A New Connection Concept to Connect an Offshore Wind Park to an Onshore Grid

Transcription

1 A New Connection Concept to Connect an Offshore Wind Park to an Onshore Grid Marc VAN DYCK, Shantanu DASTIDAR and Kristof VAN BRUSSELEN CG Holdings Belgium NV Systems Division, Mechelen, Belgium Tel: +32 (0) Fax: +32 (0) Topics: Wind Power, Transportation and distribution Power System Abstract The rapid increase of power generation from renewable energy sources, in particular wind energy, is posing considerable challenges to the power utility grids. Previously when smaller wind turbines and wind parks were built and connected to a local grid, a fault on the network resulted in the disconnection of these wind turbines. However, as the size of these wind parks are getting bigger and bigger, any disconnection of these wind parks would cause serious stability problems in the grid. This necessitates the transmission system operator (TSO) to impose certain connection conditions for connecting these large wind parks to the grid. These conditions, amongst others shall be: supply of reactive power, limiting the voltage fluctuation and the fault ride through capability. In this paper a new connection method, used for the offshore wind park Belwind, in the North-Sea, in front of the Belgian coast, will be presented. Index Terms grid code, grid connection, reactive power, voltage control, offshore wind farm 1. Introduction Over the past few years, the amount of installed electricity generation capacity from wind energy is rapidly increasing. Previously, there were small, single onshore turbines connected to the distribution grid at different locations. Presently, the large wind parks, mostly offshore, are being connected at one point in the transmission grid. The installation of these offshore wind parks, with the aggregate output power of a single offshore wind park comparable to the output of a conventional electricity generation plant, necessitated strict connection requirements [1] in order to maintain the stability of the grid. This paper will give a short description of presently used technique to do the interconnection of an offshore wind park to an onshore grid and compare this technique with the new connection technique used in the Belwind project. The focus of the new technique will be on the reactive power requirements and switch-in transients. Some information about reactive power compensation and grid requirements for various wind turbine types is given in [2], [3] and [4]. 2. Presently used connection method The standard concept to connect offshore wind turbines to the onshore transmission grid is shown in Fig. 1. All turbines are connected in a few strings on a medium voltage level, between 20 and 33 kv (depending on the size of the turbines and the wind park), to the offshore high voltage substation (OHVS). In this OHVS the medium voltage is transformed to a high voltage level between 132 and 220 kv. The step-up transformer is equipped with an on-load tap changer and is responsible for the voltage regulation. The offshore substation also contains high voltage switchgear for the outgoing export cable. At the connection point of the high voltage switchgear with the export cable, the short-circuit current is almost equal to the short-circuit current in the transmission grid. The reactive power regulation (as required by the grid code for large wind parks), is done onshore. Onshore reactors, capacitor banks and Static VAR Compensators are connected to the high voltage level so that the generating plant (wind park) capability curve meets the requirements of the grid code. Fig. 1. Basic concept to connect an offshore wind park to the onshore grid for the Belwind project. To compensate the voltage and current transients when switching in the export cable, onshore high voltage resistors along with high voltage switchgear are used. The resistors are switched in to temporary raise the impedance thereby limiting the inrush current. 3. Belwind The new connection concept was developed for 1

