Study On The United Transient Process For Hydraulic, Mechanical And Power System

In hydropower systems, when the plants' operation status changes, the transient process will happen in the flows from upstream reservoir to downstream channel and in the operating hydraulic machines, even in the electrical instruments, even the whole electrical system. Moreover, those are interconnected and influence the others. So the transient process should include three aspects, i.e. hydraulics, mechanism and electricityAccording to the definition by Chinese researchers, the transient process in hydropower plants can be divided into three parts, namely great fluctuation process, slight fluctuation process and hydraulic disturbance process. In short, the abbreviations can be noted as great fluctuation, slight fluctuation and hydraulic disturbance. Generally, great fluctuation is independent with electrical system or components, because when mechanical failure or accident occurs which make oil switch in generators cut off, the units enter the phase of load rejection. Now the electromagnetic valves for emergent stop in governors are of some kinds of linkage, so the automatic control parts in governors will be cut off, which cause the servomotors in charge of wicket gates will be closed quickly as fixed rules. Therefore, great fluctuation is actually a coupled transient process by hydraulics and mechanism.Slight fluctuation means some small changes in hydro-system just as its name implies. For instance, when the units incorporate in power grids simultaneously, the control system usually accepts some slight disturbance signals discontinuously, so as to cause the units' frequency approach the grid frequency gradually; the units with loads usually accept some load disturbances outside discontinuously, sequentially let the frequency depart the specification, and hence, the control system will regulate it into the required range consecutively. So slight fluctuation is actually a coupled transient process by hydraulics, mechanism and electricity, or united hydraulic-mechanical-electrical process in short.Slight fluctuation is caused by some small disturbances of outside loads, and contrastively hydraulic disturbance will happen between two or more units, which are linked by hydraulic system (say one surge tank shared). When one or more of units are throwing loads suddenly for the sake of accidents, the rotate speeds will increase and servomotors will shutter the wicket gates quickly. Currently, the discharge in turbines will change drastically, which consequently causes water hammer in penstocks. So other units will definitely be affected. If the units are stopped for emergency and the other units are still running under control of governors, thenhydraulic disturbance will cause some fluctuations in those units' power that are against the correspondence with the loads and afterwards the governors will work to control automatically. So, hydraulic disturbance is a combination of great fluctuation and slight fluctuation. However, for running units, the types and quantities of disturbance in hydraulic disturbance are different from those in slight fluctuation.Since the research for water hammer problems by Italian engineer Menabrea from 1858, people have done a lot of jobs and discussions in hydraulic transient process. From 80's in the 20th century, based on the theories of Willey and Streeter, Wuhan University of Hydraulic and Electrical University (WUHEE), Tsinghua University (TSH), Hohai University (HHU) and Chengdu Institute of Technique (CDIT) have all put much energy into the programs for transient process in hydropower stations. Those programs not only satisfy the practical requirements for designs, but also solve quite a lot of complex computation puzzles like optimization for the closure rules of wicket gates, the installment altitude of turbines, clusters of surge tanks and great fluctuation simulations in complicated conduit systems. In comparison with the practical operations in built plants to the numerical simulations, the computation theory for great fluctuation is nearly perfect.As we all know, the electrical consumers are very strict to electricity quality. First of all, power failures are absolutely prohibited during power supply, and then the voltage and frequency should keep stable, because if their varieties are bigger, not only the products quality of some customers will be influenced, but also some electrical devices will be damaged. So, the control system for hydro-units must keep steady with good adjustment characters. This is the main objective for researching slight fluctuation and hydraulic disturbance. Till now, the previous study mainly focused on the controls only for water inertia in conduit systems and rotate speed changes of units. For generators themselves, only mechanical inertia moment and load self-regulation coefficients are considered. The transient characteristics in generators and grids are not explored deeply during rotate speed changing. For example, during the change of rotate speed, namely of frequency, the terminal voltage of generators also changes accordingly. The whole electrical system is in a dynamic situation, where the