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Why is direction of electricity flow relevant?

A standard electricity grid is a set of networks, operated at different voltages, connected by transformers. As a physicist a know the working of a transformer and ik know that transformers work symmetrically, meaning that the energy flow can be in both directions.

Yet public understanding is that these networks can hardly cope with re reversion of the electricity flow by decentralized renewable generation.

I can think of three reasons:

  1. the transformers are not symmetrical in their working, for some reason.
  2. the direction of the energy flow is irrelevant, it's the size of the peaks that are usually larger.
  3. the standard low voltage side of the transformer is usually set at a high level, because consumption in this segment lowers the voltage. In this way the voltage of the end-point of the segments are kept high  enough. When the energy flow is reversed. This "high default setting" only makes things worse.

Can someone clarify which "reason(s)" is/are "true"?

Thanks in advance.


Charles -- Here are some brief answers for you to consider: 

1) distribution transformers are symmetrical and don't care about the direction of power flow.

2) The directional issue is mostly about 'peaks' if we take the term peak liberally -- if there is more energy generated than consumed on the customer side of a distribution transformer, power will flow back to the distribution feeder.  During the daytime when solar 'peak' generation is higher than customer load, which peaks in the early evening, there can be instances when power flow reverses direction and flows back tot he utility.  This more likely to occur when many neighboring customers have rooftop solar.

3) Voltages on the customer side of the transformer can get high, and in some cases too high.  Distribution secondary tap settings can contribute to the problem. 'High' or boost tap settings generally occur on long and/or heavily loaded distribution feeders and secondary service drops where the voltage drop from the substation transformer (source) to the customer (load) is significant.  As customer solar generation begins increasing during the day, the loading on the feeder (and secondary service drop) appears lower and so the voltage drop is lower and customer voltages increase.  If there is no voltage regulation on the feeder, customer voltages can get too high.

The direction of power flow on distribution systems can be a problem in these and other examples:

Distribution systems are designed with highest current carrying capacity nearest the substation with conductor sizes generally decreasing closer to the customer.  When enough customer-sited generation begins delivering power back to the grid, conductors nearest the customer could be too small.

System protection elements like fuses, reclosers, and protective relays are generally designed assuming no fault current contribution from customers.  As solar penetration increases and more and more inverters are connected to the grid, now fault current contributions can increase and protection systems may need to be reconsidered.

Distribution system workers used to know that equipment was energized from the substation so if the equipment they are working on is disconnected from the substation, they should be able to safely work without being electrocuted.  With customer-sited generation, workers must be extra diligent to ensure that equipment is properly isolated and grounded before working.

I'm sure these answers aren't perfect but hope they help.

Bob Meinetz's picture
Bob Meinetz on Jul 29, 2019 4:10 pm GMT

Steven, thanks for your  response. EC would benefit by having more input from people with your knowledge and experience.

Question: does phase coherence present any difficulties on the grid as solar penetration increases?

In the transmission system (with the exception of trunk lines) direction is not relevant. Transmission is designed and built - in general in rings and power can flow in any direction in the ring. 

In distribution systems there are several issues:

1) Mechanical voltage regulators have coils that can be excited by reverse power flow and mis-operate - those mis-operations can include fires in the equipment

2) the circuits are typically designed with the idea that voltage decreases from the substation to the last customer. If reverse power flow happens - that is not true, reclosers, relays and other protection equipment may need to be re-set to handle this

3) if the protection system is permissive for reverse power flow, lightning can potentially travel the whole circuit (depending about installation of lightning protection and lightning arresters) and potentially damange customer owned equipment

4) Reverse power flow can lower the fault current, if a wire is down in the right conditions of reverse power flow, the protection system may not see the downed wire and so it remains in the ground fully energized and a hazard to people, pets and property

5) Typically the conductor sizes and fuse sizes are reduced as you travel from the substation to the last customer - so putting generation in the circuit may not overload the substation transformer ,but could overwhelm the fuses and conductor sizes. Typically the best places to install large solar/generation is in the less dense population areas - which tend to be near the last customer on most circuits - overloading conductor can result in overheating and long term reductions in the conductor capacity - as well as weakening the conductor - making faults more likely

6) Reverse power flow is likely to raise the voltage locally - if the increase is enough, then the local generation will probably shut off (IEEE 1547 voltage limits) and not generate any power - resulting in unhappy owners of the generator and fluxuating voltage, since it will try to generate about every half second and then shut off again - makes the lights brighter/dimmer on each cycle. 

there are other less likely issues. 

Proper planining and interconnection studies can find and allow the distribution system owner to fix the issues, before they become problems. 

But in general distribution was not designed for reverse power flow. All these problems can be fixed, it just costs money - in many cases far more than the value of the power being generated over the life of the generator. 

Others, more knowledgeable than I in the EE aspects of the system can provide answers to your embedded questions. There  are however some very significant Economic impacts on the direction of electricity flow that are noteworthy. Financial transmission rights (FTR) are directional and can go from being a $$$ positive gain to a loss. Direction of flow is vitally important from an economic perspective in determining who gets paid and who pays.

I know this is not directly answer your embedded questions, but it does provide an economic perspective to the headliner question.

Your attempt to tie the physics of how transformers work with the direction of the flow is getting way too down in the weeds. You must back up and look at how one operates the entire power system (transmission, distribution, generation). Knowing the direction of the flow on the transformers from the distribution side where the DER is going to be installed is important to the system operator. Monitoring those flows and factoring them into managing voltage profile and reactive resources, maintaining all system loadings within acceptable equipment limits, committing appropriate generation to meet load plus losses plus interchange schedules, etc. all require the system operator know the direction and magnitude of the flow at all times.

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