Transformer

A circuit breaker is more reliable for tripping big machines than a drive. Hence some engineers prefer tripping a machine from the breaker via the PLC/safety controller. The machine could be pump or compressor.

After the trip, the startup means energizing the primary side of the transformer, whose secondary side is connected to the variable frequency drive. This subjects the transformer to the startup inrush current which could degrade it.

Tripping the machine may be due ...

A number of things would cause a transformer to run hot; if you load it continuous 95-100% which is not good practice, solution-install bigger transformer if cannot permanently reduce load. Transformer winding temperature is measured indirectly unless your transformer has fiber optic sensors embedded in the winding.

Never re-energize a transformer that has tripped on Over Pressure (Dev 63) without first performing IEEE routine test on the transformer. Most oil filled transformers ...

In Europe we have a 1-phase 220v/240v power supply to home, which is ONE hot line, unlike America that have TWO hot lines (120v of each) into their homes which is therefore logically and technically a 2-phase power supply, even if they will admit the obvious.

There are two ways we can be feed our 1-phase supply from the pole mounted 3-phase 11kV line. The primary of that transformer can be feed from either one of the 3-phase lines, that would be 6.4kV to Earth, or feed from two of ...

Having the motor star point earthed won't have any effect on protecting the motor windings during a fault condition.

A power system must be referenced to earth at a single point, usually at the supply transformer or generator star point. Referencing the system to earth at multiple points on a distribution system, for example, at the star point on your motor (in addition to the supply transformer star point), would be bad practice as it could lead to the undesirable condition of cre ...

A three-winding transformer can be seen as two transformers in one and presenting a short-circuit level that will be dependent on the coupling reactance between the two secondaries. For ex., two generators feeding one common bus and with only one transformer has the least transformation cost, but the highest short-circuit level on that bus. The same generator, each one with their transformers present now the lowest Icc, but the highest transformation cost. With the 3-winding transformer, the ...

Metering current transformers are designed to be within quite accurate values up to say two x rated current, and down to <0.01 x rated. Consequently a protection relay connected to a metering current transformer will get a heavily saturated waveform during fault conditions and it won't work properly.

IEC Class P CTs for standard IDMT OC or even distance may only be 10% accuracy - it is obviously better of they are mo ...

I've spent a good deal of time in the field recently trouble shooting motor problems and observed the following. Often the bus voltages run high. When the voltage is high the magnetizing requirement of induction motors is also higher. Often the assumption is that if the voltage is higher the current will be lower without considering motor efficiency or power factor. Motors are designed to perform best at their rated nameplate voltage. First make sure that your bus voltages are where they shou ...

What is the only case that makes the single line-to-ground fault bigger than the 3-phase short circuit fault?

The L-G fault current will exceed the 3Ø fault current whenever you are near the terminal of a D-Y transformer. The Delta winding effectively blocks the zero sequence impedance contributed by the source. Thus Z0 source is zero and the only Z0 contribution to the impedance is the transformer impedance. As you move out from the transformer, the zero component should qu ...

While reading a generator AVR operation description: "When a transformer is used to boost the generator voltage, the transformer impedance provides the DROOP characteristic (voltage drop) and so compensates for any reactive cross current flow. In these circumstances, the AVR does not provide a DROOP characteristic".

AVR of generator has a circuitry which senses reactive component & hence called quadrature droop compensation. When we operate multiple generators in parallel exci ...

I'm an EE with almost 30 years' experience and I just looked up the same thing a couple years ago because zig-zags are used extensively in wind farms (for now anyway, not really required anymore). I used them over the years but just assumed they worked. Take the standard WYE phasor diagram with a-phase at zero degrees and divide each phasor in two. The part attached to the origin is the positive sequence.

The second part of the phasor has the polarity reversed and is the negative s ...

I wonder what is criteria for this test, which reasons important when design transformer for this test? Axial, radial forces or distance between low voltage and high voltage coils or other things?

The meaning of the short circuit proof design of a transformer is easy to understand: in case a short circuit happens in your grid and the damages are repaired, you don't really like to find after you switch on the power again that your transformer fails after short time.

IEEE C57.12 ...

Q: I have a situation where our UPS servicing data center rack systems are failing repetitively after replacements.

We know our mains transformer is undersized in terms of load capacity and under voltages are measured going to the input of the UPS that is providing back up power to data center in a rack mount. However these voltages measured were 110VAC and still within operating range. In another facility with the same rack systems, colleagues are saying they're seeing "brown out ...

We know that harmonics are generally due to the non linear loads connected at the load end. But whether harmonic distortion is increased or decreased by changing the X/R ratio of the source, feeder, transformer etc. And whether injecting power factor correction equipment into the grid will have an influence on harmonics.

Are harmonics produced only due to the non linear loads or also due to various other aspects in the trans ...

I have a 7KVA isolation transformer and having issues where it's tripping the circuit breakers. I have two circuit breakers, one main and one acting as a cutoff switch both installed on the primary side. Transformer manufacturer specification says 235A of inrush current with 7KVA purely resistive load. I have two questions:

1. I am fearful to load the transformer with equipment, so will the load on the second ...

Yes you can do it but need to exercise some precaution: The LV winding that was intended by design to be the secondary winding, will serves as the primary & the value of the magnetizing inrush current actually will be greater than expected. When a transformer is reverse fed, the taps move to the output side and so their operation is reversed. Taps will control the output voltage so chances of over excitation will be there. This is not serious concern till the input voltage variation is wi ...

I have an interesting dilemma at work. We have never had the ability to monitor each individual feeder emanating from the substations. I recently installed a modern recloser on one circuit which allows detailed measurement of line characteristics and discovered that we have 1) a serious imbalance among the phases, and 2) a terrible power factor on this feeder - .33 on one phase, and .65 or worse on the other two. The meter that monitors the overall bus reads a combined power factor for the tr ...

Situation
I have recently encountered a fairly unusual installation where a YNyn0 transformer has been used to supply a low voltage pump installation incorporating a large VFD.
The transformer was initially chosen to offset harmonic distortion with an adjacent Dyn11 supplying an identical VFD.
The transformer has been installed with the primary neutral connection floating in an effort to prevent zero sequence currents from causing nuisance tripping at the su ...

The Buchholz relays I am used to dealing with have two levels, a Gas Alarm (Volume) and Gas Trip (Surge). They have a nitrogen injection port on the tank side of the Buchholz to inject to test each of these contacts, and a bleed port from the top of the Buchholz which allows for the nitrogen or fault-gasses to be captured for laboratory analysis.

Obviously an open flame is a low-tech way of determining if there are short-chain hydrocarbons present in the gas in the Buchholz, but if ...

There are three ways to explain Percentage Impedance:
1) It is the % voltage drop in secondary no load voltage when a rated full load zero power factor lagging current is taken from secondary. It is the price one has to pay for transferring the current from primary to secondary side. Higher the % impedance, higher the secondary voltage drop during loading. This drop (or rise with leading loads) depends on the X/R ratio of transformer and PF of the load. % voltage drop= % ...

The % impedance is formally referred to as impedance voltage. It is the supply voltage, expressed as a % of rated voltage, that is required to circulate rated current through the transformer.

It is measured in the factory by a short circuit test at rated frequency. With the low-voltage winding shorted, the supply voltage to the high-voltage winding is increased until rated current flows in the transformer. ...