Experience with Connecting Offshore Wind Farms to the Grid

Transcription

1 Oct.26-28, 2011, Thailand PL-22 CIGRE-AORC Experience with Connecting Offshore Wind Farms to the Grid J. FINN 1, A. SHAFIU 1,P. GLAUBITZ 2, J. LOTTES 2, P. RUDENKO 2, M: STEGER 2, A. ZSCHAU 2 1 Siemens, United Kingdom 2 Siemens, Germany SUMMARY Siemens Transmission and Distribution Ltd (STDL) are involved in connecting five of the UK Round 2 offshore wind farms to the Grid system. These projects range in size from 250MW to 630 MW in capacity. The paper intends to share STDL s experience on the design, installation, commissioning and delivery of these five wind farms. This will mainly be focused on scope split models used, onshore and offshore substation layout considerations, system study requirements and commissioning. KEYWORDS Grid Code, OFTO, Reactive Power Compensation, Voltage Sourced Converter, offshore platform layout, onshore substation layout, system studies. ahmed.shafiu@siemens.com.

2 1. Introduction Siemens has been involved in the design, installation and commissioning of five of the UK Round 2 offshore wind farms to the UK transmission and distribution network. These wind farm projects vary from 250MW to 630 MW in capacity. Two of these wind farms are connected onshore at 132kV whilst the other three are connected at 400kV. In the UK, the transmission assets at or above 132kV cannot be owned by a Generation Company and to this definition, the offshore transmission assets must be owned by an Offshore Transmission Operator (OFTO) for wind farm connections. Two of the wind farms have been commissioned and are in successful operation at present. The following sections share experience on design and delivery of offshore wind farms. In particular, the paper will look at the different contractual/scope split models used on these projects and present how the final solution of the grid connection has been engineered and delivered. The system studies required for the design of these wind farms are discussed considering the whole of the connection including how the steady state and dynamic reactive power requirements of the Grid Code are fulfilled by means of the use of hybrid Static Var Compensators (SVC) located at the onshore substation. The reasons for the different solutions adopted on the different wind farms are explained and considerations given to onshore and offshore substation layout/arrangement are discussed. 2 Overview of an offshore wind farm The main items of a Round 2 offshore wind farm are array cables, an offshore substation, export cables and an onshore substation as shown in Fig. 1. offshore substation onshore substation array string cables export cable Fig. 1. Main elements of an offshore wind farm The array cables form the Wind Turbine Generator (WTG) collection system which is connected to the offshore substation. The medium voltage collection system has always been at 33kV for the wind farms in UK. At the offshore substation, the collection voltage is stepped up to a suitable voltage for transmission which has mainly been from 33kV to 132kV except for one project where this has been from 33kV to 150kV. The export cable transmits the bulk power from the offshore substation to the onshore substation. The export cable in many instances has a significant land run which depends on the onshore connection point with the transmission or distribution network and the cable corridor for which the planning permission is available. For the five wind farms export cable lengths range from 23km to 60km. One significant feature of the onshore substation is the presence of a dynamic reactive power compensation plant which is required to meet the site specific connection agreement which basically requires wind farms to behave like conventional synchronous generators in terms of reactive power capability. The size of this compensation plant, depends on the wind farm MW entry capacity at the connection entry point (as stated in the Connection agreement), the export cable length and the available fixed compensation on the wind farm. The fixed compensation is from shunt reactors, filters and WTGs. 1

