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Salient pole wound field synchronous motors find many industrial applications thanks to their favorable characteristics: reactive power regulation, stiff mechanical characteristic and overall outstanding efficiency. Nevertheless, their competitiveness towards the induction motors, especially for medium and small power sizes, depends crucially on their capability to be asynchronously started as well. Regrettably, the asynchronous run-up of a synchronous motor can be sometimes very problematic because of thermal issues, torsional vibrations and grid voltage disturbances. This paper presents an alternative method of starting salient pole wound field synchronous machines by activating the field winding in a special manner, which makes it possible to mitigate the three problems at once. The suggested method is validated through a 2D finite elements simulation and by starting a 60 kVA prototype generator. The requirements for the application of the proposed run-up strategy are critically discussed together with related pros and cons.

The upcoming modifications to the criteria for providing Frequency Containment Reserves (FCR) in the Nordic synchronous area present severe challenges for some hydropower units. By quantifying the difference between the required FCR power response and the actual response provided by the unit, an additional power response from an energy storage system can be specified to augment the hydropower unit, thus securing its participation in FCR in the future. A practical method to control the storage system, by applying a band-pass filter on the negative frequency deviation, is presented in this paper. The method is verified using a set of field measurements from Swedish hydropower units currently providing FCR. It is found that all tested units are able to satisfy the upcoming criteria with the specified storage systems. The optimality of the method is discussed by comparing the storage system power and energy ratings to the theoretical minimum. Obtained power ratings between 3% and 24% of the FCR power, combined with a short storage system duration of up to 2.5 minutes, indicate that the method presents a possible application for high-power and low-energy storage technologies.

Spillways are very important dam safety devices that can discharge large amounts of water to prevent over-topping a dam. However, spillways are expensive to build and have operational issues. This paper is a report from a test of a system capable of discharging the full turbine flow even in the absence of an external power grid absorbing the electrical power. Instead the power is dissipated in a local load. The scope of the tests is to verify the use of the discharge capacity of the turbine under continuous operation as spillway flow for emergency discharge from a Hydropower reservoir. A dumpload consisting of a hot water electrode boiler cooled by water taken from the turbine spiral case was used as a local load. The results show long-term stability of using a local load and turbine discharge as an electrical alternative to building additional traditional spillways to increase spillway capacity from a hydropower reservoir.

The purpose of this paper is to carry out an alternative to the present transient models for field wound synchronous machines, which is able to take into account the nonlinearity of the magnetic materials as well as the cross-magnetization. After presenting the principal model structures according to the state variables, a model based on two lookup tables for the magnetizing flux linkages is introduced and built step by step. The resulting signal flowchart shows an algebraic loop within the model, where the main flux linkage rapidly converges to its instantaneous value by simple iteration. The proof of this convergence is given for both saturated and unsaturated machine. Even though the proposed model uses the total linkage flux as state variable, as many alternative models do, it does not require the inversion of the current to flux linkage function (i.e., of lookup tables). This can spare a heavy computational task, especially with very large lookup tables. In the proposed model, the computational effort in the worst case scenario is reduced to few iterations (<10). Finally, the nonlinear behavior of the model is verified in four different transient scenarios by comparing its outcomes with those of a linear model for the same test machine.

The core of dam safety is to safely store and discharge water and thereby prevent uncontrolled release of water. Excess water, e.g. in a flooding situation, is commonly discharged via different types of spillways (gated, weirs, surface, bottom outlets etc.). The turbine discharge is typically not included in the capacity as flooding scenarios could cause a grid-blackout rendering the turbine incapable of operating. The paper will describe a project that has tested island operation of a hydropower unit equipped with a developed dump-load device. The dump-load device will allow the unit to operate even during a black-out without risk for failure of the unit and thus assure the discharge capacity via the waterways connected to the unit. Stable operation is verified in the tests and this opens up for the turbine discharge to be included in the total discharge capacity of a facility. A cost-benefit analysis of the suggested solution and a brief description of the past and planned development of the device will be given. In addition to the extra discharge capacity, the new system can also be used to reduce the risk for surge during start- and stop sequences. Implementation of the system in two different facilities, with different purpose, is planned and details on the coming installations and the technical solutions around them will be given.

