HVDC connection of offshore wind power plants

November 13th, 2015, Published in Articles: Energize

 

Driven by the EU’s 20% renewable energy target by 2020, the first wave of voltage-source converters (VSC) for high voltage, direct current (HVDC) connected offshore wind power plants (WPPs) have been commissioned around the world, but with a notable high concentration to be found in the North Sea. These early WPP projects have been located 130 to 200 km from the point of common coupling (PCC), including both offshore and on shore cables to the converter terminal, thereby making HVDC the most appropriate technology to use for power transmission to mainland grids, recognising the limitations in AC submarine transmission at such distances.

In addition, VSC HVDC technology offers several unique advantages suitable for such environmentally harsh and difficult sitings, with yet greater energy yield potentials.
A partial list includes:

  • Ability to continuously transfer any power level (zero to maximum rating) in both directions, thereby facilitating WPP start up, and operation at low wind speeds.
  • Ease of integration with wind turbine generators (WTGs) in islanded grids with very low fault current levels (i.e., no need for synchronous condensers, as would be the case with line-commutated converters (LCC) HVDC applications).
  • Normally, no need for harmonic filters and additional reactive power resources.
  • Improved performance during onshore AC disturbances: VSC converters are self-commuted, so commutation failures will not occur in case of AC grid disturbances.
  • Blackstart capability: the ability to supply the auxiliary power needs of the offshore WPP when WTGs are not operating (e.g., due to low wind, or excessively high wind, conditions).
  • Feasibility of building compact, partially or fully tested and assembled, converter stations resulting in lower costs and risks for the off-shore platform.
  • Ability to use XLPE cables since the operating voltage polarity is unchanged and independent of the direction of the power flow (XLPE cables are not capable of withstanding the voltage reversal which happens with LCC HVDC applications). It should also be mentioned that MI cables are still a viable alternative, particularly at higher DC voltages.
  • Allow implementation of future multi-terminal expansion.

The first VSC HVDC connected off-shore WPP project (BorWin1, 400 MW, ±150 kV, 125 km off the coast of Germany) was commissioned in 2009. Even though a number of similar projects have been commissioned, or are under various stages of design and construction, it is generally recognised that this method of transferring energy harvested from offshore WPPs is in its early stages of maturity.

Compared to a large population of WPPs connected to AC grids, VSC HVDC transmission completely changes the electrical environment, presenting new challenges and opportunities for operation during normal and abnormal conditions. Similarly, most HVDC links have been developed as point-to-point connections between AC transmission systems. From the electrical point of view, offshore WPPs constitute weak islanded grids.

Fig. 1: Communication system of an offshore WPP integrated in an AC system through a VSC-HVDC link.

Fig. 1: Communication system of an offshore WPP integrated in an AC system through a VSC-HVDC link.

At present, the industry is developing standards and commonly accepted grid codes, while gaining deeper understanding of the integrated WPPs and VSC HVDC systems. Furthermore, there is also a growing knowledge of, and experience with, the design and operation of such projects. So, even though current projects have to develop their own design and operational philosophies, with deeper experience, and longer reported operational history, future optimally designed projects are expected.

VSC-HVDC configurations

During the last several years there has been a significant increase in the number of offshore WPPs. The primary reasons include: lack of suitable onshore locations for additional WPP developments, and offshore WPP potential to generate significantly higher levels of energy when compared with an onshore WPP project of the same rating. To transfer the offshore WPP energy to the onshore AC grid, the VSC-HVDC link provides technical features and economic advantages when the distance between the offshore WPP and the onshore AC grid extends beyond 100 km.

It must be noted that the choice of HVAC vs HVDC transmission requires further cost-benefit analysis based on an individual project’s needs. The ongoing developments in HVDC technologies in general, and specifically in the VSC-HVDC technology, indicate a growing trend in further construction and utilisation of point-to-point VSC-HVDC connection and multi-terminal VSC-HVDC grids.

