How To Size A Power Supply For Diagnostic Imaging Systems

Transcription

1 Properly Sizing UPSs for Diagnostic Imaging Systems by David A. Rush, CPQ Power Quality Team Leader General Electric Medical Systems ABSTRACT Due to the dynamic power characteristics of diagnostic imaging systems like Magnetic Resonance Imaging (MRI), Computerized Tomography (CT), and X-ray imaging additional demands will be made on Uninterruptible Power Supplies (UPS) products that support them. When UPS products are used to protect diagnostic imaging systems they have to be selected and validated to assure that they will handle the unique power characteristic requirements of these systems. Not all UPS products are capable of supporting these types of load requirements and just making them bigger does not mean or guarantee that they will work. The UPS specifications, selection, validation, and testing with these diagnostic imaging systems will be discussed as well as possible problems that will be encountered and their solutions. Case studies of UPS products that do not support diagnostic imaging systems will also be discussed. Why UPS for Diagnostic Imaging Systems? Diagnostic imaging systems have historically been supplied power direct from the facility power distribution system with little if any power quality devices placed in front of the imaging systems. This started to change in the late 1970s and early 1980s with the introduction of Computerized Tomography Scanners (CT or CAT scanners). Problems started to appear in systems that did not make sense and appeared to be random in nature. Investigations determined that there were power problems that were creating operational or image quality issues with these systems. The initial efforts to address these problems were focused on reducing the exposure of the systems to transient voltages by placing Transient Voltage Surge Suppressor (TVSS) devices on the CT systems. This helped but did not fully address all of the power quality problems that continued to impact the CT scanners. In the late 80s voltage regulators and isolation transformers were also being installed in front of a lot of the systems being installed to try and mitigate the power problems that were impacting the operation of the systems. At the same time and in parallel with the power problems that were being experienced on CT systems new diagnostic imaging systems were being introduced that experienced similar types of power quality problems. Magnetic Resonance Imaging (MRI) was one of these systems that required quality power to operate and provide reliable as well as consistent imaging performance. TVSS and regulation DAR10/15/2003rev5 1 of 7

2 transformers were also being used in front of these systems to address the power quality problems. By the late 80s and early 90s the design engineering teams for all diagnostic imaging systems had began to incorporate power mitigating devices into their Power Distribution Unit (PDU) designs. These enhancements eliminated the need for the end use customer to install the TVSS or regulating transformers. This made the imaging systems more robust enabling the diagnostic imaging systems to work in environments that if installed in the same location ten years prior would have created system imaging or performance problems. Diagnostic imaging systems are an integral part of providing quality as well as timely care for patients so minimizing equipment down time and increasing patient throughput has become very important to all medical facilities. Medical facilities wanting to improve the operation of the diagnostic imaging systems have looked at power quality as one way to make that improvement. Because of the new PDU designs that added power-mitigating components, adding a TVSS or regulating transformer in front of the system provides minimal if any improvement to the system performance, the next logical step is to install a UPS in front of the imaging system. CT, MRI, Ultrasound, Positron Emission Tomography (PET), Nuclear Medicine, Cardiac Cath Labs, Vascular Labs, General Radiography X-ray, Mammography, and Information Technology systems are all potential candidates for the installation of a UPS to optimize their performance. Identifying the Proper UPS X-ray, MRI, and CT present some interesting problems that have to be considered when sizing a UPS for these systems. Each of these systems has very dynamic load characteristics and voltage regulation requirements that have to be considered. As an example, a typical CT system will have a Continuous power demand of 20 kva but a Maximum power demand of 90 kva for a few milliseconds or possibly seconds. During this time frame the voltage has to stay within 6% of the nominal line voltage value. As can be seen the CT system has a very dynamic power demand but some imaging systems are even more dynamic. A vascular x-ray generator capable of delivering 100kw of energy to an x-ray tube requires 171 kva of input energy. This is also a very dynamic load because the vascular system will only have a Continuous power demand of 5-10 kva. The power demand for a vascular system could go from 5 kva to 171 kva for milliseconds and back to 5 kva, this will be repeated up to 12 times/second. The described loads are very dynamic and in a lot of cases are not handled well by a number of UPS systems. To identify the proper UPS the operating characteristics of the CT, X-ray, and MRI diagnostic imaging systems needs to be clearly defined and provided to the UPS vendor s engineering team so they fully understand what type of loads will be placed on the UPS they will be providing. Following is a list of the critical operating characteristics that need to be provided to the UPS vendor: 1. Continuous power demand. 2. Instantaneous or maximum power demand. 3. The cycle time for these load demands. a. Definition of how long the maximum load demand will be required. b. Definition of how often the maximum demand will be repeated. 4. What the voltage regulation requirement is for the specific imaging system. DAR10/15/2003rev5 2 of 7

