Bài giảng ngành Điện: Giải các bài toán trong mạch điện điện thế thấp một pha và ba pha (Phần B) gồm các chủ đề sau Solve problems in single and threephase low voltage circuits Part B Content Topic 1: AC power and power factor Topic 23: Multiphase systems Topic 4: Threephase STAR connection Topic 5: Threephase, Fourwire systems Topic 6: Threephase DELTA connection Topic 7 Interconnected Star and Delta Devices Topic 8 Energy and Power Requirements of AC Systems Topic 9: Harmonics
Trang 1Solve problems in single and three-phase low voltage
circuits
Topic 5: Three-Phase, Four-Wire Systems
Trang 2Three-Phase, Four-Wire
Systems
A three-phase, four-wire system consists of three ACTIVE
conductors plus a NEUTRAL conductor (note: the earth
conductor does not normally carry current therefore is excluded).
This type of system must be a STAR-connected system, due to the existence of the neutral conductor, and is most often
connected to unbalanced loads.
Unbalanced loads produce an out-of-balance current at the point (the phases have unequal current flows in them, thus they cannot take away all of the current that is supplied by the other phases).
star- The NEUTRAL conductor’s primary purpose is to carry this of-balance current away from the star-point.
Trang 3out-Determining the Neutral
8ΩPƒ=1
Trang 4Determining the Neutral
Step 3: Draw a scaled phasor diagram (based on VA as the
reference phasor) of Currents at their appropriate phase angle to their respective phase voltage.
Step 4: Use phasor addition to find the resultant current (IABC)
Step 5: Draw this current 1800 out-of-phase and label IN.
Step 6: Measure and label the phase angle (ØN) compared to the reference.
Trang 5Determining the Neutral
Trang 6Neutral Current: Answers
ZA ZB ZC
5Ω
Pƒ=1
12ΩPƒ=1
8ΩPƒ=1
L1 L2 L3 N
3Ø, 400 V
Answers:
1) IABC = 23.4A2) Neutral current IN = 23.4A3) Phase angle ØN = 1590 Lag
VA
Trang 7Neutral Current Exercises
Complete the exercises on three-phase,
four-wire systems (calculating neutral
current)
Trang 8Neutral Current Exercise
0.8lag
10ΩPƒ=
5) Phase angle ØN
Trang 9Answer – balanced load
Trang 10Neutral Current Exercise
0.8lag
10ΩPƒ=
0.8lag
L1 L2 L3 N
Answers:
1) IA=IB=IC=23.1A2) ØA =ØB =ØC=36.90E Lag3) IABC = 0A (BALANCED LOAD)
4) Neutral current IN = 0.0A5) Phase angle ØN = 00 IN-PHASE
Trang 11Neutral Current Exercise
0.6lag
20ΩPƒ=
5) Phase angle ØN
Trang 12Neutral Current Exercise
0.6lag
20ΩPƒ=
0.4lag
L1 L2 L3 N
Answers:
1) IA=IB=IC=12A2) ØA=36.90 lag VA, ØB =53.10
lag VB, ØC=66.40 lag Vc3) IABC = 4.9A
4) Neutral current IN = 4.9A5) Phase angle ØN = 1680 lag reference
Trang 13Neutral Current Exercise
0.8lag
20ΩPƒ=
5) Phase angle ØN
Trang 14Neutral Current Exercise
0.8lag
20ΩPƒ=
0.6lag
L1 L2 L3 N
Answers:
1) IA=IB=IC=12A2) ØA=36.9 lag VA, ØB =53.1 lag
VB, ØC=66.40E lag Vc3) IABC = 4.9A
4) Neutral current IN = 4.9A5) Phase angle ØN = 1680 lag reference
Trang 15Neutral Current Exercise
0.8lead
50ΩPƒ=
5) Phase angle ØN
Trang 16Neutral Current Exercise
0.8lead
50ΩPƒ=
lead VB, ØC=25.80 lag Vc3) IABC = 241A
4) Neutral current IN = 241A5) Phase angle ØN = 1130 lead reference
Trang 17Functions of a Neutral Conductor
Trang 18Functions of a Neutral
Conductor
the star-point.
To maintain the phase voltages equal in value.
To allow the connection of single-phase loads.
To carry fault currents in a distribution system that uses the MEN system.
To carry third harmonic currents produced in circuits with certain types of loads.
Trang 19Neutral Conductor Size
Specifications: AS3000
New Specifications AS3000:2007 Clause 3.5.2
Multiphase Circuits
The current-carrying capacity of the neutral conductor
of the consumer’s mains, mains, and final
sub-circuits shall be not less than the CCC of the largest associated active conductor.
Trang 20Effects of a Broken or High Impedance
Neutral
A Effects on the load
B Effects on the MEN system
Trang 21Effects of a Broken or High
Impedance Neutral
Trang 22Effects of a Broken or High
Impedance Neutral
Effects on Three-Phase Load
by the star system
causes system instability.
Higher currents will flow in the phases.
The phase voltages will become irregular
some phase voltages higher, some lower.
The star-point will “FLOAT” above zero potential.
Trang 23Effects of a Broken or High
Impedance Neutral
Trang 24Effects of a Broken or High
Impedance Neutral
The out-of-balance current will attempt to travel via the
MEN link through to the earth electrode, and back to the supply transformer.
Since the earth return path is normally poor, insufficient
current will flow thus causing the star-point to float above zero potential.
This floating voltage will be impressed onto the earthing system, causing it to become live.
This represents a significant risk of electric shock Note: The circuit protection is unlikely to recognize this situation
as a fault, thus will remain energized.
Trang 25Effects of a Broken or High
Impedance Neutral