The Root Cause Analysis of Sub-synchronous Oscillation for Offshore Wind Project

Li Bao, Xiaodong Liu (Eversource Energy)

Jigisha Desai (POWER Engineers)

2023 IEEE PES General Meeting

Orlando, July 16-20, 2023

Outline

• Overview of Eversource Offshore Wind Projects
• Subsynchronous Oscillations
• Large scale Offshore wind farm with AC network connection
• Low frequency oscillations under grid contingency
• Root Cause Analysis
• Impact of various factors
• Impedance model analysis
• Summary

Introduction to Eversource

• Largest utility (Gas & Electricity) in New England
• 4.4 million customers across three states
• Operate 49% of ISO-NE transmission
• Serve 50% of ISO-NE load

Overview of Eversource Offshore Wind Projects

• Developing 4,000 MW OSW in the Atlantic coast with Orsted
• First utility-scale OSW in federal waters, COD 2023
• First US-built offshore substation - a major milestone for the US offshore wind industry
• US’s first VSC-HVDC OSW
• Developing and constructing transmission projects enabling the interconnection of 2.4 GW OSW in ISO-NE, with additional 4.8 GW OSW enablement in the queue for approval

OSW under contract

In approval process

Subsynchronous Oscillations (SSO)

• SSO is defined as oscillations with frequencies below the synchronous frequency

Reference: CIGRE and IEEE

SSO Observations, PSCAD Study

STATCOM

Grid

Wind Farm

• Offshore wind project over 100MW with AC network connection
• Type 4 wind turbines
• Grid forming STATCOM to regulate the voltage at POI
• 8 to 10 Hz oscillations were observed, following assumed grid contingency events.
• It was concluded that a low system strength excited the SSO oscillations

POI

P, Q

Root Cause Analysis, Control Parameters

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• Various STATCOM controller gains (Integral gain) were analyzed for extreme contingency condition
• A larger gain improved the system stability, particularly for the weak grid condition

Root Cause Analysis, Control Parameters

• Dq-domain impedances of the STATCOM were calculated at different gains by harmonic injection method in PSCAD
• The dampings of the STATCOM were validated through small disturbance tests
• Smaller gain showed better damping for tortional interaction modes (>10Hz)
• However, larger gain gave better damping at 8-Hz dq-frame

Root Cause Analysis, Active Power Output

• The system performance was analyzed under weak grid scenario
• Project was assumed at full and partial dispatch
• The SSO was mitigated at reduced generation output, i.e.10% to 25% generation curtailment, from the farm

Root Cause Analysis, System Strength

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• The SSO was observed at light load condition when the grid strength is low
• The system was observed stable at peak load condition when more generators are online

Root Cause Analysis, Network/Load Model

• The composition of loads can have significant impacts on dynamic simulation results, in this case the SSO.
• Different load model representations were simulated, including 50% and 100% motor loads
• 50% motor load showed better SSO damping than 100% motor load i.e. the extreme case
• It was concluded that more accurate load representation was needed to assess the system’s SSO stability

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• SSO stability risk does exist in actual offshore wind projects
• The project may be required to curtail generations to mitigate the SSO
• All control parameters of FACTS and wind turbines in the system affect the SSO damping, while the STATCOM gain has the largest impact in this project
• Correct load modeling is important to assess the SSO stability in addition to traditional transient stability
• The SSO is likely easier to be excited at weak grid conditions

Summary

Questions?