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Title MANITOBA HVDC RESEARCH CENTRE, a Division of Manitoba Hydro International Ltd. Sub Synchronous Oscillations – An Introduction October 2016 Presented by: Dharshana Muthumuni © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Ltd. Presentation Outline Title • General background • AC Network characteristics • Series compensation and impact on SSR • Types/Classification of SSO – SSR • IG and SSTI – Transient Torques – HVDC – SSTI – SSCI – Wind • Simulation examples © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Title Introduction / Background © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Introduction / Background Title Transients in the Electric Network • Transients following a system disturbance should damp out in a reasonable timeframe. © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Introduction / Background Title Transients in the Electric Network Voltage and current transient frequencies – High frequency transients – ‘Traditional’ electromechanical oscillations (close to system frequency on system side, 0.1 – 5 Hz on mechanical side) – Frequencies below system frequency (60 Hz) • Sub Synchronous Oscillations 9.7 Hz © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Introduction / Background Title Network Characteristics System Impedance Vs Frequency Plots Base Case - Case1 1.4k case0 case1 1.2k Z+(ohms) 1.0k 0.8k 0.6k 0.4k 0.2k 0.0 Hz 0.0 0.2k 0.4k 0.6k 0.8k 1.0k 1.2k A Typical frequency characteristic Following a system disturbance currents (and voltages) corresponding to network resonance frequencies can flow in the system (and hence into generators stator windings). © Manitoba HVDC Research Centre | a division of Manitoba Hydro International ... ... ... Introduction / Background Title Network Characteristics Following a system disturbance currents (and voltages) corresponding to network resonance frequencies can flow in the system (and hence into generators stator windings). Base Case - Case1 Main : Graphs Eb 300 1.4k 200 1.2k case1 1.0k Z+(ohms) 100 y 0 -100 0.8k 0.6k 0.4k 0.2k -200 0.0 -300 0.170 case0 0.190 0.210 0.230 0.250 0.270 ... ... ... © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Hz 0.0 0.2k 0.4k 0.6k 0.8k 1.0k 1.2k ... ... ... Mechanical Oscillations Title Electric network Gen Speed - W Constant speed mechanical rotation => 60 Hz on electrical side 1 Hz mechanical oscillations => 59 Hz and 61 Hz 8 Hz mechanical oscillations => 52 Hz and 68 Hz • © Manitoba HVDC Research Centre | a division of Manitoba Hydro International fm Hz mechanical oscillations => (60-fm) Hz and (60 + fm) Hz Mechanical shaft-mass system Title δ IP HP LP • G • Speed - Wk • • © Manitoba HVDC Research Centre | a division of Manitoba Hydro International In steady state (constant torque transmission) each shaft rotates at a constant speed and with a constant ‘twist’. Following a disturbance (network or mechanical system side), the shafts will go through a ‘transient’ period. The oscillation frequencies will correspond to ‘natural frequencies’ of the mass-shaft system. Damping associated with the mechanical system and the damping provided by the response of the electrical system will determine the nature of the oscillations. Mechanical shaft-mass system Title • Interaction between the mechanical and electrical system: – Damping associated with the mechanical system and the damping provided by the response of the electrical system will determine the nature of the oscillations. © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Mechanical Resonance Frequencies Title δ IP HP LP G Mechanical modes can be calculated from shaft data: Inertias of individual masses (J) Shaft Spring Constant (K) Speed - Wk fi= 60Hz - 25 Hz => 35 Hz © Manitoba HVDC Research Centre | a division of Manitoba Hydro International 1 𝐾𝑖 .𝑊𝑏 √ 2𝜋 2𝐻𝑖 Typical resonant frequencies of Thermal Generator Turbine units are in the range of 15-25 Hz. Title Series compensation and impact on SSR © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Series compensation and impact on SSR Title Impact on network characteristics (System Impedance Vs Frequency) Network resonance points can shift to the sub synchronous frequency range (<60 Hz) Higher the level of series compensation, the resonant point moves toward the system frequency • Increases potential risk of SSO related issues Base Case - Case1 1.4k case0 case1 1.2k Z+(ohms) 1.0k 0.8k 0.6k 0.4k 0.2k 0.0 Hz © Manitoba HVDC Research Centre | a division of Manitoba Hydro International 0.0 0.2k 0.4k 0.6k 0.8k 1.0k 1.2k .. .. .. Series compensation and impact on SSR Title Higher the level of series compensation, the resonant point moves toward the system frequency Frequency Scan - Generator Terminals 1.4 |Z+| (ohms) 1.2 1 45% compensation 0.8 55% compensation 0.6 60% compensation 0.4 75% compensation 0.2 Frequency (Hz) 𝑋𝑐 𝑓𝑒 = 𝑓0√ 𝑋𝑙 © Manitoba HVDC Research Centre | a division of Manitoba Hydro International 57 53 49 45 41 37 33 29 25 17 21 13 9 5 0 Title Types / Classification of SSO © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Types/Classification of SSO Title 1. Self Excitation (Induction generator effect) Purely electrical phenomenon Self-excitation of Sub Synchronous Oscillations Generator ‘acts’ as a NEGATIVE resistance at the sub synchronous frequencies. If this effective negative resistance is greater in magnitude compared to the positive resistance as seen from the system, an unstable resonance condition can take place. Machine Output Volts p.u. 1.100 1.050 1.000 0.950 0.900 0.850 0.800 