[Spir Georges GHALI]

@guest said:
i am studying short circuit calculation for power systems.pls send me the data for designing and selection of protective devices for sc

Dear ;

As we know all, at the end of life of any SPD a short circuit is happened between “ one phase or more ” and the “ earth point ”, therefor the SPD’s supplying should absolutely be disconnected, for this reason, we have 2 products can be used to disconnect the supplying once that short circuit happen, that are :

• Circuit Breakers “ C.B. ”: 2 ranges can be used “ MCB  &  MCCB ”, and it will better if they have only a Magnetic Protection.
• Fuses : they should be of “ Network Protection ” range.

 

After defining the used Earthing System for the network, and the SPD’s main specifications especially : the Number of Poles & the prospective Discharging Current, the more important specifications of the Protective Devices that should be defined are the following :

• Number of Poles : it should be exactly the same as the number of poles of SPD. Noting that if the SPD has a Neutral pole the protective device should absolutely has it.

 

• Nominal Current for C. B. : if they have only a Magnetic Protection, the usable Nominal Current are “ 25, 40, 63, 80, 100, 125, 160A ”, where the choice will be done depending on the discharge current’s value, as well as this value increase the nominal current’s value will be increased, but if C.B. have also a Thermal Protection, we define first the value of that protection ( see bellow ), and the Nominal Current will be defined accordingly ( depending on the manufactures’ ranges ).

 

• Breaking Capacities : this value can be defined after knowing the Maximum Short Circuit Current at the installation’s point, where “ Breaking Capacity  ≥  Iscmax ”. Noting that :

  • The values of the Maximum Short Circuit Currents are normally calculated when we design any Electric Panel ( Electric Board ).
  • If the SPD’s number of poles is “ 3P with or without Neutral ” the “ Iscmax ” is for a short circuit between “ 3 phases ”, but if the SPD is “ 1P+N ” the “ Iscmax ” is for a short circuit between “ 1 phase & neutral ”.

 

• Magnetic Sitting ( Short-time Sitting ) for Circuit Breakers :as at the end of life of SPD a short circuit between “ one phase or more and the earth point ” called an Earth Faull, we define the value of this protection depending on the CB’s type as follow :

  • “ MCB ” : the type of this protection should be “ C curve ”.
  • “ MCCB ” : the value of this protection will be defined after knowing the Earth Fault Current at the installation’s point, where “ Magnetic Protection  <  Id ”.

 

We define the value of the Thermal Protection either for Fuses or Circuit Breakers if they have also this protection ( called also Long-time Protection ) as follow :

• Thermal Sitting ( Long-time Sitting ) : if possible, we don’t need for C.B. an adjustable Thermal Protection, the usable settings are “ 20, 25, 32, 40, 50, 63, 80, 100, 125, and 160A ”, and the choice will be done depending on the discharging current’s value, as well as this value increase the sitting value will be increased. For example : for  discharging current “ 5kA ” we can use “ 20 or 25A ”, and for “ 65kA ” we can use “ 125A ”.

 

Rem. 1 : as the SDP should be replaced at the end of life, therefor we should know when should be, to know that we have 2 possibilities :

• By regular check-up of each installed SDP especially during the season of Lightning Strikes.
• By using SPDs equipped with an “ Auxiliary Contact ” that change his state at the end of life.

 

Rem. 2 : some manufactures produce SPDs  equipped with suitable “ Disconnectors ”, so, no need to use with those SPD’s any protective devices.

 

Rem. 3 : if the SPD has “ 2P ” ( 2 phases ), the same Breaking Capacity’s value for “ 3P ” can be used.

[Spir Georges GHALI]

Dear Mr. Steven ;

First, it’s a good subject to be discussed but certainly not the principal basics as mentioned, because all Electrical Engineers should absolutely know it.

Let me please mention to the following points:

 

Leading and Lagging Power Factor :

• When mentioned “ Voltage phaser or Current phaser ” that means “ Voltage Vector  or  Current Vector ” .
• When you say “ we add capacitance to our system to minimize inductance and vice versa ”, you certainly mean that we add the capacitance to have a capacitive current that will minimize the inductive current ”.

