Standby Power Supplies
D. C. STANDBY SYSTEMS
Figure 6.6 shows a d.c. system of the kind that is used for many industrial and com- mercial applications. Incoming a.c. power is converted to d.c. in a charger/rectifier.
Fig. 6.6 Typical d.c. system.
Under normal circumstances (that is, when the mains supply is present), this d.c.
power performs two functions: it meets the power requirements of emergency light- ing, switchgear, controls and other d.c. loads, and it float-charges a battery.
If the mains fails, these d.c. loads draw their power from the battery until the mains is restored, a standby generator is started, or the battery becomes fully dis- charged. The d.c. system is designed to recharge the discharged battery and con- tinue supplying the standby loads.
Battery chargers
Unsuitable battery and charger combinations account for the majority of problems encountered by users, while inadequate maintenance accounts for many of the rest.
To a large extent these two aspects are linked because the maintenance require- ments of a standby battery installation are greatly influenced by the choice of the battery charger.
It is important that the charger is accurately set to the voltage levels appropriate to the type of cell in use (Table 6.3). For sealed lead-acid batteries, typical float volt- ages are:
2.275 V/cell for 5-year design life types 2.23 V/cell for 10-year design life types.
In flooded-cell types, a charger voltage of 10–15% above the recommended level can dissipate electrolyte within days, leaving the battery at best temporarily useless, and possibly with only a fraction of its nominal capacity recoverable. For recombi- nation batteries this overcharging will cause gassing and shorten service life. Con- versely, a charger voltage of 10% below the recommended level means that the battery will not be fully recharged.
If the charger output is poorly regulated, and unpredictable, and has no meter- ing or alarm facilities, very frequent – perhaps daily – checks of charger voltage and current, and topping up of electrolyte, are required.
Temperature management is more important for sealed lead-acid cells. Placement near or above heat sources should be avoided, and where necessary, temperature compensation of float voltage should be used (Figs 6.7 and 6.8).
The simplest form of charger is the taper charger, for which a typical circuit diagram is shown in Fig. 6.9. It should be noted that this type of charger is not suit- able or recommended for use with URLA or nickel-cadmium cells. In this type of circuit, the components are selected so that the battery is trickle-charged to balance any losses. An electromechanical switch (manual or automatic) connects a lower resistance to give the higher current needed for boosted charging.
The taper charger is a relatively simple and low-cost device, but it does have some serious disadvantages. If the mains varies, the unit has to be reset, otherwise under- or overcharge will damage the battery. Electrolyte consumption varies, so the level must be examined at regular intervals and topped up when necessary.
If switching from trickle-charge to boost charge is automatic, the control unit must be 100% reliable or there is a risk of not switching to boost charge after the battery has discharged. Worse still, there is also the risk of the charger not switching back
Fig. 6.7 Relationship between float service life and ambient temperature.
Fig. 6.8 Relationship between temperature and charging voltage.
to the trickle charge after a boost charge, so that the electrolyte will rapidly be boiled off.
A far better approach to battery charging is to use a constant-voltage charger. In this type of charger, the voltage applied to the battery is controlled automatically and kept constant, independent of any mains or load fluctuations. Constant-voltage charging is recommended for both sealed lead-acid and nickel-cadmium cells.
Constant-voltage chargers use a power electronic regulator, with a current- limiting circuit to protect the components in the event of an excessive overload. The circuit of a constant-voltage charger with series regulator is shown in Fig. 6.10.
From a practical viewpoint, the main benefit of the constant-voltage charger is its ability to compensate automatically for variations in load and mains voltage. The battery is kept is a healthy state and maintenance intervals are extended. There is no need to incorporate a battery load test circuit in order to check that the battery is fully charged.
Another benefit is that the size of the charger does not depend on the size of the battery installation. So extra battery capacity may be added to meet changing requirements without necessarily incurring the cost of a replacement charger (as would be the case with a taper charger). The constant-voltage charger automatically compensates for the increased charge requirements of some batteries as they age.
Meters and alarms
Some meters and alarms are desirable on any battery charger, although it is not uncommon for users to request far more complex metering than is strictly justified.
