Full-size Converter Research Articles

Fault Ride-Through Control Strategy for Variable Speed Pumped Storage Unit with Full-Size Converter

Fault Ride-Through Control Strategy for Variable Speed Pumped Storage Unit with Full-Size Converter

Deployment of a full‐size converter utilised hydropower plant to enhance inter‐area oscillation damping

AbstractDue to the increasing share of inverter‐based renewable energy sources and the increased retrofitting of conventional hydropower plants with power electronics, novel possibilities arise for providing damping power in case of occurring inter‐area oscillations in power systems. This paper presents a complete model of a variable‐speed hydropower plant (VSHP) in a single‐machine infinite bus system. The demonstrated analytical model is applied for small‐signal stability analysis that contains all relevant converter dynamics and is verified by an additional non‐linear time‐domain simulation. The damping controller approach located in the full‐size converter (FSC) consists of the classical power system stabilizer with additional phase compensation, to which an appropriate wide‐area input signal is fed via a phasor measurement unit device. Subsequently, the FSC‐VSHP with additional damping algorithm is examined with respect to the significantly higher damping performance with regard to inter‐area modes in a three‐machine model.

Modeling and Operating Characteristics of Excitation System for Variable Speed Pumped Storage Unit with Full-Size Converter

The variable speed pumped storage unit with a full-size converter (FSC-VSPSU) can provide fast and flexible regulation capacity for the power grid, assisting the rapid development of the new energy-dominated power systems, and its application is gradually becoming widespread. The excitation system of FSC-VSPSU is crucial for maintaining the motor-port voltage and further supports the stable operation of the entire unit. However, there is currently no research on excitation system modeling, and their essential operating characteristics have not been discussed in detail, which needs further study. With this aim, this paper firstly establishes a detailed transfer function model for FSC-VSPSU, which is then used to study the influence rules of FSC-VSPSU parameters on the operating characteristics. In addition, based on the summarized operating characteristics, the excitation system can be optimized, and its complexity is greatly reduced while ensuring the original performance of the unit. Finally, the results of the simulation can verify the correctness of the established model and the explored operating characteristics.

Stability characteristics analysis and ultra-low frequency oscillation suppression strategy for FSC-VSPSU

The research on control strategies of the variable speed pumped storage unit with full size converter (FSC-VSPSU), mainly focuses on the rapidity of power regulation. But the speed oscillation and slow speed regulation performance caused by fast power regulation are ignored, which is not conducive to the safe and efficient operation of the unit. This article investigates the dynamic stability characteristics of the FSC-VSPSU in detail. In turn, the stability risks are clarified and serve for the design of its control strategy and the improvement of operational efficiency. The small-signal model of FSC-VSPSU considering the variation of water head and load in fast power mode is established. The dominant instability control link and key parameters are revealed based on the eigenvalue analysis. Oscillation mode analysis shows that the unit has a wide-band oscillation characteristic. The dominant oscillation mode is the ultra-low frequency oscillation (ULFO) caused by the governor control system. To suppress the ULFO and improve the optimal speed tracking performance of the unit to the greatest extent, a coordinated control strategy of governor variable parameter control and governor power system stabilizer (GPSS) is proposed. The electromagnetic transient simulation based on PSCAD verifies the effectiveness of the proposed measures.

Suppression strategy of coupled oscillation and power anti‐regulation of variable speed pumped storage unit with full size converter

AbstractThe ultra‐low frequency oscillation and poor speed regulation performance caused by the fast power control decrease the stability and efficiency of the variable speed pumped storage unit with a full size converter (FSC‐VSPSU). This paper presents a fast frequency control strategy to solve this problem. The small‐signal model of the FSC‐VSPSU in fast frequency mode is established, and the key parameters for the system stability are studied by the eigenvalue method. Aiming at the coupling oscillation mode caused by the governor and generator side converter (GSC), the corresponding control parameters optimisation method is proposed. In order to solve the power anti‐regulation problem, the strategy of combining rotor kinetic energy release, power slope adjustment, and delay acceleration control is proposed. PSCAD and RTDS simulation results show that the proposed strategy can effectively avoid the ultra‐low frequency oscillation caused by the fast power control and significantly improves the frequency and power control performance.