regulation for voltage will play roles in controlling the electrical transient process. Therefore, it is very necessary to generalize and combine the hydraulic, mechanical and electrical transient process together in order to make clear the mechanism in the slight fluctuation and hydraulic disturbance problems.TSU has compared three hydraulic models, i.e. elastic water hammer model, rigid water hammer model and simplified model to research the hydraulic system's influence on low frequency oscillation and transient stability in electrical system. HHU also did some jobs onlow frequency oscillation in electrical system. They adopted four hydraulic models suitable for the stability analysis of slight disturbance, i.e. stable mechanical power model, simplified hydraulic model, linear turbine model in the sense of elastic water hammer and rigid water hammer respectively. They computed and analyzed the low frequency oscillation problem in electrical system. Although all these jobs implemented kinds of combination of hydraulics, mechanism and electricity, their emphasis were merely put on the influence of hydraulic system to electric system.Dr. Chen, Y. Y is the first people who really came true to take the hydraulic, mechanical and electrical interaction mechanism into consideration by and large. In his PhD thesis, he discussed the influences on the governors' adjustment process by electromagnetic transient process and the influences on electrical transient stability by hydraulic system, by the control of turbines' rotate speed and by excitation adjustment parameters etc. The basic idea is acceptable, but still worth of deliberating.As the rapid development of hydropower in China, underground and long tailrace plants become more and more popular. For satisfying the requirements, tail-surge tanks are usually necessary. Many of those building or to be built plants stand key positions in grid, which not only should supply main electricity power, some of those will also sustain to supply the peak loads or adjust peak frequency. So those plants need excellent stability and adjustment character to guarantee the high electricity quality. From this point, the design for tail-surge tanks will also made allowances for the adjustment character in plants.However, as mentioned before, most researchers didn't consider the surge tanks when dealing with the hydraulic-mechanical-electrical process. The author of this thesis will explore and discuss those new practical problems theoretically based on existent research fruits. The main contents are listed as following:1) Collect, clear up and summarize the existent fruits, analyze the mathematical model of ? control system in hydro-units, especially those models of controlled objectives. Built anon-linear mathematical model including hydraulic transient process, mechanical operation process and electrical transient process, and compile a computation program.2) For those plants without surge tanks, research how the excitation adjustment, governor regulation and hydraulic system will influence the adjusting traits of rotate speed under 5% (10%) stepped disturbance of rated load. Compare the differences in the simulation of slight fluctuation by using three different generator's models, i.e. stable moment, stable power and third-order models. So go deep into the ways on how high and mediumfrequency adjustments will act on the oscillatory process with medium frequency by surge wave.3) For the inapplicability of the setting-up condition for surge tanks in current criterion to the practical projects, provide the new setting-up conditions for tail-surge tanks.4) For the plants with surge tanks, research how the excitation adjustment, governor regulation and hydraulic system will influence the adjusting traits of rotate speed under 5% (10%) stepped disturbance of rated load. Analyze and compare the differences in the simulation of slight fluctuation by using three different generator's models, i.e. stable moment, stable power and third-order models respectively. So go deep into the ways on how high and medium frequency adjustments will act on the oscillatory process with low frequency by mass wave.5) For the plants with surge tanks, research how the section areas of the surge tank and the load self-adjustment ability will influence the adjusting traits of rotate speed under 5% (10%) stepped disturbance of rated load. So penetrate into the ways on how low frequency adjustments or other measures will act on the oscillatory process with low frequency, hence provide a reference for surge tank designs.6) Analyze and compare the differences of hydraulic disturbance process in running units between isolated grid and infinite grid, and compare the simulations by using stable moment, stable power and third-order generator's models under those two situations respectively.7) Research the influence on hydraulic disturbance by section areas and the location of surge tanks and governor's parameters so as to seek the measures to improve the adjusting character in those plants with surge tanks, and meanwhile, provide another reference for surge tank design as well.Finally, summarize the whole thesis and point out the future study directions.