Converting the Malta Oberstufe pumped storage plant to variable speed with full converter during an overhaul

By Steve Aubert, Christoph Häderli, Christian Ladreiter-Knauss, Stefan Polster and Peter Steinmann

At its Malta Oberstufe pumped storage power plant, Verbund executed an overhaul that involved replacing the existing generator and hydraulic systems with new variable speed solutions. The utility used the latest converter fed synchronous machine (CFSM) topology, including a full-size direct modular multilevel converter (MMC) of the newest generation.

This article introduces the converter solution and provides information about operational experiences, benefits and grid compliance evaluations after the plant was returned to commercial operation in January 2022.

Introduction to the overhaul project

Verbund in Austria is one of the largest producers of electricity from hydropower in Europe, with a fleet of over 100 hydropower plants of different sizes and age.

The pumped storage power station Malta Oberstufe in the state of Carinthia was designed to connect Kölnbrein Reservoir (annual storage) with the main stage pumped storage plant and to pump water collected in Galgenbichl Reservoir to fill up Kölnbrein Reservoir. The plant was originally rated at 125 MW in turbine operation and 116 MW in pump operation. The plant had been operating for more than 40 years when Verbund initiated a major overhaul and refurbishment program.

To operate under the large head variation (see Figures 1 and 2), the original motor-generators were realized for a two-speed operation, i.e., 500 rpm at 70 MVA (12 pole configuration) and 375 rpm at 42.5 MVA (16 pole configuration). The original machines were of Isogyre-type, with both runners (pump and turbine) on the same shaft with one common spiral case, both designed for the specific requirements of efficiency and operability.¹

Figure 1. Overview of the Malta-Reisseck scheme with the Malta Oberstufe pumped storage power station and related reservoirs in the red square.

Figure 2. Operation area for a single machine before and after upgrade.

With the state-of-the-technology and proven maturity of variable speed technology for motor-generators, a variable speed pump-turbine solution — with advantages regarding hydraulic efficiency improvement by adjusting actual speed to actual head and higher operational flexibility with controlled power consumption also in pumping mode — was selected for the overhaul. The CFSM solution was chosen to get high hydraulic efficiency for an increase of production but also maximum flexibility in operation for grid services.

The overhaul included replacement of the pump-turbine arrangement with a reversible pump-turbine with an increased rated power of 80 MW in both pump and turbine mode. It is integrated into the existing spiral casing, reusing the maximum of the existing structures. The new generator (rated 80 MVA) and main transformer (85 MVA) are connected to the full-size converter (86 MVA) delivered by Hitachi Energy, enabling variable speed operation from 240 to 575 rpm.

The first unit was handed over for commercial operation in April 2021, after an overall downtime of 11 months. For the second unit, overhaul activity was completed within only eight months, with handover in January 2022.

Variable speed solution description

Recent technological progress on power electronic frequency converters has opened the door for a type of variable-speed pumped storage hydropower solution referred to as the CFSM. The CFSM produces variable shaft speed by supplying the stator of a synchronous motor-generator with a variable frequency. The latter is generated by a full-scale power electronic frequency converter, i.e., rated at the full power of the motor-generator. The solution has become state-of-the-art with the recent appearance of the MMC, which enables ultralow conversion losses and high rated power.

The CFSM topology increases the operational flexibility and efficiency of pumped hydropower plants and can provide a variety of ancillary services to the grid.² Many elements — such as grid circuit breaker, phase shifting disconnectors and other short-circuiter — are eliminated. The converter can control the speed from 0 to maximal operation speed, allowing fast start-up and transition times. For conventional operation, the variation range is defined by the hydraulic system. The converter ensures compliance with grid connection requirements, such as reactive power supply, response to grid failure events and provision of virtual inertia. Hence, the motor-generator simply transfers active power and is designed for power factor 1, allowing a reduction in size and price of the machine.

Figure 3 shows the principle of the direct MMC topology. Each input terminal is connected with a string of cells (phase leg) to each output terminal, forming a matrix with nine phase legs. The bipolar cells behave as a direct current (DC) voltage source, with related switching elements for the definition of positive, 0 or negative DC voltage at its output. The DC source is a capacitor, and for the discussed project, the switching elements are reverse conducting integrated gate-commutated thyristor semiconductors. Connecting a sufficiently large number of cells in series allows to generate an output signal close to the targeted sinusoidal wave for the electrical load with high efficiency. It allows a converter voltage design increasing almost linearly with the rated power, offering a solution fitting with the machine design, for even a large unit power up to 300 MW.

Figure 3. Simplified diagram of direct MMC converter.

The direct MMC topology is a matrix conversion solution without a common DC link. A specific characteristic of this topology is related to a frequency difference between both converter sides to ensure rated power operation. With the grid frequency of 50 Hz, the machine side frequency can be up to 40 Hz. Consequently, the selection of the number of machine poles needs to be applied for rated speed operation.

System operation characteristics and performances

The general characteristics and performance parameters have been verified upon commissioning and site acceptance tests.

