Wide Range Load Variation Research Articles

Design of Multi-Input Multi-Output Non-linear Model Predictive Control for Main Steam Temperature of Super Critical Boiler

Flexible operation of coal-fired power plants is becoming increasingly necessary for successful integration of large-scale renewable power generation into the power grid. The maximum ramp rate and the number of load cycles are generally limited by the thermal stress experienced by the boiler pressure parts, turbine metallurgy and creep and fatigue of critical thick-walled components Main steam temperature is a critical operating parameter that must be controlled within acceptable limits for safe operation. Main steam temperature deviation beyond acceptable limit has impact on boiler pressure parts and turbine material of construction due to creep and fatigue effect. Base load operating units do not require steep ramp rate and hence recommended ramping rates are kept low within the safe operating zone in comparison to the flexible operation of the units with wide range load change width. Thermal stresses are caused by the temperature changes inside the thick-walled components and turbine steam admission parameters. Hence, the quality of main steam temperature control plays a vital role in flexible operation of the coal fired units. Conventional cascaded PID temperature control loop architecture performs well at steady state condition within a limited variation of load change at low ramp rate but it acts slowly and performs poorly at transient operating conditions of flexible operation of the boiler turbine with wide range load variation and load cycle with high ramp rate and remains far from rated conditions. In this paper, a Multi-Input Multi-Output (MIMO) Non-linear Model Predictive Control (MPC) design for regulation of the main steam temperature of a Once-Through supercritical Boiler is proposed. The controller is based on a non-linear dynamic model which incorporates dynamics of the variables of interest. It has the capability to operate effectively across a wide load range while maintaining main steam temperature within acceptable limits. A notable advancement in this design of MPC is the incorporation of coal flow demand and feedwater flow demand as additional control inputs alongside primary and secondary spray flows. In simulation test cases, the MPC controller demonstrates satisfactory performance and computational efficiency.

Flexible Operation of Concentrating Solar Power Plant with Thermal Energy Storage Based on a Coordinated Control Strategy

With the ambition of achieving carbon neutrality worldwide, renewable energy is flourishing. However, due to the inherent uncertainties and intermittence, operation flexibility of controllable systems is critical to accommodate renewables. Existing studies mainly focus on improving the flexibility of conventional plants, while no attention has been paid to the flexible operation of concentrating solar power with thermal energy storage (CSP-TES) systems. To this end, the ultimate goal of this work is to investigate the potentiality and realization of CSP-TES systems to flexibly operate in grid system regulation. With this goal, the dynamic characteristics of a 50 MW parabolic trough collector CSP plant with molten-salt-based TES is analyzed, and its dominant control characteristics are concluded to demonstrate the possibility of the ideal. After that, a coordinated control strategy is proposed. Specifically, a disturbance observer-based feedforward–feedback control scheme and a feedforward–feedback controller are designed, respectively, for the solar field and the energy storage subsystems, while the power block subsystem is regulated by a two-input and two-output decoupled controller. Based on the decentralized structure, three simulation cases are, respectively, performed to testify the capacity of the CSP-TES system to wide-range load variation tracking, strong disturbance rejection, or both. The results show that the CSP-TES system can adequately track the grid commands based on the proposed coordinated control strategy, even under strong fluctuation of irradiation, demonstrating the flexibility of CSP-TES participating in grid regulation. In the context of continuous penetration of renewable energy into the grid system, research on the role transition of the CSP-TES system from its own optimization to grid regulator is of great importance.

A Novel IPT Converter With Current-Controlled Semi-Active Rectifier for Efficiency Enhancement Throughout Supercapacitor Charging Process

During the typical constant current (CC) charging process, the supercapacitor has a pretty wide-range load variation. However, an inductive power transfer (IPT) converter can maintain high efficiency only within a certain load range around its optimum load point. Once the supercapacitor load resistance is far away from the optimum point, which usually happens in low charging power levels, i.e., the smaller load resistances, the IPT converter will suffer from the dramatic efficiency degradation and thus a step-up load transformation is required throughout the whole charging process. In this article, a novel current-controlled semi-active rectifier (CCSAR)-based IPT converter is proposed to fulfill this requirement. The characteristic of step-up load transformation is first identified and implemented by controlling the conduction angle of the CCSAR to enhance the efficiency in the charging process. The desired charging CC is then regulated by tuning the operating frequency of the IPT inverter. To coordinate these two objectives, a bivariate control is adopted here to achieve fast, direct, and precise current output with enhanced efficiency performance over the whole load range during the CC charging process. Finally, experiment results validate the theoretical analysis well.

