http : //www.cigre.org B4-053 AORC Technical meeting 204 Optimized Operation of Hydropower Plant with VSC HVDC Unit Connection Xiaobo Yang, Chao Yang, Chengyan Yue, Dawei Yao, Chunming Yuan Corporate Research, ABB (China) Limited China SUMMARY Among various renewable sources, hydropower is still the cheapest clean energy. As most of the new constructed hydropower plants are located at remote areas, HVDC lines are usually employed for hydropower transmission. In this paper, hydropower plant with VSC HVDC unit connection is proposed, in which the generator(s) of a hydropower station is connected to a voltage source converter (VSC) station directly and the power is transmitted by DC line. By using VSC HVDC unit connection, the hydraulic turbine realizes variable speed operation and thereby improves the hydraulic turbine efficiency substantially. In the paper, the progress of hydropower unit connection is reviewed and a typical configuration of hydropower plant with VSC HVDC station unit connection is introduced; the advantages of employing VSC HVDC unit connection technology are presented. A calculation model is established to calculate the maximum hydraulic turbine efficiency. Finally, the turbine efficiencies are compared between the variable speed operation of hydropower plant with VSC HVDC unit connection and fixed speed operation of a conventional hydropower plant; the power frequency variation range during variable speed operation is analyzed. KEYWORDS DC rid- HVDC- Hydropower- Unit Connection- Variable Speed- VSC HVDC xiaobo.yang@cn.abb.com
. Introduction The synchronous generators used in the conventional hydropower plants are usually operated to match the rated power frequency of integrated AC grid, i.e. 50 Hz or 60 Hz. The turbines are also designed to run at the speed specified by the frequency, under rated hydraulic head and power. It means that the design and control of hydropower plant are strictly constrained. Any deviations of hydraulic head or power order will result in decrease of efficiency. However, if the rotating speed of turbine can be adjusted, the efficiency of hydropower generation will be increased significantly. It is estimated that by applying appropriate variable speed control to the hydraulic turbines, the annual average efficiency of the power plant can be increased from 2% to 5%, the larger the water head may change, the higher the efficiency can be increased []. Variable speed operation of hydropower plant results in a substantial improvement in system efficiency and performance. It also brings benefits from additional control flexibility, optimized reservoir siting, less environmental impact, relaxed turbine design parameter and hydraulic turbine service time, etc. By far, there are basically 3 solutions in terms of variable speed operation of hydropower plant, namely based on doubly fed induction generator, full power converter and HVDC converter station, respectively.. Hydropower plant with doubly fed induction generator The adjustable speed operation of hydraulic turbine can be achieved by using doubly fed induction generator (DFI). Today, two converter technologies for DFI control are available, viz. thyristor based cycloconverter and forced turn-off device based PWM converter, as shown in Figure (a) and (b) respectively. Although the power of cycloconverter or the PWM converter connected to the generator is only a part of the rated generation power (depending on the variable speed range requirement), the DFI is an expensive wound rotor asynchronous machine. Therefore existing commercialized DFIs are only installed for pumped storage power plants which have relatively small power. AC grid Smoothing reactor Pw Pm Asynchronous generator Rectifier (a) DFI with cycloconverter Inverter AC grid Pw Pm Capacitor Asynchronous generator Rectifier Inverter (b) DFI with PWM converter Figure Hydro power plant with DFI for variable speed operation.2 Hydropower plant unit connection with a back-to-back full power converter Figure 2 shows a hydropower plant with connection of a back-to-back full power PWM converter (FPC). In past years, ABB had commissioned two FPC systems for pumped storage power plants (60MW Panjiakou pumped storage, China, 99 and 4 265MW oldisthal pumped storage, ermany, 2004). By using FPC, the generator is fully decoupled from the AC grid. The nominal power of the generator can be reduced because the additional capacity for negative sequence component during grid fault is not needed. The whole system performance is also improved because of the decoupled control of active power and reactive power at grid side converter.
