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Reactive power compensation project

Design of reactive power compensation panel is much different and not that simple like standard distribution panel. When dealing with such panels, there are dozen of parameters to specify and other things to take care of.

Step-by-step tutorial for building capacitor bank and reactive power compensation panel
Step-by-step tutorial for building capacitor bank and reactive power compensation panel (photo credit: Elpro Križnič)

This article is the part of Mr. Jakub Kępka’s excellent thesis work on subject ‘Reactive Power Compensation‘. I haven’t read such a good work for a long time. Excellent.

The aim of project called „Reactive power compensation panel” was to design capacitor bank with rated power of 200kVar and rated voltage of 400V adapted for operation with mains, where higher order harmonics are present. The capacitor bank was to be power capacitor based with automatic control by power factor regulator.

This type of device was chosen as a compensator, because of its price compared i.e. to active filters. The capacitor bank will be launched as a new product of the company, so it is necessary to meet all the standard`s requirements in terms of the elements, dimensions, connections, cross section of the wires, capacitor protection since it needs to be tested and accepted by certified laboratory.

Bearing above in mind, first thing to do is to investigate basic requirements for capacitor banks according to the polish standards. The most important standards, that were used during design process was:

  • EN 61921:2005
  • EN 60439-1:1999
  • IEC 60831-2

EN 61921:2005 describes the general requirements for the capacitor bank. The most important of them are listed below:

  • Access to the particular elements within the capacitor bank should be easy, so that there is no problem to replace an element in case of failure
  • Index of protection depends of the place of the installation of a capacitor bank. If the capacitor bank is to be placed in the same place as the main switchgear or utility room next to it, IP 20 is enough.
  • Section construction – in a device for reactive power compensation particular sections can be determined, placing them in separate partitions or within the same cubicle.
  • Marking – each capacitor bank has to have nameplate, which contains information about: manufacturer, identification number, date of manufacture, rated power in [kVar], rated voltage in [V], min and max ambient temperature, index of protection, short circuit strength in [A]

Contents:

  1. Enclosure
  2. Arrangement of the elements
  3. Power capacitors and detuning reactors
    1. Acceptor circuit
    2. Number and type of capacitors
  4. Contactors
  5. Protection
  6. Connection diagram
    1. Main circuit
    2. Control circuit

1. Enclosure

Having above information, it is possible to find fitting cubicle for the elements of the capacitor bank. Because the device is going to operate at the mains, where higher order harmonics are present, power capacitors must be protected by reactors. Each capacitor emits additional amount of heat as well as a reactor.

For this reason cooling fan are needed to be install in the cubicle, in order to force the air flow inside the enclosure which will cool the elements down.

The maximum temperature around the power capacitors cannot be higher than listed in table below.

Table 1 – Thermal Conditions according to IEC 60831-2

Maximum55°C
Maximum average within 24h45°C
Annual maximum average35°C
Minimal-25°C

The issue of cooling is very important. Capacitors and reactors working in improper thermal conditions are exposed for danger of overheating and its life expectancy gets shorter.

In order to avoid this, one needs to follow few rules, that will prevent unwanted effects. These are as follow (generally for switchgear cubicles):

  1. The distance between air inlet and outlet should be possibly far in order to provide the maximum speed for the air stream.
  2. The dimension of the inlet should be at least 10% bigger than the outlet
  3. Vertical dimension of the inlet/outlet should be the bigger one
  4. Avoiding air flow at the right angle or zigzag line
  5. In case of forced cooling, ventilators should be placed at the bottom of the cubicle in order to launch a cold air into the switchgear.
  6. Choosing the fun, the real air flow should be considered, since theoretical one can be can be higher in terms of counterpressure effect
Since one knows that ventilator has to be placed, it is needed to calculate the efficiency of the cooling system. Generally, we can assume that the power loss of the power capacitor (including wires, discharging resistor and contactors) is approximately 7W per / kvar – for acceptor circuit (capacitor and reactor).

