1
IEEE copyright, PES meeting Montreal 2006
Interconnecting offshore wind farms using multiterminal VSC-based HVDC D. Jovcic, Member IEEE
Abstract—This paper presents a large off shore wind farm interconnected to the grid using a multiterminal HVDC link. The 200W wind farm consists of 100 individual 2MW turbines connected using 25 VSC (Voltage Source Converter) converters to a common DC bus. The transmission system converters enable variable speed operation and therefore additional converters are not needed with individual generators, implying savings in converter costs. The paper presents PSCAD simulation of the proposed concept for various changes in wind speeds. The results confirm the ability to operate at optimum coefficient of performance and no synchronization problems occur even for severe wind speed changes. Further tests with faults on AC grid demonstrate satisfactory recoveries. The proposed concept may enable integration of large offshore wind farms at considerable distances, and using optimal number of converters. Index Terms— Multiterminal HVDC Power Transmission, Wind farms, Variable speed converter control.
I. INTRODUCTION A. Background ecause of the projected energy shortage and the concerns about greenhouse emissions, there has been significant development in renewable energy sources worldwide, in the past decade. In particular, the UK government aims to achieve the goal of 20% (up to 40% in Scotland) energy production from renewable sources by 2020. It is projected that the increase in renewable energy share from the present 3% will be largely based on increase in wind energy generation, which is likely to become the main source of renewable energy in the UK and in many other countries. Because of the economy of scale and increasing demand, future wind farms will have a larger capacity, exceeding a hundred MW in many cases, implying hundreds of individual 1-5MW units. Considering also the environmental issues, it is recognized that large size offshore wind farms are the well placed to accommodate the future increase in the wind energy generation [1-2]. Presently, there are a number of small-scale offshore wind farms in Europe, including several in the UK, where the largest is the (160MW) farm at Horns Rev, Denmark. Under the “Round two” offshore program, the British government has recently granted permission for fifteen more off-shore plants, and many of these are expected to be rated at 100-500MW. Because of the environmental and social aspects these wind farms might be located at larger distances, some approaching 100-150km from the shore [1].
B
D. Jovcic is with University of Aberdeen, Engineering Department, King’s college, Aberdeen, AB24 3UE, Scotland.
[email protected]
Traditionally, wind energy generation has been connected to the network grid assuming that its size and influence are small and therefore the connection requirements have been less stringent. Typically, wind farms do not contribute stabilization or regulation of AC grid and in many cases no detailed transient studies or stability studies are performed. With the projected power injection in the order of hundreds of MW, power plants might have significant influence on the host grid and the interaction issues need to be carefully investigated. New integration solutions are sought, taking into consideration the AC system properties including stabilization, regulation and fault recovery, but also examining cost effective wind farm topologies, their dynamics, transients and efficiency. At present, none of the wind farms, including the large Horns Rev offshore installation, can contribute to the AC system control or stability enhancement and they simply disconnect in case of AC faults. As the power share from wind farms increase, it is necessary that wind farms should take more active role in the AC systems regulation and support. The network operators have raised many issues with wind power generation, especially for large-scale generation, in order to enable secure and reliable system operation. The Danish network operator Eltra, has recently issued a unique specifications document for wind farms connections to the transmission grid [3], and comparable documents are in consultation stages in England, Wales and Scotland. Similarly as with conventional generators, wind farms are now required to comply with stringent connection requirements including: reactive power support, transient recovery, system stability and voltage/frequency regulation, power quality, whereas scheduling and reserve availability are also considered. The conventional wind generation concepts based on doubly fed induction generators may have difficulties in meeting all the above interconnection requirements [2]. B. Wind farm interconnection using HVDC Theoretically, future offshore wind farms at distances below 60km from the shore can be connected to the grid using either an AC or DC link whereas at a greater distance only DC links are applicable [1]. In searching for the adequate wind energy integration solution, it has been recognized that many of the above network-connection issues would be eliminated, and even AC system stability might be enhanced, if the wind power connection point incorporates a converter system [2]. AC connection link would therefore in many cases require an additional converter system (like SVC or STATCOM) at the connection point for reactive power and voltage support. Still, this shunt converter does not resolve the issues with low inertia, power control and frequency control/stabilization of
2
