Unless you’re one of the pole-climbing, substation-fixing, electrical-testing superstars, you’re probably not thinking about transformers all the time.
Well, that changes now.
Transformers are everywhere. And you’re reaping the benefits of them every day– whether you realise it or not.
In our homes, we use alternating current (ac) because it is easier to generate and transmit. AC is typically transmitted at higher voltages and then transformed into a safer and usable lower voltage – powering the electricity we all know and love and can’t fathom living without!
Now, we’re not going to get into the details of how transformers work today, since this blog is all about the types of transformers. But, at the most basic level, transformers take higher voltages and convert them down to lower, useable voltages, like we mentioned above.
So, what types of transformers are there?
Power transformers
A power transformer transfers electricity between a generator and the distribution primary circuits. It gets a little bit confusing because many use the term ‘power transformer’ to encompass a bunch of transformers, rather than a specific design type. Likewise, some even refer to large transmission transformers as power transformers, to easily differentiate between distribution transformers.
Regardless of the exact definition, power transformers can have one of three jobs – step up generator output voltage to the transmission system voltage level, step down transmission voltages to safe levels for distribution, or step down voltage to the auxiliary power system level in a generating station.
Power transformers can also fall into one of two class – class I or class II. Class I power transformers have high-voltage windings of 69 kV and below, while class II power transformers have high-voltage windings between 115 kV and 765 kV.
Just to make things a tad more complicated, you can also categorize these by size – small, medium, or large. Small power transformers fall under 69 kV, medium up to 230 kV, and large power transformers are between 138-765 kV.
Autotransformers
Now, let’s make things even more complicated. Autotransformers technically fall under the category of large power transformers, but these are generally used as transmission inter-tie transformers, which can be used in either step up or step down mode. What’s an inter-tie transformer? Great question. An inter-tie transformer helps connect ac networks of various voltages to each other, which is a really important feature in a power network.
Typically, your autotransformers are going to be the largest rated power transformer on your transmission system – operating with a pretty balanced and constant load. They’re also more economical than having separate winding power transformers, since there’s a physical connection between the series and the common winding. Basically, this means that the high voltage winding is made up of the series winding in series with the common winding, while the low voltage winding is the common winding.
Ideally, you don’t want your autotransformer to be any less than half the size of a conventional transformer though, since you need to account for the space that taps and tertiary windings take up. Any less than half of the size is not ideal for performance.
There is one downside to autotransformers, though – low impedance. With a low impedance, the short circuit current of an autotransformer is way higher than a conventional transformer. To counteract this, autotransformers are usually designed with a higher than normal impedance, which just makes the actual size of the unit bigger, contradicting the positive we mentioned above. Ugh.
Generator step-up transformers
Moving right along to GSUs or generator step-up transformers. GSUs (sometimes also called main or unit transformers) step up the voltage from a generator to the highest transmission voltage for a transmission grid.
Connected directly to the generator, GSUs are typically operated at a constant load close to their full rating. Since they’re constantly working at their rated temperature, they’re going to age much faster than other transformers. If you’ve read any of these blogs before, you’ll know that excessive heat is never a good thing. Unless you’re a cactus…
GSUs aren’t usually protected by a circuit breaker between the generator and transformer, so these can get hit pretty hard with fault current too (and for long periods of time), which can lead to huge overvoltages. If a generator breaker is used, then a GSU can actually be used to power a grid’s auxiliary systems.
Auxiliary transformers
Auxiliary transformers supply power to a generating plant’s auxiliary loads (think feed pumps, coolant pumps, and safety devices that a power plant needs to run). There are a few different types of auxiliary transformers to keep track of, but fortunately, we have more acronyms to make our life easier.
A unit auxiliary transformers (UAT) is connected to the same bus as the generator, stepping down voltage to feed the auxiliary power system busses. Whenever the generator is running, the UAT is supplying the auxiliary load.
The reserve auxiliary transformer (RAT) or startup auxiliary transformer (SAT) are backup transformers which are connected to an offsite high voltage system – supplying the plant auxiliary power during startups or outage periods.
All auxiliary transformers are relatively critical to a plant’s safe operation, so you don’t want to see problems with these, or you may be facing a possible plant shut down. Not good.
Converter transformers
Before we get to converter transformers, we need to talk about HVDC – or high-voltage, direct current. In case you’re not familiar with power industry jargon, HVDC electric power transmission systems are commonly referred to as electrical (or power) superhighways, since they use direct current to transfer electrical power across long distances, quickly.
So, a converter – specifically a HVDC converter – adjusts voltage between the ac transmission and HVDC transmission systems, while making sure that no direct current enters the ac system (or vice versa). A converter transformer can also supply power in both directions, thanks to rectifier and inverter mode. In case you are new to the world of transformers or too lazy to google it, a rectifier changes current from AC to DC, while an inverter does the opposite.
We should mention that converter transformers require extra special care and testing, since they’re insulation structures are hammered with both high ac and dc voltages on the regular.
Phase shifting transformers
PSTs – also called phase angle regulators (PAR) – are transformers that either speed up or slow down the voltage phase-angle relationship of one circuit to another. These are used to connect power systems together, while controlling the level of voltage flow between them.
By adjusting the phase angle between the two systems, the operator can control the split of power flow between them.
Reactors
Reactors have just a single winding and can’t transform voltage, but you can connect multiple reactors together – in series or parallel – to introduce impedance into the system. Therefore, because they control or limit power flow, we can consider them transformers too. Seems like we’re getting a little ‘lax on the rules right now, but we don’t make the rules here; we just write about them.
There are two types of reactors to know – series connected and parallel connected reactors. While series connected reactors can control or limit power flow, parallel connected reactors can compensate for times of light loading in connected lines and cables in HVDC systems.
Grounding transformers
In a utility distribution network, grounding transformers are used to provide a neutral point in a three-phase system – providing a source for zero sequence current. Essentially converting a 3-phase, ungrounded circuit into a 4-phase system with a neutral grounded source.
A grounding transformer only draws a small current from the system – carrying current only in the event of a ground fault or connection to a ground load.
Regulators
Step-voltage regulators are a special type of autotransformers that regulate system voltage. A regulator can add or subtract voltage – up to a maximum of 10% of the exciting voltage – to keep the system at a constant output. Regulators can also be either single-phase or three-phase.
Distribution substation transformers
Distribution substation transformers are pretty easy to understand, since the name basically says it all. They’re basically small power transformers that step-down voltage. Like regulators, they’re available in either single or three-phase configurations, and typically sit within a few percent of maximum load – occasionally surpassing the nameplate rating.
Mobile transformers
In the event of a storm or natural disaster, mobile transformers are used to restore power fast following an outage. By providing temporary service, mobile transformers fill in for the more permanent equipment, while facilities are being repaired. These are also useful for supplying power during new construction or during regularly scheduled maintenance.
Usually, mobile transformers are part of a larger ‘mobile substation’, so switches and circuit breakers are added at the same time. These transformers typically fall in the low to medium power range, as well, so they tend to be too small to replace grid level, critical high-power transformers.
Due to their mobile nature, these transformers need to be easily transported, thus they are designed to be smaller and lightweight. In order to achieve this, mobile transformers sometimes have adjusted short circuit integrity, decreased dielectric design, and increased operating temperatures. Since they’re not used frequently or for lengthy time period, this is justified.
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