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Electric power systems: secondary networks

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AlbertL:
There have been several threads discussing various aspects of power distribution, but one interesting topic I haven't seen is secondary networks.

The most common distribution system, called a "radial" system, consists of a feeder circuit (generally in the 4-35 kV range) originating at a substation and supplying multiple transformers, each of which serves one or more customers.  The key characteristic of this system is that any given customer is supplied by only one feeder circuit.   

In a secondary network, customers are served by two or more transformers paralleled on the secondary side and supplied by different feeder circuits, possibly from different substations.  The simplest form is the "spot" network, having only one customer on the secondary bus.  In an "area" network, the bus serves multiple customers.

A key component of a secondary network is the network protector.  This is a form of circuit breaker inserted between the secondary of each transformer and the bus.  Its purpose is to protect the system in case of a short circuit ("fault") or outage on a feeder by preventing reverse power flow from the secondary bus through the transformer into the primary feeder.     

richard.cs:

--- Quote from: AlbertL on October 22, 2020, 05:49:24 am ---The most common distribution system, called a "radial" system, consists of a feeder circuit (generally in the 4-35 kV range) originating at a substation and supplying multiple transformers, each of which serves one or more customers.  The key characteristic of this system is that any given customer is supplied by only one feeder circuit.

--- End quote ---
In the UK we have distribution circuits like this at 11 and 33 kV (phase-phase), but ours tend to be a bit different. In most cases the feeder forms a ring from the primary substation, around some number (perhaps 10-15) secondary substations each with 1 or 2 transformers supplying a few hundred customers each, and back to the primary substation. The ring is broken at one point, and is fed from both ends essentially creating two radial circuits. The nice feature though is that the break can be remotely switched to occur at any of the secondary substations, or at two of them, such that any one faulty length of cable can be isolated for repair without loosing the 11 kV feed to any of the secondary substations.


--- Quote from: AlbertL on October 22, 2020, 05:49:24 am ---In a secondary network, customers are served by two or more transformers paralleled on the secondary side and supplied by different feeder circuits, possibly from different substations.  The simplest form is the "spot" network, having only one customer on the secondary bus.  In an "area" network, the bus serves multiple customers.

--- End quote ---
We do have substations with multiple transformers and a common busbar, but so far as I am aware it's more common to have the busbar split into sections rather than operate transformers in parallel in normal operation. We commonly have LV feeders where each end connects to two secondary substations, which may or may not be on the same 11 kV feed. Most often these are connected as radial feeders, fed from one end only or fed from both with a break in the middle, but in areas with very high load density (central London for instance) the network is sometimes operated meshed with two or more substations feeding in. This gives rise to very high levels of fault current, and exciting failure modes if HV is lost at one site but not another. It gets interesting when a sizeable area ends up with its 11 kV supply powered via a distribution transformer operating in reverse and a bit of very hot-running LV cable down the street from the next substation.


--- Quote from: AlbertL on October 22, 2020, 05:49:24 am ---A key component of a secondary network is the network protector.  This is a form of circuit breaker inserted between the secondary of each transformer and the bus.  Its purpose is to protect the system in case of a short circuit ("fault") or outage on a feeder by preventing reverse power flow from the secondary bus through the transformer into the primary feeder.     

--- End quote ---

I don't have a huge amount of experience with UK meshed networks (only a few towns have them), but as I understand it in most cases this protection is not provided and reverse power flow is allowed to persist until remote network reconfiguration can be used to restore power to the faulted section or until simple overload protection operates. Withstanding short-term operation like this is part of the design.

NiHaoMike:

--- Quote from: AlbertL on October 22, 2020, 05:49:24 am ---A key component of a secondary network is the network protector.  This is a form of circuit breaker inserted between the secondary of each transformer and the bus.  Its purpose is to protect the system in case of a short circuit ("fault") or outage on a feeder by preventing reverse power flow from the secondary bus through the transformer into the primary feeder.     

--- End quote ---
How does that work when there's solar or other grid tie energy sources on the customer side? Is the reverse power breaker set to trip only if the backflow is greater than the sum of all distributed energy generation in that zone?

jc101:
There is a group who keep an Infrastructure map, like open street maps, but for services which might be of interest to some...

https://openinframap.org

AlbertL:

--- Quote from: richard.cs on October 22, 2020, 09:40:40 am ---
--- Quote from: AlbertL on October 22, 2020, 05:49:24 am ---The most common distribution system, called a "radial" system, consists of a feeder circuit (generally in the 4-35 kV range) originating at a substation and supplying multiple transformers, each of which serves one or more customers.  The key characteristic of this system is that any given customer is supplied by only one feeder circuit.

