Setting principles for comprehensive motor protection

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This post was last edited by jennifer12580 on 2016-9-13 09:19. Principles for setting comprehensive protection parameters for motors: 1. Differential current quick-break protection: The settings are determined to avoid the imbalance currents caused by the maximum transient current when the motor is started under no-load conditions, as well as the imbalance currents that occur during short circuits. Generally, Idz = KIe/n, where Idz is the operating current for differential current quick-break protection; Ie is the rated current of the motor; and K is typically set between 6 and 12. 2. Longitudinal differential protection: 1) Setting of the minimum operating current for longitudinal differential protection: This minimum operating current should be greater than the imbalance current that occurs during motor startup. Idz.min = KKΔmIe/n, where Ie is the rated current of the motor, n is the turns ratio of the current transformer, KK is a reliability factor ranging from 3 to 4, and Δm represents the error resulting from an imperfect match in the turns ratios of the current transformers, usually taken as 0.1. In practical engineering calculations, Idz.min can be set at (0.3–0.6)Ie/n. 2) The ratio braking coefficient K is calculated based on the condition that the differential protection does not malfunction under the maximum external short-circuit current. The maximum braking coefficient K is given by the formula: K = KKKfzq Ktx Kc. Where: Ktx is the similarity coefficient of the current transformer, with a value of 0.5; KK is the reliability factor, taken as 2–3; Kc is the relative error of the current transformer, taken as 0.1; Kfzq is the coefficient for non-periodic components, taken as 1.5–2.0. The calculated value of Kmax is 0.3, but considering the effects of saturation and transient distortion in current transformers, a practical value for K in engineering applications is 0.5–1.0.

3) Phase current quick-break protection:
1) The high-value current for quick-break operation is Isdg, calculated as Isdg = Kk / Ist. Here, Ist is the motor starting current in amperes, and Kk is the reliability factor, which can be set at 1.3.
2) The low-value current for quick-break operation is Isdd, which can range from 0.7 to 0.8 times Isdg; generally, it is set at 0.7 times Isdg.
3) The time required for quick-break operation is tsd. When vacuum switches or low-oil switches are used as the switching devices in the motor circuit, tsd is set at 0.06 seconds. When FC switches are used, an appropriate delay is necessary to ensure that the fuse blows before the quick-break protection activates. 4. The motor start time tqd is set based on the actual start time of the motor, with a certain margin added; it can be taken as 1.2 times the actual start time. 5. Negative-sequence overcurrent protection 1) Negative-sequence operating current I2dz: I2dz is set to a value that allows it to bypass the negative-sequence current permitted during normal operation. Generally, for protecting against severe imbalances such as phase loss or reverse phase, I2dz can be set at (0.6~0.8)Ie; whereas for providing sensitive protection against imbalances, it can be set at (0.2~0.4)Ie. 2) Negative-sequence operating time constant T2: In the case of a two-phase short circuit on the busbar, a large negative-sequence current exists in the motor circuits; therefore, T2 should be set to a value greater than the longest clearing time required for external two-phase short circuits. In the FC circuit, it is necessary to avoid the fuse blowing during an asymmetric short circuit; in other words, the negative-sequence protection must not activate before the fuse blows. 3) Two types of time-based protection settings are employed: 6) Grounding protection. The primary operating current of the motor protection and control device is set such that it can handle the capacitive current flowing from the protected components in the event of a single-phase ground fault outside the protected circuit. This value is determined using a minimum sensitivity factor of 1.25, with the formula Idz ≥ Kk Icx and Idz ≤ (Ic∑ – Icx)/1.25. Where: Icx is the capacitive current flowing from the protected components when there is a single-phase ground fault outside the protected circuit; Ic∑ is the total single-phase ground capacitive current in the power grid; Kk is a reliability factor, which can be taken as 4–5. 7) Overheating protection involves two parameters: the heating time constant Tfr and the cooling time Tsr. 1) Heating time constant Tfr: The heating time constant Tfr should be provided by the motor manufacturer; if the manufacturer does not provide this value, it can be estimated using one of the following methods. A is estimated using the motor overload capacity data provided by the manufacturer; if the motor can operate for t seconds under an X-fold overload, then Tfr = (X² – 1.052) × t. If there are multiple sets of overload capacity data, the smallest value of Tfr calculated from these values is taken. B: If the temperature rise of the motor and the current density are known, the value of Tfr can be estimated using the following formula: Tfr = (150×θe)×(θM/θe – 1)/(1.05×Je²). Here, θe is the rated temperature rise of the motor’s stator windings; θM is the maximum allowable temperature rise for the insulation material used in the motor; Je is the rated current density of the stator windings. For example, if the motor uses Class B insulation, with a maximum allowable temperature rise of θM = 80°C, a rated temperature rise of θe = 45°C for the stator windings, and a rated current density of Je = 3.5 A/mm², then: Tfr = {(150×45)/(1.05×3.5²)}×(80/45–1) = 408 seconds.
