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Symmetrical system components
Decomposition of unsymmetrical (unbalanced) voltage:
0C2C1CC
0B2B1BB
0A2A1AA
UUUU
UUUU
UUUU
Positive sequence (1), negative (2) and zero (0) sequence.
Hence (reference phase A)
022
1C
0212
B
021A
UUaUaU
UUaUaU
UUUU
02
21C
0212
B
021A
IIaIaI
IIaIaI
IIII
where 3
2je
23j
21a
3
4j2 e23j
21a
Matrix
120
0
2
1
2
2
C
B
A
ABC UTUUU
1aa1aa111
UUU
U
Inversely
ABC1
C
B
A2
2
0
2
1
120 UTUUU
111aa1aa1
31
UUU
U
3ph power
*ABCT
ABC
*
C
B
A
CBA*CC
*BB
*AA IU
III
UUUIUIUIUS
*120*TT
120*
120T
120 ITTUITUTS
E3300030003
1aa1aa111
111aa1aa1
TT2
22
2
*T
*120T
120 IU3S
*00
*22
*11 IUIUIU3S
Series symmetrical segments in ES
C
B
A
C
B
A
III
ZZZZZZZZZ
UUU
ABCABCABC IZU 120ABC120 ITZUT 120120120ABC
1120 IZITZTU
TZTZ ABC1
120
Z2Z00
0ZZ000ZZ
Z000Z000Z
Z
0
2
1
120
Multiple lines
ABC
2V
1V
ABC
ABC
2V
1V
...II
Z...UU
120
2V
1V
ABC
120
2V
1V
...II
...000T000T
Z...UU
...000T000T
120
2V
1V
ABC
1
120
2V
1V
...II
...000T000T
Z...000T000T
...UU
120
2V
1V
ABC1
1
120
2V
1V
...II
...000T000T
Z...000T000T
...UU
Shunt symmetrical segments in ES
ABCNABCABCABC IZIZU
NNN
NNN
NNN
N
ZZZZZZZZZ
Z
120N1
120ABC1
120 ITZTITZTU
TZZTZ NABC1
120
N
120
Z3Z2Z000ZZ000ZZ
Z
Symmetrical components voltages in the symmetrical segments depend only on the corresponding component current and component impedance.
Inductors and capacitors in ES
a) Series inductors
- reactors are used to limit short-circuit currents - used in grids up to 35 kV, single-phase (In > 200A) or three-phase
(In < 200A), usually air-cooled (small L) - the same design in LC filters for harmonics suppression
Input: Xtl%, Stl, Un, In
Calculation: nntl .I.U3S
tl
2nt%
n
nt%tl S100
UXI3100
UXX
IZI)Xj(RUUU tttf2f1f EZZZ tt012tabc - 3ph inductor → self-impedance tZ , mutual impedances 0
In fault-free state the inductor can be bypassed by a fuse to reduce a voltage drop.
Rtl << Xtl
b) Shunt (parallel) inductors
- in the systems UN > 220 kV, oil cooling, Fe core - used to compensate capacitive (charging) currents of
overhead lines for no-load or small loads → U control:
n tl
2n tl
n tl
n tltl Q
UI3
UX
0tlt2tl1tl Z,ZZZ Connection in the system:
a) galvanic connection to the line - Y winding, neutral point connected to the ground
through V only during auto-reclosing (disturbances)
b) inductor connection to transformer tertiary winding - lower voltage Un ≈ 10 ÷ 35 kV - problem with switch-off (purely inductive load)
Kočín 400 kV
c) Neutral point inductors
- used in networks with indirectly earthed neutral point to compensate currents during ground fault
- value of the fault current does not depend on the ground fault position and the current is capacitive
- inductor reactance Xtl should assure that the value of the inductive current is equal to the value of capacitive current → arc extinction
- for voltage 6 to 35 kV (rated at Ufn), reactor is single-phased!, oil cooling - capacitive current change (network reconfiguration) → change in
inductance (air gap correction in the magnetic circuit) = arc-suppression coil (Peterson coil)
- it doesn’t occur in positive and negative sequence component, tl0 3XX
6 MVAr, 13 kV, Sokolnice
d) Series capacitors
- capacitors in ES = capacitor banks = series and parallel connection - to improve voltage conditions (MV lines) or adjusting parameters (long
