Iacdrive|Automation Solution

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.

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.

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.

Why transformer rating is shown in KVA?

Transformers are rated in {VA, kVA, MVA etc.} due to flows of active and reactive power through transformer. In case of transformer we have active power losses as consequence of existence inside resistance of windings (primary and secondary) and existence of active losses of ferromagnetic core and other side we have reactive power losses as consequence of existence losses of magnetic flux (primary and secondary) and existence of reactive power losses of ferromagnetic core.

[VA]=sqrt(sqr[W]+sqr[VAr])

Transformer is rated in kVA by the manufacturer to inform users about the maximum power (voltage and current) that support it, the reason for not rating it in KW is that the active power (kW) is depend on the loads (lighting, machines..)

The simple answer is: It is because the kVA (or MVA) rating is only rating that matters to express a transformer’s “capacity” to allow the “passage” of power. That capacity is the thermal capacity dictated by the current it can carry at a given ambient temperature, regardless of the power factor. So combined with its voltage ratings, kVA (or MVA) is the value that matters. kW rating does not matter as transformer can handle unity power factor or in other words, a transformer can handle kW equal to its kVA rating at any time.

Remember that a transformer, as the name suggests, is only a transformation device or a pass through device and not a power producing device like a generator or an UPS, where their capacity to produce real power (kW) is an independent limit from the thermal ( kVA) limit.

To take it a step further, if you have an ability to cool the transformer further, you can augment the kVA (or MVA) rating of a transformer. This would explain having multiple kVA/ MVA ratings on transformers with forced cooling aids installed on them.

If you think of it, this is not different from a cable or a conductor’s capacity expression. Except that a transformer can have more than one voltage levels and different ampacities on primary and secondary, but the kVA rating remains the same on either side. So that makes kVA a more convenient way to express its thermal capacity vs. the amperes alone.

Transformer Magnetic Design

Control Servo motor with a variable frequency drive

Looking at those AC drives they recommend an Induction motor. A servo motor with permanent magnets which is not quite an induction motor. So, if a servo with permanent magnets can be used instead an induction with these kinds of AC drives.

Actually, the term “Servo” makes a reference about “feedback”, it means, whether we need a control loop, we are talking in terms about Servo, in this case, we have, or we know, the “feedback” by an encoder. Typical variable frequency drive doesn’t have a input for an encoder, so, if you want to control a Servo Motor with a VFD, you can move the motor, but you can’t control it.

A servo motor can be an induction servo, a brushless servo, a reluctance servo a dc servo – each of these can be either linear or rotary and can come with a variety of feedback such as tachometer, resolver of various pole counts, incremental or absolute encoders discreet or serial interface with different bus options, laser feedback, halls etc.

Then you come to the term variable frequency drive. Brushless servo amplifiers are also vfds. Do standard inverters have proper control of induction, and brushless motors. Some allow for a software switch, some allow for a firmware download, some don’t. Will inverters accept feedback – some have it built in, most that allow it do so by option cards, many do not.

Normal input in a variable frequency drive is, digital to start or stop, and we could have an analogic input to control by potentiometer.

Using AC Drives for the servo application is quite possible, provided the application is less demanding in critical positioning purpose.
There are number of makes that showcases pinpoint positioning of motor shaft being driven by AC Drives like Hitachi SJ700 / Emerson Uni drive SP / Danfoss FC etc.

Its beneficial to opt for the AC Drives as it supports SLVC [ VFD gives almost servo-like torque at low rpms if you give it encoder feedback ], multiple motors can be accessed, torque requirement can be met if required, power dips can be sustained using VFD’s.

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.

Transformer harmonics

The harmonics are created by the loads that the transformer supplies power to. If your loads include a high percentage of electronic loads like IT equipment, electronic ballast lighting, electronic motor controls, etc., there can be a very high amount of harmonics that circulate back to the transformer. The harmonics create an increase in the neutral currents. Most standard transformers are not designed to handle the higher harmonics and corresponding high temperature. Type K rated transformers are designed withstand the higher harmonics, without derating the transformer or limiting its maximum load. There are harmonics filters on the market as well as the use of isolation transformers.

B/H curve of the magnetic material forming the transformer core is not linear, so if a sinusoidal voltage is being applied for a sinusoidal current (and hence sinusoidal flux & a sinusoidal secondary voltage), the magnetizing current is not sinusoidal. Thus the magnetizing current of a transformer having an applied sinusoidal voltage will comprise a fundamental component and various harmonics. The magnitude and composition of these harmonics will depend on the magnetizing characteristic of the core material and the value of the peak flux density.

By the way:
– The standard Transformer “Non-saturated” generate Harmonics only in transient case when the power is supplied, and after this too small time it doesn’t generate any kind of Harmonics
– The Transformer generates Harmonics if it’s saturated
– We should take care when selecting the Transformer’s Power if there are a lot of installed Non-linear Loads, so for this case, we can select the power after define the correction factor by using the special curve done by “IEC”, or calculating this factor by using a special Formula done by “UTE – France”.

All AC signals are sinusoidal and periodic. These periodic signals can be resolved into a kind of trigonometric series – fourier series which is a summation of a fundamental and multiples of fundamental frequency.

The moment there is slight distortion from sinusoidal nature , it leads to harmonics in addition to the fundamental signal.. One way is to use DC signals… no harmonics.

As long as the AC signal is perfect sinusoidal , load & source is linear, there will be no harmonics. The way to get rid of harmonics is to have perfect source and perfect loads.

The non linearity introduced due to energy storing magnetic circuits, switching circuits, energy converting & inverting circuits distort the waveform to non sinusoidal. Therefore leads to harmonics. .

The way, there are antibiotic medicines for diseases. One needs to install the filter devices, which produce counter currents to suppress the effects of harmonics. The filters contain capacitors, inductors & power electronic components which are switched in anti-phase to harmonics producing elements. Thereby absorb harmonics.

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.

Frequent tripping in Unit Station Transformer

We are facing problem of frequent tripping in Unit Station Transformer -2 during heavy rain. During checking, not found any abnormalities
1. Fault UST2 OTI Trip Observation- * Direct OTI trip initiated without Alarm
* Oil temperature is normal
* Tripping contacts are not physically operated
* OTI trip contacts are operated frequently as per in Disturbance record Action taken-Spare Core of another cable used for OTI Trip contact

2. UST2 PRD Trip
* PRV contacts are not physically operated
* PRV trip contacts are operated frequently as per in Disturbance record New cable used for all contacts

Make sure for rainy conditions the marshalling box is properly closed and weather protected as moist condition also leads to tracking and may simulate tripping inadvertently. Please also make sure the tripping impulse from OTT/WTT/BT/MOG/PRD etc are driving a mechanical hand reset type VAJH type relay and the output contacts of this relays are used for trip ckt initiation.
If these are substituted by numerical relay binary inputs then also there will be a problem of spurious tripping.

Make sure the grounding of the multicore control cable if sheathed the grounding should be at TRPP panel end only, if armoured cable is used the armour grounding at the panel end only.’

I am assuming that the trip circuit is floating DC (ungrounded). If so, the moisture could be causing a “sneak” circuit, otherwise known as a “hot short” in the tripping circuit, which essentially bypasses the sensing relay contacts and actuates the tripping relay coil. I would check the cabling between the sensing relay contacts and the trip relay coil and the cabling on the hot side of the sensing relay contacts for insulation problems.