19 Nov 2015


3.1       GENERAL

There are a number of systems, both offshore and onshore, whose functioning is so important that they must not be allowed to cease, even momentarily, even if there were a total power failure.  Such systems include:
·         Emergency lighting
·         Communications
·         Certain process instrumentation and control
·         Operation of electrical switchgear
·         Navigational aids
·         Fire and Gas detection.
In order that these systems shall not fail when main power is lost, they must be supplied with power from a source of stored energy which means, in practice, a battery.  And since a battery can store only d.c. energy, these systems must in general operate from d.c.
The d.c. voltages are normally 110V and 24V, but others are used.  Separate systems are provided for each, sometimes more than one.  Where several similar services are supplied from a common d.c. unit, it is referred to as a ‘central’ d.c. system.
There are also a number of d.c. power units that supply individual equipments such as navigation lights, foghorns or diesel engine starters; these are referred to as ‘dedicated’ systems (as distinct from the ‘central’ systems).
Most d.c. systems are unearthed, but those supplying telecommunications equipments usually have one pole earthed.
Because the main power system of an offshore or onshore installation is a.c., this power must be converted to d.c. before being fed to these systems.  The modern method is to convert the a.c. electronically into d.c. through a solid-state ‘rectifier’.  It is entirely static and has no moving parts; it is also very easily controlled.  The d.c. voltage output depends on the level of the a.c. voltage input, so an in-built transformer on the a.c. side causes it to produce any d.c. voltage desired from the 415V or 440V a.c. system.  The principle of rectification is discussed more fully in the manual ‘Fundamentals of Electricity 3’.


Figure 3.1 shows the basic circuit of a d.c. supply system.  An a.c. supply is led to a transformer, which converts the a.c. to whatever voltage is necessary for producing the desired level of d.c. and thence to a 6-element rectifier bridge.  The d.c. is taken from the bridge to a distribution fuseboard from which it is fed to all the d.c. services.
Figure 3.1 shows the unit as 3-phase, but d.c. supply systems are also used supplied from a single-phase source. In that case the rectifier bridge is a 4-element one.
In order to regulate the outgoing d.c. voltage level, the rectifiers on one side of the bridge are replaced by ‘thyristors’ (controlled rectifiers) by which the d.c. output level can be adjusted.  The firing of the thyristors is automatically controlled so as to maintain the d.c. voltage at the correct level for the load or for battery charging.


This system will produce d.c. from a.c., but, if the a.c. itself fails, the whole thing stops, and no d.c. is produced either.  Such a system therefore would not give the continuity needed.  But if now a battery is connected in parallel with the d.c. side, as shown in Figure 3.1, the d.c. current from the rectifier will not only go to the system load but will also keep the battery charged.  Suppose now the a.c. system fails.  No d.c. current comes out of the rectifier, but the battery remains connected to the load and continues to supply it with d.c. current without operator action and without interruption. Indeed, the d.c. loads would not even know that there had been a failure.
This state of affairs would continue so long as the battery held its charge.  It would of course begin running down and there would be a small but progressive fall of voltage.  How long it continues to supply the load depends on the capacity, or size, of the battery, which may be regarded as an electrical ‘ready-use’ tank.  The designer, knowing the current load on the battery (in amperes) and the time during which it is desired that it continue to operate (in hours), decides the capacity (in ampere-hours, or ‘Ah’) of the battery to be installed.  Thus if the d.c. load is 80A and if it must continue for a minimum of four hours after an a.c. failure, then the battery must have a capacity of at least 80 x 4 = 320 ampere-hours.
When a.c. voltage is restored after the mains failure, the rectifier takes over its original function and starts to convert it to d.c. again.  It relieves the battery of its emergency duty and supplies d.c. directly to the load once more. In addition it starts to recharge the battery.  It is important to note that the prime duty of the rectifier is to supply d.c. to the load.  This is not done by the battery normally, which occurs solely on failure of the a.c. It only charges the battery after such a failure.  The rectifier unit is usually called a ‘Charger’, but this is not its principal duty.  The rectifier must be rated to carry out both functions (d.c. load and battery recharging) together.


