Grid-forming control strategies for blackstart by offshore wind farms

Abstract

Abstract. Large-scale integration of renewable energy sources with power-electronic converters is pushing the power system closer to its dynamic stability limit. This has increased the risk of wide-area blackouts. Thus, the changing generation profile in the power system necessitates the use of alternate sources of energy such as wind power plants, to provide blackstart services in the future. This however, requires grid-forming and not the traditionally prevalent grid-following wind turbines. In this paper, four different grid-forming control strategies have been implemented in an HVDC-connected wind farm. A simulation study has been carried out to test the different control schemes for the different stages of energization of onshore load by the wind farm. Their transient behaviour during transformer inrush, converter pre-charge and de-blocking, and onshore block-load pickup, has been compared to demonstrate the blackstart capabilities of grid-forming wind power plants for early participation in power system restoration.

Toward Self-Reliant Wind Farms

“…However, large offshore WPPs today consist of upto 100s of WTGs connected in a large inter-array network of upto 70 km of subsea cables, with long HVAC or HVDC transmission corridor that transports the bulk power onshore, requiring either special reactive power compensation or large converter substations both offshore and onshore to manage the power flow efficiently. This makes the offshore WPP an aggregated unit with a converter-dominated environment and a very rich resonance spectrum that must be first operated in a stable and robust manner before providing onshore grid services [17].…”

“…Today wind turbines are grid-following (GFL) in that they rely on an external grid voltage 9 to which the control latches using a PLL 10 for stable operation, effectively making the WTG behave as a current source injecting controlled power. Such wind turbines are currently exempt from network restoration services as not only can they not create their own voltage and lack controlled islanding capabilities but also connection in early stages of blackstart process can result in a recurrence of blackout due to the grid not being strong enough for large wind farms [17]. Now as the penetration of PEC increases, assumptions valid for stronger, traditional grids may no longer hold resulting in improper PLL behavior causing a negative impact of the controller power sharing and system stability [22].…”

“…Moreover, GFM-WTGs can potentially minimize dependence on the offshore auxiliary diesel generator that is associated with capital and operational costs 11 , not to mention the high emissions-especially when the diesel generator is operating at fullload during unscheduled outages that can last upto 4-6 months. Since a GFM-WTG can produce produce power to keep itself warm avoiding the risk to its health 12 as long as the wind blows, replacing the diesel genset with few of these can yield significant economic and reliability benefits over the project's lifetime [17]. The CO 2 displacement can also contribute to reducing carbon-footprint taxes and ensure a smoother/ faster granting of permits in the future.…”

“…In conventional GFL-WTG today, voltage is provided by an external grid, and the WTG connects with the aim to normally supply maximum power extracted from wind. This is achieved by controlling the generator speed to operate at optimal tip-speed ratio for each wind speed 17 . For this, the RSC uses standard vector-based (or field-oriented) torque control to extract electrical power from the generator based on a reference (torque/power) obtained from the turbine controller, which ensures maximum power point tracking.…”

Wind farm flow control oriented to electricity markets and grid integration: Initial perspective analysis

“…Black start support from WPPs can be essential for power systems with high share of wind power following a blackout of the power system. Converters of modern wind turbines can have grid forming capabilities to support black start 95,96 . However, in order to provide black start support to the grid, the WPP needs to produce way less than the available wind energy, which may impact the structural loading of the wind turbines 97 .…”