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HOW TO CHOOSE CAPACITOR BY INTERNATIONAL STANDARDS ( IEC )I.Tai stars need to improve the power factor :1 . Reduced cost for electricity :- Improve power factor brings the advantages of technology and [...]
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I.Tai stars need to improve the power factor :

1 . Reduced cost for electricity :

- Improve power factor brings the advantages of technology and economics , especially reducing electricity bills .

- In the use phase power limit prescribed . The reactive power consumption exceeds 40 % of the energy effects ( tgφ > 0.4 : this is worth a deal with electricity supply company ) , the use of reactive power to pay monthly prices current .

- Consequently , the total reactive power charges for time used will be :

kVAr ( pay ) = kWh ( tgφ - 0.4 )

- Although the benefits of reducing electricity bills , the user needs to consider factors by shopping costs , maintenance and installation of capacitors to improve power factor .

2 . Economic optimization - techniques

- Improved power factor enables the use of transformers , switchgear and cable smaller VV ... while reducing power loss and voltage drop in the power network .

- High power factor allows to optimize the power supply element . Then the electrical equipment does not need the excess . However, to achieve the best results , it should be placed next to each wing element of the device consumes reactive power .

- Improved power factor

- To improve the power factor of the electricity network , the capacitors do need a source of reactive power . The solution is called reactive power compensation .

- Download brings emotional low power factor will receive reactive current components from the transmitter leads to transmission through the distribution system . Hence pulling power loss and voltage drop phenomenon .

- When the capacitor in parallel with the load , taking into account the current capacity of the capacitor will be the same way as components of the load current sense resistance . so these two currents will cancel each other out IC = IL . Thus defunct flow resistance through the front mesh placement convergence .

- In particular we should avoid too large for the engine as well as modes of engine idling . At this point the power factor of the motor is very small ( 0.17 ) due to the amount of active power consumption at idle is very small .

II . The power compensation device :

1 . Clearing the grid voltage

In the low voltage network , power compensation is accomplished by :

- Capacitors with a fixed amount of compensation ( offset background) .

- Equipment or automatically adjust the compensation capacitor allows a continuous adjustment to changing load requirements .

Note: When should compensate the reactive power load exceeds 800KVAr and continuity and stability , the installation in the medium-voltage capacitor often better economic efficiency .

 2 . Capacitor background

Offset layout includes one or more capacitors to create a constant amount of compensation . the controller can perform :

- Manual : Using CB or LBS (load - break switch)

- Semi-Automatic : use contactor

- Having played directly into the power load simultaneously compensate for the closed circuit load .

The capacitors are placed :

- In connection position of the device power consumption inductive ( motors and transformers ) .

- In position to busbars supply many small motors and inductive load resistance for them to make each device a proved too costly .

- In the case when the load changes .

3 . Department of Automatic Control Capacitor ( animation offset )

- Power compensation is usually represented by means of controlled switching capacity of each department .

- This device allows power compensation controller automatically , keeping the power factor in a permissible limit around the power factor value is chosen .

- The device is installed at the location where the active power and reactive power change in a very wide range . for example, at the busbar distribution board , the connector of the cable shaft bearing large .

The principles and reasons for using automatic compensation :

- The capacitor consists of many parts and each part is controlled by the contactor . The shutdown of a contactor will close some capacitors in parallel with the capacitor operation . So the amount of compensation capacity can increase or decrease tiered done by closing or cutting contactor control convergence . One roley control power factor control of power networks will perform opening and closing the contactor corresponding to a capacity factor of the system changes ( with errors by stepwise adjustment ) . To control relay transformers to put up a determined line of power supply cables leading to the control circuit . When done exactly offset by the required load value will avoid the phenomenon of over-voltage and low load reduced by eliminating the conditions that arise , and to avoid over-voltage damage to equipment .

- Over voltage occurs due to the residual offset depends in part on the value of the source impedance .

The rules cover general

- If the output capacitor ( kVar ) less than or equal to 15% of the rated power supply transformers , should use ground cover .

- If above 15 % , use the offset type automatically .

- Location voltage capacitors installed in taking online mode power compensation ; focus or offset , offset groups , local offset , or offset schemes combine last two .

- In principle , the ideal compensation means compensation for each application consumes time and the extent to which the requirements for each load point .

- In practice , the choice of compensation methods based on economic factors and techniques .