the AC system, which remain significant issue with large wind farms. Traditional HVDC controls have in many cases been used for AC frequency stabilization [6] or AC voltage regulation. HVDC systems based on VSC converters have more versatile and faster controls [4],[5], which may be utilized at the wind farm interconnection point. A VSC converter enables independent voltage, frequency and power control. A wind farm interconnected with an HVDC link therefore has the potential to offer grid control functions similar to a conventional generator. On the downside, cost of HVDC link is considerably higher than comparable AC link because of converter stations. This paper analyses the option of reducing converter costs by eliminating primary converter systems associated with common variable speed wind generators. Such concept has the potential to offer variable speed operation and all the benefits of HVDC interconnection without significant escalation in converter costs. II. WIND FARM TOPOLOGY
4 synchronous generators
Figure 1 shows the electrical circuit for the considered wind farm. It presents a 200MW off shore wind farm consisting of 100 individual 2MW, 4kV wind generators. It is assumed that the farm distance from the shore is approximately 100km. The wind generators resemble the commercially available 2MW units based on permanent magnet synchronous generators. However, converter systems are not used with generators, since transmission system converters enable variable speed operation. This concept implies savings in the converter costs. The total converter rating in Figure 1 is same
as with conventional fully-fed variable speed wind generators and Ac interconnection. The nominal operating frequency of the offshore network is 50Hz (at full power), and the 4-pole generators use gearboxes (approximately 77 ratio). Note that directly coupled generators are not suitable in the proposed concept since they would require operation at very low offshore electrical frequency and therefore transformers with large cores would be needed. The offshore electrical network consists of 25 generator groups each connected through a single Voltage Source Converter (VSC). Each 8MW group includes 4 generators. There is a single 4kV/90kV transformer per group, which elevates the generator voltage to the transmission level. The wind turbines operate at variable speeds in order to maximize energy capture, reduce stresses and reduce noise. The generator speed is controlled using the VSC converters, and all the generators in a group operate at the same speed. The frequency in a group, and the speed of all generators in the group, is derived as the average speed considering wind speeds at individual machines. The inability to operate individual machines at most optimum speeds is not considered as great loss in efficiency, since it is expected that the wind profile will largely be similar on the four closely located turbines. Note that each group can operate at most suitable speed, which is independent of speeds for other groups. The 25 VSC converters are connected in parallel to a common DC bus, thus operating at the same DC voltage in a parallel multiterminal HVDC connection. The DC voltage is maintained at the nominal level (150kV) by the single VSC inverter located on-shore.
G (2MW)4kV +75kV 4kV
VSC 1
∆
0.0135H 1.85Ω
0.0135H 1.85Ω
100kM DC cable
Y
4kV/90kV Xl=12% 8MW 150kV
120uF 80uF
25 VSC converters
90kV/110kV 110kV Xl=10% 0.052H 4.36Ω
34uF
∆
80uF 120uF
4 synchronous generators
+75kV
200MW 150kV
G (2MW)4kV
VSC 25
4kV
∆
Y
4kV/90kV Xl=12% 8MW -75kV
-75kV
VSC - Voltage Source Converter
Figure 1. 200MW off-shore wind farm with parallel multiterminal HVDC connection
Y
AC Network on shore SCL=10
110kV
3
III. WIND FARM MODEL A. Electrical circuit A suitable model for the system in Figure 1 is developed on PSCAD/EMTDC platform [7]. It would be extremely difficult to model such complex system in detail, and a series of simplifications is adopted. The schematic of the PSCAD model is shown in Figure 2. The number of the off-shore converters is reduced to four in order to save simulation time. The converters are rated 50MW and they are connected in parallel to represent multiterminal HVDC operation in the actual system. Only one of the converters is connected to four machines, to enable studies of the dynamics within a group. All the machines models are based on a single 2MW permanent magnet synchronous machine, which has common parameters from PSCAD library. The large 50MW generators are also based on the same 2MW machine model, which uses PSCAD ability to represent 25 coherent 2MW machines in a single model. B. Offshore VSC controller The adopted principle of a parallel multiterminal HVDC control is explained with reference to Figure 3. The inverter station regulates the DC voltage which is common for all converter stations. Each of the rectifier stations (the offshore converters) regulates the DC current in its own branch. The generator speed control can be achieved using the known principles of flux oriented synchronous machine control and using position encoder to synchronize the coordinate frame [8]. However, since a transformer is placed between machine and converter, it is found more suitable to +75kV
VSC 1
G1 (50MW)
∆
use the torque control based on regulation of power transfer through the transformer. The rotating coordinate frame position is determined using a PLL, which measures the 4kV generator voltage. This signal is a good estimate of the rotor position and therefore the danger of loss of machine synchronism is avoided. The control system for each of the offshore converters is shown in Figure 4. The machine power, and consequently machine torque, is varied by changing the angle of the VSC converter voltage MΦ, with respect to the generator terminal voltage. The power control in VSC transmission is commonly achieved either using the VSC voltage angle or VSC voltage D component [9-11]. As shown by the lower control diagram in Figure 4, the VSC angle control is based on two series connected controllers. The inner control loop regulates the DC current (in the concerned DC branch) which improves performance of the of the outer speed control loop. The inner DC current loop also prevents overcurrents in the DC system. The generator speed operating range is approximately 65rad/s