For most users, alarms are more useful than meters.
A charge-fail alarm is probably the most valuable alarm because there are con- ditions where the charger can be working satisfactorily within current limits, but the battery is discharging. A high-voltage alarm, which indicates if the charger is left in Fig. 6.9 Circuit and waveform of a typical taper charger.
the boost charge position, enables positive action to be taken to prevent possible permanent damage to the battery.
Boost charging is not usually required with sealed lead-acid cells. However, if external requirements call for this facility, care should be taken to follow the battery manufacturer’s specifications.
A low-voltage alarm, set at a level towards the end-of-discharge voltage, reveals that the battery is approaching the end of its design performance. Other alarms that may be useful are an earth-fault alarm, and for flooded cell type batteries an electrolyte level-low alarm, which indicates that the electrolyte needs topping up.
As far as meters are concerned, a voltmeter indicates that the system is operat- ing correctly; an ammeter, however, is an unnecessary expense in most cases.
Current levels in constant-voltage chargers are factory preset, and in any case during the final float stage, the current would typically be only 0.005 the rated capacity of the battery.
Charger techniques for regulated d.c. loads
Where the load includes telecommunications equipment and instrumentation, and the d.c. standby supply must be regulated to within 5–10% of the nominal voltage, problems may arise unless precautions are taken to boost charge the battery in such a way that the system voltage does not rise to an unacceptable level.
Set point
Regulator with battery voltage sensing Series regulator
+ –
regulator
Fig. 6.10 Circuit and waveform of a typical transistor constant-voltage charger.
There are several ways of achieving this. The cheapest and simplest approach is to use a technique known as end-cell tapping.
This involves arranging the system so that, during boost charging, the load is sup- plied from less than the full number of cells, by automatically switching out a certain number of cells using a voltage-sensing relay and contactor. Figure 6.11 shows this arrangement. An alternative approach is to switch a diode into the load circuit so as to introduce a voltage drop to compensate for the increased boost charge voltage.
It may be that by deliberately oversizing the battery, so that it achieves its desired performance with only an 80% charge, the need for boost charging can be elimi- nated. Whether or not it is acceptable to provide an extra 20% battery capacity that will never be used depends on the particular application. Apart from considerations of cost, the size and weight of the battery may be a problem – especially in restricted locations such as offshore platforms.
Another alternative, which avoids the voltage fluctuations caused by boost charg- ing, is a split battery with a charger for each half. By ensuring that the boost charge only takes place on the battery that is off-load, a very closely regulated supply is assured. With this arrangement, shown in Fig. 6.12, it is necessary for each battery/charger to have sufficient capacity to supply the load on its own.
Valve regulated lead-acid batteries
With valve regulated lead-acid batteries, end-cell tapping and similar techniques are not required because:
(1) Maintenance boost charges are redundant, and
(2) Normal operating characteristics of valve regulated lead-acid batteries cover a much narrower voltage band.
Figure 6.13 illustrates the performance of these batteries.
Fig. 6.11 End-cell tapping.
Fig. 6.12 Duplicate/split battery.
This narrower band complies with the equipment regulation and as an additional benefit removes the problems associated with possible switching spikes corrupting stored and transmitted digital data.
ALTERNATING CURRENT SYSTEMS
A wide range of equipment in industrial and commercial premises requires a secure a.c. power supply, and with the increasing use of computers and other sophisticated electronic equipment, the demands placed on the quality and continuity of the a.c.
supply have grown correspondingly.
Even in the industrialised nations, where the a.c. mains supply is very reliable, it is estimated that a computer will on average suffer from approximately 130 power surges, sags or minor interruptions per month. In addition, between 80 and 90% of users will be subjected to voltage drops greater than 10% of the nominal. In devel- oping countries, mains outages of three or four hours a day are not uncommon.
Uninterruptible power supplies (UPSs) allow a critical or sensitive electrical load to operate reliably in spite of mains disturbances or outages. Battery-based UPSs use static inverters or rotary converters to convert the d.c. supply from a battery into a.c. at the voltage and frequency required by the load. Diesel rotary UPSs are also available and use inertia energy storage to maintain supplies until the engine starts.