According to the formula:

D = 0.3 × Ps [m3/h]
D = 0.3 × (200 × 7) = 420 [m3/h]

Where:

  • D – Minimal efficiency of ventilators
  • Ps – Total power loss od acceptor circuit

Taking into account the rules above, following cubicle was selected:

Enclosure dimensions
Figure 1 – Enclosure dimensions

Table 2 – Enclosure dimensions

Height [mm]2000
Width [mm]1050
Depth [mm]500

The photography below shows the interior of the cubicle:

Enclosure for reactive power compensation
Figure 2 – Enclosure for reactive power compensation

As you can notice, there is no floor in this enclosure. This type of construction lets the air stream flow easily up to the top of the cubicle, which is slightly lifted up for better ventilation.

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2. Arrangement of the elements

The arrangement of the elements inside the enclosure should be easily available for maintenance and replacement, and each element should be clearly marked according to the technical documentation.

In the project, in terms of the construction of the enclosure, following solution was taken into account (see figure 3 below).

  • Elements no. 1,2 (violet font) these are the metal plates which constitute panels for contactors and protection equipment of particular sections of capacitor bank.
  • Element no. 3 represents the barrier between capacitor and reactor.

All the elements 1,2,3 come from the same manufacturer, taken from the same catalogue, in order to make easier construction of next device of similar type and decrease parts diversity.

Arrangement of elements in reactive power panel (CAD drawing)
Figure 3 – Arrangement of elements in reactive power panel (CAD drawing)

The next requirement for the reactors is to be placed above the capacitors, since they evolve much more heat than capacitors which is lighter and could go up causing the capacitor temperature to rise. If one wants to place the reactors in the same cubicle, they should be physically separated by a barrier.

That is what was mentioned in EN 61921:2005 Section construction.

In the project, the barrier was carried out by means of a metal plate placed between capacitors and reactors.

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3. Power capacitors and detuning reactors

The next step is to chose appropriate power capacitors. It means, that one needs to pay attention to its rated voltage and power. Since the capacitors will be working in series with reactors, what will cause the voltage at the capacitors’ terminals to rise.

According to data sheet given by the manufacturers most of the capacitors cannot withstand the voltage of 1,1×Un longer than 8 hour per day. For this reason, there is a need to apply the power capacitors with the rated voltage higher than the voltage of mains.

By reason of this one must take under consideration a statement below:

As the voltage rises or drops, the reactive power of the capacitor changes as well, according to the formula:

Reactive power of the capacitor

where:

  • QR – calculated power of the capacitor
  • QN – nominal power at rated voltage
  • US – voltage of a mains
  • UN – rated voltage of capacitor

Project assumed rated power of the capacitor bank equal to 400V. Let’s carry out an example calculation. Considering power capacitor with rated power of 20 kvar and rated voltage of 440V supplied by mains at Un=400V.

Reactive power of the capacitor

This type of calculation is true, if there is no reactor connected in series with capacitor.

Once we know the total reactive power of the capacitors, we can choose series of capacitors for PF correction. There is 200kvar to be divided. Taking this into account, at his point, one needs to consider the number of capacitors that will be used.

However, before the capacitors will be chosen, one needs to take a closer look at the power factor regulators output number, and reactor which will change the total power of the section of capacitor bank.

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3.1 Acceptor circuit

Power electronic based devices have significant, negative influence on the power quality. Since its number is increasing nowadays, it leads to designing more and more capacitor banks, that are well prepared to work with distorted voltage and current.

This is obtained by, so called detuning reactors, which are interconnected with capacitors within the circuit breaker (CB). Capacitor and reactor connected in series is referred to as an acceptor circuit.

This connection is depicted in the picture below.

The section of detuned capacitor bank
Figure 4 – The section of detuned capacitor bank

The capacitance and inductance of the series connected capacitor and inductor create a resonance circuit with the natural frequency fr. For the frequencies below the fr, including 50Hz, circuit has capacitive behaviour, which makes possible compensation of inductive reactive power. For all the frequencies higher than natural frequency, acceptor circuit has inductive behaviour.

This prevents from the resonance phenomenon between the capacitor bank and supplying network.

In detuned filters, parameters L and C must have such a value, that the natural frequency value of the capacitor bank is smaller than the frequency of the lowest order harmonic present in the supplying electric network.

As an example, if it was found, that in the grid there are following harmonics: 5th, 7th, 11th, 13th the LC parameters has to be selected so that the resonance frequency is included in range 174 – 210Hz (usually 189Hz). This type of filtering is being used in the automatic capacitor banks.