HV lines) - voltage and power of the capacitor varies with the load - during short-circuits and overcurrents there appears overvoltage on the
capacitor (very fast protections are used)
IC
1jUC
- C must be insulated against the ground (insulated platforms) – C under voltage
- drawback – allows harmonic currents flow - current distribution among parallel transmission lines could be achieved
Canada 750 kV
e) Shunt capacitors
- used in industrial networks up to 1 kV - connection: a) wye - Y
b) delta - Δ (D)
Y2fCff CUIUQ CUIUQ 2
Cf Y
2Y
2f CUCU3Q CU3Q 2
- with the same reactive power
CU3CU 2Y
2 → C3CY → rather delta
- reactive power compensation a) QC < Q under-compensated b) QC = Q exact compensation c) QC > Q over-compensated
→ power factor improvement, lower power losses, voltage drops - individual or group compensation could be used
Transformer parameters
350 MVA, 400/110 kV YNauto - d1, Sokolnice
a) Two-winding transformers
- winding connection Y, Yn, D, Z, Zn, V Yzn – distribution TRF MV/LV up to 250 kVA, for unbalanced load Dyn – distribution TRF MV/LV from 400 kVA Yd – block TRF in power plants, the 3rd harmonic suppression Yna-d, YNynd – power grid transformer (400, 220, 110 kV) YNyd – power grid transformer (e.g. 110/23/6,3 kV)
- equivalent circuit: T – network σppσp jXRZ σssσs jXRZ qqq jBGY
- each phase can be considered separately (unbalance is neglected), i.e. operational impedance (positive sequence) is used
- values of the parameters are calculated, then verified by no-load and short-circuit tests
o no-load test – secondary winding open, primary w. supplied by rated voltage, no-load current flows (lower than rated one)
o short-circuit test – secondary winding short-circuit, primary w. supplied by short-circuit voltage (lower than rated one) so that rated current flows
P0 (W), i0 (%), Pk (W), zk = uk (%), Sn (VA), Un (V) uk ≈ 4 ÷ 14 % (increases with TRF power) pk ≈ 0,1 ÷ 1 % (decreases with TRF power) p0 ≈ 0,01 ÷ 0,1 % (decreases with TRF power)
- shunt branch:
n
0q S
ΔPg 100iy 0%
q 2q
2qq gyb
2
n
02
0%
n
0q bjg
SΔP
100ij
SΔPy
2
n
02
0%
n
02n
n2n
nqq BjG
SΔP
100ij
SΔP
US
USyY
- series branch:
n
kk S
ΔPr 100
uz k%k
2k
2kk rzx
kk
2
n
k2
k%
n
kk xjr
SΔP
100uj
SΔPz
kk
2
n
k2
k%
n
k
n
2n
n
2n
kk XjRSΔP
100uj
SΔP
SU
SUzZ
σsσpspkσps XXjRRZZ
- we choose σsσpsσp ZZ5,0Z
- this division is not physically correct (different leakage flows, different resistances)
- usage of T-network to calculate meshed systems is not appropriate sometimes (it adds another node)
- therefore calculation using -network, -network
b) Three-winding transformers
- parameters are calculated, then verified by no-load and short-circuit measurements (3 short-circuit tests: 1 winding no-load, 1 short-circuit and 1 supplied): P0 (W), i0 (%), Pk (W), zK = uK (%), Sn (VA), Un (V)
- powers needn’t be the same: e.g. SSn = STn = 0,5·SPn
- equivalent circuit:
- no-load measurement: related to the primary rated power and rated voltage SPn a UPn (supplied)
2
Pn
02
0%
Pn
0qqq S
ΔP100ij
SΔPbjgy
denominated value (S) – related to UPn
2
Pn
02
0%
Pn
02Pn
Pnqq2
Pn
Pnqq S
ΔP100ij
SΔP
USBjG
USyY
- short-circuit measurement: (3x, supply – short-circuit – no-load)
provided: SPn ≠ SSn ≠ STn
measurement between P - S P - T S - T short-circuit losses (W) PkPS PkPT PkST short-circuit voltage (%) ukPS ukPT ukST measurement corresponds to power (VA) SSn STn STn
short-circuit tests S – T: parameter to be found:
