reactive compensation & transmission losses

Er. K.K.Shrivastava Add.SE.O/o.C.E.(T&C)

REACTIVE-COMPANSATION & TRANSMISSION - LOSSES

(USE OF SHUNT & SERIES CAPACITORS AND REACTORS)

MP Power Transmission Company is transmitting the electrical power in bulk from the generating stations up to the consumers. MPPTCL receive the power from Generating stations installed in MP at 400KV, 220KV & 132KV level and also from the 400KV EHV Substations of PGCIL, and supply the electrical power to EHV consumers (at 132KV & 220KV) and Distribution company at 33KV level. The main consumer for MPPTCL is the Distribution Company at 33KV voltage level.

During the process of transmission of electrical power from point A to G, two main transmission elements come into picture.

(1) EHV power transformers - B to C D to E F to G 400/220KV 220/132KV 132/33KV

(2) EHT Transmission lines - A to B C to D E to F 400KV lines 220KV lines 132KV lines

MPPTCL has been entrusted the job, to transport electrical power from A to G efficiently, and the efficiency depends on transmission losses, and to achieve higher efficiency we have to consider each and every transmission element individually. (Each Xmer & each line individually). Efficiency can be improved through reactive power compensation (improving the power factor the Transmission system). As we know Electrical active power in three phase AC system can be represented as- P = √3 V 1 Cos Φ

M,P.Power Transmission Co. Ltd. In-house Technical Class room Training. For private circulation only 29th March 2010.

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A B

C

D

E

F

G

400/220 KV S/S

400KV line

220KV line

132KV line220/132KV S/S

132/33KV S/S

33KV feeders to DISCOM

Generating Station


MPPTCL has adopted the transmission voltage as 400KV, 220KV & 132KV, so the voltage is fixed. Principally, if we keep the power factor at the higher level, (unity power factor (Cos Φ = 1.0) is the max limit), the load current (I) and the transmission Losses (I2R losses) can be reduced for the same power. And this is the line of action to reduce the losses through PF improvement, and PF improvement comes through reactive power management. It is evident that the load current will be minimum for unity PF and so is for the (I2R) losses. Considering these minimum values of load current (I) & losses (I 2 R) as reference (=1.000) , the affect of PF on load current and losses, (it is better to identify this as loss factor) can be tabulated as under-

For the same power and same conductor size variation of load current & I2R losses are tabulated for the different power factors.

Power factorLoad current

proportional to (1/pf)Losses proportional to square of load current

1.000 1.000 1.0000.995 1.005 1.0100.990 1.010 1.0200.985 1.015 1.0310.980 1.020 1.0410.975 1.026 1.0520.970 1.031 1.0630.965 1.036 1.0740.960 1.042 1.0850.955 1.047 1.0960.950 1.053 1.1080.940 1.064 1.1320.930 1.075 1.1560.920 1.087 1.1810.910 1.099 1.2080.900 1.111 1.2350.850 1.176 1.3840.800 1.250 1.5630.750 1.333 1.7780.700 1.429 2.0410.650 1.538 2.3670.600 1.667 2.7780.550 1.818 3.306

0.500 2.000 4.0000.450 2.222 4.9380.400 2.500 6.2500.350 2.857 8.1630.300 3.333 11.1110.250 4.000 16.0000.200 5.000 25.0000.150 6.667 44.444

0.100 10.000 100.000In transmission system, it is easier to monitor the active power (MW) and reactive power (MVAR), instead of monitoring the power factor (PF), as the direct MW meter & MVAR


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meters are provided on each and every C&R panel for individual Xmers & Lines. The PF may be represented in terms of MW & MVAR also, as such inclusion of MW & MVAR parameters in the previous table will make it more convenient and user friendly to understand and monitor the affect of PF on current and losses. (Better to identify it, as loss- factor) Electrical Power = Voltage X Current = ( V ) X ( I ) = ( V ) X ( I Cos Φ + I Sin Φ ) = V I Cos Φ + VI Sin Φ

Three phase Power = √3 V 1 Cos Φ + √3 V 1 Sin Φ VA = Active Power (Watt) + Reactive Power (VAR)

MVA = MW + MVAR (Vector Sum)

For the same power and same conductor size variation of load current & I2R losses are tabulated for the different power factors & load flow conditions