AC grid Pw Pm Capacitor enerator Turbine Rectifier Inverter Figure 2 Hydro power plant with full power PWM converter.3 Hydropower plant unit connection with HVDC converter station Nowadays, more and more HVDC lines are employed for hydropower transmission due to most of the new constructed hydropower plants are located at remote areas. The coordination of hydropower plant and HVDC converter station brings the possibility to develop variable speed operation technologies for considerably large hydraulic turbines at conventional hydro power plant without additional investment. The direct connection of hydropower plant and HVDC was referred to unit connection in some earlier literature [8, 2, 5]. A typical hydro power unit connection scheme with line commutated converter HVDC (LCC HVDC) is shown in Figure 3, in which a is the guide van opening and n is the turbine speed. overnor a Hydrolic Turbine n Synchronous Machine uabc Converter Transformers + HVDC converter station AC grid uabc Excitation System Aux. Power supply Unit Connection Figure 3 Hydropower plant unit connection with LCC HVDC Hydropower plant unit connection with HVDC was firstly presented by Brown Boveri & Cie (BBC) in 973 [2], but the turbine was assumed to operate at a fixed speed. Since 980s, many literatures had proposed HVDC converter station for adjustable speed operation of hydropower plant [3-2], including variable speed pumped storage power application [3, 4], harmonic measurement and analysis of generator [6, 0, ] and unit connection system design [7, 9,, 2, 5]. In all these papers, only LCC HVDC system for hydro power unit connection was studied. The applications of VSC HVDC for hydropower unit connection were rarely discussed, because the capacity and voltage rating of VSC HVDC were thought as major limitations for transmission level application. However, in recent years, the VSC HVDC technology has been developed significantly and its application has been extended from sub-transmission and distribution level to transmission level. In 2008, a short discussion on application of VSC HVDC technology in hydropower exportation was published [3], while no in depth information, such as circuit topology, control and protection scheme, has been introduced. In this paper, hydropower plant with VSC HVDC unit connection is addressed. Firstly, the advantages of employing VSC HVDC unit connection technology are presented in Section 2; then in Section 3, a calculation model is established to calculate the maximum hydraulic turbine efficiency; finally, the turbine efficiency of variable speed operation is compared with that of fixed speed operation, based on a typical conventional hydropower plant. The frequency variation range during variable speed operation is also analyzed, to investigate the requirement on the generator circuit breaker. 2. Hydropower plant with VDC HVDC unit connection A VSC connected hydropower plant to DC grid will give fully advantages of variable speed operation of turbines and bring additional commercial interests. The hydropower plant with direct VSC HVDC connection can be regarded as a DC source in the DC grid, in which the DC voltage and DC power will be adjusted according to the power order from the dispatch center of the DC grid. Further, it s - 2
possible to connect several hydropower plants into one DC grid. This is particularly profitable when several hydropower plants are located along one river and cascaded constructed. The hydropower plant with VSC HVDC unit connection is proposed as shown in Figure 4. Hydrolic Turbine a overnor a* Synchronous Machine VSC HVDC Unit Connection n u abc Excitation System I exc* VSC HVDC Converter Station Converter Transformers u abc Power plant controller U dc_ref + - Pord U dc Hydraulic head DC grid Figure 4 Hydropower plant with VSC HVDC unit connection and its integration to the DC grid The major advantages of hydropower plant with VSC HVDC unit connection come from variable speed operation