Evaluating the Impact of Increasing System Fault Currents on Protection

Transcription

1 Evaluating the Impact of Increasing System Fault Currents on Protection Ilia Voloh, Zhihan Xu GE Grid Solutions Mohsen Khanbeigi Hydro One 7th Annual Conference for Protective Relay Engineers

2 Outline Overview of increasing short circuit fault current levels Impacts of increased fault levels on protection rela ys Impact evaluation techniques Case studies

3 Causes of increasing fault current Installa tion of new tra nsmission fa cilities, transformers and generators Additions and changes to existing generators Reconfigurations of the bulk electric system (BES) network New distribution generation

4 OEB Maximum Allowable Fault Levels Ma ximum a llowa ble fa ult levels set out by the Ontario Energy Board Nominal Volt age Maximum 3-Phase Fault Maximum SLGfault (ka) (kv) (ka) 5 8 (usually limited to63 ka) 8 (usually limited to63 ka) (usually limited to 63 ka) (usually limited to 8 ka) 27.6 (4-wire) (3-wire)

5 Hydro One Data Maximum fault currents in some major substations Nominal Volt age (kv) Fault Current (up to ka) CB interrupting capacity requirement Nominal Volt age 199s (ka) Present (ka) (kv) 5 4/63 63/8 23 4/5/63 5/63/ /5 4/5/63 Increased by 27% in the last twenty years

6 Questions from Protection Engineers What will be the impact of increasing fault current on protection relays? Will relays be reliable or not under such situations? What are the effects on the dependability and security of a relay? How to eva luate a specific a pplication? How to upgrade relay or adjust relay settings to increase dependability and security?

7 Impacts ADC Range and Clamping Due to the conversion range of ADC, digital current samples will be clamped if they exceed the conversion levels. Cl amp l evel = 1 pu 2 15 Ideal Current Clamped Current 1 Cur r ent ( pu) Ti me ( s ) Ratio of clamped level to true peak determines phasor magnitude error.

8 Impacts ADC Range and Clamping The lower cla mp level results in the sma ller current magnitude and rms values Affecting protection functions that are related to the current magnitude or rms values. Negligible effect on current phasor angle Rat i o of magni t ude & RMS t o i deal mag Ratio of Mag to Ideal Ratio of RMS to Ideal X: Y:.6255 X: Y: Rat i o of cl amp l evel t o peak ( %)

9 Impacts ADC Range and Clamping Erroneous odd harmonics are induced For third harmonic, the larger fault current will result in the larger ratio of odd harmonics to fundamental magnitude 35 The fifth harmonic ratio may be used to inhibit 87T function during overexcitation No even harmonics induced Rat i o of har moni cs t o f undament al mag ( %) Rat i o of cl amp l evel t o peak ( %) Third Harmonic Fifth Harmonic Seventh Harmonic Ninth Harmonic

10 Impacts CT Saturation AC saturation: caused by the symmetrical current with no DC component Ratio Current CT Ratio: 8:5 Burden: 2.1 ohms Vs@1A: 7V DC offset: % To avoid AC saturation V > I X S Z S Cur r ent ( pu of f aul t ) Saturated Secondary Current (4-8 ka) Ti me ( s )

11 Impacts CT Sa tura tion (AC) 1 4 ka 6 ka 8 ka Ideal 1 4 ka 6 ka 8 ka Ideal Magnitude (pu of fault) RMS (pu of fault) Angle shift (degree) ka 6 ka 8 ka Time (s) Time (s) Ratio of harmonics to fundamental (%) Time (s) Time (s) 8 ka, 2nd Harmonic 8 ka, 3rd Harmonic 8 ka, 4th Harmonic 8 ka, 5th Harmonic

12 Impacts CT Saturation DC saturation: caused by DC component in the fault current, unipolar half wave current or remnant flux in the CT To avoid DC saturation VX > I S ZS (1 + X R ) Cur r ent ( pu of f aul t ) Saturated Secondary Current (2-8 ka) Ratio Current Ti me ( s ) CT Ratio: 8:5 Burden: 2.1 ohms Vs@1A: 7V DC offset: 1% X/R: 17

13 Impacts CT Sa tura tion (DC)A

14 Impacts CT Saturation Reduced magnitude and rms values, which affect current-based protection functions Result in the leading angle, which may affect directional functions. o DC saturation will cause more leading angle shift compared to AC saturation Increased ratios of harmonics to fundamental

15 Evaluation Techniques Test in a high current laboratory o Apply high primary fault current to CT, as the true fa ult o Needs special equipment, costly and rarely a va ila ble to most users Test using a real time power system simulator o Model the power system, simulate different system and fault conditions in real time, generate analog signal, which can be applied to a signal amplifier and then injected into input of the relays o Ca pacity of the signal a mplifier is a ma jor concern for high level secondary fault currents

16 Evaluation Techniques Simula te in electroma gnetic tra nsient a na lysis software o Rela y model built-in in the s/w: relay performance can be directly tested by simulating different system and fault conditions o Rela y model not built-in in the s/w : the specific function in a relay can be modeled but requires a lgorithm details a nd skills. o Save simulated raw waveforms as COMTRADE files and inject to the relay by using a test set. o Program analysis software, such as MATLAB, to load raw waveforms, simulate signal processing, model relay functions, and analyze relay response-very complicated.

