There are several categories of static UPS systems available. Broadly speaking, UPS Modules fall within one of three operational design architectures, namely off-line, line interactive and on-line. However, irrespective of their individual design criteria certain features are common to all forms of static UPS systems - i.e., they all contain batteries which store energy when the mains supply is available and a means of converting the battery charge into an alternating current (ac) supply in times of mains failure. All systems must therefore include a battery charger and a power inverter circuit. As described above, the battery provides a power source for the inverter when the mains supply fails, whereupon it discharges at a rate determined by the critical load connected to the UPS output. The inverter automatically shuts down when its dc supply falls below a certain voltage, therefore the duration for which the critical load can be supported in times of mains failure depends upon the battery capacity and the percentage applied load. A typical UPS system, which may be referred to as, Uninteruptible power supplies, Uninteruptable power supplies, Uninterruptable power supplies, Uninterruptible power supplies, Energy Systems, or simply UPS, will contain sufficient battery capacity to support its fully rated output load for 5 to 15 minutes. However, in most cases this can be extended by adding further battery cabinets or selecting batteries of a higher capacity. The battery backup time is often referred to as the autonomy time. Virtually all systems contain a `bypass` system which, in conjunction with some form of output switching circuit, provides a means of connection the critical load directly to the mains supply. In most cases the output switching circuit is implemented using solid-state switching devices. The rules governing the static switch control depend on the UPS operating mode. Line-Interactive System This type of UPS covers a range of hybrid devices that attempt to offer a higher level of performance than conventional off-line designs by adding voltage regulation features in the bypass line. The two most popular types of system in this category employ either a buck/boot transformer or a ferroresonant transformer. Like off-line models, line-interactive UPS normally supply the critical load through the bypass line and transfer it to the inverter in the event of a bypass supply failure. The battery, charger and inverter power blocks are utilised in the same manner as in an off-line system but due to the added `regulation` circuits in the bypass line the load is transferred to the battery-fed inverter supply less often, making this type of system slightly more efficient in terms of running cots and batter `wear` compared with an off-line system. Buck/Boost Transformer Design One of the drawbacks of the straightforward off-line design is that the load must be transferred to the inverter immediately the bypass supply voltage reaches voltage limits acceptable to the load. This means that the UPS might transfer between bypass and inverter quite frequently if it is set up to operate with a critical load having a tight voltage tolerance. Apart from the power break each time this occurs, this method of operation incurs frequent battery usage which reduces battery life and might perhaps result in a battery that is inadequately charged when it is called upon to support a prolonged mains blackout. A buck-boost transformer connected in the bypass line helps overcome this problem. The transformer has tapped secondary windings which are selected by relays to either step-up or step-down the bypass voltage as appropriate to maintain the UPS output voltage within the required output voltage limits. This means of controlling the output voltage permits a wider variation of bypass voltage to exist before the output voltage reaches its limits and initiates a load transfer to inverter. A typical UPS in this category will sustain the load voltage over a bypass voltage range of +20. Note that although the output voltage is maintained within its preferred window using this method, buck/boot switching unavoidably leads to a degree of step voltage changes as tap changes take place. On-Line System An immediate difference between this design and the previously described off-line system is that the battery charger is replaced by a `rectifier/charger` black. The rectifier/charger may be two separate units or a combined power black. When the mains supply is present this black float charges the battery and supplies the inverter with a stable de voltage. In the absence of the mains supply the charger shuts down and the inverter dc supply is provided by the battery, which begins to discharge. The connection between the rectifier/battery and inverter is often known as the dc busbar , or dc bus. As part of its control function the rectifier/charger generally includes an input current limit feature to provide overload protection an a dc overvoltage shutdown mechanism to protect the batter/inverter and dc filter components This UPS design, which is sometimes also referred to as a double conversion UPS, offers the greatest degree of critical supply integrity in that the load is supplied with processed power at all times. That is, when the UPS