The machine side voltage waveform is crucial for compatibility with the machine. Notably, the limitation of reoccurring voltage peaks and dv/dt (rate of voltage change over time) are important for the machine’s lifetime. Figure 4 shows the measured machine side voltage and current waveforms. The measurements match well with previous simulation results, confirming the switching characteristics and suitability of the chosen modulation scheme.

Figure 4. Measured machine side voltage in kV on left and machine side current waveforms in A at 29 MW in turbine mode on right.

To get the converter connected to the grid, the machine remains in standstill. The converter cells are charged, the transformer is magnetized, and grid voltage is built up and synchronized to busbar voltage. Closing of the high-voltage circuit breaker is bumpless (no transformer inrush) (see Figure 5, negative time).

To disconnect from the grid, the converter ramps the currents to 0 and the high-voltage circuit breaker is opened close to 0 current. Afterwards (t>45s), the transformer is demagnetized and converter cells are discharged (see Figure 5, positive time, after main circuit breaker opening).

Figure 5. Start with charging cells and synchronization (negative time slot). Stop and discharge cells (positive time slot). Figure is merged from two independent recordings.

For the fast change from generator to motor mode, the machine is unloaded and brought to standstill using regenerative braking with the converter. Once at standstill, the sequences of the unit control system need a few seconds to initiate the start sequence into pump mode. Start-up and braking are coordinated with the governor control, which provides setpoints for speed and speed gradients to the converter. As soon as speed is in the operating area of the turbine/pump, the converter control is switched from speed control to power control (see Figure 6).

Figure 6. Fast changeover from generator mode to pump mode at top. Zoom on current (top) and voltage (low) at the end of the braking at bottom left and the beginning of the start-up process at bottom right. Phase reversal realized by converter.

The pump is started without dewatering and the frequency converter provides constant high torque from 0 to maximal operation speed, unlike with a doubly fed induction machine, where torque can be limited in start-up mode. Figure 6 shows a pump start. For system shut-down, mode transition (from/to turbine mode to/from pump mode) or changeover to VAR compensation mode, the machine needs to be braked quickly. After removing the primary power input by closing the guide vanes (and main valve in the case of a shutdown or changeover to VAR compensation mode), the electrical braking sequence of the static frequency converter (SFC) is initiated and the machine is brought to 0 speed within a few seconds (see Figure 7).

Figure 7. Start sequence in pump mode with guide vanes opening from t=43s on top and electrical braking sequence on bottom.

The point of connection (POC) at the Malta Upper Stage pumped storage plant is connected to the point of common coupling (PCC) at Malta Main Stage by a 42-km-long 110-kV overhead line. The PCC is a strong network node connected to the 220-kV transmission grid and the 110-kV distribution grid with strong local generation. This setup with a low impedance at the PCC and a high impedance at the POC, combined with the inherently low harmonic distortion of the MMC (high number of levels), results in a low harmonic content at the PCC.

The overhead line also impacts resonances in the network, which could be an issue when excited by the converter. This must be considered for the harmonic mitigation design of the SFC. The choice of a suitable modulation method (flux error compensation method, see also ³) and appropriate switching frequency made it possible to choose a filter-less approach for Malta Oberstufe. The assessment of the harmonic performance is done at the PCC, see single line diagram Figure 8. Therefore, portable measurement equipment with a sampling rate of 50 kHz was installed and different operation points across the possible active power range in turbine and pump mode were recorded.

Figure 8. Single line diagram of the grid connection of Malta Upper Stage (DMO) at the switch gear of Malta Main Stage (VMH). The diagram is simplified to only highlight the local generation connected at VMH.

The depicted data in Figure 9 refers to the maximum 10 min-moving-average values of the voltage and current harmonics. Hence the results correspond to a worst-case-scenario with the highest distortion levels in all harmonics at the same time. The base harmonic data is calculated within the measurement software according to IEC 61000-4-7.⁴ The allowed level of the single harmonics is set by the grid operator to half of the planning values of the IEC 61000-3-6.⁵

Figure 9. Envelope of harmonic distortion at Malta Main Stage for turbine mode and pump mode. The depicted data refers to the maximal 10 min-moving-average of the 5-seconds distortion values calculated according to IEC 61000-4-7.⁴

The typical 6-pulse harmonics (5th, 7th, 11th, etc.) are caused by other sources of distortion in the network, as could be shown by no-load measurements (SFC not in operation and disconnected). The distortion caused by the SFC is in the higher order range (starting from around the 20th) and remains constantly low (<0.2%) for all orders (voltage and current distortion). The current spectrum shows a peak at a system resonance located near the 43rd order but remains within the imposed limits. There is little dependency on the operating point, so that the picture remains similar throughout the system’s operating range.

The comparison between one and two SFCs in operation shows that the overall distortion level does not change significantly, but the resonance frequency shifts slightly to higher orders. The differences in the total harmonic distortion (THD) values between pump and turbine mode are explainable by changing operation settings of the surrounding power plants (see Table 1).

Table 1. Maximum 10 min-moving-average values of current and voltage THD at PCC.

Summarizing the results, the SFC does not exceed the harmonic distortion levels set by the grid operator. Further, there is little impact of the point of operation and the number of SFCs in operation.

