GSU construction and protection

 "He who laughs at himself never runs out of things to laugh at." - Epictetus

Today's post will initially seem really over the top technical, but it isn't.   This is about big power plant transformers and a few of the different techniques we use to protect them from external and internal faults.  The step-up voltages of these transformers is really high - 115,000 volts, 230,000 volts, and even 500,000 volts are pretty common.  

It should be pretty obvious that a fault involving half a million volts will involve a lot of fireworks.  Such fireworks can cause a great deal of damage if they aren't quickly interrupted.  So this post is about discussing how we go about sensing problems and rapidly cutting power when faults show up.

First, let's look at a one-line electrical diagram, courtesy of https://top10electrical.blogspot.com

Let's start at the bottom left, with the power plant generator.  The generator typically has an output voltage of 13,000 - 24,000 volts, at a current of about 1000 amps.  These voltage and current values are based on the practical and physical limitations of generator design.  The generator output goes onto the generator bus, and follows two paths.  The path most of the power takes is upwards through the "UT" (or Unit Transformer), where the voltage is stepped up to 230KV, through the Breaker, and out to "System".  We build power plants to make electricity to sell to customers, and that is the main path - the one we will discuss shortly.

The other path that the power can take is to the right, through the "UAT" (Unit Aux Transformer), where the voltage is stepped down to 4160 volts, to power the "Auxiliary Bus".  This supplies power to electrical loads inside the power plant, and allows the power plant to be self-sustaining once the generator is online.  During outages when the generator is offline, there is still equipment that must be running, so the "System" can back-feed power through the "SST" (Station Service Transformer).

This post focuses on the "UT", or Unit Transformer, which is also known as a "GSU" or Generator Step-Up transformer.  

First off, why do we step the voltage up in the first place?  

To understand why we step the voltage up to transmission line voltage, we first need to understand  resistive heating.  Resistive heating is what intentionally occurs on your electric range top.  Electrical current flows through a high-resistance heating element, usually made of 80%-20% Nickel-Chromium.  The NiChrome conducts electricity, but not very well, and as a result, it gets hot - this is essentially due to the friction of electrons trying to get through something that doesn't want to let them pass. 

Below:  Do not touch!  Resistive heating.

There is an electrical formula that explains how much power resistive heating generates, and that formula is P=I^2 x R,

Where P = Power, I^2 = the current squared (or times itself), and R = the resistance of the circuit.

All transmission lines have electrical resistance - at least until we can make them out of superconductors.   Transmission line conductors are made from aluminum to save weight on tower construction, with a steel core to provide tensile strength.  Like most engineering, this is a compromise between cost and benefit.

So we have a transmission line with a low, but (mostly) fixed resistance.  How can we reduce the resistive heating loss as we send the power hundreds of miles away?  Jack up the voltage.  This reduces the current.  There is a formula for that too.

P=I x E.  Power equals current times voltage.  If you send 10 watts of power, you can send it at 1 volt and 10 amps, or you can send it at 10 volts and 1 amp.  Or you can send it at 100 volts and 0.1 amp.  Etc.

The higher the voltage, the less amps need to flow for a given power.  So the less amps, the less resistive heating (and electrical loss) in the transmission line, and the more power you can push through it before it succumbs to overheating.  This is why very long-distance power lines (with greater resistance) often have very high operating voltages - like 750,000 or even a million volts.  These lines require a great deal of separation between conductors and very long insulators to hang the lines from the towers.  The expense of building such costly transmission lines can only be justified by the large distances involved.

Now that I've explained why the generator voltage has to be stepped up to 115KV or 230KV (or higher), let's look at the process.  It's pretty cool.

Below:  Y-Y connected transformer, courtesy of https://electrical-engineering-portal.com/3-phase-transformer-connections  Power flows in from the left on phase A, B, and C.  There are more coils on the right side than the left side, so voltage will be increased, and power at higher voltage (but less current) flows out on phase a, b, and c to the right. 

So far so good:  Power at 18KV and 1000 amps flows in on the left, and the same amount of power flows out on the right, but now it's at 230KV and just 78 amps.

Most people have seen a small transformer at one time or another.  They look like this:

A utility-size transformer that has to handle a lot of power looks considerably different, even though both transformers do the exact same thing.

First off, let's look at the electrical guts of one of these things.  We have three coils, one for each phase.

The primary side of the transformer is lower voltage and higher current, and the current tends to generate a lot of heat due to resistive heating.  The secondary side is high voltage and low current, and so it needs a lot of insulation.  To solve both the heating issue and the insulation issue, these windings are submerged in a tank, containing a special oil that has very high electrical resistance - also called dielectric. The oil not only insulates the high voltage coils from electrical arcing, it also cools them and prevents breakdown of the paper insulation that the windings are wrapped in.

Flow path for cooling:  The hot oil rises off the windings to the top of the transformer tank, flows out into headers, down through cooling fins that have fans blowing air across them, then flows back into the transformer tank at the bottom.  Sometimes an oil pump is used but not in all cases.

 

As you would expect, day-to-night and seasonal temperature swings, as well as internal heating of the transformer due to the amount of power it is passing, cause the oil to expand and contract, so the transformer oil tank is equipped with an expansion tank.  This keeps a solid oil bath in the main transformer tank, while also giving the oil a place to expand and contract to.  This expansion tank is called a "conservator"

Below: The oil conservator is the cylinder at the top right.

 

Below:  A conservator level indicator.  Notice the electrical wire connected to the bottom of the gauge in the right-hand image.  If the oil level gets too low, a trip signal is sent to the transformer protective relay (called the 87T relay), de-energizing both the high voltage and low voltage sides of the transformer windings - and this will happen well before the windings are ever uncovered of oil, because this tank sits well above the main tank.