σTσSST ZZZ σSSσS XjRZ - recalculated to UPn σTσSST zzz - recalculated to UPn, SPn
Pk for ITn → PkST = 3·R+ST·I2Tn , Tn
TnTn U3
SI
R+ST….resistance of secondary and tertiary windings (related to UTn) 2Tn2
Tn
kSTST U
SPR
2Tn
2Pn
STST UURR
→ 2Pn2
Tn
kSTTSST U
SPRRR
RS (RT)…resistance of sec. and ter. windings recalculated to primary
Pn2Tn
kST2Pn
PnSTST S
SP
USRr
- impedance:
Tn
PnkST%ST S
S100
uz , Tn
2PnkST%
Pn
2Pn
STST SU
100u
SUzZ
STSTST xjrz , 2
ST2STST rzx , σTσSST xxx
- based on the derived relations we can write: P - S:
2
Pn2Sn
kPS
2
Sn
PnkPS%Pn2
Sn
kPSPSPS S
SΔP
SS
100ujS
SΔPxjrz
PS
22Pn2
Sn
kPS
2
Sn
2PnkPS%2
Pn2Sn
kPSPSPS U
SΔP
SU
100ujU
SΔPXjRZ
PS
- analogous for P – T and S – T
- leakage reactances for P, S, T: )ZZZ(5,0XjRZ STPTPSPPσP )ZZZ(5,0XjRZ PTSTPSSSσS )ZZZ(5,0XjRZ PSSTPTTTT
- knowledge of the series impedances and shunt admittances allows to study voltage and power conditions of 3-winding transformers
- mentioned impedances are valid for positive and negative sequences
Transformers zero sequence impedances
Series parameters are the same as for the positive sequence, the shunt always need to be determined. Assumptions:
- Zero sequence voltage supplies the primary winding. - The relative values are related to UPn and SPn. - We distinguish free and tied magnetic flows (shell x core TRF).
Z0 depends on the winding connection.
note: reluctance, inductance
magnetic resistance (reluctance)
Sl1R m
analogy Sl
Sl1R e
magnetic flux (Hopkinson’s law)
mRIN
analogy (Ohm’s law) eR
UI
m
2c
RN
IN
IL
LLRR Fe0mmFe0Fe
TRF magnetic circuit
LL
NI
NIL
NIL
RIN
RIN
FeFe
0mmFeh
a) Y / any connection 0i3 0
0
00 i
uz 0p0z ps0z
b) D / any connection Zero sequence voltage is attached to D → voltage at each phase
0uu 00 → 0iii cba → 0i0
0
00 i
uz 0p0z ps0z
Primary winding Secondary winding
Any connection
Primary winding Secondary winding
Any connection
c) YN / D Currents in the primary winding i0 induce currents i0’ in the secondary winding to achieve magnetic balance. Currents i0’ in the secondary winding are short-closed and do not flow further into the grid.
0qp0p0 zzz
0qs
0qsp
0
00 zz
zzz
iuz
shell s
1q0q zyz → k1ps0 zzz
3-core
1q0q yz → ps0 z9,07,0z
Primary winding Secondary winding
d) YN / Y Zero sequence current can’t flow through the secondary winding. Current i0 corresponds to the magnetization current.
ps0z 0qp0p00 zzzz
shell
1q0q yz → 0z
3-core
1q0q yz → 13,0z0
Primary winding Secondary winding
e) YN / YN If element with YN or ZN behind TRF → points a-b are connected → as the positive sequence. If element with Y, Z or D behind TRF → a-b are disconnected → as YN / Y.
Primary winding Secondary winding
f) ZN / any connection Currents i0 induce mag. balance on the core themselves → only leakages between the halves of the windings.
ps0z ps0p00 z3,01,0zz
p0 rr
Primary winding Secondary winding
g) impedance in the neutral point Current flowing through the neutral point is 3i0. Voltage drop: 0u0uuz iz3i3zu → in the model uz3 in series with the leakage reactance
h) three-winding TRF
Tertiary winding
Primary winding
Secondary winding
System equivalent
Impedance (positive sequence) is given by the nominal voltage and short-circuit current (power). Three-phase (symmetrical) short-circuit: )kA(I),MVA(S kk
knk IU3S
k
n
k
2n
s I3U
SUZ
CR: 400 kV MVA300006000Sk kA459Ik 220 kV MVA120002000Sk kA302Ik
110 kV MVA3000x100Sk kA15xIk