If one wants to consider operation of the capacitor bank without resonance reactors, the fact, that higher order harmonics sources can be either receivers from the spot, where the CB is installed or supplying network.

Before decision is made, whether install the reactors or not, it is strongly recommended to make measurements at the place of CB installation, it the higher order harmonics are present in supplying current and voltage.

As one can notice, the reactors are very important part of capacitor bank, and they cannot be omitted in the designing process. They also cause the voltage rise of series connected capacitor. Increased voltage changes the power of the capacitor.

So, there is a row of calculations that are required to be carried out during designing process.

First of all, as mentioned above, basing on detailed network analysis, knowing the harmonic content in supplying voltage/current, a detuning factor can be found. Since the capacitor bank in the project has no determined particular network to operate with, but was built for demonstrations by the ELEKTROTIM company, it was assumed that it has to be able to work at resonance frequency of 189Hz.

Usually, this piece of information is drawn from network analysis. Once the frequency is known, first step is to calculate the detuning factor.

Detuning factor indicates the capability of the acceptor circuit to filtrate the higher order harmonics. It is denoted as p and expressed in percents. It can be defined as ratio of reactor’s reactance with respect to reactance of capacitor.

However, it can be calculated basing on the network frequency and natural frequency of the circuit according to the formula:

Detuning factor

Typical range of higher order harmonic limiting includes 5th and 7th harmonic , which usually are present in mains and have the biggest share in supplying current.

Table 3 – Detuning factor and corresponding resonance frequency

Detuning factor p%5%5.76%7%12,5%14%
Resonance frequency fR≈224Hz≈210Hz≈189Hz≈141Hz≈134Hz

Since the detuning factor for the project was given as p=7%, one knows that the capacitor bank needs to be equipped with reactors. For this reason, some calculations have to be performed, in order to fit the power of the capacitors and its rated voltage taking into account reactive power of a detuning reactors. This power has to be considered when resultant power of capacitor bank section is being determined.

First, capacity of the capacitor has to be found basing on the rated power and rated voltage value of the capacitor, according to the formula:

Capacity of the capacitor

where:

  • f – frequency,
  • Qcn – rated reactive power of capacitor,
  • Ucn – Rated voltage of capacitor,
  • C – capacitance of the capacitor

In compliance with the project assumptions, for p=7% and taking into account value of C calculated above, one can determine capacitive and inductive reactance:

Capacitive reactance

Inductive reactance

Resultant reactance of acceptor circuit is:

Resultant reactance of acceptor circuit

Having calculated the values above, one can find phase inductance of the reactor:

Phase inductance of the reactor

as well as the current forced by the capacitor:

Capacitor forced current

The reactor connected in series will step the voltage at the capacitor terminals up what can found by following formula:

Capacitor voltage

All above calculations allow to find out, what is the reactive power of the capacitor bank, when the voltage across its terminal has changed:

Reactive power of the capacitor bank

In the next step, the reactive power of detuning reactor will be calculated:

Reactive power of detuning reactor

Then, the resultant power of the acceptor circuit is going to be:

Resultant power of the acceptor circuit

Table 4 – Results of calculation

CXCXLXCBLRISUCQREQLQRES
μFΩΩΩmHAVkvarkvarkvar
3299,860,6892,1625,6543019,111,3418

In the table above all the results of calculation are listed. Since, as mentioned above, capacitor bank working with the mains where higher order harmonics are present, needs to be equipped with reactors, which affect the total reactive power value of the capacitor bank.

In order to find the total rated power of the capacitor bank including reactors, all the calculations above has to be carried out. Data taken for the calculations above:

Table 5 – Data for calculation

Capacitor
Rated powerQcn20 kvar
Frequencyfn50 Hz
Rated voltageUcn440 Volts
Other
Mains rated voltageUn400 Volts
Detuning factorp7%

The rated voltage of the capacitor that was taken for calculations is not random, since it is known, that reactor will increase the voltage across the capacitor terminals according to formula above Uc=Us (1-p).