input mains supply is resent the rectifier, charger and inverter power blocks are all active and the load is connected to the inverter output via the static switch. As the load is powered from the inverter under normal circumstances it is well protected from input supply aberrations because the rectifier and inverter act as a barrier to mains borne noise and transient voltage excursions, in addition to providing a well regulated output voltage. If the input supply goes outside a preset voltage range (typically +10), or suffers a total failure, the inverter continues operating from battery power and the event is totally transparent to the load as there is no transfer operation involved. When operating from battery power the inverter supplies the same degree of supply regulation as when the main is present. If the mains is not restored before the battery reaches its end-of-discharge voltage the inverter shuts down and, in some models, the static switch may attempt to transfer the load to the bypass line. The result of the transfer action depends on whether or not the module`s bypass line is connected to the same mains supply as the modules rectifier, and if the bypass supply is live. (Known as a split bypass system.) What Happens if the UPS Fails? A UPS Fault is generally seen as the inability of the inverter to provide the correct voltage or requency at the UPS output terminals and the resulting actions that take place may vary between models. Usually, the UPS control logic will detect the failing output voltage/frequency as the fault occurs and immediately signal the static switch control system to transfer the load to the bypass line in a make-before-break fashions. However, if the inverter is not synchronised to the bypass supply when the transfer is called fro it will be impossible to perform a break-free transfer operation, consequently there will be a brief supply break while the transfer takes place. These are the only circumstances under which the load is subjected to a (brief) supply break in a true on-line ups system. Note that although the break-free transfer to bypass is transparent to the load it is no longer supplied with processed power once it is transferred to the bypass supply; also, if the bypass supply is unavailable when the `fault` transfer is necessary a total loss of power to the critical load is unavoidable. The static switch usually transfers the critical load back to the inverter automatically once the inverter fault clears. This feature is occasionally described as auto-retransfer. The response of an on-line system to an output overload is usually similar to that of the UPS failure described above in that the load is transferred to bypass until the cause of the overload clears whereupon it automatically re-transfers back to the inverter. If the bypass supply is unavailable this will lead to a total loss of load supply, therefore some systems allow an overload condition to continue to be supplied from the inverter for a finite time ? that is the UPS equipment is able to supply enough current to a faulty piece of load equipment to ensure that the load protection fuse or circuit breaker will automatically disconnect if from the UPS. While feeding the overload under these circumstances the inverter operates in a current-limit mode and its output voltage may be reduced deliberately, but in most cases this is preferable to total power loss and of course conditions will return to normal if the overload is cleared during the allotted time. Parallel Systems This type of system comprises two or more UPS modules sometimes referred to 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 deign their UPS modules such that they can be used in either configuration without the need for complex modification. 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 module's 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 which 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 low voltage 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 that is otherwise possible with 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 - i.e. it is not possible to parallel a 30kVA unit with one of 120kVA. Installing the UPS Delivery and Positioning The importance of planning the installation and delivery of the UPS system cannot be overstated. Having chosen a particular system and topology it is important to decide:1. Will the system fit into the space reserved for it? 2. Is the proposed location suitable? 3. Can and how will the system be transported to the location Size & Weight Improvements in UPS technology and design have provided much higher power densities which when combined with the flexible installation options for modern parallel systems make it much easier to find space for UPS systems. Also, because the most modern designs no longer need bulky and heavy input transformers, installation of very powerful UPS systems is no longer limited to the ground floor or basement plant room. The manufacturer or supplier will provide details of space requirements and details of module weights in the UPS system specification. Be sure to consider possible future expansion when choosing a UPS location and if you can allow extra space over and above the manufacturers recommended minimum, maintenance and service will be easier. A UPS system is not just a big battery box. It contains electronic components similar to those found in computers and therefore requires careful handling when being transported. Additionally, large UPS equipment will be heavy and unwieldy and will require specialist contractors using `air-ride` suspension vehicles and specialised lifting equipment to unload and position it. The UPS supplier should be able to recommend handling procedures and suitable contractors with experience in this field. Choosing a Suitable Location The choice of a particular installation location for the UPS depends on many things: 1. How much space is available? 