 

Active Power and Reactive Power :

• The 2 formulas are mentioned to the “ Active Power ” where the 2^nd should be :

 “ Reactive Power : V(rms)I(rms)sin(angle between V and I) ”

• It’s mentioned that “ both inductor and capacitor (pure) are lossless elements means that they don’t consume any   power they only cause voltage drop across themselves”, but sorry to say : electrically it’s not right, because when there’s a drop voltage, that means there’s a current runs in the circuit, and if there’s a current that means there’s a Power’s consumption. By another wards:

  • All “ pure Capacitive or pure Inductive loads consume a Reactive Power ”.
  • The Reactive Power is a kind of power without obtaining a useful work.
  •  It’s mentioned also that “ They only absorb power from the source and release it after wards ” and “ Thus Reactive power oscillates or moves to and forth in the circuit … ”, but sorry to say : electrically it’s not right, they absorb ( consume ) power but they don’t release it, and also the absorbed Reactive Power doesn’t oscillate or move to and forth in the circuit.

 

Power Factor (Practically) :

To be more clear, the relation between the “ Cos ϕ ” ( Power Factor for the Fundamental Voltage and Current ) and the Power Factor for Network containing Harmonics ( Polluted Network ) is done by the following formula :

Power Factor = Cos φ / √(1+THD I²) 

Where the “ THD I ” is the Total Harmonic Distortion of Current.

So, relative to your formula the Distortion Factor is : 1/√(1+THD I²)

 

[Spir Georges GHALI]

Dear Mr. Steven ;

Refer to the connection’s schematic diagram of the Capacitors to a motor, please note that this connection is not too much used, especially for medium and big Motor’s Power, because it may be cause a risk of “ Self Excitation ”,  therefor to avoid that phenomena, we should either change the connection way by using another contactor to supply the Capacitors where the closing of it should be done a few seconds after the transient running state and opining with the main contactor, or take into consideration of the following conditions : 

Ic ≤ o.9 x Io

Qc  ≤ 2 x P ( 1 – Cosφ )

[Spir Georges GHALI]

Dear Mr. Steven ;

 

first, thanks for your reply, and be sure for me there’s no any confusion.

 

Please let me clarify the following :

– The percentages of Harmonics are certainly ” THD-I “ ( Total Harmonics Distortion of Current ) and not ” THD-V

– The mentioned formula between ” K-Factor ” and ” the Transformer’s Power ” is ” K-Factor = S ( kVA ) * IL ” as IL is the load harmonic, please let me clarify the following points :

• This formula is ” linear “, but the relation between K-Factor and the Transformer’s Power is not a linear.
• As I mentioned, there’s a Curve ( not linear ) between ” the Percentage of Non-linear Loads compared with the total installed loads ” and the ” Derating Factor “, when we obtain this factor, we use the following formula : S(new) = S / Derating Factor. Noting that I use this curve during designing step of any site if the derating of transformer is necessary, and if you want, send me your e-mail to send it you.

– About the type of used motors : from my experience, most of LV motors even till 300kW are Squirrel Cage Motors. noting that all Motors are Induction Motors. Please clarify if there’s another kind, and what?

[Spir Georges GHALI]

@saipurushotham said:
sir , can i know what are the different types of harmonics in power systems and can you please list out the power factor ranges in different industries such as lighting,textile industry etc..

Dear ;

About The Harmonics :

The “ Harmonics Currents ” generating by “ Non-linear Loads ” are different depending on these loads, where we found hereafter some of Non-linear loads and the relative generated Harmonics Currents :

– Computers with single phase power supply : 3^rd, 5^th, 9^th

– Computers with three phase power supply : 5^th, 7^th

– DC power supply with three phase power supply : 5^th, 7^th

– Electronic Ballasts : 3^rd, 5^th, 9^th

– Variable Frequency Drives with 6kHz switching : 5^th, 7^th

– Variable Frequency Drives with 12kHz switching : 11^th, 13^th

– Greatz Rectifiers : 5^th, 7^th, 11^th

– Single Phase Welding machines : 3^rd, 5^th, 7^th, 9^th

– Fluorescent Lamps : 3^rd, 5^th, 9^th, 11^th

Rem. : each of these Non-linear Loads generates also another Harmonics Currents but their amplitudes are too small.