Although classical LCC HVDC systems currently offer advantages for specific applications (e.g., bulk power transfer over very long distances), VSC HVDC systems are necessary for integrating long distance offshore WPPs into the onshore AC grids due to the lack of synchronous generation offshore. The basics of the VSC HVDC technology, protection, and control have been widely discussed in technical literature [1, 2] and are therefore not discussed in this article.

However, when necessary, the relevant specific characteristics and features are briefly described and highlighted. The VSC HVDC system was first implemented as a test installation in 1997, with a growing installed base over the past decade. Currently, there has been an increasing trend in the development of semiconductor technologies, resulting in further consideration of VSC HVDC technology for transmission projects around the world. By the end of 2014, advancements in semiconductor devices have seen ratings up to 900 MW at ±320 kV for WPP connection.

One of the considerations regarding the VSC HVDC technology, compared to the LCC HVDC technology, is the potentially higher converter station losses. The latest developments in the semiconductor switches as well as developments in the VSC configuration, which can operate at lower switching frequency without compromising the waveform quality, indicate that the VSC HVDC converter station losses can be reduced to the levels comparable to that of the LCC HVDC converter station.

The first VSC HVDC technology was based on the two-level and the three-level VSC configurations and the classical PWM switching strategies, e.g., the sinusoidal PWM (SPWM). The VSC HVDC technology for future installations, including for offshore WPP applications, consider the multilevel VSC configuration, e.g., the modular multilevel converter (MMC) configuration, and accordingly different switching strategies. The development of multilevel VSC for HVDC applications promises:

  • Decrease in the losses
  • Potential for increasing the voltage level and thereby also the power level
  • Reduction in the size or even elimination of the AC- and DC-side filters
  • Higher degree of reliability and availability
Fig. 2: Point-to-point VSC-HVDC connection between an offshore WPP and an onshore AC system.

Fig. 2: Point-to-point VSC-HVDC connection between an offshore WPP and an onshore AC system.

The developments in VSC technology for HVDC applications are accompanied by developments of the extruded DC cable technology. The extruded DC cable technology for commercial use is currently available for DC voltages of up to 320 kV. For future developments 525 kV extruded HVDC cable system has been tested [3]. Higher voltage ratings are possible based on the use of the mass-impregnated cable technology [4]. References [2 to 5] provide further details on various VSC HVDC technologies, applications, control and protection strategies, merits and limitations, and for future onshore and offshore WPP applications.

General VSC-HVDC design considerations

The actual design of the offshore VSC HVDC for the WPP applications should consider several basic factors, including but not limited to: availability, reliability, fault-ride-through capability and requirements, range of voltage variations (voltage band), reactive power requirements, transient overvoltages, cable voltage rating, converter and the overall system losses, maintenance requirements and accessibility. As compared to a completely onshore VSC HVDC link, the VSC HVDC link which connects an offshore WPP to the onshore AC system may have special requirements. For example:

  • A braking chopper in the onshore converter station
  • Multiple/parallel transformers in both converter stations. Each transformer is typically rated to transmit more than 50% of the WPP power (sometimes up to 100% in case of another transformer outage), and requires a more sophisticated mechanical design to withstand the harsh offshore environmental conditions. It must be noted that selection of the transformer also requires cost-benefit analysis.

Other main considerations include outage time and reliability. For example, accessibility of the offshore VSC HVDC platform and maintenance (e.g., repair and replacement of components and apparatus) are major considerations and influence the level of redundancy of the:

  • Auxiliary subsystems and components
  • Redundant transformers
  • System for cooling
  • Auxiliary diesel-generator unit(s)

The adoption of the VSC HVDC grid configuration for offshore WPPs or integration of an existing VSC HVDC converter station of an offshore WPP in a future VSC HVDC grid also necessitates consideration for additional power rating under various contingency scenarios and operational conditions.