of hydropower plant, as listed below: Significant energy savings because of higher efficiency of hydraulic turbine; Longer life time of turbine thanks to reduced noise, vibration and cavitation problems; New flexibility in site selection and sizing of hydro units; Relaxation of parameter requirements on turbine design; Fast generation control for wind/solar power integration; Less environmental impacts, etc. Figure 5 Hill Chart curves of a normalized Francis type turbine The characteristic of variable speed operation with higher efficiency can be explained by the so called Hill Chart curve. Figure 5 presents the Hill Chart curve of a Francis type hydraulic turbine [4], in which is the unit water flow quantity and is the unit hydraulic turbine speed. In this example, for the constant rotating speed operation of the hydraulic turbine at certain water head, the efficiency of the turbine will decrease from 89% to 87% (point A to B) when unit flow changes from about 0.62 m 3 /s to 0.55 m 3 /s. In contrast, with variable speed operation, approximately 2% efficiency increase is obtained by adjusting the turbine speed from 69 r/min to 62 r/min (Point B to C) under the same flow quantity. Consequently, the adjustable speed of turbine permits a maximum efficiency tracking. 3. Calculation model for operation optimization Based on the Hill Chart in Figure 5, a calculation model for the Francis turbine is established to calculate the maximum efficiency from variable speed operation of hydropower plant with VSC HVDC unit connection. The model consists of the following calculation sections: 3
[m 3 /s] [r/min] ) Unit power P : P P / ( H D ), () 3 2 e where P is the electric power; η e is the efficiency of generator; H is the water head and D is the nominal diameter of hydraulic turbine. 2) Maximum efficiency curve f : f ( Q ), (2) H max where is the maximum turbine frequency, which is a variant of unit flow quantity The curve is obtained from the turbine Hill Chart. 3) Maximum Power curve f 2 The unit flow quantity Q for given unit power P can be expressed as Considering (2), Hmax Q. Q P / 9.8. (3) P becomes function of Q : P 9.8 Q f ( Q ) f ( Q ) (4) 2 Then Q can be obtained from inverse function of (4): Q f2 ( P ). (5) 4) uide opening curve f 3 : a0 f3( Q ), (6) where a 0 is the guide van opening angle. The curve is obtained from the turbine Hill Chart. 5) Efficiency peak curve f 0 n f0(q ), (7) H H max where n is unit turbine rotating speed. The curve is obtained from the turbine Hill Chart. 6) Rotating speed n: n n H / D. (8) The efficiency peak curve f 0, maximum efficiency curve f, maximum power curve f 2 and guide vane opening curve f 3 are shown in Figure 6 (a)-(d), respectively. 70 0.95 0.95 68 0.93 0.93 n 66 64 Hmax 0.9 0.89 Hmax 0.9 0.89 62 0.87 0.87 60 0.5 0.6 0.7 0.8 0.9 Q [m 3 /s] 0.85 0.85 0.5 0.9 0.6 0.5 0.6 0.7 0.8 0.9 Q [m 3 /s] (a) (b) f ( Q ) n f0(q ) H H max H max 0.9 24 24 0.8 22 20 22 20 0.7 8 8 Q 0.6 a 0 6 a 0 6 0.5 4 2 4 2 0.4 4 5 6 7 8 P [kw] 0 0 0.5 8 0.6 0.5 0.6 0.7 0.8 0.9 Q [m 3 /s] (c) Q f2 ( P ) (d) a0 f3( Q ) Figure 6 Hydraulic turbine characteristic curves 4
The calculation model is established in Matlab/Simulink to calculate the maximum efficiency during variation of water head or power order. The calculation diagram is shown in Figure 7. ηmax=f(q) Maximum Efficiency a0=f3(q) uide vane opening Water Head Turbine diameter Power order H(m) D(m) P(kW) P(kW) Unit Unit power flow Q=f2 - (P) n=f0(q) Unit rotating speed H(m) n D(m) n (r/min) f (Hz) Rotating speed Power freqency Figure 7 Simplified diagram for maximum efficiency calculation under variable speed operation The turbine efficiency under fixed rotating speed is also calculated and compared with the maximum efficiency under variable speed. The simplified calculation diagram for efficiency under fixed rotating speed is shown in Figure 8, in which the 2-dimensional interpolating method is used for calculation of unit flow and efficiency. Rated