A range of UPS configurations are available to suit the sensitivity, criticality and demands of the critical load.
Fig. 6.13 Discharge performance (CA is the rated capacity in amperes).
Off-line UPS
Figure 6.14 shows an off-line (or standby) UPS using a simple electromechanical changeover device to select the source of supply for the maintained load. Static switching devices can be used to achieve the same function but with a shorter inter- ruption. The inherent energy storage of power supply units within computers and other information technology equipment allows continuous operation provided the changeover is less than 20 ms. Under normal circumstances, the load is supplied direct from the a.c. mains. The mains also supplies a charger/rectifier which float charges the storage battery.
If the mains fails, the load is switched automatically to the a.c. output of an oper- ating but unloaded static inverter, which gives the prerequisite voltage, frequency and output rating for the load. During mains failure, the static inverter powers the load, drawing its imput from the rechargeable battery, until either the mains is restored, an alternative primary source is switched in, or the battery becomes fully discharged.
Figure 6.15 shows a standby unit designed for non-critical desktop equipment such as personal computers and word processors.
The storage time required for the off-line UPS depends on the application. For emergency lighting requirements in public buildings such as cinemas, regulations dictate that the discharge time shall have a minimum period of 3 hours. Office equip- ment tends to require no more than 10 minutes duration.
In addition, the power supply module within the computer is more generally of the switch mode type. These can tolerate short periods of fairly crude input wave- forms such as square-wave (quasi) or step-wave configurations with their resultant high harmonic contents.As the control circuitry for these types of inverters is simple, the overall cost of producing these units is kept to a minimum thus offering the various markets a cheap, if not crude, UPS system.
Fig. 6.14 Simple standby static inverter supply for non-critical a.c. coads.
Some standby UPS systems have been designed with faster and more sophisti- cated static transfer switching arrangements to meet the needs of less critical com- puter and telecomms applications. However, the various manufacturers of standby UPS all carry one common flaw within their designs. This type of unit is not strictly uninterruptible as the output will have a break of at least 2 ms whilst the unit detects a mains failure and switches to inverter mode and battery back-up.
In order to counter this effect and still produce cost effective units, manufactur- ers have utilised the latest innovations to produce line-interactive UPS which has the capability of supplying no-break power with off-line technology.
Line interactive UPS
As with all significant changes the concept of line-interactive UPS was born of a market requiring cheaper alternatives to the more traditional on-line designs while maintaining the integrity of no-break principles. Thus older more established and, in some cases, forgotten designs such as constant voltage transformers (CVTs) and ferro-resonant transformers were given a new lease of life and incorporated within the modern UPS packages.
In principle a CVT or ferro-resonant transformer provides the heart of the UPS, as clearly defined in Fig. 6.16. This acts as a full time power conditioner filtering out Fig. 6.15 A standby unit designed to protect non-critical desktop equipment, such as
personal computers and word processors, from mains-borne hazards like electrical noise, overvoltage spikes and blackouts.
any harmful spikes, surges or sags with the additional benefit of providing isolation between input and output sources. A rectifier ensures the battery is maintained during normal conditions with the inverter synchronised but not conducting.
When the mains fails, the off-line inverter is switched on and supplies the load via the CVT. This in turn discharges its residual energy during the few milliseconds it takes for the inverter to become active, thus achieving an effective no-break trans- fer from supply failure to inverter mode. The inverter itself can be a simple square- wave system as the CVT will condition the waveform to provide a sinusoidal output.
This concept can be known as single conversion as well as line interactive.
The static inverter on the input stage ensures there is no back-feed on to the mains through the transformer.
Several problems exist in this type of design. The CVT will not correct for any significant frequency deviations in excess of ±1 Hz without reverting to its inverter and free running on the output. While this effectively deals with the immediate problem, this condition cannot be sustained as it is limited by the amount of avail- able battery back-up and therefore actual running time. Some manufacturers over- come this difficulty by widening the input frequency tolerance to ±3 Hz. However, as there is no effective control of the input stages, the output from the UPS and subsequently the load will be subjected to the same levels of variation. It is therefore necessary to check equipment tolerances before selecting this mode of operation.
