The present invention relates to an electric power generating
Electric power generating systems are known comprising
a number of alternators, each driven by at least one internal combustion
engine (e.g. a diesel or turbogas engine) and generating an alternating output
voltage; and the engines are closed-loop controlled to run at substantially the
same speed, so that the output voltages of the alternators have the same frequency
In such known systems, the outputs of the alternators are
arranged parallel to sum the output currents which are used to supply a local electric
network powering a number of electric loads.
The above systems can also be used to advantage on ships
to power on-board electric user devices (motors, lighting, electronic equipment,
As stated, the internal combustion engines must operate
at the same constant speed to sum the alternating output currents.
This can pose serious drawbacks, in that, in many practical
applications, the speed of the internal combustion engine does not correspond to
the speed which maximizes efficiency and/or reduces consumption and/or minimizes
wear of the engine.
As a result, in known power generating systems, consumption
is normally high, efficiency less than optimum at the various power outputs, and
wear of mechanical component parts is severe.
It is an object of the present invention to provide an
electric power generating system designed to eliminate the drawbacks of known systems.
According to the present invention, there is provided an
electric power generating system as claimed in Claim 1.
The invention will be described with particular reference
to the accompanying drawings, in which:
- Figure 1 shows an electric power generating system in accordance with the teachings
of the present invention;
- Figure 2 shows a reconfigurable power distribution network in accordance with
the teachings of the present invention;
- Figure 3 shows an operating flow chart of the Figure 2 network;
- Figure 4 shows a variation of the Figure 2 network.
Number 1 in Figure 1 indicates as a whole an electric power
generating system connected to a reconfigurable power distribution network 3.
System 1 and network 3 may conveniently, though not exclusively,
be used to advantage for generating and distributing electric power on naval vessels,
e.g. system 1 may be installed on a warship (not shown), and network 3 used to distribute
the power generated locally by system 1 to a number of electric user devices 5 (shown
schematically in Figure 1).
System 1 comprises a number of alternators 10, each driven
by a respective internal combustion (e.g. diesel) engine 11 to generate an alternating
output voltage. In Figure 1, alternators 10 are shown schematically as single-phase,
but may obviously be other types, e.g. three-phase.
The alternators are driven by engines controlled by electronic
central control units 13, which run the engines at normally different speeds
&ohgr;1, &ohgr;2, ... &ohgr;n, so that the alternating output voltages
V(&ohgr;1), V(&ohgr;2), ..., V(&ohgr;n) of the alternators have different
The speed of each internal combustion engine 11 is conveniently
selected by electronic central control unit 13 on the basis of the technical operating
characteristics of engine 11, so as to maximize efficiency and/or reduce wear and/or
minimize consumption of the engine in relation to the power demanded of the engine.
System 1 comprises a number of rectifiers 14, each of which
receives a respective alternating output voltage V(&ohgr;1), V(&ohgr;2)
, ..., V(&ohgr;n), and generates a rectified voltage V(r1), V(r2)
, ..., V(rn). According to one aspect of the present invention, closed-loop
control devices 16 are provided, each of which determines the rectified voltage
at the output of a respective rectifier 14, and acts on respective alternator 10
to keep the respective output voltage V(r1), V(r2), ..., V(rn) close to a
common target value, so that all the output voltages are substantially equal.
Each control device 16 may conveniently operate by regulating
excitation 17 of respective alternator 10. Alternatively, output voltage can be
controlled by acting in negative feedback manner on the respective rectifier, or
on a chopper/booster (not shown) downstream from the rectifier.
System 1 also comprises a number of circuit breakers 20,
each interposed between the output of a respective rectifier 14 and a common adding
node 22 defining an output of the electric power generating system.
Number 3 in Figure 2 indicates a reconfigurable direct-current
power distribution network in accordance with the teachings of a further aspect
of the present invention.
Network 3 only represents the positive pole of a direct-current
system, and is therefore shown schematically as single-pole; the same diagram also,
or alternatively, applies to the negative pole of the distribution network.
It should be pointed out that the network layout shown
(in this case, an H network) is purely indicative to illustrate operation of network
3, and may be any of various widely differing layouts, such as the loop layout (Figure
4) described in detail later on.
The example shown comprises a first electric power line
(BUS) 30 and a second electric power line (BUS) 32, both of which may be supplied,
for example, by the output of generating system 1.
Network 3 comprises a first one-way switch 40 having a
first terminal 40a connected to line 30, and a second terminal 40b connected to
a first terminal 41a of a second one-way switch 41 also forming part of network
3 and having a second terminal 41b powering an electric load 5a.
Current, and therefore also power, can only flow in switches
40 and 41, when closed, from the a terminal to the b terminal.
Network 3 comprises a third one-way switch 42 having a
first terminal 42a connected to line 32, and a second terminal 42b connected to
a first terminal 43a of a fourth one-way switch 43 also forming part of network
3 and having a second terminal 43b powering an electric load 5b.
Current, and therefore also power, can only flow in switches
42 and 43, when closed, from the a terminal to the b terminal.
Network 3 also comprises a two-way switch 49 interposed
between terminals 40b, 41a and 42b, 43a, and which permits current (and power) flow
in opposite directions between its two terminals 49a, 49b.
