IEEE Energy2030 Atlanta, GA USA 17-18 November, 2008

Wind Farm Grid Integration Using VSC Based HVDC Transmission - An Overview

S. K. Chaudhary Department of Energy

Technology, Aalborg University, Denmark

skc@iet.aau.dk

R. Teodorescu Department of Energy

Technology, Aalborg University, Denmark

ret@iet.aau.dk

P. Rodriguez Department of Energy

Technology, Aalborg University, Denmark

pro@iet.aau.dk ‘ Abstract - The paper gives an overview of HVAC and HVDC connection of wind farm to the grid, with an emphasis on Voltage Source Converter (VSC)-based HVDC for large wind farms requiring long distance cable connection. Flexible control capabilities of a VSC-based HVDC system enables smooth integration of wind farm into the power grid network while meeting the Grid Code Requirements (GCR). Operation of a wind farm with VSC-based HVDC connection is described. Keywords : VSC, HVAC, HVDC, Wind Farm, Grid Code requirements, Black Start,, Reactive Power Support, Voltage and Frequency regulation

I. INTRODUCTION

Wind power generation has received a major impetus due to ever increasing demand for energy and depleting fossil fuel reserves. Energy is the critical resource for development, without which the world will come to standstill. Fossil fuels like coal, oil and gas have been the chief energy source till now. However, fossil fuels are limited in supply and the reserves are concentrated in a few countries; thereby raising energy security concerns. Therefore, significant research and development has been directed towards harnessing renewable energy sources world-wide for sustainable development. Wind energy is an abundant renewable energy resource. In the last couple of decades, there has been a lot of research and development in the field of electricity generation from wind power. While the development is still going on, now wind turbine technology has matured enough to generate electrical energy from wind on a massive scale (see Fig. 1).

Another attractive factor for wind energy is its cleanliness. It does not use water for its operation and the greenhouse gases emission (GHG) from

wind farms, both onshore and offshore, is of the order of only 10 to 30 kg CO2 equivalent per MWh of energy. This is a major environmental advantage over fossil fuels, like coal, gas and oil, which emit around 400 to 550 kg CO2 equivalent per MWh of energy. In the European Union (EU), energy accounts for 80% of all GHG emissions. In order to reduce GHG emission and ensure energy security, EU is committed to develop renewable energy sources to the level of 20% by the year 2020[1]. A major portion of this is expected to be from wind farms, both onshore and offshore. As shown in Fig 1, in 2030 a reference scenario from the European Wind Energy Association (EWEA) assumes that wind energy will generate 180GW of power, out of which 120GW will come from offshore wind [2]. While this paper specifically refers to offshore wind farms, most of the concepts are applicable for onshore wind farms as well.

EWEA's Wind Power Reference Scenario

56 77113

146 165 180

35

75

120

41

12

0

50

100

150

200

250

300

350

2007 2010 2015 2020 2025 2030

Year

Pow

er (G

W)

Offshore

Onshore

Fig. 1. EWEA’s Wind Power Development Scenario

Wind turbine generators (WTG) convert wind

energy into electrical power. Now large wind turbines of up to 5MW size have been developed. A wind farm, also known as wind power plant (WPP), is a collection of a few tens or a few hundreds of WTG installed in close

vicinity. They are connected to the collector bus by cables. For instance, 160MW Horns Rev offshore wind farm comprises of 80 WTG of 2MW each. The proposed 400MW Borkum-II wind farm in the North Sea will have 80 WTG of 5MW rating each. The outputs of individual WTG’s are aggregated at the collector bus. While WTG’s operate at a low voltage level, typically 690V, step-up transformers are used to step up the voltage to the collector bus voltage, usually around 30 to 40 kV.

Fig. 2. Single line diagram of HVAC and HVDC

interconnection of offshore wind farm to the grid.

