Electronic control systems are the foundation of modern, efficient industrial processes. The reliability and performance of instruments, actuators, sensors, electric motors, and relays can all be affected by the quality of power supplied to them.
Programmable logic controllers (PLC) that unexpectedly experience faults, variable speed drives (VSD) that trip, motors that tend to overheat, as well as erroneous sensor signals can interrupt and shutdown industrial processes. This can lead to product scrapping, major restart delays, customer dissatisfaction, significant costs to business, and the resultant losses.
Unbalanced Voltage Causes
Power quality issues are seen in all electrical networks in different degrees and frequencies. While short sags and surges are common, networks can have voltage supply irregularities that are constantly present on the network, or may last for a long period of time. If a voltage imbalance is present on a supply network, it is often due to unmatched impedance on transformer banks, generation faults, or huge single phase loads on the three phase network.
Voltage imbalances caused by customer installation are mostly due to single phase loads not connected uniformly across the 3 phase system. Heating and cooling loads and single phase motors are connected in such a way that a single phase conductor carries more amount of current than the other two. Compared to the other two phases, the Line to Neutral Voltage of one phase is lower. Likewise, one Line to Neutral voltage is higher than the other two, where most of the load is connected over just two phases. In both cases, Line to Line voltages are affected.
Shown in Figure 1 is an over voltage on one line and an under voltage on another line at the High Voltage (HV) or Medium Voltage (MV) transformer, while the third is at a specified voltage. On the Low Voltage (LV) secondary, the Line to Neutral voltages on two phases is clearly over and under by 10% respectively; measuring the Line to Line vectors (dashed lines) displays voltage varying from specification.
This article deals only with the simple under and over voltage, the effect on electronic rectifiers and motors, and the resultant effects of those devices. Additional reading is available in other papers referencing phase angle shift that takes place with unbalanced voltage in three phase installations.
Unbalanced Voltage and Induction Motors
Most technicians and plant engineers are aware of the effect of unbalanced voltage on induction motors. The motor may generate excessive noise and motor torque and speed are negatively impacted. The voltage imbalance can also increase the current imbalance and promote a temperature rise that is much greater than the voltage imbalance percentage.2 Due to voltage imbalance, the increased temperature in an induction motor winding can be measured. In a 3 phase system, voltage imbalance is expressed as a single percentage. As shown in Figure 1, an over voltage and an under voltage may exist.
The following formulae are used to calculate the system imbalance and the resulting temperature rise in the winding:
|Voltage imbalance % = (Maximum imbalance/Average voltage) * 100
|Measured line to line voltages
|L1 - L2 = 392 V (-2%)
||Largest imbalance = 12 V
|L2 - L3 = 400 V (0%)
|L3 - L1 = 412 V (+3%)
||Average voltage = 401.33 V
|Voltage imbalance % = (12/401.33) * 100
||Voltage imbalance % = 2.99%
|Temperature rise % = 2 x (voltage imbalance %2 ) * 100
|Temperature rise % = 2 x 2.992 * 100
||Temperature rise % = 17.88%
Table 2 that demonstrates the formula above outlining the exponential increase in winding temperature compared with the increase in voltage imbalance. Greater than 2% imbalance is unacceptable2 because it causes a temperature rise in the winding that will be beyond the specification of motors; the motor life may also be reduced. No more than 5% unbalanced voltage is specified by NEMA limits. Studies demonstrate that with every 10° of temperature increase, the average life expectancy of insulation is reduced by 50%.3
|Effect of unbalanced voltage on winding temperature for 3 phase induction motors
|% Voltage imbalance
It is important to de-rate the three phase induction motors according to the chart shown in Figure 2.5
The increased operating heat makes the motor to expire prematurely and excess current is also drawn with no extra power output, thus over-stressing the supply cables and perhaps reaching the levels where the current overloads and the Variable Speed Drives (VSD) over current protection will trip.
Rectifier power supplies, VSD diodes, and DC link capacitors will experience extra thermal stress because of the increased AC line currents-to offset the voltage imbalance. In addition, triplen harmonics can be generated due to increased stress on the rectifier diodes.
Unbalanced Voltage and Rectifiers
Electronic equipment that changes AC to DC will have a rectifier, PLCs, computers, VSDs, and Uninterruptible Power Supplies (UPS). Rectifiers are a non-linear load in which the current output waveform is not linear to the voltage input waveform.
In a typical rectifier, the diodes will switch when the switching threshold voltage is surpassed in the positive direction. Two phases will surpass the switching threshold voltage during each half cycle, as one is decaying another is rising and vice versa, thus creating two peaks in the line current draw.
