In 2003, the Nysted Offshore Wind Farm, with an Installed Capacity of 165.6 MW, was commissioned on the island of South Zealand in southern Denmark. The wind turbines are equipped with asynchronous generators with squirrel-cage rotors. Studies supported by operational experience have confirmed the wind farm was the source of unacceptably large and frequent voltage fluctuations on the 132-kV transmission system.

The voltage disturbances were due to a combination of a weak 132-kV transmission system and large variations in the reactive power-flow exchange between the wind farm and the transmission system. The reactive power flow at the Radsted substation was due to the absorption of the reactive power in the magnetisation of the wind turbines, transmission cable and 132/33/33-kV transformers. The grid interconnection at Radsted was more than 100 km (62 miles) from the nearest conventional power plant and the country's strong 400-kV transmission system.

To reduce the voltage fluctuations, SEAS-NVE (Svinninge, Denmark) decided in the spring of 2005 to install a static VAR compensator (SVC).

TRANSMISSION SYSTEM IN EASTERN DENMARK

The transmission system in eastern Denmark comprises 400-kV and 132-kV transmission lines, cables and existing interconnectors with southern Sweden and Germany, with plans in the works for yet another interconnector to the Danish island of Funen.

The Swedish interconnector — which includes four AC cables, comprised of two 400-kV cables and two 132-kV cables — has a transfer capacity of 1900 MW and links Denmark to the Nordic grid. The interconnector to Germany is a 400-kV high-voltage direct current (HVDC) transmission system with a transfer capacity of 600 MW. The planned interconnector to Funen will be a 600-MW HVDC transmission system. Currently, in eastern Denmark, the installed generating capacity is 4360 MW supplying a maximum demand of some 2870 MW.

SELECTION OF THE SVC

To comply with existing European Union Grid Code legislation, it is necessary to provide power and dynamic-controlled grid voltage to enable a grid interconnection. The equipment installed to control the grid voltage can be incorporated in the wind turbines or sited in the onshore AC substations. Wind farms that are increasing the installed generating capacity are now regarded as power plants, and wind turbines are becoming more complex with the need to install more equipment to comply with the Grid Code. In addition to increasing the cost and maintenance of wind turbines, there are concerns these demands could adversely affect availability of the wind turbines.

The Nysted Offshore Wind Farm was designed to minimise the equipment installed in the harsh offshore environment, so the SVC was installed onshore, providing additional supply opportunities to support the transmission system during faults.

THE RADSTED PROJECT

In Denmark, transmission system projects require approval by the energy and local authorities to ensure the proposal satisfies all environmental standards, thereby securing local acceptance.

SEAS-NVE in cooperation with Energinet.dk (Fredericia, Denmark), the Danish transmission system operator, considered the dynamics of the complete transmission system by undertaking system stability studies to determine the rating of the SVC. To allow the normal operational requirements and for fast changes in the wind farm's production, ratings of 80-MVAR capacitive and 65-MVAR reactive were specified for the SVC.

The SVC for the Radsted project was based on conventional technology using two thyristor-controlled reactors (TCR) and two filters. Because of the requirements on size and permitted noise, the SVC was designed as a 12 pulse; a six pulse is the norm used in an SVC. The 12-pulse SVC optimises the size of the building required to accommodate the complete SVC and reduces the filtering requirements, thereby making the SVC silent.

The project specification was completed in late 2004. January 2005 to March 2005 was allocated for tenders; SEAS-NVE invited pre-qualified bidders to offer costs for a SVC solution. In June 2005, Siemens Wind Power A/S (Brande, Denmark) was awarded the turnkey contract for the supply, installation and commissioning of the SVC in accordance with the unit rating determined by SEAS-NVE.

During the design and engineering phase of the Radsted project, technical issues were clarified and SEAS-NVE prepared the site for onshore installation work:

• Re-routing existing 10-kV overhead lines

• Constructing a new road and leveling the building site

• Tree planting to screen the proposed installation

• Extending the existing 132-kV bus bar.

The building that would house the SVC was completed by the fall of 2006. It has an area of approximately 825 sq m (8880 sq ft) and a height of 6 m (20 ft). The building works were performed in parallel with the installation of the indoor SVC.