Capacitor installation location :

3.1 Clearing a concentrated load applied to the stability and continuity .

Principle : the lower contribution from the input bar pressure distribution board and is played during the loading operation .


- Reduce fines for problems of reactive power consumption .

- Reduces the apparent power .

- Make light load for the transformer and therefore more likely to develop the required load .


- The current protests continue to expose all of the distribution board voltage network .

- For this reason the conductor size , power losses are not offset improvements in focus mode .

3.2 Bu groups ( segments ) .

Bu groups should use when electric network is too large and the load mode of consumption over time segments vary .

Principle : the convergence is input from regional distributors . effectively offset by the group gives the conductor comes from distribution board to the cabinet put together the areas most evident .


- Reduce fines for problems of reactive power consumption .

- Reduces the apparent power requirements .

- Dimensions cables to the distribution panel or area will decrease over the same cables can increase the load distribution panel area .


- The current protests continued in all wires comes from regional distributors .

- For this reason, the size and power losses in overhead wires is said to improve compensation mode group .

- When there is a significant change of the load , the risk always exists and accompanying compensation residual overvoltage phenomenon .

3.3 Clearing own :

Own compensation should be considered when engine power significantly greater than electricity network .

Principle : capacitor with the wires directly to the device using inductive power ( mainly motor ) .

Capacitors norms ( kVAr ) to about 25 % of the engine power . Bu additional power at the beginning can also bring good results .


Reduce fines for consumption of reactive power ( kVAr )

Reduced apparent power requirements .

Reduce size and lossy conductor wires for all .


The current protests have greater value will not exist in the power network .

3.4 optimal level of compensation

The common method

The table calculates the reactive power needed in the design phase . which can determine reactive power and active power for different levels of compensation .

The problem of economic optimization techniques for operating a power network .

The calculation of the optimal level of compensation for an existing network can comply with the following note :

+ Electricity charges before placing compensation

+ Future costs of electricity after capacitor installation .

+ The cost includes :

- Buy and capacitor control circuit .

- Installation and maintenance

- Losses in capacitors and losses on cables , transformers after capacitor installation .

III . Choose from Bu :

1 . Straightforward calculation method :

( to select the capacitor for a certain load , we need to know the power ( P ) of load and power factor ( Cos φ ) of downloading it ) :

Suppose we have the load capacity of the P

The power factor of the load is tg Cos φ1 → φ1 ( before compensation )

Power factor Cos after compensation is tg φ2 → φ2 .

Reactive power compensation is required QC = P ( tgφ1 - tgφ2 ) .
From power should compensate us accordingly selected capacitor in your catalog supplier of capacitors .

For ease of understanding we will give the following example illustrates :

Suppose we have the capacity load is P = 270 ( KW ) .

Power factor before compensation is cosφ1 tgφ1 = 0.88 = 0.75 →

Power factor after compensation is Cosφ2 = 0.95 → tgφ2 = 0:33

So reactive power compensation is needed Qbu = P ( tgφ1 - tgφ2 )

Qbu = 270 ( 0.88 - 0.33) = 148.5 ( KVAR )

From this data we selected capacitor in the manufacturer's catalog tables assume that we have gathered 25KVAr . To compensate enough for the load , we need to make 6 of 25 total reactive power KVAR is 6x25 = 150 ( KVAR ) with 6 This capacitor was chosen level controller 6 .
2 . Compensation method based on optimal conditions not pay a fine :

Considering electric bills related to the amount consumed and kVArh record number kVArh pay then select bills kVArh highest price paid ( not considering exceptions ) .

            For example, in January 15965 kVArh

Calculate the total operating time of the month for example : 220H reviewed to calculate the number of hours that the system is now the largest electrical load and peak load reached the highest value . outside the above time consuming reactive power is free .

Power values ​​should compensate :

[ kVAr ] = kVAr / T = Qbu

kVAr : No. kVAr pay .

T : number of hours of operation

 Capacity is usually chosen offset higher calculated value a bit .

Some vendors provide rules logarithmic size specially designed for this calculation according to the particular price bracket . And tools on the accompanying data helps us offset equipment selection and appropriate control scheme , while binding of the voltage harmonics in the power system . harmonics requires the use of residual norm convergence ( related to heat , pressure, and current norms ) and the reactor or harmonic filtering circuit .

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