Abstract title Provision of primary frequency support and inertia emulation by offshore wind farms connected through multi-terminal VSC-HVDC links. Authors and affiliations Sotirios Nanou *, Argiris Spetsiotis, Stavros Papathanassiou School of Electrical and Computer Engineering, National Technical University of Athens (NTUA), 9 Iroon Polytechniou str., 15780 Athens, Greece (e-mails: sotnanou@central.ntua.gr, argispets@gmail.com, st@power.ece.ntua.gr) * presenting author Introduction Technical requirements imposed to wind farms and other power stations are gradually extending to HVDC connections, including offshore wind farms [1]. Among these, particularly important is the provision of over- and under-frequency response, combined with inertia emulation. In the case of offshore wind farms connected through multi-terminal DC (MTDC) grids, a coordinated control approach is required between the onshore and offshore converter stations, in order for the onshore converters to provide a frequency-dependent active power modulation and meet the relevant requirements. In this paper, such a control concept is proposed and evaluated for a 4-terminal MTDC grid. Approach To evaluate the frequency response capability of the MTDC grid, a suitable simulation model has been developed in SimPowerSystems Toolbox of Matlab/Simulink using the phasor simulation method. For this purpose, all high-frequency components related to the switching of power converters are neglected and the WT and HVDC converters are described by the fundamental frequency model of [2]. The study case system is depicted in Fig. 1 and comprises two 300 MW offshore wind power plants (OWPPs) connected through a 4- terminal MTDC grid to an indicative two-area four-generator power system [3]. So far, relevant publications on frequency support by MTDC grids rely on DC voltage modulation techniques, performed by the onshore Voltage Source Converters (VSCs), upon detection of onshore frequency deviations [4], [5]. In this paper, it is assumed that the offshore VSCs (#1, #2) are able to receive frequency measurements by the control units of the onshore VSCs (#3, #4), via a suitable communication link, i.e. fiber optic cables installed with the HVDC submarine cables. When frequency deviations are detected by the onshore VSCs, an estimate of the frequency deviation is transmitted throughout the MTDC grid, in order for the offshore VSCs to emulate onshore frequency excursions at the offshore side. The provision of frequency response is achieved by: 1) activating the frequency response capabilities of the individual wind turbines (WTs), 2) delivering the modulated active power to the onshore grid, via the DC voltage-power droop controllers of the onshore VSCs, which compensate the induced DC voltage variations. The communication
delay introduced by the aforementioned approach has also been taken into account, including a brief parametric analysis showing its effect on the expected frequency response. The frequency response capability of the MTDC grid is evaluated via simulation, assuming step changes of the load connected at bus 7. Alternative frequency control implementations are comparatively assessed, namely the droop-type and inertia frequency controllers or their combined application. The amount of power reserves that should be maintained by the offshore WTs for frequency regulation purposes is also assessed, by testing the system response at different reserve levels. Figure 1: Study-case system comprising a 4-terminal MTDC grid connected to a two-area four-generator power system. Main body of the abstract The power system of Fig. 1 consists of four 900-MVA conventional generators, split into two areas. Each generator incorporates an automatic voltage regulator and a generic power system stabilizer. For the purposes of this study, generators 1-4 comprise steam turbines, whereas an IEEEG1 speed governor model has been assumed. The overall control scheme employed for the onshore VSCs is depicted in Fig. 2(a). VSCs #3 and #4 export DC power to the onshore grid by regulating the HVDC voltage v dc via a DC voltage-power droop control concept [6]. The droop constant ρ dc is assumed 4% for both VSCs. A conventional current vector controller operating in the Synchronous Reference Frame (SRF) is considered for the inner control loop, while the estimate of the frequency deviation ΔωPLL,on, provided by the Phase-Locked Loop (PLL) unit, is transmitted to VSCs #1 and #2, assuming a communication delay T com of 20 ms. The offshore VSCs (#1 and #2) operate as grid-forming power converters, where a Proportional-Integral (PI) AC voltage controller (Fig. 2) controls the magnitude of the grid voltage v off, whereas a frequency
controller emulates onshore grid conditions, by superimposing the frequency deviation signal ΔωPLL,on on the default value ω n. To simplify wind farm modeling, an aggregate WT represents each wind farm connected to the offshore VSCs. Full-power Converter WTs (FCWT) based on Permanent Magnet Synchronous Generators (PMSGs) are assumed, whose typical configuration is illustrated in Fig. 3. The PMSG is controlled by the Machine Side Converter (MSC), which employs a Rotor Field Oriented (RFO) control scheme, while the outer WT controller (Fig. 3(a)) implements the Maximum Power Point Tracking (MPPT) strategy, which determines the power order p ord, employing a speed control loop [7]. A droop-type and inertia frequency controller is integrated into the outer WT controller (Fig. 3), utilizing the estimate of the grid frequency ΔωPLL,WT provided by the PLL unit of the Grid Side Converter (GSC) controller. Fig. 4 demonstrates the under-frequency response capability of the entire MTDC grid, following a 200 MW step increase of the load connected at bus 7, at t=10 s. Each OWPP initially generates approximately 250 MW, while the reserve command r * to the offshore WTs (see Fig. 3(a)) is set to 10%. The droop constant and the virtual inertia gains (Fig. 3) are R WT=5% and K in=20 respectively. In Fig. 4, different frequency control approaches are tested. If the frequency controller is suspended, a maximum frequency dip of approximately 0.38 Hz occurs (Fig. 4(a)), following the load connection (blue curve). Droop control alone (green curve) achieves a reduction of post-disturbance frequency deviation by approximately 8%, while a slight increase of the damping ratio of the dominant electromechanical mode is observed. When inertia control is applied (red curve), both the maximum frequency and the Rate Of Change Of Frequency (ROCOF) are notably reduced. The combined droop and inertia (PD type) control (black curve) expectedly provides best results, as the maximum frequency excursion is reduced by 18%. In all cases, the DC voltage-power droop controllers of VSCs #3 and #4 successfully compensate the induced DC voltage fluctuations (Fig. 4), thus transposing the desired frequency-dependent active power modulation to the onshore VSCs (Fig. 4(c)). As for the WT rotor dynamics, the rotor speed deviations (Fig. 4(d)), assisted by the action of the pitch regulator, are acceptable in all cases. The robustness of the communication-based approach in the presence of different communication delays T com is demonstrated in Fig. 5, where it is evident that even large delays in the range of 100-500 ms do not alter the expected frequency response characteristics. The ability of the MTDC grid to provide adjustable power reserves during normal operation, exploiting the de-loaded operation of the offshore WTs, is demonstrated in Fig. 6, for different levels of the power reserve command r * (0, 10% and 20%), applying the combined frequency controller. Comparing Figs. 4(a) and 6(a) it is evident that the MTDC grid is still able to filter-out fast frequency excursions even in the case where no power reserve is maintained, however the provision of sustained under-frequency response is degraded, since the WTs are unable to release additional active power permanently.