The electric power aggregated at the collector bus has to be fed into the power grid network for transmission and distribution to the load centers and utilities located hundreds of km away. This requires that the wind farm to the grid. Cable transmission is required for the submarine power transmission (in case of offshore wind farms), and underground transmission onshore so as to connect at a sufficiently strong point in the grid. Though overhead line (OHL) provides an economic connection, there are problem in securing right of way for OHL. Hence, only cable connection of wind farms have been considered in this overview.

High Voltage AC (HVAC) and High Voltage DC (HVDC) are the two alternatives for the connection of the wind farm to the grid as shown in Fig. 1. The figure shows Voltage source converter (VSC) based HVDC system.

On the basis of the type of converters used, modern HVDC transmission has three major variations. The three types of converters are – a. Line-commutated Converter (LCC) b. Capacitor Commutated Converters (CCC)- c. Voltage Source Converters

CCC-based HVDC is a special type of LCC-based HVDC, with series capacitors between the converter transformer and the thyristor-bridge. It has some advantages with respect to lower reactive power requirement and lower risk of

commutation failure when compared with LCC-based HVDC. It is not referred in this paper.

The paper is divided into five sections. A brief description of HVAC and LCC-based HVDC transmission system for wind farms is given in section II Prevalent grid codes are briefly discussed in Section III. Section IV describes the VSC-based HVDC system. In the end, section V concludes the paper.

II. HVAC AND HVDC OPTIONS FOR WIND FARM INTEGRATION

A vast majority of generation, transmission, distribution and consumption of electric power is in the form of AC. Hence, HVAC transmission is the obvious choice for the grid connection of wind farm. Most of the operational wind farms are connected using HVAC connection. Horns Rev Wind farm uses 21 km of submarine cable and 36km of onshore cable for the HVAC transmission of. 160MW at 150kV

P

ower

Tra

nsfe

r Cap

acity

(MW

)

Fig. 3. Choice of transmission technology for different wind farm capacities and distances [3].

However, HVAC cable transmission suffers

from the excessive reactive current drawn by the cable capacitances. Not only this increases the cable losses and reduces the power transfer capability of the cables, but also demands reactive shunt compensation to absorb the excessive reactive power and avoid over-voltage. Presently AC cables have a maximum cable rating of about 200MW per three phase cable, on a voltage level of 150 - 170kV, compensation at both ends and maximum cable length of around 200km. For a shorter distance of a 100km, voltage ratings may be raised to 245kV, thereby increasing the power transfer capability to 350MW [3]. As shown in Fig. 3, HVAC transmission is not feasible for large offshore

power plants requiring cable transmission over long distances.

Unlike HVAC transmission systems, there is no reactive power generation or absorption in HVDC transmission systems. Hence, HVDC transmission is very suitable for bulk power transmission over long distances. This has been one of the driving factors for the development of HVDC systems since the first commercial installation in Gotland in the year 1954.

The advantages of HVDC systems are fast and reversible power flow, asynchronous and decoupled connection of two grids, frequency control and power oscillation damping capabilities. Though a large number of large LCC-based HVDC systems are operational for bulk power transmission and/or asynchronous connection between two grids, none of them is associated with wind farms.

LCC-based HVDC terminals use thyristor-bridge converters, which require a stable AC voltage for commutation. These converters absorb reactive power as current is always lagging behind the voltage. The reactive power (VAr) requirement for the LCC terminals is of the order of 60% of the active power rating; though actual reactive power absorption depends upon the power flow level. Large capacitive filters are used to provide reactive power to the terminals and filter out the low frequency harmonics. Another problem lies in the fact that LCC-based HVDC cannot be connected to weak ac grids due to risk of commutation failures. The strength of ac grid with respect to the LCC-based HVDC rating is measured in terms of short circuit ratio (SCR) and effective short circuit ratio (ESCR), which accounts for the reactive power compensation provided at the terminals [4].

VSC-based HVDC transmission overcomes the shortcomings of the LCC-based HVDC system, albeit at increased converter cost and higher converter losses.

