Motor Protection. May 31, Tom Ernst GE Grid Solutions

Transcription

1 Motor Protection May 31, 2017 Tom Ernst GE Grid Solutions

2 Motor Relay Zone of Protection -Electrical Faults -Abnormal Conditions -Thermal Overloads -Mechanical Failure 2

3 Setting of the motor protection relay is based on the motor datasheets information and system configuration Datasheets are normally provided by motor manufacturer System configuration data can be obtained from single line diagram 3

4 CT Selection 869 4

5 Phase CTs The CT should be nominally sized at or greater than motor FLA The CT must have an accuracy class high enough so that the current waveform presented to the relay will allow the overcurrent to operate Higher CT ratio is generally better from a saturation point of view than a lower CT ratio 5

6 Phase CTs Our Motor has a FLA of 413 Amps Our maximum fault current is 22KA 6

7 Phase CTs C /5 INPUT PARAMETERS: ENTER: Saturation Curve CALCULATED: Inverse of sat. curve slope = S = V Rt = Total burden resistance = Rw + Rb = s RMS voltage at 10A exc. current = Vs = 125 volts rms slope pf = Total burden pow er factor = Turns ratio = n2/1= N = = 1/S Zb = Total burden impedance = mf gr's Winding resistance = Rw = ohms V Tau1 = System time constant = Burden resistance = Rb = ohms e data Lamsat = Peak flux-linkages corresponding to Vs Burden reactance = Xb = ohms volts log-log plot, w = Radian freq = System X/R ratio = XoverR = rms equal RP = Rms-to-peak ratio = Per unit offset in primary current = Off = <Off<1 decade A = Coefficient in instantaneous ie spacing Per unit remanence (based on Vs) = lrem versus lambda curve: ie = A * l^s : 4.89E+06 Symmetrical primary fault current = Ip = 22,000 amps rms dt = Time step = I e amps rms 10 Lb = Burden inductance = Thick lines: Ideal (blue) and actual (black) secondary current in amps vs time in seconds. Thin lines: Ideal (blue) and actual (black) secondary current extracted fundamental rms value, using a simple DFT with a one-cycle window Saturated Magnitude Trace 7

8 Phase CTs C /5 INPUT PARAMETERS: ENTER: Saturation Curve CALCULATED: Inverse of sat. curve slope = S = V Rt = Total burden resistance = Rw + Rb = s RMS voltage at 10A exc. current = Vs = 125 volts rms slope pf = Total burden pow er factor = Turns ratio = n2/1= N = = 1/S Zb = Total burden impedance = mf gr's Winding resistance = Rw = ohms V Tau1 = System time constant = Burden resistance = Rb = ohms e data Lamsat = Peak flux-linkages corresponding to Vs Burden reactance = Xb = ohms volts log-log plot, w = Radian freq = System X/R ratio = XoverR = rms equal RP = Rms-to-peak ratio = Per unit offset in primary current = Off = <Off<1 decade A = Coefficient in instantaneous ie spacing Per unit remanence (based on Vs) = lrem versus lambda curve: ie = A * l^s : 4.89E+06 Symmetrical primary fault current = Ip = 22,000 amps rms dt = Time step = I e amps rms 10 Lb = Burden inductance = Thick lines: Ideal (blue) and actual (black) secondary current in amps vs time in seconds. Thin lines: Ideal (blue) and actual (black) secondary current extracted fundamental rms value, using a simple DFT with a one-cycle window Increase the ratio - less saturation 8