The core of dam safety is to safely store and discharge water and thereby prevent uncontrolled release of water. Excess water, e.g. in a flooding situation, is commonly discharged via different types of spillways (gated, weirs, surface, bottom outlets etc.). The turbine discharge is typically not included in the capacity as flooding scenarios could cause a grid blackout rendering the turbine incapable of operating. This paper describes a project that has tested island operation of a hydropower unit equipped with a developed dump-load device. The dump-load device allows the unit operating even during a black-out without risk for failure of the unit and thus assure the discharge capacity via the waterways connected to the unit. Stable operation of the dump-load verified in the tests together with high reliability opens for the turbine discharge to be included inthe total discharge capacity of a facility. A cost-benefit analysis of the suggested solution and a brief description of the past and planned development of the device is given. In addition to the extra discharge capacity, the new system can also be used to reduce the risk for surge during start and stop sequences. Implementation of the system in two different facilities, with different purpose, is planned and details on the coming installations and the technical solutions around them is given. A cost comparison between conventional spillway gates and dump-load indicates a factor of ten less for dump-load for a given increase in discharge capacity. The safety analyses shows that the turbine discharge should be included in the total discharge capacity of a facility if a dump load device is present., QC 20230825

An electrical machine (1) comprises a stator (20) having armature windings (24), a rotor (10) having salient poles (12) and being rotably arranged with respect to the stator (20), and a rotor excitation system (30). The rotor excitation system (30) has a rotor winding arrangement (14), a rotor power supply (34) for exciting the salient poles (12), and a rotor driving unit (32) configured to control currents provided to the rotor winding arrangement (14). In a first operation mode, the rotor excitation system (30) is sconfigured for providing a rotating magnetic field by supplying alternating currents to the rotor winding arrangement (14), while the armature windings (24) being short-circuited or closed on external resistors. A method for operating an electrical machine is also disclosed.

The ability to discharge water plays a vital role in hydropower dam safety. There are several different designs that provide discharge capacity, with the two main types being surface and bottom spillways. Here, an alternative concept is presented, whereby the regular waterway through the turbine is utilized. In the event of grid loss a local load is connected which enables the unit in question to operate isolated from the main grid. This dump-load concept has been tested at pilot scale in Sweden with positive results and therefore proposed to be applied in existing power plants. Additional discharge capacity via a dump-load has a number of benefits compared to conventional spillgates, the most obvious being its significantly lower investment cost, fast installation and provision of redundancy., StandUP research initiative

The load angle of a synchronous generator connected to a power grid has an eigenfrequency that depends on the operating conditions. The existence of an eigenfrequency can make the generator sensitive to electrical and mechanical disturbances and motivates the use of damper windings and power stabilizing systems. Another reason for keeping load angle oscillations small is that they increase the eddy current losses in the stator core and clamping structure due to axial leakage flux. This is shown with transient finite element analyses and can be explained by a loss model derived from a phasor model of the eddy current loss density., Title in thesis list of papers: A Loss Model and Finite Element Analyses of the Influence of Load Angle Oscillation on Stator Eddy Current Losses in a Synchronous Generator

A magnetization system with active compensation of unbalanced magnetic pull for synchronous machines with rotating exciters is demonstrated. The system used switched power electronics and a digital control system to control the currents in four rotor pole groups, each consisting of 3 poles. It was mounted on the shaft of a synchronous machine, providing an interface between a permanent magnet outer-pole brushless exciter and the segmented field winding. Measurements of magnetic flux density on each pole face and current control made it possible to control the airgap magnetic flux density to balance the machine magnetically, thus removing flux density space harmonics in the airgap and also the unbalanced magnetic pull. The construction of the system is presented along with results from experiments and simulations. Tests were performed with the stator winding both in series and with two parallel circuits. Approximately 80% reduction of static forces and 60% reduction of dynamic forces between the stator and rotor were observed when the system was running.

Herein, a set of experimental procedures is presented for determining the main electrical distributed parameters of the field winding in salient pole synchronous machines. It applies to the electrical characterisation of iron-core power inductors and transformer windings as well, in a range of frequency useful for power electronics applications. A first estimation of the parameters is obtained by forcing the winding into resonance with capacitors of known capacitance. The obtained estimates are then refined through an iterative process, which makes use of the winding natural frequencies. The presented procedures are applied step-by-step to the field winding of a 60-kVA salient pole synchronous generator with solid poles. The distributed parameters model, featured using the outlined procedures, accurately reproduces the winding voltage distribution in a large range of frequency. Finally, it is explored how the interaction between armature and rotor influences the field winding parameters, pointing to the differences of measuring them with the rotor inside or outside the machine bore.