A Comparative Analysis of S-S and LCCL-S Compensation for Wireless Power Transfer with a Wide Range Load Variation

Wireless power transmission (WPT) has great potential for charging electric vehicles. Constant voltage (CV) and constant current (CC) are two major types of battery charging modes. In this paper, we analyze the output characteristics of series-series (S-S) topology and double capacitances and inductances-series (LCCL-S) topology. Voltage gain variation is achieved in the LCCL-S compensation structure without additional components, and the system is still kept in resonant condition. A WPT experimental platform was also built and tested based on the theoretical analysis. When the load resistance is 300 Ω, a voltage gain of 0.7 or 2.22 is achieved for the LCCL-S with a compensating inductor of 100 μH or 33 μH, respectively. The experimental results fit the theoretical analysis. The CC/CV output characteristics and efficiencies of S-S and LCCL-S topologies in a wide load resistance range are also demonstrated. Moreover, zero voltage switch (ZVS) is also implemented in both two systems.

Dual‐active‐bridge DC–DC converter with auxiliary parallel networks for wide load range ZVZCS application

In this study, a wide load range zero-voltage zero-current switching (ZVZCS) dual-active-bridge DC–DC converter using auxiliary parallel networks is proposed. The outstanding feature of the proposed converter is that all of the switches are allowed to operate with zero-voltage switching while four of the switches are allowed to operate with zero-current switching, due to the use of two auxiliary parallel networks, which were paralleled to two active bridges, respectively. Furthermore, the circulating current losses can be reduced significantly by using unified boundary trapezoidal modulation method. Based on these merits, the proposed converter has some advantages for the wide range load variation applications with less circulating current losses. The operation principles, soft-switching conditions and loss analyses comparison are given in this study. These benefits and characteristics mentioned above are verified from the experimental results by using a 1 kW prototype.

Hybrid mode control for wide range soft‐switched full‐bridge converter with auxiliary parallel inductor networks

Aiming at the limitations of conventional phase-shifted full-bridge (PSFB) converter, such as, narrow zero-voltage switching (ZVS) range and large duty cycle losses, here, a wide range soft-switched full-bridge (FB) converter using hybrid mode control is proposed. Given using of two auxiliary inductors under light load, the energy that required for soft switching is satisfied and thus, ZVS can be achieved, and, therefore, efficiency of converter can be improved significantly under light load. Furthermore, the duty cycle losses of the secondary side are reduced by using a transformer with the small leakage inductor. Based on these merits, the proposed converter has some advantage for the wide range load variation applications without duty cycle losses. This paper presents the circuit configuration of the proposed converter and analyses its operating principles. Experimental results of an 800 W 200/50 V prototype are presented to validate the effectiveness of the proposed converter.

Reinforced coordinated control of coal-fired power plant retrofitted with solvent based CO2 capture using model predictive controls

Solvent-based post-combustion CO2 capture (PCC) provides a promising technology for the CO2 removal of coal-fired power plant (CFPP). However, there are strong interactions between the CFPP and the PCC system, which makes it challenging to attain a good control for the integrated plant. The PCC system requires extraction of large amounts of steam from the intermediate/low pressure steam turbine to provide heat for solvent regeneration, which will reduce power generation. Wide-range load variation of power plant will cause strong fluctuation of the flue gas flow and brings in a significant impact on the PCC system. To overcome these issues, this paper presents a reinforced coordinated control scheme for the integrated CFPP-PCC system based on the investigation of the overall plant dynamic behavior. Two model predictive controllers are developed for the CFPP and PCC plants respectively, in which the steam flow rate to re-boiler and the flue-gas flow rate are considered as feed-forward signals to link the two systems together. Three operating modes are considered for designing the coordinated control system, which are: (1) normal operating mode; (2) rapid power load change mode; and (3) strict carbon capture mode. The proposed coordinated controller can enhance the overall performance of the CFPP-PCC plant and achieve a flexible trade-off between power generation and CO2 reduction. Simulation results on a small-scale subcritical CFPP-PCC plant developed on gCCS demonstrates the effectiveness of the proposed controller.

Generalized active disturbance rejection control for the boiler-turbine unit using multi-objective optimization and extended state observer

This paper proposes a generalized active disturbance rejection controller (GADRC) based hierarchical control structure for the boilerturbine unit. In the lower layer, a multivariable extended state observer (MESO) is developed to estimate the values of the lumped disturbances caused by modelling mismatches, fuel quality variation and wide range load variation. The influence of the disturbances is then compensated at the input side as a feedforward control. In the upper layer, the multi-objective optimization is devised to obtain the set-points by removing the plant behaviour variation from the optimized model in a feasible way. The lowpass filter acting on the lumped disturbances is designed to bridge the gap between the lower and upper layer. The impact of the feedthrough item is approximated by a first-order system and a two degree-of-freedom (2-DOF) control strategy is established to illustrate the set-point tracking and disturbance rejection properties of the controller. Simulation studies on a 1000MWe coal-fired ultra-supercritical boiler-turbine unit demonstrate that the proposed control strategy can achieve a satisfactory performance in cases of fuel quality variations, model-plant mismatches and wide range load variation.