Fig. 3: Multi-infeed VSC-HVDC connection between an offshore WPP and an onshore AC system.

Fig. 3: Multi-infeed VSC-HVDC connection between an offshore WPP
and an onshore AC system.

The control and protection of the offshore WPP through a VSC HVDC link is more sophisticated, when compared to a land-based point-to-point VSC HVDC link, where the conventional approach is to control the DC power flow and the HVDC voltage each by one of the VSC HVDC converter stations. The control of VSC HVDC converters that connect an offshore WPP to the onshore AC system must be designed to start the operation of the offshore converter station and energise cables and transformers. This can be accomplished only by the VSC technology and not by LCC technology.

During normal steady-state power transfer from the offshore WPP to onshore, the function of the offshore VSC HVDC converter station is also different than that of its counterpart land-based VSC HVDC converter station. The offshore VSC HVDC station must be capable of accepting and delivering the power generated by the WPP and transfer it to the onshore station and enforce the power balance and protect integrity of the operation of the WTGs units within the offshore WPP.

The offshore VSC HVDC station, under this mode of operation, operates as a “power slack bus”, and the onshore VSC HVDC station controls the DC voltage of the DC link and voltage or reactive power on its AC side.

The onshore VSC HVDC station can also operate in a STATCOM style mode. During some operational scenarios (e.g., faults on the AC side of the onshore VSC HVDC station) the power balance between the WPP and the AC system is transiently violated and the generated power by the WPP cannot be fully injected in the AC side. In such a situation, the excessive power at the WPP side (with respect to the power that is injected into the onshore AC-side) results in the voltage rise of the DC link.

Fig. 4: Multi-terminal “back-bone” VSC-HVDC connection.

Fig. 4: Multi-terminal “back-bone” VSC-HVDC connection.

The DC link voltage must be kept within pre-specified bounds to comply with the protection requirements and to prevent HVDC shut down. Thus the balance of power needs to be provided to maintain the DC voltage to limit variations of the DC link voltage. One approach to achieve this objective is to utilise a DC chopper at the DC side of the onshore VSC HVDC converter station to drain the excessive power from the DC link and dissipate it in a properly designed resistor. This provides ride-through capability to the VSC HVDC system during AC system faults and maintains the WPP operation uninterrupted.

However, since cost consideration imposes a practical limit on the amount of power that can be dissipated by the operation of the chopper system, the VSC HVDC must be tripped if the limit is reached. The DC chopper, used in several schemes, is able to absorb a limited amount of energy from the DC system and limits the voltage in the DC cable system by preventing cable overcharge to the trip level, or prolongs the DC cable charging time interval, to enable the other countermeasures to address the problem.

Overall, the DC chopper must provide:

  • Robustness, reliability, and ease of maintenance
  • Independent control based on local voltage measurement
  • Coordinated control with the converter station controller (if applicable)
  • Low or no degree of dependency on communication systems
  • Compliance with the local grid codes

In addition, higher level controls are also required by the VSC HVDC for offshore WPP connection. These include:

  • Control of the WPP output power
  • Automatic WPP output power control: The power generated by the WTGs can be controlled by assigning a control function to the offshore VSC HVDC converter to control the WPP power output, based on frequency control of the WPP AC collector system [6].

Excess power in the onshore AC system results in the AC system frequency increase. In such a case, it is desirable to actively control and reduce the injected power of the WPP into the AC system. This can be accomplished by communicating the main AC network frequency deviations to the offshore VSC HVDC converter station to vary the offshore frequency, or direct the WPP governor to vary their active power generation. This mode of combined WPP VSC HVDC operation is analogous to that of a classical power plant operation under AGC control [7].

Fig. 5: VSC HVDC grid configuration.

Fig. 5: VSC HVDC grid configuration.

To guarantee reliable and appropriate control functions of the VSC HVDC in compliance with the requirements of the offshore WPP and the onshore AC system, there is a need for a bi-directional communication system between the VSC HVDC stations.

