Cluster Interconnection of Offshore Wind Farms Using Direct AC High Frequency Links

Transcription

1 Cluster Interconnection of Offshore Wind Farms Using Direct AC High Frequency Links Alejandro Garcés, Marta Molinas Norwegian University of Science and Technology Department of Electric Power Engineering Trondheim, Norway Abstract Cluster connection of wind turbines is a promising topology which will enable to exploit the advantages of series and conventional shunt connection for offshore wind farms. This paper discusses the advantages and disadvantages of this kind of connection for high voltage direct current transmission and presents five different possible configurations for conversion system based on high frequency AC links. A hierarchical control strategy is proposed: in the first level, each turbine is controlled by using PI controls in rotor flux oriented frame while in the second level an optimal dispatch based on a non linear optimization model is used for coordinated onshore/offshore control. A case study of one of the configurations is presented with simulation results using Matlab and PSIM. Although only one of the proposed configurations is tested by simulations, the concept is general and can be easily extended to all the proposed topologies. β λ Φ Ψ r Ψ s ρ A w C p f I DC I kdc I km I r I s J m m np P M P T T E T m U s V On sh V w w m w r w s NOMENCLATURE blade pinch angle tip speed ratio blade diameter flux in rotor flux in stator air density rotor swept area performance coefficient switching frequency Current in the HVDC transmission line Current in the cluster k at the offshore grid DC voltage in the conversion system m and cluster k current in rotor current in stator inertia in the generator and the wind turbine duty cycle number of pairs of poles output mechanical power total generated power electrical torque mechanical torque voltages in stator DC voltage in the converter onshore wind velocity mechanical rotational speed in the turbine mechanical rotational speed in the generator electrical speed I. INTRODUCTION Wind energy is a promising technology for electrical power generation because of it well known environmental advantages over conventional ones [1]. In recent years, there has been an increasing interest in offshore wind farms since the longer the distance from the shore the higher and more constant wind velocity. Offshore technology takes advantage of this characteristic but poses new challenges to conversion and transmission. Due to this long distances, conventional AC transmission is not an economical technology anymore and the HVDC appears to be the most suitable option [2]. Design of the conversion system offshore requires taking into account not only efficiency and reliability but also size and weight, inasmuch as expensive platforms must be placed to support each new component. Although many topologies have been proposed for offshore connection, there are two main well defined alternatives: series or parallel connection. According to [3], series connection leads to less transmission losses and increases power density without heavy high power transformers, while parallel connection permits a more reliable operation. In spite of efficiency in transmission series connection is superior compared with other topologies; global efficiency is still less due to the converters losses. A good compromise between reliability and efficiency could be achieved using a composed topology called cluster connection by Lungberg [4]. Converter topology and its modulation are key factors to make viable series and cluster connection with reduced losses in conversion system and the transformer. In [5] a comparison between conventional back to back topology and one of the proposed topologies in this paper was presented. This paper proposes four additional converters topologies based on high or medium frequency AC links. Squirrel carrel induction generators are used for electricity production. Each generator is connected using a high frequency link which consists on an AC/AC converter, a high frequency transformer and an AC/DC converter. Four converters topologies are described and compared with the conventional backtoback converter. A case study for one of the four topologies is presented including a detailed comparison of losses analytically and by simulations. A simplified model for the transmission cable is also taken into account when modeling the system. A coordinated control based on the

