Paralleling 2 alternators Flashcards
(21 cards)
what is paralleling 2 alternators
connecting 2 alternators to same power system so they can share load together
why parallel them
back up if one fails
generator can be added/removed depencding on load
how to do introduce one alternator to the system
use a switch to connect end of alternator 2 to just before the load of alternator 1
which conditions do you need to check before closing switch
RMS val and freq of VPH2 must be equal to Vph1
Vph2 must be in phase and have the same phase sequence with Vph1
what do you have to make sure abt power when sharing between alternator
total active power is shared proportionally and stably - done using speed droop control
what happens to power when we parallel them
initally alternator 1 is supplying entire load -> Pload = Pm(1)
p(m) = Pmech(in) ideally is P3phi
alternator 2 is newly synchronised so it is spinning at same speed as system ωnl(Y)
It is not yet supplying any power:
Pm(2)=0
alternator 2 starts supplying power
by slightly increasing no load speed setpoint of altenator 2 ou slightly increase the no-load speed setpoint of Alternator Y
ωnl(2) >ωdp(1)
This small increase causes Alternator Y to “push” against the system, beginning to supply power
infinite bus bar def
idealised type of power system with many alternators are joined in parallel and have negligible influence on frequency and system V
adavantages of infitie bus bard
power it can supply is bigger than just one
having many alternators increase reliability of power system
allows one or more of them to be removed for maintenance
to connect a alternator to infinite bus bar
V magnitude, phase and freq must be exactly the same
what is no load speed of incoming alternator
set to value of the system freq so no power supplied in beginning
loading the alternator connected to infinite bus bar
to increase power supplied by incoming alternator, no load speed increases so that it is higher
this decreases the power supply by bus bar whilst system freq stays same
how to increase active power
- no power flow - Eph = Vph and they are in phase, no I -> no power flow, alternator is turning at synchronous speed
- apply more mechanical power - increase input pmech making rotor accelereate briefly
- new steady state reached - once 3phase power mactched pmechin rotor stabilises at synchronours speed
why does this work
increasing torque ships eph forward in phase increasing theta, as sintheata increase more power flows to bus but theta cant exceed 90 or machine becomes unstable
what happens to reactive power
supplys active power and absorbs the reactive power (VARs)
what do we want to regulate in the power
keep active power contant
control reactive by adjusting excitation
hwo to change reactive power
Reactive power:
Q 3ϕ =3Vph *I ph sinϕ so to change reaective we change iph sinphi
varying reactive power means eph moves in and out
what happens in over exicited case (supplying Q)
Eph > sqrt(Vph^2+(Xs*I ph)^2)
Field current is higher than normal
Iph lags Vph→ 𝜙>0
Q 3ϕ >0: Alternator is supplying reactive power
what happens in normally excited (no reactive flow)
Eph = sqrt(Vph^2+(Xs*I ph)^2)
Field current is at nominal
Iph in phase Vph→ 𝜙 = 0
Q 3ϕ =0: Alternator is supplying only real power
Scenario 3: Under-Excited (Absorbing Q)
Eph < sqrt(Vph^2+(Xs*I ph)^2)
Field current is lower than normal
Iph leads Vph→ 𝜙<0
Q 3ϕ >0: Alternator is absorbing reactive power
how to adjust eph
adjust Bm by adjusting field current