This transformer centred line interactive approach is currently offered by several manufacturers, although not using a CVT in all cases. The non-CVT based variants offer higher efficiency than on-line UPS and can impose lower harmonic current distortion on the mains supply even if the UPS load is harmonic rich.
Equally, some designs have difficulty in discriminating between mains failure and some other faults on the line. For example, motors can induce a back e.m.f. into the circuit at the point of mains failure. The CVT seeing the back e.m.f. as a valid voltage may continue to try and function as normal, thus the detection circuit and transfer mechanism would be delayed. By the time the back e.m.f. has diminished and been recognised as a loss of supply, the load would be dropped. Other types of Fig. 6.16 Typical on-line interactive UPS system.
line interactive designs use a multitap auto-transformer on the input stage and replace the CVT with a capacitive reservoir. This allows the input voltage to be boosted under low voltage conditions with typical tolerances of 164 V to 264 V and the output regulated between 187 V to 264 V. When the mains fails, the capacitive reservoir discharges in the same manner as the previous CVT system, thus provid- ing a no-break transfer.
The alternative to standby inverter configurations is on-line operation, where the load is powered by the static inverter at all times. In the event of a mains power failure, the battery charging function ceases, but the inverter continues to drive the load without interruption. These systems are usually referred to as double conver- sion or continuous mode on-line.
Continuous mode on-line
This design topology is ideally suited for more sensitive operational environments;
for example, ultra-critical computer systems require a much higher degree of input to output isolation in order to avoid transference of power disturbance such as common and normal mode line noise in addition to providing a totally secure output.
Figure 6.17 shows a schematic of a typical on-line UPS system, which provides continuous, conditioned a.c. power for the computer. Incoming a.c. from the public supply (or other prime power source) is rectified by a rectifier/charger. The d.c.
output serves two purposes: it float charges a storage battery, and it supplies a high- performance static inverter. The output of the static inverter, at appropriate voltage and frequency, supplies the load directly.
This arrangement means that the quality of the a.c. supply to the load is deter- mined solely by the characteristics of the static inverter. Any transients or mains- borne interference on the a.c. supply does not reach the computer load due to the principle feature of the continuous mode design, which is the full time operation of its static inverter. This negates the need for any switchover mechanism as required by standby UPSs or the use of residual energy sources for fly-wheel effects as required by line interactive UPSs. As discussed previously, the line interactive concept is unable to deal with excessive frequency fluctuations. However, the double conversion UPS can sustain tolerances of ±5 Hz without switching to battery as the rectifier absorbs such variations.
Fig. 6.17 Typical on-line UPS system.
In the event of an interruption to the a.c. supply, the load continues to draw its power from the static inverter, which in turn draws its d.c. input requirements from the storage battery. The capacity of the battery is chosen at the design stage to suit the power requirements of the load and the standby duration it must provide.
When the supply is restored, the rectifier/charger once again takes over as the source of d.c. power for the static inverter.
UPS systems above 1 kVA usually employ a static switch to transfer the critical load in the event of inverter failure (Fig. 6.18).
If the sensing circuitry in the static transfer switch detects that the output from the static inverter is about to exceed its limits, the switch transfers the load to an auxiliary a.c. supply. In many installations, this auxiliary supply is in fact the a.c.
mains.
The possible risk of the static inverter experiencing a fault at the same time as a mains power failure is regarded as small enough to ignore. Where extra levels of security are required, the auxiliary a.c. supply could be another static inverter which would probably be maintained on active standby.
Battery rotary UPS
Modern battery rotary UPS tends to use a similar topology to the continuous mode on-line static UPS but with an inverter fed rotary motor generator (MG) set in place of the inverter fed output transformer. Although this may at first seem like an unnecessary added complication, the use of a MG set allows certain enhance- ments to be claimed by the designers of such options compared with static UPS.
These include:
(1) An internal bypass that allows the MG set to draw most of its power direct from the input supply. This leads to lower harmonic pollution on the input supply compared with power flow via an input rectifier.
(2) A simpler naturally commutated inverter.
Fig. 6.18 An on-line static UPS system.