Understanding short-circuit motor contribution - EE … 2013/vector_june2013... · Understanding...
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June 2013 - Vector - Page 45 DRIVES & SWITCHGEAR Understanding short-circuit motor contribution Motor contribution can be used to calculate maximum short circuit current and the short circuit rating of electrical equipment. by Darrell Broussard, GE Industrial Solutions It can be demonstrated that motor contribution is present during faults regardless of the size or voltage rating of the motor. System voltage decays under short circuit conditions where stable voltage supply no longer exists. The rotating magnetic field in the rotor will attempt to support the reduced voltage condition by becoming a power source. The motor now provides additional current into the faulted electrical system. This phenomenon is called "motor contribution". The amount of current generated in this way depends on the motor's impedance. At first, an asymmetrical current containing both AC and DC components is present, but lack of a stable voltage supply causes the AC component to decay when the rotor flux begins to drop. The transient DC component also decays without a stable voltage supply. The current supplied by the motor initially differs in frequency from the system frequency because of motor slip. The motor and load inertia determine the rate of decay. It is understood that the difference does not have an effect on the short circuit calculation. The motor impedance Z m determines the AC components' value. Motor impedance has a resistive R m component and a reactive component X m . The resistive component is always much smaller than the reactive component and the inductive component X m will therefore provide a conservative value for calculating motor contribution. The electrical system's X/R ratio determines the rate of decay of the DC component. When X = 0, there is only symmetrical current with no DC component. When R = 0, the DC component would never decay. It can be assumed that there will always be both resistance and reactive components in the system. Induction motor contribution typically lasts from one to four cycles from T = 0 during a short circuit condition. Synchronous motors' short circuit contribution can, however, last between six and eight cycles. The main difference is that the induction motor does not have the excitation ability of a synchronous motor and can therefore not maintain voltage for the same amount of time. In either case, the contribution is present during the first cycle. ANSI standard C37.010 offers guidance when calculating motor contribution for a group of LV motors if detailed motor data is not available. Assuming a motor contribution of four times rated full load current is acceptable. The standard arrived at this value by assuming that the motor contribution of 3,6 times rated current came 75% from induction motors and 4,8 times rated current from 25% synchronous motors. It has become accepted practice to use four times rated full load current in the absence of detailed data on synchronous and induction motors of unknown size on the system. Multipliers of 3, 6, 4 and 4,8 times are less than the motor lock rotor current. This could account for a reduction of motor contribution at the start of the short circuit. This assumption seems to explain the reduction. Approximations are a useful tool when detailed motor data is not available. For large motors or groups of large motors, lock rotor current, typically five to seven times full load current, is used in stead of the motor. Actual impedance will provide a very conservative value. For small motors or groups of small motors, it is recommended that a value of 0,2 to 0,28 per unit be used in place of actual motor impedance data. Detailed motor data or even complete motor lists are not available in the very early project stages and it is acceptible to use approximations or "rules of thumb". At this point, the engineer must determine the orders of magnitude of short circuit current. The engineer knows that the assumptions provide a conservative value. The important points are that the engineer has motor contribution factored into has calculations and bearing in mind that his assumptions have provided a conservative value. When all the equipment data is gathered, the engineer may conduct a detailed short circuit study before purchasing equipment. References [1] "Application guide for AC high-voltage breaker on symmetrical current bases", ANSI C37.010 1999. [2] "Calculating ahort circuit current with contribution from induction motors", II Transactions on industry application, Vol. 1A-18, No. 2, March/April 1982 [3] "Short circuit calculations for industrial and commercial power systems", General Electrical Company, GET-3550F, 0489. Contact Richard Wood, GE, Tel 011 236-7000, [email protected]
Transcript of Understanding short-circuit motor contribution - EE … 2013/vector_june2013... · Understanding...
June 2013 - Vector - Page 45
D R I V E S &S W I T C H G E A R
Understanding short-circuit motor contribution
Motor contribution can be used to calculate maximum short circuit current and the short circuit rating of electrical equipment.
by Darrell Broussard, GE Industrial Solutions It can be demonstrated that motor contribution is present during faults regardless of the size or voltage rating of the motor.System voltage decays under short circuit conditions where stable voltage supply no longer exists. The rotating magnetic field in the rotor will attempt to support the reduced voltage condition by becoming a power source. The motor now provides additional current into the faulted electrical system. This phenomenon is called "motor contribution". The amount of current generated in this way depends on the motor's impedance. At first, an asymmetrical current containing both AC and DC components is present, but lack of a stable voltage supply causes the AC component to decay when the rotor flux begins to drop. The transient DC component also decays without a stable voltage supply. The current supplied by the motor initially differs in frequency from the system frequency because of motor slip. The motor and load inertia determine the rate of decay. It is understood that the difference does not have an effect on the short circuit calculation. The motor impedance Zm determines the AC components' value. Motor impedance has a resistive Rm component and a reactive component Xm. The resistive component is always much smaller than the reactive component and the inductive component Xm will therefore provide a conservative value for calculating motor contribution. The electrical system's X/R ratio determines the rate of decay of the DC component. When X = 0, there is only symmetrical current with no DC component. When
R = 0, the DC component would never decay. It can be assumed that there will always be both resistance and reactive components in the system. Induction motor contribution typically lasts from one to four cycles from T = 0 during a short circuit condition. Synchronous motors' short circuit contribution can, however, last between six and eight cycles. The main difference is that the induction motor does not have the excitation ability of a synchronous motor and can therefore not maintain voltage for the same amount of time. In either case, the contribution is present during the first cycle. ANSI standard C37.010 offers guidance when calculating motor contribution for a group of LV motors if detailed motor data is not available. Assuming a motor contribution of four times rated full load current is acceptable. The standard arrived at this value by assuming that the motor contribution of 3,6 times rated current came 75% from induction motors and 4,8 times rated current from 25% synchronous motors.It has become accepted practice to use four times rated full load current in the absence of detailed data on synchronous and induction motors of unknown size on the system. Multipliers of 3, 6, 4 and 4,8 times are less than the motor lock rotor current. This could account for a reduction of motor contribution at the start of the short circuit. This assumption seems to explain the reduction. Approximations are a useful tool when detailed motor data is not available. For large motors or groups of large motors,
lock rotor current, typically five to seven times full load current, is used in stead of the motor. Actual impedance will provide a very conservative value. For small motors or groups of small motors, it is recommended that a value of 0,2 to 0,28 per unit be used in place of actual motor impedance data. Detailed motor data or even complete motor lists are not available in the very early project stages and it is acceptible to use approximations or "rules of thumb".At this point, the engineer must determine the orders of magnitude of short circuit current. The engineer knows that the assumptions provide a conservative value. The important points are that the engineer has motor contribution factored into has calculations and bearing in mind that his assumptions have provided a conservative value. When all the equipment data is gathered, the engineer may conduct a detailed short circuit study before purchasing equipment.
References [1] "Application guide for AC high-voltage
breaker on symmetrical current bases", ANSI C37.010 1999.
[2] "Calculating ahort circuit current with contribution from induction motors", II Transactions on industry application, Vol. 1A-18, No. 2, March/April 1982
[3] "Short circuit calculations for industrial and commercial power systems", General Electrical Company, GET-3550F, 0489.
Contact Richard Wood, GE, Tel 011 236-7000, [email protected]