2 rotational speed of the generator is proposed due to the fact that small variations in turbine speed could be the source of abnormal operation in the system due to series connection. The paper is exposed as follow: First, introduction of cluster connection is given, followed by the general concept of high frequency link. Then, five different topologies based on this concept are exposed, and finally, results from a case study of one of the topologies are discussed. II. CLUSTER CONNECTION Series connections could become an interesting alternative for offshore wind farms due to the fact that this concept makes possible to raise the voltage to a transmission level without a heavy high power transformer which needs an offshore platform as is shown in fig 1(a). On the other hand, there are well known attractive characteristics in conventional parallel connection [6]. Therefore, a combination between series (fig 1 b) and parallel connection (cluster topology), as shown in fig 2 is studied in this paper. Cluster topology could increase the reliability of the system holding the advantages of both connections. Wind Turbines (a) Parallel connection OffShore High Power Transformer OffShore High Power Converter HVDC Transmission HVDC Transmission Fig. 2. HVDC Transmission Cluster connection in a wind farm for HVDC transmission losses are important issues to take into account. Comparison between AC and series DC transmission was also discussed. A series topology based on cycloconverters using fast thyristors and a medium frequency transformer was also proposed in [9]. However, an offshore platform is required for the AC/DC conversion step. In [10] a medium frequency AC link was proposed and a control strategy was presented. Meyer [3] shows a comparison between different offshore grids configurations, considering operative and investment cost. Series connection is presented as an attractive option if DC converter technology can maximize efficiency and reduce investment cost. Series configuration presents less cable losses and require less conversion stages and investment cost, however, the efficiency and reliability in the conversion system must be increased to make this configuration economically and technically feasible. In addition, a coordinated control must be used to operate the system; series converter needs a constant and equal current in each turbine, otherwise, there could be risk of abnormal operating conditions. III. ENERGY CONVERSION TOPOLOGIES BASED ON AC/AC LINKS Conv Conv 50 Hz AC 10 khz Square AC DC Nancelle (b) Series Connection Fig. 1. Parallel vs series connection Recent studies have shown different topologies for series connection using current source converters. In [7] a thyristor based current source converter is proposed for series connection of permanent magnet synchronous generator. Two and three turbines were simulated showing a good performance of this kind of topology. Current source converters based on self commutated switches has been also proposed in [8] showing that insulation and optimal management of transmission cable Generator AC/AC Converter High Freq. Transformer AC/DC Converter Fig. 3. Proposed High Frequency Link to increase efficiency of AC/DC conversion This section presents five possible AC/AC conversion topologies that can be used in the suggested cluster interconnection. In fig 3 the general concept of high frequency link

3 is shown. A high frequency transformer is used for isolation purposes. The AC/DC converter allows transformation of the DC voltage in a square wave high frequency voltage signal. This voltage is finally transformed in a 50 Hz three phase wave through an ACAC converter based on reverse blocking IGBTs (RBIGBT) and using the PWM technique presented in [11]. The new RBIGBT increases the efficiency and permits to reduce significantly the number of semiconductor devices compared with the conventional back to back topology [5]. Fewer elements in the converter topology means less switching and conducting losses and more reliability. Five topologies are going to be discussed for use in the proposed high frequency AC link. These topologies are: BacktoBack converter (BTB), Reduced Matrix Converter with Full Bridge (RMCFB), Reduced Matrix Converter with Three Winding Transformer (RMCTW), Three Phase Matrix Converter (TPMC) and the Double Input Matrix Converter (DIMC). The main characteristics of these topologies are given bellow: 1) BTB (Fig 4): This topology [10] uses the classical conversion stages for series connection, a full bridge permits to convert the 50 Hz voltage in a DC signal and then in a medium frequency square voltage which is delivered to the AC/DC converter though a medium frequency transformer. No bidirectional switches are required since there are an intermediate DC stage with an electrolytic capacitor. Nevertheless, the number of semiconductor devices is 28 and the use of electrolytic capacitor is inconvenient for offshore applications were reliability and weight are key factors for operation, maintenance and investment. 2) RMCFB: Reduced Matrix Converter with full bridge DCAC converter (Fig 5): This configuration presented in [11] uses a single phase high frequency transformer and reduced matrix converter with 6 bidirectional switches (12 RBIGBTs). In [5] this topology was compared with BacktoBack converter, showing superior performance regarding conduction and switching losses for the RMCFB. This is result of the use of RBIGBTs and the reduced number of switches. This paper includes results discussing the controllability of this topology and clarifies some aspects about how the cluster connection can be implemented. 3) RMCTW: Reduced Matrix Converter with a three winding transformer (Fig 6): It uses the same philosophy than the RMCFB topology but in this case the AC/DC conversion stage is made by means of two bidirectional switches and a three winding transformer. In [12] RMC TW was successfully implemented for particle accelerator applications. The number of semiconductor devices are reduced and RBIGBTs are implemented for both AC/AC and AC/DC conversion stages. However, the high frequency transformer is bigger. This topology will be studied in future works but results presented here about control and operation are equally applicable to this converter topology. 4) TPMC: Three phase matrix converter (Fig 7): This topology uses a three phase high frequency transformer and a conventional matrix converter [13]. A three phase transformer could be more efficient than a single one [3] but the number of semiconductors and switching complexity is highly increased. In spite of the fact that conventional matrix converter is a well known technology, few proposals have been done in wind energy. In [14] a study about reactive capacity in matrix converter for onshore applications was presented. 5) DIMC: Double Input matrix converter (Fig 8): In [15] a nine switches double output converter is proposed to feed two independent three phase loads. In this paper, this topology is adopted for series connection following the layout presented for the previous cases. The concept is modified to include bidirectional switches based on RBIGBTs, moreover, the capacitor originally proposed in [15] was eliminated. This new kind of converter can be used with a double fed induction machine, a six phase induction generator or a double rotor wind turbine [16] and will be investigated later as part of this research. AP AN BN Fig. 4. BP CN CP Case I: Back to Back converter (reference case) AP BP CP AN BN CN Fig. 5. Case II: Reduced Matrix Converter with full bridge DCAC converter and single phase high frequency transformer AP BP CP AN BN CN Fig. 6. Case III: Reduced Matrix Converter with two primary windings transformer Although these topologies could use different operation frequencies (high or medium), high frequency reduces not only the weight and size of the transformer but also the harmonics and filters. The output of the full bridge is the output of the

