Category: Blog

Home  ›  Blog  ›  Page 8

AQ: Hysteresis and eddy currents

Hysteresis would also lead to harmonics, complicating things even further. And, when considering unbalanced three-phase systems and/or the presence of harmonics, the conventional tools for power system analysis might not be applicable.

The losses due to hysteresis are limited by using better materials in transformer core. Eddy current losses are limited by using laminated construction. These losses are a relatively small portion of the total losses in a power system. Most of the losses are Joule losses (currents and resistances).

Because “energy” might be misinterpreted. Sure, But they do so twice (one positive, one negative) on every cycle of the AC system, so the average energy is zero.
There is an energy “exchange” between magnetic and electric fields. But no, that is not an oscillation in energy (kWh), not something that you could measure, for instance, in the torques on a mechanical shaft (that is purely kW, active power).

AQ: Why there are different type’s conductor cables, like EPR, XLPE

As far as the cables insulation material is concerned, EPR and XLPE insulated cables to some extent are having similar properties. In this respect, there are different types of Electrical cables such as ETFE ,FP, HOFR , LSF,LSOH, MI, PILC, TRS, VR, CTS, CSP, PTFE, etc.

However, it may be necessary to conduct a rough comparison (insulation) between the PVC and XLPE cables to clear the picture.
1. PVC/SWA/PVC multicore sheathed cables are manufactured in all sizes up to 400 mm² in accordance to BS 6346, the allowable operating temperature up to 70 °C.
2. XLPE Cables are used at max. ambient temp. of 90°C and are made to BS 5467. These cables have better insulation qualities than PVC and available in sizes up to 400 mm² or 1000 mm² Single Core.

Both type of cables are easy to lay and bending and they have less bending radius up 8 times nominal diameter.

These Different types of cables are not only based on the insulation material, are also either classified as cables of Aluminum conductors or Copper Conductors. Regardless, each has it own characteristics which can be appropriate to a range of installation / application since there are many wiring systems that may be adopted. In deciding the type of wiring system for particular, many factors have to be taken into consideration e.g….

a. Whether alteration & extensions are expected or not. Also, whether is going to be executed during the construction, in a completed project or as an extension of existing system.
b. Type of Project / building, function, purposes and ambient and environmental conditions.
c. Expected duration (life time) of the Installation.
d. The required layout, safety & constraints.
e. Feasibility & Cost

Eventually, I confirm that armored PVC & XLPE Insulated cables are now being used widely for feeders, submain cables & Industrial Installations.
Such Cable consists of multi conductors insulated by PVC or XLPE, with PVC sheath and steel wire armor (SWA), and PVC sheath overall.

AQ: Circulating current in parallel transformers

When two transformers are in a parallel group, a transformer with a higher tap position will typically have a higher (LV side) no-load voltage than the other one with a lower tap position. These unequal no-load voltages (unequal tap positions) will cause a circulating current to flow through the parallel connected transformers. A transformer with higher no-load voltage (typically higher tap position) will produce circulating current, while a transformer with lower no-load voltage (typically lower tap position) will receive circulating current.

When load is connected on these two parallel transformers, the circulating current will remain the same, but now it will be superimposed on the load current in each transformer, i.e. for a transformer producing circulating current, this will be added to its load current, and for a transformer receiving circulating current, this will be subtracted from its load current.

Thus voltage control of parallel transformers with the circulating current method aims to minimize the circulating current while keeping the voltage at the target value.

In case of a parallel operation of transformers, the electric current carried by these transformers are inversely proportional to their internal impedance. Think of it as two parallel impedances in a simple circuit behind a voltage source, you will have equal currents through each impedance only if you have two identical impedances, in some cases as stated above, tapping could be a problem, the other one is the actual manufacturing tolerances which could diverge by almost 5-10%, if the transformers are manufactured by different suppliers or not within the same batch. So, the difference in current between the currents through these two impedances is basically the circulating current as it is not seen outside these parallel impedances.

