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Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
This presentation will deal with three phase
machines and for ease of reference will
assume alternators are four pole and engines
are running at 1500rpm giving 50Hz
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
SynchronisingThe easy bit
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Synchronising is NOT load sharing.
To synchronise generators the engines and alternators
need not be of equal size, speed or number of poles
The phase to phase angles need to be zero
The frequency needs to be equal
The voltages need to be equal
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
The three phases are commonly referred to as R S T
or UVW (also historically red, yellow and blue).
As a three phase machine rotates each phase is passed
by an alternator pole in turn every 120° of rotation,
when two machines are ‘in sync’, like phases are at the
same angle relative to each other and each machine.
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
In the example below imagine generators ‘a’ and ‘b’
are rotating in the same direction, at similar speeds.
On the left the alternators are out of phase whilst on
the right they are in phase, providing the engine
speed difference is not too great a circuit breaker
could be closed connecting the units together. The
coupled generating sets are then in parallel.
a b a b
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Synchronisation is achieved by adjusting engine speed,
therefore frequency, and alternator voltage by either
electronic or manual means, then closing a circuit
breaker to connect two generators together.
The method normally adopted is to close the lead
engine circuit breaker onto the bus bars (bus) and all
other generators are then individually synchronised to
the bus.
There is no real limit to the quantity of units that can
be connected together.
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Automatic synchronisation is carried out by a synchroniser unit, this compares the bus and incoming generator voltage, frequency and difference of phase angle. The unit will adjust speed about a set point but will generally not adjust voltage, this is an alternator AVR or voltage matching device rather than governor system function. The phase angle is reduced by speed variation to a preset level (<5° for diesel, greater for gas due to instability). If it remains within that 5° window for a preset time and the phase to phase voltages are equal then the unit gives a signal to enable incoming generator circuit breaker closure.
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
allowed phase errorsetpoint (mains)generator
voltage difference forsynchronization window
sine.exe
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Manual synchronising can be carried out in two basic
ways,
• A synchroscope which is in the form of a meter
and indicates synchronisation when the pointer is at
12 o’clock.
• Lamps cross connected between phases R to R, S to T and T to S, these form a triangle with the R to R at the apex. When ‘in sync’ The top lamp will be off whilst the two bottom lamps will be bright.
Paralleling is then a matter of closing a circuit breaker
by whatever means available.
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Mis-synchronising will cause high mechanical stresses
and may lead to serious damage. For this reason it is
normal to include a ‘sync check’ device in the control
circuit of a manual synchronising system to prevent
operator error.
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Any Questions?
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Load SharingThe difficult bit
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Defining Load
Load can be discussed in two ways, true power (kW)
and reactive power (kVa) As engine manufacturers kW
is the only unit that can be dealt with.
Load is often referred to in amperes, this may as well
be ducks or elephants, it means nothing in terms of
power without either a kW value or known voltage and
cos φ (power factor).
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
The formula to work out load isV x I x √3
1000
For a generating set running at 415 Volts, and 700 Amps the reactive load is 502 kVa, if the power factor is introduced into the equation then the true active power (kW) can be seen.@ cos φ 0.7 true power is 502 x 0.7 = 351kW@ cos φ 0.95 true power is 502 x 0.95 = 476kWFrom this it can be seen for the same voltage and current true power (kW) varies widely
= kVa x cos φ = kW
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Cos φ is dependent on three things;
• The power factor of the load if the application is island operation.
• The difference in voltage between the generators in parallel with each other and the load.
• The difference in voltage between a generator and grid when paralleled together.
What the figures in the previous slide indicate is that if
two or more generators are in parallel and they show
the same current they are not necessarily sharing load
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
There are two methods of load sharing
• DroopA decrease of desired speed for an increase in load
A increase of desired speed for an decrease in load
• IsochronousNo change of desired speed with a change of load
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Reactive load sharing
Like an engine, an alternator requires voltage droop to
share load, in this case reactive (kVa) load sharing. As
kVa is affected by power factor the closer the power
factor is to cos φ =1 then the better the reactive load
sharing is.
Even if a group of generators are running with
isochronous load sharing, alternator voltage droop is
still required.
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
To successfully load share there must be
• Engine droop or an isochronous load sharing device to share active (kW) load
AND
• Alternator voltage droop or, when parallel with the grid, a power factor controlling device to share reactive (kVa) load
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Load Sharing With Droop
The origins of droop lie in the mechanical losses
associated with purely mechanical governing systems.
With the advent of electronic and hydraulic governors
droop could be set to zero however to share load in a
simple group of generating sets droop is required.
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Droop is expressed as a percentage and is the
difference between no load speed and full load speed,
droop is calculated thus,
No load speed - full load speed
Full load speed
1560 - 1500
1500
x 100 = Droop %
x 100 = 4 %
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Droop will give a fixed load for a given speed.
For example;
If an engine, regardless of power output, with a rated
speed of 1500 rpm has 4% droop then with no load the
engine will run at 1560 rpm, at 1530 rpm the load will
be 50% and at 1500 rpm the load will be 100%.
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
If this engine is connected to a generator and is in
parallel with another engine driven generator with the
same droop setting then at no load the engines will run
at 1560 rpm, as load is added and speed droops to
1530 both engines will be at 50% load, as more load
is added the speed will droop to 1500 rpm at 100% of
rated load for each engine.
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Droop gives stability to a system, higher droop settings result in more accurate load sharing, normally droop will be set in the region of 4 –5%.
Without droop 1500 rpm could equate to any load between 0 and 100%. If two or more generators were connected in parallel the result would be very unstable load sharing.
Electronic load sharing systems do not rely on droop and can control engine output by means of continual measurement and adjustment of output power.
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Load
Sp
eed
100%0
1500
1500 +
% Droop
Isochronous
Droop
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Isochronous Load Sharing
Isochronous load sharing allows generators to run in
parallel at a constant speed regardless of the load on
each engine. The kW output from each engine is
measured by a load sharing device, generally these
devices ‘talk’ to each other and continually adjust
engine fuelling to maintain an equal percentage kW
load on each engine.
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Any Questions?
Name of Meeting (Date)Proprietary Information of Perkins Engines Company Limited 2002 - All Rights Reserved
Perkins Confidential ‘Green’
Synchronising and Load Sharing
Poles v Hz v Speed
Number of pairs of poles x speed = Hz
60
Example for a four pole machine running at 1500 rpm
2 x 1500 = 3000 = 50Hz
60 60