Taking resultant reactive power of acceptor circuit and denoting it as QRES and rated power of the capacitor Qcn, one can find the ratio:

Coefficient M

The idea to determine the coefficient M is to make easier finding the total capacitor bank rated power when equipped with reactors. For the project:

Total power of the capacitors

Summing up, the total power of the capacitors that are used in capacitor bank will be bigger, than assumed rated power of CB. It arose due to reactors connected with capacitors in series. Since voltage will be increased at the capacitor terminals, up to the 430V, overrated capacitors had to be used with the nominal voltage of 440V.

However, nominal power of the capacitor is reached at its rated voltage, so i.e. 20kvar at 440V. If the mains voltage is 400V, capacitor nominal voltage 440, and reactor cause voltage change at the capacitor terminals as well as launch additional reactive power to the circuit, all the calculations introduced in this article must be done.

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3.2 Number and type of capacitors

Once coefficient M was calculated as well as the total power of the capacitors that needs to be installed, one may consider how many capacitors should be selected. At this point, it is important to match the capacitor which will be the first one in the series. However, before it happens, the “series of type” has to be explained.

Power factor regulators are manufactured with 6 or 12 outputs. It means that maximum 6 or 12 power capacitors can be switched on or off.

Let`s take a closer look at the series below:

  1. 1:1:1:1:1:1…
  2. 1:2:2:2:2:4…

1:1:1:1:1:1… – The first series (case a.) says, that in a capacitor bank there are six capacitors with the same rated power. This uniform staging allows to switch the capacitor on without waiting until it is discharged and ready to be switched on one more time. The first number in the series (representing multiplication of rated power of a capacitor) has to be chosen very carefully.

Usually, it is dependent on the load fluctuation at the network the capacitor bank is going to operate with. It is important issue, since the next capacitor in the series has to be equal or integer multiplication of the one on the first place. The series has to be increasing.

1:2:2:2:2:4… – In case b. one can notice, that i.e. if the rated power of the first capacitor in the series is equal i.e. 10kvar, then, second one is 20kvar, and so on.

Once the total power of 220 kvar that is supposed to be distributed among certain number of capacitors, one should find out, what are the typical ratings of capacitors offered on the domestic and international market.

For the project purposes, the products of ZEZ SILKO company was bought, because they were competitive comparing to the other suppliers.


Characteristics of chosen capacitors

The company produces Capacitors in MKP and MKV systems. Both dielectric systems are self−healing. Metal plated layer is evaporated in case of the voltage breakdown. Formed insulating surface is very small and does not effected the functionality of the capacitor.

Capacitors windings are inserted into aluminum container. Container is equipped with the overpressure disconnector.

Dry Cylindrical Capacitor by ZEZ SILKO
Dry Cylindrical Capacitor by ZEZ SILKO

MKP capacitors are made of one−side metalized PP film. Contacting of the winding is performed by zinc spraying. This configuration is dry without impregnant. As for MKV capacitor, electrodes are of metallized paper on both sides and PP foil serves as a dielectric. The system is impregnated by mineral oil.

MKV capacitors are suitable for higher power loading and higher ambient temperature. In the meantime the capacitors are produced mainly in MKP system, MKV”.

Table 6 – Capacitors that are being offered by ZEZ Silko

Capacitors that are being offered by ZEZ Silko
Table 6 – Capacitors that are being offered by ZEZ Silko

Having at disposal the list of capacitors, it is possible to figure out its total number for the capacitor bank.

The first capacitor in the series will have a power of 20kvar. If the remaining power will be managed in a smart way, it will be possible to reduce the cost of the power factor regulator choosing the one, that has 6 outputs instead of 12.


Capacitor type description

Each capacitor by the company is described by the specific name such as CSADG or CSADP. In this notation, each letter indicates a feature of the capacitor:

Table 7 – Capacitor type description

Sequence
of a letter
FeatureLetterDescription
1ApplicationCPF protection
2Number of phases impregnatSThree phase without impregnat
3Cooling case constructionASteel insulated case
4Configuration protection degreeDBuilt-in discharge resistor for indoor use IP20
5Dielectric systemGMKP (metalized PP film, dry, gas filled)
PMKP (metalized PP film, dry, gel filled)

In order to check, if the capacitors are suitable for reactive power compensation and match the project assumptions, one can decode the capacitor type description in compliance with Table 7.