The currents that are produces due to magnetic flux circulation in the core are called eddy currents and these eddy currents are responsible for core losses in transformer.
While the circulating currents are the zero sequence currents that may be produces due to following causes.
1- when there is three phase transformer the (3rd, 5th, 7th….) harmonic currents which are called zero sequence currents from all the three winding of three phase transformer add up and become considerable even in loaded conditions these currents have no path in Y/Y connection of transformer so a tertiary winding is provided co conduct these currents but in Y/d or D/y connection these currents circulate in delta winding.
2- Whenever there is unbalanced loading in transformer. In which with positive sequence, negative sequence and zero sequence currents are also produced which cause circulating currents.
3- When the transformer banks are used and the transformers have phase between them then circulating currents are produced between them, than transformers in the bank get loaded without being shearing the power to the load.

AQ: What is the Reactive Power?

For a “physical” interpretation, reactive current (power/KVA flow), in my opinion is best looked at from the perspective of a generator connected directly to an infinite bus (in LV generators this is the norm).

The generator when connected to the system, “see’s/feels” the parallel impedance combination of all other generators (circa 3 ohms each) with respect to ground – which basically parallel to equate to a zero impedance in terms of restriction to any current flow out of our generator.

Post initial synchronization, the system voltage prevents currents from flowing into or out of the generator due to pressure (voltage) balance of our generator matching that of the system voltage.

If you (as the generator operator), try to lift the generator voltage, the result will only be heaps of current output flowing into the system – but with no actual extra power generated!

This is due to the fact that to achieve the extra generator voltage setpoint you desired, the generator must send out enough current into the system impedance to create the back emf required to achieve the new desired generator terminal voltage setpoint.

But because the system impedance to ground is very low (as it actually is) – then despite the extra current sent out in that fruitless attempt, the generator is near impotent to make any substantial effect on raising the “system” voltage – “fruitless” current sent out.

In a DC sense you can equate this to a small DC generator trying to lift the voltage of a load system that has a zener diode installed across that system load.

Back to the AC world, ….that current sent out in the fruitless attempt to lift system voltage must flow through the parallel low impedance of the other connected generators (each of those working against you – lowering their own generator excitation, hell bent on keeping their own same old voltage set points), thwarting our futile attempt to achieve a raise in the system voltage.

All those generators, although collectively of low impedance, compose virtually no resistance, compared to their inductive reactance. Hence all our little generators current flow – in its futile attempt to lift system volts – is virtually purely inductive.

So we have heaps of current flowing out in our attempt to lift generator volts, but because the current is 90 degrees lagging the voltage, the only power imposed on the generator prime mover is that due to the resistance of the generator windings (circa 1% of the full load current rating – hence basically un-noticeable).

Hence the physical interpretation of VAR’s, is actually simply a look at the voltage balance perspective of an electricity network. It’s the collective attempt of many parallel-connected generators to influence the system voltage – either trying to raise the voltage at a particular node (positive VAR’s) or trying to reduce the voltage at a particular node (negative VAR’s flowing back through our generator due to our attempt to lower our generator setpoint – which “lets current in”).

Reactive Power is an electrical parameter that exist in a sinusoidal (AC circuits). It maybe zero or a certain magnitude. It maybe capacitive in nature or it maybe inductive nature. In the power triangle, it is the vertical power component (plus or minus / capacitive or reactive). It may be supplied from power sending end (grid or generator) on from the power receiving end (load). A capacitor bank connected on the grid provides capacitive reactive power. An inductor bank connected on the grid provides inductive reactive power. Both of them have magnitude. Reactive power also influences the between phase angle displacement between the voltage and the current. It is power but reactive power.

AQ: Is frequency inverter better than soft starter in motor control?

There are hundreds of applications for a frequency inverter. I use them on a pump to test pumps with voltages from 208-600VAC 3PH 50 and 60 HZ. You just have to size the frequency inverter to the largest 208 HP motor, so it can handle the current. Many people are installing them on pumps, fans and air compressors to get the energy savings of lowering the speed on the motor to maintain the pressure, temperature and flow. Frequency inverters also have the ability to ride through power dips, since the DC bus to store in a capacitor bank.

