Parallel lines

In an increasing number of mission critical applications, IT and communications managers are not simply looking for uninterruptible power support but for redundancy in power support systems. But a fault tolerant strategy can have its drawbacks – notably huge costs and space penalties.

In a single-module ups installation there are potential single points of failure. If a fault develops at any one, the critical load may be exposed to raw mains until the fault is repaired. But two or more ups can be connected in parallel to provide a fault tolerant system.

This comprises two or more ups modules operating in parallel to feed a common critical load bus, and is generally applicable to medium/high rated modules of on-line design.

Units forming part of such a multi-module system are almost identical in operation to that of their corresponding single module counterparts. In fact, some manufacturers design their ups modules so they can be used in either configuration without the need for complex modification (see figure 1).

Each module contains a static switch to provide a means of transferring the load between inverter and bypass. However, a certain amount of inter-module electronic control logic is added to ensure that all the modules’ static switches operate simultaneously when transferring from one power source to the other.

Damage would result if one module attempted to transfer its output to the bypass line while the others remained on inverter. Additional inverter control functionality is also required to facilitate inter-module load sharing and frequency synchronisation. These control signals, and others, are passed between the modules over lv control cables which are normally connected in a ring configuration to allow each module to communicate with every other module in the system.

One advantage of providing external input and output isolators for each of the modules is that it allows modules to be fully isolated and hot-swapped if necessary without disrupting the remainder of the system.

There are two major reasons for installing a parallel system. The first is to increase the effective ups capacity to enable the ‘system’ to power a larger load than is otherwise possible with a single module. The second is to introduce a measure of module redundancy to improve the anticipated system reliability. Parallel ups systems are therefore commonly categorised as either capacity or redundancy systems, although some are intelligent enough to operate as either depending on the prevailing circumstances.

Irrespective of the intended mode, all the modules forming part of a parallel system must be of the same type and rating – ie it is not possible to parallel a 30 kVA unit with one of 120 kVA.

Capacity systems

A capacity system is implemented by using the appropriate number of modules of a particular rating necessary to supply full load power when they are all operating and connected in parallel – for example three 60 kVA units might be used to serve a 170 kVA load.

Under normal circumstances each module will supply a maximum of approximately 57 kVA. However, if one module fails the remaining two modules will be expected to supply 85 kVA and would be substantially overloaded. In this situation the load will immediately transfer to bypass via the simultaneous operation of the static switch in every module.

If the faulty module is unable to operate its bypass-side static switch, the static switches in the remaining healthy modules will be sufficiently rated to sustain the full load supply.

When the problem is corrected on the faulty module and it again becomes available (ie the inverters are now operational on all three ups modules) the load will be transferred from the bypass back to the inverters automatically.

Redundancy systems

In a parallel redundant system, the number of modules forming the system is a minimum of one module over and above that required by the capacity system – for example, four 60 kVA modules used to serve a 170 kVA load. This allows the system to continue to support the load with inverter power if any one module shuts down, and thereby increases the system reliability.

In a four module system operating with one redundant module, the load will be maintained on processed inverter power if one module fails and will automatically transfer to bypass only after the failure of a second module.

If an ultra secure supply is required, the number of redundant modules can be increased. This type of system is referred to as an N+1 or N+2 system, depending on the number of redundant modules.

In the past, parallel systems were managed in a manner less ideal than is possible today using the latest technology. For example, in some older systems, the module redundancy was often based on ‘module availability’ rather than the prevailing load itself. Once the number of offline modules exceeded the designed number of redundant modules the load transferred to bypass irrespective of the load current demand and available remaining module kVA.

Modern systems are able to treat the matter of redundancy more effectively and flexibly.

Traditionally, parallel systems were configured as either capacity systems or redundancy systems. If a single module failed in a capacity system the critical load would be transferred to bypass irrespective of the size of the critical load and the available modules to handle it. Similarly, if the critical load exceeded the available non-redundant power, it would be transferred to bypass irrespective of the amount of redundant power available.

Uninterruptible Power Supplies is one company that offers a flexible yet tidy way of implementing an expandable parallel ups system by using a purpose-designed panel incorporating input and output switchgear for each ups module together with a system output isolator and a wrap-around bypass switch.

Whether paralleling is required for redundancy or upgradeability of power system output, this panel reduces the system design burden, simplifying cabling and shortening cable runs. It also minimises design and implementation complexity, saving time and costs.

Spare ways can be made available on the panel to permit the addition of further ups modules to provide extra capacity or redundancy at a later date, and additional ups can be connected and commissioned without any need to disrupt the load.

The panel enables electrical and physical isolation of individual ups modules for service and maintenance without a break to the load.

Electrical interlocks should be included to prevent the ups modules from being back fed due to incorrect switching.