For the VSC HVDC integration of offshore WPP a two-level communication system (e.g., fast and slow) are suggested in Fig. 1. The high-speed communication could be used for control actions and the low-speed communication could serve the needs for monitoring functions. The slow communication provides transmission of measured RMS values, device status for interlocking, start-up/shut-down/reconfiguration commands, alarms, and detailed status of equipment parameters for monitoring or maintenance.
The fast communication for control and protection functions covers the delivery of actual DC voltage, actual active power limits, DC current limits, converter status (blocked, unblocked), converter protective block actuation, and converter emergency switch off related actuations.

Since the existing trends indicate that the point-to-point VSC HVDC system for offshore WPPs will be extended to the VSC HVDC grid configuration, it may be desirable to design the point-to-point VSC HVDC system to accommodate the future requirements in the grid context (e.g., provisions for additional space to allow new pieces of equipment and connection of new DC cables to the offshore VSC HVDC platform provided this additional space does not substantially increase the cost of the platform). However, the main changes in the evolution of the point-to-point VSC HVDC to the VSC HVDC grid will be related to the control strategies and algorithms. Availability of cost competitive HVDC circuit breakers, and DC/DC converters, along with potentially new technologies for fast and effective DC power flow control and DC fault detection, will also impact future HVDC grid developments.

VSC HVDC configurations

This section discusses the point-to-point and the multi-infeed configurations for connecting WPPs to the host AC power system and also briefly addresses emerging configurations.

Point-to-point connection

Fig. 6: Basic offshore platform configuration.

Fig. 6: Basic offshore platform configuration.

In the point-to-point connection, as shown in Fig. 2, the offshore WPP is connected to the onshore AC system through two VSC HVDC converter stations and the submarine cable. The DC chopper at the onshore station provides the fault ride-through capability. Other options to maintain the DC voltage within the desired limits, during the AC system faults, also exist [8].

Multi-infeed connection

In Fig. 3, two independent VSC HVDC links (each a point-to-point configuration) connect one offshore WPP to two different buses of the onshore AC system through VSC HVDC converter stations. The potential reasons for adopting this configuration include:

  • Upgrading and increasing the power rating of the WPP beyond the nominal capacity of single existing VSC HVDC system
  • Staged development of the DC connection by different developers
  • Increasing the reliability of the connection and maintaining power flow from the WPP to the AC system even if one VSC HVDC is out of service due to planned or forced outage events.

When this configuration is chosen, full redundancy of the offshore and onshore converters is required.

Emerging configurations: multi-terminal VSC HVDC

In this configuration, shown in Fig. 4, multiple VSC HVDC stations are connected to the main DC trunk. In this configuration, multiple independent AC systems can exist and one or more of these AC systems can be offshore and/or onshore WPPs. The AC systems do not need to be synchronised. This configuration enables:

  • Transfer of active power among the (onshore) AC systems
  • Collection and redistribution of the output power for each WPP
    The WPP can participate in the frequency control (primary regulation) of the AC systems [10]. The drawback of the multi-terminal VSC HVDC (without DC circuit breaker or full bridge converter) is that a DC side fault requires a complete shut down and restart of all converters and the DC system, as the fault can only be cleared by opening the AC side circuit breakers of all the VSC HVDC converters. When considering this configuration, the following must be noted:
  • Offshore DC switching stations (especially if air insulated) add potential project cost and require considerable maintenance due to harsh offshore conditions.
  • High equipment ratings, to accommodate high wind/power flow conditions, may result in an additional cost burden for the DC system.
  • Without economical and reliable DC breakers, or other fast fault clearing measures, the stability of the connected AC systems may constrain or not allow such configuration.