3 5. Details on what type of front-end rectifier will be used in the diagnostic imaging system that will be pulling the power, for example is it a 6 or 12 pulse design. All diagnostic imaging systems do not have dynamic power demands. Nuclear Medicine, PET, Ultrasound, and Information Technology systems have a fairly constant power demand so as a general rule it is easier to size a UPS for them. Although there are a couple of caveats that need to be consider: 1. Ultrasound systems are very sensitive to RF noise. RF noise, which could compromise the operation of the Ultrasound system, can be either radiated or conducted. The inverters in the UPS can potentially be a major source of RF interference for the Ultrasound system and cause imaging problems. 2. Nuclear Medicine cameras can have an imaging head gantry that is driven by motors that could require large momentary current demands that might be larger than the kva available from the UPS. Because of this the UPS will possibly need to be sized significantly larger than the continuous power demand the system would appear to require. UPS Validation After a decision has been made to test a specific UPS for a diagnostic imaging system, then validation testing in a controlled environment will need to be done. The preferred location to do this testing would be in an engineering test bay. This takes a lot of variables out of the testing and allows the product design engineering team to participate in the UPS validation testing. A validation testing document should be developed that details who, how, what, when, and where for the testing that will be done. Following is a list of some of the things that should be included in this document. 1. Recommended test equipment that will be used to record the performance of the UPS. a. Where the test equipment will be connected during the testing. b. How the test equipment will be connected. c. Data recorded from the test equipment should be electronic so it can be pasted into the validation document. d. Test Equipment Utilized Manufacturer: Model Number: Serial Number: Cal date: 2. Imaging system to be tested defined by model number, name, and serial #. 3. UPS being tested. a. Model Number: b. KVA Rating: c. Manufacturer Date d. Data Section: [Data acquired while performing the testing should be pasted into the document, along with any other pertinent information regarding the testing that was performed.] 4. Sign offs from design engineering team that the UPS is supporting the diagnostic imaging system. DAR10/15/2003rev5 3 of 7

4 5. Details of exactly how the system will be operated and exactly what tests will be done to assure that the system will have been operated at it s highest or worse case power demand. (It is extremely important that this be done in as much detail as possible with the system design engineers before testing is started.) UPS Validation Testing Once all the plans and processes are in place the validation testing can be done with the diagnostic imaging system and the UPS that has been selected to support it. If at all possible have a technical service engineer from the UPS vendor present when the tests are being done. There are two main reasons for this coordination with the UPS vendor: 1. If there are any technical problems or questions concerning the UPS system or installation issues they can be taken care of immediately. This is more critical when testing large UPS systems that are hardwired, for the smaller plug and play UPS products having a representative on site is not as critical but has always been beneficial. 2. If the UPS does not carry the intended load the UPS vendor s representative is a very good channel for carrying that information back. By them being on site they will be in a better position to explain how the UPS was connected and explain exactly where and how the data was collected. This can dramatically shorten the time that it will take to resolve problems. After the testing has been completed in a controlled environment such as an engineering cell then the same tests need to be done at an end users site. The testing done at the customer site tests not only the operation of the UPS but also the shipping, installation directions, and commissioning of the product. The best product in the world if installed wrong will perform very poorly and create more problems than it solves. UPS Sizing Misconceptions If the diagnostic imaging system has a continuous load of 20 kva then a 20 kva UPS should handle it. Wrong, the continuous, average or nominal power rating listed for most diagnostic imaging systems is the power demand when the system is setting at an idle state. When a procedure is done on the system the power demand could go many times larger than the continuous value listed. The load is only momentary so the UPS can be undersized. Wrong, the diagnostic imaging system has voltage tolerance variations that it has to stay within to work as it was designed. If the UPS is undersized the peak load of the system will cause the voltage to go out of that tolerance range and cause image artifacts, system errors, lockups, or possibly hardware failures. If the diagnostic imaging system load is 60 kva then just doubling the size of the UPS up to 120 kva will take care of the voltage regulation concerns. Wrong, the ability of the UPS inverters to handle the dynamic step load is the deciding factor. Doubling or tripling the size of the UPS might not enable the UPS to meet the voltage regulation requirements of the diagnostic imaging system. DAR10/15/2003rev5 4 of 7