17 Evaluation Techniques Playback recorded waveforms o Analyze the relay performance and corrective actions by playing back waveforms recorded from a misoperation event due to a heavy fault Program in a simple Excel spreadsheet o For exa mple, once the ma ximum fa ult level is determined and CT parameters are known, the PSRC CT saturation calculator can generate the secondary current with saturation or without it, and calculate the fundamental magnitude

18 Case Studies 1 Transformer differential relay in a 23/27.6kV substation 23 kv Zs F1 F2 F3 Three fa ult locations a t F1, F2 and F3 Different fault types Two different fault current levels, 6 and 73 ka (F1 point) Two fault inception angles, and 9 degree (phase A voltage) 87T 27.6 kv Load Di f f er ent i al Cur r ent ( pu) CT: HV 8/5, burden 1.6 ohms LV 16/5, burden 1.73 ohmw OPERATE Percentage 87T Settings Pickup:.87 pu Break point: 2pu Slope 1: 3% Slope 2: 5% Unbiased 87T Setting: Pickup: 2.79 pu RESTRAINT Rest r ai nt Cur r ent ( pu)

19 Case Studies 1 CT saturation example I SLG fa ult a t F1 Sat ur at ed Cur r ent ( pu at HV si de) Ti me ( s )

20 Case Studies 1 CT sa turation exa mple II three-phase fault at F1 Sat ur at ed Cur r ent s ( pu at HV si de) A B C Ti me ( s ) Percent 87T PKP, but inhibited by 2 nd <.5 ms (all phases) Percent 87T operates, 2 nd inhibit unblocked Phase A: 89.6ms B: 93.7ms C: 11.7ms Fault Inception <1 ms (all phases) Unbiased 87T operates

21 Case Studies 1 Effect of CT saturation o Percentage differential function does not have fa ilure to opera te for internal fa ults or misoperations for external faults (studied case only) o The instantaneous (unbiased) differential protection function should be enabled to avoid a slow operation, if the percentage differential function can be potentially blocked by the second harmonic inhibit

22 Case Studies 1 Effect of clamping levels, with 1pu pickup setting Cur r ent s ( pu at HV si de) Secondary Current Clamped Current (32pu) Clamped Current (64pu) Di f f er ent i al Cur r ent s ( pu) Clamped by 32pu Clamped by 64pu X:.151 Y: 1.2 X:.2578 Y: Ti me ( s ) Ti me ( s ) Unbiased 87T clamped by 32pu will be slower by 1.68ms

23 Case Studies 1 Results: o In the studied substation, the existing CT and relay settings are able to handle the increased fault current level, when both biased and unbiased differential elements are enabled. o The increased fault level has effect on the operate time of the biased differential function, but this can be mitigated by proper setting of the unbiased differential function.

24 Case Studies 2 A 1MVA Yd11 transformer in a 138/19.5kV substation Similar fault scenarios o Internal and external faults o Fault levels: 5 and 65 ka o Clamping levels: 32 and 64 pu

25 Differential Current (pu) Misoperation due to increased fault current and lower clamping level 65kA - 64pu Clamping 65kA - 32pu Clamping 5kA - 64pu Clamping 5kA - 32pu Clamping Restraint Current (pu) Case Studies 2 Exa mple: External fa low voltage side Experience CT saturation

26 Conclusions CT saturation would reduce the magnitude and cause a leading phase angle shift of the current phasor Rela y interna l cla mping level limits the measurement range of waveforms, therefore, the calculated magnitudes are decreased potentially endangering differential function and erroneous harmonics are generated

27 Conclusions By analyzing 87T function, o Always enable the unbiased differential function to avoid slow operation due to the inrush inhibit of the percentage differential function, thereby improving the relay dependability o The security of the percentage differential function may be jeopardized due to the increased fault current and lower clamping level. The security can be ensured by increasing the differential settings or by employing dedicated CT saturation function a va ila ble in the rela y. o The 87T function with the larger clamping level is typically able to respond faster.

28 Thank You Questions?