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1
REQUIREMENTS OF POWER SYSTEM
It must supply energy practically everywhere the customer demands. The load demands vary with time. The system must able to supply this ever changing demand. The delivered energy must meet certain minimum requirements in regard to quality. The following factors determine the quality:
a) The system frequency must be kept around 50Hz with a variation of +0.05Hz to -0.05Hz.
b) The magnitude of the bus voltages are maintained within prescribed limit around the normal value. Generally the voltage variation should be limited to +5 to -5%.
The energy must be available with high reliability. The energy must be delivered without overloading any element in the power system. The energy must be delivered at minimum cost.
2
REAL POWER (P): The real power, P is defined as the average value of P and therefore, physically, means the useful power being transmitted. Its magnitude depends very strongly on the power factor cosΦ.REACTIVE POWER (Q):The reactive power, Q is by definition equal to the peak value of that power component that travels back & forth on the line, resulting in zero average, and therefore capable of no useful work.
3
TYPE OF LOADS:TYPE OF LOAD PHASOR PHASE
ANGLE
POWER ABSORBED BY THE LOAD
P Q
V
I
R VI Ф = 0° P > 0 Q = 0
V
I
L
V
I
Ф Ф = +90° P = 0 Q > 0
V
IC
V
I
Ф Ф = - 90° P = 0 Q < 0
R
L
R L
I
V
V
ΦV
I 0°<Φ<+90° P > 0 Q > 0
4
TYPE OF LOADS:TYPE OF LOAD PHASOR PHASE
ANGLE
POWER ABSORBED BY THE LOAD
P Q
-90°<Φ<0° P > 0 Q < 0
LI
V -90°<=Φ<=+90° P = 0 Q = 0
R
C
RC
V
V
I
IVΦ
C
Ic IL
Tuned to
Resonance
IL = Ic
PL = Pc
Energy travels
Back & forth
Between C&L
5
TYPE OF LOADS• Inductive load absorbs positive Q. i.e., an inductor consumes reactive power.• Capacitive load absorbs negative Q. i.e., a capacitor generates reactive power.• Sign change in Q simply means a 180° phase shift.• Resistive load consumes real power.• Inductive load consumes positive reactive power• Capacitive load consumes negative reactive power.• Combination of R & L load consumes real & positive reactive power.• Combination of R & C load consumes real & negative reactive power.• Reactive power is bi-directional power. It travels from
source to load as well as load to source.
6
CAPABILITY DIAGRAM OF A 110 MW ALTERNATOR
• I) COLLECT THE INFORMATIONS FROM T.G. NAME PLATE / MANUAL:1. Terminal Voltage : 11,000 V2. Rated MVA : 137.53. Rated p.f. (cos Ф) : 0.8 Lagging4. Rated Armature Current : 7220 A5. Rated Field Current : 1500 A6. Short Circuit Ratio : 0.5
• II) CALCULATED VALUES:1. MW = MVA X p.f. = 137.5 X 0.8 = 110 MW2. MVAR = MVA X SCR = 137.5 X 0.5 = 68.75 MVAR (Max. permissible zero
p.f. leading MVAR)3. Ф = cos-1(0.8) = 36.87°4. To ensure operational safety, there should be a margin of at least 12.5 % (given by the manufacturer) of the power rating of the generator between the working point & the theoretical stability (load angle ‘δ’) limit line. The operational limit of a generator rated at 0.8 p.f. lagging can be tabulated below:p.u. MW 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
p.u. MW + Margin 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
7
p.u. MW
Lagging p.f.Leading p.f.
Unity p.f.
0.10.2 0.3 0.4 0.50.60.70.80.91.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
A
B
C
D
O
TOTAL ROTOR (O
R) FIELD C
URRENT
TOTA
L S
TATO
R (
OR
) AR
MAT
URE
CU
RR
ENT
TH
EO
RIT
ICA
L S
TA
BIL
ITY
LI
MIT
LIN
E
0.1
0.2
0.3
0.4
0.5
0.6
0.7
TURBINE LIMIT LINE
SCR MVA X SCRMAXIMUM PERMISSIBLE
MVAR IN ZERO p.f. LEADING.
