DTU Wind Energy 18 June 2019

Black Start and Self Energization of HVDC Connected Offshore Wind Power Plants

Dr. Jayachandra Naidu Sakamuri, DTU Wind Energy

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DTU Wind Energy 22 February 2019

Agenda Why HVDC for OWPPs

Need for Self-Energization of OWPPs

Grid Forming Capabilities of WTs

Energization of onshore AC Grid

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DTU Wind Energy 22 February 2019

Why HVDC for OWPPs

• OWPPs are located far from the shore to avail the higher energy yield from the steady wind.

• The length of the AC cables has the limitation due to the charging current requirement

• DC cables are preferred if the offshore transmission length is more than 80 km

• The OWPPs don’t directly get impacted from the onshore faults due to HVDC decoupling

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New HVDC Corridors in Germany- Source: www.entsoe.eu

DTU Wind Energy 22 February 2019

Need for Self-Energization of OWPPs Current Practice:

– The offshore AC network is energized from the onshore AC grid by utilizing the grid forming controls of the offshore HVDC converter.

– Hence, in case of outage of the HVDC link or an onshore black out, other auxiliary power supplies are needed for supplying the WT auxiliary loads.

– Diesel generators/batteries are installed at offshore platforms. Future Prospects:

– The modern type-4 WTs with fully rated power electronics power electronics have the grid forming capabilities.

– These grid forming WTs can form the offshore AC grid and few WTS can be up and RUN to supply the auxiliary loads of other WTs and HVDC converter.

– Similarly, these grid forming WTs can be utilized to energize offshore HVDC converter, DC link and onshore AC grid black start and power system restoration.

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DTU Wind Energy 22 February 2019

Grid Forming Controls of WTs

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Distributed PLL based frequency control

DTU Wind Energy 22 February 2019

Energization of Onshore AC Grid from OWPP- Steps

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Onshore Converter – Vac.f control/ P,Q control if AC gird is already energized

Offshore Converter 1. Energize transformer & Pre-charge the

MMC sub-modules 2. Bypass Pre-insertion resistor 3. Deblock converter (in Vdc control) 4. Regulate Vdc

Onshore Converter 1. Ensure Vdc is regulated by offshore

converter 2. Deblock the converter in Vac/f mode 3. Regulate Vac 4. Start power transfer

Offshore Converter- Vdc/Q control

DTU Wind Energy 22 February 2019

Energization Sequence

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DTU Wind Energy 22 February 2019

Simulation Results- Stage 2 – Energization of Offshore VSC

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Offshore Converter

Onshore Converter

PIR ensures controlled charging of

the transformer (inrush currents) and MMC sub-modules.

The offshore converter controls the DC voltage.

DTU Wind Energy 22 February 2019

Simulation Results- Stage 3 – Energization of Onshore VSC

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Offshore Converter

Onshore Converter

Once the onshore converter is de-

blocked at t=2.75s, there is considerable oscillations in the sub-module voltages and DC voltage both at onshore and offshore. This is due to the un-controlled charging of the onshore MCC Sub-modules and deblocking them when the voltage is below the rated voltage.

These transients can be minimized by the controlled charging of the MMC sub-modules, on which we are working now.

DTU Wind Energy 22 February 2019

Simulation Results- Stage 4 – Onshore Load Connection

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Offshore Converter

Onshore Converter

A small dip (5 %) is briefly observed in

the HVDC link voltage, which recovers quickly once the offshore HVDC converter and WPP start supplying the required active power to the onshore ac network.

DTU Wind Energy 22 February 2019

Conclusions

The simulation results have shown that the grid-forming capabilities of WTs can be used to establish the offshore ac voltage and thereafter energize the offshore HVDC converter, HVDC link, and onshore HVDC converter. The pre-insertion resistor connected at the offshore HVDC converter ac terminals helps protect the

WTs against large inrush currents and control the charging of the HVDC converter submodules. It is shown that HVDC-connected OWPPs using the modern fast-responding WTs can behave as

black-start units in the restoration of future power systems with high shares of converter-interfaced energy resources.

Future Work Implement controlled charging of onshore MMC sub-modules to eliminate the oscillations in the sub-

module and DC voltages.

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DTU Wind Energy 22 February 2019

Thank You

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DTU Wind Energy 22 February 2019 14