(a) Figure 2. (a) Onshore VSC and controller structure, offshore VSC controller with integrated frequency modulation technique. (a) Figure 3. (a) FCWT and controller structure, outer WT controller in block diagram form, with integrated droop & inertia frequency controller [7].
(a) (c) (d) Figure 4: (a) System frequency, HVDC voltage at VSC #3 station, (c) onshore active power (VSC #3), (d) WT rotor speed of OWPP #1, following a 200 MW step increase of bus 7 load at t=10 s, for alternative frequency control approaches (R WT=5%, K in =20). Figure 5: Impact of communication delay T com on the frequency response.
(a) (c) (d) Figure 6: (a) System frequency, HVDC voltage at VSC #3 station, (c) onshore active power (VSC #3), (d) WT rotor speed of OWPP #1, for the same disturbance as in Fig. 4, assuming operation at different reserve levels with combined (droop & inertia) control. Conclusion In this paper, the contribution of OWPPs connected through MTDC grids to onshore frequency regulation has been investigated, employing a communication-based approach to emulate onshore frequency fluctuations at each offshore AC grid. The communication-based method relies solely on the existing communication infrastructure of the VSC-HVDC links, while the DC voltage fluctuations, induced by the frequency response of the offshore WTs, establish a virtual communication link to the onshore VSCs in order for them to readjust their active power via the DC voltage-power droop controllers. Results obtained from time-domain simulations demonstrate the potential for a substantial contribution of the MTDC grid to frequency control. The droop-type controller contributes to the reduction of postdisturbance frequency deviations, while the inertia controller increases the apparent system inertia and provides substantial damping to the dominant electromechanical modes of the power system. In this study, a reserve level of 10% suffices in order for the MTDC grid to have a positive effect on frequency regulation. It can be concluded that the communication-based approach is entirely feasible in practice, while the impact of communication system latency on the frequency response is insignificant, due to the relatively slow variations of the onshore frequency. The DC voltage-power droop controllers successfully compensate the
induced DC voltage excursions, thus the provision of frequency response becomes feasible without the need to implement additional droop-type or inertia frequency controllers in the control units of the onshore converter stations. Learning objectives The main objective is to provide insight to the possibility offered by OWPPs to contribute to onshore frequency control through MTDC grids. It was shown that only slight perturbations of HVDC voltage levels are necessary in order for the onshore VSCs to provide the stipulated frequency support, while the impact of communication system latency is insignificant. References 1. ENTSO-E Draft Network Code on High Voltage Direct Current Connections and DC-connected Power Park Modules, ENTSO-E, Brussels, Belgium, Apr. 2014. 2. H. Saad, J. Peralta, S. Dennetiere, et al, Dynamic Averaged and Simplified Models for MMC-Based HVDC Transmission Systems, IEEE Trans. Power Del., vol. 28, no. 3, 2013. 3. M. Klein, G. J. Rogers, P. Kundur, A Fundamental Study of Inter-Area Oscillations in Power Systems, IEEE Trans. Power Systems, vol. 6, no. 3, Aug. 1991. 4. N. R. Chaudhuri, R. Majumder, B. Chaudhuri, System Frequency Support Through Multi-Terminal DC (MTDC) Grids, IEEE Trans. Power Systems, vol. 28, no. 1, Feb. 2013. 5. J. Zhu, J. M. Guerrero, W. Hung, et al, Generic inertia emulation controller for multi-terminal voltagesource-converter high voltage direct current systems, IET Renew. Power Gener., vol. 8, no. 7, 2014. 6. W. Wang, M. Barnes, Power Flow Algorithms for Multi-Terminal VSC-HVDC with Droop Control, IEEE Trans. Power Systems, vol. 29, no. 4, July 2014. 7. K. Clark, N. W. Miller, J. J. Sanchez-Gasca, Modeling of GE Wind Turbine-Generators for Grid Studies, GE Energy, New York, 2010.