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’.
3.2 D.C. SUPPLY SYSTEMS
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.
FIGURE 3.1
BATTERY
SUPPORTED D.C. SYSTEM
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.
FIGURE 3.2
D.C. DUAL SUPPLY SYSTEM
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.
3.3 FLOAT OR BOOST CHARGING
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.
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.
3.4 CONTROL, ALARMS AND INDICATION
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).
FIGURE 3.3
TYPICAL CHARGER CONTROL SYSTEM
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.
3.5 D.C. POWER SUPPLY UNIT
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.
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FIGURE 3.4
TYPICAL D.C. POWER SUPPLY UNIT
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.
3.6 D.C. SUPPLIES FOR OTHER PURPOSES
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.
FIGURE 3.5
TYPICAL DEDICATED D.C. SUPPLY –
DIESEL ENGINE STARTING
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.
3.8 BATTERY CHARGING
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’.
FIGURE 3.6
RECHARGE CYCLE FOR DISCHARGED BATTERY
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.
3.9 VENTILATION
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.
3.10 BATTERY CARE
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.
FIGURE 3.7
BATTERY
SUPPORTED A.C. SYSTEM
3.11 BATTERY SUPPORTED A.C. SYSTEMS
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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