What is the Thermal protection for motors ? | Schneider Electric Nigeria
This function is used to protect a transformer against overloads, based on the measurement of the current taken. IEC standard 60076-2 proposes 2 thermal models ...
Protection functions Thermal overload for transformers ANSI code 49RMS DE81253 Operation This function is used to protect a transformer against overloads, based on the measurement of the current taken. IEC standard 60076-2 proposes 2 thermal models for evaluating the winding thermal capacity used during an overload, depending on whether the transformer is dry-type or immersed. Taking account of harmonics The equivalent current Ieq measured by the transformer thermal overload protection is the highest of the phase rms currents (the rms current takes account of harmonic numbers up to 13). Taking account of 2 operating conditions The choice between thermal sets 1 and 2 is made by the "switching of thermal settings" logic input. This means you can have thermal set 1 for normal transformer operation and thermal set 2 for unusual transformer operation. Dry-type transformer For dry-type transformers, the thermal model used in the Sepam relay conforms to 3 standard IEC 60076-12 (with 1 time constant). Block diagram AN / AF Insulation class Switching of thermal settings Insulation class I1 rms I2 rms I3 rms Ieq Max Dry-type transformer thermal model + + Ambient sensor 20 rC a Use of temperature sensor Inhibition by logic input or TC > alarm > trip Alarm Trip Dry-type transformer thermal model The thermal limit for dry-type transformers is determined by the thermal limit for insulating components in order to avoid damaging them. The table below defines the maximum permissible temperature and the winding temperature gradient according to the insulation class: Insulation class (°C) 105 (A) 120 (E) 130 (B) 155 (F) 180 (H) 200 220 Gradient n 75 °C (67 °F) 90 °C (194 °F) 100 °C (212 °F) 125 °C (257 °F) 150 °C (302 °F) 170 °C (338 °F) 190 °C (374 °F) Maximum permissible winding temperature max 130 °C (266 °F) 145 °C (293 °F) 155 °C (311 °F) 180 °C (356 °F) 205 °C (401 °F) 225 °C (437 °F) 245 °C (473 °F) The winding maximum permissible thermal capacity used equals: max a Where: a : ambient temperature (rated value equals 20 °C or 68 °F) n : temperature gradient at rated current lb max : insulating component maximum permissible temperature according to the insulation class SEPED303001EN - 01/2013 131 Protection functions 3 Thermal overload for transformers ANSI code 49RMS The temperature build-up in the dry-type transformer winding is calculated as follows: Ieq 5 % Ib:n = n 1 + n I--Ie--b--q-- q n 1 d-----t Ieq < 5 % Ib:n = n 1 1 d-----t Where: : dry-type transformer time constant q : equals 1.6 for transformers with natural cooling (AN) equals 2 for transformers with forced cooling (AF) The protection trips when the temperature build-up in the winding reaches ma x . a Evaluating the time constant The thermal protection function protects the MV winding as well as the LV winding. Therefore the time constant corresponds to the lowest value of the MV winding and LV winding time constants. The time constant is evaluated, for each winding, according to standard IEC 6007612 as follows: = C------------(---------n------------e---) Pr Where: Pr : total winding loss in Watts C : winding thermal capacity in Watts min, given by the winding material: b Aluminum: 15 times weight of Al conductor (kg) + 24.5 times weight of epoxy and other insulating component (kg) b Copper: 6.42 times weight of Cu conductor (kg) + 24.5 times weight of epoxy and other insulating component (kg) e : contribution of the core to the thermal capacity used: b 5 °C (41 °F) for MV winding b 25 °C (77 °F) for LV winding 132 SEPED303001EN - 01/2013 Protection functions Thermal overload for transformers ANSI code 49RMS DE81254 Time constant (in mn) Example of a class B dry-type transformer: Regardless of the winding material, the LV winding has the lowest time constant. The following graph gives the values of the time constant for different 20 kV / 410 V dry-type transformer power ratings: 80 70 60 50 Cu 40 Alu 30 20 10 0 3 0 500 1000 1500 2000 2500 3000 Power (in kVA) 20 kV / 410 V dry-type transformer time constant. Saving the thermal capacity used On loss of the auxiliary power supply, the winding thermal capacity used is saved. Operating information The