Master self‐tuning VDCOL function for hybrid multi‐terminal HVDC connecting renewable resources to a large power system
2017; Institution of Engineering and Technology; Volume: 11; Issue: 13 Linguagem: Inglês
10.1049/iet-gtd.2017.0094
ISSN1751-8695
AutoresMai Huong Nguyen, Tapan Kumar Saha, Mehdi Eghbal,
Tópico(s)High-Voltage Power Transmission Systems
ResumoIET Generation, Transmission & DistributionVolume 11, Issue 13 p. 3341-3349 Research ArticleFree Access Master self-tuning VDCOL function for hybrid multi-terminal HVDC connecting renewable resources to a large power system Mai Huong Nguyen, Corresponding Author Mai Huong Nguyen maihnguyen410@gmail.com Queensland Geothermal Energy Centre of Excellence, The University of Queensland, Brisbane, QLD, 4072 Australia Faculty of Energy Management, Electric Power University, Hoang Quoc Viet, Hanoi, VietnamSearch for more papers by this authorTapan Kumar Saha, Tapan Kumar Saha School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, QLD, 4072 AustraliaSearch for more papers by this authorMehdi Eghbal, Mehdi Eghbal School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, QLD, 4072 AustraliaSearch for more papers by this author Mai Huong Nguyen, Corresponding Author Mai Huong Nguyen maihnguyen410@gmail.com Queensland Geothermal Energy Centre of Excellence, The University of Queensland, Brisbane, QLD, 4072 Australia Faculty of Energy Management, Electric Power University, Hoang Quoc Viet, Hanoi, VietnamSearch for more papers by this authorTapan Kumar Saha, Tapan Kumar Saha School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, QLD, 4072 AustraliaSearch for more papers by this authorMehdi Eghbal, Mehdi Eghbal School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, QLD, 4072 AustraliaSearch for more papers by this author First published: 24 July 2017 https://doi.org/10.1049/iet-gtd.2017.0094Citations: 13AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract To take advantage of line commutated converter (LCC) and voltage source converter high-voltage, direct current (HVDC) technologies a hybrid multi-terminal HVDC (MTDC) is designed to exploit the power from renewable resources. A previous study showed that the voltage dependent current order limiter (VDCOL), an emergency function to reduce the current order of an HVDC converter station when voltage is depressed, should be installed in all of the LCC stations of the hybrid MTDC. This study presents a design for a master self-tuning VDCOL function implemented in a hybrid multi-terminal HVDC system. This controller aims to generate an adaptive current reference for each LCC station of the VDCOL in a low-voltage condition. The parameter of this adaptive VDCOL can be chosen according to the summary table of practical experiences; therefore, no tuning technique is required. The developed HVDC control algorithm is implemented on the simplified South East (SE) Australian 14 generator system using DIgSILENT PowerFactory. The simulation results demonstrate the effectiveness of the proposed control function in reducing the complexity of the control system and operation along with improving system transient stability. 1 Introduction The South East Australian network is the world's largest interconnected AC power system which is about 5,000 km long with the total installed capacity of more than 50 GW [1]. The largest geothermal resource is found in Cooper Basin near the border of Queensland and South Australia. The main load centres are unevenly distributed along the South East coastline, from North Queensland to South Australia. Therefore, one of the formidable technical challenges to promote large-scale utilisation of hot rocks geothermal energy in Australia is the transmission distance to a large power plant which is located more than 500 km from major load centres and the national grid. Line commutated converter (LCC) high-voltage, direct current (HVDC) is a cost-effective technology for bulk power transmission over very long distances. However multi-terminal LCC HVDC operation and control are problematic tasks due to the requirement of current balancing [2] and mechanical switches to change the direction of the power flow [3]. Voltage source converter (VSC) HVDC offers more flexibility for power flow control on the DC link. This advantage of the VSC HVDC technology makes it a proper option for intermittent renewable sources (e.g. wind) integration into an AC network [4]. Therefore, to utilise both above-mentioned technologies, a hybrid multi-terminal HVDC (MTDC) with LCC and VSC stations was designed to connect the power from geothermal and wind resources to the Australian system in the authors’ previous work [5]. Voltage dependent current order limiter (VDCOL) (sometimes referred to as VDCL) is used to decrease the current reference to the LCC HVDC converter station when the voltage at that station is depressed during disturbances. The application of this function to