C: The heating time constant Tfr is determined by the temperature rise of the stator under the motor’s starting current: Tfr = (θ×Ist²×tst)/θ1st. Here, θ is the stable temperature rise when the motor operates at its rated capacity; Ist is the multiple of the motor’s starting current; tst is the starting time of the motor; θ1st is the temperature rise of the stator windings during the starting time.
D: The value of Tfr can be estimated based on the motor’s operating specifications. For example, if it is specified that a motor should not be started from a cold state and reach full speed more than twice, and if the motor’s starting current multiple Ist and starting time tst are known, then: Tfr ≤ 2(Ist²–1.05²)tst.
2) The cooling time Tsr is determined based on the time required for the motor to cool down to normal conditions after overheating. 8. Protection against restarting the motor when it is overheated. The overheating lockout value θb is set based on the principle of enabling the motor to restart successfully under normal conditions; generally, θb can be set at 0.5. 9. Long-start protection: Long-start protection involves two fixed values, namely the motor’s rated starting current Iqde and the allowable stall time tyd of the motor. 1) The rated starting current Iqde of the motor is taken as the starting current during startup under the motor’s rated operating conditions (A). 2) The allowable locked-rotor time tyd of the motor is taken as the maximum safe locked-rotor time (S) of the motor. 10. Forward sequence overcurrent protection: Forward sequence overcurrent protection involves two fixed values, namely the forward sequence overcurrent operating current I1g1 and the forward sequence overcurrent operating time t1g1. 1) The operating current I1gl for positive-sequence overcurrent protection can generally be set at I1gl = (1.5~2.0)Ie. 2) The operating time t1gl for positive-sequence overcurrent protection can generally be set at t1gl = (1.5~2.0)tyd.
11. Low-voltage protection:
1) It is set based on the condition of shutting down less important motors. The low-voltage operating value is as follows: for medium-temperature and medium-pressure power plants, Udz = 60~65% Ue; for high-temperature and high-pressure power plants, Udz = 65~70%. To enable the automatic start-up of important motors, a minimum time limit of t = 0.5S is used.
2) It is set to ensure that the minimum allowable voltage of the supply busbar is maintained, taking into account the reliability factor and the return coefficient of the voltage relay. For medium-temperature and medium-pressure power plants, Udz = (60~65%)Ue/(KKKf); generally, 40%Ue is used. For high-temperature and high-pressure power plants, Udz = (65~70%)Ue/(KKKf); generally, 45%Ue is used. The setting is made based on conditions that ensure technical safety and meet the requirements of the process, with a sufficiently long time limit – the motor is disconnected only when the voltage drops significantly or disappears over time. Generally, t = 9S is used.