3 3 Wind farm Design Studies A number of studies are required to establish the design basis for the wind farm primary equipment. Such studies include load flow and losses, short circuit, harmonics, dynamic stability, insulation coordination, voltage fluctuation, protection coordination, earthing and electro magnetic interference studies. The size of the dynamic reactive power compensation plant is determined in the load flow study. An important point to note in the load flow study is the consideration given to the possibility of a harmonic filter for the wind farm. However, generally it has not been possible to do the harmonic studies upfront to establish the filter size. This is due to unavailability of connection point network impedance loci and background harmonic distortions at an early stage of the project. The approach adopted is to make an engineering judgment on the size of filter required and use this information in the load flow study. In all our designs, where the client has agreed, the WTG reactive power capability is used to provide fixed compensation. By using the capability of the WTGs, shunt reactors for fixed compensation is avoided which reduces the offshore platform weight and space requirements. In establishing an optimum size for the dynamic compensation plant, an appropriate size of fixed compensation is used to place the net reactive power of the system so as to achieve symmetrical compensation at 50% and 100% MW loading against the grid code requirement. 4 Onshore substations layout and connections On each of the five wind farms, a single onshore substation has been used to connect the wind farm to the transmission or distribution network regardless of the number of offshore platforms. An exception to this is in one wind farm where the dynamic reactive power compensation plant is installed in special elliptical pods outside the main substation. This has been done mainly for aesthetic reasons rather than being technical. In all the wind farm designs, the connections from the wind farm to the transmission or distribution network interface point is made using cable connections. In one of the projects, the onshore utility breaker that was provided for the export cable connection was not able to meet the capacitive duty of the wind farm export cable. In this case a late change to the layout was necessary to introduce an additional breaker in the wind farm onshore compound to cater for this requirement. In this particular design, the utility breaker is allowed to energize the wind farm onshore busbar only. Special electrical interlocks were put in place to ensure that during switching in/out of the export cable, the utility breaker cannot switch in/out the export cable prior to the operation of this dedicated breaker which was introduced later in the design. At the onshore substation, a dedicated dynamic compensation plant is provided to meet the requirements of the grid code. The STATCOM used by Siemens for this purpose is a state of the art Voltage Sourced Converter type (VSC), also referred to as SVC PLUS. In all of the five projects this technology has been used together with Mechanically Switched Capacitor banks and Mechanically Switched Reactors as a hybrid solution to reduce the overall cost of the compensation plant. A +/- 50MVAr SVC PLUS occupies approximately 21x25m including external equipment that goes with the SVC PLUS system. This includes SVC PLUS phase reactors, a high frequency blocking filter, medium voltage switchgear and auxiliary transformers and external coolers. In most of the onshore connections, a single busbar arrangement is used at 132kV or 150kV level except for one wind farm where a double busbar has been used due to the client s specific request. Due to its compact design and limited space requirements, Siemens Highly Integrated Switchgear (HIS) type 8DN8 suitable for out door application has been use in the onshore substation at 132kV level. On the project that used 150kV, Siemens Gas Insulated Switchgear (GIS) type 8DN9 was used. Among the five projects, at 132kV level a total of 28 HIS bays were used of which 24 had Circuit Breakers (CBs), at 150kV level a total of 4 GIS bays with CBs were used and at 420kV 2 HIS bays were used without CBs. The 420kV switchgear used was Siemens HIS type 8DQ1. What is also critical in the layout as mentioned earlier is that it is not possible to confirm the requirement for a harmonic filter at the initial stage of layout design and for this reason space reservation has to be made for a possible harmonic filter. 2