a LCC HVDC link has been verified in [6, 7]. The dynamic VDCOL function for multi-terminal LCC HVDC having four special complex ramps was presented in [8]. The normal VDCOL function has only one relationship between the input DC voltage and the current order of the VDCOL [8–10]. The current order is normally limited by the reference current of the HVDC link. Adaptive neuro-fuzzy VDCOL unit combining a radial basis function neural network output with a fuzzy inference mechanism to produce an adaptive current order has been developed in the literature [11, 12]. Self-adaptive VDCOLs based on the voltage of ac side of converter transformers are proposed in [13, 14]. Adaptive fuzzy VDCOL is designed based on a simplified model of DC P-Q coupling recovery in [15]. These adaptive VDCOLs are superior to the conventional VDCOL in terms of transient stability of the HVDC system under several dynamic operating conditions. In the authors’ previous work [5], the controller scheme for a MTDC was designed for the simplified SE Australian system. Simulation results show that VDCOL function should be installed in the LCC rectifier and inverter of the MTDC. However, the parameters of VDCOL need to be properly tuned for each operating condition [5]. This paper presents the design of a master self-tuning VDCOL function replacing all the VDCOL required in all LCC converters to reduce the complexity in designing and operating a hybrid MTDC system. The master self-tuning VDCOL can be used for all LCC converters of the MTDC system. The product of the output of this master controller and the reference order of each MTDC station will be the VDCOL current order of the corresponding station in the low-voltage condition. Therefore, this VDCOL current order is dependent on the reference current of each station, varying with the loading condition of the system. This adaptive VDCOL is based on the basic linear function of the common VDCOL in practice; therefore, no parameter tuning technique is required. The parameters of this function are selected for three operating conditions of the test system based on the empirical data [16]. The simulation is conducted in DIgSILENT PowerFactory software [17]. In this paper, Section 2 demonstrates the case study and the operating scheme of MTDC with three loading conditions. Section 3 describes detailed design for the proposed VDCOL function. The effectiveness of the proposed VDCOL on transient stability of the system is investigated in Section 4. Finally, Section 5 concludes the paper. 2 Case study 2.1 Simplified southern and eastern Australian grid The proposed controller scheme is implemented on the simplified Southern and Eastern Australian 14 generator power system, depicted in Fig. 1. Areas 2–5 of this system represent New South Wales, Victoria, Queensland and South Australia regions, respectively [18]. The information of three loading conditions of the system: heavy, medium heavy and peak load is presented in Table 1. Fig. 1Open in figure viewerPowerPoint Diagram of the simplified SE Australian power system Table 1. Loading conditions of the simplified SE Australian power system [18] Case 1 2 3 load condition heavy medium heavy peak ∑ generation, MW 23,030 21,590 25,430 ∑ load, MW 22,300 21,000 24,800 power flow 4-2-1-3-5 5-3-1-2-4 1-2-4 1-3-5 ESCRa of LCC inverter system 3.37 1.62 3.72 a ESCR–Effective short circuit ratio. A new geothermal power plant is connected to the grid by a hybrid MTDC proposed in [5]. The LCC rectifier is connected to the geothermal power plant, LCC inverter is connected to bus 416 in area 4 (QLD); the VSC station is connected to bus 508 in area 5 (SA) of the system, as described in [5]. Wind power is accounted for 21% of the capacity in SA area. The load in each area is increased by the average annual growth rate of demand of each corresponding region obtained from [19] to consume the new installed generation. 2.2 Hybrid multi-terminal HVDC The hybrid MTDC, designed based on the CIGRE HVDC Benchmark model [20] and the ABB VSC HVDC model [2], consists of two bipolar ± 250 kV HVDC links that are an LCC link (1000 km) and a VSC link (500 km) as shown in Fig. 2. Fig. 2Open in figure viewerPowerPoint Hybrid MTDC In heavy and peak loading conditions, the active power is transferred from the geothermal source to area 4 (QLD) and area 5 (SA) as 700 and 300 MW, respectively. In this case, the VSC station of the hybrid MTDC is operated in inverter mode. In the medium heavy loading condition, area 4 is assumed to receive 1000 MW from the geothermal source and 300 MW from area 5. In this case the VSC station is operated in rectifier mode. The master current controller is designed to generate DC current order for the LCC rectifier, as the summation of measured DC currents at LCC inverter and VSC station. Table 2 summaries the operating parameters for the MTDC controller scheme which was presented in details in the authors’ previous work [5]. Table 2. Operation mode of the MTDC system [5] LCC rectifier LCC inverter VSC station Connection point Geothermal site QLD network SA network Control mode Current control DC voltage control Active power control Reactive power control Controller set-point Heavy and peak loading condition Iord = 0.97 p.u. Vdc = 0.99 p.u. Inverter mode Iord = 0.6 p.u.a Pord = 0.86 p.u. (300 MW) Qord = 0.06 p.u. (20 MVar) Medium heavy loading condition Iord = 0.97 p.u. Vdc = 0.96 p.u. Rectifier mode Iord = 1.25 p.u.a Pord = 0.86 p.u. (300 MW) Qord = 0.06 p.u. (20 MVar) DC voltage set-point ± 250kV ± 250 kV ± 250 kV a The current order for LCC inverter station is for VDCOL function only, the current controller at LCC inverter was inactivated. 