Harmonic Analysis Flashcards
When harmonics are involved what percentage of Breaker settings is advisable for Other load breakers and capacitor bank breaker?
As per IEEE standard
Considering harmonics :
Capacitor bank breakers pick up= 1.4 times *full load amps
And add additional 20 % margin which is
1.6 times *full load amps
Capacitor bank voltage should be equal to
Bus voltage it is connected ,otherwise kvar value varies
What equipment is needed for 12 pulse converter?
Transformer
In this circuit or device current leads voltage by 90 degree and power factor is zero
Pure capacitor because true power equal to zero
In this circuit or device current lags voltage by 90 degree and power factor is zero
Pure inductive circuit because true power qual to zero and no dissipitative resistive component in circuit and circuit is reactive
In this circuit current and voltage are in phase and Power factor is one
Resistive circuit because reactive power equals to zero
Average power is zero in
Inductive load which leads to current is flowing without no work done by inductive load but current circulates in line thereby increasing losses and voltage
Average power is not zero in
Resistive load
Frequency of power is double of its voltage frequency in which load?
Pure inductive load
Real power is positive or unidirectional in which load ?
Resistive load
Power is negative for small duration and power is positive for longer duration for which power factor?
Cos(0.8)
Power factor=0.80
True power factor formula
Cos pi* sqr root of 1/1+(thd)^2
When try power factor and thd are used
When non linear load are present
Which is voltage harmonic devices?
Arc furnace and all other devices are current harmonic device
What are harmonic sources?
A devices which changes the nature of voltage and current waveforms
If inductive reactance increases?frequency?
Frequency increases
When capacitive reactance increases?frequency?
Frequency decreases
Which equipment provides low impedance path for high frequency harmonics?
Shunt capacitance so harmonic current goes to capacitance instead of source
Which equipment opposes high frequency harmonics?
Inductor
Formula for peak value
Square root of 2 * rms value
If source impedance of grid is increased what happens to source short circuit current and voltage thd? And current THD?
Source short circuit current decrease which results in increase in voltage THD
If source impedance of grid is decreased what happens to source short circuit current and voltage thd?
Source short circuit current increases which results in decrease in voltage THD
If x/R ratio of source increases?
THD increase
If x/R ratio of source decreases?
THD decreases
R value remains same while X value varies
If Transformer impedance is increased
THD below Transformer increases and THD above Transformer decreases
If transformer impedance is decreased
THD below TRANSFORMER decreases and THD above transformer increase
When connecting capacitor bank in Down stream system filled with harmonic source?
Smart thing is add reactor with capacitor bank because reactor limits harmonic current
cable selections is depends on what factors?
1) voltage drop
2) short circuit withstand capacity
3) current carrying capacity
what is formula for MIN cross sectional area of cable
A= SHORT CIRCUIT CUURENT * (SQRT ROOT OF t)/K
t= FAULT CLEARING TIME OR DURATION OF SHORT CIRCUIT k= S.C TEMP RISE CONSTANT
WHAT IS K VALUE FOR XLPE AL CABLE AND XLPE CU CABLE?
K=0.094(FOR XLPE AL)
K=0.143(FOR XLPE CU)
FLA FORMULA?
KVA/1.732*V
VOLTAGE DROP FORMULA?
IRCOS(PI)+ jXSIN (PI)
WE NEED GRID DATA FOR HARMONIC STUDY OR NOT?IF WE DON’T HAVE GRID DATA?
YES WE NEED GRID DATA.IF WE DONT HAVE GRID WE SHOULD SEE SWITCH GEAR DATA OR SLD
THE WORST CASE SCENERIO FOR HARMONIC ANALYSIS OCCURS DURING
MIN SHORT CIRCUIT CURRENT
HARMONIC ANALYSIS SHOULD BE DONE AT WHAT SC CURRENT?
MIN SHORT CIRCUIT CURRENT
AC RESISTANCE OF CONDUCTOR SHOUL BE CONSIDERED AT WHAT TEMPERATURE?
