3phase system n power concept - mrs lieyna
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EEE3233
POWER SYSTEM
CHAPTER 3:CHAPTER 3:3-Phase System and Power3-Phase System and Power
ConceptConcept
Noorlina MohdNoorlina Mohd
ZainuddinZainuddin
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IntroductionIntroduction
q The generation, transmission anddistribution of electric power isaccomplished by means of
three-phase circuit.qAt the generating station, three
sinusoidal voltage have equal
magnitude and an equal 1200
-phase difference between anytwo phase.
q This is called a balanced source.
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qPositive phase sequence when thevoltages reaches their peak value in
the sequential ABCqNegative phase sequence when the
phase is order is ACB.
ECn
EBn
EAn
EAn
ECn
EBn
Positive or ABC phase
sequence
Negative or ACB phase
sequence
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Y-Connected Load Y-Connected Load
- -A Y connecter generator supplying a Y.connected load
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Y-Connected Load (cont.) Y-Connected Load (cont.)
§ Assume a positive sequence to find therelationship between the line voltage(line-to-line voltage) and phase voltage
(line-to-neutral voltage).§ Line –to-neutral voltage of the phase ischosen as a reference.
|Vp|= magnitude of phase voltage
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Y-Connected Load (cont.) Y-Connected Load (cont.)
§ The line voltages at the load terminals interms of the phase voltage are found by
application of Kirchhoff’s voltage law.
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§ - -C o m p a riso n o f th e se lin e to lin e vo lta g e s w ith th e
- -lin e to n e u tra l vo lta g e s o f le a d s to th e fo llo w in g
:co n clu sio n
- -In b a la n ced th re e p h a se Y con n e cte d
- , - -syste m w ith p o sitiv e se q u e n ce so u rce th e lin e to
√ - -lin e vo lta g e a re 3 tim e s th e lin e to n e u tra l
°. ;vo ltag e s an d le ad b y 30 Th a t is
V ab = √ 3V an + °30
Y-Connected Load (cont.) Y-Connected Load (cont.)
V bc = √ 3V bn + °30V ca = √ 3V cn + °30
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§ T h e re la tio n sh ip b e tw e e n th e lin e vo lta g e a n d p h a se vo lta g e :is sh o w n a s b e lo w
Y-Connected Load (cont.) Y-Connected Load (cont.)
Phasor diagram showing phase and linevoltages
Vab
Vbc
Vca
Van
Vbn
Vcn
Voltages Triangle
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§ RMS value of any of the line voltages is
denoted by VL, then one of the importantcharacteristic of the Y-connected tree-phaseload may be expressed as
Y-Connected Load (cont.) Y-Connected Load (cont.)
§ - -T h re e p h a se cu rre n t a lso p o ssess th re e p h a se sym m e try a n d a re g ive n b y
IL = IP
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§A balanced ∆-connected load (withequal phase impedances) is shown in
below:
∆ ∆-Connected Loads-Connected Loads
§ Lin e v o lta g e a re th e sa m e a s p h a se
.vo lta g e s
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§ The phase current Iab
is chosen as reference
∆ ∆-Connected Loads (cont.)-Connected Loads (cont.)
§ T h e re la tio n sh ip b e tw e e n p h a se a n d lin e
cu rre n ts ca n b e o b ta in e d b y a p p ly in g
’ .K irch h o ff s cu rre n t la w a t th e co rn e r o f ∆
I p = magnitude of
phase current
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§ The RMS of any of the line currents andphase is denoted by IL
∆ ∆-Connected Loads (cont.)-Connected Loads (cont.)
§ √ T h e m a g n itu d e o f lin e cu rre n t is 3 tim e s th e
,m a g n itu d e o f th e p h a se cu rre n t a n d th e lin e
°.cu rre n ts la g s o f p h a se cu rre n ts b y 3 0
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§ The relationship between the line currents and phase currents is showing
.in phase diagram
∆ ∆-Connected Loads (cont.)-Connected Loads (cont.)
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∆ -Y Transformation
§ ,Fo r a n a lyzin g n e tw o rk p ro b le m s it is-con ve n ie n t to re p la ce th e ∆ con n e cte d
-circu it w ith a n e q u iv a le n t Y co n n e cte d
.circu it
§ -C o n sid e r th e Y co n n e cte d circu it o f Z Y Ω /phase which isequivalent to a balanced
-co n n e cte d circu it o f Z ∆ Ω / .phase
∆ -connection -Y co n n e ctio n
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∆ - ( .)Y Transformation cont
§ -For ∆ co n n e cte d circu it
( )1
§ ,Fro m th e p h a sor d ia g ra m th e re la tio n sh ip
- -b e tw e e n b a la n ce d p h a se a n d lin e to lin e
;v o lta g e( )2
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∆ - ( .)Y Transformation cont
§ .( ) .( ),S u b stitu tin g eq 2 in to eq 1 w e g et
( )3
o r
( )4
§ - ,For Y co n n e cte d circu it w e h a ve
( )5
§ .( ) .( ),S u b stitu tin g e q 4 in to e q 5 w e fin d th a t
( )6