Smart grid: variable ratio distribution transformers 2

[electricuwe]

Smart grid currently is often regarded as a topic requiring lots of highly advanced new equipment and a lot of communication in between the different stations.

However, in Germany a quite pragmatic solution is gaining significant ground: The variable ratio distribution transformer, either using a scaled down on-load tap-changer well known from larger units or some power electronic equipment (rated at some per cent of the nominal power) to vary the secondary voltage. With both approaches the secondary voltage is controlled, mainly to allow for higher photovoltaic power beeing injected without exceeding the voltage limits.

Do you see similar solutions in other regions ?

How do you assess this approach compared to the solid-state-transformer (processing the full power with power electronics and hence operating the core at higher frequency) that is favoured by research in US ?

 

[davidbeach]

Gee, anybody ever heard of a (On) Load Tap Changer? Or, its close cousin the voltage regulator? Sounds, yet again, like "smart grid" is a ploy for those that don't know how the system works to reinvent the wheel.

 

[Skogsgurra]

Easy now! Read twice.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[davidbeach]

So many of these "smart grid" things, including some I've seen personally, strike me as solutions in search of problems.

Like the grand idea that appliances could have frequency sensors and shut down on low frequency. Great in theory, but not in practice. With low penetration they won't matter one way or another. But, in higher penetration if set in such a manner as to not trip on transient frequency events it won't trip until the system is gone, no added value. If set more sensitively, it would remove load during frequency events while the system is designed and operated with the expectation a certain load response. Almost any under frequency we see is the result of a huge dynamic break being used to arrest an over frequency event in conjunction with significant generation shedding. Last thing we need is load going away on its own.

 

[Skogsgurra]

I read Uwe's post another way. It is about avoiding "smart" solutions and trying to find a solution to a problem that is getting more and more difficult to handle. Especially in Germany after Frau Merkel's decision to shut down nuclear (and perhaps also fossile).

This problem is common to all micro-generation plants where power quite often is supplied at the end of the "arteries" and thus rises voltage above acceptable limits. There are tap changers, voltage regulators and other devices that can handle the situation. I think that Uwe wanted to know how this problem is handled in other parts of the World (outside Germany, that is).

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[davidbeach]

Perhaps I read too hastily. What I think needs to happen in that regard is a combination of bi-directional regulators and inverters that can actively regulate voltage. The inverter manufacturers won't be too happy about that as it requires larger power electronics for a given real power output. But it really can't be the expense of the utility as the benefit is only to the customer with the inverter and not to the system as a whole.

 

[PHovnanian]

I think David makes a valid point. Although I suspect its not just the inverter manufacturers who will complain. The people with an investment in solar installations won't like being told when to go off line or throttle back their systems. And right now they have political power (thanks to pro alternative energy agendas). From a cost and technology standpoint, it makes sense to regulate at the PV inverter. The power electronics and supporting software are already there. The utility may need the regulatory power to dictate VR/droop parameter settings in connected inverters. Or even remote control of PV generation from the utilities sys op (watch the owners scream when the utility sends a shutdown command during light load conditions).

On the other hand, LTC (either switching types, solid state or hybrid) could be useful to utilities for other conditions. If the prices could be kept low (not much more than the typical pole pig), they could be a useful tool for long line distribution systems that exhibit wide voltage swings and require line regulators, capacitors and other equipment. Think about how this technology could improve single wire earth return system performance on long rural distribution systems.

 

[cranky108]

Just a thought, but just how much maintenance do these things require? Would switched capacitors require less investment, and maintenance?

You know if you keep your distribution feeders short, these won't be needed. Many of these gee wiz produncts only solve a few problems without them selves becomming a problem.

Our position currently is if the customer wants to ruin there own power, then OK. However if they ruin somone elses our legal people make them fix it.

 

[electricuwe]

Thank you very much for your feedback.

To David: Inverters that actively regulate the voltage by feeding in reactive power are already state-of-the-art in Germany . However, even this is sometimes not enough to cope with high PV-generation at the end of long feeders. Therefore the variable ratio distribution transformer is a solution to complement this approach.

Feeding in reactive power can be done with little penalty on performance if the inverter is designed properly.

 

[prc]

Providing on-load tap-changers in distribution transformers( as low as 500 kVA) is some thing that we were doing in India well before the advent of smart grid. Our problem then was acute low voltage and fluctuation of distribution voltage, mainly due to demand exceeding much above the power supply . We developed very simple OLTC for that purpose with very wide on-load tap range in distribution transformers.Today the famous german tap changer manufacturer has come out with a simple OLTC specially for distribution transformers to meet the challenge of renewable power generators feeding at distribution voltages.

 

[Grinc]

I read this thread and many others about on-load tap changers in distribution transformers. Prc, it seems that in India you use on-load tap changers...i would like to know more about this on-load tap changers. In our network we have off-load tap changing in distribution transformers (< 500 kVA), so i am very curious to know more. When did you use on-load tap changers ? How they impact the whole grid (when did they change the tap of transformer) ? Did this on-load tap changer in transformer work automaticaly and i can lead this tap changing from distance ?

Best regards,
Grinc

 

[HornTootinEE]

I agree with davidbeach on the smart grid in general. I'm seeing alot of "Solutions" in search of a problem. People trying to sell these "smart meters" as the get all end all of the "smart grid" I don't really see them as all that special to tell you the truth.

Alot of these people are pushing solutions (voltage regulators, distribution capacitors, etc.) that any engineer worth his salt has already implemented on their distribution systems. If you have long feeders and haven't looked into reactive compensation via capactiors, shame on you as this concept is taught in basic circuits in EE school.

So really, all this "smart grid" talk on the vendor side is just re-packaging existing technology for no-experience managers who don't know any different and are too bull headed or stupid to listen to their experienced engineering staff.

A true smart grid? there are some out there probably already. Heck, given the right spin the basic recloser could be "smart grid" all by itself!

I have seen alot of systems-especially small rural electrics and small municples-that have never had an engineer on staff paying attention to load growth, system performance, etc, and those are exactly the markets the "smart grid" vendors are targeting. Large rural Coops, large municiples, and IOUs that have engineers consistently on staff over the years already know it's just a repackage. You'll never get them to bite. With the current sales pitch that is...

 

[prc]

In India we were using DT with OLTC to take care of the violent fluctuations in incoming voltage. One or two companies in India are making such low cost simple on-load tap changers. of course they can be controlled in local/remote electrical control and auto control also. Recently MR Germany has brought out a simple tap changer for distribution transformers esay to fit over the active part of transformer working with vaccum contactors. You can have details from their website.

http://www.reinhausen.com/en/desktopdefault.aspx/tabid-1515/1834_read-4692/

 

[Grinc]

So i understand that you are using DT with OLTC when primary voltage fluctuating dramatically to stabilize secondary voltage by automaticaly change DT taps. And how often do you have these fluctuatings ? OLTC is mechanical device so it needs maintance and i suppose that OLTC have also limited numbers of actions. Is it cost effectively ? And do you use OLTC in urban or rural areas, or both ?

 

[prc]

Grinc, it was used mainly for industrial factories. OLTC operation will be limited depending on the fluctuation, may be 3-4 times a day.Even in auto mode it will not hunt as there will be an adjustable time delay for intiating tap changer operation.

 

[Grinc]

Prc, thank you.