Daudi Mushamalirwa

1. Daudi Mushamalirwa Luanda June, 2014 Technical issues of the stability of small size electric systems composed of wind generators and conventional generating units – A case study Senior Expert

2. Agenda • Introduction • Studied system • Performed studies • System modeling • Simulation results • Spinning reserve issue • Conclusions and recommendations

3. Introduction Introduction In different parts of the world the power generation by wind turbines has been developed, to a major part besides the classical power plants based on hydro, coal, gas or nuclear fuel. Since some years the installation of big wind farms located offshore or onshore is investigated and in implementation. Such wind farms have been connected to the transmission grids. Major activities are in progress in several countries to install wind farms of several Megawatts even Gigawatts.

4. Introduction Introduction For so called “strong networks” the impact of the wind farms to the operation of the system is relatively low. However depending on the wind energy penetration ratio this impact can become relevant. Wind generators being in general “non dispatchable”, their participation to the frequency control of the system is not required. Therefore before connecting the wind farms to the network it must be proven that the wind farm will fulfill all related grid code requirements and will not jeopardize the operation of the whole system. We present here the results of the study of the stability conditions of a small size power system of about 550 MW installed capacity and a peak load of 530 MW. The stability is assessed when a 125 MW wind farm is connected to the system

5. Studied system Studied system • Onshore wind farm of 50 WTG, 2,5 MW each, total of 125 MW. • National transmission grid 225 kV and 90 kV levels of Senegal • Annual peak load 530 MW and a total installed capacity of 550 MW. The generating units are of type gas turbines and Diesel. The power generated by the wind turbines represents 23 to 25 % of the total installed power. • Taking into consideration forced and scheduled outages, the wind power may reach 30 % of connected conventional generating units.

6. Performed studies Introduction Performed studies • In order to assess the robustness of the system, different studies were carried out: • Steady state power flow and reactive power control along with short-circuit current assessment • Power quality: harmonics and flicker • Small signal stability • Transient stability • Switching transients • The paper is focused on transient stability

7. System modelling Introduction System modelling • For a weak system and for a good accuracy and confidence of the results, it is important to model the whole system including both transmission and sub-transmission. • The load is connected to the MV bus bars • The wind farm is represented in details and connected to the transmission network through a step-up transformer and a 225 kV line

8. System modelling Introduction Dynamic modelling • The conventional generators (gas turbines and diesels) are modelled using standard IEEE models including the turbine and speed governor, the alternator, the excitation system and voltage controller • The wind turbine generators WTG are modelled taking into account the dynamics of the turbine and the generator: • The doubly-fed asynchronous machine (Two-Mass Turbine Model) • The DC/AC converter • The active and reactive power control • The voltage, frequency and speed protection of the machine • The pitch control • The WTG models are provided by the manufacturer and are available in most of the simulation software

9. System modelling Introduction Stability assessment – incident modelled • Different incidents are studied with emphasis to the system frequency behavior. • Peak load and off-peak load network configurations are tested. • Loss of the whole wind farm • Variations of wind speed • Short-circuit and Low Voltage Ride Thru (LVRT) • The critical amount of primary spinning reserve is determined and the load shedding scheme is tested to avoid excessive system frequency drop and preserve the stability of the network. • Following main parameters are monitored: • The system frequency • The bus bar voltage • Rotor angles of synchronous machines • The generator active and reactive power

10. Simulations Results Introduction • Loss of the whole wind farm • The loss of the whole farm can happen in case of short-circuit on the 225 kV connection bus bars. • The peak load case is stable in case of the lost of the wind farm (125 MW). This is a favorable case where all available generating units are connected, with high inertia and spinning reserve. • The off-peak case is also stable but the oscillations are poorly damped Generator active power

11. Simulations Results Introduction • Sudden variation of wind speed • A sudden variation of wind speed from 11 m/s to 6 m/s is assumed. The wind farm power output is consequently reduced • The system remains stable in case of a sudden decrease of the wind speed from 11 m/s to 6 m/s • Other speed variations may be critical depending on the magnitude System frequency

12. Simulations Results Introduction • Short-circuit and LVRT • A 200 ms three-phase short-circuit is applied on the 225 kV wind farm connecting bus bars. • The system is stable but oscillations poorly damped. • This is an extreme fault case, bus bar faults are generally cleared within 100 ms. System frequency

13. Spinning reserve issue Introduction Spinning reserve issue • The purpose of frequency control and regulating reserves is : • to keep a permanent balance between demand and supply • the balance is measured with system frequency which should be kept as close as possible to 50 Hz • a deficit of supply shall cause frequency drop while an excess shall result in a frequency increase • therefore a primary reserve of power must be available and activated at any instant to face any frequency deviation. • typical activation time: 5 to 30 seconds

14. Spinning reserve issue Introduction Spinning reserve issue • To keep the frequency within limits, a primary reserve of about 30 MW should be necessary. This is an extreme case when the wind farm is suddenly disconnected. • The spinning reserve can be provided: • by operating the connected units with technical margins • by installing a special generating unit (diesel) ready for starting immediately, typically in the range of 10 seconds to few minutes • disconnect some loads • other systems such as stored energy, superconducting magnetic energy storage, super capacitors can be envisaged but they do not have yet practical applications

15. Conclusion Introduction Conclusion • The stability conditions of a small size system, the Senegal network has been assessed with respect to the connection of a 125 MW wind farm. • The study showed that the system is stable for most of major incidents including a sudden loss of the wind farm, short-circuit at grid connection busbars and wind speed variations. • Low load operating conditions when few generators are connected are critical if the wind farm is operating at its maximum level. It is important in that case to reduce the wind farm output and have more conventional generators on line. • A maximum of 30 MW is needed for spinning reserve. In practical system operating conditions less than 30 MW are needed since the lost of the entire wind farm is an exceptional event.