2 the Belwind wind farm. In the following paragraphs a short description of the project and the most important grid code requirements are given Short project description The Belwind wind park is located in the North Sea, in front of the Belgian coast. The total output power of the first phase of the park is 165 MW. It consists of 55 turbines of 3 MW, of type VESTAS type V90. These wind turbines are equipped with doubly-fed induction generators. All turbines are connected at 33 kv level to the offshore high voltage substation (OHVS) by means of 33kV array cables. In this substation a 3-winding transformer steps up the voltage to 150 kv. The offshore substation is located at a distance of around 45 km from the landing point at the beach in Zeebrugge, Belgium. and not the total output power of the wind farm. In case the network operator does not ask a strong capacitive reactive power delivery, the wind farm is allowed to absorb a small amount of inductive reactive power. When all turbines are out of service, the wind park is treated as a consumer and the reactive power should be between and of the total maximum output power of the wind farm. Fig. 3 shows the required and allowed reactive power delivery in function of the output power. The curve is shifted to the capacitive side in order to make the inductive compensation of the export cable as small as possible and still meeting the grid requirements. The wind farm must be able to meet these reactive power requirements for grid voltages between 90 % and 105 % of the operating voltage (which is 155 kv for 150 kv lines). In Zeebrugge the wind farm will finally be connected to the Belgian 150 kv transmission system, operated by Elia the TSO. Construction and commissioning of this wind farms was completed in December Fig. 2 shows a picture of the OHVS after offshore installation on its monopole foundation. Fig. 3. Minimum reactive power requirements in function of the total output power of the wind turbines in service [ELIA] Fig. 2. Picture of the installed offshore high voltage substation 3.2. Elia grid code requirements The Belwind wind farm is connected to the Belgian grid, so all connection requirements stated by the Belgian grid operator, Elia, have to be fulfilled. The full grid code can be found in [5]. In this section the most important requirements are highlighted. A production plant with the size of the Belwind wind park is treated by Elia like a regulating unit (or conventional power plant). The Belgian grid code states that a regulating production unit should always be able to absorb or deliver a minimum reactive power between -0.1 P nom and 0.45 P nom. The nominal power (P nom ) is equal to the maximum output power of the number of turbines in service, 4. The Belwind connection principle The concept for the new connection technique used for the Belwind project is shown in Fig. 4. In this concept, there is no static VAR compensator onshore. The required plant capability curve, given by the transmission system operator is met by using: a) the reactive power of the export cable, b) the regulating capabilities of the wind turbines and c) the installation of offshore reactors. However, compared to the basic concept of Fig. 2, the changes are described in the following paragraphs. An onshore booster transformer is added. This transformer is a series-compensated autotransformer 150/150 kv and has 2 main functions. The most important function is the voltage regulation of the 33 kv offshore grid. In order to benefit from the reactive power capabilities of the wind turbines, this voltage needs to be constant with only small deviations (+/-1 %) of the nominal voltage. To limit the switch-in current (from the export cable capacitive loading / transformer inrush) and voltage transients occurring when the onshore high voltage 2

3 breaker is closed, this transformer is also equipped with a soft-closing device. This device will be connected to an internal circuit on medium voltage. different reactors connection states is necessary. For this reason the two reactors (one on each medium voltage winding of the 3-winding step-up transformer) can be switched in in two steps. The main controller also controls the tap changer of the onshore booster transformer in order to keep the medium voltage level constant. A simplified version of the control system architecture is shown in Fig. 5. Fig. 4. New concept to connect an offshore wind park to the onshore grid used for the Belwind project. The offshore step-up transformer has a fixed transformation ratio. This makes the transformer more reliable and lowers the need for maintenance because there is no on-load tap changer. There is no offshore high voltage switchgear / circuit breaker. The onshore booster transformer with soft-closing feature makes it possible to switch in the export cable and offshore step-up transformer at the same time without causing excessive transients. With the conventional connection method this has to be done separately and even then, the charging of the export cable still causes larger disturbances compared to this new concept. The onshore compensation equipment (SVC, reactors, capacitors) is moved to the offshore high voltage substation. However, only reactors are needed. When the grid requests an amount of capacitive reactive power, this will be supplied by the export cable by disconnecting the reactors. Smaller reactive power changes will be compensated by the doubly fed induction generator inside the wind turbines. The reactors will not be connected at 150kV level but at 33kV. This reduces their size and their noise production. 6. Advantages The new connection concept used for the Belwind wind farm has a number of advantages compared with the standard connection principle. This section describes the most important advantages Reduction of short circuit current The use of a booster transformer in the onshore substation gives a large reduction in short circuit current on the export cable and on the high voltage offshore substation (up to one fourth of the initial current). This makes it possible to use cables and switchgear with a lower rating. For the export cable, the lower earth fault current could, for example, result in a cable with a smaller metallic screen section Use of offshore reactors The placement of the reactors offshore has the advantage that these reactors can be connected on medium voltage level. This makes them smaller.. Another advantage is that switching phenomena will be damped by the export cable and less visible for the onshore grid. 5. Control philosophy To control the wind park, the grid operator, Elia gives a reference voltage and reference reactive power output to the operator of the wind park. With these values and the actually measured voltage a reference MVAR value for the whole wind park is calculated. The wind park control system uses this value to control the reactive power of the individual wind turbines and to switch in or disconnect the reactors. The reactive power regulating capability of the doubly fed induction generators of the wind turbines will be used first. When these generators reach the limit of their control range, the control range is shifted into the opposite direction by switching in or disconnecting the reactors. To avoid a lot of switching of the reactors, enough overlap between the regulating range of the turbines and the Fig. 5. Belwind wind farm control system for reactive power and voltage control 6.3. Soft-closing The soft-closing feature has the dual advantage of minimizing the transients during switching in of the export cable and offshore transformer but also makes some part of the offshore high voltage switchgear not necessary. Because the export cable and the offshore step-up can be energized at the same time, there is no need to install a high voltage circuit breaker on the 3

