Alexander Schwery

Cooling Airflow, Losses, and Temperatures in Large Air-Cooled Synchronous Machines

At steady-state operation, power losses cause a heating of rotating electrical machines. In air-cooled machines, these losses are evacuated by a forced cooling airflow through the active parts. When designing and optimizing such a machine, the design engineer must be able to get a full picture of the power losses, the cooling airflow, and the temperatures inside the active parts (e.g., core laminations, windings) and the periphery (e.g., winding overhangs). The aim of the designer is to fulfill the customers requirements regarding the guaranteed temperatures. This paper presents a computation method, where the power loss, airflow, and temperature calculations for the worlds largest air-cooled hydrogenerators are coupled in an iterative process. The new contribution of this paper is a calculation software developed by the authors. It includes a state-of-the-art loss computation, an automated airflow network, and a set of linked thermal networks. These computations result in a complete overview of the temperature gradients and allow fine tuning of the cooling airflow and, consequently, optimization of ventilation losses.

Investigation of Control Strategies for Variable-Speed Pump-Turbine Units by Using a Simplified Model of the Converters

This paper presents the modeling, simulation, and analysis of the dynamic behavior of a fictitious 2 × 320 MW variable-speed pump-turbine power plant, including a hydraulic system, electrical equipment, rotating inertias, and control systems. The modeling of the hydraulic and electrical components of the power plant is presented. The dynamic performances of a control strategy in generating mode and one in pumping mode are investigated by the simulation of the complete models in the case of change of active power set points. Then, a pseudocontinuous model of the converters feeding the rotor circuits is described. Due to this simplification, the simulation time can be reduced drastically (approximately factor 60). A first validation of the simplified model of the converters is obtained by comparison of the simulated results coming from the simplified and complete models for different modes of operation of the power plant. Experimental results performed on a 2.2-kW low-power test bench are also compared with the simulated results coming from both complete and simplified models related to this case and confirm the validity of the proposed simplified approach for the converters.

Additional Losses in the Damper Winding of Large Hydrogenerators at Open-Circuit and Load Conditions

Large salient-pole synchronous machines are typically equipped with a damper winding. At steady-state conditions, parasitic voltages are induced in the damper bars which lead to a current flow with associated power losses. This paper describes an analytical algorithm for the calculation of currents and corresponding losses in the damper winding. The presented method is based on an equivalent network of the damper winding containing all the bars of a repetitive section of the machine. The inductances are calculated with an air-gap permeance model. Contrary to similar existing approaches, the induced damper-bar voltages are computed using a numerical integration. This allows more precise results when it comes to higher harmonics. In order to validate the analytical computation, the results are compared with the results of 2-D transient finite-element studies and with a conventional analytical method based on the d-and q -axis equivalent circuits of the machine.

Double-fed asynchronous motor-generator equipped with a 3-level VSI cascade

The present paper investigates the complete regulation strategy for a large double-fed asynchronous motor-generator equipped with a 3-level VSI cascade for applications mainly in the field of pumped storage plant. For the present application, the specific problem of the DC link capacitors voltage balancing is investigated. Also a sensorless method for the determination of the rotor angle is proposed and verified in practice. Measurements of the system performed on a laboratory low power system are discussed and compared to simulations.

Dynamical Behavior Comparison Between Variable Speed and Synchronous Machines With PSS

The dynamic performances of variable speed and fixed speed units are compared in the case of 2 × 320 MW pumped storage power plant. The case study of the pumped storage power plant with standard synchronous machine with PSS is presented and, then, the corresponding variable speed system is designed for this case. Unsteady numerical simulations of both architectures enable to demonstrate that the variable speed architecture improves the power system stability characteristics for both the local and the inter-area modes of oscillation. Moreover, the damping of active power oscillations originated by the part load operation of the pump-turbine in generating mode is investigated as well.

Analytic calculation of the voltage shape of salient pole synchronous generators including damper winding and saturation effects

A novel analytic method for determining the no‐load voltage shape of salient pole synchronous generators is presented. The algorithm takes into account the full influence of the damper winding and the saturation effects in the stator teeth. Main interest is an easy and very fast calculation method, which can be used as a criteria for the selection of the number of stator winding slots in the initial design calculation or an optimization process. The analytical results obtained are compared with the results of transient finite element analyses.

OPTIMIZED CALCULATION OF LOSSES IN LARGE HYDRO-GENERATORS USING STATISTICAL METHODS

A very important issue during the electrical design of hydro-generators is the reliability of the loss calculation in the manufacturers design calculation program. The design engineer who has to guarantee the losses must be able to estimate the risk of liquidated damages when defining the guarantee values. This paper presents the optimization of the loss calculation in a design program for salient pole synchronous machines. Statistical methods are used to calibrate the loss calculation with measurements made during commissioning. Within this paper special importance is attached to the optimization of the no-load electromagnetic losses.

Calculation of power losses in the damper winding of large hydrogenerators at open-circuit and load conditions

This paper presents an analytical method for the calculation of currents and power losses in the damper winding of large salient-pole synchronous machines at various steady-state operational conditions. The algorithm is based on a damper winding network containing all damper bars of a repetitive section of the machine. The self and mutual inductances as well as the induced voltages are computed taking into account the air-gap permeance function. The presented algorithm is used to compute currents in the damper winding at open-and short-circuit test conditions. The obtained analytical results are compared with the results of transient finite element studies.

Start-up and synchronization of a variable speed pump-turbine unit in pumping mode

This paper presents the start-up and synchronization procedures of a variable-speed pump-turbine unit of 250 MW in pumping mode. These procedures applied to a doubly-fed induction motor-generator with three-level voltage source inverters VSI cascade in the rotor side, can be achieved without any supplementary equipment. The control strategy used in the start-up procedure and based on a stator flux oriented control, is described in details as well as different steps in order to reach the minimal speed required for synchronization. The controls of the amplitude, frequency and phase of the stator voltage related to the synchronization procedure are presented. Simulated results confirm the validity of the proposed approach.

Advanced calculation of temperature rises in large air-cooled hydro-generators

During the design of large air-cooled synchronous machines for hydro power plants special attention must be paid to efficient cooling of the machine. It is necessary that the design engineer gets a complete picture of the losses, the cooling airflow and the temperatures inside the different parts of the machine. Not only the active parts but also the end regions of stator and rotor must be carefully checked. This paper presents a new calculation method which is based on an iterative numerical coupling of power losses, airflow and temperature calculation. This computation gives a complete overview of the temperature gradient in the stator core, the stator winding including overhang and the pole coil.