Sync n Droop

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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




SynchronisingThe easy bit




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




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.




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




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.




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.




allowed phase errorsetpoint (mains)generator

voltage difference forsynchronization window

sine.exe




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.




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.




Any Questions?




Load SharingThe difficult bit




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).




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




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




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




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.




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




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.




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 %




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%.




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.




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.




Load

Sp

eed

100%0

1500

1500 +

% Droop

Isochronous

Droop




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.




Any Questions?




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

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Sync n Droop