1) AC RESISTANCE OF CONDUCTOR SHOULD BE CONSIDERED AT MINIMUM TEMP FOR SHORT CIRCUIT STUDY
2) AC RESISTANCE OF CONDUCTOR SHOULD BE CONSIDERED AT MAXIMUM TEMP FOR LOAD FLOW STUDY AND HARMONIC STUDY AND TRANSIENT STABILITY
IF TEMPERATURE RISES WHAT HAPPENS TO RESISTANCE
RESISTANCE INCREASES
R t FORMULA
R t = R o(1+ ALPHA*(t - t o)
AL[PHA FOR CU CABLE
ALPHA FOR AL CABLE
YOU HAVE TO GET DC RESISTANCE OF CONDUCTOR AT WHAT TEMP
AT ALL TEMP
KVA FORMULA
KVA= 1.732 VI
FOR HARMONICS WHAT THREE LIMITS OR THINGS COME INTO PICTURE?
1) VOLTAGE DROP LIMIT
2) STABILITY LIMIT
3) THERMAL LIMIT
WHY WE CANT GET PARAMETER SUCH AS LINE REACTANCE AND CAPACITANCE?
BECAUSE LINE REACTANCE AND CAPACITANCE DEPENDS UPON
1) TOWER STRUCTURE AND HEIGHT
2) SPACING BETWEEN CONDUCTORS
3) HOW MANY GROUND WIRES
4) SPACING BETWEEN GROUND WIRES
IF WANT TO CONSIDER RLC PARAMETERS FOR BUSDUCT MODELLING.WHAT YOU HAVE TO DO
MODEL BUSDUCT AS AN IMPEDENCE(MODEL) IN ETAP
HOW TO FIND CABLE TYPE WHENT IS MENTIONED?
YOU CAN FIND CABLE TYPE BY SEEING MIN CROSS SECTIONAL AREA OF CONDUCTOR
(A= I sh * SQRT OF t/K)
(REG 5 WINDING INTO 3 WINDING TRAFO)
WHAT IS IMPEDANCE VOLTAGE (U k) OF WINDING UNDER TEST(SC TEST)?
PRIMARY WINDING VOLTAGE AT WHICH YOU GET RATED CURRENT AT SHORTED SECONDERY WINDING
PER UNIT IMPEDANCE FORMULA
IMPEDANCE VOLTAGE/RATED VOLTGE (IF NECESSCARY MULTIPLY TIS VALUE BY 100%)
Z P.U NEW FORMULA
Z p.u OLD * (KVA OLD/KVA NEW)^2 *
MVA NEW/MVA OLD
X/R RATIO FORMULA
LOAD LOSS or Cu loss/BASE MVA
HOW WILL DETERMINE MAX LOAD VALUE (i.e of trafo)IF IT IS NOT KNOWN
ASSUME LOAD VALUE AS 80% OF TRAFO MVA
HOW PF OR KVAR CAN BE IMPROVED AT SOURCE(BUS)?
IMPROVED BY ADJUSTING GENERATING VOLTAGE(AVR) OR ADJUSTING OLTC
WHAT IS A STANDARD FOR HARMONIC ANALYSIS?
IEEE 519 2014
WHAT IS TDD
TOTAL DEMAND DISTORTION
AS WHICH BUS GRID WILL ACT AS ALWAYS
GRID WILL ALWAYS BE SWING BUS
WHATEVER MISMATCH HAPPENS THAT WILL BE HANDLED SWING BUS
WHEN GRID(SYSTEM )CONSIDERED WEAK.
WHEN IT HAS
1) LESSER S.C CURRENT
2) HIGH SOURCE IMPEDANCE
3) HARMONICS IS HIGH
WHAT IS Q VALUE
QUALITY FACTOR
WHAT IS FORMULA FOR h?
h = n*P *(+ OR - 1)
n- INTEGER
P- NO OF PULSE (i.e 6 PULSE CONVERTOR )
WHY WE AVOID ADD HARMONIC FILTER TO HV AND EHV LINES
BECAUSE IT IS EXTREMELY EXPENSIVE
SHOULD WE PERFORM HARMONIC ANALYSIS IF TOTAL THD VALUE IS WITHIN LIMIT AND ONE PARTICAL HARMONICS(i.e ; 5th HARMONIC) THD VALUE EXCEEDS LIMIT?