9 Phase CTs C /5 INPUT PARAMETERS: ENTER: Saturation Curve CALCULATED: Inverse of sat. curve slope = S = V Rt = Total burden resistance = Rw + Rb = s RMS voltage at 10A exc. current = Vs = 300 volts rms slope pf = Total burden pow er factor = Turns ratio = n2/1= N = = 1/S Zb = Total burden impedance = mf gr's Winding resistance = Rw = ohms V Tau1 = System time constant = Burden resistance = Rb = ohms e data Lamsat = Peak flux-linkages corresponding to Vs Burden reactance = Xb = ohms volts log-log plot, w = Radian freq = System X/R ratio = XoverR = rms equal RP = Rms-to-peak ratio = Per unit offset in primary current = Off = <Off<1 decade A = Coefficient in instantaneous ie spacing Per unit remanence (based on Vs) = lrem versus lambda curve: ie = A * l^s : 4.04E+00 Symmetrical primary fault current = Ip = 22,000 amps rms dt = Time step = I e amps rms 10 Lb = Burden inductance = Thick lines: Ideal (blue) and actual (black) secondary current in amps vs time in seconds. Thin lines: Ideal (blue) and actual (black) secondary current extracted fundamental rms value, using a simple DFT with a one-cycle window Increase ratio & C-rating almost no Saturation 9

10 Motor Data Sheets Motor Performance Data Thermal Limit Curves 10

11 Motor Thermal Limit Curves Thermal Limit Curves: B A D C A. Cold Running Overload B. Hot Running Overload C. Cold Locked Rotor Curve D. Hot Locked Rotor Curve E F E. Acceleration 80% rated voltage F. Acceleration voltage 11

12 Motor Thermal Parameters G I H J Motor Data Sheet Parameters G. Temperature Rise, Insulation Class H. Full Load Current I. Locked Rotor Current J. Locked Rotor Time; Cold/Hot K. Number of Starts per hour; K 12

13 Motor Specifications Information required to set Thermal Model: Motor FLA Locked rotor current Locked rotor time hot & cold Stopped & running cool time constants Service factor Motor thermal damage curves 13

14 Settings Example Select CT Rating, Voltage Sensing Voltage Sensing : Enter the connection type, secondary volts and ratio. VTratio = 14400/120 = 120:1 Vsec = Vnom/VTratio = 13800/120 = 115 V Phase CT: The phase CT should be chosen such that the FLA is 75% to 150% of CT primary. Since the FLA is 297 a 300:5 CT may be chosen. Ground CT: Zero sequence core balance CT is used for high impedance grounded systems. The primary rating should be large enough to assure that the CT can handle all potential fault ground levels without saturating. 50 A >> systems with less than 50 amps of ground fault current. 200 A or 300 A >> systems with up to 300 amps of ground fault current. No ground CT required on low impedance or solidly grounded systems (Use neutral functions (3I0 is calculated from the phase CTs). Secondary rating can be same as phase CTs (1A/5A) or special 50:0.025 A. 14

15 Settings Example Select FLA, Ground CT Motor FLA: Set as specified by the data sheets. Overload Factor: This is the pickup of the OL curve. Set 10-15% above data sheet service factor. NP Voltage, HP & Poles: Set as specified in the data sheets. Load Average Calc. Period: Set this longer than the oscillatory duration of oscillating loads like reciprocal compressors. Set at 0 for non-oscillatory loads. Max Acceleration Time: Set this to the longest acceleration time expected plus a margin (the acceleration time trip function is enabled separately - see Protection > Group X >> Motor). 15

16 Settings Example Select Overload Curve for Thermal Model Overload Curve Set the overload curve below cold thermal limit and above hot thermal limit. If only hot curve is provided by manufacturer, then must set at or below hot thermal limit The best fitting curve is time dial multiplier 9 in this example. Note that this is a 3 dimensional curve: f(a,t, TCU), TCU = thermal capacity used. Curve values given are for TCU = 0 (40 C stator temp). The curve represents TCU = 100%. 16

17 Settings Example Select Overload Curve TD Multiplier for Thermal Model Overload Curve Set the overload curve TD multiplier below cold thermal limit and above hot thermal limit. If only hot curve is provided by mfgr, then must set at or below hot thermal limit. The best fitting curve TD multiplier is 9 in this example. This can be verified with Hot Stall Time of 30s at 540% FLA by using the standard overload curve equation above. 17