It is correct the frequency inverter will reduce the staring current of and induction Motor, but as all of you know that the motor have to drive a load the starting torque is related to starting current, also the main role of frequency inverter is to control the speed.
The starting current is related to the rotor conductor structure or classes because we can get direct starting currents within 1.5-4 times the full load depend on the squirrel cage design or construction.

The effect is, that at the reduced frequency during start, the full torque can still be developed at nominal current. As soon as the frequency hits the nominal slip frequency, the nominal torque will also be developed, at nominal full-load current. (The slip frequency is the nominal frequency multiplied by the full load slip percentage, i.e. around 2.5Hz for a 50Hz motor with a full load slip of 5%).

It really depends on the application. If you are only interested in starting current, then soft start is what you need.

AQ: Circuit Breakers tests

1- For small circuit breakers we can do the test of Magnetic protection behavior by using “Injection Current Apparatus”, and suppose the CB’s results were good, do you think it’s enough? I’m sure not, because by this apparatus we can inject the necessary current with a very low voltage value (5-15V), so, do you think that the arc will be the same if we have the same current but with “400V”?

2- The same question for “Short Circuit Tests”

Personally, I done the tests of many MCBs for different manufactures by using “Injection Current Apparatus”, and I saw the same tests in laboratory in France for the same MCBs by injection the same currents values with 230V or 400V depending on the CB, be sure, the results weren’t the same, we found some differences for Magnetic protection tests, and big differences for Short Circuit tests.

AQ: How to select a breaker?

Before breaker’s selecting for your electrical system, you need to calculate value of expected short circuit current at the place of breaker’s installation. Then you need to calculate value of heat pulse and 1s current (expected value of current during one second). After that you need to calculate power of breaker and finally, after all, you can select appropriate breaker. Values of characteristics of selected breaker need to be higher from calculated values of characteristics of your power system.

You can calculate operational current of breaker using this expression:

Inp=SnT/((sqrt(3))*Un)

After that, you need to calculate expected value of surge current:

kud=1+e(-0,01/Tae)
Iud=(sqrt(2))*kud*I’

After that, you need to calculate expected value of heat impulse:

A=(sqr(I0″))*Tae*(1-e(-2*ti/Tae))+(sqr(I’))*(ti+Td”)

And finally, you need to calculate 1s current (expected value of current during 1s):

I1s=sqrt(A/1s)

So, current of interruption of your breaker and power of interruption of your breaker are:

Ii=I’
Si=(sqrt(3))*Un*Ii

Additional expressions that you can use during your calculation:

I0″=Un/((sqrt(3))*Ze”);
I”=1,1*Un/((sqrt(3))*Ze”);
I’=1,15*Un/((sqrt(3))*Ze’);

where are:

ti-time of interruption
Inp-operational current of breaker
SnT-rated power of transformer
Un-rated voltage
kud-surge coefficient
Tae-time constant of aperiodic component of short circuit current
Iud-surge current
A-heat impulse
I0″-short circuit current in subtransient period (generators are in no-load conditions)
I’-short circuit current in transient period
Td”-time constant of subtransient component of short circuit current
I1s-current during one second
Ii=expected value of current of interruption of your breaker
Si=expected value of power of interruption of your breaker
Ze”-equivalent impedance of power system in the place of fault (subtransient period)
I”-short circuit current in subtransient period (generators are in full-load conditions)
I’-short circuit current in transient period
Ze’-equivalent impedanse of power system in the place of fault (transient period)

For a branch circuit feeding a single pump, you would generally size the circuit at 125% of the pump’s full-load amperage. If you’re not using a variable frequency drive or soft starter (which have built-in overload protection), you would use a Motor-circuit protector (MCP) breaker that has both thermal and magnetic trip capability. Sizing would be according the breaker manufacturer’s recommendations for a motor of a given horsepower, but not larger than would be required to protect the circuit conductors.