Modeling and sliding mode predictive control of the ultra-supercritical boiler-turbine system with uncertainties and input constraints

The coordinated control system (CCS) serves as an important role in load regulation, efficiency optimization and pollutant reduction for coal-fired power plants. The CCS faces with tough challenges, such as the wide-range load variation, various uncertainties and constraints. This paper aims to improve the load tacking ability and robustness for boiler-turbine units under wide-range operation. To capture the key dynamics of the ultra-supercritical boiler-turbine system, a nonlinear control-oriented model is developed based on mechanism analysis and model reduction techniques, which is validated with the history operation data of a real 1000 MW unit. To simultaneously address the issues of uncertainties and input constraints, a discrete-time sliding mode predictive controller (SMPC) is designed with the dual-mode control law. Moreover, the input-to-state stability and robustness of the closed-loop system are proved. Simulation results are presented to illustrate the effectiveness of the proposed control scheme, which achieves good tracking performance, disturbance rejection ability and compatibility to input constraints.

Closed‐loop waveform control of boost inverter

The input current of single-phase inverter typically has an AC ripple component at twice the output frequency, which causes a reduction in both the operating lifetime of its DC source and the efficiency of the system. In this paper, the closed-loop performance of a proposed waveform control method to eliminate such a ripple current in boost inverter is investigated. The small-signal stability and the dynamic characteristic of the inverter system for input voltage or wide range load variations under the closed-loop waveform control method are studied. It is validated that with the closed-loop waveform control, not only was stability achieved, the reference voltage of the boost inverter capacitors can be instantaneously adjusted to match the new load, thereby achieving improved ripple mitigation for a wide load range. Furthermore, with the control and feedback mechanism, there is minimal level of ripple component at the DC bus during steady state, and the transient response is rapid with negligible effect on the output voltage. Analysis, simulation and experimental results are presented to support the investigation.

Fuzzy modeling and stable model predictive tracking control of large-scale power plants

This paper develops a stable model predictive tracking controller (SMPTC) for coordinated control of a large-scale power plant. First, a Takagi–Sugeno (TS) fuzzy model is established to approximate the behavior of the boiler–turbine coordinated control system (CCS) using fuzzy clustering and subspace identification (SID). Then, an SMPTC is designed based on the fuzzy model to track the power and pressure set-points while guaranteeing the input-to-state stability and the input constraints of the system. An output-based objective function is adopted for the proposed SMPTC so that the controller could be directly applicable for the data-driven model. Moreover, the effect of modeling mismatches and unknown plant variations has been overcome by the use of a disturbance term and steady-state target calculator (SSTC). Simulation results for a 600MW power plant show that an off-set free tracking performance can be achieved over a wide range load variation.

A High-Frequency-Link Secondary-Side Phase-Shifted ZVS-PWM DC-DC Converter with ZCS Active Rectifier

Performance evaluations of a secondary-side phase shift (SPS) pulse-width-modulation (PWM)-controlled zero voltage soft-switching (ZVS) DC-DC converter with a zero current soft-switching (ZCS) active rectifier are presented in this paper. In the proposed DC-DC converter, the soft-switching operations are achievable from full to null loads by utilizing the parasitic inductances of the high-frequency (HF) transformer. In addition, no circulating current emerges in either the primary- or secondary-side full-bridge circuits, and thus the related idling power losses can be minimized. In this paper, the soft-switching operations over the wide-range load variations are verified, and the characteristics of output power and voltage versus phase shift angle regulations are demonstrated in an experiment based on a 1kW-50kHz laboratory prototype of the proposed DC-DC converter. Furthermore, the converter power losses, which include the switching and conduction power losses, are analyzed, after which the effectiveness of the proposed DC-DC converter is evaluated from a practical point of view.

Induction Machine Control in Presence of Magnetic hysteresis Modelling and Speed reference tracking

The problem of controlling induction machine speed has generally been dealt with using standard models i.e. those based on the assumption that the magnetic characteristic is linear. In coherence with this assumption, most control strategies involved rotor flux regulation around a fixed nominal value. As a matter of fact, optimal performance operation (e.g. maximal efficiency, maximal torque, unitary power factor…) are not achievable using constant flux reference especially when facing wide range load variations. But, control strategies involving wide range flux reference variations can not be obtained without accounting for the nonlinearity of the machine magnetic characteristic. In the present work, we propose a new speed control strategy based on a new machine model that accounts for the magnetic characteristic hysteresis and saturation. A speed controller is designed using a nonlinear design technique. The performances of the proposed control strategy are formally analysed and their supremacy with respect to the standard control solution is illustrated through a simulation study, using a 7.5 KW machine