Technology  »   bus differential relay & transformer differential relay

The new unit has two switchgear buildings. The first gets 34,500 volts from the local utility company through a bunch of cables as thick as your wrist. The power comes to the building and hits a circuit breaker. Closing this circuit breaker puts power on the “bus”. A bus in electrical parlance is a means of distributing power. Ours takes the power and delivers it through other circuit breakers to the cables feeding our other switchgear building a quarter mile away. We have two 34.5 kV buses like this, each with one incoming breaker, three outgoing breakers and between the two buses is a breaker that lets us tie the two buses together. This breaker is cleverly named the tie breaker.

Now, with a million or more dollars worth of equipment, you’re gonna want some protection, and we’ve got it. Think of cutting the cord to your table lamp with a pair of scissors. You’d see a flash, hear a pop, and the breaker’d trip. Your house might be able to put up a few hundred amps worth of fault current at 120 volts until that breaker tripped. My power system is 300 times the voltage, and the fault current is in the thousands of amps. A fault releases plasma hotter than the surface of the sun. We want to minimize that sort of event so we have clever protective devices.

One of those protective devices is called a “bus differential relay”. Bus, because it’s protecting the electrical bus. Differential, because it looks at the amount of current flowing into the bus and compares it to the amount flowing out of the bus, and reacts if there as difference. And realy, because it uses small quantities to cause big actions.

One of my problems yesterday was that both bus differential relays operated. Each thought that at one point in the operation the current flowing into one breaker did not match the currents flowing out of the other breakers. Since it knew in its tiny little brain that this was “bad thing” it tripped all the breakers on the bus. This is what it’s supposed to do if it reads things right.

However, it did not read things right. Seems that there was a disconnect between the people who designed and built the switchgear and the people who programmed the relay. The relay read the data coming into it, applied the wrong math, and operated.

This morning we found the inconsistency in the drawings and that problem was fixed in five minutes by reprogramming the relays.

So that brings us to the second problem. The power leaves the first building through another set of big cables and goes to a couple of transformers at the second building. These transformers take 34,500 volts and change it to 4160 volts. The 4160 then goes into the building to main breakers that each feed 4160 volt buses.

These transformers are protected too, because they’re about a million bucks a copy and contain 2000 gallons of oil and make a big mess when they blow up. They have protective relays too. One function looks at the amount of current flowing into and out of the transformer and says it can’t be above a definite value. If the current gets over that value, the relay starts thinking about tripping. The higher the current, the faster it trips. Again responding to fits of whimsy, we call this a “time overcurrent relay”. But there needs to be a lot of current for this event to happen, and we want our million bucks to be rather better protected, so we use another element, too.

Complete article :

http://mostlycajun.com/wordpress/?p=4552

Question:I’m sure there’s a good reason, but I have to ask: why the complexity of harmonic detection? The relay “knows” it’s protecting a transformer — it can’t do anything else; that’s what it’s built for. It would seem, to this ignoramus, that a relay that detected when current was first applied, and expected an inrush peak that decayed rapidly and a slowly growing output current, would serve the same purpose. Clearly this isn’t the case. Why not?

Answer:Good question, and it gets into a bit of history. Early transformer differential relays were electromechanical devices of a class called “induction disk” relays. These were fun things. They’re still out there, too, 1930’s technology working exactly as designed seventy years later.

Here’s how they work: A transformer (not all of them, but we’re trying to make things simple here) has two windings, a high voltage and a low voltage winding. Usually power flows into the high voltage winding, magnetizes an iron core and induces a voltage in the low voltage winding. Power flowing into one winding equals power flowing out the other. Yes, bucky, there are some tiny amounts of loss, but we can ignore that. The relay does. To make our ‘differential’ relay work, we have a current input from each winding. In the old electromechanical relays, each current passed through an electromagnet, producing a pull. In our “induction disk’ relay, the pull on one of those magnets tended to turn and induction disk. If the disk turned far enough, it closed a set of contacts that would trip the transformer’s circuit breakers, disconnecting it from the power source. Either input from the transformer would do this, but if both inputs saw the same amount of energy, one going in, one going out, their pulls canceled each other out and the disk didn’t turn.

When we energized (turned on) the transformer, current rushed in one winding to magnetize the core, but nothing was leaving the other winding. This takes a few cycles, a couple of tenths of a second, at most. Only one of the electromagnets in the relay sees current in this condition, and the induction disk starts turning. If it is, however, only “inrush”, the current drops to almost zero before the disk turns far enough to trip the breakers.

This is simple, and it works, but it guarantees that if after the transformer is energized and you do have a REAL problem, that real problem can go on for a certain amount of time while the disk gets around to tripping things. That small amount of time is often the difference between a transformer REPAIR or a transformer REPLACEMENT.

The more modern relays like the ones I wrote of earlier don’t have to depend on the delay. On inrush, they look at the electricity going in and evaluate it. If it meets “inrush” characteristics, the realy shrugs it off, but if it doesn’t meet those characteristics (as defined by the protection engineer) then it says “Game over, Man!” and trips. Instantaneously, which is defined as “no intentional delay”.

And that’s only ONE part of the transformer protection. We also have “straight” overcurrent protection (there’s no “gay” overcurrent protection, but there is “directional” and “voltage-restrained”) that says the transformer is rated for X amount of current and if the actual current is X plus something, we trip. It’s on a time-current curve, so the more current the less time it takes, but for example an overcurrent element is set at 1.25 times the rated current and at that level it would take several minutes to trip, whereas the differential element will trip RIGHT NOW with only 15% of rated current IF it thinks there’s a problem in the transformer. Again, the difference is between a transformer we can fix and one we have to replace.

Another protection monitors the temperature of the transformer. It gives us an alarm if it thinks the transformer is too hot, and if it gets too much hotter, it will trip the transformer off before the poor thing cooks itself.
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