A Novel Virtual Resistor and Capacitor Droop Control for HESS in Medium Voltage DC System

A Novel Virtual Resistor and Capacitor Droop Control for HESS in Medium Voltage DC System

Abstract: The hybrid energy storage system (HESS) plays an essential role in the shipboard medium voltage DC (MVDC) system to provide backup power, buffer large load change, as well as improve power quality. In this paper, a novel virtual resistor and capacitor droop (VRCD) control is first proposed for HESS including battery, super capacitor (SC), and flywheel. It enables the generator and multiple types of energy storages (ESs) with different characteristics to respond to multiplefrequency unbalanced power instead of simple high-frequency and low-frequency power sharing by the conventional droop control method. A detailed design procedure for the VRCD control is presented. Also, the secondary state of charge (SoC) recovery control is proposed to maintain high energy reserve of ESs for further power demand. Simulations are conducted to demonstrate the effectiveness of the control scheme proposed in a shipboard MVDC under particular loads fluctuation, such as the propulsion motor and the pulsed load.

Existing system:

Effective power sharing among ESs is a major challenge if the HESS is utilized in the shipboard. It is desirable for ESs with high energy density to balance steadystate power mismatch and for ESs with high power density and fast dynamics to compensate for transient power. A topology of HESS for shipboard MVDC application in can achieve superior fault response with fault current limiting function. In, a fuzzy logic controller is proposed to provide total storage reference power for the battery and SC. A low pass filter (LPF) is intended for the power splitting between the battery and SC. However, the LPF method belongs to the centralized control. It highly relies on communication for the sake of the unbalanced power acquirement and thus reduces the system reliability. The decentralized control methods, such as droop control, are independent of communication. But the existing droop based control can only realize the steadystate power sharing. In , an extended droop control (EDC) composed of a virtual resistance droop (VRD) controller.

Proposed system: Researches on MVDC are mainly associated with mitigating the influence of particular loads such as the propulsion motor and the PPL. One method is through improving the control strategy. In, an adaptive current–voltage control scheme is designed for actively controlled hybrid dc micro grid to reduce the negative impact of the PPL. In, a control strategy is proposed to mitigate the adverse effect of PPLs by identifying the optimal charging profile. Reference proposes a control method based on a Linearization via State Feedback (LSF) to crack the destabilizing problem of constant power load and to ensure the MVDC bus voltage stability.

Advantages: VRCD control enables different ESs to respond to power mismatch under different time-scale. It is independent of the type and the number of ESs, and can takes full advantages of varying ESs features. Therefore, this method possesses strong applicability and generality.

The decentralized VRCD control proposed is a generalized power splitting method, and is suitable for multiple types of ESs with different response characteristics under various scenarios. Up until now, no control method is reported for power splitting among multiple Esses.

The particular loads in shipboard MVDC have strong demand for multiple types of ESs and impose stringent requirements on the control strategy. This method is firstly applied in shipboard MVDC system to realize the power sharing among different ESs and the conventional generator to buffer load fluctuation.

Disadvantages: The battery can respond to the low-frequency component via VRD control.

However, the high-frequency power sharing between SC and flywheel encounters problems with the conventional control.

In, a control strategy is proposed to mitigate the adverse effect of PPLs by identifying the optimal charging profile. Reference proposes a control method based on a Linearization via State Feedback (LSF) to crack the destabilizing problem of constant power load and to ensure the MVDC bus voltage stability.

Modules: Medium – voltage DC: SHIP electrification, based on medium-voltage DC (MVDC) power system, is a development tendency in the future ship, for the sake of the benefits such as high efficiency and reliability, low noise and environmental protection. The ship loads can be divided into three major parts: pulsed power load (PPL), propulsion load and ship service load. The PPL appears as a special one with periodic fast varying power demands, ranging from kilowatts to megawatts in a charge interval on the order of seconds to minutes. On the one hand, the intermittent PPLs degrade the power quality. On the other hand, the change of the propulsion speed will also cause significant impact on the voltage of MVDC. Traditional generators have limited capability to follow sudden changes in the load. Energy storages (ESs) technologies are capable of satisfying rapid power demand. Consequently, the ESs are of essential importance for MVDC to supply the PPL, minimize the impact of the PPL and propulsion motor and improve power quality.

Super capacitors:

It has been investigated that the most widely applied nonfuel ESs in shipboard MVDC are based on batteries, super capacitors (SCs), and flywheels. They serve in different situations due to their distinct power density and energy density features.

Batteries have relatively low power density, slow dynamic response but high energy density. SC has high power density, fast dynamic response but low energy density. Therefore, batteries and SCs are usually combined as the backup energy storage devices to support the generators in shipboard. A flywheel has the capability of frequent and rapid charging and discharging with high efficiency, and it can compensate for the adverse effect of the PPL on the MVDC system. When the three type of ESs are installed in the MVDC, the hybrid energy storage system (HESS) formed is an effective way to maximize battery lifecycle and retain the benefits of high power and high energy density.

Virtual capacitance drop: Virtual capacitance droop (VCD) controller for ESs is proposed to realize dynamic current sharing between the battery and SC. Similarly, in, the coordination of the integral droop and conventional voltage-power droop can also achieve the transient power allocation in HESSs in a decentralized manner. In, the concept of frequency-coordinating virtual impedance is proposed to enable battery and SC to absorb low-frequency and high-frequency power fluctuations. However, all these power sharing methods are constrained to two types of ESs, and there is no method proposed for more than two types of ESs. During the steady state, the unbalanced power is mainly assumed by the generator and the MVDC bus voltage is regulated by the droop-controlled excitation controller of the generator. In the transient, the unbalanced power is shared among different types of ESs. Virtual and capacitor droop: In the transient, the unbalanced power is shared among different types of ESs. To coordinate the droop control of the generator with the HESS and improve MVDC bus voltage quality, in this paper, a decentralized virtual resistor and capacitor droop (VRCD) control is proposed to realize automatic power splitting among the generator, batteries, SCs, and flywheels, so as to fully take advantages of all power sources. The major contribution of this paper is highlighted below. The decentralized VRCD control proposed is a generalized power splitting method, and

is suitable for multiple types of ESs with different response characteristics under various scenarios. Up until now, no control method is reported for power splitting among multiple Esses.

VRCD Control: For most HESSs consisting of two types of ESs, VRD and VCD control can realize favorable power sharing between the ES with a fast dynamic response and the ES with a slow dynamic response. When multiple types of ESs are installed in the system, this control is not efficient due to rough low- and high-frequency power splitting. Take a system with battery, SC and flywheel for example, the dynamics and capacity of the three types of ESs are totally different. The battery can respond to the low-frequency component via VRD control. However, the high-frequency power sharing between SC and flywheel encounters problems with the conventional control. To improve the flexibility and effectiveness of the control, a generalized power splitting method based on the virtual resistor and capacitor droop (VRCD) control is proposed. In terms of circuit, the resistor in the ES branch possesses the damping characteristics. Compared with the capacitor branch, the RC branch has lag property. By changing the resistor and capacitor, desirable response characteristics of the ESs can be derived.

Pulsed Load Operation: This case studies the impact of the PPL operation on the dc bus voltage level. The propulsion motor and the service load maintain unchanged. The PPL is put into operation at 5s, and the power demand rises to 15MW at a period of 0.1s. The operation quits at 7s, and its power reduces from 15MW to 0MW in 0.1s. The simulation results without HESS are shown in Fig.11. The bus voltage exists a deviation above 0.5kV. The output power of the generator is almost beyond its maximum limit during transients, and the voltage cannot be fully regulated to achieve high quality.