Functions of a substation with LV metering: Difference between revisions

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== Connection to the MV network ==
<br>


A consumer substation with LV metering is an electrical installation connected to a utility supply system at a nominal voltage of 1 kV - 35 kV, and includes a single MV/LV transformer generally not exceeding 1,250 kVA.  
Connection to the MV network can be made:
* By a single service cable or overhead line,
* By dual parallel feeders via two mechanically interlocked load-break switches
* Via a ring main unit including two load-break switches.


The different functions of the substation are:
== MV/LV Transformers ==


'''The substation<br>'''All component parts of the substation are located in one room, either in an existing building, or in the form of a prefabricated housing exterior to the building.<br>
Since the ban of PCB in most of the countries, the remaining available insulation technologies for the transformers are:
* Oil-immersed for transformer preferably located outside premises
* Dry-type, cast-resin preferred for transformers located inside premises such as buildings receiving the public.


'''Connection to the MV network<br>'''Connection at MV can be:
Local regulations define where the use of cast resin transformers is mandatory.


*Either by a single service cable or overhead line, or
== Metering ==
*Via two mechanically interlocked load-break switches with two service cables from duplicate supply feeders, or
*Via two load-break switches of a ring-main unit<br>


'''The transformer'''<br>Since the use of PCB<sup>(1)</sup> - filled transformers is prohibited in most countries, the&nbsp;preferred available technologies are:
Most of the LV metering and billing principles take into account the MV/ LV transformer losses.


*Oil-immersed transformers for substations located outside premises
The characteristics and the location of the VT’s and CT’s dedicated to the metering must comply with the utility’s requirements.
*Dry-type, vacuum-cast-resin transformers for locations inside premises, e.g. multistoreyed buildings, buildings receiving the public, and so on...


{| style="width: 65%; height: 15px" cellspacing="1" cellpadding="1" border="1"
The metering current transformers are generally installed in the LV terminal box of the power transformer, alternatively they can be installed in a dedicated compartment in the main LV switchboard.
|-
| valign="top" align="left" | (1) Polychlorinated biphenyl
|}


'''Metering'''<br>Metering at low voltage allows the use of small metering transformers at modest cost.<br>Most tariff structures take account of MV/LV transformer losses.<br>
The compartments housing the metering VT’s and CT’s are generally sealed by the utility.


'''LV installation circuits'''<br>A low-voltage circuit-breaker, suitable for isolation duty and locking off facilities, to:
The meters are mounted on a dedicated panel accessible by the utility at any time.


*Supply a distribution board
== Local emergency generators ==
*Protect the transformer against overloading and the downstream circuits against short-circuit faults.


'''Simplified electrical network diagram<br>'''The diagram '''(Figure B10)''' shows:
Emergency standby generators are intended to maintain the supply to the essential loads, in the event of failure of the utility power supply.


*Methods for connecting to the network (4 options):
A substation with LV metering may include one single emergency generator connected at low voltage level on the main LV distribution switchboard.


&nbsp; - Spur network or single-line service<br>&nbsp; - Provisional network (can be transformed into a loop)<br>&nbsp; - Parallel feeders service<br>&nbsp; - Loop or ring-main service
The generator may be sized either for the supply of the whole installation or for a part only. In this case a load shedding system must be associated to the generator. The loads requiring an emergency supply may also be grouped on a dedicated LV busbar (see {{FigRef|B31}}).


*MV protection and MV/LV transformation methods
An '''U'''ninterruptible '''P'''ower '''S'''upply (UPS) may be added when required at LV level to avoid the interruption of the supply during the starting of the emergency generator.
*LV metering and LV general isolation methods
*LV protection and distribution methods
*Zones accessible to different parties&nbsp;


----
{{FigImage|DB422033_EN|svg|B31|Emergency generator at LV Level}}


<br>[[Image:FigB10.jpg|left]] <br><br>
== Capacitors ==


<br>
Capacitors are intended to maintain the power factor of the installation at the contractual value specified by the utility. The capacitor banks are connected on the main LV switchboard and can be fixed or adjustable by means of steps controlled by a regulator.