2. Can the floor safely support the weight of the equipment? 3. Will the installation cause continued inconvenience to the existing personnel and business? 4. Are the environmental conditions at the chosen location suitable? 5. Can access to the UPS equipment be made secure yet convenient? 6. Does the UPS comprise one module or several in parallel? 7. What is the effect of the installation on existing air flow and air conditioning equipment? 8. Will the switchgear controlling the UPS be in the same area? 9. Can the chose area safely accommodate the battery installation? In general the location chosen for modern UPS can be summarised as follows: Small UPS - less that about 20kVA, can be installed in a normal office environment although care should be taken to ensure that the additional noise and heat does not adversely effect the office environment. Medium UPS - between 20 and 100kVA are designed to be installed in computer rooms.Large UPS - greater that 100kVA, will usually be located either in a separate UPS room or in an existing plant room Transporting the System Having chosen a suitable location to suit the UPS system it is vital to survey the proposed transportation route. If a specialist delivery contractor has been employed for the task they will usually undertake a site access survey before attempting to deliver any equipment. Even if the location chosen for the installation could in fact accommodate an addional three or four UPS, access to the area may prove problematic. Check the access route: 1. Is the site easily accessible by road? Bear in mind the size of the delivery vehicle and the equipment required to off-load the UPS. 1. Are all doorways large enough for the UPS equipment and any transportation equipment to pass through? 2. Ensure the equipment can be moved along the entire route especially around corners. 3. Will the UPS need to be carried across soft or uneven surfaces? 4. Are there any stairs between the off-loading point and the final location? 5. If the equipment must be transported using a goods lift, chick that the lift has the required capacity. 6. Ensure that site staff are aware the equipment is being delivered and have made every effort to ensure that access along the route is unhindered on the day of delivery Environmental Considerations Heat All UPS manufacturers will quote a maximum operating temperature for their equipment (typically +40C). The air conditioning plant must have sufficient capacity to maintain the conditions stated. Obviously the overall efficiency of the UPS will have a significant effect on both the size and the operating cost of the air conditioning plant. The high efficiency figures (up to 97). Whilst most UPS equipment is well designed, high relative humidity levels may promote corrosion of cabinets and internal parts. Simple dehumidification equipment is available for sites where this may be a problem. Audible Noise The unit of sound intensity is the decibel (dB) and it represents the ratio between the sound level measured with a microphone and a reference sound level, Odb, which is defined to be approximately equal to the threshold of human hearing. However as the human ear is less sensitive to very low and very high frequencies, an additional `A` Filter is applied when measuring background or other intrusive noises, hence the dBA unit used by all UPS manufacturers. Typical audible noise figures for fully loaded UPS equipment range from 50dBA for 5dVA to 60dBA at 60kVA. Electrical Installation Installation Contractors Electrically installing a UPS, sometimes refer to as or protected power system is a specialised task and should only be performed by a qualified and experienced electrical contractor. The supplier of the UPS equipment should be able to undertake the installation work or supply a list of suitable contractors who can provide references of previous installations. Take the time to: 1. Check the credentials of the staff who will be installing the equipment 2. Contact and investigate previous installations and discuss their work with the staff on the other sites. It is important to ensure that the installation is carried out in strict accordance with the supplier's instructions and it complies with local and national electrical installation regulations. Installation Design Small and medium sized UPS equipment will probable require very little installation work and minimal changes to the existing electrical wiring. However, if larger, high-power UPS equipment is being installed then careful consideration of the switchgear and cabling arrangements must be made. Considerable time and therefore