3 VDCOL control function design Simulation results in authors’ previous work showed that VDCOL function should be installed in both LCC rectifier and inverter. Moreover, the parameters of VDCOL need to be properly tuned for each operating condition [5]. This section presents an overview of VDCOL design in the literature and the design of master self-tuning VDCOL for the hybrid multi-terminal MTDC used to connect geothermal power plant to SE Australian system. 3.1 Conventional and adaptive VDCOL The characteristic of a conventional VDCOL are illustrated in Fig. 3. The maximum allowable DC current after VDCOL, IVDCOL, will be the order current to LCC converter when voltage drops. Fig. 3Open in figure viewerPowerPoint Conventional VDCOL characteristic [8] The effectiveness of four VDCOL ramps, including linear, double linear, delayed double linear and exponential ramps of the dynamic VDCOL characteristics was introduced in [8]. The exponential ramp A is the simplest and most effective one among them. Therefore, in this paper the proposed VDCOL is designed based on an exponential ramp. The adaptive neuro-fuzzy VDCOL in [11] stored seven ramp functions of the output current, similar to the ramp D shown in Fig. 3. Each ramp function corresponded to each value of the reference current of the HVDC station where the VDCOL is installed. Another adaptive fuzzy VDCOL is designed based on a simplified model of DC P-Q coupling recovery in [15]. In this VDCOL, voltage lower and upper limit of VDCOL is selected based on limited numbers of active power value delivered by the MTDC. The advantage of these adaptive fuzzy VDCOL to the conventional one is that this fuzzy VDCOL generates an adaptive current order to the HVDC system based on the reference current and DC voltage of the HVDC station [11, 12] or based on the change of active power delivered by the DC link [15]. However its disadvantage is the limited number of current order that can be generated and its complexity due to Fuzzy logic and the neutral network algorithm. The parameters of adaptive VDCOL proposed in [13, 14] can be adjusted by themselves based on ac voltage of the converter transformer. This type of adaptive VDCOL offers more flexibility with simpler design than the neuro-fuzzy one but need to be installed in each LCC converter of a MTDC system. 3.2 Proposed master self-tuning VDCOL The master self-tuning VDCOL is a combination of a conventional VDCOL and a number of multipliers. The input signal of the proposed master self-tuning VDCOL is the minimum DC voltage signal of LCC stations, adjusted with a time delay TU. Its output signal is called delta. In a particular LCC station, the product of the reference current (Iord) and delta produces the maximum allowable DC current after VDCOL (IVDCOL) for that station, as shown in Fig. 4. Therefore, the current order created by the proposed VDCOL is dependent on the reference current of each station, varying with the loading condition of the system in a simple way. Fig. 4Open in figure viewerPowerPoint Proposed master self-tuning VDCOL The output signal delta of the proposed VDCOL ramp is calculated by (1) where: Udc_min is the minimum measured DC voltage at LCC converters. u1 is lower voltage limit of the VDCOL. u2 is upper voltage limit of the VDCOL. i1 is minimum DC current of the VDCOL. i2 is maximum DC current of the VDCOL. The current order for LCC converter n generated by proposed master self-tuned VDCOL is calculated as (2) where: IVDCOL_n is the maximum allowable DC current after VDCOL at LCC converter n. Iord_n is the reference current at LCC converter n. Udc_min is the minimum measured DC voltage at LCC converters. u1 is lower voltage limit of the VDCOL. u2 is upper voltage limit of the VDCOL. i1 is minimum DC current of the VDCOL. i2 is maximum DC current of the VDCOL. The relationship between maximum allowable DC current after VDCOL with the LCC station reference current (Iord) and DC voltage is demonstrated in Fig. 5. The number of ramp functions is unlimited and depends on the number of reference current settings. That is one of the advantages of the proposed VDCOL. Fig. 5Open in figure viewerPowerPoint Static characteristics of the proposed VDCOL 3.3 