MW + MVAR MVA PF CURRENT LOSS-FACTOR

x y z = √( x2+ y2) x/z 1/pf 1/( pf)2

10 0 10.000 1.000 1.000 1.00010 1 10.050 0.995 1.005 1.01010 2 10.198 0.981 1.020 1.04010 3 10.440 0.958 1.044 1.09010 4 10.770 0.928 1.077 1.16010 5 11.180 0.894 1.118 1.25010 6 11.662 0.857 1.166 1.36010 7 12.207 0.819 1.221 1.49010 8 12.806 0.781 1.281 1.64010 9 13.454 0.743 1.345 1.81010 10 14.142 0.707 1.414 2.0009 10 13.454 0.669 1.495 2.2358 10 12.806 0.625 1.601 2.5637 10 12.207 0.573 1.744 3.0416 10 11.662 0.514 1.944 3.7785 10 11.180 0.447 2.236 5.0004 10 10.770 0.371 2.693 7.2503 10 10.440 0.287 3.480 12.1112 10 10.198 0.196 5.099 26.0001 10 10.050 0.100 10.050 101.000

SHUNT CAPACITORS—(Reactive compensation on 33KV Bus bar.)


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132/33KV power transformers are generally supplying inductive load through 33KV Bus bar. Let us examine the reactive compensation and its affects. Consider a 132/33KV Substation having 40MVA transformer capacity with 4 No’s outgoing 33KV feeders and 12MVAR capacitor bank with average load of 30 MW with 15MVAR reactive loading as shown in the diagram. In transmission system flow of power out from the bus bar is identified as positive direction of power flow (for active as well as reactive power)

Φ Φ

Load diagram without cap bank Load diagram with cap bank

The affect of 12MVAR capacitor bank on the loading parameters can be calculated as under:

Load particular Without Cap bank

Cap bank in service

Net saving in parameters

Remarks

Active power 30 MW 30 MW -Reactive power 15 MVAR 3 MVAR 12 MVARApparent power 33.54 MVA 30.15 MVA 3.39 MVAPower Factor (Cos- Φ)

0.894 0.995 -

Load current LV 587 Amp 527 Amp 60 AmpLoad current HV 147 Amp 132 Amp 15 AmpTransmission loss-factor

1.25 1.01 0.24

The load reduction of 15 Amps on 132KV side of transformer reflects through out the whole transmission system vide 132KV EHV lines, 220/132KV Xmers, 220KV lines & up to generating end, and reduces the losses of each and every transmission elements


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132/33KV, 40 MVA

30MW

15MVAR 12MVAR Cap. Bank

30MW 30MW

15MVAR

3MVAR

33KV feeders


(Transformer and EHV line) coming into entire circuit up to generating end. That is why it is necessary to install more and more capacitor banks on 33 KV bus bars only , which is the load point as for as the MPPTCL is concern. Use of 132 KV capacitor banks is to be avoided in preference to 33 KV capacitors for reactive compensation.

Benefits of Cap bank (Reactive compensation)

Primary benefits (1) Reduced reactive loading on the system (2) Reduced I2R losses (Transmission losses)

Secondary benefits (1) Capacity improvement of the Xmer as well as whole transmission system,

(2) Improvement in voltage profile of the system, as the load current reduced, hence IR voltage drop will also be reduced in the system

The transformer is a static equipment with approximately 99.7 % efficiency; alternatively we can say the normal transformer losses are 0.30 %. Now with the help of reactive compensation (12 MVAR Cap bank) we have further reduced the variable part (I2R) of this loss, by reducing the loss-factor from 1.25 to 1.01.

From 0.30% to approx 0.24% (0.30 x 1.01) 1.25

As such net reduction is losses are approx 0.06% (0.30 - 0.24)

0.06% of 30MW comes out to be 18 KW, and if we are utilizing the capacitor bank 20 hrs per day for 200 days only, the energy saved during the whole year will be 18KW x 20hrs x 200 days = 72000 units and in the terms of money @ Rs.5/- per units, it is Rs. 3.6 lacks per annum. (More than the salary of a new engineer). MPPTCL is having around 425 No’s of such 132/33 KV Xmers and capacitor banks, imagine the amount of savings an engineer can produce with his excellence in power engineering. Careless utilization and casual approach towards the capacitor banks may harm also, to the same extent??

