Pumped storage for balancing wind power fluctuations in an isolated grid

Transcription

1 Wind Power to the Grid EPE Wind Energy Chapter 1 st seminar Delft University of Technology, 8-9 March 008 Pumped storage for balancing wind power fluctuations in an isolated grid Jon Are Suul, Professor Tore Undeland Norwegian University of Science and Technology Department of electrical power engineering N-7491 Trondheim, Norway jon.are.suul@elkraft.ntnu.no Kjetil Uhlen SINTEF Energy Research N-7465 Trondheim, Norway 1

2 Pumped storage hydro power plant Upper and lower reservoirs Pumped storage units are feasible for a wide power range Small unit of a few KW Large units with ratings of several hundreds of MW Roundtrip efficiency up to around 85 % Reversible Francis turbine Optimal speed depends on turbine head Maximum efficiency for different speeds in generator mode and in pumping mode Efficiency and utilization range can be improved with variable speed operation Variable speed operation allows for controllable power in pumping mode Upper reservoir Water flow Reversible Francis-turbine Lower reservoir

3 Types of controllable pumped storage units Examples of existing units: Large variable speed units in operation for load balancing in power systems Mainly doubly-fed asynchronous machines Japan Ohkawachi with X395 MVA and 7 MVA rotor circuit cycloconverters Units reaching 475 MVA are under construction with rotor circuit VSC Germany Goldisthal with x300mva and 100 MVA rotor circuit cycloconverters Few examples of LCI-drives Low power units in small isolated systems Islands in Greece and Spain Stand-alone systems with separate pump and turbine units Standard industrial induction motor drives with speed/power command for smaller pump units Synchronous generator for the turbine unit Cycloconverter Voltage source converter = LCI-converter = = DFAM DFAM SM 3

4 Investigated system configuration Salient pole synchronous machine Full scale voltage source converter Medium voltage (3.3 kv) Three-level topology based on IGCT switches Converter can be bypassed and the system operated as a normal hydropower plant Redundancy Keeps familiar technology available for operator Diode rectifier as grid side converter if variable speed is not needed in generator mode Improved operation can be obtained with Active Front End converter Reversible power flow Control of reactive current and grid voltage Relevant for pumped storage units in medium power range kv Filter = = 3.3 kv SM Pumped storage 4

5 Case study: The Faroe Islands 5

6 The Faroe Islands Isolated power system Minimum load: 14 MW Maximum load: 70 MW Existing electricity supply 60% based on diesel generators 35% traditional hydropower Case study II 5 % wind power, 4 MW installed High CO emission per capita Need for energy storage and grid stabilization to integrate higher amount of wind power 6

7 Case study III Simplified grid model of the Faroe Island power system 10 MW wind farm with induction generators directly connected to the grid Assumed worst case with respect to controllability Diesel generators DE SG < 80 MW Wind turbines IG Z Customer load Z Z Z Hydropower SG HT 5 MW Synchronous machine for pumped storage 10 MW Z Rating of 10 MW Pumped storage Full scale converter Aggregated model for diesel generator sets PT SM 10 MW DC AC AC DC Converter 5 MW hydropower plant 7

8 General challenges of the system System operation with high share of wind power Minimum load and maximum wind power production corresponds to 10 MW wind and 14 MW load without pumped storage Pumped storage can be introduced as a controllable load Maximum share of wind power in the system is more than 40% even with pumped storage running as load Wind generators and pumped storage will dominate the system under minimum load conditions Momentary power balance Controllability of available conventional production units Power control of pumped storage synchronous machine and turbine Power control of wind turbines 8

9 Grid simulation model Torshavn 30 km cable Vestmanna Diesel SM Z kv 66 kv 66 kv kv 3.3 kv SM Hydro T1 60 MVA T6 60 MVA C d,1 C d,1 T 5 MVA C d, T3 5 MVA Load Vestmanna 5 MVA Wind IM 10 MVA 690 V C IM T4 10 MVA kv kv Load Torshavn 1 km cable Z 3 C d,3 C d,3 C d, Z kv 3 km cable T5 1 MVA Simulation model in PSCAD/EMTDC of simplified power system with pumped storage drive Investigation of the pumped storage unit for balancing fluctuations from the wind power production 3.3 kv SM 10 MVA Pumped storage 9

10 Synchronous machine control system overview Grid p pref, Power & Torque control w ψ s i lref, ψ s, ref i mref, i mref, Flux control i m ψ s Power factor control im i f ψ s ξ Δi f, ref i dqref,, ξ i ml, ψ s Current controllers v dqref,, Current estimation & Flux calculation i f w ψ dq, Simulation model based on vector control principles Three controllable variables under variable speed operation Torque or power Flux Power factor Fast and accurate response in torque or power i f i dq, v f, ref w v dq, i dq, q - D θ θ θ Q u q f + - v αβ,,ref b s v αβ, i αβ, + u f u β - u α v, 3 abc ref θ V SM _ abc u d I SM _ abc i f + a s d w θ w = = SM = 10

11 Pumped storage power control Investigation of two main ways of controlling pumped storage unit Balance power fluctuations directly Minimize the influence from the wind power on the rest of the system Frequency bias of the power system is indirectly increased since more production is kept online during low load condition Closed loop frequency control Pumped storage is participating in the primary frequency control like normal synchronous generators Pumped storage unit will contribute directly to the frequency bias of the systems and will improve the response to all changes in load or production f ref Frequency control Droop Δp p set p ref v abc, ref Drive control Measurements Synchronous machine drive τ mech Hydraulic control Pumpturbine τ em 1 Tm w s p em f g Grid p g Grid connection Production Consumption 11

12 Power flow in the system Simulated cases: Minimum load in the system Pumped storage in pumping mode Two or more pump units for controllability in the range of 3 to 1 MW Medium average wind speed with severe fluctuations 18 MVA diesel online with 0.7 pu power set-point 5 MVA hydropower with 0.8 pu power set-point Investigation of control strategies The diesel generators must cover most of the fluctuations when the pumped storage is run with constant power Droop control of the pumped storage is reducing the power fluctuations from the diesel generators Only small fluctuations are remaining with compensation of variation in measured power from the wind turbines The remaining influence is mainly because of oscillations in the output power of the small hydropower plant These fluctuations can be reduced by improving the excitation system of the 5 MVA generator Pumped storage power [MW] Power from diesel generators [MW] Pumped storage controlled to constant power Pumped storage controlled by frequency droop Pumped storage controlled to balance measured power fluctuations Time [s] Time [s] 1

13 Influence on system frequency The system would be difficult to control without the pumped storage as additional load The amount of diesel generators corresponding to the wind power production could not be turned off The system is able to operate with constant power input to the pumped storage unit but the frequency fluctuations of the system is large The frequency bias of the system is significantly improved with pumped storage taking part in the primary frequency control Influence on frequency can be further reduced by compensating measured fluctuations in output from the wind turbines but this will not increase the frequency bias of the system Frequency [Hz] Pumped storage controlled to constant power Pumped storage controlled by frequency droop Pumped storage controlled to balance measured power fluctuations Time [s] 13

14 Main results Variable speed pumped storage units can improve the dynamic power balance of isolated system and balance power fluctuations from wind turbines both in generation and pumping mode The power system can be kept stable with a high share of wind power as long as there is enough capacity for fast power control Synchronous machine drive system allows for quick control response without electrical limitations of the operating range Variable speed operation in pumping mode allows for control strategies that can increase the frequency bias of the system and improve system response to disturbances The pumped storage unit can be controlled to perform additional auxiliary services to improve power system operation Different power control strategies can be combined to obtain suitable characteristics 14