Protecting Mission-Critical Systems with Parallel UPS Technology

By Jorma Mannerkoski
Dec 16 - 22, 2002

We are increasingly dependent on electrical and electronic equipment in our everyday lives. The proper functioning of this equipment depends on a good supply of electricity. Unfortunately, many people wrongly assume that the only time to worry about the effect of power on computer and other electronic equipment is when there is a total power cut. Of course this kind of protection is essential and one must never underestimate the effect total power cuts can have on a business and on our lives. However, nowadays power cuts in the western world are very infrequent and what has the greatest impact on businesses are the fluctuations and disturbances in mains power, which are unseen. Companies need to rid themselves of the risk of the chance that a power disturbance or blackout could interrupt their bushiness and yet figures still show that many organisations fail to protect their business with the correct UPS back up solution.

What a UPS does is not only provide back up power in the event of complete power failure but also takes the input electricity, cleans it up and outputs what is called 'pure sinewave'. For the most part this solution works well for those seeking reliable power protection and is highly recommended for industries running any kind of mission critical systems.


However, as the world around us continues to change the need for businesses to be up and running 24/7 mean organisations need an even higher level of reliability. Today, competition. extended shopping hours more demanding customers, virtual shopping, mobile connectivity, the Internet, to name just a few things, have combined to change the face of the world in which companies and organisations must compete.

To have computer systems, electrical and electronic equipment, phone lines up and running is now so critical to an organisation's survival that failure due to power problems cannot be tolerated. Organisations such as banks, Internet Service Providers, telecommunication companies and anyone surviving in the thriving e-commerce industry cannot afford to have their business to go down at anytime. If it does it can lead to loss of profits and loss of customers perhaps the most damaging effect of all.

For piece of mind they need to know that their chosen back up solution can never fail. They need to know that if anything goes wrong with one UPS then another takes over at once. Organisations need redundancy. And the solution? To run a parallel UPS system.


Put simply a parallel UPS system means the linking together of two or more UPS units in parallel so that in the unlikely event one fails the other can automatically take up the load. When buying a UPS in a situation that requires a redundant solution there are two types to investigate. You can install a parallel redundant system or an isolated redundant system.

Redundancy in an isolated (or standby) system configuration depends completely on the on the successful transfer of the load by the on-line (primary) UPS to its bypass and the graceful acceptance of that load by the stand-by (secondary) UPS. Theoretically, it could be a 100% load step, which is a serious transfer.

To successfully obtain a transfer it requires the following four step procedure:


The primary unit must recognize that a transfer is required.


The primary unit must ensure that the transfer is feasible:

- Bypass source is available - Bypass source is in sync

* Voltage is within accepted limits

* Frequency is within accepted limits


The actual transfer action has to take place:

- Static switch must turn on

- The primary unit's inverter must disengage

- If there is a wrap-around breaker, it must close successfully.


The secondary UPS must accept the load step and maintain a voltage variation within limits that are acceptable to the critical load. Actually, since the secondary unit is not supporting any load, one cannot be certain it is operational and will accept the transfer at all until it is attempted. What a time to find out there is a problem!

These four steps are in series. If any one of the four steps fail, the transfer fails and the load will be dumped. Remember. this is an emergency situation. There is no room for a slip. If the reason for transfer is a faulted bus, the transfer must take place TWICE before the load bus is attached to a power source capable of clearing the fault (through the bypass of the secondary UPS). If the reason for transfer is a failed primary UPS, the module cannot be isolated for repair because the secondary UPS is feeding the load through the bypass of the primary UPS. If there is a maintenance bypass, the load can' of course, be transferred to it but if UNPROTECTED power is acceptable, why spend the money for redundancy?

A parallel redundant systems, and the systems which Invensys Power Systems strongly supports, has a number of advantages that increase the availability of protected power to the load and gives the customer fewer things to worry about where support of the critical load is concerned.

With a parallel redundant system the following six points outline the advantages:

1. The two most critical elements in the isolated redundant scheme are eliminated: Execution and Acceptance of the load transfer.
2. The Qualification stage is minimised because the normal operation of a parallel redundant configuration is for the modules to share the load, which means they must be in sync for both voltage and frequency.
3. If a module is taken off line for maintenance, the remaining module AT WORST will suffer a 50% load step, which substantially reduces the risk of voltage variations outside the limits of the critical load (its safe to say the risk is eliminated). You can be sure the on-line module is operational and will accept the transfer because it is already sharing the load.
4. The unit that is off-line is isolated so maintenance can be performed.
5. Protected power to the load is maintained by the on-line unit and automatic transfer to bypass is still an option if necessary.
6. If a faulted bus occurs, only one transfer is required to reach a power source (the feeder bus) capable of clearing the fault.