Efficiency improvement

One of the main project objectives was to increase hydraulic efficiency.¹ This is achieved by operating the turbines at an optimum speed depending on the water head. Verbund also targeted to be able to continuously vary the power in turbine and pump mode, and consequently provide balancing energy to the grid (see Figure 2).

In turbine mode, the original machines could be operated at 375 rpm or 500 rpm depending on relative water head, with hydraulic efficiency between 85.6% (at 375 rpm) and 87.6% (at 500 rpm). With the large speed variation range provided by the CFSM solution, the variable-speed machines reach a maximum hydraulic efficiency of about 94.5% over the whole range of head, when operated at the optimal speed. The hydraulic efficiency in turbine mode is above 86% over the whole continuous output power range, compared to 77% of the original machines near the switchover point from 375 rpm to 500 rpm.

In pump mode, the advantages depict a similar trend. While the old pumps had an efficiency range of 70% (at 50 m water head and 375 rpm) to 87% (at 190 m water head and 500 rpm), the new pumps reach a hydraulic efficiency of 92% over the whole range of head when operated at optimal speed.

Besides the increased efficiency in pump and turbine mode, the new variable speed machines add a powerful possibility to provide balancing energy to the grid when operating in pump mode. The maximum hydraulic efficiency in this mode is 92.75% at high water head and does not go below 84% for all possible operating points in pump mode.

To sum up, the average increase in hydraulic efficiency thanks to the use of modern variable-speed pump turbines in combination with a SFC is 7% to 9% in turbine and 6% to 22% in pump mode. In addition, the balancing power window can be increased in turbine mode and the provision of balancing energy in pump mode is possible with the new design. All these aspects help to decrease the payback time for the project.

Operation flexibility

In today’s energy systems and market situations, the operational flexibility of power plants becomes more important. The chosen CFSM configuration gives the opportunity to increase the operational area in the turbine mode by avoiding hydraulic constraints using different speeds. However, more important is the fact that with the CFSM the pump load can be controlled. This allows the implementation of grid-supporting measures such as frequency support and primary control functionality in pump mode too and a higher flexibility for an optimal unit commitment.

In Figure 10, the pump operation of both generators of Malta Upper Stage are evaluated for a summer month for 2017 to 2022. The depicted data show the sorted 5-min mean values of the active power of the generator units. Before the change to CFSM configuration, the pump power is basically defined by the net head (see Figure 2). This results in the quasi-straight active power lines for 2017 to 2019 for unit M1 respectively 2017 to 2020 for unit M2. Focusing on the lines corresponding to the years after the overhaul, it can be seen that the pump load is varying, and the possibility offered by the CFSM is actively used in the unit commitment.

Figure 10. Evaluation of operation hours in pump mode and their related consumed active power for the related unit – evaluation for a summer month.

As can be observed in Figure 10, the operation time increases after the overhaul. This is caused by the higher flexibility but also influenced by external events such as the energy crisis and electricity prices.

In Table 2, the number of starts per 15 minutes decreases. This is caused on the one hand by the increased operation time in the evaluated time span leading in general to a more continuous operation. On the other hand, the number of starts per 15 minutes is affected by the fact that with the CFSM configuration, a controlled reduction of the pump load is possible, allowing a more flexible unit commitment and leading to longer operation time periods.

Table 2. Overview of number of starts per machine and the related evaluation per 15 minutes.

Conclusion

Completion of the Malta Oberstufe overhaul project confirms the advanced benefits and features the variable-speed CFSM solution can provide. Thanks to an appropriate design including electrical and hydraulic systems at an early project stage, numerous benefits could be realized, making the power plant ready for today’s and future challenges, such as grid stability or the need for operational flexibility. At the same time, the practical use of the direct MMC for hydro pumped storage has been successfully demonstrated in a commercial plant.

Notes

¹Mayrhuber, J., et al, Upgrading of hydropower plants for the EU-Green Deal: efficiency upgrade projects for more generation and flexibility, Hydro 2022 Conference.

²Haederli, D., et al, Project Malta Upper Stage – World’s First Direct Modular Multilevel Converter for Variable-Speed Pumped Storage Hydropower, Hydro 2022 Conference.

³Vasiladiotis, M., et al, “IGCT-Based Direct AC/AC Modular Multilevel Converters for Pumped Hydro Storage Plants,” 2018 IEEE Energy Conversion Congress and Exposition (ECCE), pp. 4837- 4844.

⁴IEC 61000-4-7

⁵IEC 61000-3-6

The authors are listed in alphabetical order, with all contributing equally. Steve Aubert is sales specialist hydro in the Global Center of Competence and Christoph Häderli is a principal system engineer for larger power conversion projects, both with Hitachi Energy Ltd. Christian Ladreiter-Knauss is project manager for the electrical part of the Malta Oberstufe upgrade and Stefan Polster is a power systems expert, both with Verbund Hydro Power GmbH. Peter Steinmann is a principal engineer with Hitachi Energy Ltdand was commissioning leader for both converter systems at Malta Oberstufe.