Before moving on to the VSC-based HVDC, the grid code requirements are briefly reviewed in the following section.

III. GRID CODE REQUIREMENTS FOR WIND FARMS

At present a vast majority of the generating power plants are thermal, hydro or nuclear power stations with large synchronous generators. These plants have a very controllable generation capability of both the active and reactive powers within their capability limits. Moreover, the power system network has evolved around these

machines; hence they go together very well. In the last few decades several large wind farms (also referred as wind power plants, WPP), each having capacities of a few hundreds of MW, have been connected to the grid, and a larger number of WPPs are in the planning and development stages. The characteristics and capabilities of wind WPPs are very different from the conventional power plants. Their operational behavior, dynamics, controllability and capability are dependent upon the type of wind turbine generators used, farm control architecture as well as instantaneous wind availability. For a given wind farm, the power generated is inherently stochastic in nature as it depends upon the instantaneous local wind conditions.

In the past, wind power penetration in the power grid network was relatively small and grid operators treated them as negative load, rather than a power generation source. They were not expected to provide grid support. On the contrary, they used to get disconnected whenever there were disturbances. The conventional power houses were required to provide controlling power to make up for the lost wind power generation and support grid recovery [5].

With increasing wind penetration, grid operators are now imposing grid code requirements to specify the steady and dynamic requirements that wind farms must comply with for getting connected to the grid. Wind farms need to participate in the frequency and voltage regulation by continuously controlling their active and reactive power outputs. Rather than, disconnecting from the grid during fault conditions, they are expected to exhibit low voltage fault ride through capability and support the grid recovery [6, 7].

E.ON Netz Grid Code [7] states that every generating plant with a rated capacity of over 100MW must be capable of supplying the control power. Phase swinging or power oscillations must not trigger the generating plant protection or lead to capacity disconnection. On the other hand, the plant regulation must not stimulate phase swinging or power oscillations.

IV. VSC-BASED HVDC TRANSMISSION

Voltage Source Converters (VSC) use high voltage Insulated gate Bipolar Transistor (IGBT) capable of carrying high currents and switching at high frequency of a few kHz for pulse width modulation A comparison of LCC-HVDC (referred as HVDC Classic) and VSC-based HVDC is given in Table 1 [8].

Table 1. Comparison of LCC-based HVDC and VSC-based HVDC Light [8]

LCC-based HVDC VSC-based HVDC 1 Size range single convertor 150 – 1500 MW 50 – 550 MW

2 Convertor/Semiconductor technology Line commutated, Thyristor Self commutated, IGBT

3 Relative volume 4 – 6 times 1

4 Type of cable Mass Impregnated Paper Oil/Paper XLPE

5 Control of active power Yes yes

6 Control of reactive power No (only switched regulation) Yes, continuous control

7 Voltage control Limited Extensive

8 Fault ride-through No Yes

9 Black start capability No Yes

10 Minimum short circuit capability in AC grid >2.0 x rated power No requirement

11 Power reversal with-out interruption No Yes

12 Generator needed on off-shore platform Yes No requirement

13 Minimum DC power flow 5-10% of rated power No minimum DC power

14 Typical losses per convertor 0.8% 1.6%

15 Operating experience > 20 years 8 years

16 Operating experience off-shore No Yes

Fig. 4. Capability Curve of a VSC on active power (P) and reactive power (Q) plane [9].

VSC’s are capable of operating in all four quadrants on the active power (P) and reactive power (Q) plane as shown in Fig. 4 [9]. It allows for the fast control of active and reactive powers independent of each other. Depending upon the requirements, the VSC can be operated to supply or absorb reactive power from the grid.

By virtue of PWM controlled IGBT switches, the operation of VSC-based HVDC system is independent of the grid strength, it is even capable of supplying to a passive load and energize a dead network during black start.