following information is available to the operator: b the winding relative thermal capacity used E as a %: Ek= 100 ----k-------n-----a- b the time before tripping in minutes (at constant current) Accounting for ambient temperature The characteristics of dry-type transformers are defined for an ambient temperature of 20 °C (68 °F). When the Sepam is equipped with the temperature sensor module option, the ambient temperature is measured by sensor no. 8 and added to the winding temperature. SEPED303001EN - 01/2013 133 Protection functions 3 Thermal overload for transformers ANSI code 49RMS Characteristics Settings Measurement origin Setting range I1, I2, I3 / I'1, I'2, I'3 Choice of transformer or thermal model Setting range Dry-type transformer Natural ventilation (AN) Forced ventilation (AF) Generic model(1) Insulation class Setting range 105 (A) 120 (E) 130 (B) 155 (F) 180 (H) 200 Alarm set point ( alarm) Setting range 220 class 105: 95 °C to 130 °C (203 °F to 266 °F) class 120: 110 °C to 145 °C (230 °F to 293 °F) class 130: 120 °C to 155 °C (248 °F to 311 °F) class 155: 145 °C to 180 °C (293 °F to 356 °F) class 180: 190 °C to 225 °C (374 °F to 437 °F) class 220: 210 °C to 245 °C (410 °F to 473 °F) Resolution Tripping set point ( trip) Setting range 1 °C (1 °F) class 105: 95 °C to 130 °C (203 °F to 266 °F) class 120: 110 °C to 145 °C (230 °F to 293 °F) class 130: 120 °C to 155 °C (248 °F to 311 °F) class 155: 145 °C to 180 °C (293 °F to 356 °F) class 180: 190 °C to 225 °C (374 °F to 437 °F) class 220: 210 °C to 245 °C (410 °F to 473 °F) Resolution 1 °C (1 °F) Transformer time constant ( ) Setting range 1 min to 600 min Resolution 1 min Accounting for ambient temperature Setting range yes / no Characteristic times Operating time accuracy Inputs ±2 % or ±1 s Designation Reset protection Inhibit protection Outputs Syntax P49RMS_1_101 P49RMS_1_113 Equations b b Logipam b b Designation Time-delayed output Alarm Inhibit closing Protection inhibited Hot state Thermal overload inhibited Zero speed Syntax P49RMS _1_3 P49RMS _1_10 P49RMS _1_11 P49RMS _1_16 P49RMS _1_18 P49RMS_1_32 P49RMS_1_38 Equations b b b b b b b Logipam b b b b b b b (1) See settings associated with generic thermal overload. Matrix b b b 134 SEPED303001EN - 01/2013 DE81255 Protection functions Thermal overload for transformers Code ANSI 49RMS I1 rms I2 rms I3 rms Max 20°C restricted Immersed transformer For immersed transformers, the thermal model used in the Sepam relay conforms to standard IEC 60076-7 (with 2 time constants). The thermal limit for immersed transformers is determined by the thermal limit for the oil, to avoid the formation of bubbles that could damage the dielectric strength of the oil. Block diagram wdg oil Transformer type Change of thermal settings Ieq Winding thermal model + wdg + oil Oil thermal model > alarm > trip Alarm Trip 3 Use of temperature sensor ambient Inhibition by logic input or TC oil Immersed transformer thermal model The immersed transformer thermal model takes account of thermal exchanges between the winding and the oil. To this end IEC standard 60076-2 proposes a model for each of the transformer components: b a thermal model with 2 time constants for the winding b a thermal model with 1 time constant for the oil. The winding thermal model transfer function is as follows: Ieq y wdg Ieq Ib k21 k21 -1 - wdg 1+ p k22 Where enr : winding temperature gradient at current Ib y : winding thermal capacity used exponent heu22n12irl e : thermal exchange coefficient between the winding and the oil : multiplying factor applied to the time constants : winding time constant : oil time constant DE81256 SEPED303001EN - 01/2013 135 Protection functions 3 DE81257 Thermal overload for transformers ANSI code 49RMS ItErTaCrnasnsftosarfnmodreamrr,detrh6e00fo7l6lo-w72ip1nrgovpaolsueess2,:2dependinegnorn the nature y of the immersed enr huile ONAN (distribution) 1 2 23 °C 1,6 4 min 180 min ONAN (power) 2 2 26 °C 1,3 10 min 210 min ONAF 2 2 26 °C 1,3 7 min 150 min OF 1.3 1 22 °C 1,3 7 min 90 min OD 1 1 29 °C 2 7 min 90 min Note: For distribution ONAN and OD transformers, the winding thermal model only reacts with the winding time constant. When the winding and oil time constants are given by the immersed transformer manufacturer, the user can enter them in place of the default values proposed by the standard. For transformers in which the oil flow can be