Network 3 comprises at least one electronic control unit
50 for each switch in the network, to safety control the switches (40, 41, 42, 43,
49 in Figure 2) and reconfigure network 3, when a short-circuit or overload is detected,
on the basis of signals from units 50 of adjacent switches, and regardless of control
by a higher network monitoring system (50b). Units 50 conveniently communicate with
one another over a high-speed communication system; and each unit 50 may be integrated
in the respective switch to reduce sensitivity to electromagnetic noise.
The Figure 3 flow chart shows operation of each electronic
control unit 50.
As shown in Figure 3, an initial block 100 monitors current
flow in each of the switches in network 3 to determine short-circuiting/overloading
of network 3.
A short-circuit/overload can be determined in known manner
by determining when the current Iswitch flow in each switch exceeds a threshold
value Ilim, i.e.
Alternatively or in parallel with the above, a short-circuit/overload
can be determined when the derivative of the current Iswitch flow in each
switch exceeds a threshold value Dlim, i.e. :
When short-circuiting/overloading of a switch is detected,
a block 110, downstream from block 100, sends a lock signal to all the switches
upstream, with respect to the power flow direction, from the selected switch
on which the fault has been detected.
Since switches 40-43 are all one-way, the power flow direction
through each switch 40-43 is predetermined, so control unit 50 of each one-way switch
knows which one-way switches are located upstream from its own position. For example,
switches 40 and 42 are located upstream from switch 41 or 43. Power flow in two-way
switch 49 on the other hand is determined by a current sensor (Hall-effect sensor)
52 cooperating with unit 50 of switch 49.
The lock signal results in locking by all the units 50
of the upstream switches, i.e. the switches for which a lock signal has been generated
are maintained in the (open/closed) position preceding generation of the lock signal.
Block 110 is followed by a block 120, which determines
- 1) whether a standby period has elapsed since the lock signal was generated;
- 2) whether, during the standby period, no further lock signals have been generated
from switches downstream from the selected switch (with respect to the power flow
For example, switches 41 and 43 are located downstream
from switch 40 or 42.
In the event of a positive response, a block 130, downstream
from block 120, opens the selected switch - since there are no other switches closer
to the short-circuit/overload, i.e. downstream from the selected switch - and then
goes back to block 100.
In the event of a negative response, a block 140, downstream
from block 120, maintains the preceding status of the selected switch, since at
least one switch has been found closer to the short-circuit/overload, i.e. downstream
from the selected switch.
Block 140 then goes back to block 100.
The following is an example to explain the above operations
Assuming a short-circuit CC (shown by the dash line) occurs
close to switch 41, between switch 41 and load 5a.
In this case, electric line 30 being grounded directly,
the current in switches 40 and 41 increases rapidly, and, if switches 49 and 42
are closed, there is a rapid increase in current in these too.
Electronic units 50 of switches 41, 40, 42, 49 therefore
detect a fault, emit lock signals for the switches upstream from the switch (in
this case, switches 40, 42, 49), and switch to standby awaiting lock signals from
the downstream switches.
In the example shown, there being no more switches between
switch 41 and load 5a, switch 41 is opened at the end of the standby period.
On detecting the fault, electronic unit 50 of switch 40
sends a lock signal to the switches immediately upstream from the selected switch
(in the example shown, there are no upstream switches) and then switches to standby
to await a lock signal from other switches downstream from switch 40.
In the example shown, a lock signal is received from switch
41 downstream from switch 40, so switch 40 is kept closed at the end of the standby
The same also applies to switches 42 and 49 if the short-circuit
current also flows through switches 42 and 49 to switch 41; in which case, switches
42 and 49 are kept closed when the short-circuit occurs.
Only switch 41 closest to the short-circuit is therefore
opened, and power is only cut off to electric user device 5a, whereas electric user
device 5b can be kept supplied by switches 42 and 43 and/or 40, 49 and 43.
Even in the presence of a short-circuit, therefore, power
is cut off from a minimum number of electric user devices, but is maintained to
the electric user devices not close to the short-circuit.
The same also applies in the event of a short-circuit or
anomalous absorption by electric user device 5a, in which case too, only switch
41 is opened.
In an alternative embodiment (not shown), alternators 10,
engines 11 connected to them, and rectifiers 14 may be formed into two or more groups,
each supplying a respective output adding node 22 by means of a circuit breaker
of the type indicated 20 in Figure 1.
For example, assuming two groups, one output node may supply
electric power line 30 in Figure 2, and the other may supply electric power line
In another embodiment shown in Figure 4, the reconfigurable
network comprises the same switches 40, 42, 49, 41, 43 as in Figure 2, and the same
loads 5a and 5b. The switches have the same layout as before, and therefore not
described in detail.
In addition, a second two-way switch 71 is provided, with
a first terminal connected to the common terminals of switches 40, 41, and a second
terminal connected to a loop bus 70.
Similarly, a third two-way switch 73 is provided, with
a first terminal connected to the common terminals of switches 42, 43, and a second
terminal connected to a loop bus 72.
Loop buses 70, 72 are connected to other networks of the
type shown in Figure 2.
The network may thus comprise a number of H networks 3
interconnected by loop buses 70, 72, in turn protected by two-way switches 71 and