Since VSC-based HVDC uses high frequency PWM technique for the AC to DC conversion

and vice-versa, it does not introduce any low frequency harmonics. The first characteristic harmonics appears at the switching frequency which of the order of a few kHz (typically at 1-2 kHz) and unlike in the case of LCC-based HVDC, these converters do not require reactive power support. Hence compact high frequency filters can be used.

At the cost of more expensive converter terminals and higher losses in the converters, VSC-based HVDC is far better than the classic HVDC system.

Key features of the VSC-based HVDC are listed below – a. Fast and reversible control of power flow b. Frequency and voltage control on the wind

farm grid to attain maximum power tracking. c. Grid frequency regulation by fast power flow

control d. Power Oscillation damping by modulating

the power flow. e. Fast and reversible control of the Reactive

power generation or absorption at the point of common coupling.

f. Voltage regulation by virtue of reactive power control over a wide range from inductive to capacitive.

g. Dynamic voltage stability and flicker mitigation by the dynamic modulation of reactive power injection.

h. Black start capability. i. Improvement in power system stability by

fast and dynamic control of both the active

and reactive power injection at the point of common coupling.

j. Asynchronous mode of operation, providing decoupling from power grid disturbances.

k. Compact converter terminals compared to the conventional HVDC terminals – ideal for platform based offshore applications.

l. Low filter requirements as the first characteristic harmonics is in the order of a few kHz.

On the flip side, the VSC-based HVDC

system is expensive due to high VSC-terminal costs and the converter losses are higher than those in LCC-based HVDC because of high frequency switching of the IGBT switches.

However, VSC development in the last decade has brought down the losses by more than 60% since the development of the first system in 1999 [8] Liu and Arrillaga et al [10] proposes a VSC with current re-injection scheme to reduce the switching losses.

In [11] Weber, has proposed the use of LCC-based HVDC with STATCOM on the basis of lower overall losses. When LCC-based HVDC is used, the losses are lower, and it can be designed to meet the grid code requirements using VSC-based STATCOMs at its terminals.

A. VSC-based HVDC for Wind Farm Grid Integration

VSC-based HVDC provides a flexible control of both active and reactive power flow, and it can be controlled to achieve a variety of objectives like voltage and frequency control. Therefore, it can be designed to operate with different types of farm grids irrespective of the type of wind turbine generators used, such as squirrel cage induction generators, doubly fed induction generators or synchronous machines with (or without) full converters.

When the wind farm is connected to the grid by VSC-HVDC, it may be operated as a generator with controllable active and reactive power. Though the maximum amount of active power is limited to the availability of wind power at any given instant, power generation can be reduced as and when needed by the farm control. Unlike LCC-based HVDC, VSC-based HVDC poses no limitation on the minimum active power flow. Further, reactive power flow can be efficiently regulated to comply with the grid connection requirements. The wind farm side converter may be controlled to regulate the wind farm grid voltage and frequency. As power flows through the farm side converter into the

DC-link capacitors, DC-link capacitor voltage tends to rise. The DC voltage control logic implemented on the grid side converter, transfers the power supplied by the farm side converter (minus the losses) to the grid, so as to maintain the constant DC link voltage. The grid side converter also tends to regulate the grid voltage at the converter terminal by regulating the reactive power flow. Under normal operating conditions, the two converters can operate independently of each other.

B. Operation of the Wind Farm with VS-based HVDC under Fault Conditions

When there is a fault resulting in a low voltage on the grid side converter terminal bus, the power transfer capability to the grid is reduced. In such a case the wind farm may be commanded to reduce the power generation. Any excess power fed into the dc link would result in DC over-voltage. Chopper controlled resistors may be used to dissipate the excess energy and avoid DC over-voltage [12].

A fault on the wind farm side would result in reduction of power generation or reduction of farm side grid voltage. The farm side converter can be regulated to operate at a lower AC voltage and low power generation.

A fault on the DC link would result in the outage of the HVDC link. The wind farm will have to be tripped whereas the grid side will have to be isolated by the operation of AC circuit breakers.