--- End quote ---
In the UK we have distribution circuits like this at 11 and 33 kV (phase-phase), but ours tend to be a bit different. In most cases the feeder forms a ring from the primary substation, around some number (perhaps 10-15) secondary substations each with 1 or 2 transformers supplying a few hundred customers each, and back to the primary substation. The ring is broken at one point, and is fed from both ends essentially creating two radial circuits. The nice feature though is that the break can be remotely switched to occur at any of the secondary substations, or at two of them, such that any one faulty length of cable can be isolated for repair without loosing the 11 kV feed to any of the secondary substations.
--- End quote ---
My utility (Dominion Virginia Power) has something similar in their overhead systems.  Single or two-circuit high-voltage (230kV I think) transmission lines feed large substations, which step down to 34kV feeders.  These feeders either supply distribution transformers directly or (in older neighborhoods) supply smaller substations which step down to 12.5kV feeders for final distribution.  Where a feeder from one substation is routed near a feeder from another, a "tie switch" is installed so that, if a feeder fails, the entire load can be served by one substation after the other end of the feeder is disconnected.  A lineman told me that, when a feeder has to be opened for maintenance, they sometimes close the tie switch while both feeders are energized, thus paralleling the transformers at the two substations, to avoid a service interruption.         


--- Quote from: richard.cs on October 22, 2020, 09:40:40 am ---
--- Quote from: AlbertL on October 22, 2020, 05:49:24 am ---In a secondary network, customers are served by two or more transformers paralleled on the secondary side and supplied by different feeder circuits, possibly from different substations.  The simplest form is the "spot" network, having only one customer on the secondary bus.  In an "area" network, the bus serves multiple customers.

--- End quote ---
We do have substations with multiple transformers and a common busbar, but so far as I am aware it's more common to have the busbar split into sections rather than operate transformers in parallel in normal operation. We commonly have LV feeders where each end connects to two secondary substations, which may or may not be on the same 11 kV feed. Most often these are connected as radial feeders, fed from one end only or fed from both with a break in the middle, but in areas with very high load density (central London for instance) the network is sometimes operated meshed with two or more substations feeding in. This gives rise to very high levels of fault current, and exciting failure modes if HV is lost at one site but not another. It gets interesting when a sizeable area ends up with its 11 kV supply powered via a distribution transformer operating in reverse and a bit of very hot-running LV cable down the street from the next substation.
--- End quote ---
Yes, that's the problem network protectors are intended to prevent.  I've read about incidents where the only way to disconnect the supply was to send linemen into manholes with hacksaws to cut live LV cables.  BTW, there's a type of fuse called a "cable limiter", which is rated by conductor size rather than amperage, that's made expressly for the protection of LV network cable insulation against thermal damage.   


--- Quote from: richard.cs on October 22, 2020, 09:40:40 am ---
--- Quote from: AlbertL on October 22, 2020, 05:49:24 am ---A key component of a secondary network is the network protector.  This is a form of circuit breaker inserted between the secondary of each transformer and the bus.  Its purpose is to protect the system in case of a short circuit ("fault") or outage on a feeder by preventing reverse power flow from the secondary bus through the transformer into the primary feeder.     

--- End quote ---

I don't have a huge amount of experience with UK meshed networks (only a few towns have them), but as I understand it in most cases this protection is not provided and reverse power flow is allowed to persist until remote network reconfiguration can be used to restore power to the faulted section or until simple overload protection operates. Withstanding short-term operation like this is part of the design.
--- End quote ---
That's an interesting approach - potentially better if there's good (fast) telemetry in place.  My understanding of US practice is that the feeder breakers are not set low enough to operate on secondary faults, so they won't trip a feeder that's supplying a fault through a backfed transformer.

For example, in a simple spot network with Feeder A supplying Transformer A, and Feeder B supplying Transformer B, a fault on Feeder A will operate the Feeder A breaker at the substation, but the fault will still be powered by Feeder B through Transformer B and (backfed) Transformer A, and the current on Feeder B will not be high enough (due to the impedances of the two transformers back-to-back) to trip the Feeder B breaker.  So, without the network protector to disconnect Transformer A from the bus, the fault will continue to burn.     

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