The building, which externally has the appearance of a conventional barn to blend into the existing landscape, was designed to limit noise emission. Also, due to the high-created magnetic fields present in the area that accommodate the TCRs, this enclosure had to be constructed without introducing closed metallic loops in the roof structure and floor. Special design features in the building allow a large volume of airflow to dissipate the heat generated by the reactors and filters.

FIELD TESTING

The SVC transformer is an oil-immersed, naturally cooled transformer type with separate coolers. The transformer is positioned on an anti-vibration foundation, designed to retain any oil that leaks from the transformer or radiator, and is placed in a separate soundproof enclosure. The outdoor coolers from the valve cooling system have low-noise fans.

In the course of the design, special attention was given to the limited space within the building to ensure there was sufficient access for the tools and equipment required to complete all future maintenance on the SVC.

The pre- and post-commissioning tests were performed by a team comprising Siemens A/S and SEAS-NVE staff, an exercise that also provided training for the utility personnel. The tests proceeded as planned, but minor design changes were necessary because of the high magnetic field generated by the TCRs.

To determine the rating of the filters in the SVC, harmonic measurements were taken simultaneously with the main circuit design of the SVC. The harmonic level measured in the preliminary studies indicated the need for a SVC with a substantial rating; therefore, SEAS-NVE decided that further monitoring was required prior to energisation of the SVC.

The harmonics were measured on the 132-kV bus bar at Radsted in 10-minute intervals for a six-month period. Table 1 shows the harmonic levels recorded in each circuit for week 27 and the maximum values recorded during the period of monitoring.

ACCEPTABLE SOUND LEVELS

As expected, the SVC filters reduced the harmonic voltages on the 132-kV bus bars but created currents in the SVC filters. These currents can be calculated using the design criteria for filters (see CIGRÉ Working Group 14.30's “Guide to Specification and Design Evaluation of AC Filters for HVDC Systems”) by applying the harmonic voltage to the impedance:

Z _GRID + Z _TRANSFORMER + Z _FILTER.

ACCEPTABLE MAGNETIC FIELD LEVELS

The harmonic currents calculated using the CIGRÉ criteria when considering a 132-kV grid with cables instead of overhead lines are quite significant in deciding the rating for the filters. Table 2 shows the currents for the SVC based on the rating study compared with the harmonic currents measured during the trial period

In Denmark, acceptable disturbance levels in a rural environment have been a key issue for many years. The public's focus on the issue is increasing while the allowable and acceptable noise levels are decreasing. There are no exceptions to the legislation, and expensive remedial measures have been made to existing substations in order to comply with the new requirements.

The SVC and Radsted substation are close to a residential area, which means the noise from the complete installation at the neighbour's boundary must be below 35 dBA. When the SVC specification was decided, various authority approvals had not been granted, including the precise location of the substation, so that residences could have been between 10 m to 50 m (33 ft to 164 ft) from either side of the SVC. To avoid delays to the construction project, a larger site for the Radsted substation was obtained to ensure the boundary noise level did not exceed 30 dBA. Following commissioning of the SVC, the sound level at the closest neighbour was measured at 18 dBA.

OPERATIONAL PERFORMANCE

Magnetic field levels are regulated and must comply with the April 29, 2004, European Union Directive 2004/40/EC on the minimum health and safety requirements on exposure levels for workers and the general public. This directive includes action values on the magnetic field and the limits on induced current density for head and trunk. Action values can be measured but the limit is more difficult to obtain. As a safety precaution for the Radsted SVC project, SEAS-NVE decided to apply the action value as the limit value. The current in the main reactors and filter reactors both contribute to the SVC magnetic field. Since these currents have a harmonic content, the magnetic field limits are frequency-dependant. It was necessary to calculate the magnetic field strength under different operating conditions as the maximum value may not correlate with the maximum current. To verify the calculations, measurements were taken at three different positions for inductive loads of 65 MVAR and 20 MVAR and for a 30-MVAR capacitive load. Table 3 shows the results obtained using the 20-MVAR inductive load.

The measured and calculated values are in close agreement, with the minor differences probably being due to the deviation between the positions used for measurement and calculation. The calculations were repeated 30 times from full inductive (firing angle of 90 degrees) to full capacitive (firing angle of 180 degrees).