4 AX BX CX AY BY CY AZ BZ CZ Fig. 7. Case IV: Three phase matrix converter with three phase high frequency transformer AP BP CP AM BM CM AN BN CN Fig. 8. Case V: Double output matrix converter turbines are connected in series, the objective is not to achieve maximum tracking point in each single turbine but to achieve maximum power generation in each cluster. Series connection imposes some restrictions which are taken into account in the coordinated control strategy. Therefore each turbine will be operated to guarantee constant DC current. According to [17] a general numerical approximation for C p can be used since differences between different commercial wind turbines are very small: ( ) 151 C p (λ, β) = β 0.002β e 18.4 λ i with λ i = B. Induction machine model 1 1 λ 0.02 β β 3 1 λ i (3) A squirrel cage asynchronous machine is used as generator; the stator current is represented using space vector theory and considering that I A I B I C = 0, so only two phases are needed to calculate the current, as follow: (4) wind turbine nacelle which is then connected in series with the other turbines in the park until a suitable level for high voltage DC transmission is reached (see fig. 2). A capacitor must be placed in the output of this nacelle to achieve a smooth DC current. Power generation with asynchronous machine depends on the input velocity and the three phase voltage; this voltage can be controlled using any of the proposed converter topologies. In the following section, case II is selected for a detailed investigation of performance and feasibility of the cluster operation of the wind turbine. IV. CASE STUDY: OPERATION OF THE RMCFB BASED CLUSTER This section shows the operation of the RMCFB. First the model of a single turbine and generator will be explained, followed by the switching pattern of the converter, and finally the coordinated control concept is going to be presented. A. Wind Turbine Model. Mechanical power in a wind turbine is mainly a function of wind speed and rotational speed in the generator, as is showed in equation 1. P M = 1 2 ρ C p(λ, β) A w V 3 w (1) I s = I A j IA 2 I B 3 (5) The induction generator model in α, β coordinate axis is as follow: U s = R s I s d Ψ s U r = R r I r d Ψ r (6) j (ω r ) Ψ r = 0 (7) Ψ s = L s I s L m I r (8) Ψ r = L m I s L r I r (9) T E T m = J m dω m (10) Stator flux can be estimated by using the measured stator voltage and current: Ψ s = and the rotor flux is: t 0 ( U s R s I s ) (11) The tip speed ratio is defined as: λ = w m V w ( Φ 2 A Wind turbine can be driven to keep a constant value of λ for maximum power generation. However, since the ) (2) where Ψ r = L r L m ( Ψ s L σ I s ) (12) L σ = L s L2 m L r (13)

