Operational Transients of Hydraulic Machinery - A New Test Rig

Due to the increase of fluctuating electricity generation by wind and photovoltaic power, the stabilization of the electrical grid is increasingly becoming a focus of research. The question of the extent to which hydropower technology is able to provide balancing power on a large scale in the shortest possible time requires the investigation of the dynamic behavior of hydraulic machinery. A new high-performance and highly flexible test rig is being set up in the institute's laboratory to investigate these operational transients such as, e.g. start-up sequences.

The new test rig was built as a closed-loop system in addition to the existing test facilities in the laboratory at the IHS. The test rig is designed with a pressure rating PN 25 on the pressure side and a pressure rating PN 10 on the suction side. The majority of the piping has a nominal diameter of DN 500. The layout of the loop and the spatial position of the components is presented in Figure 1. The test rig consists of two pump units that can operate either individually, in parallel or in series to cover the widest possible operating range. With this setup, the potential operating range on the test machine can achieve a head of up to approx. 140 m water column and a maximum flow rate of more than 1 m³/s.

The pumps are connected on the suction side to the horizontally adjustable tailwater tank via pipes. The tailwater tank is equipped with an air-pressurized dome to allow adjustment of the static pressure for cavitation studies. A vacuum pump is connected to the dome to lower the pressure level below atmospheric pressure. Additionally, there is a compressed air connection to raise the pressure level.

On the pressure side, the pumps are connected via a pipeline system to the headwater tank. A magnetically inductive flow meter (IDM) is installed in this pipeline system for volume flow measurement. The headwater tank, which is adjustable in height and laterally, is also equipped with a dome. The headwater tank has the connection for the pipe from the pumps on one end. On the other end, the pipeline or the inlet to the pressure-side connection of the test machine is attached.

The hydraulic connection of the test machine is between the headwater and tailwater tanks. The test machine is attached to four concrete columns and connected to a speed-controlled motor generator. Both rotation directions are selectable, allowing for four-quadrant operation of the test machine. The electrical energy generated during turbine operation is fed into the DC link and absorbed by the simultaneously operating drive motors of the pumps.

To ensure flexible operation, various valves (shown in light blue in Fig. 1) are installed, allowing switching between different operating modes. An additional bypass line with a further IDM is noticeable, being inserted parallel to the test machine and therefore also parallel to the pump units. This pipeline is used, for example, to examine a test machine in pump mode. For efficiency reasons, an energy recovery turbine is installed in this line, which also feeds into the DC link. Another short line with an ring piston valve is installed parallel to the energy recovery turbine. This pipeline section with the energy recovery turbine and the ring piston valve greatly expands the operational flexibility of the test rig.

In addition to the investigation of the flow characteristics of steady state operating points, the investigation of operational transients is also planned and feasible (see Fig. 2). Since the operating pumps as well as the energy recovery turbine and the ring piston valves, can be specifically regulated and controlled during operation, a replication of transient operating conditions of the prototype plant on the test machine is possible with this arrangement. This includes start and stop processes, but also, for example, operational transients from pump to turbine  mode and, in particular, vice versa. All types of hydraulic machinery enable operation at variable speed allowing the adjustment of the conditions on the test machine for the investigation of all four quadrants of it.

We express our gratitude to the German Research Foundation DFG and to the Ministry of Science, Research and the Arts of Baden-Württemberg, as well as to the University of Stuttgart for funding this large-scale research device.