Synchronous motors are hard to line start due to torque pulsations at zero rotor speed and low starting torque when started using induced current in a damper squirrel cage. By inverting the rotor pole polarity at appropriate times it is possible to, in principle, achieve uniform torque, albeit pulsating with twice the line frequency at zero initial rotor speed. This has been demonstrated in an earlier work. In this paper we demonstrate that high torque starting using the back-emf in the field winding as triggering signal for the rotor polarity inversion is possible. We further discuss the origin of the rotational energy and active and reactive power pulsations. Finally, we show that it is possible to operate a synchronous motors at continuous asynchronous speed by inverting the polarity of the rotor current and adjusting the field current accordingly, although down rated.

Developments of the high-speed response brushless excitation systems (HSRBESs) are ongoing in the power industry. This is because the transient response of the excitation system (ES) is a key performance indicator for the grid owner. In dealing with this problem, accurate prediction and control of the ES ceiling voltage are desirable. However, the brushless exciters' nonlinear armature reaction causes the ceiling voltage to be unknown under varying operating conditions. This article proposes a numerical average-value model (AVM) that captures all the main dynamics of the HSRBES. It is shown that the AVM relationships can be utilized for online prediction of the ceiling voltage and employed in a dual-control scheme. The proposed model is validated against a dynamic voltage build-up test. Moreover, it is derived from a detailed model, which is verified from instantaneous field measurements and further from finite-element analysis. Finally, the accuracy and effectiveness of the AVMs' transient relationships prove the feasibility of the proposed dual-control scheme and shows that it can be easily implemented in existing systems.

The frequency in the electrical grid is, on the short time-scale, stabilized by the total rotational mass given predominantly by synchronously connected devices. These devices include the generators themselves and, on slightly longer timescales, the control system acting on turbine governors. As the inverter-connected technologies increase their share of total power, the total inertia in the grid is significantly reduced. One way to stabilize the frequency and reduce low frequency oscillations is to add synthetic inertia provided by a small energy, high power, storage devices combined with a fast control system. This paper describes a possible hardware topology for linear synthetic inertia. It uses an inverter coupled to a local energy storage unit comprised of supercapacitors. The paper presents the implementation and some selected experimental results. The system response is fast enough on a small test grid to act as inertia.

The excitation system plays a critical role in the operation of synchronous generators. An equipment failure could impact the voltage quality for smaller grids. Further, it can lead to cost penalties and reduced production for the power plant owner. Recently, a new high-speed-response rotating brushless exciter was developed that employs remote control of the rotating thyristors on the generator shaft. This has led to new possibilities for improving the performance of brushless exciters. This contribution investigates the failure modes of a dual-star outer pole exciter that feeds two separate thyristor bridges connected in parallel during normal operation. The possibility of redundant postfault operation due to open-thyristor or open-phase faults are demonstrated using experimental testing. The system is compared with the fault performance of a conventional three-phase system. This work includes the implementation and validation of a fault-predicting double d-q exciter model. In addition, the dangerous effects of a shorted-thyristor fault are investigated. A "skip firing" protection technique is briefly demonstrated for the fast isolation of such faults, yielding nondestructive postfault recovery and redundant failure-mode operation. The evidence shows that the dual-star exciter is a competitive choice for the future development of fault-tolerant brushless exciters., Title in thesis list of papers (Nøland): Failure-Modes of a Thyristor-Controlled Six-Phase Rotating Brushless Outer Pole PM Exciter with 12-Pulse Hybrid-Mode Operation

Fault detection of electrical machines can avoid unplanned outage of the electricity generation at a power plant. Research related to a different type of faults in the synchronous generator, the broken damper bar (BDB) fault attracts less attention due to the low statistical population. However, a comprehensive condition monitoring system requires a method to diagnose this few in number faults also. Detection of BDB fault is difficult since damper bars are active only during a transient period, which could be start-up intervals or as a reaction to power system dynamics. In this paper, the BDB fault is detected during the machine start-up. The Radius of Gyration (RG) was introduced using a time series data mining (TSDM) approach that was applied to induced electromotive force of the rotor field winding due to BDB fault. Location and number of BDBs effect on the nominated feature are also studied.