Master self-tuning VDCOL parameter selection In this section the parameters of VDCOL are selected for the heavy loading condition (case 1) in order to minimise the commutation failure of the MTDC system when subjected to a fault at the LCC inverter station by trial-and-error method. The commutation failure time is recorded by DIgSILENT PowerFactory software. The initial parameters of the VDCOL are chosen as the common practical value for LCC HVDC, taken from [16] and given in Table 3. Table 3. Common VDCOL parameters [16] Parameters Time delay Voltage lower limit Voltage upper limit Minimum DC current Maximum DC current TU, s u1, p.u. u2, p.u. i1, p.u. i2, p.u. Value 0.015 0.25 0.75 0.345 1 Fig. 6a demonstrates the extinction angle (gamma) and the DC voltage at the LCC inverter station when it is subjected to a 50 ms balanced fault. As can be seen from this figure, after the fault is cleared there is a commutation failure at 0.637 s. Fig. 6Open in figure viewerPowerPoint Inverter signal with initial and tuning VDCOL (a) System responses with initial VDCOL, (b) System responses with tuning VDCOL in heavy loading condition (case 1), (c) System responses with tuning VDCOL in medium heavy loading condition (case 2), (d) System responses with tuning VDCOL in peak load condition (case 3) When parameter i1 is adjusted to 0.35 the commutation failure does not happen as shown in Fig. 5b when the system is subjected to the same disturbance. Figs. 6b–d shows the extinction angle (γ) and the DC voltage at the LCC inverter station when subjected to the 50 ms balanced fault in three considered loading cases. The system is stable after the disturbance and the commutation failure time in all the cases is <100 ms. 4 Master self-tuning VDCOL performance In authors’ previous work [5], two conventional VDCOL function was designed to for LCC rectifier and inverter. The parameters of these VDCOLs were tuned for each operating condition in order to maintain system stability during low-voltage condition. The tuning parameters for MTDC system in heavy loading and medium heavy loading conditions are shown as conventional VDCOL_1 and conventional VDCOL_2 in Table 4, respectively. Table 4. VDCOL parameters Parameter Proposed VDCOL Conventional VDCOL_1 Conventional VDCOL_2 Rectifier Inverter Rectifier Inverter time delay (TU), s 0.015 0.015 0.015 0.015 0.015 lower voltage limit (u1), p.u. 0.25 0.25 0.25 0.25 0.25 upper voltage limit (u2), p.u. 0.75 0.75 0.75 0.75 0.75 minimum DC current (i1), p.u. 0.35 0.3395 0.2345 0.3395 0.4375 maximum DC current (i2), p.u. 1 0.97 0.67 0.97 1.25 Using the proposed VDCOL, separated VDCOL function in each LCC converters are not required. Moreover, VDCOL parameters will not have to be tuned for different loading condition. Table 4 also shows the parameters of the proposed VDCOL. 4.1 Robustness in different loading conditions In this section, the performance of the proposed VDCOL (with adaptive output) with conventional VDCOL_1 (designed for MTDC system in heavy loading conditions) and conventional VDCOL_2 (designed for MTDC system in medium heavy loading conditions) is investigated when the system is subjected to a 50 ms three phase balanced fault at LCC inverter station. It should be noted that the parameters of these conventional VDCOL will not be tuned in order to assess the robustness of the proposed VDCOL. In the heavy loading condition system (case 1), the reference current at the LCC rectifier and inverter is 0.97 and 0.67 p.u., respectively. In the medium heavy loading condition system (case 2), the reference current at the LCC rectifier and inverter is 0.97 and 1.25 p.u., respectively. In both cases, the parameters of the conventional VDCOL at each LCC station are selected so that the output current of each conventional VDCOL is equal to the product of the output signal of the proposed VDCOL and the reference current of the corresponding station. Figs. 7–9 shows the DC voltage at each MTDC station with each above VDCOL when the system is subjected to a 50 ms three balanced fault at LCC inverter station in three considered loading conditions: heavy loading, medium heavy loading and peak load. Part a, b and c in each figure illustrate the voltage response of each terminal when conventional VDCOL_1, conventional VDCOL_2 and the proposed master self-tuning VDCOL is employed, respectively. Fig. 7Open in figure viewerPowerPoint Heavy loading condition (Case 1) (a) Conventional VDCOL_1, (b) Conventional VDCOL_2, (c) Master self-tuning VDCOL Fig. 8Open in figure viewerPowerPoint Medium heavy loading condition (Case 2) (a) Conventional VDCOL_1, (b) Conventional VDCOL_2, (c) Master self-tuning VDCOL Fig. 9Open in figure viewerPowerPoint Peak loading condition (Case 3) (a) Conventional VDCOL_1, (b) Conventional VDCOL_2, (c) Master self-tuning VDCOL As can be seen from parts a and c of these figures the conventional VDCOL_1 has similar performance to the master self-tuning