rotating speed Water Head Turbine diameter Power order n0 (r/min) H(m) n (r/min) D(m) Unit rotating speed calculation H(m) D (m) P(kW) P (kw) Unit rotating speed Unit power n (r/min) P (kw) Q (m 3 /s) Unit flow n (r/min) Q (m 3 /s) η Efficiency Figure 8 Simplified diagram for efficiency calculation under fixed speed operation 4. Benefits from variable speed operation Three calculation cases are studied in this section. In Case and Case 2, the efficiencies of variable speed operation and fixed speed operation are compared, in terms of water head variation and power order variation, respectively. Case 3 is performed to investigate the frequency variation range during the variable speed operation. Main parameters of the hydropower plant that used in the calculation cases are shown in Table I. Table I Main parameters of the hydropower plant Nominal power of turbine 306 MW Nominal power of generator 300 MW Pole pair number 28 enerator synchronous speed 07. r/min Efficiency of generator 0.98 Nominal diameter of turbine D 6 m Maximum water head H max 39.2 m Weighted average water head H av 9.9 m Optimized unit rotating speed 62 r/min 4. Case - Efficiency during water head variation The turbine efficiency vs. water head curves of both optimized variable speed operation and fixed speed operation are presented in Figure 9, including three scenarios with different power orders i.e. pu, 0.5 pu and 0.25 pu respectively. It can be observed that in each scenario, the variable speed operation will have higher efficiency than the counterpart of the fixed speed operation. The efficiency difference is getting larger when the water head is lower. At the nominal operation point (nominal 5
Frequency [Hz] Efficiency [%] Efficency [%] power and nominal water head), the efficiency difference is minimum, whereas there is still about.7% higher efficiency obtained. 0.9 0.8 0.7 0.6 pu (Variable speed) pu (Fixed speed) 0.5 0.5pu (Variable speed) 0.5pu (Fixed speed) 0.4 0.25pu (Variable speed) 0.25pu (Fixed speed) 0.3 60 70 80 90 00 0 20 Water Head [m] Figure 9 Case : The relationship of efficiency and water head under different power orders: variable speed and fixed speed 4.2 Case 2- Efficiency during power generation variation The turbine efficiency vs. power order curves of both optimized variable speed operation and fixed speed operation are presented in Figure 0, including three scenarios with different water heads, namely 40 m and 20 m respectively. It can be observed that compared with fixed speed operation, variable speed operation always has higher efficiency. Under certain water head, a maximum efficiency point can be obtained by variable speed operation. 0.95 0.9 0.85 0.8 40m Variable Speed 40m Fixed Speed 20m Variable Speed 20m Fixed Speed 0.75 60 80 200 220 240 260 280 300 320 340 Power [MW] Figure 0 Case 2: The relationship of efficiency and water head under different water heads: variable speed and fixed speed 4.3 Case 3- Impact of frequency variation on CB selection The power frequency of gerneator will impact the selection of the generator circuit breaker (CB) which connected between generator and converter transformer. enerally, CBs are designed and tested for normial power frequencies of 50 Hz or 60 Hz. For operation under norminal frequency, the magnitude of the current to be interrupted has to be reduced as the duration of the cycle would be longer than that at norminal frequency. According to [5], convential circuit breakers can operate over a frequency band but are subjected to a lower frequency limit of about 45 Hz on account of arc extincition times. It is necessary to investagte the frequency variation range of the generator and determine whether a commerialized CB can be used. In Case 3, the power frequency band of variable speed operation is calculated, under differenct power orders ( pu, 0.5 pu and 0.25 pu respectively). The calculation results are depcited in Figure. It is showed that in this example, a frequency band between 45 Hz- 60 Hz permits a large water head variation range. The frequency variation may not be critical issue for CB selection. Finially, because of the unit connection of VSC HVDC, the generator is not directly connected to the AC grid. Thus it should be evaluated that if a CB is really necessary in the system. 60 55 50 45 40 pu 35 0.5pu 0.25pu 30 80 90 00 0 20 30 40 Water Head [m] Figure Case 3: the power frequency vs. water head under different power orders 6