<br>
See chapter [[Power Factor Correction]]


<br>
== LV main switchboard ==


<br><br>
The MV/LV transformer is connected to a main LV distribution switchboard equipped with a LV general circuit breaker ensuring:
* The general protection of the LV installation
* The general isolation of the LV circuits, according to the rules of protection of the persons working in an electrical installations
* The protection of the MV/LV transformer against overload


<br>
To comply with the interlocking requirements defined in [[Interlocks and conditioned operations]], the circuit breaker must be equipped with padlocking facilities for locking it in open position.


<br>
== Simplified electrical network diagram ==


<br>
The diagram {{FigRef|B32}} shows:
* The different methods to connect a MV/LV substation to the utility supply:
**Single-line service
**Single line service with provision for future connection to a ring or to dual parallel feeders
**Dual parallel feeders
**Loop or ring-main service
* The protection of the MV/LV transformer, either by a load break switch or by a circuit breaker
* The LV metering
* The main LV switchboard.


<br><br>
{{FigImage|DB422034_EN|svg|B32|Consumer substation with LV metering}}
 
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<br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br>'''''Fig. B10:&nbsp;'''''<i>Consumer substation with LV metering</i> <br>
 
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Latest revision as of 09:49, 22 June 2022

Connection to the MV network

Connection to the MV network can be made:

  • By a single service cable or overhead line,
  • By dual parallel feeders via two mechanically interlocked load-break switches
  • Via a ring main unit including two load-break switches.

MV/LV Transformers

Since the ban of PCB in most of the countries, the remaining available insulation technologies for the transformers are:

  • Oil-immersed for transformer preferably located outside premises
  • Dry-type, cast-resin preferred for transformers located inside premises such as buildings receiving the public.

Local regulations define where the use of cast resin transformers is mandatory.

Metering

Most of the LV metering and billing principles take into account the MV/ LV transformer losses.

The characteristics and the location of the VT’s and CT’s dedicated to the metering must comply with the utility’s requirements.

The metering current transformers are generally installed in the LV terminal box of the power transformer, alternatively they can be installed in a dedicated compartment in the main LV switchboard.

The compartments housing the metering VT’s and CT’s are generally sealed by the utility.

The meters are mounted on a dedicated panel accessible by the utility at any time.

Local emergency generators

Emergency standby generators are intended to maintain the supply to the essential loads, in the event of failure of the utility power supply.

A substation with LV metering may include one single emergency generator connected at low voltage level on the main LV distribution switchboard.

The generator may be sized either for the supply of the whole installation or for a part only. In this case a load shedding system must be associated to the generator. The loads requiring an emergency supply may also be grouped on a dedicated LV busbar (see Fig. B31).

An Uninterruptible Power Supply (UPS) may be added when required at LV level to avoid the interruption of the supply during the starting of the emergency generator.

Fig. B31 – Emergency generator at LV Level

Capacitors

Capacitors are intended to maintain the power factor of the installation at the contractual value specified by the utility. The capacitor banks are connected on the main LV switchboard and can be fixed or adjustable by means of steps controlled by a regulator.

See chapter Power Factor Correction

LV main switchboard

The MV/LV transformer is connected to a main LV distribution switchboard equipped with a LV general circuit breaker ensuring:

  • The general protection of the LV installation
  • The general isolation of the LV circuits, according to the rules of protection of the persons working in an electrical installations
  • The protection of the MV/LV transformer against overload

To comply with the interlocking requirements defined in Interlocks and conditioned operations, the circuit breaker must be equipped with padlocking facilities for locking it in open position.

Simplified electrical network diagram

The diagram Fig. B32 shows:

  • The different methods to connect a MV/LV substation to the utility supply:
    • Single-line service
    • Single line service with provision for future connection to a ring or to dual parallel feeders
    • Dual parallel feeders
    • Loop or ring-main service
  • The protection of the MV/LV transformer, either by a load break switch or by a circuit breaker
  • The LV metering
  • The main LV switchboard.
Fig. B32 – Consumer substation with LV metering
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