restricted, exchanges between the winding and the oil are worse, so the winding thermal capacity used values are exceeded. In this case coefficient 21 takes the following values: Transformer Restricted flow OFF ON ONAN (power) 2 3 ONAF 2 3 OF 1,3 1,45 Accounting for ambient temperature The characteristics of immersed transformers are defined for an ambient temperature of 20 °C (68 °F). When the Sepam is equipped with the temperature sensor module option, the ambient temperature is measured by sensor no. 8 and added to the oil temperature rise. The oil thermal model transfer function is as follows: Ieq x 1 ho 1 + R oil Where ho : oil temperature gradient at current Ib R : ratio between the on-load losses and the no-load losses x : oil thermal capacity used exponent 11 : multiplying factor applied to the oil time constant 136 SEPED303001EN - 01/2013 Protection functions Thermal overload for transformers ANSI code 49RMS IEC standard 60076-7 proposes, depending on the nature of the immersed transformer, the following values: Transformer 11 ho x R ONAN (distribution) 1 55 °C 0,8 5 ONAN (power) 0,5 52 °C 0,8 6 ONAF 0,5 52 °C 0,8 6 OF 1 56 °C 1 6 OD 1 49 °C 1 6 Taking account of the oil temperature When the Sepam is equipped with the temperature sensor module option, sensor no. 8 can be assigned to the oil temperature measurement. In this case the oil temperature measurement is substituted for the oil thermal model. The measured oil temperature oil is added to the winding temperature rise. Saving the thermal capacity used On loss of the auxiliary power supply, both the winding and oil thermal capacity used 3 are saved. Operating information The following information is available to the operator: b the time before tripping in minutes (at constant current) b v the relative thermal capacity used Ek when the oil temperature is estimated of the transformer expressed as by a calculation: a %: Ek= 100 ----k----e----n---r-a---+m-----b----i--a-h--n--o--t v when the oil temperature is measured: Ek= 100 ----k----------e-h--n--u--r-i--l--e-- SEPED303001EN - 01/2013 137 Protection functions 3 Thermal overload for transformers ANSI code 49RMS Characteristics Settings Measurement origin Setting range I1, I2, I3 / I'1, I'2, I'3 Choice of transformer or thermal model Setting range Immersed transformer ONAN (distribution) ONAN (power) ONAF OD OF Alarm set point ( alarm) Generic model(1) Setting range Resolution Tripping set point ( trip) Immersed transfo: 98 °C to 160 °C (208 °F to 320 °F) Dry-type transfo: 95 °C to 245 °C (203 °F to 473 °F) 1 °C (1 °F) Setting range Immersed transfo: 98 °C to 160 °C (208 °F to 320 °F) Dry-type transfo: 95 °C to 245 °C (203 °F to 473 °F) Resolution 1 °C (1 °F) Winding time constant ( enr ) Setting range 1 mn to 600 mn Resolution Oil time constant ( huile ) Setting range 1 min 5 mn to 600 mn Resolution 1 min Accounting for ambient temperature Setting range yes / no Accounting for oil temperature Setting range yes / no Restricted oil flow Setting range Characteristic times on / off Operating time accuracy Inputs ±2 % or ±1 s Designation Reset protection Inhibit protection Outputs Syntax P49RMS_1_101 P49RMS_1_113 Equations b b Logipam b b Designation Time-delayed output Alarm Inhibit closing Protection inhibited Hot state Thermal overload inhibited Zero speed Syntax P49RMS _1_3 P49RMS _1_10 P49RMS _1_11 P49RMS _1_16 P49RMS _1_18 P49RMS_1_32 P49RMS_1_38 Equations b b b b b b b (1) See settings associated with generic thermal overload. Logipam b b b b b b b Matrix b b b Glossary of transformer type abbreviations: b AN: air-cooled transformer with natural ventilation b AF: air-cooled transformer with forced ventilation b ONAN: transformer immersed in mineral oil, cooled by natural air convection b ONAF: transformer immersed in oil with forced circulation b OD: transformer immersed in oil with forced circulation, directed into the windings b OF: transformer immersed in oil with forced circulation 138 SEPED303001EN - 01/2013 DE81258 Protection functions Thermal overload for motors ANSI code 49RMS Operation This function is used to protect the stator and the rotor of an asynchronous motor. T max Block diagram The stator thermal overload protection is provided by a thermal model with 2 time constants ( long and short). The rotor excessive starting time thermal protection is provided by an adiabatic thermal model. I alarm Ambient