When voltage is balanced and operating correctly, the AC supply current waveform will have a double pulse per half cycle shape (Figure 3). The area below the graph is effectively power, in watts, needed to operate the connected DC load.
Figure 3. Normal line current for 1 phase of a three phase rectifier.
Due to supply inductance, current flow cannot commutate or transfer instantly from one diode to another in a rectifier. The rectifier starts to display varying conducting modes where varying quantities of diodes are concurrently conducting. Additional supply imbalance leads to overlapping of these modes, where more varying numbers of diodes are conducting. The angle of overlap is established by the time taken to complete commutation.4
Figure 4. Line current with 5% voltage imbalance.
Shown in Figure 4 is the AC input line current waveform, representing an AC voltage supply imbalance of 5%. The line current waveform begins to resemble a single pulse, suggesting an increased switching of the diodes and an increase in current via the diodes. Still, the same amount of power has to be supplied which means the area below the graph should remain the same as shown in Figure 4. The AC line current peak is therefore larger.
Figure 5. Line current where one phase of the rectifier is no longer conducting.
If the voltage supply imbalance is larger, the AC line current waveform becomes a more single pulse shape. The same amount of watts is again needed so the amplitude of the waveform has to increase as the period decreases to sustain the area. Larger voltage imbalance considerably increases the line current (Figure 5).
Increased line current flows via the diodes and related capacitors of the rectifier, which raises the stress on these components; the heat produced is due to the increased switching load. The high switching frequency causes a ripple on the supply line, leading to the existence of uncharacteristic triplen harmonics.1
Current industrial plants use contemporary VSDs that mostly include a Pulse Width Modulator (PWM) rectifier, where unbalanced voltage supplies generate increased line current — a reactive power (kVAR) increase and a 100 Hz ripple on the DC bus of the VSD. These phenomena can make the VSD to trip on under voltage, or over current on the DC bus. Technicians may find it difficult to trace a fault because the connected load may not show any apparent or instant faults.
Correcting Unbalanced Voltage Supply
It is almost impossible to supply a perfectly balanced voltage to industrial plants. Since single phase loads are switched unpredictably or distributed unevenly, a degree of asymmetry will always be there in the power supply system with little to no control of events on the distribution or transmission network.
ABB has designed and developed the PCS100 AVC-20 that has been specifically built to provide continuous and balanced voltage at the utility supply level. Whether the unbalanced voltage is due to an unbalanced single phase loading or unstable network supply within the plant, the PCS100 AVC-20 can continuously correct up to a 20% unbalanced voltage, even if there is a permanent voltage imbalance on the supply.
Network issues, or faults and loads, within the customers’ installation can cause the unbalanced voltage to randomly and continually fluctuate. The PCS100 AVC-20 corrects the unbalanced voltage in less than one cycle, or within 20 milliseconds, and keeps on correcting and adjusting the correction level as required to ±1% of the set nominal voltage.
The presence of a properly sized PCS100 AVC-20 on the low voltage supply to an industrial plant can prevent all the issues, costs, premature aging, and process interruptions that unbalanced voltage supply can cause to 3 phase rectifiers and 3 phase induction motors used in industrial plants.
1. Bollen, M. H. (2000). Understanding Power Quality Problems: Voltage Sags and Interruptions (1 ed.). (P. M. Anderson, Ed.) Gothenburg, Sweden: IEEE Press
2. Christopherson, N. (n.d.). Protecting Motors From Improper Voltages and Amperages. Retrieved October 20, 2015
3. Gosbell, V., Perera, S., & Smith, V. (2002). Voltage Unbalance. University of Wollongong, School of Electrical,Computer & Telecommunications Engineering. Wollongong: University of Wollongong. Retrieved October 20, 2015
4. Kazem, H., Zahawi, B., & Giaouris, D. (2009, February). Modelling of Six-Pulse Rectifier Operating Under Unbalanced Supply Conditions. ARPN Journal of Engineering and Applied Sciences, 4(No. 1), 71-75. Retrieved October 21, 2015: https:// www.staff.ncl.ac.uk/damian.giaouris/pdf/Papers/jeas_860.pdf
5. Pillay, P., Hofmann, P., & Manyage, M. (2002, December). Derating of Induction Motors Operating With a Combination of Unbalanced Voltages and Over or Undervoltages. IEEE Transactions on Energy Conversion, 17(4), 485 - 491. Retrieved October 19,2015: http://users.encs.concordia.ca/~pillay/12.pdf
This information has been sourced, reviewed and adapted from materials provided by Innovative ABB Power Conditioning - Discrete Automation and Motion Division.
For more information on this source, please visit ABB Power Conditioning - Discrete Automation and Motion Division.