5 C. Description of switching pattern β A dedicated PWM is used as switching pattern for RM CFB. A similar approach is used for the other topologies. First switching in the AC/DC converter is generated as a square wave, then a three phase modified per unit control signal is compared with a triangular wave to generate the reduced matrix converter switching; this triangular wave can be represented as the function: q Ψβ Ψ Ψp p y(t) = 1 2 acos(sin(2 π f t)); (14) π Ψα α three phase modified signal is calculated as a product between per unit control signal and the sign of the voltage in the secondary of the transformer as shown in Fig. 9. AP,BP, and CP represents the switches in the reduce matrix converter (see fig 5) while is the switch in the AC/DC converter. Control of the Asynchronous Generator Fig. 9. Control Phase A Control Phase B Control Phase C Sign Switching pattern for RMFB DC current can be calculated as: I DC = I A S A I B S B (I A I B ) S C (15) Space vector modulation could be also used in each of the proposed topologies. D. Hierarchical Control strategy Cluster connection requires control of the output DC current since this current must be equal in each series cluster. However there are restrictions in voltage and power which must be taken into account for reliable operation of the system. Therefore, a stationary state control based on an optimization technique is used to define the reference for the DC current in each turbine. The objective function is to achieve maximum output power in whole the wind farm. On the other hand, DC current is controlled by means of the speed of the machine since there is a direct relation between these two variables. The speed control is made using the AC/AC converter in each nacelle. In the following, these three stages of control will be explained. AP BP CP Fig. 10. Flux in the rotor frame 1) Speed control: The rotor flux in equation 12 is used to calculate the rotation angle to achieve rotor flux reference frame. In this frame Ψ rq = 0 and dψ rq / = 0 (see fig 10) and the real part of the equation 7 becomes: dψ rd R r I sd = 0 (16) This equation shows a direct relation between I sd and rotor flux, therefore, using a PI control it is possible to maintain the rotor flux in a desired value using current I sd. On the other hand, the electromechanical torque for rotor flux orientation is expressed as: T E = 3 2 np Ψ rd I sq (17) Due to the small dynamics of the rotor flux and its control, the electromagnetic torque (and indirectly the speed) is controlled by the current I sq. However, the converter does not control directly the currents I s since the control is made by the voltage U s. Combining all the electrical equations in the machine model for rotor flux frame orientation, the relation between stator voltage can be expressed as: ( ) d U sd = L σ I L m sd R σ I sd ω s L σ I sq R r L 2 Ψ rd r ( ) d U sq = L σ I L m sq R σ I sq ω s L σ I sd ω r Ψ rd L r (18) (19) the therms in parenthesis in equations 18 and 19 represent the coupling voltages between d and q which can be avoided adding them to the control voltages U s. In this way, a PI control can be used to control the stator current I s. Tunning parameters are the same for both PI controllers, since once the coupling voltages are added, equations 18 and 19 would have the same constants. As is shown in equation 17, torque is controlled by the current I sq. Combining equation 17 and 10, the mechanical system considering the turbine model for constant values of V w and β is:

6 3 2 np Ψ dω m rd I sq T m (ω m ) = J m A new control variable I x is defined as: (20) 1 I x = I sq 3 np T m (ω m ) (21) 2 Ψ rd(reff) replacing 21 in 22 the mechanical equation becomes linear as function of I x and ω m 3 2 np Ψ dω m rd I x = J m (22) a simple proportional control acting over the new variable I x can be used to control the speed. The real current I sq can be calculated from I x using equation 21. A schematic view of the whole speed control is shown in the block diagram in fig 11. ωr Iq Is αβ dq Id ωreff From Current Control Fig. 11. ψreff θr P Rotor Flux ψd Vs Ix θr PI Eq (21) dq Id(reff) Iq(reff) αβ/abc Usd,Usq Decoupling Eq 18 and 19 Schematic diagram of the speed control strategy PI PI To Switching: Control Phase A Control Phase B Control Phase C As the main objective of the control is not the speed but the output current, a speed estimator based on the change in the angle of the estimated flux can be used. However, the result presented in this article, uses the real value of the speed. 2) Control of the output current: The set point for the speed is calculated using an approximated inverse function of equation 1,this model was done using a fourth order polynomial approximation of the inverse of the function 3 as follow: w reff = H(P M, V w, β) (23) λ i = 4 a k (C p ) k (24) k=0 This function is just a fast way to achieve an approximated set point for the speed, however, there is not guarantee of minimum stationary state error, since it is an open loop control. A PI control is used to achieve the expected current behavior, therefore, this open loop control is the main control for large changes in wind velocity (like in the start up) while the PI control acts as fine adjustment. Gains of the PI must be adjusted to guarantee small and smooth changes. This control structure is shown in fig 12. ωr Fig. 12. IA,IB Is Speed Control ωreff VT Vs Switching Eq 23. PI Expected Power x VDC Block diagram of the current and speed control IDC(reff) From Coordinated Control 3) Coordinated OfShoreOnShore Control: The control signal is U s. This signal is sent to the switching algorithm to achieve the desired stator voltage. Three phase AC voltage is function of the input V DC and the duty m. This voltage is controlled through the matrix converter. 3 V AC = 2 2 m V DC (25) a capacitor is placed in the DC side of the converter to reduce the ripple in the current and also to increase the stability in the DC voltage, since this voltage is important to control the machine. Stationary state output DC current in each cluster k by each generator m is calculated (eq. 26). I k(dc) = P km V km(dc) (26) Interconnection between on shore and offshore is calculated taking into account the model of the cable. In stationary state, only resistance have a real effect, this model is shown in fig. 13:... Fig. 13. farm... V11 V1m... Vk1 Vkm Vkm... Ik(DC) VOffShore OffShore Network Rk R1 Cluster 1 Cluster k Detail of each nacelle Vkm Transmission Generator AC/AC IDC High Freq Transformer I(DC) AC/DC RL VOnShore Layout of the entire cluster interconnection of the offshore wind A convenient way to control the whole the system, is to guarantee equal power generated by each turbine. This solution is possible for small and medium size wind parks were wind velocity in each generator is almost the same. However, it is OnShore Converter