So, depending on the types, numbers, and powers of installed non-linear loads, the amplitudes of some Harmonics Currents will be bigger than others, and if we know exactly the specifications of these loads, especially the VFD and UPS, we can estimate which Harmonics Currents will have an important amplitudes, because the serious manufactures of VFD and UPS add in their catalogues a special tables contain : the percentage of each generated Harmonic Current, and also the percentage of the Total Harmonic Distortion of Current “ THD-I ” where that value should be “ ≤ 48% ” accordingly to “ IEC 61000-3-12 ”, also, these manufactures try to minimize or eliminate all Multiple Third Harmonics “ 3, 9, 15, … ”, because these harmonics are “ Zero Sequence ”.

In practice way, it’s better to carry out some currents and voltages measurements in multiple places of network to know exactly which Harmonics Currents injected in the network and their amplitudes or percentages compared with the fundamental current, and in some case knowing which non-linear load or loads generate these Harmonics Currents, then decide which should be minimized or eliminated, and at any level of network or where the treatment of Harmonics should be done.

Normally, if some Harmonics Currents have a big amplitudes, we try to minimize them by using in the most of case “ Passive Filters ” and/or “ Active Filters ” depending on the site’s type, and also the currents’ values of these harmonics, as the “ Passive Filters ” can be manufactured by a big capacities of current, where the “ Active Filters ” has a limited capacity.

By the way, using of “ Passive  and/or  Active Filters ” will correct also the Power Factor’s value but certainly not to the target’s one, so, after doing the Harmonics Currents’ treatment, another measurements should be carried out to decide : the Type & Power of Capacitors that should be used, and where should be installed to achieve the target value of Power Factor. Noting that we shouldn’t install the Capacitors to the same point where the Filters are installed.

 

About the Power Factor :

It’s too difficult to estimate the Power Factor value of any site because this value depends on the types, numbers, and powers of installed loads, as for exp. : the power factor values for big motors are bigger than the small motors, and even for the same motor this value depends on the load’s percentage, where we found hereafter some loads and the relative Power Factor :

– Motor of medium power :

• ≈ 0.85  at 100% of load
• ≈ 0.80  at 75% of load
• ≈ 0.73  at 50% of load
• ≈ 0.55  at 25% of load

– Fluorescent Lamp without capacitor : ≈ 0.50

– Fluorescent Lamp with capacitor : ≈ 0.85 – 0.90

– Discharge Lamp without capacitor : ≈ 0.40 – 0.60 

– Induction Furnace with capacitors : ≈ 0.85

– Arc Furnace : ≈ 0.50 – 0.55

– Arc Welding machine : ≈ 0.75 – 0.80

So, it will better to measure The Power Factor’s value in the MDB, and also in the SMDB especially if one of them or more supply a big machines or motors, then depending on the value or values of these measurements we can choose the suitable solution of Power Factor Correction. Noting that we should also have an idea about the Total Harmonic Distortion of Current “ THD-I ” or the percentage of Non-linear Loads comparing with the total installed loads to be able to define the Capacitors’ type that should be used.

[Spir Georges GHALI]

@john2mcc said:
what is the purpose of a differential CT versus using regular ct

Dear ;

Do you mean the Special CT used with ” Differential Protection ” or ” Earth Leakage Protection ” ?  

[Spir Georges GHALI]

Dear Al-Firasah ;

 

My remarks in reply No. 6 were about some points in your reply No. 3, and your reply No. 7 is totally different, please reply to my remarks mentioned in No. 6

Regards.   