Energy saved = 18KW x 20hrs x 200 days = 72000 KWH (units)

MPPTCL has provided the capacitor bank at the S/S, and the field engineer has to utilize this bank with the increased no. of running hours & working days, it is therefore necessary for field engineers to keep close watch on loading pattern of the transformer and utilize the bank wisely.

Some of the guide lines and points to be kept in mind while utilization of capacitor banks are as under for reference:--

1- Keep the Cap bank maintained and ready to be utilized as and when required as per the reactive loading on the Xmer.


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2- If some of capacitor units are failed / defective or short, even then, keep the Cap bank balanced on de rated capacity.

3- Keep the Over Voltage setting at 105% and confirm that its accuracy and trip test is OK, along with the other protection NDR, O/C, E/F, & U/V protections. Time delay in closing, after a tripping must be assured.

4- If 33KV bus voltage is high and cap bank is tripping frequently on over voltage, lower the tap position of the transformer, and keep the Cap bank into service for reactive compensation as per requirement of the reactive loading on the Xmer.

5- Do not over compensate, over compensation will neutralize all the benefits & may even increase the losses, there fore it is recommended, just to maintain the system for power factor as nearer as possible to unity.

6- Switch off the cap bank if the MVAR loading on the Xmer is negative. 20-25% capacitive loading may be allowed during peak loads only, just to compensate for the shortage of reactive compensation at the near by EHV S/S and not more than this; i.e.12 MVAR Cap bank may be kept in service up to ( -3) MVAR loading on the Xmer during peak load season only.

7- Switch off (Hand trip) the cap bank; if it is still running, at 35 KV or more bus voltage, and check the O/V relay, its setting, operation and trip-test.

Capacitor banks on 33 KV bus bar are for reactive compensation and these should be utilized for this purpose only, voltage improvement is the inherent by-product of reactive compensation. Alternatively the capacitor banks should not be utilized for voltage improvement, for that purpose we are having tapings on the transformer winding (1 to17) with 1.25% voltage variation per tap.

Selection of proper tap of 132/33 KV Transformer

Selection of optimum tap for a 132/33 KV Transformer and correct evaluation of normal average load on the Xmer and normal average bus voltage is an art & excellence in power engineering, it is not just physics and mathematics, Suppose the 132 KV bus voltage is exactly maintained at 132 KV & Xmer is charged at normal tap (tap No 5), the output LV voltage will be 33 KV at no load. As the load increases the LV voltage goes down due to impedance of the Xmer (% impedance); at full load (at 100% load) it will go down exactly by, equal to the value of % impedance of the Xmer, i.e. if the percentage impedance of the Xmer is 10%, the LV voltage will also go down by 10% at full load, even at tap No 5. It will be great disasters if the tap position of the Xmer is increased unwisely see how?

HV voltage 132 KV No load 33.0 KV Tap No 5 Full load 29.7 KV (33 - 3.3) 10% less Tap No 5

To maintain LV voltage at Full load 33.0 KV (Raise1.25 % per tap) Tap No 13 DISCOM request if accepted for raise 34.24 KV (Further raise by 3 taps) Tap No 16Simple Physics and Mathematics will see nothing wrong in the above steps, but a field engineer foresee the sudden load throw (may be fault tripping, load shading or U/F tripping). LV voltage at tap No 16 will go high up to 37.6 KV (+14% approx) at no load


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on load throw, and the system will suffer an attack of weakness, damage to equipments, insulators & Xmer itself. Imagine the damage when HV voltage is also high (135-140 KV), very common figures, so what was wrong? And what is the solution?

1- Don’t be govern by DISCOM, it may protect up to 1.0 – 1.5 KV as the case may be. MRERC has given the flexibility (+6% to -9% ); make full use of it for MPPTCL.

2- Make-up only 50% voltage drops through Tap position of the Xmer, i.e. for 10% voltage drop, improve 5% only. (Raise of tap up to Tap No 9 only in the above case) This will ensure voltage variation limited (-5% to +5%) from full load to no load.

3- Balance 50% voltage drops will be managed to a sufficient level, through appropriate reactive compensation on 33 KV bus bar, for which O/V tripping of capacitor bank at 105% has to be ensured,+2.5% increase (2 taps) may be made if Cap is not available.

