Characteristic Analysis On Energy Storage And Control Methods Of Rapid Load Change For Heat Supply Units

Large-scale new energy integration makes the problem of grids’ frequency control and peak load regulation much severer. Increasing the operating flexibility of coal-fired power plant is an effective way to solve the problem. Traditional operation and control of coal-fired power plants focus on index such as safety, efficiency, environmental protection, and so on; while goal of increasing the operating flexibility is to realize rapid and deep load change of the units. Under the present control level, if the coal-fired power plant keeps on increasing load change rate, it will lead to severe fluctuation of main parameters such as fuel, main steam pressure, and so on, which is harm to unit’s safe and stable operation. The essential cause of this phenomenon is that:boiler energy storage is limit, while fuel energy responses slow which cannot supplement boiler energy storage in time. Therefore, the key point to realize rapid load change for coal-fired units is making full use of energy storage in units by optimization of the control system.By quantitative calculation and frequency characteristic analysis on energy storage in boiler steam-water system, condensate water systems of coal-fired units and in heat supply net circulating water systems of heat supply units, it is found that energy storage in heat supply net is much larger than other units’ energy storages, and changing heat supply regulating valves can change power load rapidly. Moreover, experiments show that using net energy storage in short time duration has little influence on heat consumers. Nevertheless, plenty of energy storage in heat supply net is not utilized due to units’ operation mode of fixing power load by heat load. Building heat supply units’control model is the foundation of comprehensively unitization of energy storage. By mass and energy balance analysis, simplified dynamic model of units including heat supply regulation methods was built. Then, on the basis of feedforward and feedback control which is widely used in engineering, coordinated control schemes for heat supply units were designed. Analysis on energy storage shows that different inputs’ energy has complementary characteristic scales. If the control assignment is decomposed, separate control to complement advantages will be derived. On the basis of this multi-scale perspective, a signal decomposition method was proposed to decompose load command or controller output into complementary signals. Matching scales of the signals and inputs’ energy, feedforward and feedback coordinated control schemes were derived.Conclusions and achievements are as follow:(1) By mechanism analysis, a set of energy storage capacity calculation method was summarized. This method does not need experiments; while only need heat supply units’ designed data and water-steam thermodynamic property data. Analysis show that, in space scale, energy stored in boiler, heat supply net and fuel increases in sequence; in time scale, their responding time to throttle pressure and unit’s load delays in sequence.(2) A rate limit multi-scale decomposition method was proposed, which can decompose input signal into multi-scales by amplitude and rate of the input’s variation.(3) Simplified dynamic model of a heat supply unit was built. The model has two kinds of heat regulating inputs which makes the model closer to engineering practice. And experiment data validates the accuracy of the model.(4) On the basis of signal multi-scale decomposition and energy storage multi-scale analysis, matching control assignments and control inputs by corresponding scales, feedforward and feedback multi-scale control schemes were proposed. Simulation show that, heat supply units can raise load responding rate by these control schemes.According to the theoretical achievements, engineering control schemes were designed and tested on LPS thermal power plant. Experiments show that:with these schemes, the actual maximum load responding rate can reach to 4%Pe/min. This rate is much larger than 1.5%Pe/min set by grid, which can effectively relieve grid’s frequency control and peak load regulation pressure caused by large-scale new energy integration.