[Spir Georges GHALI]

@Al-Firasah said:
Before installs of capacitor
kVAri = kW*tan(thetai) ……… (1)

After installs of p.f.correction
kVArf = kW*tan(thetaf) …….. (2)

The value of capacitor, kVAr = kVAri – kVArf

equation (1) – (2)
kVAri – KVArf = kW*tan(thetai) – kW*tan(thetaf)
kVAr = kW*(tan(thetai) – tan(thetaf)) …… (3)

i = initial
f = final
theta = angle of power factor
kVAr = reactive power = 117.6
kW = real power = 100

assume that,
cos(thetai) = 0.85, therefore tan(thetai) = 0.6197
cos(thetaf) = 1, therefore tan(thetaf) = 0.0000

where
thetai = initial angle
thetaf = final angle
kVAri = initial reactive power
kVArf – final reactive power
kW = real power of electrical load

The reduction in current after installs of capacitor, Ir is:

Ir = [kVAi – kW*cos(thetai)]/(sqrt(3)*V) ……. (4)

where V in kV = nominal voltage supply from supply authority = 0.415kV

The reduction of power in cable = Ir^2*resistance of cable ………… (5)
assume that resistance of cable 0.005 ohm

From equation (3): kVAr = 100*(0.6197 – 0)
= 61.97

From equation (4): Ir = [117.6 – 100*0.85]/(1.732*0.415)
= 32.6/0.7188
= 45.4 Amperes

From equation (5): The reduction in power = 45.5^2*0.005
= 10.4 Watts

Dear Al-Firasah

refer to your equations mentioned above, i have the following remarks :

1- For any application, we don’t, never ever, correct the power factor to ” Cos φ = 1 ” that means ” tg φ = 0 “. 

2- In the equation No. 4, you multiply ” kW x Cos φ ” that means ” P x Cos φ “, what is the kind of this result ?

3- For any inductive load we have the following values : P, U, Cos φ, so the nominal current will be : ” I = P/1.732 x U x Cos φ “, where : P ( Watt ), U ( Volt ), and I ( Amp ), and this Current’s value doesn’t change, because that current is one of the load’s specifications, but if we installed the Capacitors bi-side the load, in this case, the current runs in the cable supplying the load & capacitor will be changed.

[Spir Georges GHALI]

@saipurushotham said:
sir , I need the different types of capacitors ,and where we are using that different types of capacitors (their ranges)

Dear ;

As you know, in principal, each capacitor is composed of 2 conductive parts with isolator between them, and the type & kind of components are different of each manufacture depending on his design to achieve the needed capacitance that drives to the Capacitor’s Reactive Power in “ kVAR ”, and to have 3 Phases Capacitor, the manufactures connect 3 elements in “ Star  or  Delta ” connection way. Noting that :

• – That most of used connection way is “ Delta ”.
• – Most of Capacitors used on LV networks are “ 3 Phases ”.
• – There are resistances connected between phases to discharge the capacitors to “ < 50V  during 1 min ” as defined by IEC 60831

 

Most of manufactures have many ranges of LV Capacitors depending on :

– Network Frequency that’s : 50  or  60Hz

– Nominal Network Voltage value that’s : 3 Phases, “ 230,  400,  415,  525,  690V ”

– The Capacitors Reactive Powers’ values in “ kVAR ” are in general : 2.5,  5,  6.25,  7.5,  10,  12.5,  15,  20kVAR, and some manufactures produce also “ 25,  30kVAR ”

– To have more than these Reactive Powers’ values, we assemble in Parallel “ 2 or more ” of those Capacitors, noting that the IEC 60831 advise to assemble up to 60 to 65 kVAR

– The Pollution’s Percentage “ Gh/Sn ” that the Capacitors can withstand, where : Gh : the Total Powers of Non-linear Loads in kVA, Sn : the apparent Power of the Transformer ( kVA ), noting that there’s 3 ranges as follow :

1- Standard  or  Classic range, for which “ Gh/Sn ≤  15% ”

2- Overrated  or  H  or  Confort range, for which “ 15%  <  Gh/Sn ≤  25% ”

3- Detuned  or  DR  or  Harmony range, for which “ 25%  <  Gh/Sn ≤  50% ”  

Rem. 1 : the same Capacitors can be used on “ 50Hz  or  60Hz ”, but as the Capacitor’s Impedance value is not the same ( Zc with 50Hz is bigger than with 60Hz ), the Reactive Power ( kVAR ) is also different ( Qc relative to 50Hz is smaller than Qc relative to 60Hz ).