4 offshore platform. This results in an easier platform design and a lower maintenance cost Onshore voltage regulation The regulation of the system voltage by means of the onshore booster transformer has also some advantages compared to offshore voltage regulation. The voltage on the export cable will remain constant, resulting in a constant production of reactive power. This simplifies the control logic. The maintenance cost will be lower as well. Because the offshore step-up transformer has a fixed turn ratio and no moving parts this transformer needs less maintenance. Off course the booster transformer (and especially the on-load tap changer) will need maintenance, but it is much cheaper to do this on an onshore location. When the grid voltage is different from the export cable voltage, an onshore transformer substation is always needed. In this case the transformer can be upgraded to a booster transformer with soft closing without a large extra cost. As can be seen from Fig. 6, the grid code requirements are completely covered when a different amount of reactors is in service. Even without reactor switching and with all reactors in service, the requirements are almost met by the regulating capabilities of the turbines. In this case, reactor switching will only be necessary for high output power. However, absorbing inductive power becomes increasingly difficult because of the limitations of the voltage regulation by the booster transformer. The numbers on the curve indicate the tap changer position of the booster transformer. For the lowest yellow curve the tap changer will be on his lowest tap ratio but the voltage on 33kV will not remain between +/- 1 % of the nominal value. Because of this deviation, the wind turbines can absorb less inductive power. This effect is shown in the light blue curve CAP-LIM of Fig Simulation and calculation results This section shows some simulation and calculation results obtained for some scenarios that could occur when the wind farm is in service. In all simulated situations is shown that the connection complies with grid code requirements Capability curves The capability curves for the wind park where calculated for 3 different grid voltages: kv, 155 kv and kv (or 90, 100 and 105%). Fig. 6 and 7 give the curves for the low and high voltage case respectively. For each case the capability curve is calculated for the 3 reactors positions (switched off, half power and fully active) and displayed in yellow. The grid code requirements are indicated in deep blue and turquoise. Fig. 7. Calculated capability curves for the Belwind project at high grid voltage for different reactors in operation compared with the grid code requirements For the high grid voltage, grid code requirements will be met except at high output power and a high demand to deliver inductive power. Again the voltage regulation of the booster transformer will run into its limits (tap position 115), resulting in smaller capabilities of the turbines, as shown in the light blue line of Fig. 7. Because this is only a small deviation it will be accepted by the grid operator. Fig. 6. Calculated capability curves for the Belwind project at low grid voltage for different reactors in operation compared with the grid code requirements At nominal operating voltage of 155 kv, the voltage regulation of the booster transformer will be able to keep the 33 kv voltage constant. This makes it easier to comply with the grid code requirements. As can be seen from previous figures, at active power output levels between 20 and 80%, the wind park will be able to deliver or absorb more reactive power than the grid code requires. In this output power range, it can also be seen that there is a large overlap between the curves for different amounts of reactors in service. The chance that reactors will continuously switch in and out will be very small. 4