Depending upon normal average transformer loading & normal average 132KV Bus voltage (which is again a job of experience), the tap position of 132/33KV, two winding transformer, having 1 to 17 taps of 1.25% voltage variation per tap with normal tap at 5 & percentage impendence 10%, are tabulated as under to facilitate the smooth running –

For normal load 80% and normal voltage 135 KV the optimum tap is 6.Average voltage of 132KV Bus

Normal average Loading on the 132/33 KV Transformer0+% 25+% 50+% 75+% 100+%

145.2 +10.0% Two Two Two Two Two143.55 +8.75% Two Two Two Two 2141.9 +7.50% Two Two Two 2 3140.25 +6.25% Two Two 2 3 4138.6 +5.00% Two 2 3 4 5136.95 +3.75% 2 3 4 5 6135.3 +2.50% 3 4 5 6 7133.65 +1.25% 4 5 6 7 8132 KV Normal 5 6 7 8 9130.35 -1.25. 6 7 8 9 10128.7 -2.50% 7 8 9 10 11127.0 -3.75% 8 9 10 11 12125.4 -5.00% 9 10 11 12 13123.75 -6.25% 10 11 12 13 14122.1 -7.50% 11 12 13 14 15120.45 -8.75% 12 13 14 15 16118.8 -10.00% 13 14 15 16 Sixteen117.15 -11.25% 14 15 16 Sixteen Sixteen115.5 -12.5% 15 16 Sixteen Sixteen Sixteen113.85 -13.75% 16 Sixteen Sixteen Sixteen Sixteen112.2 -15.0% Sixteen Sixteen Sixteen Sixteen SixteenPOWER FACTOR IMPROVEMENT & REDUCTION OF I 2 R LOSSES IN EHV LINES (132 KV & 220 KV)--


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One may run transformer very efficiently by utilizing the shunt capacitor banks effectively and improving its efficiency from 99.7% to 99.8%, ( reducing the Xmer losses. from 0.3% to 0.2% ) but all these efforts could be vanished if our EHV lines are not efficient (I2R losses of EHV lines). The Efficiency of both the elements (Xmer as well as EHV lines combined) reduces the transmission losses of the system.

The key to control the reactive power flow on 132 & 220 KV EHV LINES lies in the fact that the Active power (MW) flows towards the load & Reactive power (MVAR) flows towards low voltage level

Active power loading and its direction of flow is not in our control, but fortunately reactive power flow can be controlled through voltage adjustment of the bus bars, through tap position of feeding EHV transformers. Reactive power flow of interconnected 132 KV lines can be controlled through taps of 220/132 KV Xmer and similarly for 220KV lines, the taps of 400/220 KV Xmers may be utilized.

0.51 lead 0.928 lead 3.8 loss factor1.16 loss factor

Reactive power (MVAR) flow on the line clearly indicates the higher 132KV Bus voltage at 220/132KV sub-station C and Active power flow indicates that the load is towards S/S C. Now the power factor of line AB+BC can be improved by reducing the 132KV bus voltage at station C and increasing the 132KV bus voltage at sub-station A (Line capacitance and Ferranti affect is practically negligible in short and loaded 132 KV lines.)

Final load.


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220/132KV160+125 Xmer

220/132KV2*160 Xmer

132KV line (45 KM) 132KV line

50MW

20MVAR

A

B

C

20MW

35 MVAR

30MW

15MVAR

33KV Bus

25MW

5 MVAR10MVAR

55MW

30MW


0.985 lag 0.981 lead 1.03 loss factor 1.04 loss factor

This was an example: -- 132KV Nagda - Khachrod - Ratlam line (NOV- 2006)

Station A -- 220KV S/S Nagda voltage increased by one tap 11 to 10 (+1.25%)

Station C -- 220KV S/S Ratlam voltage reduced by three taps 7 to 10 (-3.75%)

Station B-- 132 KV S/S Khachrod 12 MVAR Cap- bank installed afterwards and 132 KV lines made efficient to the max possible extent.