E F
G
STA
TOR
CU
RR
EN
T LIMIT
PR
AC
TIC
AL
ST
AB
ILIT
Y L
IMIT
WIT
H 1
2.5%
MA
RG
IN (δ=63°)
δ=90°
0.8
0.9
H
RO
TO
R C
UR
RE
NT
LIMIT
Ф=
36.8
7°
P.F.= 0.8 LAGGING
REAL POWER
VAR EXPORTVAR IMPORT
REACTIVE POWERp.u. MVAR (lagging)
REACTIVE POWERp.u. MVAR (leading)
CAPABILITY DIAGRAM OF A 110 MW ALTERNATOR
OE : No-load Field Current
OD : Field Current required for Armature Reaction
FGDHF : Capability Diagram of the 110 MW Alternator
8
p.u. MW
Lagging p.f.Leading p.f.
Unity p.f.
0.10.2 0.3 0.4 0.50.60.70.80.91.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
A
B
C
D
O
TOTAL ROTOR (O
R) FIELD C
URRENT
TOTA
L S
TATO
R (
OR
) AR
MAT
UR
E C
UR
REN
T
TH
EO
RIT
ICA
L S
TA
BIL
ITY
LI
MIT
LIN
E
0.1
0.2
0.3
0.4
0.5
0.6
0.7
TURBINE LIMIT LINE
SCR MVA X SCRMAXIMUM PERMISSIBLE
MVAR IN ZERO p.f. LEADING.
E F
G
STA
TOR
CU
RR
EN
T LIMIT
PR
AC
TIC
AL
ST
AB
ILIT
Y L
IMIT
WIT
H 1
2.5%
MA
RG
IN (δ=63°)
δ=90°
0.8
0.9
H
RO
TO
R C
UR
RE
NT
LIMIT
Ф=
36.8
7°
P.F.= 0.8 LAGGING
REAL POWER
VAR EXPORTVAR IMPORT
REACTIVE POWERp.u. MVAR (lagging)
REACTIVE POWERp.u. MVAR (leading)
CAPABILITY DIAGRAM OF A 110 MW ALTERNATOR
OE : No-load Field Current
OD : Field Current required for Armature Reaction
FGDHF : Capability Diagram of the 110 MW Alternator
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
9
III) III) COMPARISON
Actual MW=50 (i.e. 50/137.5=0.364p.u.)
Actual MVAR=6 (i.e. 6/137.5=0.044p.u.)
Arm. Current = 0.36p.u. X 7220A=2599A
Field Current=0.475p.u. X 1500A=712.5A
p.f.=cos(6.5°)=0.994 lag
Load Angle ‘δ’=33.4°
V=(MVA X 106)/(√3 X Iarm.)
=(√(MW2+MVAR2) X 106)/(√3 X Iarm.)
=(√(502+62) X 106)/(√3 X 2599) = 11.12KV
E.T.P.S. *** UNIT-5
DATE: 09.08.2004TIME: 11:00 Hrs.
MW = 50
MVAR = 6
Armature Current = 2600A
Field Current = 710A
p.f.= 0.98 lag
δ = -- (No measurement)
V = 11.2 KV
10
CAPABILITY CURVEA. Rotor current limit Class of insulation (to take care of rotor insulation)B. Stator current limit Class of insulation for stator.C. MW load limit Turbine limit (steam power generation capability) Turbine is designed for MW load only .D. Minimum load angle limit Leading p.f. operation Stability limit of generation E. Stator end heating limit Stressing stator winding & heating of stator 10 to 20 MVAR (leading p.f.) is safe Rotor is relieved from stress Stator end winding heated due to capacitive effect Remove capacitor banks in load centres In NCTPS 210 MW unit, running the generator at -64 MVAR
load for ½ an hour. Not able to reduce the load.
11
USEFULNESS OF CAPABILITY DIAGRAM FOR EXCITATION CONTROL
The information given by the capability diagram regarding full load rotor current (excitation), maximum rotor angle during steady state leading p.f. zone operation (<75°) etc., are essential for proper setting of the various limiters in the excitation control system.
