.gif)
RING BUS SYSTEM
In its search for a fail safe power protection system that can expand with ever-growing computer loads, the giant railroad transportation center discovered the hybrid UPS could be configured in a "Ring Bus" to meet their special requirements.
When the management team and the IS assessed mainframe computers for their new central data center, they had two missions: (1) to choose from a computer technology that moves, changes, and grows at a breathtaking speed, and (2) to select a UPS system that would meet their absolute requirements for reliability. Since the best mainframe technology in the world is only as good as its power supply, a consulting engineering firm was retained to define their UPS needs and evaluate vendor's ability to insure an Uninterruptible Power Supply technology that would allow maintenance and service, as well as growth, without disrupting the system.
The resulting design consisted of three interconnected UPS systems in a Ring Bus configuration to provide maximum flexibility and expansion. An output-paralleling switchboard configured in a Ring Bus tie allows UPS modules to operate singly or in tandem with tie breakers connecting individual switchboard sections. Not only does the Ring Bus provide paralleling of UPS modules for redundancy but also it offers near-infinite expandability. To implement the system with hardware of the highest possible reliability at the lowest possible cost, a request for proposals gave UPS vendors a great deal of latitude in module size, quantity, and technology. Vendors were required to substantiate reliability claims in their proposals. The system ultimately selected consisted of three 1250kVA hybrid UPS modules, connected in the required Ring Bus arrangement, and capable of operating independently or in parallel. The selection was based primarily on the proven performance of the selected module: 150,000 hours, which includes going to utility bypass as a failure. This level of reliability is achieved by keeping module parts count to an absolute minimum. Competing systems of the static type generally have much higher parts counts due to the requirement to filter their output to acceptable levels, and therefore generally have MTBF of approximately 20,000 hours.
Specifically, the system begins with three Caterpillar 1750kW standby engine generator sets connected to an input paralleling bus. The UPS consists of three Power Systems & Controls 1250 kVA HybridT UPS modules fed either by three separate utility feeds or the engine generators. Each UPS module is also configured so that the engine generators can be paralleled with the module, the load transferred, and then the module isolated for scheduled maintenance. All three UPS modules feed and output Ring Bus, which enables paralleling for redundancy and expandability.
The Ring Bus offers several distinct advantages over traditional paralleling schemes. Primary is the ability to have an expansion without AC power shutdown to the critical load during upgrade construction phases. Once the additional UPS modules and ring tie switchboards have been added, a four-hour window is needed to check paralleling capability during Ring Bus operations. No modification of existing equipment is necessary for expansion: new UPS modules and ring tie switchboards are "plugged" into the existing switchboard by using a bus duct raceway connected to a removable bus link. A Programmable Logic Controller (PLC) manages system operations and is configured for infinite expansion from two to three to four or more UPS modules. Additionally, the ring bus permits a site with multiple utility sources to keep the sources independent by its ability to run modules either in parallel or isolated modes facilitated by the use of multiple bypasses. This avoids the danger of any single point causing cascading system failure.
The consulting engineering firm considered that the most effective system is usually the simplest. Over designing can result in prohibitive expense or machinery so intricate that only the design engineer can operate it. Hybrid power conditioning, with its leisurely control circuits, online frequency control and stored energy in the motor-generator has proved to be a proper fit with a PLC based Ring Bus paralleling system.
Lastly, because lightning strikes can totally destroy a computer, and with the data center located in a very active thunderstorm area, UPS designs were evaluated for protection from lightning. The selected hybrid UPS, with its use of a synchronous motor coupled to a synchronous generator could provide total isolation and full hardware protection. Static UPS systems simply could not meet the requirements of achieve total isolation. The same mechanical interface that protects from lightning insures security of information, since the data from the computer cannot be transmitted via power lines to the outside world.
It goes without saying that an Uninterruptible Power Supply may not be interrupted for maintenance or repairs. The HybridT UPS permits periodic maintenance without operational impact. The installation of total maintenance bypass systems and inverter bypass systems can isolate critical power systems for maintenance as well as repair in the event of a catastrophic failure. While classic redundant UPS systems may give one extra UPS power module in a larger parallel system, they do not give infinite expandability while maintaining redundancy when the computer load grows. New configurations with Ring Bus switchgear provide numerous alternate power paths including engine generator maintenance bypass.
HybridT UPS may be the best kept secret in the industry, but there is a growing community of consulting engineers who are discovering this unique approach to difficult critical power problems. It has evolved over 37 years from rotary to static back to hybrid. Industry requirements and standards are high.
Power Systems & Controls, who produced the oldest continuously running trouble free UPS in the country, also backs its' products with a 24-hour nationwide field service operation. The track record for reliability and adaptability has won over companies as diverse as a large credit card processing center, one of the Federal Reserve Banks, and a large chip manufacturer.
As the evidence continues to build, more and more managers today can see how the hybrid system will serve them as the dynamic answer to their practical power quality problems.