In Figure 3.1 the transformer-rectifier has been drawn in a straight line with the d.c. output, and the battery has been drawn to one side.  This is deliberate and is to emphasise the role of the rectifier. Under normal conditions it is the rectifier which supplies the d.c. load; the battery supplies nothing (indeed it receives a small maintenance charge) and is said to float on the d.c. system.  It is only in the abnormal condition when the a.c. supply fails that the battery takes over as the supplier of power.
With ‘central’ battery systems it is usual to provide two chargers and two batteries, both feeding a common d.c. distribution board as shown in Figure 3.2, which is for an offshore installation.
Each charger is supplied from a different a.c. source, one of which is always a Basic Services or Emergency switchboard to which the emergency generator may be connected.  Thus, after a prolonged blackout period which leaves both batteries discharged, at least one can recharged as soon as the emergency generator can be started.
The blocking diodes seen in the figure are to prevent feedback from the batteries.  The lower ones prevent one battery feeding into the other if it is in a discharged state, and the upper ones prevent a battery feeding back into a faulty rectifier.
Both batteries are provided with heavy fuses, and a centre-zero ammeter on the unit switchboard indicates whether the battery is charging or discharging.

Both battery/rectifier d.c. sources can be isolated from the distribution board by normally closed contactors (as shown in the figure) or by manual switches.  The purpose of these is explained in para. 3.3.
The battery, which is usually of the nickel-cadmium type delivering approximately 1.4V per cell, is permanently connected through the battery fuses to the d.c. side of the rectifier.  There are about 88 cells for each 110V battery and 18 or 19 cells for each 24V.
Where d.c. power units are continuously loaded, two batteries are provided, one associated with each charger.  In some power units each battery is capable of supplying the load on its own for the required length of time (referred to as ‘100% capacity each’); in others both batteries are needed to achieve this (referred to as ‘50% capacity each’).
There are other ways in which the charger, battery and load can be interconnected, but the one described above is by far the most common in offshore and onshore installations for bulk d.c. supplies.


If the batteries become partially discharged, as may happen after an a.c. supply failure, a manual, or ‘boost’, charge is desirable to recharge them quickly.  At the same time the d.c. supply to the load must be maintained without subjecting the load to the higher boost voltage.  To do this the equipment is arranged so that only one of the two battery banks can be boost-charged at a time and that this battery, and its associated charger, is disconnected from the load while the boost-charge is in progress.  The load is meanwhile supplied from the other charger.
In one typical system the operating mode is selected by a control switch which may be on the panel-front or inside the charger cubicle.  It is normal to run both chargers in the ‘Float’ mode (sometimes also referred to as ‘Auto’).  Under this condition each charger produces a constant voltage suitable for the load, while the battery, when fully charged, receives a small ‘floating’ or maintenance charge.  In the ‘Boost’ (or ‘Manual’) mode a higher voltage is applied to the battery, which is then charged at a higher rate than normal.
Suppose that Charger No 1 in Figure 3.2 is to be used to boost-charge its associated battery.  The Float/Boost switch is moved to BOOST, but, to ensure continuity of supply, an interlock is provided to ensure that Charger No 2 is ON and switched to FLOAT before the control of Charger No 1 can be changed to the manual-boost mode of operation.  To protect the load from the higher boost voltage, the output contactor of Charger No 1 is opened when the changeover takes place, leaving the load to be supplied by Charger No 2 and its battery.  If the a.c. supply to Charger No 2 fails, then Charger No 1 automatically reverts to the Float mode, its output contactor closing to support Battery No 2 which picked up the load on the failure of Charger No 2.
Various methods are employed to achieve these ends.  The equipment described uses electrical interlocks and switching but relies on the operator to terminate the boost charge.  Other equipments may have key interlocks and manual switching with hand-set or electrical timing devices.
When a discharged battery is first put on charge, or under d.c. fault conditions, the load on the charger will try to exceed its current limit setting.  The current is sensed by a current limit sensing shunt which holds the rectifier output current at its rated maximum value by reducing its d.c. output voltage.
Provision is sometimes made to disconnect the load from certain batteries in an emergency by tripping the input moulded-case circuit-breaker to the d.c. distribution board, but this is
not common.  This would only be done when an offshore installation was abandoned and the d.c. supply was no longer necessary.
Certain power units are fitted with equipment to monitor earth leakage and identify the circuit where this occurs.  Six such earth-leakage switches are seen on the right centre panel of Figure 3.4.
It must be emphasised that this description is typical only.  Switching and control facilities vary from one make of equipment to another.


There is a wide variation in the control circuitry of d.c. power units, which are provided by many different manufacturers. However, certain principles are common to all. A typical control scheme is illustrated in Figure 3.3; only Charger No 1 is shown (the scheme for Charger No 2 is similar).