temperature Correction by the ambient temperature fcorr Exfcorr > Ialarm2 Alarm Annunciation P49RMS_1_10 long short cool I trip Stator thermal E LRT capacity used Exfcorr > I trip2 & Is_therm Id li Calculation of Ieq Metal frame thermal M capacity used Id Id > Is_therm 1 3 Tripping & Annunciation P49RMS_1_3 IL gn Rotor thermal capacity used IL Tc Th "Inhibit thermal overload" TC logic input "Inhibit thermal overload" 1 "Authorize emergency restart" logic input 49 RMS "on" "Inhibit protection" 1 logic equation P49RMS_1_113 W & W > 1 Start inhibit g g > 0.95 M > (Hot state set point)2 Hot state set point Inhibit Closing & Annunciation P49RMS_1_11 Zero rotor speed P49RMS_1_38 lnhibit thermal overload P49RMS_1_32 Hot state P49RMS_1_18 Protection inhibited P49RMS_1_16 SEPED303001EN - 01/2013 139 Protection functions 3 Motor thermal overload ANSI code 49RMS Blocking of tripping and closing inhibition The protection tripping and inhibit closing outputs can be inhibited by: b an "Inhibit thermal overload" latched logic input b an "Authorize emergency restart" latched logic input b an "Inhibit thermal overload" remote control order (TC). Start inhibit When the protection trips, circuit breaker closing is inhibited until the rotor thermal capacity used allows another motor start. This inhibit is grouped together with the "Starts per hour" protection function, and signaled by the message "INHIBIT START". The inhibit time before starting is authorized can be accessed from: b the "Machine diagnosis" tab in the SFT2841 software b the Sepam front panel. "Hot state" set point The thermal overload function provides a "hot state" data item used by the starts per hour function (ANSI code 66). It is used to distinguish between cold starts and hot starts. The number of consecutive starts per hour is stated by the motor manufacturer. Depending on the manufacturer, the previous load current defining hot state varies between 0.6 Ib and Ib. Hence the "hot state" set point can be adjusted to suit the motor characteristics. Saving the thermal capacity used On loss of the auxiliary power supply, the thermal capacity used of the rotor W, the stator E and the metal frame M are saved and reused in their current state until the relay is re-energized. Operating information The following information can be accessed from the "Machine diagnosis" tab in the SFT2841 software and the Sepam front panel: b the stator thermal capacity used b the time before the stator protection trips (at constant current) b the time before restarting is authorized. 140 SEPED303001EN - 01/2013 Protection functions Thermal overload for motors ANSI code 49RMS Characteristics Settings Inputs Measurement origin Setting range I1, I2, I3 Choice of thermal model Setting range 2 Constant Designation Reset protection Inhibit protection Outputs Syntax P49RMS_1_101 P49RMS_1_113 Equations Logipam b b b b Generic(1) Thermal model switching threshold Setting range 1 to 10 pu of Ib Resolution 0.1 pu of Ib Stator thermal settings Motor thermal capacity used time constant Setting range 1 mn to 600 mn Resolution 1 mn Is_therm long Designation Time-delayed output Alarm Inhibit closing Protection inhibited Hot state Thermal overload inhibited Zero speed Syntax P49RMS_1_3 P49RMS_1_10 P49RMS_1_11 P49RMS_1_16 P49RMS_1_18 P49RMS_1_32 P49RMS_1_38 Equations Logipam b b b b b b b b b b b b b b Matrix b b b Stator thermal capacity used time constant short Setting range Resolution 1 mn to 60 mn 1 mn 3 Cooling time constant cool Setting range 5 mn to 600 mn Resolution 1 mn Tripping current set point Itrip Setting range 50 % to 173 % of Ib Resolution 1 % of Ib Alarm current set point Ialarm Setting range 50 % to 173 % of Ib Resolution 1 % of Ib Thermal exchange coefficient between the stator and the motor Setting range 0 to 1 Resolution 0.01 Hot state set point Setting range 0.5 to 1 pu of Ib Resolution 0.01 pu of Ib Accounting for ambient temperature Setting range Yes / No Maximum equipment temperature (insulation class) Tmax Setting range 70 °C to 250 °C or 158 °F to 482 °F Resolution 1 °C or 1 °F Rotor thermal settings Locked rotor amperes IL Setting range 1 to 10 pu of Ib Resolution 0.01 pu of Ib Locked rotor torque LRT Setting range 0.2 to 2 pu of Ib Resolution 0.01 pu of Ib Locked cold rotor limit time Tc Setting range 1 s to 300 s Resolution 0.1 s Locked hot rotor limit time Th Setting range 1 s to 300 s Resolution 0.1 s Characteristic times Operating time accuracy ±2 % or ±1 s (1) See settings associated with generic thermal overload. SEPED303001EN - 01/2013 141 Protection functions Thermal overload for motors ANSI code 49RMS Help with parameter setting The function parameters are set using the motor manufacturer data and the SFT2841 software (49RMS tab in the protection functions). 