In the ure showing the magnetic field strength, the blue-green border at 100 µT (at 50 Hz) is the action limit where the public has access and yellow-red at 500 µT (at 50 Hz) is where authorized personnel have access.

An additional benefit attributable to the installation of the SVC is the prevention of induction generators from disconnecting from the transmission system in the event of an outage, reducing the stress on the wind turbines. This characteristic is limited to induction generators, which form the majority of wind turbine generators connected to the Danish transmission system.

The performance of the SVC installed at Radsted was proven when in April 2007, there was a transmission system outage on Amager Island near Copenhagen, Denmark, some 140 km (87 miles) away SVC. The SVC operated as expected, delivering full reactive capacity during the failure and thereby supporting the system voltage and ensuring system stability. The event had no impact on the output of the Nysted Offshore Wind Farm.

The rapid development of generation from wind farms in Denmark has increased the problems of supply reliability and system stability. Wind farms are now regarded as a reliable source of energy to support the transmission system during normal conditions and in outages attributable to system faults and conventional power plant downtimes for maintenance.

The Radsted SVC has demonstrated that the 165.6-MW Nysted Offshore Wind Farm is now a source of electricity generation, connected to a very weak 132-kV transmission system without any adverse effect on the system voltage or stability. The SVC installation proved to be an environmentally acceptable project, constructed with the support of the planning authorities to minimize the impact on the local community.

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Niels Andersen (na@seas-nve.dk) received a MSEE degree from the Technical University of Denmark and was employed by ABB Power Systems in HVDC until 1994. Since then, he has been employed by SEAS-NVE, where he works on HVDC and Flexible AC Transmission System (FACTS) applications. His current main areas of work include project management, system studies, feasibility studies, specifications for HVDC/FACTS and offshore wind farms.

Michael B. Hansen (mbh@seas-nve.dk)received a BSEE degree from the College of Copenhagen and was a consulting engineer working for the Denmark railways until 1994, when he joined SEAS-NVE to work on HVDC and Flexible AC Transmission System applications. His current responsibilities as a commissioning and project engineer include transformers, protection, SCADA and measurements.

Table 1. Harmonic levels on the 132-kV bus bars prior to installing the SVC.

Harmonic	Line 1 (max.) % of VN		Line 2 (max.) % of VN		Line 3 (max.) % of VN
	Week 27	Max. level	Week 27	Max. level	Week 27	Max. level
3rd	0.43	0.46	0.23	0.29	0.25	0.41
5th	2.07	2.25	2.28	2.44	2.14	2.30
7th	1.22	1.34	1.08	1.36	1.13	1.53
11th	0.01	0.20	0.02	0.03	0.01	0.05
13th	0.21	0.23	0.16	0.23	0.16	0.21

Table 2. Currents in the SVC filter.

Frequency	Calculated using the CIGRÉ criteria
	Measured current from the grid (A)	Total calculated rating current (A)	Current from the grid (A)
150 Hz (3rd harmonic)	1253	376	74
250 Hz (5th harmonic)	1347	968	98
350 Hz (7th harmonic)	406	254	47
550 Hz (11th harmonic)	64	17	4
Rated current	3550

Table 3. Magnetic field strength results with a 20-MVAR inductive load.

Position	Frequency (50 Hz) Magnetic field (µT)		Frequency (150 Hz) Magnetic field (µT)		Frequency (250 Hz) Magnetic field (µT)		Frequency (350 Hz) Magnetic field (µT)		Frequency (450 Hz) Magnetic field (µT)
	Measured	Calculated	Measured	Calculated	Measured	Calculated	Measured	Calculated	Measured	Calculated
P11	369.8	399.5	105.1	91.2	31.7	30.4	13.5	14.6	11.9	8.1
P1	82.8	67.9	22.2	33.9	6.0	4.9	2.4	2.5	1.3	3.0
P3	65.8	61.5	22.2	21.2	4.5	4.7	1.7	2.3	0.9	1.9
I5	465.6	470.9	166.5	165.5	35.6	35.9	15.2	17.3	123.4	14.8
P15	329.6	383.3	88.4	86.3	26.7	29.2	11.4	114.1	9.5	7.7