“The total load of an area” is much too ambiguous to answer. If you have lighting and receptacles, you’re going to need a different type of breaker than if you have motors or mixed types of load. There is no general approach. Circuit breaker types are very specific to the application.

Safety should not be taken lightly. Installing the wrong type of breaker could result in equipment damage and/or physical harm.

There are instantaneous breakers as well as time delay breakers. For time delay breaker, for example, you go 250% maximum of the rated current based upon the HP of a motor (look in the NEC), not on the nameplate label. The nameplate current value is for overload protection. Also try to size the breaker so that the conductors are protected.

As we kn

AQ: Motor line starting and ramp starting with VFD

Variable frequency drives are important power electronic devices. When we start an electric motor, we are increasing from 0 speed to full operating speed. A VFD ensures that the motor accelerates (increases its speed) to its full speed in a smooth manner, without causing much irregularities. In other words, VFDs make the motor accelerate uniformly.
VFDs are also easy to install and use. VFD drives are not only for starting motors (like the normal starters), but for easy speed control as well.

The difference between line starting a motor and ramp starting the motor with a variable frequency drive is that the motor/load does not pull the 6-7 times rated current of the motor, because the motor winding are not saturated with the full EMF produced to get the motor to synchronous speed it is ramped to it. If you are not trying to control the motors speed from process control then a soft start will serve the same purpose. The VFD drive main purpose is to control the V/F of the motor.

You will have to adjust the ramp time on the VFD or soft starter to over the force required to turn whatever the motor is turning, this can be accomplished with both devices. Soft starter is less expensive than variable frequency drive, thus it has limitations.

AQ: Difference between ICCB, MCCB and MCB

The aforementioned types of Circuit Breakers are used in LV System and generally based on the same operating principle.
MCB and MCCB/ICCB have a bimetal heater for overload which releases the Contact s while for short circuit the trip / electromagnet hammers itself against moving contacts. The arc created by breaking contacts is extinguished in an arc chamber. Are defined as “Thermo-magnetic “ CBs , accordingly. It is operating characteristic addressing the overload by thermal action of the bimetal strip and instantaneously dealing with short circuit occurrences by electromagnetic action.

MCB – Miniature Circuit Breaker is suitable for domestic usage. Used to protect final circuits from O/C such as Overload & Short Circuit.
i- MCB is basically made in accordance to BS 3871, is now superseded by BS EN 60898 which recognizes type B, C & D.
Type B is suitable in domestic premises.
Type C is used in commercial & industrial applications.
Type D is suitable for application where a high in-rush current is expected.
ii- MCB is of low breaking / making capacity as well as low current rating compared with MCCB/ICCB. MCBs available in different number of poles (SP, DP, TP,,).

MCCB – Molded Case Circuit Breaker & Insulated Case Circuit breaker are also current limiting devices but with high making/ breaking capacity and current ratings compared with MCB. MCCB and ICCB are almost the same and both are manufactured in accordance to NEMA AB1/AB3 to suit industrial and commercial purposes.
The advent of electronic protection increased the use of them and the scope is widened like tolerances, range of time & current adjustment. By virtue of that a good discrimination can be achieved with accuracy about ±10%.
Eventually, MCCB/ICCB has advantages in the capability of accommodating further features which can be provided as
i. RCD.
ii. Under voltage device.
iii. A shunt trip coils that enabling remote tripping.
iv. Auxiliary switches for remote monitoring and/or control.

AQ: VFD PWM and PAM definition

PWM is shorted for Pulse Width Modulation, it’s a variable frequency drive (VFD) regulate way to change the pulse width according to certain rules to adjust the output volume and waveform.

PAM is shorted for Pulse Amplitude Modulation, it’s to change the pulse amplitude according to certain rules pulse amplitude pulse train to adjust the variable frequency drive output volume and waveform.