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Centralised multivariable feedback control of steam drums in combined cycle power plants

机译：联合循环电厂蒸汽鼓的集中式多变量反馈控制

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The integration of renewable energy sources has led to a continuous increase of complexity, design and management of power systems. The installed process controllers have to be designed in such a way, which can simultaneously offer a high degree of flexibility and at the same time, should bear in mind safety and lifetime of the plant crucial elements. The process controllers have to comply with the requirements imposed by load change demands preserving the grid stability and frequency, which is processed in primary grid control. In particular, control of the steam drum unit water level is very critical for boiler operation, and the controller should maintain it within safety limits over the complete operating range of the boiler. The steam generation process using the drum unit is a multiple-input multiple-output (MIMO) system, with a strong coupling between its input channels and non-linear dynamics depending on the operating point. Additionally, it has a non-minimum phase behaviour, leading to an initial inverse response of the water level, which is associated with the shrink and swell physical phenomena of steam bubbles under the water level. This makes the control task extremely challenging with classical techniques using decentralised PID control loops, which regulate feedwater and steam flow rates separately from each other. The industry standard 1-, 2- and 3-element cascaded architecture PID controllers can be found in nearly all current power plants [1]. Although they can behave fairly well with low load changes (≤20 MW) with long time interval between each transition, their performance can become unsatisfactory for higher steps (≥30 MW), thus resulting in large deviations in the system response. The modification of the controller parameters to improve the system performance is not a straight-forward task due to the strong coupling between different actuating variables. The adjustment of a particular loop, without considering the effects on the overall multivariable system, can result in instability of the plant and even worse tripping of the boiler unit. Statistically speaking, it was reported recently, that 30% of emergency shutdowns in power plants are caused by poor level control of the steam drum unit [2].
机译：可再生能源的整合导致电力系统的复杂性，设计和管理不断增加。已安装的过程控制器必须以这种方式设计，这种方式可以同时提供高度的灵活性，同时应牢记安全性和工厂关键元件的使用寿命。过程控制器必须遵守负载变化要求所施加的要求，以保持电网的稳定性和频率，这在一次电网控制中进行处理。特别地，对汽包单元水位的控制对于锅炉的运行非常关键，控制器应将其保持在锅炉整个运行范围内的安全范围内。使用硒鼓单元的蒸汽产生过程是一个多输入多输出（MIMO）系统，其输入通道之间的耦合很强，并且取决于工作点的非线性动力学。另外，它具有非最小的相位行为，导致水位的初始逆响应，这与水位下蒸汽气泡的收缩和膨胀物理现象有关。这使得使用分散式PID控制回路的经典技术的控制任务极具挑战性，该回路分别调节给水和蒸汽的流量。在几乎所有当前的发电厂中都可以找到行业标准的1、2和3元素级联体系结构PID控制器[1]。尽管它们在低负载变化（≤20MW）且每次过渡之间的时间间隔较长时表现良好，但对于更高的阶跃（≥30 MW），其性能可能无法令人满意，因此会导致系统响应出现较大偏差。由于不同执行变量之间的强耦合，修改控制器参数以提高系统性能不是一项直接的任务。在不考虑对整个多变量系统的影响的情况下，对特定回路的调节会导致设备不稳定，甚至导致锅炉单元跳闸。从统计上讲，最近有报道说，发电厂中30％的紧急停机是由于对汽包单元的液位控制不当引起的[2]。

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