It is critical that no moisture whatsoever enter the transformer, due to the high voltages involved.  For this reason, the oil in the conservator does not have an interface with air.  Air contains water vapor, so we don't want it in contact with transformer oil.  The oil is protected from direct contact with air by a rubber bladder.  Even so, the bladder is protected.  The breather vent line contains an oil bath to remove airborne dust, and also has a dessicant cannister to ensure that any air reaching the bladder is dry.

Below:  Transformer breather.  There is an oil bath at the bottom to remove dust.  Dessicant crystals are pink when saturated and blue when dry.  These should be checked on daily rounds.

Time now to discuss troubles - how and where they can occur, and what we can do to protect this amazing and expensive piece of equipment from them.

Oil degradation:  This can occur due to moisture ingress - due to a faulty bladder or poor sealing.  It can also occur due to minor arcing in the transformer windings.  Because transformers vibrate a lot, the windings can rub against each other and damage the paper insulation, creating places where minor arcing can occur through the oil. 

This arcing causes carbon to build up in the oil, which reduces its ability to insulate.  If the process continues long enough, a flash-over can occur, causing a great deal of damage.  There are a couple of ways to avoid being surprised by this situation:  Periodic oil samples can be drawn and sent off for analysis, or you can install an online monitor.

Below: An online transformer oil condition monitor.  This one is known as a "Hydran".  It monitors for hydrogen gas, which is a large component of oil break-down.

So if minor internal arcing causes a build-up of hydrogen gas prior to a catastrophic flash-over, is there any other way we can know about it?  Yes there is.  Large transformers are equipped with a very cool device called a Buchholz relay.  This is an ingenious two-stage mechanical device that gives you a warning for the minor internal arcing condition and a trip for the flash-over condition.  Let's take a look.

Below:  A Buchholz relay.  This is installed in the oil line that connects the conservator to the main transformer tank.  By Fluppe37, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=34969018

 Location of the Buchholz Relay.

Now let's have a look at how this protects the transformer from each event, mechanically.

Below:  Cutaway of a Buchholz relay.  By Fluppe37, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=179854

If the transformer is off-gassing due to minor arcing, this gas will flow out of the main tank and up towards the conservator.  But first it must pass through the Buchholz relay.  The gas will accumulate at the top, and the upper float will begin to move down with the oil level as a gas pocket builds up.  This movement will trigger a switch and send an alarm to the control room - letting the operators know that there is an issue in the transformer.

The Buchholz relay also provides protection in the event there is a major flash-over.  Any flash-over will vaporize quite a bit of oil in the main tank due to heating.  This will pressurize the main tank and send a surge of oil rushing toward the conservator, since this is the only path for expansion.  The lower ball will be shoved over by the flow surge and trip a switch.  This switch will activate the 87T transformer protective relay, de-energizing the high and low voltage sides of the transformer.  The lower portion of the Buchholz is also called the "Sudden Pressure" relay.

There is another electrical relay that will activate the 87T transformer protective relay, called "transformer differential".  What this refers to is differential power.  Power in must equal power out.  With our transformer, the total power going into the low voltage side should be equal to the power coming out of the high voltage side, within a very tight margin.  If you have a lot of power going into the transformer and suddenly have much less coming out, then you have a big problem that is going to generate a lot of heat, and you need to de-energize the transformer right now.

So this differential relay constantly monitors to see that (per our earlier example) 18KV times 1000 amps = 230KV times 78 amps.  If this relationship changes for any power level, the transformer gets de-energized.

These transformers also have oil temperature and winding temperature gauges on the outside of the tank.  These provide indication, cooling fan start/stop function, alarms, and high temperature trip.  Usually a transformer trip is caused by the faster-acting relays rather than the temperature gauge.

It might not have occurred to you, but the transformer winding temperature gauge is an indirect measurement.  Very indirect!  You don't want to install thermocouple wires in 230,000 volt transformer windings, and route them out to a gauge that someone can touch.  That would be extremely unsafe.

The temperature displayed on the winding temperature gauge is a rough guess, based on how much electrical current is flowing through the transformer.  Here's how it works:  

A current transformer reduces the current flowing through the high side winding to a safer voltage, like 120 volts, but the current at 120 volts will be proportional to current through the transformer main windings.  This 120 volts heats a small bulb of gas that moves the needle of the temperature gauge.  So it's measuring temperature remotely and safely, by using pressure!

Lastly, there are some external faults that can damage or destroy a transformer, and the primary one is lightning hitting the transmission line and back-feeding into the transformer.  Lighting strikes deliver millions of volts - enough to break down the oil in our transformer and cause a flash-over.  But there are some ways to mitigate even lightning strikes.

Take a look at the image below of this high voltage transmission line.  At the very top is a tiny wire.  This is called the "static line", and it is connected to ground.  The purpose is to have a ground line at a higher elevation than our power lines, so the lightning will hit that and go to ground, rather than hitting one phase of the power line and blowing up our transformer.

Even so, not all lightning strikes hit the static line.  Some decide to hit the power lines, and we get a million volt surge that they transformer was never designed to handle.  But we can still avoid damage.  

Below, left: Surge Arrestors.

On the transmission line (high voltage) side of our transformer, we install a surge arrestor on each phase.  The surge arrestor is a stack of metal oxide varistors.  These resist the flow of electrical current until their breakdown voltage is reached.  Once the breakdown voltage is reached, they become conductors.  This shunts the excess voltage to ground until the condition clears - with lightning this is usually measured in milliseconds.  Then the surge arrestor stops conducting and goes back to being an insulator for the rated line voltage.  Clever!  Each surge arrestor has a counter to display how many times it has shunted excess voltage to ground.  You check this daily during your rounds, and yes, most places I've worked have had some hits.