WE SHOULD PERFORM HARMONIC ANALYSIS
IN HARMONIC SITUATION WHAT YOU ARE NEEDED TO FIND OUT?
1) WHERE THIS HARMONICS COMES FROM
2) WHAT ORDER HARMONICS
PURPOSE OF IMPEDANCE SCANNING
TO FIND OUT WHERE YOUR IMPEDANCE IS MAXIMUM AND AT THIS(IMP MAX POINT) POINT WHICH HARMONIC IS PREDOMINANT (IN GRAPH)
HOW TO MODEL CABLE LINE FOR HARMONIC ANALYSIS IF IT IS LESS THAN ONE KM?
SHOULD BE MODEL AS DISTRIBUTED PARAMETER OR FREQUENCY DEPENDANT MODEL FOR HARMONIC ANALYSIS
HOW TO MODEL CABLE LINE FOR HARMONIC ANALYSIS IF IT IS MORE THAN ONE KM?
MODEL IT AS A DISTRBUTED PARAMETER .BECAUSE NOMINAL PI METHOD WONT WORK IF CABLE IS MORE THAN ONE KM AND FREQUENCY OF HARMONICS AND RESONANCE FREQUENCY OF NETWORK ALMOST COINCIDES(VERIFY)
IF SYSTEM IS NOT GRID CONNECTED SHOULD WE PERFORM HARMONIC ANALYSIS?
HARMONIC ANALYSIS IS NECCESSARY
TO LOOK AT HARMONIC DISTORTION,RESONANCE POINT AND IMPACT OF ELEMENT
WHEN WE SHOULD SUGGEST INSTALLING FILTER AT PCC?
1) AFTER MEASURING HARMONICS AT SIT
2) DURING DESIGN ANALYSIS PROCESS
OTHER PURPOSE OF HARMONIC ANALYSIS
TO FIND WHERE RESONANCE FREQUENCY OCCURS
RESONANT FREQUENCY
?`
WHERE SHOULD BE HARMONIC FILTER PLACED
FILTERSHOUL BE PLACED NEAR TO HARMONIC SOURCE
DOES SATURATION OF TRAFO CAUSE HARMONICS
YES BUT FOR SHORT PERIOD OF TIME
DO WE NEED TO PERFORM HARMONIC ANALYSIS FOR ISLANDED SYSTEM?WHY?
YES BECAUSE SHORT CIRCUIT CURRENT AT ISLANDED SYSTEM IS LOW WHICH LEADS INCREASE IN HARMONICS.ALSO YOU NEED TO PERFORM HARMONIC ANALSIS TO CHECK WHETER DEISEL GENERATOR ARE CAPABLE OF WITHSTANDING HARMONICS
HARMONICS ORDER TYPE AND ITS RESULTING CURRENT
1ST ORDER- +VE SEQUENCE CURRENT 2ND ORDER- -VE SEQUENCE CURRENT 3RD ORDER- ZERO SEQUENCE CURRENT 4TH ORDER- +VE SEQUENCE CURRENT 5TH ORDER- -VE SEQUENCE CURRENT 6TH ORDER- ZERO SEQUENCE CURRENT ETC
WHAT IS EFFECT OF NEGATIVE SEQUENCE CURRENT IN GENERATORS?
GENERATOR ROTATES IN OPPOSITE DIRECUTION .SO TORQUE(BREAKING) IS GENERATOR WHICH CAUSES THE HEAT TO INCREASE
What can harmonics do to transformers?
Harmonics generate additional heat in transformer thus reducing the life expectancy of trafo
Why is low voltage side generators are solidly earthed?
We are going for solidly earthed because fault current will be higher and it could be easily detectable by current principle by over system protection devices.
Lv side has ACB ,MCCB,MCB.now except electronic devices all other like thermal magnetic or mcb cannot detect earth fault