Variable speed hydropower plant with virtual inertia control for provision of fast frequency reserves

In this paper, five virtual inertia control structures are implemented and tested on a variable speed hydropower (VSHP) plant. The results show that all five can deliver fast power reserves to maintain grid stability after disturbances after a disturbance. The VSHP is well suited for the purposed since its output power can be changed almost instantaneously by utilizing the rotational energy of the turbine and generator. This will cause the turbine rotational speed to deviate from its optimal value temporarily. Then the governor control will regain the turbine rotational speed by controlling the guide vane opening and thereby the turbine flow and mechanical power. With that, the VSHP output power can be changed permanently to contribute with primarily frequency reserves. Dynamic and eigenvalue analyses are performed to compare five different versions of the basic VSG and VSM control structures; VSG, power-frequency PID-controller with permanent droop (VSG-PID), VSM, VSM with power-frequency PD-controller (VSM-PD), and VSM with power-frequency PID-controller and permanent droop (VSM-PID). They are evaluated by two main criteria; their ability to deliver instantaneous power (inertia) to reduce the rate of change of frequency (ROCOF) and their contribution to frequency containment control (steady-state frequency droop response

Direct Driven Permanent Magnet Synchronous Generators with Diode Rectifiers for Use in Offshore Wind Turbines

This work is focused on direct-driven permanent magnets synchronous generators (PMSG) with diode rectifiers for use in offshore wind turbines. Reactive compensation of the generator, power losses and control of the generator are studied. Configurations for power transmission to onshore point of common connection are also considered. Costs, power losses, reliability and interface with the PMSG are discussed. The purpose of the laboratory tests and simulations are to learn how a PMSG with diode rectifier behaves. A 55kW PMSG is tested in "Vindlabben", with and without reactive compensation. The generator current and generator voltage are measured and the total harmonic distribution (THD) of the current and the voltage are calculated. The results are compared to simulations on an equal generator in PSCAD/EMTDC. A 2MW PMSG is also simulated to compare parallel and series compensation and to find how the generator efficiency varies with the wind speed. The generator is also simulated with constant DC-link voltage and varying local wind to find how much the turbine and generator efficiency decreases when a Cluster step-up configuration is used. The DC-link voltage is in this case equal for parts of the wind farm or the whole wind farms. A 3MW ironless PMSG with very low synchronous reactance is simulated to find how this generator behaves with a diode rectifier. The laboratory tests and PSCAD simulations show that the maximal generator power increases when reactive compensation of the generator is used. The measured and simulated generator voltage and current shapes are found to be approximately equal. Series compensated PMSGs have lower generator current rms and lower current THD than parallel compensated PMSGs when the synchronous reactance is large. Therefore, the generator losses are 2-15% lower and the diode rectifier losses are 0-1% lower, depending on the wind speed. The diode rectifier losses are lower than 1%. The losses can be reduced even more if the diodes are connected in parallel. If a Cluster step-up configuration is used, the turbine efficiency is reduced by 3-4%. %The generator efficiency is more or less the same. The ironless PMSG has a low synchronous reactance and reactive compensation is not needed because the reactive power produced by the generator is low. Parallel connected capacitors have no positive effect and series connected capacitances must be very large and can therefore not be used. The generator current THD is very large when no reactive compensation is used. However, the current THD can be reduced by connecting an inductance to the DC-link. Cluster step-up, two-step DC/DC system, turbine step-up and series connected wind turbine are the most relevant layouts of the wind farms transmission system discussed in this thesis. The cluster step-up system has low power losses since only one large DC/DC converter is used. Also, the power equipment in the turbine is very reliable. However, the turbine efficiency is reduced since the generator torque and generator speed could not be controlled for one specific turbine. The other transmission systems require DC/DC converters in the turbines and they are therefore probably not as reliable. The total cost is crucial for the chose of the transmission system. Further cost accountings for the different DC systems are needed

Variable Speed Hydropower Conversion and Control

The objective of this paper is to develop and analyse a variable speed hydropower (VSHP) model that can aid the design of controllers that maximize the utilization of power plant for the provision of ancillary services, considering the limitations given by the hydraulic system. The model is tested and analysed with more or less conventional controllers to identify critical modes, adverse interactions or other limitations that must be taken into account in the future design of potentially multivariable or more advanced– controllers for VSHP. Dynamic tests are performed by simulating step responses in power demand and by comparing responses of the model with the VSHP and with a conventional hydropower plant. A participation factor-based interaction analysis shows that there are no strong dynamic couplings between the hydraulic system of the VSHP and the rest of the grid. This simplifies the tuning of the control system. However, the analysis concludes that some oscillatory modes associated with the hydraulic system become significantly more excited when operating at variable speed; which is due to the larger deviation in turbine rotational speed. Extra awareness when designing the control system is therefore needed to keep the hydraulic system variables within their limits

Optimized Control of Variable Speed Hydropower for Provision of Fast Frequency Reserves

This paper deals with the design of controllers for variable speed hydropower (VSHP) plants with the objective of optimize the plants performance. The control objectives imply enabling fast responses to frequency deviations while keeping the electric and hydraulic variables within their constraints. A model predictive controller (MPC) was developed to coordinate the turbine controller with the virtual synchronous generator (VSG) control of the power electronics converter. The simulation results show that the VSG is able to deliver fast power responses by utilizing the rotational energy of the turbine and the generator. The MPC controls the guide vane opening of the turbine to regain the nominal turbine rotational speed. If this is not possible due to the constraints of the hydraulic system, the MPC adjusts the power output of the VSHP by changing the VSG power reference. The proposed control system allows the VSHP to provide fast frequency reserves (FFR)