4 5 Offshore substations layout and connections As already mentioned two of the wind farms were connected to onshore 132kV systems whilst the other three were connected to 400kV onshore networks so that onshore transformation took place. In general, the 400kV connections aligned with the larger power ratings in excess of 500MW but in one case the power rating was less than 300MW. On the larger wind farms two offshore substations were utilized to gather the power from the wind turbines. This assisted in reducing the lengths of inter array cabling and produced two almost identical offshore substations each with two transformers and two export cable connections to the shore. With this style of platform the power flow can be from one side of the platform to the other with 33kV switchgear on one side, transformers and then HV switchgear on the other side. For the wind farms with generated power of 300MW or less, only one offshore substation has been used. The majority of the offshore substations feature two transformers and two export cables with the transformer ratings varying from 160MVA via 180MVA up to 240MVA. Also the transformers have varied from three winding design to two winding design. Transformers are the item of plant which gives rise to the most variations in the design of the platform as they are by far the heaviest item of plant. On the five projects variations have included the location of the coolers with some projects featuring tank mounted coolers and other using separately located radiator banks. The separate radiator banks solve certain problems such as fire protection simplifying the use of water mist systems and enabling the transformer tank to be located in an almost enclosed environment. However the connections from the tank to the radiator banks give rise to problems due to the forces experienced during barge transportation of the substation. Another variant is the method of connection between the transformer and the switchgear on both the HV and LV sides. The original project used gas insulated bus duct connections at 132kV and solid insulated bus ducts on the 33kV side. In some of the other projects cables have been used on either the HV or LV sides of the transformers. Again each of the connection methods has advantages and disadvantages. The 33kV bus duct solutions minimizes the number of connections required at 33kV as the rating of these connections can be 2500A and it allows virtually right angle bends to be achieved. However the solution is fairly expensive and the installation can be challenging. Transportation of gas insulated bus duct connections fully gassed up has to be done carefully. One of the 132kV connected wind farms with rating of 500MW was originally to be connected by a single offshore substation using three 180MVA transformers, but due to commercial and political reasons it became necessary to locate some of the wind turbines on a separate sand bank and this gave rise to the need for a second offshore substation. In this case the three transformers were retained on the main platform and a second platform with two smaller (90MVA) two winding transformers was established on the second sand bank. In this case the smaller platform was not directly connected to the shore but was connected by a submarine cable to the master platform and connected via switchgear to one of the three export cables to the shore. As this extra length of submarine cable was added at a late stage in the compensation design shunt reactors were also located on the new offshore platform to compensate the length of the connecting cable. The amount of HV switchgear has also varied quite significantly at client request from minimal, namely only disconnectors, earth switches and surge arresters on two of the projects, through to having circuit breakers to separate the cable from the transformer to a full five circuit breaker switchboard on one of the projects providing separate CBs for the transformers, cables and a bus section. In all platforms except one, HIS switchgear shown in Fig. 2 has been used at 132kV level. The HIS is modular and can be accommodated into the available space in the offshore platform. Adapting the HIS switchgear layout in the main platform to accommodate a second platform connection at a later stage of the project as stated above was easily achieved with the HIS. In total among the projects that used 132kV voltage, a total of 18 HIS bays were used of which 12 had CBs. 3

5 Fig. 2. Siemens Highly Integrated Switchgear (HIS) being installed in one of the platforms In one project, where the offshore HV voltage was at 150kV, the HV switchgear offered is Siemens GIS switchgear 8DN9. In this project four bays were used all without CB. Out of the five projects, the platforms on four projects have used jacket foundations whilst the one which had a separate platform contract has used a monopile construction. Monopile construction was quite common on the early single transformer platforms however experience has shown that some of these are prone to excessive vibration from wind and wave forces acting on the platform. One other layout consideration is the cable deck and whether this is integrated with the topside or provided as part of the jacket foundation. The latter solution can give more flexibility in the construction programme in that the cabling can be pulled in before the arrival of the topside. Of the five projects three have had integrated cable decks with the topside whilst two have had the cable deck as part of the jacket foundation. The previous paragraphs have discussed the main plant items but the platform layout must also consider the cabling and access for terminating the cables and the material handling to enable equipment and tools to be lifted onto the platform and then moved to the appropriate locations for work. The whole issue of access to the platforms is a key consideration in the layout design. Access will normally be required from boats but helicopter access may also be required as was the case on one of the projects. This introduces a whole new set of requirements into the layout of the platform taking into account wind directions and approach direction for the helicopter as well as the access to the helideck, constraints on the crane location and the additional fire fighting requirements associated with a helideck. 6 Commissioning Commissioning for offshore wind farm connections has two major differences from normal substation commissioning activities. The first is that the pre commissioning activities associated with the offshore substation are mainly carried out onshore before the substation platform leaves the construction yard. This is important as the cost of any activities carried out offshore is approximately ten times that of the same activities performed onshore. It is important to allow sufficient time in the construction programme to carry out this onshore commissioning activity as the sail out date is linked to the very expensive hire of the heavy lift vessel. The commissioning activities to be carried out offshore should be kept to the absolute minimum to prove that the equipment has not been damaged in transit and is functioning correctly and of course those items which can only be proved when the substation is in situ and the cables connected. The second difference is the need for a set of grid code compliance tests on the whole wind farm to ensure that the wind farm meets the reactive power, voltage control and frequency control requirements. For the GB Grid Code these tests require the demonstration of the capability of the wind farm to generate and absorb reactive power up to the maximum levels defined. Also the capability of 4