 

Rem. 2 : we should be attention of the Rated Voltage value for which the manufacture defined the Capacitors Reactive Powers’ values, and if the network voltage’s value is smaller than that Rated Voltage value, the Capacitors Reactive Powers’ values should be modified ( Qc relative to 380V is smaller than Qc relative to 400V ).

 

Rem. 3 : depending on the “ Gh/Sn ” value, the above ranges are using as follow :

– Standard  or  Classic range is used for “ Slightly Polluted Networks ”.

– Overrated  or  H  or  Confort range is used for “ Polluted Networks ”.

– Detuned  or  DR  or  Harmony range is used for “ Highly Polluted Networks ”.

 

Rem. 4 : each unit of “ Detuned  or  DR  or  Harmony range ” is composed of “ Detuned Reactor with Capacitor ” connected in Serial way, and the supplying should be done first to the Detuned Reactor then to the Capacitor.

 

Rem. 5 : the “ Detuned  or  DR  or  Harmony range ” are produced in “ 3 types ” depending on the “ Tuning Order ” that are “ 2.5,  3.8,  4.3 ”.

 

Rem. 6 : if “ Gh/Sn > 50% ”, a special and deep study of network should be done to know all Harmonics and their amplitudes, then decide the suitable solution in which we use the Passive or Active Filters or both, but an economical study also should be done to choose the best solution.

 

So, depending on : the Network Frequency & Voltage Values, the Pollution’s Percentage, we choose the suitable range, then, from the available Capacitors Reactive Powers of that range, we can choose the capacitors’ units, and we assemble them to have the units we need.

 

Exp. : Network’s information : 400V,  50Hz,  Gh/Sn = 21%, and to correct the Power Factor value by Automatic Capacitors Bank we need the units : 10, 20, 30, 3×40 kVAR   

For that :

– The suitable range is : 3 Phases, 400V,  50Hz, from “ Overrated  or  H  or  Confort range ”.

– To have the needed unites, we have the following possibilities :

A- For “ 10 kVAR ” : 1×10 kVAR  or  2×5 kVAR  or  2.5+7.5 kVAR

B- For “ 20 kVAR ” : 1×20 kVAR  or  2×10 kVAR  or  5+15 kVAR

C- For “ 30 kVAR ” : 2×15 kVAR  or  3×10 kVAR  or  10+20 kVAR

D- For “ 40 kVAR ” : 2×20 kVAR  or  10+2×15 kVAR  or  4×10 kVAR

Noting that the final choosing should be done depending on the costs value of each combination.

[Spir Georges GHALI]

@guest said:
why star at the primary & delta at the secondary of a transformer???

Dear Abdul ;

First, the exact connections for ” MV/LV Transformres ” are : Delta for primary, and Star fro secondary.

Normally, most of Medium Voltage Networks are ” 3 Phases ” only, that means without Neutral, so, the connection’s of the transformer’s primary should be ” Delta “, and as we need for Single Phase Loads the ” Neutral “, the connection’s of the transformer’s secondary should be ” Star “.

[Spir Georges GHALI]

Dear;

It’s very good and important to mention the effects of Harmonics on the Transformers, but I have some remarks that are :

– I think the percentages of Harmonics currents mentioned in this topic mean the percentage of the “ THD-I ” ( Total Harmonic Distortion of Current ). If not, please clarify.

– It’s known that the effect of K-Factor is to over-size the Transformer’s power, but what is the formula between the K-Factor and the Transformer’s power ?

– You mention that “ the inductive loads like Motors are known as harmonic generating loads ”, but most of motors especially “ Squirrel Cage Motors ”, and after the transient running case are always “ Linear Loads ”, but for these loads the Harmonic currents can be generated or not depending on the kind of running equipment used to run these motors. ( for exp. Contactors with Thermal relays doesn’t generate any harmonic, VFD equipment generates harmonics ).

