Evaluating Motor and Transformer Inrush Currents

Inrush currents associated with motor starting and transformer energizing can cause interaction problems with other loads in a facility or on the power system, particularly sags that trip loads. Protection devices can misinterpret these events as fault currents, if the devices are not properly coordinated. Coupled with the tendency of other constant power devices to increase current to make up for the reduced voltage, the inrush current may cause protection devices to trip. Energizing a transformer has the additional issue of harmonics in the inrush current, which can excite system resonances and cause dynamic overvoltages.

Evaluating these concerns requires measurement equipment that can capture the waveforms over the full duration of an event, such as the starting of a motor or the energizing of a transformer, which can take a number of seconds. This measurement equipment can include expert systems that look through significant amounts of monitoring data and identify particular events and conditions. It's possible to look for abnormal conditions only after these expert systems characterize the waveforms associated with normal operation of facilities and equipment. Let's take a look at some of these voltage-affecting events and how to deal with them.

Motor inrush characteristics. Motors have the undesirable characteristic of drawing several times their full load current while starting. By flowing through system impedances, this large current will cause voltage sags that dim lights, cause contactors to drop out, and disrupt sensitive equipment. Theses sags also affect the starting itself, as large enough sags will prevent a successful start. Even small and medium horsepower motors can have inrush currents that are six to 10 times the normal steady-state current levels. High-efficiency motors can have even higher inrush currents.

Characterizing inrush currents and their effects requires a monitor capable of capturing the waveforms for a long duration. Figures 1a and 1b show an inrush of current to a motor that resulted in a significant sag lasting more than 1 second.

Ohm's and Kirchoff's Laws can help you analyze what happened. Per Ohm's Law, voltage = current × impedance. Per Kirchoff's Law, the sum of voltages around a closed loop must equal zero. If we assume a 0.5-ohm source impedance and a 10A nominal current on a 480V system, the inrush current can result in a drop of 30V to 50V. Therefore, voltage at the load would sag to 430V, down from the nominal 475V level. This sag occurs because the impedance of the motor initially (when the rotor is stationary) looks much like a short circuit. Once the rotor starts turning, the current decreases and eventually goes to a much lower, steady-state value. However, if a load change causes the motor to come close to stalling or remain in the locked rotor condition, another sag can result for similar reasons.

Transformer inrush characteristics. Transformers also exhibit inrush currents upon initial energizing. Here, the high currents occur to energize the transformer core. The steady-state magnetizing current for a transformer is very low, but the momentary current when it is first energized can be quite high.

The concerns are typically the same as with motor starting, except for one important difference — besides being a high current magnitude, the transformer energizing current is full of harmonics. Both even and odd harmonic components occur when a transformer is energized, and they can excite system resonances, resulting in dynamic overvoltages.

These dynamic voltages can cause surge suppressors to overheat, capacitor fuses to blow, capacitor failures, or misoperation of electronic equipment. Again, monitoring equipment that can characterize the waveforms over the full duration of the event will allow you to see what is going on in the system. Figure 2a shows a typical transformer energizing current waveform. Figure 2b shows the low-order harmonic components in the current.

Solving inrush problems. As previously discussed, the most significant problem associated with inrush currents is the resulting voltage sag. ANSI C50.41-2000, “American National Standard for Polyphase Induction Motors for Power Generation Stations,” states that motors must be able to start as long as the voltage is not less than 85% of the rated voltage.

In addition, most utilities limit the allowable voltage variation at the point of common coupling (PCC) caused by a single motor start to about 4%. The voltage variation on the distribution system is determined by the impedance of the distribution system supply in relation to the impedance of the step-down transformer and secondary cabling to the motor.

If the motor starting operation results in a voltage sag that causes tripping of equipment within the facility or at other customer facilities, you can use one of the following methods to reduce the voltage sag.

1. Keep large motors on a separate supply from the sensitive loads. Following this advice usually prevents problems with other equipment. The PCC will be at the distribution voltage level, where the voltage sag is less severe than at the motor terminals.

2. Use resistance and reactance starters. These initially insert an impedance in series with the motor. After a time delay, the starter bypasses this impedance. Starting resistors may be bypassed in several steps while starting reactors are bypassed in a single step. This approach requires the motor be able to develop sufficient torque with the added impedance.

3. Use delta-wye starters. These connect the stator in wye for starting, then after a time delay, reconnect the windings in delta. The wye connection reduces the starting voltage to 57% of the system line-line voltage, which causes the starting torque to fall to 33% of its full start value. The reduced voltage during the initial stage of the starting reduces the inrush current — and the resulting voltage sag.

4. Use shunt capacitor starters. These devices work by switching in, along with the motor, a large shunt capacitor bank that supplies a large portion of the motor VAR requirements during the start process. The capacitor bank then automatically disconnects once the motor is up to speed (usually based on overvoltage relay).

5. Use series capacitors on distribution circuits supplying large motors. This will reduce the effective impedance seen by the motor during starting as well as the resulting voltage sag on the motor side of the series capacitor. However, the source side of the series capacitor may still experience a more severe voltage sag.

6. Change frequency response characteristics. This will counter the potential for system resonance caused by harmonics in transformer inrush currents. You may be able to do this by switching out one or more shunt capacitors prior to energizing the transformer.

Proactive monitoring leads to prevention. Motor starting and transformer energizing operations can be a detriment to facility equipment and the supplying power system.

Characterizing these events requires sampling the voltage and current waveforms over a relatively long duration (seconds), which can be a problem for monitors that record only a few cycles of information for a disturbance. But, there's more to this. The process of turning the raw measurement data into knowledge involves data selection and preparation, information extraction from the selected data, information assimilation, and report presentation.

McGranaghan is vice president of consulting services for EPRI Solutions in Knoxville, Tenn. Kinder is a principal engineer with Dranetz-BMI in Edison, N.J.

Sidebar: Determining Sag Severity During Full-Voltage Starting

The severity of voltage sag during full-voltage starting of a motor is described by the following equation:

V_MIN (pu) = ( V(pu) × kVA_SC) ÷ ( kVA_LR + kVA_SC)

where V(pu) is the actual system voltage in per unit of nominal; kVA_LR is the motor locked rotor kVA; and kVA_SC is the system short-circuit kVA at the motor terminals.

If the result is above the minimum allowable steady-state voltage for the affected equipment, then you can use full voltage starting. If the result is below the allowable steady-state voltage, then you must compare the sag magnitude versus duration characteristic to the voltage tolerance envelope of the effected equipment.

Note that the required calculations are fairly complicated, so you'll need to use a motor-starting or general transient analysis computer program or have the motor manufacturer use its application engineering capabilities to do the calculations and provide the resulting information.