Reducing Inrush Current through an Hospital Equipment Prevented Frequent Failures Dean. A. S. Sempio Corporate Business Group – Technical Services MERALCO, Philippines Email: cbg.techsupport@meralco.com.ph Introduction Computers and electronics are found almost anywhere today. They dominate the controls of your television set, manufacturing equipment and even in the specialized field of diagnostic medicine. Computers provide flexibility and ease of operation to medical diagnostic equipment; this in turn, has pushed the capabilities of these machines to levels unheard of before. Computers in diagnostic machines can provide detailed images of the brain without the having the patient risk an operation or make doctors see how a certain drug will affect a patient. The advances in the field of electronics and computers makes one forget that some things that haven’t changed much since the last half century. One of those things is power supply. Diagnostic equipment is invaluable to medical practitioners as they rely on them to provide information that will provide insights on the condition of patients. Hospital Equipment The hospital’s diagnostic equipment are among the most expensive but very valuable tools available today. It is not surprising that advances in medical equipment aided medical practitioners to accurately diagnose ailments and literally save lives. Hospital administrators are understandably concerned about the reliability of their equipment and in turn, concerned about the quality of power of their electrical system. In this study, a hospital’s medical equipment has been experiencing frequent failures despite the presence of a UPS regulating the equipment’s power supply. The equipment vendor has given up on the frequent failures and has finally blamed the power company on the cause of the problem. The hospital is not willing to write off the equipment, as they are relatively new and expensive. Power quality engineers were consulted by the hospital administrators to seek for solution on their problem.
Power Quality Study The Power Quality engineers interviewed the actual users of the hospital equipment and gained some insights on the history of the equipment. It seems that despite being brand new, the
diagnostic equipment never functioned for more than 6 months without experiencing failure. The installation of a UPS to regulate the supply voltage and a Transient Voltage Surge Suppressor (TVSS) did not mitigate or even reduced the frequency of the failures. The Power Quality engineers installed a Power Quality Analyzer at the point between one of the diagnostic equipment and its power supply. The data collected are shown below. Parameter
Minimum
RMS Voltage
95.53%
Voltage Unbalance
0.95%
Voltage Harmonic Distortion
0.94%
Average Vab – 100.47% Vbc – 101.02% Vca – 102.67% 1.44% Vab – 2.26% Vbc – 2.08%
Maximum 106.87% 1.97% 4.54%
Vca – 2.53%
The table shows the voltage variation based on the utilization voltage of the hospital equipment as well as the values for the voltage unbalance and voltage harmonics of the power supply. The data clearly shows that the quality of the voltage supply is within the operating parameters of the diagnostic or any other electronic equipment of the plus or minus 10% of the rated voltage and should not cause any problems for the equipment. However, one most probable cause of the equipment’s problem was recorded and is seen at the graph below:
The equipment draws current up to 150 amperes; the normal operating current of the equipment is only 40 amperes. The data prompted another round of interviews with the hospital personnel. The interviews confirmed the findings of the Power Quality engineers – the most common part of the hospital equipment that failed was the power supply fuse. The next component most likely to fail was the equipment’s power supply board, particularly the board’s inverters. The data shows that inrush current during the equipment’s operation reaches levels that is almost 3 times the equipment’s normal operating current. The equipment’s line conditioning devices – UPS and TVSS – are not capable of mitigating the effects of inrush current. Solution Inrush current is a normal phenomena observed during the starting of induction motors. Electronic devices exhibit similar characteristics; when the power supply inverters conduct electricity, the inverters are virtually short circuited and cause a large amount of current to pass through them. This is the inrush current that eventually damages the components over time. A reactor is simply a coiled wire that allows a magnetic field to form around the coil when current flows through it. It resists sudden changes in current, the way a capacitor resists sudden changes in voltage. A line reactor is simply a reactor placed in the supply side of the equipment. Any sudden change in current flow will be by the reactor. This is the inherent characteristics of an inductor. It was recommended that a line reactor be placed between the UPS and the equipment; the UPS will compensate for the voltage drop caused by the reactor installation.
To ensure compatibility, the equipment manufacturer was contacted and no objections were raised to the installation of a reactor between the equipment and the power supply. After installation, another week of power quality monitoring was done. The results are striking:
The inrush current was reduced from a high of 150 amperes to less than 65 amperes. This means that there is less current passing through the equipment, reducing the damaging events to it. If further proof is needed, the equipment was able to function for more than 3 years without experiencing failure. The hospital was able to fully utilize their diagnostic equipment as well as reduce their maintenance costs. CONCLUSION Electronics and motors both experience current inrush during their start-up. However, the solution for both cases is different – reduced voltage starters for motors and line reactors for electronics. Power quality data and correct analysis are needed to find the right solution to any power quality problem; without the proper combination, foreseen solutions may not address the power quality issues involved.