Pumped storage system model and experimental investigations on S-induced issues during transients

Highlights

• A pumped storage system model with two different pump-turbines is developed.

• Runaway tests are made to get characteristic curves and study instability issues.

• Load-rejection tests are conducted to study the S-induced water-hammer pressure.

• Relevant theories and numerical analyses on S-shaped characteristics are verified.

Abstract

Because of the important role of pumped storage stations in the peak regulation and frequency control of a power grid, pump turbines must rapidly switch between different operating modes, such as fast startup and load rejection. However, pump turbines go through the unstable S region in these transition processes, threatening the security and stability of the pumped storage station. This issue has mainly been investigated through numerical simulations, while field experiments generally involve high risks and are difficult to perform. Therefore, in this work, the model test method was employed to study S-induced security and stability issues for a pumped storage station in transition processes. First, a pumped storage system model was set up, including the piping system, model units, electrical control systems and measurement system. In this model, two pump turbines with different S-shaped characteristics were installed to determine the influence of S-shaped characteristics on transition processes. The model platform can be applied to simulate any hydraulic transition process that occurs in real power stations, such as load rejection, startup, and grid connection. On the experimental platform, the S-shaped characteristic curves were measured to be the basis of other experiments. Runaway experiments were performed to verify the impact of the S-shaped characteristics on the pump turbine runaway stability. Full load rejection tests were performed to validate the effect of the S-shaped characteristics on the water-hammer pressure. The condition of one pump turbine rejecting its load after another defined as one-after-another (OAA) load rejection was performed to validate the possibility of S-induced extreme draft tube pressure. Load rejection experiments with different guide vane closing schemes were performed to determine a suitable scheme to adapt the S-shaped characteristics. Through these experiments, the threats existing in the station were verified, the appropriate measures were summarized, and an important experimental basis for the safe and stable operation of a pumped storage station was provided.

Introduction

Pumped storage stations are widely used to store electrical energy. They perform peak regulation and frequency control of a power grid as well as enable developing renewable and intermittent energy sources, such as wind power and solar energy [1]. To adjust the power system more efficiently and in a timely manner, pumped storage stations undergo numerous transition processes, which require the units to rapidly switch to steady states while maintaining the key parameters within the control range. Many engineering incidents occur in the process, such as water column separation in the draft tube [2], [3], unsuccessful grid connections under low head, and broken rotor bar faults caused by a high runaway speed. Therefore, the stability and security of pumped storage station are significant restrictions. The pump turbine is the most crucial component of a pumped storage station and determines the characteristics of the station.

The S-shaped characteristics of the pump turbine are the critical factor determining the transient characteristics of pumped storage stations. During the transient process such as load rejection, the operating point of the pump-turbine goes though turbine-working (before runaway (zero torque)), braking (from runaway to zero flow), and reverse-pumping (after zero flow) stages, forming the S-shaped region. Static tests of pump turbines [4], [5] and computational fluid dynamics numerical methods [6], [7] can reveal the complex hydraulic phenomena of the internal runner when a pump turbine is operating in the S region, such as stationary vortices, unsteady vortex, and the rotating stall that results in the blockage effect; all these phenomena are fundamental causes for the inversion of the S-shaped characteristic curve. Moreover, the S-shaped characteristics also lead to an unstable runaway, no-load operations [8], [9], [10], [11], as well as water-hammer pressure surge after load rejections. To solve these problems, numerical methods have been applied, such as the method of characteristics (MOC) [12], [13], MOC coupling with the implicit scheme [14], the coupled liquid-gas model [15], the equivalent circuit method [16], [17], and the coupled 1D-3D model [18].

In addition to numerical methods to analyze the transient issues in pumped storage stations, field experiments can also be conducted to obtain additional verification. Dörfler et al. closed the part valve in the field to achieve pump turbine no-load stability [19]. Yang et al. analyzed the pressure pulsation characteristics of a prototype pump turbine during a transition process [20]. The transient experiments of load rejections and power failure must be performed in the debugging stage to ensure safety and stability [21]. However, owing to the risks involved in field experiments, it is difficult to conduct experiments for specific engineering problems and some extreme conditions are impossible to obtain. In addition, parameters such as system inertia cannot be changed once the power station is established, which restricts the investigations.

To compensate for the limitations of field experiments, the corresponding model experiments can be developed. The existing model experiment platform for hydraulic machineries is generally a closed-loop test rig with upstream and downstream pressure tanks [22], [23], as shown in Fig. 1. The model has a large water head range and is mainly used to test the turbine characteristics in the constant state, such as energy characteristics, pressure pulsation characteristics [4], [5], and the draft tube vortex [24]. The platform can accurately measure the hydraulic properties of model runners and has played a key role in runner design and power station design. While the platform is built to only examine turbine properties, the absence of other systems such as the piping system makes it difficult to study the transient issues.

To study the transient problems of a high-head mixed-flow turbine, the Norwegian University of Science and Technology (NTNU) changed the piping system on the basis of the closed-loop test rig to set up the open-loop test rig [25], as shown in Fig. 2. The model is used not only to test the turbine characteristics [26] and the pressure pulsation of no-load operations [27] but also to measure the pressure pulsations during startup and load rejections with different closing and opening schemes [25], [28], [29], [30], [31]. However, some complex transition processes in actual power stations, such as OAA conditions, are difficult to achieve with this experimental platform.

In summary, to investigate the transient characteristics of pump turbines and pumped storage stations, a more intact pumped storage system is indispensable. The dynamic process of a pumped storage station is complicated: (1) The unstable pump turbine S-shaped characteristics make it difficult to measure the S-shaped curves, which can, therefore, be obtained by the transient method [32], [33]; (2) pump turbines often show an unstable runaway and no-load operations, which are close to the form of S-shaped curves [8], [9], [10], [11]; (3) in the load rejection process, the water-hammer pressure is also related to the S-shaped curves [34], [35]; (4) in some extreme conditions, such as OAA load rejections, numerical simulations have shown that the water-hammer pressure, especially in the draft tube, is fairly significant [35], [36], [37], [38]; (5) to control the water-hammer pressure and runaway speed during the load rejection process, multi-phase closing schemes can be used to guarantee the safety of a pumped storage station [39], [40]. Therefore, based on an actual pumped storage station, a pumped storage system was set up in accordance with a certain scale. Based on the transient issues described above, the relevant verification experiments were conducted and appropriate engineering measures were obtained.