cost savings can be made by carefully planning the electrical installation to allow for possible business growth and the addition of extra UPS Modules. Using an integrated switchgear and bushbar solution, makes the installation process for a modern parallel system much simpler by: 1. Providing a single point of entry for the incoming mains supply 2. a single point of entry for the bypass mains supply 3. a fully interlocked maintenance (or wrap-around) bypass circuit 4. correctly sized bushbars and circuit breakers 5. co-ordinated protection for the load and UPS equipment 6. straightforward connection of load distribution panels Connecting the Critical Loads In order to make best use of the UPS equipment and to ensue maximum protection of the critical load it is important to consider carefully how best to connect the load components. Large ring circuits feeding many critical load devices and unsuitable as a fault on one device may cause the circuit feeding it to trip or fuse and consequently disconnect power to other pieces of important equipment. Radial wiring with individual devices protected by their own circuit breakers is a far better approach? In this way a fault in one device will cause that device only to be disconnected and remaining critical load elements will remain undisturbed. To avoid confusion, particular attention must be paid to the labelling of circuit breakers and fuses in the load distribution panels. Earthing In any electrical installation correct earthing is essential for personnel safety and equipment protection. A protected power installation in no exception, it is a essential to ensure that all earthing points within the system are connected to a properly planned and secure earthing system. As a minimum a properly planned and secure earthing system for a computer and UPS installation must provide: 1. Protection against electrical shock 2. A short, low impedance return path for fault currents 3. A path for induced currents caused by high voltages such as lightning 4. Straightforward connection facilities for future expansion. Most earthing installations are based on star or grid configurations. Generator Maintenance and Testing A regular generator service program should include tests and checks of the following: 1. Cooling system Radiator/heat exchanger, coolant, hoses and connections, fan drive pulley and fan, fan belts, jacket water heater, water pump, thermostats. 1. Fuel systemFuel tank, water trap/separator, fuel lines and connections, governor and controls, fuel filters ? primary/secondary, fuel pressure, air induction and exhaust system, air filter, air filter service indicator, air inlet system, turbocharger, exhaust manifold, valves and valve rotators. 1. Lubrication oil system Oil, oil filters, oil pressure, crankcase breather 1. Starting system Batteries, battery specific gravity, battery charger, starting motor, alternator, engine monitor and safety controls, gauges, remote annunciators/alarms 1. Generator Bearings, slip rings and brushes, space heaters, vibration isolators 1. Control panel Start controls - manual/auto, voltmeter, ammeter, frequency meter, circuit breaker, auto transfer switch 1. Gas engine Gas lines and connections, carburettor and linkage, magneto/distributor, ignition system, spark plugs 1. Insulation test Main stator, main rotor, exciter stator, exciter rotor 1. Load testing With full load, perform a two to four hour load test Overall Energy Systems Testing As well as performing the periodic Planned Maintenance of each part of the system, consideration should be given to testing the entire system on a regular basis. An overall system test will involve putting the critical load at risk so careful arrangements and agreements must be made. Mains Failure Test Disconnect the mains supply to the protected power equipment for an extended period and check: " With no generator : " the UPS battery supports the critical load for the expected autonomy time " all alarms and control signals are correct? " the critical load responds correctly to any signals received from the UPS equipment e.g. system alarms, orderly shutdown sequence etc. If a generator is installed: 1. the Automatic Mains Fail (AMF) equipment operates correctly 2. the generator auto-starts after the expected time 3. the generator output supply is correct and within acceptable UPS input limits. 4. the UPS accepts the generator supply 5. battery recharging takes place while the UPS is being supplied by the generator. Mains Restoration TestFollowing the Mains Failure Test, re-connect the mains supply to the protected power equipment and check: With no generator: If the UPS is fitted with auto-restart facilities, 1. The system returns to normal operation automatically, otherwise all UPS modules can be re-started manually and the system restored to normal operation 2. All alarms can be reset. If a generator is installed: " the Automatic Mains Fail (AMF equipment operates correctly " the UPS signals a mains failure during the changeover " the generator shuts down after the expected time " the UPS accepts the restored mains power supply and recharges the batteries " all alarms can be reset. PowerContinuity Means Business ContinuityPlease visit our website at or call us on 0845 055 8455 for further details.