Investigation of Dynamic Loads in Wind Turbine Drive Trains Due to Grid and Power Converter Faults

Electrical faults can lead to transient and dynamic excitations of the electromagnetic generator torque in wind turbines. The fast changes in the generator torque lead to load oscillations and rapid changes in the speed of rotation. The combination of dynamic load reversals and changing rotational speeds can be detrimental to gearbox components. This paper shows, via simulation, that the smearing risk increases due to the electrical faults for cylindrical roller bearings on the high speed shaft of a wind turbine research nacelle. A grid fault was examined for the research nacelle with a doubly fed induction generator concept. Furthermore, a converter fault was analyzed for the full size converter concept. Both wind turbine grid connection concepts used the same mechanical drive train. Thus, the mechanical component loading was comparable. During the grid fault, the risk of smearing increased momentarily by a maximum of around 1.8 times. During the converter fault, the risk of smearing increased by around 4.9 times. Subsequently, electrical faults increased the risk of damage to the wind turbine gearbox bearings, especially on the high speed stage.

Dynamic characteristics of pumped storage unit based on the full-size converter

For enhancing the regulation performance and the efficient operation of the pumped storage power plant, this paper proposed to combine a hydro-generator set with a full-size converter to achieve the decoupling control of the machine and the power grid. On the basis of the model test, the regulation characteristics of a certain pump-turbine had been discussed and the implication for the converter was highlighted. At the same time, modeling and control for the machine side converter were presented, the simulation results showed that the hydroelectric generator system based on a full-size converter could track the torque change properly through the control system and achieve variable-speed operation due to the decoupling from the fixed grid frequency. The analysis revealed that it is possible to operate the plant beneficially by adopting a full-size converter between the generator motor and the power grid to improve the dynamic performance of the system effectively.

Generator Design Possibilities for Full-Size Converter Operation of Large Pumped Storage Power Plants

This article investigates the additional opportunities and aspects in the design process of large hydro power wound-field salient-pole synchronous machines provided by full-size converter (FSC) operation. As a matter of fact, thus far, power converters for hydro power plants have been mainly selected to meet the demands of conventionally designed machines and retrofit applications. The new optimization possibilities in the motor-generator design given by the FSC operation have not yet found much attention in the literature. This article shows how FSC operated hydro power synchronous machines can be designed to have smaller volumes, lower material cost, and higher efficiencies than their conventional counterparts. A cost evaluation of various generator designs including costs for materials as well as costs for power loss during operation is performed. Furthermore, the power loss evaluation cost factors are varied to show their impact on the choice of the machine parameters for an optimum generator design. For the presented evaluation, the partial load operation of large pumped storage power plants in turbine and pumping modes is analyzed, as is the effect of the free choice of the design frequency, the power factor, as well as the synchronous reactance. Finally, a suggestion for the generator design for FSC operation is presented.

A comparative survey of power converter topologies for full-size converter operation of medium-voltage hydropower generators

This paper discusses the challenges for today’s power converter topologies when operating large synchronous motor-generators in pumped storage power plants, which is a medium-voltage high-power application. First, the available ratings and characteristics of the utilized power semiconductor switches are analyzed. Then, the practical limitations of classical multilevel voltage source converters are reviewed, particularly the individual maximum achievable operating voltage. Next, the usability of direct AC–AC converters is discussed. Due to their arbitrary scalability and excellent performance, modular multilevel converter topologies gain special attention in this paper. The characteristics of the two main arrangements, the modular multilevel converter and the modular multilevel matrix converter, are examined with respect to the torque-speed characteristic of the installed reversible-speed pump-turbines. A review of commercially available power converters for medium-voltage high-power drive applications and their specifications as well as the identification of promising power converter topologies for operation in large pumped storage power plants complete this comprehensive analysis.

