Flexible control of combined heat and power units based on heat-power estimation and coordination

doi:10.1016/j.ijepes.2020.106261

International Journal of Electrical Power & Energy Systems

Volume 123, December 2020, 106261

https://doi.org/10.1016/j.ijepes.2020.106261 Get rights and content

Highlights

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The heat-power coordinated relations are set up.
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A novel heat and power coordinated control strategy is designed.
•
The heat-to-power support is proposed for rapid power control.
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The accurate energy balance is designed to coordinate heat and power.
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The electric power ramp rate is improved significantly.

Abstract

Combined heat and power (CHP) units, as integrated power and heating systems, aim for the flexibility of the electric system and efficiency of the heating system. In this study, the static and dynamic models for CHP units are presented, and a novel control strategy for heat and power coordination and distribution is proposed based on the design of heat-to-power support, heat state reconstruction, and accurate energy balance. Simulation results of a 300 MW CHP unit prove that the proposed strategy improves the unit power load-following capability and meanwhile ensures reliable heat supply.

Introduction

In recent years, combined heat and power (CHP) systems have received considerable attention on account that they convert more of the input energy into useful service [1]. CHP units feature as typical links for electric and thermal systems [2]. For the electric system, they need to operate with great flexibility to support energy balance and frequency stability. Consequently, power associations worldwide have implemented stringent requirements on the operation of grid-connected power units [3]. CHP units have weak operating flexibility due to the coupling of heat and power [4]. On one hand, their electric power adjustable range will be compressed by half when operated in heat-led mode. Heat accumulators [5], electric boilers, batteries [6], heat pumps [7], [8], and the thermal inertia of heating systems [9] have recently been used to decouple heat and power and expand the power operation ranges for CHP units. On the other hand, the electric power ramp rate can hardly be improved due to the large boiler delay and strong couplings of heat and power.

CHP units commonly adopted boiler–turbine coordinated control strategy (CCS) to perform electric power load change. The using methods were almost the same as that for pure condensing generation units. The proportional, integral and derivate (PID) form decoupling controller is the most popular [10]. Furthermore, many studies have applied robust [11], predictive [12], active disturbance rejection [13], and fuzzy [14] controls to overcome uncertain disturbances, gain-scheduled [15], data-driven [16], backstepping [17], and H∞ [18] controls to widen the load operating conditions. However, existing research separated the heating supply side and electric power side, leading to the ineffective control. Ref. [19] pointed out that the heating supply side has a large inertia, therefore short-time variations of heat source supplying hardly affects the heat consumers. Refs. [20], [21], [22] taken use of heat inertia to expand the operation ranges for CHP units.

This work aims to present the features and relations of heat and electric power, and develop a novel CHP control approach to coordinate the heat and power control. The paper is structured as follows: Section 2 presents the heat–power coordinated relations for CHP units. Section 3 designs a novel control strategy to coordinate the heat and power productions. Section 4 provides a simulation to prove the effectiveness of the proposed strategy. Finally, Section 5 shows our conclusion.

Section snippets

Constrains

CHP units can operate the heat and electricity productions to shift from one to another according to their demands. Commonly, they prioritize to meet the heat demand, i.e., the heat-lead mode [23]. Under operation time, CHP units should run in the feasible operation region as illustrated in Fig. 1 [9].

Static relations

The static relations give the expression of electricity power taking heat power as the independent variable, which will provide accurate signal support for heat and power coordinated control.

The

Control strategy design

The CHP control strategy should be capable of flexible power operation and reliable heat supply. This is because the electric power output is required to respond in real time; nevertheless, the heat system focuses the total energy supply in a certain period. According to Section 2.3, both heat and power productions are influenced three model inputs together, i.e., the coal feeding rate command (μ_B), the governor valve opening (μ_T), and the heat source valve (μ_H). The heat source valve

Simulation and results

A 300 MW CHP unit is taken for example to conduct simulations in MATLAB R2014a.

Conclusions

In this work, the static and dynamic relations for heat and power in a CHP unit is presented and a novel control strategy for heat and power control is designed. By introducing H2P support, heat state reconstruction, and accurate energy balance, the control strategy increases the electric power ramp rate to over 3% rated power per minute, but the heat supply is not influenced. The simulation results of a 300 MW CHP unit prove the effectiveness of the proposed strategy. The control strategy will

CRediT authorship contribution statement

Wei Wang: Conceptualization, Methodology, Software. Jizhen Liu: Supervision, Resources, Data curation. Zhiyong Gan: Investigation. Yuguang Niu: Resources. Deliang Zeng: Visualization.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This paper is supported by the Young Elite Scientist Sponsorship Program by CAST (2017QNRC001), National Natural Science Foundation of China (Nos. U1766204 and 51821004), Beijing Municipal Science and Technology Project (No. Z181100005118005), Science and Technology Project of the Headquarters of State Grid Corporation of China (4500-201999535A-0-0-00), and Fundamental Research Funds for the Central Universities (2018ZD05).

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View full text

Flexible control of combined heat and power units based on heat-power estimation and coordination

Highlights

Abstract

Introduction

Section snippets

Constrains

Static relations

Control strategy design

Simulation and results

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgements

Energy

ISA Trans

ISA Trans

Appl Energy

Energy

Energy

Appl Energy

Energy

The impact of CHP (combined heat and power) planning restrictions on the efficiency of urban energy systems

Energy

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Renew Sustain Energy Rev

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Energy Convers Manag