Principles for setting transformer protection:
1. Differential current quick-break protection: It is set to avoid the inrush current that occurs when the transformer is started under no-load conditions, as well as the maximum unbalanced current that occurs during external short circuits. Generally, Idz = KIe/n, where Idz is the operating current for differential current quick-break protection, Ie is the rated current of the transformer, and K is a multiplier. For transformers of 6300 KVA and below, the multiplier ranges from 7~12; for those between 6300~31500 KVA, it ranges from 4.5~7.0; for those between 40000~120000 KVA, it ranges from 3.0~6.0; and for transformers of 120000 KVA, it ranges from 2.0~5.0.
2. Longitudinal differential protection:
1) Setting of the minimum operating current for longitudinal differential protection: The minimum operating current should be greater than the unbalanced current when the transformer is operating under its rated load. Idz.min = KK(Kc+ΔU+Δm)Ie/n, where Ie is the rated current of the transformer, n is the turns ratio of the current transformer, KK is a reliability factor ranging from 1.3~1.5, Kc is the relative error of the current transformer – 0.03×2 for 10P type transformers, and 0.01×2 for 5P and TP type transformers – ΔU represents the error caused by voltage regulation, taken as the maximum deviation from the rated value within the voltage regulation range, and Δm represents the error resulting from an incomplete matching of the current transformer’s turns ratio, generally taken as 0.05. In practical engineering calculations, Idz.min can be set at (0.3~0.5)Ie/n. 2) Setting of the proportional braking coefficient K: The operating current of the longitudinal differential protection should be greater than the unbalanced current flowing through the differential circuit during an external short circuit. Ibph = (Kfzq Ktx Kc + ΔU + Δm) / IK.max, where: Ktx is the similarity coefficient of the current transformer; Ktx = 1.0. IK.max represents the periodic component of the maximum short-circuit current during an external short circuit. Kfzq is the coefficient for the aperiodic component; it takes a value of 1.0 when both current transformers are of the TP type, and 1.5–2.0 when both are of the P type. ΔU: The error caused by voltage regulation of the transformer; it is taken as the maximum deviation from the rated value within the voltage regulation range. Δm: The error resulting from an imperfect matching of the current transformer ratios; generally, this value is set at 0.05. KK: The reliability factor, with values ranging from 1.3 to 1.5. The operating current for differential protection is given by Idz.max = KK × Ibph.max. The maximum braking coefficient is Kmax = Idz.max / Izd.max; when Izd.max = IK.max, then Kmax = KK × Ibph.max / IK.max. Here, IK.max represents the maximum short-circuit current. In practical engineering calculations, a value of K between 0.3 and 1.0 can be used. 3) Setting of the second-harmonic braking ratio: It is generally set at 15% to 20%. 4) Setting of the inrush current interruption angle: The blocking angle can be set between 60° and 70°. 3) Zero-sequence differential protection: When ratio braking differential protection is used, it operates in the same manner as longitudinal differential protection. ΔU=0. When setting is done without ratio braking differential protection. 1) Idz.0 is set based on the unbalanced current that occurs when avoiding external single-phase ground faults, using the formula Idz.0 = KK(Kfzq Ktx Kc + Δm) × 3I0.max/n. Where: Idz.0 represents the operating current for the zero-sequence differential protection; Ktx is the similarity coefficient of the current transformers, with a value of 0.5 when the transformers are of the same type and 1.0 when they are of different types; KK is the reliability factor, ranging from 1.3 to 1.5; Kc is the relative error of the current transformers, with a value of 0.1; Kfzq is the coefficient for non-periodic components, with a value of 1.0 when both current transformers are of the TP type and 1.5 to 2.0 when both are of the P type. Δm: The error resulting from an incomplete matching of the current transformer ratios; generally, a value of 0.05 is used. 3I0.max: Three times the maximum single-phase or two-phase short-circuit