18 Settings Example Select Overload Curve for Thermal Model Select overload curve using Hot Stall Time and Locked Rotor Current when Overload Curves are not available: Example: For Hot Stall Time = 30s and LRA = 540% FLA Substitute in the above equation: 30s = TD MULTIPLIER x ( x x 4.4) TD MULTIPLER = 30 x ( x x 4.4) point) = 9.66 SELECT TDM 9 (which is below this intersection 18

19 Settings Example Determine Unbalance Bias K Factor for Thermal Model Unbalance Bias Of Thermal Capacity Enable the Unbalance Bias of Thermal Capacity so that the heating effect of unbalance currents is added to the Thermal Capacity Used. K=175/LRA 2 = 175/ = 6 (Typical) K=230/LRA 2 = 230/ = 8 (Conservative) 19

20 Settings Example Stopped & Running Cool Time Constants Stopped and Running Cool Time Constants This information is usually supplied by the motor manufacturer but is not part of the data that was given with this motor. If RTD s are present and will be wired to the relay biasing of the thermal model will be used so it is not critical to have these cooling times from the manufacturer: the default values of 15 and 30 minutes can be used for the running and stopped cool times respectively. 20

21 Settings Example Determine Hot/Cold Safe Stall Ratio for Thermal Model (method 1) HCR LRT LRT HOT COLD Hot/Cold Ratio = 30/35 = 0.86 Hot/Cold Curve Ratio The hot/cold curve ratio is calculated by simply dividing the hot safe stall time by the cold safe stall time or use the motor thermal limits curve. For this example, both are available. Using the data sheets the Hot/Cold Curve Ratio equals 30 / 35 =

22 Settings Example Determine Hot/Cold Safe Stall Ratio for Thermal Model (method 2) Overload Curve Method Hot/Cold Curve Ratio If the thermal limits curves are being used to determine the HOT/COLD ratio proceed as follows: LRC = 5.4FLA LRTcold = 8sec LRThot = 6sec From the thermal limits curves run a line perpendicular to the current axis that intersects the hot and cold curves at the stall point Draw lines from each points of intersection to the time axis. Record the corresponding times. In this case, 6 and 8 seconds respectively. The Hot/cold ratio can now be calculated as follows: = 6s/8s = 0.75 NOTE: If hot and cold times are not provided and only one curve is given verify with the manufacturer that it is the hot curve ( which is the worst case), then the Hot/ Cold ratio should be set to

23 Settings Example Determine RTD Bias Setpoints for Thermal Model Enable RTD Biasing This will enable the temperature from the Stator RTD sensors, to be included in the calculations of Thermal Capacity. RTD bias model determines the Thermal Capacity Used based on the temperature of the Stator and is separate from the overload model for calculating Thermal Capacity Used. RTD biasing is a back up protection element which accounts for such things as loss of cooling or unusually high ambient temperature. This measured temperature is used to bias or modify the thermal capacity value stored in the motor relay. 23

24 Settings Example Determine RTD Bias Setpoints for Thermal Model MID POINT TEMP: 130 C TCU: 25% MIN POINT TEMP: 40 C TCU: 0% MAX POINT TEMP: 155 C TCU: 100% Motor relay will use the calculated thermal capacity unless the RTD thermal capacity is higher. This feature will not trip the motor at the max point temp unless the average current is greater than the overload pickup setting RTD Bias Function Set to Enabled/YES RTD Bias Minimum Set to 40 C which is the ambient temperature obtained from the data sheets. RTD Bias Center Point The center point temperature is set to the motor s hot running temperature and is calculated as follows: Temperature Rise of Stator + Ambient Temperature. The temperature rise of the stator is 80 C + 10% hot spot allowance, obtained from the data sheets. Therefore, the RTD Center point temperature is set to 90 0 C C or 130 C. RTD Bias Maximum This setpoint is set to the rating of the insulation or slightly less. A class F insulation is used in this motor which is rated at 155 C, so setting should be 155 C. 24

25 Settings Example Determine RTD Bias Setpoints for Thermal Model MAX POINT TEMP: 155 C TCU: 100% MID POINT TEMP: 130 C TCU: 25% MIN POINT TEMP: 40 C TCU: 0% 25