1 Selection of the motor / generic thermal overload DE81197 protection function 2 Switching threshold between the stator and rotor thermal models (Is_therm) 3 Rotor thermal model parameters 1 4 Stator thermal model parameters 5 Calculated stator thermal model parameters 4 2 5 3 3 SFT2841 software: 49RMS protection parameter-setting screen for a motor application. Parameter-setting procedure 1. Select the thermal overload protection function by choosing the "2 Time constants" value from the "Thermal Model" drop-down list. Note: The "Generic" value selects the generic thermal overload protection function (see page 153 to set the parameters for this protection function). 2. Enter the rotor and stator parameters using the motor manufacturer data. b Rotor parameters: v Locked cold rotor limit time (Tc) v Locked hot rotor limit time (Th) v Locked rotor torque (LRT) v Starting current (IL) b Stator parameters: v Heating time constant: long v Cooling time constant: cool 3. Determine in graphic form the switching threshold between the stator and rotor thermal models (Is_therm). Depending on the manufacturer curves, there are 2 possible scenarios: b If there is any discontinuity between the manufacturer curves (see example on next page), choose Is_therm at the stator breaking point. b If there is no discontinuity: v Plot the locked cold rotor thermal model curve, between IL and Ib, using the equation below in order to determine Is_therm: W(I) = Tc x (IL / I)2 v Determine the value of Is_therm for which the rotor thermal model (adiabatic) no longer corresponds to the manufacturer's locked cold rotor curve. 142 SEPED303001EN - 01/2013 Protection functions Thermal overload for motors ANSI code 49RMS DE81259 Permissible operating time [s] 10000 Motor running 1000 Cold curve 100 Hot curve Locked rotor Tc 10 Th 3 0 1 Itrip 2 3 4 5 IL 6 I/Ib Stator Is_therm Rotor Determination of Is_therm in the case of discontinuous manufacturer curves. Itrip: permissible continuous current and tripping set point in pu of Ib IL: starting current in pu of Ib Tc: Locked cold rotor limit time Th: Locked hot rotor limit time 4. Determine the following stator parameters: b Tripping current set point Itrip b Stator thermal capacity used time constant short b Thermal exchange coefficient If these parameters are not available, proceed as follows to calculate them using the SFT2841 software: 4.1. Press the "Use Genetic Algorithm" button which can be accessed from the 49RMS tab in the protection functions. 4.2. Enter 4 typical points found on the manufacturer's cold stator curve. 4.3. Press the "Use Genetic Algorithm" button: the SFT2841 software calculates all 3 parameters. SEPED303001EN - 01/2013 143 Protection functions 3 DE81260 Thermal overload for motors Code ANSI 49RMS Example of parameter setting no. 1: 3100 kW / 6.3 kV motor We have the following manufacturer data: Parameter Name Value Rotor / stator insulation class F - rated current Ib 320 A - starting current IL 5.6 Ib rotor rated torque Tn 19,884 Nm rotor starting torque LRT 0.7 Tn rotor motor time constant long 90 minutes stator cooling constant cool 300 minutes stator locked cold / hot rotor limit time Tc / Th 29 s / 16.5 s rotor starting time 2.3 s - number of consecutive cold (hot) starts 3 (2) - Setting the function parameters 1. Selection of "2 Time constants" from the "Thermal Model" drop-down list to select the motor thermal overload protection function. 