5 The UPS in our facility providing power to the medical records department computer system has worked great with no problems. Because of the history with this vendor in our facility and the quality of their UPS we feel that it is a good product and will support diagnostic imaging equipment. Wrong, the UPS that has been carrying the computer in the medical records department is probably an excellent UPS for data processing equipment. When placed in front of a diagnostic imaging system the demands placed on the UPS will be dramatically different. The UPS might work but because the loads are so different it might not work well and be able to maintain the voltage regulation that is required for the diagnostic imaging system. UPS Validation Case Study #1 Request was made by one of the GE Medical Systems (GEMS) groups to test a UPS made in the country where several GEMS MRI systems were to be installed. This would save dollars in shipping, import-export taxes, and make the logistics easier for the local team in country. Arrangements were made for the UPS to be shipped in for testing on the MRI system. The UPS that had previously been validated and was working well on this particular MRI system was an 80 kva UPS provided by another vendor. All of the power specifications, and load profiles were provided to the UPS vendor so they could make the proper selection of UPS for this MRI application. After the UPS vendor s engineering team reviewed the power specifications they determined that a 150 kva UPS would be best able to carry the MRI load. The 150 kva UPS was received and moved to a testing area to validate it s ability to handle the MRI system. When the first imaging sequence was made on the MRI the UPS immediately went to by-pass. The UPS vendor s representative that was on site when the testing was done made sure that the system was fully operational. All of the current and voltage data was given to the UPS vendor representative, which he took back and gave to the UPS engineering team. A much larger UPS was selected and used with this MRI system. Just increasing the size of the UPS does not always guarantee that it will work. UPS Validation Case Study #2 UPS was being validated for use with an x-ray system. When an exposure was made the unit would immediately go to by-pass with an error code indication on the UPS that the maximum current for the UPS had been exceeded. Current readings that were recorded during the testing indicated that the unit had never exceeded the UPS maximum current limit. The UPS vendor representative that was on site during this testing called the UPS engineering team and explained the problem to them. It was determined that the fast rise time in the current demand waveform when x-ray was initiated was causing an anticipator circuit to trip prematurely. The UPS engineering team worked out a resolution that involved a minor modification to the UPS, which solved the problem. Hospital UPS Case Study #3 Customer had a series of minor power outages over a period of several months with one major power outage caused by a direct lightning strike to the utility substation that provided power to the hospital. The major outage took place when two critical DAR10/15/2003rev5 5 of 7

6 procedures were being done on two different diagnostic imaging systems at the same time. After this event the hospital investigated several different options and it was determined that a UPS for the diagnostic imaging systems would be the best option. A multiple room UPS was recommended by GE Medical Systems but the purchasing department in the hospital wanted to stay with the UPS vendor that provided UPS products for their data processing department. The decision was made by the customer to go with that vendor instead. Because that UPS vendor had never been validated with diagnostic imaging systems and it was anticipated that additional loads might be added on to it in the future the UPS was increased in kva above the kva of the UPS recommended by GEMS. The UPS was installed in the facility and put on line. The diagnostic imaging systems started to have major problems with random error codes and system lock ups that required a reboot of a system for it to become operational again. The UPS was the suspected source of all these problems and after monitoring the power with test equipment it was determined that it was indeed the culprit. It was discovered during this testing that any one system acquiring images would cause dramatic voltage regulation problems with the output of the UPS and disrupt the operation of all the other systems powered by the UPS. The UPS preferred by the hospital was replaced with the original UPS that was recommended by GEMS. Since the UPS has been installed the anticipated additional loads have been placed on it. During the time that the UPS has been installed in front of the diagnostic imaging systems the customer has experienced no down time or operational issues with any of the diagnostic imaging systems and during this time there have been several power outages experienced by the facility. During these outages procedures continued with no interruption or degradation of the performance of any of the diagnostic imaging systems supported by the UPS. Conclusion Identifying the proper UPS for diagnostic imaging systems requires more than just looking at the placard on the equipment that gives the operational specifications. UPS vendor engineering team needs to be involved with the selection of the proper UPS for each different type of diagnostic imaging system. Diagnostic imaging systems such as MRI, CT, or X-ray have very dynamic load characteristics. Written UPS validation plans needs to be agreed upon by all parties involved with the UPS validation process. UPS validation is best preformed in engineering, manufacturing, or other controlled environment test facilities. All UPS products do not respond in the same way to the very dynamic load requirements of diagnostic imaging systems. UPS products that work very well for data process applications might not work well for diagnostic imaging systems such as MRI, CT, or X-ray. DAR10/15/2003rev5 6 of 7

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