5. Conclusions The hydropower plant with VSC HVDC unit connection has an attractive prospect for hydropower integration and DC grid application. The major advantages of VSC HVDC unit connection come from variable speed operation of hydropower plant, as well as lower total initial investment and faster response etc. The maximum efficiency is calculated by using the Hill Chart of a typical Francis turbine. The calcualtion results show that the variable speed operation of of hydropower plant with VSC HVDC unit connection always has higher efficiency than that of fixed speed operation, under varaious of power order or water head. Based on the calculation, at least.7% higher efficiency can be obtained. The frequency variation band of generator is also analysed and it shows that the frequency variation may not be critical issue for CB selection. 6. Reference [] F. H. Shihang Duan, "Integrated Design of Speed Regulating Operation of Large-sized Hydropower Plant Conventional enerating Units and High Voltage DC Power Transmission," Yunnan Water Power, vol. 7, 200: pp. 4. [2] J. Kauferle, "Infeed from Power Station with enerators and Static Converters in Unit Connection," Brown Boveri Review, vol. 60, 973: pp. 205-2. [3] R. J. Kerkman et al., "An Inquiry into Adjustable Speed Operation of a Pumped Hydro Plant Part I - Machine Design and Performance," IEEE Transactions on Power Apparatus and Systems, vol. 99, 980: pp. 828-837. [4] R. J. Kerkman et al., "An Inquiry into Adjustable Speed Operation of a Pumped Hydro Plan Part II - System Analysis," IEEE Transactions on Power Apparatus and Systems, vol. 99, 980: pp. 838-844. [5] J. Arrillaga et al., "Direct Connection of enerators to HVDC Converters: Main Characteristics and Comparative Advantages," ELECTRA, vol. 49, 993: pp. 9-30. [6] D. Nhut-Quang, "Harmonic Domain Modelling of Direct Connected enerator and HVDC Convertor Units," Ph.D., University of Canterbury, 998. [7] J.. Campos et al., "uide for Preliminary Design and Specification of Hydro Stations with HVDC Unit Connected enerators," presented at the CIRE JW /4-09, HVDC Unit Connected enerators, 997. [8] M. Naidu and R. M. Mathur, "Evaluation of Unit Connected, Variable Speed, Hydropower Station for HVDC Power Transmission," Power Systems, IEEE Transactions on, vol. 4, 989: pp. 668-676. [9] L. Ingram, "A Practical Design for an Integrated HVDC Unit-Connected Hydro-Electric enerating Station," IEEE Transactions on Power Delivery, vol. 3, 998: pp. 65-62. [0] S. J. Macdonald et al., "Harmonic Measurements from a roup Connected enerator HVDC Converter Scheme," IEEE Transactions on Power Delivery, vol. 0, 995: pp. 937-943. [] S. Sankar, "Dynamic Simulation of AC/DC Systems with Reference to Convertor Control and Unit Connection," Ph. D., University of Canterbury, 99. [2] X. Pang, "Some Aspects of HVDC Unit-Connected eneration Scheme," Master of Science in Electrical Engineering, Faculty of raduate Studies, The University of Manitoba, 992. [3] Y. L. Wei Wang, Lihua Zhao, "Discussion on Application of High Voltage Direct Current Light Technique in Hydropower Exportation Delivery," Shanxi Electric Power, vol. 5, 2008: pp. 5. [4] S. Xu, C. uo, and W. Wang, "Optimum Load Regulation of the Variable Speed Hydrogenerator with AC Excitation," Large Electric Machine and Hydraulic Turbine, vol. 2, 995: pp. 0-3. [5] Joint Working roup /4.09. uide for Preliminary Design and Specification of Hydro Stations with HVDC Unit Connected enerators. IRE, August, 997: pp. 4. 7