zero-sequence current outside the protected area. 2) Idz.0 is determined to avoid unbalanced currents during external three-phase short circuits, using the formula Idz.0 = KKKfzq × Ktx × Kc × IK.max/n. Where: Ktx is the similarity coefficient of the current transformers; it is 0.5 when the transformers are of the same type and 1.0 when they are of different types. KK is the reliability factor, with a value between 1.3 and 1.5. Kc is the relative error of the current transformer, with a value of 0.1. Kfzq is the coefficient for non-periodic components; it is 1.0 when the current transformers are of the same type, and 1.5–2.0 when both transformers are of the P class. IK.max: Maximum external three-phase short-circuit current. 3) It is set to accommodate the zero-sequence unbalance current generated by excitation inrush currents; this refers to the transient current. Generally, Idz.0 = (0.3~0.4)Ie/n. The sensitivity factor is verified based on a metallic ground short circuit occurring within the zero-sequence differential protection area. 4) Calculation of the current quick-break protection setting: 1) It is set to handle the three-phase short-circuit current on the low-voltage side bus of the transformer: Idz.gl ≥ (KkKjx/Kf) × Kzq × (Ie/n). Kk: Reliability factor, taken as 1.2~1.3 ； Kf: Return coefficient, ranging from 0.8 to 0.95 ； Kzq: Load self-starting coefficient, ranging from 1.2 to 1.5 ； Kjx: Connection coefficient; n: Variation of the current transformer. 2) It is calculated based on the sum of the maximum starting currents of all motors that need to start automatically on the branch lines, namely Idz.gl = (KkKjx/Kf)×Kzqd×(Ie/n). Kk: Reliability coefficient; Kzq: The multiple of the overcurrent caused by the simultaneous restart of all motors that need to start automatically, which can be approximated using the formula above. 3) Verification based on sensitivity: Klm = I(2)d.min/Idz.gl(n); I(2)d.min: the minimum two-phase short-circuit current when there is a fault on the low-voltage side of the transformer ； Klm: Sensitivity in the event of a fault on the low-voltage side of the transformer. Generally, it refers to the setting time for the time-delayed overcurrent protection to operate. tfz: Operating time of the low-voltage circuit breaker ； tdz = tfz.max + △t; the operating time should be smaller than that of the overcurrent protection with the higher timing threshold by a certain time difference (take △t=0.5S). Typically, the time it takes for a low-voltage circuit breaker to operate in order to cut off a fault is around 0.1 seconds; therefore, the overcurrent operating time should be greater than 0.5–0.8 seconds. △t: time difference; take △t = 0.5 S. 5. Setting calculation for the zero-sequence overcurrent protection on the low-voltage side: 1) Determine the value to ensure that it can withstand the maximum unbalanced current flowing in the neutral wire of the transformer’s low-voltage side under normal operating conditions, i.e., Idz = Kk(0.25Ie). Kk is the reliability factor, taken as 1.2 ； Ie: Rated current of the transformer. 2) Coordination with the operating current of the protection devices connected to adjacent components: a) When the low-voltage service transformer has no branch circuits, it is coordinated with the inter-phase protection of the low-voltage motors: Idz = Kk × Kph × Kqd × Ie. Here, Kk is the reliability factor, taken as 1.2; Kph is the coordination factor, taken as 1.1; Kqd is the self-starting factor (a multiple of the motor’s starting current), taken as 5; Ie is the rated current of the largest motor. b) When the low-voltage service transformer has branch circuits, it is coordinated with the zero-sequence protection on those branch circuits: Idz = Kph × Kqd.fz. Here, Kph is the coordination factor, taken as 1.1; Kqd.fz is the operating current of the zero-sequence protection on the branch circuits. c) Sensitivity check: Klm = I(1)d.min / Idz. Here, I(1)d.min is the current value flowing through the current transformer on the neutral side of the service transformer in the case of a single-phase ground fault under the minimum operating conditions. The required value for Klm is between 1.5 and 2. d) Setting of the operating time for the zero-sequence protection on the low-voltage side: It is set in coordination with the quick-break protection on that side: Tgldz = t0.4 / sddz + △T. 6. Overload protection on the high-voltage side: The operating current for the symmetric overload protection is set such that it can handle the rated current: Idz = Kk / KF × (Ie / n). Here, I2 is the negative-sequence current of the low-voltage transformer; Kk is the reliability factor, taken as 1.05; KF is the feedback factor, taken as 0.85; Ie/n is the rated current at the location where the protection is installed. 