On December 10, over 300 clients, professionals, and the academic community joined together at a technical seminar on modular uninterruptible power supply (UPS), which was hosted by China Data Center Committee (CDCC) and China Power Supply Association (CPAS) and co-organized by Huawei.
At this UPS industry event, Huawei, for the first time, demonstrated the super-capacity system of eight paralleled 800 kVA/W UPSs, which is composed of 160 40-kVA UPS modules. Huawei conducted stringent onsite tests which were witnessed by engineers from TL Certification Center (TLC) and over 300 seminar attendees.
A total of 18 test procedures were performed; nine on single UPS and nine on the parallel system. The Procedures are tightly interwoven, aiming to relieve customer’ major concerns, as illustrated in the following figure.
Right from the beginning, Huawei modular UPSs preceded conventional tower-mounted UPSs by a landslide in same-scenario and maintenance duration comparison tests. Then, as the most eye-catching part, an array of reliability tests were carried out, convincing the attendees that reliability comes from the design and is verifiable.
The three themes for the testing were module reliability and performance, fault isolation in extreme conditions, and loop current of the super-capacity system. The following are the details of this unprecedented technical event.
Round 1: How to Guarantee the UPS’s Reliability and Performance in Poor Power Grids and Harsh Environment?
Reliability of the entire UPS or parallel system depends on reliability of each single UPS module. According to Lv Yihang, General Manager for the UPS Product at Huawei, a reliable UPS module shall exhibit the characteristics of “Three Friendly”: grid friendly, load friendly, and environment friendly.
In onsite tests, Huawei showed highly distorted power-grid waveforms that are very commonplace in reality, such as those captured in the cities of Chengde and Anshan in China. As verified in the tests, the UPS can still operate stably under a square wave input where the harmonic wave components accounted for up to 30%.
Onsite tests verified the full-load working capability of the UPS under various types of loads, such as RCD, pure capacitive, pure inductive, and pure resistive.
The environment friendly characteristic poses requirements on UPSs’ adaptability to diversified temperatures, humidity levels, dust levels, and weather conditions such as salt spray. Huawei selected a coastal field in Hainan, which is only 300 meters away from the sea, for conducting the test on environment friendliness. In this test, the UPS module demonstrated its capability to operate continuously in full-load, high-temperature, and low-pressure conditions.
With high temperatures, high humidity, frequent thunderstorms, and high salt spray in the air, the test field in Hainan is the best choice for verifying equipment reliability. In such an environment, the aging rate of the equipment is dozens of times that in general poor environment. By constantly improving the design weaknesses exposed in the Hainan test field, Huawei has managed against all odds to have the modular UPSs run for over two years.
Round 2: Fault Isolation in Extreme Conditions
Lv Yihang pointed out that achieving reliability of a single UPS module does not necessarily guarantee that of the modular UPSs. The system is composed of multiple modules connected in parallel. A fault in any module will incur faults in other modules and even power output failures in the whole system. A system without any fault isolation mechanism is less reliable even than the conventional tower-mounted UPS system, which is also the concern of some customers when they consider using modular UPSs.
To ensure that faults in a single module do not affect other modules or the system, Huawei employs redundancy designs on critical components, such as the main control unit, communication bus, auxiliary power supplies, and fans, in Huawei modular UPSs. Also, fault isolating mechanisms, such as rapid detecting and mutual backup between software and hardware, are implemented for the input, output, and battery ports of the UPS. In this way, any module with faults is promptly isolated from the system.
At the instantaneous moment when the short circuit occurred, the module blared out and released light produced by electricity. Field engineers immediately ran to determine the UPS output state. Although the insulated gate bipolar transistor (IGBT) in the inverter was damaged, the UPS kept working in inverter mode without transferring to bypass mode or generating any flame or smog. In addition, the monitoring system correctly reported the address of the faulty module to prompt for maintenance.
Round 3: Paralleling of 10 UPS Modules Already Reaches the Limit? No!
The appropriate number of paralleled UPS modules has long been the topic of debate in the UPS industry. At early stages, most people held the opinion that no more than eight or ten UPS modules shall be paralleled. This was based on the following reasoning: The UPS is an alternating voltage power supplier; to parallel such devices, synchronization in the phase, frequency, and amplitude must all be achieved, which requires powerful processors as well as high-speed and real-time inter-module communication. This, indeed, was a big challenge before 2000. However, with the rapid development of digitization, the communication data rates have increased by at least 100 times from the kbit/s level to the Mbit/s level, and the processing rates of DSP chips have also surged from the MHz level to the GHz level. These two combined helped eliminate the technical bottlenecks in module paralleling.
Asynchronization in any of the three aspects will result in uncontrollable loop current of the parallel system. Therefore, the most important indicator of paralleling performance and reliability is the loop current of the parallel system, which must be less than 5% according to the current industry standards. When four UPSs are paralleled using the mainstream tower-mounted mode in the industry, the loop current approximates 5%.
Lv Yihang invited engineers to conduct an onsite test on the loop current of the system of 160 paralleled UPS modules. The test result was 1.7% at maximum, significantly lower than the industry standard.
According to Lv Yihang, with the current technologies, mainstream industry manufacturers can implement paralleling of 20 to 40 modules. Continuously and unremittingly, Huawei has invested in and accumulated extensive experience from the research and development (R&D) in such areas as communication, chip designing, and power electronics. This, in combination with design concepts that are digital, network-based, and intelligent-oriented, guarantees Huawei an unmatched advantage and a leading position in modular UPS design. In the future, Huawei will continue its investment in modular UPS design and bring products with higher reliability and lower total cost of ownership (TCO) to its customers.