Future configurations: VSC HVDC grid

The VSC HVDC grid configuration, shown in Fig. 5, is based on a meshed DC line configuration with an N-1 contingency requirement (i.e., subject to the loss of one converter station or a DC line, the rest of the system should maintain operation). Since the VSC HVDC converter stations directly control the power flow between the corresponding AC and DC sides, the power flow in the DC network imposes severe challenges and requires elaborate communication among the VSC HVDC converters, regulation algorithms, flow dispatch and/or additional components to impose the desired DC grid power flow.

Fig. 7: A schematic diagram of a VSC HVDC link for an offshore WPP.

Fig. 7: A schematic diagram of a VSC HVDC link for an offshore WPP.

The technical brochures from Cigré WGs B4.57 (Guide for development of models for HVDC converters in a HVDC grid), B4.58 (Devices for load control and methodologies for direct voltage control in meshed HVDC grid), and B4/B5.59 (Control and protection of HVDC grids) provide further details on the subject.

Basic offshore WPP configuration

Two basic configurations, i.e., the radially connected configuration and ring connected configuration, for the offshore WPP are shown in Fig. 6. The radially connected configuration (WPP1), through the common offshore AC substation, is connected to the main onshore AC grid by a point-to-point VSC HVDC system. The ring connected configuration (WPP2) also provides connections to the off-shore AC substation, in parallel with the radially connected configuration [10, 11].

Basic configuration of point-to-point VSC HVDC for offshore WPP

Based on the schematic diagram of Fig. 6, Fig. 7 shows the basic configuration and the main components of the point-to-point VSC-HVDC connection for the offshore WPP.

References

[1] Cigré Technical Brochure No 269: “VSC Transmission”, Cigré WG B4.37, 2005.
[2] Cigré Technical Brochure No 492: “Voltage Source Converter (VSC) HVDC for Power Transmission – Economic Aspects and Comparison with other AC and DC Technologies”, Cigré WG B4.46, 2012.
[3] A Gustafsson, M Saltzer, A Farkas, H Ghorbani, T Quist, and M Jeroense: “The new 525 kV extruded HVDC cable system”, ABB Grid Systems, Technical Paper, August 2014.
[4] N Mahimkar, G Persson, and C Westerlind: “HVDC Technology for Large Scale Offshore Wind Connections”, Proceedings of Smartelec 2013.
[5] EM Callavik, P Lundberg, MP Bahrman, and RP Rosenqvist: “HVDC technologies for the future onshore and offshore grid”, Proceedings of Cigré Symposium “Grid of the future”,Kansas City, USA, October 2012.
[6] Y Phulpin: “Communication-Free Inertia and Frequency Control of Wind Generators connected by an HVDC-Link”, IEEE Transactions on Sustainable Energy, 27(2), May 2012.
[7] T Haileselassie: “Control, Dynamics and operation of Multi-terminal VSC-HVDC Transmission Systems”, Ph.D. Thesis, NTNU Trondheim, Norway, 2012.
[8] R Sharma: “Electrical Structure of Future Off-Shore Wind Power Plants with a High-Voltage Direct Current Power Transmission”, Ph.D. Thesis, Technical University of Denmark, Lyngby, 2011.
[9]    VF Lescale, P Holmberg, R Ottersten, YJ Hafner: “Paralleling offshore wind farm HVDC ties on offshore side”, Cigré Session 2012, paper C1-107, Paris, 2012.
[10]    Offshore Grid Development Plan 2013, first draft, German TSOs, 2013, www.netzentwicklungsplan.de/content/offshore-netzentwicklungsplan-2013-ersterentwurf
[11]    Cigré WG B3-36 interim draft report: “Special considerations for AC collector systems and substations associated with HVDC connected wind power plants”.

Acknowledgement

This article is an extract from Cigré WG B4.55’s report “HVDC connection of offshore wind power plants”, May 2015, and is republished here with permission.

Contact Rob Stephen, Cigré, Tel 031 563-0063, rob.stephen@eskom.co.za

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