Selection of proper tap of 220/132 KV Transformer

SELCTION CRITERIA -1:-( Normal load & Normal HV (220KV) bus voltage)

Selection of optimum taps for a 220/132 KV Transformer is also an art & excellence in power engineering. However depending upon normal loading & normal 220 KV bus voltage at the S/S, the tap position of 220/132KV, auto transformer, having 1 to 17 taps of 1.25% voltage variation per tap with normal tap at 9 & percentage impendence 7.5 - 8.0 % , are tabulated as under to facilitate the smooth running –

For normal load 70% and normal voltage 225 KV the optimum tap is 9.

Average voltage of 220KV Bus

Normal average Loading on 220/132 KV Transformer0+% 30+% 60+% 90+% remark

242.0 +10.0% Sixteen 16 15 14239.5 +8.75% 16 15 14 13236.5 +7.50% 15 14 13 12233.75 +6.25% 14 13 12 11231.0 +5.00% 13 12 11 10228.25 +3.75% 12 11 10 9225.5 +2.50% 11 10 9 8222.75 +1.25% 10 9 8 7220 KV Normal 9 8 7 6217.25 -1.25. 8 7 6 5214.5 -2.50% 7 6 5 4211.75 -3.75% 6 5 4 3209.0 -5.00% 5 4 3 2206.25 -6.25% 4 3 2 Two203.5 -7.50% 3 2 Two Two200.75 -8.75% 2 Two Two Two198.0 -10.00% Two Two Two Two

SELCTION CRITERIA - 2 :-(Reactive power flow on interconnecting 132 KV lines)


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15MVAR


Tapings provided on the winding of a 220/132 KV Auto transformer, not only controls the output 132 KV bus voltage at the S/S, but it can also be made useful to control the reactive power flow on inter connecting 132 KV EHV lines between the different 220 KV S/S. As already explained above how 30 MVAR reactive power flow was controlled to make the 132 KV Nagda – Khachrod – Ratlam line efficient by razing the 132 KV bus voltage (+1.25% ) at 220 S/S Nagda and lowering it (-3.75% ), at 220 S/S Ratlam. Similar exercises had been performed for other inter-connections also

1- 132 KV Nagda – Ratadia – Ujjain2- 132 KV Ratlam - jaora - Mandsaur - Mlhargarh - Neemach3- 132 KV Neemach – Manasa – Gandhisagar.4- Excersise on 132 KV Ujjain - Badnagar & 132KV Ratlam – Badnagar inter-

connections could not be performed due to over loading problem at Badnagar, being single 160 MVA Xmer at that time (NOV - 2006)

MPERC has given the flexibility (+10% to -10%) for variation in 132 KV bus voltages. In present scenario this is a great opportunity in our hands and every effort is to be made to make EHV lines most efficient. SELCTION CRITERIA -1 (the normal load & normal HV bus voltage) provides almost perfect guide lines to decide the optimum tap position to maintain the rated bus voltage 132 KV.

Besides that, depending upon the reactive power flow on the interconnecting 132 KV lines, it may be required to maintain 135 KV at one S/S -A, and at the same time 130 KV only at the other S/S-B. Both are within the limits of MPERC, so don’t be mesmerized to maintain the bus voltage exactly equal to 132 KV at both the S/S-A & B, instead maintain the required difference. This deliberate 5.0 KV difference can control 30-35 MVAR reactive power flow, and 30-35 MVAR for 132 KV line is sufficient to make the line efficient with reduced losses. Alternatively the voltage difference introduced due to innocent or careless tap selection may reflect as higher losses in EHV lines. It is there fore recommended to analyze and revise the working tap position of the 220/132 KV Xmer decided as per criteria -1 accordingly.

Experiences have shown that, if the tap selection of 132/33 KV Xmers and after that selection of taps for 220/132 KV Xmers as per criteria -1, are properly followed, then the revision as per criteria-2 is normally not required.

Selection of optimum tap of 400/220 KV Auto -Transformer

Exactly similar and for the same purpose as it is for 220/132 KV auto transformers--

1- Select optimum tap according to normal load & normal HV (400KV) bus voltage.2- Revise the selection-1, according to reactive power flow on interconnecting 220

KV lines, to reduce the losses in 220 KV lines.Series compensation on EHV lines:


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The impedance of EHV line increase with the increase in the length of line and represented as (R + j XL). The resistance part is lesser, but it is the inductance of the line which contributes the major part of the impedance. Hence, it becomes very difficult to transmit power to a longer distance with reasonable voltage drop. To neutralize the affect of this series inductance of the line, series capacitors and synchronous condensers are used in between the longer lines as such the total impedance of line becomes less (R + j XL – j XC) and as a result more power can be transmitted over the same distance or, the same power can be transmitted to a longer distance.

