Issues and practical experiences with connection of wind farms to weak ac networks

1. Issues and practical experiences with connection of wind farms to weak ac networks June 2013 Presented by Babak badrzadeh

2. Background • At present installed capacity of wind generation in the Australian national electricity market (NEM) is around 2.7 GW. • An additional 8-9 GW of wind is anticipated by 2020. • Fault levels are often lower relative to the Europe and North America. • Displacement of synchronous generators by wind and other forms of variable generation technology causes network fault levels to reduce. • Large scale penetration of variable generation—and consequent displacement of synchronous generators—may give rise to network-wide issues, as well as local issues.

3. Relevance At present only a limited number of wind farms are connected to weak connection points, but concerns will grow in the future in particular for: South Australia: • Connected to the NEM power system through ac and dc links, but: • High wind penetration rate • More wind farms are being connected • Retirement or semi-retirement of some of the large conventional power plants • Network faults levels are becoming lower and lower so as the system inertia Tasmania: • Connected to the NEM power system through dc links only with future concerns on: • Minimum fault level and inertia • Maximum rate of change of frequency during fault conditions • Sympathetic response of wind farms to remote faults

4. Related issues • Model requirements • Steady-state stability • Transient stability • Minimum inertia • Fault level and protection coordination • Harmonic resonance • Temporary overvoltage • Sub-synchronous interaction (one committed project and one proposal with series compensated ac lines)

5. NEM EXPEREINCE (1) • Positive sequence models may be used for conditions for which their accuracy has not been validated. • The use of positive sequence models could give rise to optimistic results compared with the EMT models. • A carefully designed overall voltage control strategy coordinating the response of wind turbine and all reactive support plant is essential. • Contribution of the wind farm to active power and frequency control can be crucial in maintaining system stability, and hence mandatory.

6. NEM EXPEREINCE (2) • Giving priority to reactive power during fault recovery could give rise to excessive energy deficit, and potentially high temporary overvoltages. • Turbine control may need to be modified to give priority to active power recovery. • Understanding the precise behaviour of the fault ride-through function is essential. • This many need to be demonstrated through on-site fault ride-through test.

7. recommendations • Identify the criteria and conditions where the use of electromagnetic transient (EMT) type models may be required. • Investigate and identify the minimum short circuit ratio (and other) requirements for secure and reliable power system operation with respect to wind generation. • Study the implications of using type 3 and 4 wind turbines in weaker parts of the network, in terms of active power recovery and energy deficit upon fault clearance. • Investigate scenarios resulting in high temporary over-voltages. Should the results identify high TOVs (in the range of 1.3 pu), identify whether system-wide controls might be required. • Investigate the effects of reactive current injection limits of around 1 pu in weak and isolated parts of the power system.