A Generic EMT-Type Model for Wind Parks With Permanent Magnet Synchronous Generator Full Size Converter Wind Turbines

Utilities are under considerable pressure to increase the share of wind energy resources in their generation fleet. With the increasing share of wind energy resources, the dynamic behavior of power systems will change considerably due to fundamental differences in technologies used for wind and conventional generators. There is a very little standardization in the ways to model wind turbines (WTs) and wind parks (WPs) in sharp contrast to conventional power plants. Hence, there is an international interest to deliver generic models (i.e. standardized and publicly available) for WTs and WPs that are able to capture all performance aspects as good as manufacturer-specific models. This paper presents an electromagnetic transient (EMT) simulation model for full-size converter (FSC) WT-based WPs that can be used for stability analysis and interconnection studies. The considered topology uses a permanent magnet synchronous generator. Although the collector grid and the FSC WTs are represented with their aggregated models, the overall control structure of the WP is preserved. FSC WT and WP control systems include the non-linearities, and necessary transient and protection functions to simulate the accurate transient behavior of WPs.

Analysis of the stability and dynamic responses of converter‐based generation in case of system splits

Here, a methodical research on the stability and response of a converter-based generation model is conducted, focusing on the accompanying effects of a system split scenario in the (European) electricity transmission network. The purpose of this research is to analyse the behaviour of a generic but detailed model in case of extreme contingencies. In particular the focus is set on reduced short-circuit power, high rates of change of frequency, and sudden voltage phase jumps at the point of common coupling (PCC). These effects are analysed by means of electromagnetic transient (EMT) simulations. In addition, the stability of the converter control is evaluated qualitatively. For the investigations carried out here, a single reduced wind turbine model with full-size converter is used focusing on the interactions with the accompanying phenomena of a system split. The results show that reduced short-circuit power as well as a large and sudden voltage phase jumps can be potential reasons for the converter system to disconnect from the grid.

Full-size converter operation of hydro power generators: a state-of-the-art review of motivations, solutions, and design implications

This paper reviews motivations and solutions for variable-speed operation in large hydro power plants with a special emphasis on full-size converter operated synchronous generators. First, the established concepts of conventional pumped storage power plants are briefly described. Then, the implemented applications with use of power converters and synchronous machines in hydro power plants, their power ratings, power converter topologies used, and the experiences obtained during operation are reviewed. Concerning its state-of-the-art position in variable-speed pumped storage power plants, the variable-speed capability offered by doubly-fed induction machines is also briefly described, including the power range that can be achieved in pumping mode as well as the power converter ratings of existing power plants. Following this comprehensive analysis, this paper identifies the requirements of today’s power converters and synchronous machines for utilization in large pumped storage power plants as full-size converter operated synchronous generators. It analyzes the design implications on the electric machines driven by such converters, and investigates the advantages of the additional possibilities given by the full-size converter operation that may outweigh the added cost, system complexity, and power loss.

1D simulation of pump-turbine transition

Variable speed motor-generators based on full size converter offer new options for operation of reversible pump-turbines. To explore the challenges related with this new technology a specific test rig configuration is developed to perform such transition with a PT reduced scale model. Transition from pump to turbine and vice versa are performed with various guide vanes opening and transition time. In order to extrapolate the results obtained at model scale to prototype, it is important to understand the system behaviour including the waterways and its influence on the flow in the pump-turbine. For this purpose, a 1D model of the test rig is realised. After validation with measurement results, the 1D model has been used to test the influence of the waterway on the pump-turbine transition.

Analysis of pressure pulsations during the fast transition of a pump turbine from pumping to generating mode

Variable speed motor-generators based on full size converter offer new options for operation of reversible pump-turbines. To explore the challenges related with this new technology a specific test rig configuration is developed to perform such transition with a PT reduced scale model. Transition from pump to turbine and vice versa are performed with various guide vanes opening and transition time. In order to validate the predictive capabilities of state-of-the-art CFD methods to predict the flow field under fast transition through off design operating conditions, pressure pulsations are measured as well in the rotating as in the stationary frame. Since some phenomena are likely to occur only for short periods of time, wavelet transformation methods are used to compare the pressure pulsations occurring during the transitions for different times of transition ranging from 20s to 4s.