Basing on the two tables above, following capacitors were selected:

  • 1 capacitor – CSADG 1-0,44/20
  • 5 capacitors – CSADP 3-0,44/40

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4. Contactors

The last step is to select the protection of the capacitors as well as the contactors. In order to do so, one has to skim the catalogue cards of the manufacturers.

Contactors for the capacitor banks are specially designed, taking into account life expectancy of the contacts, as well as an extra module limiting the inrush current of the capacitor.

Table 8 – Contactor ratings by LG

Contactor ratings by LG
Table 8 – Contactor ratings by LG

In the table above, there are listed contactors from LG company. In order to select proper contactor for each capacitor, one needs to pay attention for the rated power that can be handled by the device at given rated voltage.

Therefore, for the project, where there are capacitors of rated power of 20kvar and 40kvar, following contactors were selected: MC – 32 and MC – 50. The last column of the table shows, what type of module should be used for particular contactor. The module is available separately.

One also needs to supply the coils of the contactors with the voltage of 230V. It can be obtained by mounting the transformer with the ratio 400/230.

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5. Protection

The short circuit protection of the capacitors is provided by the switch disconnectors. For the capacitors the fuse link rated current should be 1.6 time of the rated reactive current of the capacitor.

In=Q / (Un×√3)

where:

  • Un – rated voltage of the mains,
  • Q – rated power of the capacitor at rated mains voltage.

Not only capacitors should be protected against short circuit, but the whole capacitor bank as well. Usually, in the switchgear from which the CB is supplied, there is an additional circuit breaker for the capacitor bank.

Its value should be selected as:

  • Standard capacitor bank : 1,36 × In
  • Overrated capacitor bank: 1,50 × In
  • Capacitor bank with reactors (n=4.3): 1,21 × In

The next important issue is to provide proper section of the wires and conductors, which has to be able to withstand at least 1,5 of the nominal reactive current.

One needs to remember, that the control and cooling circuits also need protection. It is provided by the fuse links with rated current of 6A in compliance with technical documentation of PFR.

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6. Connection diagram

6.1 Main circuit

The next task, which designer has to handle is to create the connection diagram for all the elements that were selected to be used in the capacitor bank. The capacitor bank should has two technical drawings, namely, main circuit diagram and control circuit diagram.

The main circuit diagram should provide information how to connect the capacitor bank to the supplying switchgear:

Supplying network
Figure 5 – Supplying network

There is three phase network incoming to supply the capacitor bank (Low Voltage switchgear). From the feeder, the incoming power is distributed through the bus bars mounted in the capacitor bank. The cross section of the bus bars is chosen so that it can easily withstand the current flowing through the device.

Moreover, it is important to know the proper number of isolators holding the busbars, since it determines short circuit strength of the device. In case of the capacitor bank, there are three insulators which gives short circuit strength of about 20 – 30kA.

The connection points (red dots) L1, L2, and L3 represents the point of connection of the capacitors and reactors with the bus bars.

The main circuit of circuit breaker (CB)
Figure 6 – The main circuit of circuit breaker (CB) – click to expand scheme

The three cooper busbars (cross section 30×10 mm) L1, L2 and L3 are connected through the wires to the switch disconnectors F1 – F6. All switch disconnectors has the same current strength of 160A, the only thing that differs them from each other is rated current of the fuse link.

The terminals 2,4,6 of each disconnector are connected to the three phase reactor (D1 – D6). Each reactor has thermal protection (contact 11 and 14).

Next, the reactors are connected in series through the contactors (K1 – K6). The terminals A1 and A2 (coil of the contactor supplied by 230V AC source) trip the contacts of the contactor.

The second drawing “Control diagram” explains in details how to connect the contactor’s coils with and thermal protection of the reactors with the power factor regulator. The procedure of creating control circuit diagram will be shown in few steps in the next subsection.

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6.2 Control circuit

In order to connect all the control equipment and protection one needs a terminal stripe. Terminal stripe will cross all necessary wires in order to make the circuit work.