5 7.2. Active power variation Simulations show that changes in active power output will only have a small effect on the tap changer position of the booster and the switching of reactors. A variation of the output power from 160 MW to 0 MW was simulated for different amount of reactive power output. Fig. 8 gives an example of this calculation for 0 MVAR reactive power output. No reactors are switched and the tap changer only moves three steps. Other simulations give the same result. It can be concluded that active power variations will not have a large influence on the switching behavior. power rates of the wind farm. Fig. 9 gives the result of the first case at a 100 % active power output. In the beginning of the simulation the two reactors (4 parts) are in service. When the voltage starts to drop, the requested amount of capacitive reactive power delivery to grid rises. First this will be compensated by the wind turbines but after a certain time the turbines will reach their limit and the first part of the two reactors will be switched off. When the voltage keeps on dropping, some time later also the second part of the two reactors will be switched off. At that time the reactive power delivery to the Elia grid will be around 75 MVAR, which is a little bit more than required. When the voltage rises again, the first part of the reactors will be switched in again. It is important to see that the second part will not be switched in again, because of the overlap between the different reactor quantities in service. This result confirms that the number of switching operations will be limited. Fig. 8. Voltage and reactive power variation when active power drops from 160 MW to 0 MW (including tap changer position changes of booster) with no delivery of reactive power to the grid 7.3. Reactor switching In order to fully comply with the grid code requirements some reactor switching will be necessary. It is important to know the voltage variations this switching will cause. Simulations done in the case of a low short circuit power in the grid have shown that when 2 reactors are fully tripped, the 150 kv grid voltage will only change with a value of around 0.7 %. The temporary variations on the 33kV level will be a bit higher (+/- 3.5 %), but will have no effect on the booster tap changer positions because this tap changer action is delayed. The voltage variations due to reactors switching will be small Voltage regulation To test the reaction of the new connection type to voltage variations different simulations were done: Slow voltage drop to minimal voltage followed by a low voltage rise back to nominal voltage Slow voltage rise to maximal voltage followed by a slow voltage drop back to nominal voltage Voltage step drop of 4 % Voltage step rise of 4 % Each simulation was done for different output Fig. 9. Reaction of the MVAr regulation (reactors and WTG) on a slow voltage drop of the grid with output power equal to 100% For other percentages of the output power, results will be equivalent. The reactive power delivered to support the voltage in these cases will be much higher than the minimum requirement from the grid code. In the case of a voltage slope to the maximal voltage, results are comparable as well, but here in all cases the reactive power support will be higher than requested. It can also be noticed that in the simulated cases reactor switching will not be necessary, as the wind turbines will be able to adjust the reactive power output. In Fig. 10, results are presented for a step voltage rise of 4 % with an output power of the wind park of 50 % and all reactors in service. In this case the reactors remain in service and the wind turbines will absorb the reactive power. The voltage support to the grid will again be much larger than the minimum requirements stated in the grid code. 5

6 requires a strong voltage support from wind farms, simulations done on the connection technique prove it will be able to comply with these requirements. The proposed technique makes it possible to connect a relatively high amount of wind power with an AC-connection to shore. Fig. 10. Reaction of the MVAr regulation (reactors and WTG) on a step voltage rise of the grid of 4 % with output power of the wind farm equal to 50% 8. Conclusion The new (patent pending) connection technique used to connect an offshore wind park to an onshore grid, which has been used for the Belwind project, has advantages compared to the standard connection techniques. The need for an extra onshore transformer is compensated by more standard, less maintenance requiring equipment that will be used offshore. There is no need for a high voltage circuit breaker and a tap changer in the offshore transformer. This makes the installation more reliable and needing less maintenance offshore, which is expensive. In cases where the export cable will be operated at a different voltage than the grid voltage, the onshore transformer only needs to be upgraded with soft closing. This can be done without large additional cost. Especially in countries where the grid code 9. References [1] A. Arulampalam, G. Ramtharan, N. Jenkins, V.K. Ramachandaramurthy, J.B. Ekanayake and G. Strbac, Trends in Wind Power Technology and Grid Code Requirements, International Conference on Industrial and Information Systems, ICIIS, Aug 9-11, 2007, pp [2] E.H. Camm, M.R. Behnke, O. Bolado, M. Bollen, M. Bradt, C. Brooks, W. Dilling, M. Edds, W.J. Hejdak, D. Houseman, S. Klein, F. Li, J. Li, P. Maibach, T. Nicolai, J. Patiño, S.V. Pasupulati, N. Samaan, S. Saylors, T. Siebert, T. Smith, M. Starke and R. Walling, "Reactive Power Compensation for Wind Power Plants," IEEE Power & Energy Society General Meeting, 2009 [3] E.H. Camm, M.R. Behnke, O. Bolado, M. Bollen, M. Bradt, C. Brooks, W. Dilling, M. Edds, W.J. Hejdak, D. Houseman, S. Klein, F. Li, J. Li, P. Maibach, T. Nicolai, J. Patiño, S.V. Pasupulati, N. Samaan, S. Saylors, T. Siebert, T. Smith, M. Starke and R. Walling, "Characteristics of Wind Turbine Generators for Wind Power Plants," IEEE Power & Energy Society General Meeting, 2009 [4] I. Erlich, W. Winter and A. Dittrich, Advanced Grid Requirements for the Integration of Wind Turbines into the German Transmission System, IEEE Power Engineering Society General Meeting 2006 [5] Belgian Royal Decree of concerning the technical rules for the operation and access to the Belgian transmission network, Belgisch staatblad/moniteur Belge, Ed. 6