VDCOL when the system is operated in heavy and peak loading conditions. The reason is that the reference currents of the HVDC converters in these two loading conditions are identical. It appears that the conventional VDCOL_1 works properly for only cases 1 and 3. When the system is operated in the medium heavy loading condition (case 2), VDCOL_1 cannot help the voltage to recover after the disturbance. Similarly, VDCOL_2 can stabilise the system in case 2 only. Meanwhile, the proposed master self-tuning improves the voltage stability of the system in all three considered loading conditions. Therefore, the proposed VDCOL is found to be more robust than the conventional ones. As the reference current at each station varies under different loading conditions, the conventional VDCOL would be more effective if its parameters are accordingly tuned. However, it will make the operation process more complicated, especially when the changes are required in different LCC stations. The more LCC stations in an MTDC, the more complex the tuning process will be. 4.2 Improves the start-up process for LCC inverter In this section, the system is first operated with the LCC inverter in the blocking mode, i.e. there is no power transferred from the geothermal site to area 4 of the AC grid. Only 300 MW geothermal power is generated to supply area 5 through the VSC station. At 0.5 s, the inverter is started and operated in extinction angle control mode. The reference current of the LCC rectifier is increased from 0.26 to 0.97 p.u. to supply 700 MW power for area 4. Fig. 10 illustrates the DC voltage at the LCC inverter and VSC station with the conventional VDCOL_1 and the proposed VDCOL. The start-up process of MTDC is considered for the heavy loading condition only. Fig. 10Open in figure viewerPowerPoint Start-up process with the proposed VDCOL (a) With conventional VDCOL, (b) With proposed VDCOL As can be seen from Fig. 10, if the conventional VDCOL is used the DC voltage at the LCC inverter station fluctuates around 0.4 s in 100 ms and then settles down at 0.9 p.u. Therefore, the system is operated in low-voltage condition and the active power transferred to area 4 does not reach its rated value. If the proposed VDCOL is used the DC voltage of the LCC inverter station is up to its rated value (0.99 p.u.) at 0.9 s. 4.3 Enhances system stability when subjected to large disturbances In the previous sections, the proposed VDCOL was proven to be effective in damping the oscillation of the system when subjected to a fault at the LCC inverter of the system. In this section, the performance of the proposed VDCOL (with adaptive output) is investigated when the system is subjected to a 50 ms three-phase balanced fault at other locations. Only the heavy loading condition is considered here. Figs. 11a–c show the DC-voltage magnitude of each station of the MTDC system when subjected to a fault at the LCC geothermal power plant, VSC station, DC link, respectively. Fig. 11Open in figure viewerPowerPoint DC voltage magnitude of each station of the MTDC system when subjected to disturbances (a) Fault at the geothermal power plant, (b) Fault at the VSC station, (c) Fault at the DC link As can be seen from those figures the proposed VDCOL helps to stabilise the system when subjected to large disturbances. 5 Conclusions A master self-tuning VDCOL is designed to improve the performance of a hybrid MTDC proposed to connect largescale renewable energy sources to an existing HVAC network. As conventional VDCOL is designed for a specific loading condition, the parameters need to be tuned when loading conditions and load flow direction change. Therefore to overcome this complexity a master self-tuning VDCOL is proposed to effectively improve the transient stability of the system under different loading conditions. In the study case, one master self-tuning VDCOL replaces two VDCOL in two LCC converters, having unlimited ramp functions. Simulation results illustrate the robustness and effectiveness of the proposed VDCOL in improving the transient stability under different loading conditions. 6 Acknowledgment This work was supported by Queensland Geothermal Energy Centre of Excellence (QGECE), the University of Queensland, Australia. 7 References 1AEMO, AEMO Australian Energy Market Operator [23-11-2012]; Available at http://www.aemo.com.au/corporate/aboutaemo.html 2ABB Library, HVDC and HVDC Light. Available at www.abb.com/hvdc 3Liang, J., Jing, T., Gomis-Bellmunt, O. et al.: ‘Operation and control of multiterminal HVDC transmission for offshore wind farms’, IEEE Trans. Power Deliv., 2011, 26, (4), pp. 2596– 2604 4Arrillaga, J., Liu, Y.H., Watson, N.R.: ‘ Flexible power transmission’ ( The HVDC option, John Wiley, 2007) 5Nguyen, M.H., Saha, T.K., Eghbal, M.: ‘Hybrid multi-terminal LCC HVDC with a VSC converter: A case study of simplified south east Australian system’. 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