2. Set the rotor and stator model parameters using the manufacturer data: Rotor parameter Locked cold rotor limit time Locked hot rotor limit time Locked rotor torque Starting current Stator parameter Alarm current set point Heating time constant Cooling time constant Name Tc Th LRT IL Name Ialarm long cool Value 29 s 16.5 s 0.7 pu rated torque 5.6 Ib Value < Itrip 90 minutes 300 minutes 3. Determination of Is_therm switching threshold between the 2 models: In this example there is a clear distinction between the rotor and stator manufacturer curves. Therefore the Is_therm switching threshold at the rotor curve breaking point is selected. Hence Is_therm = 2.8 Ib 10000 6000 1500 1000 Cold curve Ttrip in sec 400 250 Hot curve 100 10 1 1.4 1.8 2 2.4 2.8 3 Stator Is_therm 4 5 6 l/lb Rotor 144 SEPED303001EN - 01/2013 Protection functions Thermal overload for motors ANSI code 49RMS 4. Determination of the stator parameters: For example on the cold stator curve (previous graphic) the following 4 points are selected, spread between Ib and Is_therm: I/Ib Ttrip 1.4 6000 s 1.8 1500 s 2.4 400 s 2.8 250 s The SFT2841 software calculates the missing stator parameters on the basis of these 4 points: Calculated stator parameter Name Value Tripping current set point Itrip 1.2 Ib Stator heating time constant short 5.5 mn Thermal exchange coefficient between stator and motor 0.7 3 The function parameter setting is complete: On the graphic below the manufacturer curves are bold lines, whereas the curves generated from the configured model are fine lines. The function protects the motor beyond its stated characteristics. 10000 1000 Ttrip in sec 100 10 1 2 3 4 5 6 l/lb Stator Is_therm Rotor Comparison of the manufacturer curves and the configured model. DE81261 SEPED303001EN - 01/2013 145 Protection functions 3 DE81262 Thermal overload for motors ANSI code 49RMS Example of parameter setting no. 2: 600 kW / 6 kV motor We have the following manufacturer data: Parameter Name Value Rotor / stator insulation class F - rated current Ib 69.9 A - starting current IL 6 Ib rotor rated torque Tn 392.2 kgm rotor starting torque LRT 0.9 Tn rotor motor time constant long 60 minutes stator cooling constant cool 180 minutes stator locked cold / hot rotor limit time Tc / Th 33.5 s / 25 s rotor starting time 1.2 s - number of consecutive cold (hot) starts 2 (1) - Setting the function parameters 1. Selection of the "2 Time constants" value from the "Thermal Model" drop-down list to select the motor thermal overload protection function. 2. Set the rotor and stator parameters using the manufacturer data: Rotor parameter Locked cold rotor limit time Locked hot rotor limit time Locked rotor torque Starting current Stator parameter Alarm current set point Heating time constant Cooling time constant Name Tc Th LRT IL Name Ialarm long cool Value 33.5 s 25 s 0.9 pu rated torque 6 Ib Value < Itrip 60 minutes 180 minutes 3. Determination of Is_therm switching threshold between the 2 models. 10000 1000 Ttrip in sec 100 10 1 2 3 4 5 6 Is_therm l/lb In this example the rotor and stator manufacturer curves (in bold lines) merge into one another. We therefore plot the rotor model curves (in fine lines) defined by: b cold curve W(I) = 33,5 (6 / I)2 b hot curve W(I) = 25 (6 / I)2 We can see that the rotor model curve coincides with the manufacturer curve over the whole current range I/Ib. We therefore select the Is_therm switching threshold = 1.01 Ib. The rotor model thus protects the motor over its whole operating range. 146 SEPED303001EN - 01/2013 Protection functions Thermal overload for motors ANSI code 49RMS 4. Determination of the stator parameters: The SFT2841 software calculates the following stator parameters: Calculated stator parameter Name Value Tripping current set point Itrip Stator heating time constant short Thermal exchange coefficient between stator and motor 1.01 Ib 60 Minutes 1 In this example, the stator thermal overload protection is only used to define the thermal state of the motor, in order to be able to: b change the locked cold rotor limit time value to its corresponding hot value b define the hot / cold thermal state of the motor. The function parameter setting is complete. 3 SEPED303001EN - 01/2013 147 Protection functions Thermal overload for motors ANSI code 49RMS Additional information about the models cu R1 fe Stator thermal model The stator thermal model takes account of thermal exchanges between the stator DE81177 rl 2 C1 eq C2 winding and the motor metal frames using 2 time constants. R2 a Having used to designate the ratio R2/(R1+R2), the stator winding relative thermal Stator thermal model. rIeq² : heat generated by the copper losses at capacity used E transfer function is expressed as follows: E(p) = (---1-----+---(--p-1-------s---h---)-o-----r--t---) + (---1-----+-----p--------l--o----n----g----) equivalent current Ieq where 0 < < 1. C1 : stator thermal capacity The thermal model time response with two time constants is