Capability diagram give the basic information regarding the limiting zones of the operation so that limiters can be set / commissioned suitably for safe operation of the units.
12
FREQUENCY IS RELATED TO REAL POWER ( P – f )
SMALL DROP IN SYSTEM LOAD.
VALVE SETTINGS ARE IGNORANT OF THE LOAD CHANGE.
INPUT TORQUE TO EACH MACHINE REMAINS UNALTERED.
DECREASE IN CURRENT SUPPLIED BY EACH ALTERNATOR.
DECREASE IN ELECTRO-MAGNETIC TORQUE BY EACH ALTERNATOR.
EACH ALTERNATOR EXPERIENCES SURPLES ACCELERATING TORQUE.
SLIGHT INCREASE IN SPEED AND FREQUENCY.
13
EFFECT ON OTHER LOADS:
AT HIGHER FREQUENCY, THE REMAINING LOAD ROTATES AT HIGHER SPEED AND TAKES MORE CURRENT.
HENCE THE LOAD DEMAND INCREASES. POWER GENERATION AT HIGHER FREQUENCY EQUALS THE LOAD
DEMAND POWER. TO DECREASE THE FREQUENCY, THE VALVE MUST BE CLOSED
SLIGHTLY.
EXAMPLE: PUMP SET (INDUCTION MOTOR)
At high frequency, the speed of IM increases.
Ns = 120f / P Nr = Ns ( 1 - s )
The current taken by the IM will be more. Hence the demand on the system increases.
ADJUSTINGINPUT VALVES
CONTROLSFREQUENCY
CONTROLSREAL POWER
14
VOLTAGE IS RELATED TO REACTIVE POWER ( Q – V )
V1 V21 2
G1
jX
P jQ
I
1. Bus Voltage V1 is kept at constant magnitude.2. Transmission line has reactance only i.e. jX.3. Power flow is P Q.
Take V1 as reference.V2=V1-jXI -----------------------------------(1)V1 * I = P jQI = (P-jQ) / V1 ------------------------------(2)Substitute (2) in (1)V2 = V1 – jX [(P/V1) – j(Q/V1)]V2 = [V1 – (X/V1)Q] – j(X/V1)P]
15
VECTOR DIAGRAMS:
V2 = V1- X Q
V1
- j X P V1
BOTH DROPS EQUAL
V1
V2
DOUBLE P DOUBLE Q
X
V1
P
XV1
QV1 V1
V2
V2
X
V1Q
2 X
V1P
2 X
V1Q
X
V1P
DOUBLE “P ”: VOLTAGE ANGLE WILL CHANGE. NO CHANGE IN MAGNITUDE.
DOUBLE “Q ”: VOLTAGE MAGNITUDE IS VERY MUCH RELATED TO REACTIVE POWER.
MORE “Q ” FLOW WILL AFFECT THE VOLTAGE
EXCITATION MORE
EXCITATION LESS
LAGGING MVAR
LAGGING MVAR
GEN. VOLTAGE
GEN. VOLTAGE
16
REACTIVE POWER INJECTION AT LOAD SIDE BY USING SHUNT CAPACITORS, IMPROVES THE VOLTAGE.
UNDER LIGHT LOAD CONDITIONS, RECEIVING END VOLTAGE > SENDING END VOLTAGE (FERRANTI EFFECT) DUE TO CAPACITIVE LOAD. CONNECT SHUNT REACTORS TO CONTROL VOLTAGE.
SYNCHRONOUS CONDENSER IS USED TO ABSORB or TO DELIVER THE REACTIVE POWER.
SYNCHRONOUS MOTOR UNDER NO-LOAD CONDITION IS SYNCHRONOUS CONDENSER.