7 also possible to use an optimal load flow to maximize the power on shore and minimize losses as follow: max P T = k subject to P km m k R k I 2 k (DC) R L I 2 (DC) (27) TABLE II PI TUNNING FOR THE CONTROLLERS variable input K P K I I DC ω ω s I sq Ψ r I sd I sd, I sq U sd, U sq V On sh = V Off sh R L I (DC) (28) 5000 V off sh = R k I k(dc) m V km (29) 0 I (DC) = k I k(dc) (30) I dc 5000 P km = V km I k(dc) (31) V km(min) V km V km(max) (32) P km(min) P km P km(max) (33) This optimization model could be solved using any nonlinear programming technique, and represents a control in stationary state. This control is coordinated between offshore and onshore, but since it is stationary state, not fast communication system is required. A. Single turbine V. SIMULATION RESULTS A single turbine and RMCFB was simulated. Parameters of the turbine and the generator were taken from [18] and are shown in table I. Simulation was made using Matlab and PSIM. Start up is shown although it is no the objective of the proposed control. PI tunning values are shown in table II TABLE I PARAMETERS OF TURBINE AND GENERATOR Parameter Value Unit Nominal Power 2 MW Number of poles 6 V DC 1 kv V DC(max) 1.2 kv V DC(min) 0.8 kv I DC(max) 2 ka f 10 khz L s mh L r mh L m mh Φ 75 m R k Ω R L Ω J m s Wind velocity was fixed in 9 m/s during the first 0.5 s and after that it was changed to 10 m/s, output reference current was fixed in 700 A as is shown in fig 14. Startup is taking into account 0.1 s Time [s] Fig. 14. Output DC current Over currents are observed during startup as is shown in fig 16, however these over currents can be reduced putting limits in the I dq PI gains, this action will increase the time of the start up. The change in wind velocity at t = 0.5 s produces a moderate change in the rotor speed reference which is necessary to achieve a small transient in the output DC current (see 14 and fig 15) B. Wind turbine in cluster connection Consider a system of 2 series connected cluster with 10 generators each one. Parameter of each generator are shown in table I. Wind speed in all generators is a value such as the available power is 2 MW except in one generator which has an available power of 1.5 MW. Optimal results were found using a software for nonlinear programming (General Algebraic Modeling System GAMS [19]) and are shown in table III TABLE III OPTIMAL OPERATING PINT FOR CLUSTER CONNECTION Cluster Turbine V DC I DC P M (max) (max) Except generators 2 to 10 in cluster 1 have a capacity to generate 2 MW this capacity can not be achieved due to voltage and current restrictions in the other generator. However, generators in the rest of the clusters can generate at maximum capacity independently. It is clear that individual maximum

8 Speed ω r I abc Time [s] 3 x Fig. 15. Generator rotor speed Time [s] Fig. 16. Generated threephase currents tracking point is not possible, but coordinated maximum total power is achieved considering all restrictions. VI. DISCUSSION A cluster connection for offshore wind farms was presented and discussed in details. This connection increases reliability compared with series connection but keep the possible advantages of series connection. Four different topologies with high frequency AC link based on matrix converters can be used in the conversion stages. An efficient control was designed to keep output current or power constant. A coordinated control has been proposed to maintain the system within proper operative limits. Operation of the entire wind farm was tested for normal conditions. The control strategy proposed has proved appropriated performance. Maximum power point tracking in each generator is possible but only if all generators in the same clusters have small wind velocity variations, otherwise, restrictions in one generator could limit capacity at the other generators in the same cluster. More sophisticated control strategies can be implemented to improve the transient behavior. The next step in the research will be to test the wind park when a wind turbine is lost an the effect in the onshore converter if a three phase fault in the AC will have in the performance of the entire park, in order to deliver the expected voltage and power. REFERENCES [1] B. Snyder and M. J. Kaiser, Ecological and economic costbenefit analysis of offshore wind energy, Renewable Energy, vol. 34, no. 6, pp , [2] N. Negra, J. Todorovic, and T. Ackermann, Loss evaluation of hvac and hvdc transmission solutions for large offshore wind farms, ELSEVIER Electric Power System Research, no. 76, pp , [3] C. 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