To make a parallel system work you will always rely on the transfer to bypass happening successfully with the secondary module always accepting the load gracefully. But if it doesn't you are back to where you started. A parallel redundant system eliminates the possibility of this happening.


When installing a parallel redundant system, in other words to get two or more units to run parallel and share the load the units must be locked to exactly the same phase. You must then synchronise the modules, connect them together through the collective buses of the system thus allowing them to share the load between them.

The system is now ready to achieve redundancy. Load the system with power equal to the total demand of all the loads minus the demand of a least one module. This ensures that failure of any one UPS will not corrupt the power, will allow one to isolate the damaged module whilst the functioning UPS modules seamlessly take over without the load ever noticing the transfer.


Doesn't all this sound great. However we still have one problem. Traditionally a parallel redundancy configuration is achieved by having a common paralleling logic circuitry in the system. This logic circuitry gives out individual commands to all the different units. Unfortunately this can lead to a single point of failure for the whole system because if the logic circuitry system fails the whole UPS system will be in trouble. And so what you are left with is a very complicated configuration that at the end of the day and whilst this gives you a 98% solid power protection it does not eliminate the risk of a single point of failure keeping you from receiving 100% peace of mind.

That is exactly why the Powerware Hot SyncŪ technology was developed. An industry leading paralleling technology in its own right, the patented Hot SyncŪ enables you to setup a parallel redundant system giving you 100% conditioned power at all times. Its unique digital design eliminates the system-level single point-of-failure inherent in traditional parallel UPS modules and exponentially increases the reliability of the overall system.


Hot SyncŪ allows two or more UPS units to cover the same load in parallel and redundant configuration, by working in complete synchronisation without being wired together. No common logic is needed h1 this patented design It provides automatic load sharing and module level redundancy with nothing other than the power connecting the modules.

Hot SyncŪ does not require any centralised commands to function. All units are able to function on their own and make decisions simply by looking at their own output, the UPS modules do not need any communication between them to load share or to remove a faulty module from the load bus. If one module fails the other or others continue supplying the load and the failed UPS module is immediately isolated from the critical bus while the other UPS takes over the full load. This means there is no single point of failure, no crashes and most importantly no downtime. In short the critical load remains 100% UPS protected.

It manages to share the load by making minute changes to the output phase angle 1000 times per second. If one of the UPS modules detects that it is getting ahead of the other it will push its extra load back to the other module until finally the load is evenly shared. If the load changes all UPSs will see it instantaneously and in tile same the way. That's why an even load share is maintained even if the load varies.


Phenomenal growth of super-servers, telecommunication switching centres, and other large, centralised computing operations, has created a market need for extremely large power solutions. This need typically exceeds the capacity of a single UPS module. To address this shortfall, Hot SyncŪ enables UPS modules to be tied together to create a UPS solution that protects the entire system, and allows additional modules to be added as capacity needs grow. It allows customers to expand effortlessly from a single-module system to a two-module redundant system and, potentially, to an eight-module system. The scalability of this system provides customers the flexibility to add the power protection as their application grows. And it also makes redundancy for these very large systems a reality. increasing system availability and overall reliability.


Invensys Powerware is a leading global provider of engineered power solutions. Offering the broadest range of product and services available today, Invensys Powerware integrates a full line of AC and DC power systems, power management software, remote monitoring, turnkey integration services and site support, providing a seamless solution. Powerware systems and services deliver the high nines of availability demanded by today's digital economy. Powerware products and services are utilized in local and wide area networking, data and voice over IP, co-location facilities, fixed-line and wireless communication networks, and industrial manufacturing. Invensys Powerware is headquartered in Raleigh, NC, and is part of Invensys plc.


Invensys plc is a global leader in production technology and energy management. The group helps customers improve their performance and profitability using innovative services and technologies and a deep understanding of their industries and applications.

Invensys Energy Management works with clients involved in the supply, measurement and consumption of energy and water, to reduce costs and waste and improve the efficiency, reliability and security of power supply. The division includes Energy Management Solutions, Appliance Controls, Climate Controls, Global Services, Metering Systems, Powerware and Home Control Systems. These businesses focus on markets connected with power and energy infrastructure for industrial, commercial and residential buildings. The company also serves the specialized rail, wind-power and electronic manufacturing (power components) markets through Invensys Rail Systems, Hansen Transmissions and Lambda, respectively, in its development division. Invensys operates in more than 80 countries, with its headquarters in London.

The writer is Product Marketing Director EMEA, Invensys Powerware