Beside this these series capacitors and synchronous condensers has an extra inherent advantage of being automatic voltage regulator, as it generates reactive power and this reactive power generation (MVAR- Capacitive) increases with the increase in load current of the line.

The synchronous condensers and series capacitors are normally provided with bypass arrangement using circuit breakers. In MPPTCL we are having series compensation on the following EHV lines,

1- 220KV Itarsi-Barwaha line at Handiya (series capacitors)2- 220KV Bina-Gwalior line at Pichhore (synchronous condenser)3- 132KV Damoh-Tikamgarh line at Bijawar (series capacitors)4- 132KV Motijheel-Sheopurkalan at Sabalgarh (series capacitors)

Presently only at Bijawar, it is in utilization, that too in case of failure of Tikamgarh supply to the area due to low voltage. After the commissioning of 220 KV S/S Chhatarpur it will not be in use for any more in MPPTCL. Although, with the expansion of EHV network, increased no. of EHV Substations & interconnecting EHV lines, the necessity of series compensation becomes less & less required, but the importance of this ideology still has its importance in Transmission System.

Generally applied means in practice, to regulate reactive power:--

S. No.

Particulars Brief working principle

1 Generator excitation

Under excited generator absorbs the reactive power i.e. it can feed capacitive load, on the other hand the generator when over excited, produce reactive power and can feed the reactive loads. So as per the requirement of system the generator can be utilized to regulate the reactive power also.

2 Shunt compensation

Capacitor generates reactive power, so as per the reactive power requirement of the load capacitor banks are used at load end. In MPPTCL 33 KV bus bar is the load end.


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3 Series compensation

Series capacitor banks and synchronous condensers are utilized in series, in between the long EHV lines to neutralize the inductance of the line.

4 OLTC of EHV Transformer

Reactive power flows towards the low voltage level; this fact is utilized to regulate the flow of reactive power by adjusting the working tap position of the EHV Xmers installed at both the ends of the line, within the limits of voltage variation.

5 Switching of EHV lines

EHV lines generate reactive power when switched ON in to the system (being highly capacitive) and unloaded or lightly loaded line produces high voltage due to well known FERRANTY EFFECT, as such making IN or OUT the EHV lines from the system, one can control the flow of reactive power in the system. 100 KM 220 KV line generates approx 10MVAR and 300 KM 400 KV DCDS line generates approximately 2*180 MVAR when charged without reactors. and produces approx 30-35 KV voltage difference ( rise ) at remote end.

6 Shunt reactors Shunt reactors are used to neutralize the capacitance of the EHV lines and to suppress the voltage rise due to Ferranti effect, and are used at both the ends of EHV lines. 50MVAR reactors are used for 200-300KM long 400 KV lines and 63 MVAR reactors are used for 400 KV line of length more than 300 KM at both ends. Balance un neutralized part of line capacitance is used to meet the reactive demand of the load, there fore unloaded lines are required to be switched OFF. Reactors when utilized on BUS-BARS to suppress the voltage rise called bus reactor. 25 MVAR 33 KV reactors are used in 33KV LV of 400KV 315 MVA auto Xmer for reactive compensation as per requirement.

7 Series reactors Series reactors in transmission system are generally used as NGR (Neutral grounding reactor) connected between the star point of shunt reactor and the ground, limits the secondary arc current to ensure complete deionization of arc path during single phase to ground faults. Arc extinction during single phase auto reclose dead time is assisted by this NGR.

8 Synchronous AC Machines

Synchronous AC Machines when over excited generates reactive power in stable manner and when under excited absorbs reactive power but with reducing stability. These M/Cs can generate full reactive power on its own, while the capacitor banks require voltage across its terminals to generate reactive power, hence Synchronous AC M/Cs are called dynamic compensating device.

9 Static VAR Compensators

It is a combination of two elements, Shunt capacitor and Shunt reactor. Shunt capacitor can not give step less variation there fore shunt reactor is used as continuous variable element for


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smooth variable compensation in both generation mode and absorption mode of reactive power.