proportional to the R1 : thermal resistance between the stator and the square of the current. motor metal frame C2 : motor thermal capacity R2 : motor thermal resistance a : ambient temperature 3 cu : stator winding temperature fe : motor metal frame temperature short = R1C1: stator winding time constant (Ieq,t) = (1 ) 1 e----s----h--t--o----r---t + 1 e----l--o--t--n-----g-- Ie2q The stator thermal overload protection trips when E(Ieq,t) = K², K being the permissible continuous current in pu of Ib. long = R2C2 : motor metal frame time constant For = 0, there is no thermal exchange between the stator and the metal frame since the motor thermal resistance R2 is zero. Thus the stator heats up with the lowest time constant short. Conversely for = 1, the thermal exchange between the stator and the metal frame is perfect, therefore the stator and the metal frame only make one, resulting in the stator heating up with a time constant close to that of the metal frame long. For 0 < < 1, thermal management with 2 time constants makes it possible: b to protect the stator winding correctly against strong overloads, since the resulting time constant is close to the stator time constant b for the motor to run at low overload as close as possible to the limits defined by the manufacturer data, since the resulting time constant is close to that of the metal frame. Illustration of the influence of the coefficient on a motor whose time constants are as follows: b stator winding: short = 4 mn b metal frame: long = 60 mn. Ttrip in sec 100000 DE81263 10000 1000 Maximum thermal exchange 100 No thermal exchange 10 1 1.5 2 Influence of the coefficient on a motor. 2.5 l/lb 3 0 0.4 0.6 1 148 SEPED303001EN - 01/2013 Protection functions Thermal overload for motors ANSI code 49RMS Additional information about the models Stator thermal model (continued) Equivalent current Ieq The presence of a negative sequence component accelerates the motor temperature build-up. The current negative sequence component is taken into account in the protection function by the equation Ieq = II---db-- 2 + Ki I--I-b-i- 2 where Id is the current positive sequence component Ii is the current negative sequence component Ib is the motor rated current Ki is the negative sequence component coefficient. For an asynchronous motor, Ki is calculated using the following parameters: b LRT: locked rotor torque in pu of the rated torque b IL: starting current in pu of the rated current Ib b N: rated speed in rpm. 3 The number of pairs of poles np is defined by the expression: np = i n t 6----0-----N------f---n-- The rated slip gn is defined by the expression: gn = 1 6-N--0---------n--f--pn-- where fn is the network frequency in Hz. The coefficient Ki is defined by the expression: Ki = 2 ----L----R----T-----gn IL2 1 Accounting for ambient temperature Asynchronous motors are designed to run at a maximum ambient temperature of 40 °C (104 °F). Where Sepam is equipped with the temperature sensor module option (with sensor no. 8 assigned to measuring the ambient temperature), the stator thermal capacity used is multiplied by the correction factor fcorr, from the time when the ambient temperature is higher than 40 °C. fcorr = -T---m------a--T--x--m------a-T---x-a----m----4--b-0---i--a----n----t where Tmax is the maximum temperature in the thermal class for the motor insulating components defined in accordance with standard 60085. Class Tmax in °C Tmax in °F 70 Y A E B F H 200 220 250 70 90 105 120 130 155 180 200 220 250 158 194 221 248 266 311 356 392 428 482 SEPED303001EN - 01/2013 149 Protection functions 3 DE81264 Thermal capacity used Thermal overload for motors ANSI code 49RMS Additional information about the models Stator thermal model (continued) Metal frame thermal capacity used Having used to designate the ratio ----l--o----n-----g---l--o----n----gs----h----o----r---t the motor metal frame relative thermal capacity used M transfer function is expressed as follows: M(p) = (---1-----+---(--p-1-------s---h---)--o----r--t---) + (---1-----+-----p--------l--o----n----g----) where > 1. Example: Starting with a zero initial thermal capacity used and applying a current the same as the rated current Ib, the stator and metal frame relative thermal capacity used reach 100 %. Initially, the metal frame thermal capacity used has a zero slope, until the heat transfer is established between the stator and the metal frame. 