7. Negative-sequence overcurrent protection: 1) The operating current for negative-sequence protection, I2dz, is set such that it can handle the negative-sequence current allowed during normal operation (negative-sequence current can occur when the bus voltage is asymmetric). Generally, in the case of severe imbalances such as phase loss and reverse phase, I2dz = (0.8~1.0) Ie can be adopted as the value for sensitive imbalance protection; whereas for less severe imbalances, I2dz = (0.2~0.4) Ie can be used. 2) The negative sequence operating time constant T2: In the event of a two-phase short circuit on the busbar, there is a significant negative sequence current in the motor circuits; therefore, T2 should be set to be greater than the maximum switching time required for external two-phase short circuits. In the FC circuit, it is necessary to avoid the fuse blowing during an asymmetric short circuit; in other words, the negative-sequence protection must not activate before the fuse blows. Setting principles for comprehensive protection of plant service branches
1. Time-limited current quick-break protection
1) The current operating value Idz is set to ensure that it can withstand the maximum three-phase short-circuit current at the end of adjacent components, or to be compatible with the operating current of the quick-break protection of those adjacent components; Idz is determined as the larger of the two values:
Idz = Kk × Kjx × Id.max(3) / n
Where:
Kk: Reliability factor, which can be taken as 1.2–1.5
Id.max(3): Steady-state three-phase short-circuit current at the end of adjacent components under the maximum operating conditions
Kjx: Wiring factor, equal to 1 when phase current is involved, and equal to √3 when phase current difference is involved
n: Turns ratio of the current transformer
2) The quick-break operation time tsd should be longer than that of the quick-break protection of adjacent components, typically ranging from 0.5 to 0.7 seconds.
3) Sensitivity factor verification: In the event of a short circuit at the end of the line, Km = Id.min(2) / Idz

2. Overcurrent protection
1) The current operating value Idz is set to accommodate the sum of the maximum starting currents of all motors that need to start automatically on the plant service transformer or branch lines:
Idz = KK × Kzq × Ie
Kzq = 1/(Ud%/100 + Se / Kqd × Sd∑)
Where:
KK: Reliability factor, taken as 1.2
Ie: Rated current of the plant service transformer or branch lines
Kzq: Multiplier of overcurrent caused by all motors that need to start automatically during startup
Ud%: Percent impedance value of the transformer
Sd∑: Total capacity (in KVA) of all motors that need to start automatically
Se: Rated capacity (in KVA) of the transformer
Kqd: Current multiplier during motor startup, typically taken as 5
2) Sensitivity factor verification: In the event of a short circuit at the end of the line, Km = Id.min(2) / Idz

3. Overload protection
Idz is set based on the rated load current:
Idz = Kk × Ie / n × KF
Where:
Kk: Reliability factor, which can be taken as 1.1–1.2
Ie: Rated current of the plant service transformer or branch lines
n: Turns ratio of the current transformer
KF: Return factor, taken as 0.85

4. Grounding protection
The primary operating current of the protection device is set to ensure that it can handle the capacitive current flowing from the protected components in the event of a single-phase ground fault outside the protected branch, with a minimum sensitivity factor of 1.25:
Idz ≥ Kk × Icx
Idz ≤ (Ic∑ – Icx) / 1.25
Where:
Icx: Capacitive current flowing from the protected components in the event of a single-phase ground fault outside the protected line
Ic∑: Total single-phase ground capacitive current in the power grid
Kk: Reliability factor, which can be taken as 4–5