26 Settings Example Enable Start Inhibit Enable Start Inhibit This function will limit starts when the motor is already hot. The motor relay learns the amount of thermal capacity used at start. If the motor is hot, thus having some thermal capacity used, the relay will not allow a start if the available thermal capacity is less than the required thermal capacity for a start. If Start Inhibit is not used, must wait until Thermal Capacity Used (TCU) falls below 15% before the motor can be re-started. Using Start Inhibit allows one to start a motor sooner. 26

27 TCU / Start Inhibit Example Thermal Capacity required to start For example, if the THERMAL CAPACITY USED for the last 5 starts is 24, 23, 27, 25, and 21% respectively, the LEARNED STARTING CAPACITY is 27% 1.25 = 33.75% used. Thermal Capacity used due to Overload If the motor had been running in an overload condition prior to stopping, the thermal capacity would be some value; say 80%. If Motor is Stopped: When the motor has cooled and the level of thermal capacity used has fallen to 66%, a start will be permitted. 27

28 Settings Example Starts/Hr, Time Between Starts Starts/Hour Starts/Hour can be set to the # of cold starts as per the data sheet. For this example, it is 2 Time Between Starts In some cases, the motor manufacturer will specify the time between motor starts. In this example, this information is not given so this feature can be disabled or set at a typical 20 min between starts. 28

29 Settings Example VFD Support Functions Bypass Switch If the VFD has a bypass switch then set this for the contact input that is ON when the switch is closed. Starting Frequency Traditionally, the frequency tracking function started at 50/60 Hz and then looked at zero crossings of several cycles to determine the correct actual frequency. This caused the first 5 10 cycles of current measurement to be wrong when the motor was started from a VFD. Starting frequency feature allows the tracking to start at a more realistic frequency (6 Hz in this case). 29

30 Advanced Diagnostics Broken Rotor Bar Detection The Broken Rotor Bar element uses two different algorithms to detect broken or cracked rotor bars: Power Based Coherent Demodulation: This technique uses multiplication of voltage and current samples thereby shifting the fundamental to DC and fault frequency to lower closer to DC value, to detect the broken rotor bar component. This method is running when voltage is available and is meeting MOTOR VOLTAGE SUPERVISION setting check. Conventional current based FFT method: In case voltage is not available or the voltage magnitude is lower than the MOTOR VOLTAGE SUPERVISION setting value, the algorithm switches to analyzing the frequency spectrum from current samples only, to detect the broken rotor bar component. Alarm settings are based on an increase in db as each motor will exhibit a different signature when healthy. 30

31 Advanced Diagnostics - BRB FFT of Stator Current of Induction Machine with Rotor Bar Fault signature is only about 12 Hz off of fundamental The FFT of the resultant multiplied signal more robust signature than with the current only FFT method signal. 31

32 Advanced Diagnostics BRB Pickup is set based on initial in-service db measurements taken when the motor is known to be healthy plus a change margin (~15% increase). 32

33 Advanced Diagnostics Stator Inter-Turn Fault The Stator Inter-Turn Fault element uses sequence components to detect stator turn failure of the induction machine. Local heating caused by shorted turns can rapidly cause additional damage to adjacent windings and stator iron Alarm to avoid additional damage Normalized cross-coupled impedance ratio: Z np /Z pp = (V 2 Z nn *I 2 )/V 1 ~ 0 under balanced non-fault conditions Z pp = positive sequence impedance Z np = cross-coupled negative-to-positive sequence impedance V 1 = positive sequence voltage (motor terminals) V 2 = negative sequence voltage (motor terminals) I 2 = negative sequence current (motor terminals) Z nn = negative sequence impedance 33

34 Advanced Diagnostics Stator Inter-Turn Fault Operating quantity: OP = Z np /Z pp Z UBbase Z UBbase = normalized cross-coupled impedance ratio under non-fault conditions 34

35 Thank You Questions?