C. Energizing the Wind Farm Grid from the AC Grid

Black start capability of the VSC-based HVDC may be used to start the wind farm grid. When the grid side converters are connected to the AC grid, the DC link capacitor gets charged to the required DC voltage level. Then the farm side converter can energize the farm side grid to the desired voltage and frequency in a controlled manner. The wind turbine generators can be connected to the farm side grid when voltage and frequency is stable.

V. OPERATIONAL EXPERIENCES WITH VSC-BASED HVDC

ABB has developed VSC-based HVDC under the trade name HVDC-Light. [13 - 15]. Recently Siemens has developed VSC-based HVDC under the trade name, HVDC-plus.

A. Gotland

Gotland HVDC Light (50MW, ±80kV, 70km) connects southern Gotland to the center of the island This link is operating in parallel to the existing 70kV/30kV AC grid since 1999. A loss minimization program within the AC network utilizes the power flow controllability of the HVDC system. The voltage control reduces voltage and frequency variations so that the wind power does not synchronize with flicker and no separate flicker controller is required. Staged fault test study has demonstrated the voltage support capability of the HVDC light system. Installation of Gotland HVDC has improved the overall stability and voltage quality of the Gotland Energy AB (GEAB) to the extent that the number of wind turbines installed in Gotland has doubled from the number in 1997 [13-14].

70 kV

HVDC to Mainland

HEMSE

NÄS 10kV

70 kV

NÄS 2

30 kV

± 80 kV50 MW

65 MVANÄS

65 MVABÄCKS

Simplified diagram of the Gotland Network

Fig. 5. Simplified Diagram of Gotland Network

B. Tjæreborg

The 7.2MW, ±9kV, 4.4km long HVDC Light at Tjæreborg was commissioned in 2000 to demonstrate the VSC-based HVDC technology on a small scale. The farm consists of 4 wind turbines of different types and makes, with a total generation of 6.5 MW. The DC cable is laid in parallel with the existing AC cable, thus enabling three different operation modes: AC mode via the AC cable only, the DC mode via the DC cable only or the AC/DC mode via the DC and the AC cable in parallel. The offshore

converter can be operated within the frequency range 30 - 65 Hz in isolated operation mode.

2MWGB

2MWGB

1.5MWGB

1MWGB

10kVEnge

8MVA 8MVA±9kV DC

7MW

10kVTjæreborg

Tjæreborg wind farm with AC and DC Cable Feeders

Fig. 6 Tjæreborg Wind Farm with AC and DC Cable Feeders

C. Platform mounted Offshore HVDC-Light HVDC terminal at Troll.

The Troll A HVDC-Light link, consisting of 2x41MW, ±60kV, converters and 67km long submarine cables, is the only HVDC system with offshore platform terminal. It supplies power to the offshore platform from mainland grid. This system proves the technical viability of offshore platform mounted converter terminals and VSC-based HVDC supply to passive networks.

D. Others

Murray link, in Australia (220MW, ±150kV, 180 km) and Estlink (350MW, ±150kV, 105km) between Finland and Estonia, demonstrate the technical aspects with regard to high power ratings and long cable transmission lengths. Murray link is used for facilitating power trading as well as AC voltage control at both ends.

E. Nord E.ON 1 HVDC

Nord E.ON 1 HVDC, due for commissioning in 2009, will be the first HVDC connection to a large wind farm. It will connect the 400MW Borkum-2 wind farm with the German Grid The wind turbine generators, of 5MW size each, will feed to a 36kV local AC grid, which will eventually be stepped up to 170kV AC voltage. HVDC Light offshore converters will convert it to ±150 kV DC voltage. The transmission link will comprise of 128 km of submarine cable and 75 km underground cable before connecting to the 380kV AC grid at Diele. [17]

F. HVDC.plus from Siemens

The first VSC-based HVDC from Siemens, known as HVDC Plus, will be transmitting 400MW at ±200kV over 88km submarine cable between San Fransisco City Center and Pittsburg from March 2010[17].

