking energy.When dc u is within the range of

Figure 6 .Figure 7 .Figure 8 .
Figure 6.DC bus support voltage waveform.When the flywheel is switching modes, the experimental waveforms of this FES system's speed, torque, voltage, and this simulated three-phase inverter current and voltage are illustrated in Figures 7-9, where Figure (a) is the traditional control of the waveform under the control mode, Figure (b) is the waveform under control after the improvement in this paper.