The incoming a.c. supply to each charger is controlled by a contactor operated from an On/Off switch on the front of the board.  The supply is armed by a switch in the Ventilation Monitor (see para. 3.8) which in this case only allows charging while ventilation is on.
After transforming, the a.c. supply goes to the diode/thyristor bridge whose d.c. output level is regulated by an Electronic Control Unit.  When switched to BOOST this unit raises the d.c. output voltage of the bridge.  It also ensures, by sensing the d.c. current and regulating the voltage, that the current output never exceeds a preset level.  The d.c. current then passes through the blocking diodes and output contactor to the load distribution fuseboard.
Instruments consist of a d.c. voltmeter and ammeter: also a centre-zero battery ammeter.  Alarms are given by flag relays which sense some or all of control unit failure, loss of a.c., charger failure or high or low d.c. voltage.  These may also give a common alarm at some remote control point.
Operation of the Float/Boost selector switch to BOOST also causes the corresponding normally closed d.c. output contactor to open.  On some systems this function is carried out by extra contacts on the selector switch itself, interlocked by key with the selector switch on the other unit to ensure that it has first been set to FLOAT.


A typical d.c. power supply unit, incorporating a dual charger and two batteries, is shown in Figure 3.4.
On the centre two panels are the two charger controls, instruments and flag relays.  The chargers themselves, the transformers and the various control fuses are inside the cubicles behind doors.  The common d.c. distribution fuseboard is also inside.

Output MCCB



At either end are cubicles containing the batteries, the cells being arranged in shelves.  The 110V battery cells are small and numerous, whereas 24V cells are fewer and generally much larger.  Sometimes batteries of high capacity occupy several cubicles on either side.


The d.c. power supply unit applies particularly to the ‘central’ or bulk d.c. supplies to many consumers throughout the installation.  In addition there are separate ‘dedicated’ d.c. supplies used solely for particular equipments.  Among them are the supplies for navigational aids, for turbine and diesel engine starting, etc.  These are not individually described here, but in principle they are similar -  that is, they comprise a transformer-rectifier (charger), floating battery and d.c. output.  Dedicated systems usually consist of only a single charger and battery, similar to the arrangement of Figure 3.1.
A special note, however, must be made regarding the d.c. starting of diesel engines and of some gas-turbines.  Starting requires a heavy-duty battery (usually 24V) and a large starter motor which consumes power very rapidly while actually starting.  The recharge after a start is normally given by a d.c. generator (‘dynamo’) on the engine itself, as on a car.  If however the engine has been standing idle for long periods or has made only short runs after test starts, the battery may gradually lose charge, and there is a danger that it may not be able to start the engine when an emergency arises.


To prevent this, diesel starting batteries are provided also with a static charger powered from the offshore a.c. mains, as shown in Figure 3.5.  It is of the same transformer-rectifier and floating battery type as already described, but it is smaller.  It is located close to the engine, and in most installations it is left on permanently to give the battery a continuous maintenance charge.
It should be noted that in this case the rectifier is not the normal source of d.c. supply; it is the battery which powers the motor, as shown in heavy line, and the rectifier is purely a charger.

3.7       OTHER D.C. SOURCES

The d.c. sources described so far all employ solid-state rectifiers, but these are not the only methods available, especially when large d.c. powers are required.

3.7.1    Rotating Machines

It is also possible to obtain d.c. supplies from rotating machines.  In former times a ‘rotary converter’ was used, having a common a.c./d.c. armature and both commutator and sliprings.  Later these came to be replaced by motor-generator sets, having a standard a.c. motor driving a completely separate d.c. generator.  Such rotating equipment however posed a maintenance problem, which has now been largely overcome by the present static rectifiers.
One advantage of the motor-generator type of conversion is that the d.c. system is completely independent electrically of the a.c., and transients in the one are not carried over into the other.  This can be important in communications and other electronic systems.

3.7.2    Mercury Arc Rectifiers

An alternative, and static, method uses a mercury arc.  This allows electron current to flow only in one direction, as in a thermionic valve, from cathode to anode.  A ring of anodes is sealed into an evacuated glass bulb in which a pool of mercury acts as the cathode.  Each anode in turn carries the peak voltage in a multi-phase a.c. system, and the arc rotates from each anode to the next, so providing a continuous d.c. current, always in one direction.  Some mercury arc rectifiers have six, twelve or even twenty-four anodes supplied from special 6-, 12- or 24-phase transformers.  They provide a d.c. supply with very low ripple content, and they also cause lower harmonics in the a.c. system.
An alternative design of mercury arc rectifier uses a steel tank instead of the glass bulb.