Influence of huge renewable Power Production on Inter Area Oscillations in the European ENTSO-E-System

Abstract: After the enlargement of the European ENTSO-E power system towards Turkey at the end of 2010, the East-West Inter-Area Oscillation mode in the enlarged the European ENTSO-E power system has been identified in the frequency range of 0.15 Hz (T P = 7s) accompanied by insufficient damping. By the end of 2012, more than 107 GW of wind generation capacity had been installed across Europe, representing about 25% of the peak demand of ENTSO-E power system. In this paper, the impact of large scale wind power generation in the European ENTSO-E system on the North-South Inter-Area Oscillation mode using a detailed dynamic model of the European ENTSO-E system is investigated by gradually replacing the power generated by the synchronous generators in the system either Full Size Converter or Doubly Fed Induction Generator (DFIG) wind turbines. Because the whole system is extremely nonlinear, the analysis method in state space is senseless; therefore the damping behavior of Inter-Area-Oscillations of the whole system was analyzed in detail using the analysis method in time domain. The model was created using DIgSILENT software.

Numerical comparison of the effects of different types of distributed generation units on overcurrent protection systems in MV distribution grids

The integration of distributed generation (DG) units into traditional distribution grids causes several significant changes in their characteristics like power flow direction, voltage profile and short circuit level. Therefore, the currently used control and protection strategies can no longer work properly and have to be revised and modified. The most important protection problems are e.g. blinding of protection, false tripping, unsynchronized reclosing. For a reliable and efficient protection system, both transient and steady states of fault current contributions should be considered. In this paper, in order to study the real impact of DG units on a given protection scheme, the fault current contributions generated with exact models of DG units including their interfaces with the grid and control system (photovoltaic generator, PSMG with full size converter, DFIG with partial size converter, commonly met on Belgian grids, and directly connected IG) are presented and compared with the ones that are generated by ideal models of DGs in the same conditions. The PSCAD software is used for the simulation of transient contributions of DGs under several faulty conditions in a tested medium voltage distribution grid.

Impact of Increasing Wind Power Generation on the North-South Inter-Area Oscillation Mode in the European ENTSO-E System

After the enlargement of the European ENTSO-E power system towards Turkey at the end of 2010, the East-West Inter-Area Oscillation mode in the enlarged the European ENTSO-E power system has been identified in the frequency range of 0.15 Hz (TP = 7s) accompanied by insufficient damping. By the end of 2012, more than 107 GW of wind generation capacity had been installed across Europe, representing about 25% of the peak demand of ENTSO-E power system. In this paper, the impact of large scale wind power generation in the European ENTSO-E system on the North-South Inter-Area Oscillation mode using a detailed dynamic model of the European ENTSO-E system is investigated by gradually replacing the power generated by the synchronous generators in the system either Full Size Converter or Doubly Fed Induction Generator (DFIG) wind turbines. Because the whole system is extremely nonlinear, the analysis method in state space is senseless; therefore the damping behavior of Inter-Area-Oscillations of the whole system was analyzed in detail using the analysis method in time domain. The model was created using DIgSILENT software.

Small and Mid-Size Pump-Turbines with Variable Speed

The stability of the grid is jeopardized with the large percentage of non-dispatchable renewables like wind power and also with increasing solar power. This creates various problems because these forms of energy are very volatile and difficult to predict. In most countries the in-feed of these sources must not be curtailed. In addition most of the renewables do not provide short circuit capacity and inertia in the same way as classical units and so further worsen the stability of the grid. The growing exploitation of wind and solar might be limited due to grid stability problems. In order to compensate those problems a large amount of reserve capacity is needed and therefore new technologies for electricity storage are required. Hydraulic pumped storage—the classical storage technology—has some disadvantages. These plants are in mountain regions often far away from wind farms. The distance to the wind farms mean additional loading for the already stressed grid and additional transmission losses. To compensate the very volatile wind energy, the pump input power should be varied continuously. This is so far only possible with variable speed units. Up to now double-fed asynchronous motor-generators are used which are rather expensive. In order to provide a solution for the described situation, ANDRITZ HYDRO has developed a new innovative concept of decentralized pump storage plants. Small standardized pump turbines are combined with a synchronous motor-generator and a full size converter which allows speed variation in pump and turbine mode over a wide range. These plants can be built locally close to wind farms and other sources to be balanced, allowing the increase of renewable energy without increasing the transmission line capacity. For the future smart grids this will be a key storage technology. This concept is reliable, innovative and more economic than other storage technologies.