Terminal stripe of capacitor bank
Figure 7 – Terminal stripe of capacitor bank

The terminal stripe needs to be provided together with the control circuit diagram for the wireman, who was going to connect the equipment. The bottom part of the terminal stripe is dedicated for the wires coming from :

  1. Current transformer l – CT and k – CT from the supplying switchgear
  2. Short circuit protection of regulator, ventilators (FS – 1…F2.4) as well as for the reactors (D1 – D6)
The upper part of terminal stripe contains the outputs, which are connected through the wires with the control, protection and cooling equipment. The letter “R” denotes the Power Factor Regulator i.e. “l – R “ is a connection of the terminal “l” of the current transformer with the terminal “l” on the power factor regulator and so on. W1 – W3 terminals are assigned to ventilators.

Extraction of terminal stripe

According to the terminal stripe, one can wire the circuit step by step.

Control circuit of capacitor bank
Figure 8 – Control circuit of capacitor bank (click to expand scheme)

First, on needs to use the phases L2 and L3 in order to supply the transformer Tr 40/230V (250VA). The transformer is protected from short circuit by two-pole switch disconnector F1 whit the fuse link of 6A. At the output of the transformer, one gets the phase L (230V) and the neutral N.

The transformer will supply the equipment which needs 230V AC source to operate, that are:

  • Ventilators
  • Power factor regulator
  • Coils of the contactors

The ventilators are controlled by the thermostat T which will turn them on when the temperature will rise above 35 degrees of Celsius. The phase L2 and L3 is connected to the power factor regulator through the fuse FS2.

The next picture will continue from the points 1, 2, 3, 4 and 5 at the very bottom of the figure above.

Wiring of capacitor bank control circuit
Figure 9 – Wiring of capacitor bank control circuit (click to expand scheme)

Starting from points 1,2,3,4 and 5 one continue designing the control circuit. The figure shows the layout of the power factor regulator RMB 10.6.

Power factor regulator RMB 10.6
Power factor regulator RMB 10.6 – A miniature version of the regulator, designed for small, low-cost capacitor batteries. Mounting on a 35mm DIN rail. Has 6 outputs on contact relays.

The regulator has got three terminal stripes:

  • l, k, alarm
  • L1, L2, L3, N
  • C, 1 – 6

The terminals “l” and “k” provide connection for the current transformer mounted on the phase L1 in the main switchgear. The input alarm is connected in series with the lamp “LA” mounted on the door of capacitor bank. The lamp will light up every time, when contact “ALARM” inside the power factor regulator will close down.

The light will signalize each improper operation or error in the capacitor bank, Thanks of mounting it on the doors, it will be visible from far distances.

The second terminal stripes contains terminals L1, L2, L3 and N. The terminal L1 and N is connected to the phase L and neutral wire N of the transformer respectively. In this way, one provides the supply source for power factor regulator.

Phases L2 and L3 are connected to terminals L2 and L3 respectively. These terminals are responsible for the measuring of the voltage. Moreover, phases L1, L2 and L3 are leaded through the switch WL1. This solution lets disconnect the capacitor bank without disconnecting it at the main switchgear i.e. for the maintenance.

The last terminal stripe will control the coils of the contactors. The terminals 1 to 6 are connected to the coils of the contactors which trip the contacts in order to switch the capacitor on or off. They are followed by the contacts D1 – D6. These are responsible for the thermal protection of the reactors.

In case, when the temperature rise above the limit, that is safe for the reactors, the contact “D” of the reactor will switch off the circuit capacitor reactor .

Putting all these diagrams together, one obtains complete control circuit diagram for the capacitor bank.

Wiring scheme of power factor regulator RMB 10.6
Figure 10 – Manufacturer’s wiring scheme of power factor regulator RMB 10.6 (click to expand scheme)

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Reference // Master thesis: Reactive Power Compensation by Jakub Kępka and Supervisor PhD. Zbigniew Leonowicz at Wroclaw University of Technology

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About Author

author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

One Comment


  1. George F
    Jul 01, 2018

    This is a very nice article on CBs. However, I cannot understand how you end up with the results in table 4 as far as the various Qs are concerned.As per my understanding the formulas are as below.
    QRE=Qrz =2*3.14*50*0.000329*430=44.45
    QL= 3*2*3.14*50*0.00216*25.65=52.14
    QS=QRES=Qrz-QL=-7.69

    Above results are far from what are given in Table 4 unless I am missing something.

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