The rate of charge which is put into a battery by a charger depends on the d.c. voltage applied to its terminals and to the back-emf developed within the battery, which rises with its state of charge.  Under normal running conditions the charger, while supplying the d.c. load at its nominal voltage, maintains the voltage applied to the battery at just over its charged voltage.  This results in a minimal charge current, or maintenance charge, going continuously into the battery to maintain its state.  The battery contributes nothing to and takes virtually nothing out of the system.  It is said to be ‘floating’.
After a period of discharge following an a.c. failure and consequent use of the battery as a back-up source, power will eventually return.  It is essential that the battery be recharged as quickly as possible, but not at a rate that would damage it.  This high charging rate is



called ‘boosting’.  Most chargers are provided with electronic circuits which control the rate of charge. Initially, on switching to BOOST, the charger voltage is controlled so that the charge current is limited (the ‘constant current’ period).  After that a constant voltage is applied to the battery so that, as its emf rises, the charge current tapers off.  On completion of the charge, the charger reverts to its ‘float’ mode either by manual switching to FLOAT or automatically, and the battery thereafter receives a maintenance charge only.  These stages can be monitored on the charger’s d.c. ammeter and are shown graphically in Figure 3.6.
On most chargers the change from float to boost and back again must be done manually by the operator (some systems have a timed return to float).  Panel lamps indicate whether the battery is on boost or floating. In some cases a dial may be provided on which the operator can set the hours of boost required.  At the end of the set period the dial has worked back to zero and switched the charger automatically back to the ‘float’ mode.  This can be checked from the FLOAT or BOOST lamps. (‘HI RATE’ is sometimes used instead of BOOST.)  With manual control the operator must estimate from his experience how many hours of boost are needed to replace the discharge.
It should be appreciated that while these varying voltages are being applied to the battery to recharge It, they are at the same time being applied to the d.c. loads.  They are all higher than nominal, especially during the initial constant-current stage, and they could do damage to some of the d.c. equipments.  For this reason only one battery at a time may be boost-charged, and it must be isolated from the d.c. bars while doing so, using the other half of the system to provide the d.c. loads, as described in para. 3.3.  On certain makes of equipment this is done automatically, as already described.  Most systems have d.c. overvoltage protection to disconnect the load if the voltage exceeds a certain limit.  A flag relay or lamp gives an indication if this has happened.

When a battery is being charged, especially when near the top of charge, it starts to ‘gas’.  This is because the charging current, having no more charge to give, electrolyses the water in the cells and breaks it down into hydrogen and oxygen gas.  This gas is a very explosive mixture indeed over a wide range of hydrogen concentrations (between 4% and 96%).  It is therefore essential that batteries - or at least battery rooms where batteries are concentrated - are well ventilated.
The ventilation of the battery rooms is continuously monitored; if the ventilation fails, any charge that is in progress is automatically stopped.  This is done by the ventilation flow monitor tripping the a.c. supply into the charger - see Figure 3.3.  This condition is indicated by a VENTILATION FAIL lamp.  Similarly, if there is no ventilation, a battery boost charge cannot be started.
An uncharged battery however may create a difficult situation, and on many offshore installations facilities are provided to override the Ventilation Fail trip.  This is usually a key-operated switch (seen in Figure 3.4), but it may only be used by an Authorised Person after he has satisfied himself that it is safe to do so.  This might entail installing temporary fans in the battery room.  After use, care must be taken to reset the switch.