6 the wind farm to respond to dynamic changes in voltage in accordance with the set point and slope settings applied has to be demonstrated. In all of the five Round 2 wind farms the IGBT based SVC PLUS with mechanical switched components has been used. In the majority of cases both switched reactors and switched capacitors have been employed but on one project only switched reactors were required. 7 Way forward A lot of experience has been gained on wind farm design and installation from working on these five Round 2 projects and the experience gained on transformer and platform design will be fed back into the new designs. However, this does not mean that there will not still be significant challenges in responding to the connection of Round 3 wind farms in which the AC onshore system and the offshore AC system is likely to be coupled via a HVDC system. If voltage sourced converter HVDC is used, which seems most likely, then the need for SVC systems should be removed. However the fundamental design of the offshore AC systems will need careful consideration because of the effective decoupling between the onshore and offshore systems caused by the HVDC link. The opportunity to choose an optimal frequency and voltage for the offshore system can be considered. Higher frequency may lead to smaller lighter transformer designs but the higher frequency will lead to higher VAR generation from the power cables. Higher voltages will hopefully lead to lower losses but again higher voltage will lead to higher VAR generation. One of the fundamental issues to be decided is what is the correct size for the offshore collector substations? This will need to be a compromise between minimizing the number of offshore substations and hence HV connections required taken together with keeping down the length and losses of inter array cables. With so many large wind farms being considered in Round 3 thus requiring a large number of HVDC connections with ratings of the order of 1000MW there will be a need for some form of interconnection between these links to cover for the loss of one HVDC link. Should this be achieved by multi terminal DC or should the interconnection be achieved on the AC side? However some issues now well understood with regard to fault levels and protection will need to be reconsidered. With AC connected wind farms the transformer impedance had to be chosen to limit the fault levels and then the impact of these high impedances on the reactive compensation balance had to be considered. With a HVDC link the fault level from the onshore system will be limited to approximately 1.2 times full load current at most. This means that the whole concept for choosing transformer impedance values will need to be reconsidered and this may avoid the need for tap changers on the offshore transformers. The reactive compensation for the offshore network can no longer be achieved by locating compensation plant onshore, again because of the effective decoupling of the two systems. This leaves the question as to the optimum way to control reactive power on the offshore network. Should this be achieved by using the STATCOM capabilities of the HVDC converters, or should there be static devices such as shunt reactors mounted on the offshore platforms or can the reactive capability of the wind turbines be utilized? The answer may lie in an intelligent combination of a number or all of these possible sources. The effective decoupling of the onshore and offshore networks will certainly have a significant effect upon the protection systems used particularly for the inter array cables where either plain or directional overcurrent protection has been used in the existing projects. Because of the lack of fault infeed from the onshore network this method will not be possible in the future and the use of distance protection or possibly voltage controlled overcurrent protection may provide the answer. 5

7 Short Bio-data of Main Author Study of electrical engineering at St. Petersburg State Polytechnic University in Russia, graduation as engineer in PhD in the field of power networks at RWTH Aachen University in Germany, graduation as Dr. in Power system analyst at engineering consulting company Lahmeyer International, Germany, till Sales and marketing responsibility at Siemens AG in Germany for the high voltage substations in CIS countries till Now responsible for the technical marketing of the high voltage substations at Siemens AG. 6