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 5000 10000 15000 Stator and metal frame thermal capacity used for a load current Ib. Stator Metal frame t(s) 20000 The metal frame relative thermal capacity used is used to: b adapt the rotor protection rotor limit time b define the hot state of the motor. Cooling time constant When the current Ieq is less than 5 % of Ib, the motor is deemed to have stopped. In this case it is the cooling time constant cool of the metal frame that is taken into account to estimate stator cooling. 150 SEPED303001EN - 01/2013 DE81180 DE81181 Protection functions Thermal overload for motors ANSI code 49RMS Additional information about the models Rs Xs Xr Rr Rotor thermal model For the rotor, guide IEEE C37.96-2000 on protection of asynchronous motors defines an adiabatic thermal model, dependent on the slip, which is based on the equivalent Rm Xm Rr(1-g)/g Steinmetz diagram. Steinmetz diagram. Rs: stator resistance Xs: stator reactance Rr: rotor resistance Xr: rotor reactance Rm: magnetic loss Xm: magnetizing reactance g: slip During the asynchronous motor starting phase, rotoric currents travel across the rotor conductors to a depth that depends on the slip. Therefore the rotor inductance Xr and the rotor resistance Rr vary as a function of the slip g as follows: Rr = Kr g + Ro Xr = Kx g + Xo Kr: coefficient taking account of the increase in the rotor resistance Kx: coefficient taking account of the decrease in the rotor reactance 4 3 R1 3.5 3 2.5 Kr 2 Kx 1.5 R0 1 0.5 0 0 0.2 0.4 0.6 0.8 1 g Coefficients Kr and Kx as a function of the slip. Assuming that the positive sequence rotor resistance Rr+ varies almost linearly between Ro and R1: Rr+ = (R1 R0) g + R0 The proportion of negative sequence current can be high during the motor starting phase. As a result the negative sequence rotor resistance Rr- is high in order to evaluate the rotor thermal capacity used. It is obtained by replacing the slip g with the negative slip sequence (2 - g). Thus: Rr- = (R1 R0) (2 g) + R0 The thermal model used in the Sepam relay measures the active part of the positive sequence impedance during the motor starting phase to evaluate the slip g. Depending on the motor status, the positive and negative sequence rotor resistances are as follows: Motor status Stop (g=1) Rated speed (g 0) Rr+ Rr- R1 R1 R0 2 R1 - R0 SEPED303001EN - 01/2013 151 Protection functions 3 152 Thermal overload for motors ANSI code 49RMS Additional information about the models Rotor thermal model (continued) The mechanical power developed by the motor equals the electrical power drawn in the resistance Rr (1 - g) / g. The torque Q equals: Q = w-P--- = 1-----P-----g-- = -R---------r------(----g---------)-----------g----1------(----1--------g----------g----------)----------I--L2-- = IL2 -R----r---(--g-----) g Thus: Rr(g) = -Q---IL2 g When the motor has stopped, g = 1. We can therefore deduce that: R1 = L-----R----T--IL2 (in pu of Zn) Where LRT: locked rotor torque in pu of the rated torque IL: locked rotor current in pu of Ib When the motor is at rated speed, the torque Q equals the rated torque Qn and the current equals the rated current In, thus R0 = gn (in pu of Zn). Where: Zn = ---U-----n----- 3Ib gn: rated slip When the motor is at its rated speed of rotation, the ratio between the positive and negative sequence resistances is: 2RR-----10-- 1 = 2-g---n-L----R-----T--I--L2--- 1 During the starting phase the rotor thermal capacity used W is defined by the following expression: Wn = Wn 1 + R-----r---+-R1 I-I-L-d-- 2 + R-----r---R1 I-I-L--i 2 T----d-(--M--t----) Where T(M): locked rotor limit time depends on the thermal state of the motor M: T(M) = Tc - (Tc - Th) x M, where 0 M 1. Tc: locked cold rotor limit time at the starting current IL Th: locked hot rotor limit time at the starting current IL. Example for a motor whose starting time is 5 s and the locked cold rotor limit time is 20 s. b When the rotor is locked, the slip g = 1, as a result Rr+ = R1. Thus the thermal capacity used is 5/20 = 25 %. b When the slip g changes from 1 to 0 in 5 s, the rotor thermal capacity used is 17 %. DE81265 Rotor thermal capacity used (in pu) 0.3 0.25 0.2 S=1 0.15 S#1 0.1 0.05 0 0 1 2 3 4 5 Starting time (in sec) Comparison of the rotor thermal capacity used during normal starting with locked rotor. SEPED303001EN - 01/2013GPL Ghostscript 9.10