PEAK LOADCONDITION
LIGHT LOADCONDITION
CONNECTCAPACITORS
CONNECTREACTORS
TO CONTROLVOLTAGE
17
POWER DIAGRAM (CAPABILITY DIAGRAM):
• ASSUMPTION: I.R. drop is negligible. CASE-I: In Δ ABC, BC=E Sinδ In Δ BCD, BC=IXd
CosФ E Sinδ = IXd CosФ
Multiply both sides by V Xd EV Sinδ = VI CosФ = REAL Xd POWER
At δ=90°, We get the maximum power i.e. the theoritical stability line.• CASE-I I: In Δ ABC, CD=AC – AD; In Δ BCD, CD=IXd SinФ
In Δ ABC, AC=E Cosδ & AD = VIXd SinФ = E Cosδ - V ; Multiply both sides by V , We get
Xd EV Cos δ – V2 = VI Sin Ф = REACTIVE POWER Xd Xd
E
V
IXd MW
MVARIΦ
δ ΦA
B
CD
18
SHORT CIRCUIT RATIO ( SCR ):SCR = FIELD CURRENT REQUIRED TO PRODUCE RATED VOLTAGE ON O.C.
FIELD CURRENT REQUIRED TO CIRCULATE RATED CURRENT ON S.C.
SCR = o Fo c Fo c Fo 1 1 1
o Fs b Fs a Fo a Fo / c Fo Per unit voltage on open circuit Xd
Corresponding per unit current on short circuit
= RECIPROCAL OF SYNCHRONOUS REACTANCE
FIELD CURRENT
O.C.C.
S.C.C.
1.0
PE
R U
NIT
VO
LT
AG
E
PE
R U
NIT
CU
RR
EN
T
a b
c
o Fo Fc
A B
C
D
E
AD AE DE
AB AC BC
19
TYPICAL S.C.R. VALUES: For 500 MW T.G., SCR= 0.48 For 210 MW T.G., SCR= 0.49 For 110 MW T.G., SCR= 0.50 For 60 MW T.G., SCR= 0.59 The SCR value may have to be raised to 1.0 to 1.5, if the loading is likely to be
capacitive i.e. leading MVAR supply. For modern Turbo-alternator, the SCR is normally between 0.48 to 0.7
EFFECT OF S.C.R. ON MACHINE PERFORMANCE: Higher value of SCR has higher stability limit. Better voltage regulation for high SCR. High value of SCR has a long air gap which means that the mmf required by
field is large. Hence machine with higher SCR is costlier to build.
AIR GAP SCR WEIGHT SIZETRANSPORTATION
PROBLEM
Present trend is to build low value of SCR since fast acting excitation system available.
20
GENERATOR – IMPORTANT TIPS• T.G. CAPACITY IN M.W.: 50
6062.5100110120200210 – Weight: 250 tonnes235250500800 future1000 future
21
GENERATOR – IMPORTANT TIPS
• T.G. TERMINAL VOLTAGE IN KV :10.511 – ETPS 60 MW, 110 MW13.815 – Neyveli-Stage I15.75 – BHEL 210 MW16 – Nuclear 235 MW18.4 – NTPC 210 MW21 – 500 MW
22 - 500 MW 33 (or) 34 – Future (800 MW/1000 MW)
requires 800 KV line (year 2010)
22
GENERATOR – IMPORTANT TIPS Higher capacity Hydro machine in India : 250 MW, KOINA (Maharastra), Air cooled.
Higher capacity T.G. in India : 500 MW.
Higher capacity T.G. : Advantage : Reduction of cost of Generation. Limitations : (i) Transportation problem
(bigger size) (ii) Do not have adequate
transmission lines.
Higher capacity G.T. in India : 315 MVA, 3 phase, single unit, 400 KV.
Maximum voltage : 400 KV AC.
National Grid : 800 or 765 KV line – year 2010.
Regional Grid : 400 KV line.
World highest : FRANCE, 1500 MW T.G., Nuclear , 1600 MVA, 1200 KV.
23
GENERATOR – IMPORTANT TIPS
SPECIFICATION FOR ROTATING MACHINES:
IEC 34 Part – I, II, III (International Electro-Technical commission)IS 5422
2*105 hours guaranteed operating time (23 years) 8760 hrs/year. 104 start/stop times. Total life time : 25 years. Capital O/H : Once in 4 to 5 years (25 days). Annual O/H : < 10 days.