VI. CONCLUSIONS

VSC-based HVDC system is a viable transmission connection for large offshore wind farms, especially when long distances are involved. Relatively higher cost and higher converter losses may be acceptable in view of its flexibility to ensure compliance with the Grid Code requirements, facilitate ancillary services like reactive power support, voltage stability, power flow regulation, etc. Further development and competitive research on VSC technology is expected to bring down the losses and price as well.

However, for certain onshore wind farms, where space availability is not a concern, a combination of HVDC classic and STATCOM or other hybrid topologies may be evaluated.

When there are a number of large wind farms in a certain region, VSC based multi-terminal HVDC may be optimal. In such cases a detailed simulation study is required with regards to GCR, possible ancillary services and power system operation and stability. Further the stability and performance of the VSC based converters has to be studied for unbalanced grid conditions and asymmetrical faults.

ACKNOWLEDGEMENTS

The research is a part of Vestas Power Program sponsored by Vestas Wind Systems A/S, Denmark and Department of Energy Technology, Aalborg University, Aalborg, Denmark.

REFERENCES

[1] An Energy Policy Update, Communication from the Commission to the Eurpoean Council and the European Parliament, Commission of the European Communities, Brussels 10.1.2007.

[2] Pure Power, Wind Energy Scenarios up to 2030, European Wind Energy Association, March 2008. Available at http:// www.ewea.org

[3] T. Ackermann, Wind Power in Power Systems, John Wiley and Sons Ltd., England.

[4] P. Kundur, Power System Stability and Control, McGraw-Hill Inc, New York.

[5] S. Haier,. Grid Integration of Wind Energy Conversions Systems, John Wiley and Sons Ltd., England.

[6] Z. Chen, ‘Issues of connecting wind farms into power systems’, Proceedings of the IEE/PES Transmission and Distribution Conference & Exhibition: Asia and Pacific Dalian, China, 2005.

[7] Grid Code High and extra high voltage, E.ON Netz GmbH, Bayreuth, 1.04.2006.

[8] B. Normark, E. K. Nielsen, ‘Advanced power electronics for cable connection of offshore wind’, Paper presented at Copenhagen Offshore Wind 2005

[9] S. G. Johansson, G. Asplund, E. Jansson, and R. Rudervall, ‘Power system stability benefits with VSC DC-transmission systems’, Proceedings of CIGRÉ Conference in Paris, Session B4-204, 2004

[10] Y.H. Liu, J. Arrillaga and N. R. Watson, ‘A new High-Pulse Voltage-Sourced Converter for HVdc Transmission’, IEEE Transactions on Power Delivery, Vol. 18, no. 4, Oct 2003, pages 1388-1393.

[11] T. Weber, L. Yao, M. Bazargan and T Pahlke, ‘Grid Integration of Sandbank 24 Offshore Wind Farm Using LCC HVDC Connection’, Proceedings of Cigré Session 2008, B4-302

[12] P. Sandberg and L. Stendius, ‘Large Scale Offshore Wind Power Energy Evacuation by HVDC Light®’, Paper presented at EWEC 2008, March 31st –April 3rd, 2008, Brussels, Belgium

[13] K. Eriksson,‘Operational experience of HVDC LightTM’, Seventh International Conference on AC-DC Power Transmission’, 2001.

[14] G. Asplund, ‘Application of HVDC Light to Power System Enhancement’, IEEE Power Engineering Society Winter Meeting, 2000.

[15] ABB Web Pages on HVDC, Available at: http://www.abb.com/hvdc

[16] ABB web page on Nord E.ON 1, Available at: http://www.abb.com/cawp/gad02181/306c726f332f36d3c1257353003b91f0.aspx

[17] ‘Siemens to deliver HVDC technology for submarine cable to San Francisco’, PEI International, Available at: http://pepei.pennnet.com/display_article/308576/6/ARCHI/none/PRODJ/1/