The utilization of shunt capacitor banks and OLTC of EHV transformers has already been discussed in details for reactive compensation and reduction of I2R losses in Power transformers as well as in EHV lines, which are the only means available with MPPTCL for 132 KV and 220 KV systems. As for as the utilization of shunt reactors in 400KV lines & bus bars and NGR along with switching IN/OUT of 400 & 220 KV EHV lines for the benefits of Transmission losses are concern, these will be discussed separately in details in Part -2 of the subject.

&&&&&&&&

QUESTIONS


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Q1- Power factor of a load has been improved from 0.45 to 0.90. What will be the changes in line current and line loss in the feeding line?

(a) Load current will be doubled & line losses will be 4 times.(b) Load current will be half & line losses will be double(c) Load current will be half & line losses will be one fourth (d) Load current will be doubled & line losses will be half

Q2- The directions of active power flow (MW) is from S/S –A to S/S –B, while the direction of reactive power flow (MVAR) is from S/S –B to S/S –A. in a 132KV line AB. What can be assessed about the 132KV Bus voltages at the two EHV Substations A & B?.

(a) Bus voltages of the S/S –A will be higher than that of S/S -B (b) Bus voltages of the S/S –A will be lower than that of S/S -B (c) Bus voltages of the S/S –A & B will be equal(d) Bus voltages of the S/S –A & B do not depend on direction of power flow.

Q3- Capacitor banks are used in transmission system to;-

(a) Generate reactive power .(b) Consume reactive power.(c) Neither to generate nor to consume reactive power. (d) It can be used to generate or to absorb the reactive power as per requirement.

Q4- Reactors are used in Transmission System to

(a) Generate reactive power.(b) Consume reactive power .(c) Neither to generate nor to consume reactive power. (d) It can be used to generate or to absorb the reactive power as per requirement.

Q5- Name the Devices which can generate and absorb the reactive power as per the requirement of the system

(a) Capacitor banks (shunt & series)(b) Reactors (shunt & series)(c) Synchronous machines & static VAR compensator. (d) OLTC of EHV Xmers & Switching of EHV lines.

Q6- To feed a reactive load with lagging PF the generator has to run with(a) Over excitation . (b) Under excitation

(c) Excitation do not depend on PF of the load (d) Generator can not feed reactive loads.Q7- Series capacitor are used in transmission lines to; -- (find the one incorrect reason)


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(a) To reduce the voltage rise of the line. (b) To reduce the impedance of the line.(c) To increase the transmission capacity of line.(d) To reduce the voltage drops in the line.

Q8- A transformer is having impedance volt (percentage impedance) as 8%. What will be the voltage drop in the Xmer at full load?

(a) 8% (c) Voltage drop depends on working tap position of the Xmer(b) 4% (d) Voltage drop do not depend on % impedance of the Xmer

Q9- What are the voltage limits fixed by the MPERC for 33KV and 132KV bus voltage to be maintained in MPPTCL.

Ans: The MPERC has fixed the upper and lower limits for bus bar voltages as under- For 33KV bus voltage - +6% to -9%- For 132KV bus voltage - +10% to -10%

Q10- Name the means used in Transmission System to regulate the reactive power flow.

Ans: Following means are used in MPPTCL to regulate the reactive power flow in transmission system.

(1) Shunt and Series capacitors. (2) OLTC of power transformers (3) Shunt and Series reactors (4) Switching IN/OUT of EHV lines

Q11- What are the benefits of utilization of shunt capacitors in transmission system?.

Ans.: The benefits of shunt capacitors are as follows: Primary benefits (1) It reduces the reactive loading of the system

(2) It reduces I2R losses in the system Secondary benefits: (1) It improves the capacity of Transmission capacity

(2) It improves the voltage profile of the system

Q12- How the reactive power flow in interconnecting 132 KV & 220 KV EHV lines can be controlled in MPPTCL?

Ans:- Reactive power flow in interconnected 132 KV & 220 KV EHV lines can be controlled through proper selection of working Tap position of EHV Xmers at the feeding S/S, within the limits of permissible voltage variations. 132 KV lines -- Through OLTC of 220/132 KV Transformers 220 KV lines -- Through OLTC of 400/220 KV Transformers **************


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reactive compensation & transmission losses

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