– It’s mentioned that “ the working principal of K-rated Transformers involves the use of a double sized neutral conductor ”, but in general, depending on the Harmonics Currents value or percentage, we over-size “ double or even more ” the cables’ sections of Phases & Neutral “, and also depending on the value or percentage of the “ Third multiple Harmonics ” and the “ Unbalance current’s value ” we decide the final section of neutral conductor.

– It’s mentioned that “ K-rated Transformers could be used at places where the loads are either inductive or nonlinear ”, and as known, some inductive loads generate harmonics but not all inductive loads.

– As mentioned, the K-Factor Value’s range is from “ 1 ” to “ 50 ”, and also, the rules mentioned the K-Factor Transformers that should be used depending on the Harmonic Current, ( exp. for Harmonics Currents more than 75% the “ K-20 Transformers ” should be used ) but, at which levels or where the values “ K-30  or  K-35 Transformers ” or more should be used ?

– The K-Factor value is the percentage of Harmonics current’s percentage to fundamental current, but this percentage’s value can be only known after industries’ running, so, what the Electrical Engineers Designers should do at designing step ? For this point, the IEC determines a curve that helps the Electrical Engineers Designers to define the transformer’s powers that should be used depending on the percentage of “ Non Linear Loads ”.

[Spir Georges GHALI]

@guest said:
i want to knw more about this topic i didnit understand it well.

FROM IVAN

Dear Ivan

 

to be able to reply, please let me know what information you want to know about capacitors.

[Spir Georges GHALI]

@sturnbull720 said:
The no load voltage at the origin of the circuit is 242V and the no load voltage at the outlet where the welder is connected is 241V. With a total circuit load of 32 amps (26A for the welder plus another couple of small appliances at 6A) the voltage under load at the origin of the circuit is 237V and at the outlet where the welder is connected the under load voltage is 208V.

 

Dear ;

As you said that the voltage at the circuit’s origin with ” 32A ” is ” 237V ” ( the circuit origin is the place where the cable 2.5mm² is connected ), and at the connection points of the welder is ” 208V “, that means the voltage drop is caused by this cable, so :

– The distance is ” 50m “.

– The total load current is ” 32A ” where the welder current is ” 26A “.

As the welder current contains a lot of Harmonics that can’t be measured by the standard equipment, the real welder’s current is bigger than ” 26A “. For this case it’s better to use a cable’s section around ” 6mm² ” where the method of installation of this cable should be taken into consideration.

[Spir Georges GHALI]

@emdias16 said:
Somebody may tell me what means the mobile pins in LR1 thermal riley?

 

Dear ;

What kind of ” Thermal Relay ” that you use ? and if possible to mention the complete part number.

[Spir Georges GHALI]

@sturnbull720 said:
Just wondering if anyone can help me, I have recently come across a large voltage drop issue when a client is using a mig welder. When the welder is in operation at full load current it draws 26 amps (single phase load). The voltage at the outlet at no load is 242V and under full load current is 208V. The voltage drop is due to a pre-existing large run (over 50 metres) of 2.5mm2 twin and earth cable which is grossly under rated for its application. I am looking at upgrading the circuit cabling to larger CSA conductors which will overcome the voltage drop issue. My question is however, if the welder is drawing 26 amps at 208V, what will the current draw be at nominal voltage of approx 240V? I understand with a purely resistive load that, impedence remaining constant, as voltage decreases, current will do the same. I’m trying to avoid installing circuit wiring suitable for circuit length/voltage drop/load current, only to find out at nominal voltage the load will be considerably higher. Any information will help. Thanks

 

Dear ;

Normally, the voltage drop is from the source till the load, and the percentage at your case is around ” 14% ” ( too much ), so, is the measured value at the input points of welder or at the beginning of the 2.5mm² cable ? if at the input points of the welder, it’s better to measure it at the beginning of cable to decide if the changing this section will be enough or not.

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