0.750 0.700 Gens Generator Series cap. 2.50 2.00 1.50 1.00 0.50 0.00 -0.50 -1.00 -1.50 -2.00 4.0 3.0 2.0 1.0 0.0 -1.0 -2.0 -3.0 -4.0 1.00 © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Capacitor Volts, A Machine Current, A,out+ 2.00 3.00 4.00 ... ... ... Types/Classification of SSO Title 1. Self Excitation (Induction generator effect) Synchronous generators as well as induction machines can contribute to this condition Easy to explain (in a 20 min introduction) using an induction machine example R1 I1 I 2' X1 V1 Pag Xm S R2' s X 2' System nominal frequency R2' s Higher effective NEGATIVE resistance) © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Negative Slip (S) Electrical resonant frequency Types/Classification of SSO Title 1. Self Excitation (Induction generator effect) Synchronous generators – Field (and damper) winding characteristics contribute to IG – At frequencies (w) below the system frequency (wo), the effective resistance is negative. ( w w0 )T ' d 0 w( x x' ) Rs wb (1 [(w0 w)T ' do ]2 ) © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Types/Classification of SSO Title 2. Torsional Interaction Electrical resonance frequency (fe) of system caused by series capacitors close to natural torsional resonance frequency (fm) of mechanical system (turbine-generator unit) Condition for SSR-TI Risk =>: fe fo - fm Due to interaction between the electrical (generator/line series capacitor…) and mechanical systems (long shafts and masses of thermal units) Result in shaft fatigue & undesirable stress, damage Typical mechanical resonance frequencies (15 – 30 Hz) The risk is with higher Series Compensation (SC) levels This condition should be avoided through design considerations © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Types/Classification of SSO Title 2. Torsional Interaction Electrical damping from the machine/electric-network mechanical shaft oscillations – derived from EMT simulations (damping analysis using EMT simulations). 0.4 0.2 0 -0.2 0 10 20 30 40 50 -0.4 -0.6 -0.8 -1 -1.2 -1.4 -1.6 © Manitoba HVDC Research Centre | a division of Manitoba Hydro International 60 70 80 Types/Classification of SSO Title 3.Transient Torques • • Shaft oscillations following disturbances will result in shortening of the shaft life time due to material ‘fatigue’. The critical items that determines the level of severity are: • Amplitude of the mechanical transient oscillations • Decay of the oscillations (damping) © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Types/Classification of SSO Title 3.Transient Torques Peak transient torque produced on each shaft segment can be used to determine the level of risk. Machines are designed to withstand for “terminal short circuit” and the peak torque produced for this event can be used as a reference. If the torque produced for an evet is larger than about 75% of the value for terminal short circuit, it is considered as in the zone of vulnerability. S-N curves (number of stress cycles as a function of stress amplitude before crack initiation). Shaft Torque (PU) 3 2 1 Source: EPRI report: “Torsional Interaction Between 103 105 107 © Manitoba HVDC Research Centre | Cycles to failure) a division of Manitoba Hydro International Electrical Network Phenomena and Turbine-Generator Shafts” Types/Classification of SSO Title 4. Interaction with HVDC converter controls - SSTI • An issue associated with conventional (LCC) HVDC schemes • A parameter ‘Unit interaction Factor’ is used to determine the level of risk (a screening level indicator) – How close the generator is to the HVDC converter – Other parallel AC connections (improved damping) • Radial connection of a generator to the rectifier AC bus is the likely worst case. • Can be easily mitigated through proper design of HVDC control schemes and settings. Rbus Gens Rectifier © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Types/Classification of SSO Title 4. Interaction with HVDC converter controls - SSTI • SCTOT – Short circuit level at the HVDC terminal • SCi – Short circuit level at the HVDC terminal with the generator under SSTI investigation disconnected. • • UIF > 0.1 is recommended for further analysis UIF should be calculated for carefully selected (credible) N-x outage conditions Source: EPRI report: “Torsional Interaction Between Electrical Network Phenomena and Turbine-Generator Shafts” © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Types/Classification of SSO Title 5. SSCI involving T3 Wind Units • Modern wind turbines use power electronic converters (connected to the generator) to improve performance • Wind farms located far from the AC grid has necessitated ‘Series Compensated (SC)’ Transmission lines • Problem: DFIG controls act to ‘amplify’ sub synchronous currents entering the generator – Negative Damping – Rotor side converter plays the dominant role © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Types/Classification of SSO Title 5. SSCI involving T3 Wind Units Dynamic frequency scan to estimate damping provided by the DFIG to sub synchronous frequency transients 10 0 -10 0 -20 -30 -40 -50 -60 -70 -80 -90 © Manitoba HVDC Research Centre | a division of Manitoba Hydro International 5 10 15 20 25 30 35 Title Do I need to perform studies? © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Performing studies Title Identifying potential risks Series Cap Locations WGR Location © Manitoba HVDC Research Centre | a division of Manitoba Hydro International Title Thank you © Manitoba HVDC Research Centre | a division of Manitoba Hydro International