Nickel-cadmium batteries are now used on most offshore and onshore installations.  They are robust and will withstand relatively high rates of charge and discharge.  The electrolyte is an alkaline solution requiring great care in handling as it attacks the skin and destroys clothing; always wear rubber gloves and wash hands in 10% solution of boric acid or under running water after contact.  The electrolyte takes no chemical part in the charge/discharge cycle and has a constant normal specific gravity of 1.210.  The state of charge of an alkaline battery is indicated by its cell voltage, not by its gravity.  (See the manual ‘Fundamentals of Electricity 1’.)
The capacity of a battery is expressed in ampere-hours (Ah); a fully charged 200Ah battery will produce 40A for 5 hours before it is discharged; this battery is then discharging at its so-called ‘5-hour rate’.  If the same battery were discharged at 100A it would not last 2 hours but would be serviceable for appreciably less time - say 1.5 hours - and its ‘200Ah capacity at a 5-hour rate’ would be reduced.  It is usual to quote the capacities of nickel-cadmium batteries at their 5-hour rate and to make allowance for heavier discharges.
Because a battery is not 100% efficient, it requires more electrical energy to recharge it than was taken out on discharge; a typical value of this ‘charge coefficient’ is 1.4 for a nickel-cadmium cell.
Ah (charge) = 1.4 x Ah (discharge)
The only way to determine the state of charge of a nickel-cadmium battery is by measuring the overall voltage and so determining the average cell voltage.  This should not be allowed to fall below 1.1V while discharging at the 5-hour rate.  A fully charged battery will receive a satisfactory floating maintenance charge when the output of its charger is maintained at 1.4 to 1.45V per cell.  These figures vary slightly between different makes.
On boost a battery may be charged at its 5-hour rate, which will bring a discharged battery to full charge in 7 hours.  These figures are given as a guide only; they may be varied to suit individual circumstances.

When on charge, a fully charged battery loses electrolyte by gassing.  The degree of gassing depends on the charging current; loss of fluid will also take place by evaporation. In both cases only water is lost, not the chemical salts.  The level of the electrolyte in each cell must be checked periodically and must not be allowed to fall below the top of the plates; loss of fluid is made up by adding distilled water.
When a battery is on continuous floating charge, it should be discharged periodically and then given a boost-charge, say every six or twelve months, to keep it in good condition; it should also be boost-charged after a mains failure.  The tops of battery cells should be kept clean and dry and the terminals lightly coated with a suitable grease to resist corrosion.  The terminal connections and inter-cell links must be kept tight.
Lead-acid batteries are not now much used offshore.  Where they are used, the above general description still applies, except that the cell voltage at full charge is about 2.1V.  The electrolyte is diluted sulphuric acid which, at full charge, has a specific gravity of between 1.200 and 1.300.  This falls during discharge, and it must not be allowed to fall below 1.150.  The battery must then be recharged at once and never be left in a discharged state.
When sulphuric acid is being diluted for use in a battery, ALWAYS ADD THE ACID SLOWLY INTO THE WATER - never the other way round, as adding water to acid can cause a violent reaction and result in serious danger to the operator.


So far only d.c. systems having battery support have been described.  In some cases, such as instrumentation, equally assured a.c. supplies are needed, and clearly they cannot come direct from a battery.  What is done, as shown in Figure 3.7, is to provide a battery-supported d.c. system exactly as was done in Figure 3.1, but to take its d.c. output and pass it through an inverter.  This is a solid-state static device that converts d.c. into a.c., and moreover at any voltage and frequency desired, so as to distribute the output to the vital a.c. loads.  Thus, if the mains a.c. fails, the battery will continue to provide d.c. without operator action or interruption.  This unbroken d.c. is inverted to unbroken a.c. and distributed to the various loads.  The voltage level of the ‘d.c. link’ and battery is not important, and any d.c. voltage may be used; in some cases this may even be 220V d.c.  Normally only one charger and one battery are needed.
It is usual, with such an arrangement, to provide a standby a.c. supply from the 415V or 440V mains direct to the inverter-fed distribution board, as shown dotted in Figure 3.7.  If this distribution were, for example, at 110V a.c. single-phase, a single-phase connection would be taken from a separate main board, transformed to 110V and passed through a ‘Static Switch’.  This is an electronic switch with no moving parts which normally connects the distribution board to the inverter.  If the inverter itself should fail, there would be loss of voltage at the static switch, and it would change over automatically to the direct transformer supply, so re-energising the 110V distribution board.  To effect a smooth changeover there is usually an electronic synchronising circuit.  This back-up feature is shown dotted in Figure 3.7.
Thus if the charger’s a.c. supply or the rectifier should fail, the battery takes over without interruption and the static switch stays on the inverter.  But if the inverter itself should fail, the static switch changes over to the alternative supply (normally from an emergency switchboard).  Note that this back-up supply is not itself battery-supported.
The actual power unit would look like half the dual d.c. system shown in Figure 3.4, together with an extra cubicle housing the inverter and static switch.

Those systems which are described here, both the battery-supported d.c. and the battery-supported a.c. systems, are sometimes referred to as ‘Uninterruptible Power Supplies’, or ‘UPS’ for short.

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