Specific protection of prosumer electrical installations

From Electrical Installation Guide


When local sources for self-consumption are included in an electrical installation, some specific points need to be taken into account regarding the protection devices, in particular the ones described below.

Protection against electric shock in islandable PEI

As seen before, protective measures against electric shock inside a grid connected installation rely on the connection to the distribution network. When an installation is intended to be used disconnected from the grid (islandable PEI) additional measure shall be taken to ensure safe operation in this mode.

  • Selection of a type of system earthing for island mode
  • Adaptation and checking of effectiveness of automatic disconnection of the supply protective measure in particular with low level of earth fault current compare to grid connected operation.

IEC 60364-8-82 2022 introduces the possibility to introduce a switching device in the link between Neutral and earthing arrangement: Switching device for system referencing conductor.

Note: The conductor connecting a LV power system to earth (usually the Neutral in AC 3 Phase system) according to type of system earthing was not specifically described in IEC 60364-1 and by default considered as a PE or PEN. This conductor is now defined as "System Referencing Conductor (SRC)" and a switching device may be inserted in it which is not possible with a PE.

IEC 60364-8-82 table 1 describes possible type of system earthing for non-galvanically isolated PEI.

Fig. K59 – Combinations of possible types of system earthing in connected mode and island mode for PEIs and associated requirements for system referencing conductor switching device (IEC 60364-8-82 Ed 2022, Table 1)
AC type of system earthing in grid connected mode AC type of system earthing in island mode Island mode possible Y/N Need for system referencing conductor switching device
TT TT Y Y
TN-S Y Y
IT Y N - but need to start insulation monitoring or switch on artificial neutral point
TN-S TT Y Y
TN-S Y Y - except if neutral to earth connection on load side
IT Y N - but need to start insulation monitoring or switch on artificial neutral point
TN-C-S TT N
TN-S Y Y
TN-C-S Y N - but local earthing electrode needed
IT N
TN-C TT N
TN N
IT N
IT TT Y Y
TN-S Y Y
IT Y N
  • This table deals with AC 3 lines + neutral system.

In the paragraphs below we provide some examples and guidance on different ways to implement islandable PEI.

Protection by automatic disconnection of the supply in islandable PEI without galvanic separation from the Distribution Network

The simplest way to create a local neutral to earth link for prosumer supplied by LV utilities in TNS or TT is to add a 3P+N (or P+N for single phase installation) SDFI combined with a single pole switching device for system referencing conductor, as shown in the figure below:

Fig. K60 – Example of PEI TT or TN grid connected, TN-S islanded with local grid forming source 3P+N (capable of supplying unbalance loads)
DB431210.svg

[a] PV local source
[b] Energy storage or other grid forming source 3P+N
[c] Distribution network in outage (or intentional islanding)
[d] Loss-of-mains protection
[e] Loads supplied in island mode
[2a+2b] 4P Switching Device For Islanding (SDFI) combined with 1P switching device for system referencing conductor (SRCSD)

In Grid Connected mode:
[1] Main OCPD: Closed
[2a] SDFI: Closed
[2b] SRCSD: Open

In Islanded mode:
[1] Main OCPD: Closed
[2a] SDFI: Open
[2b] SRCSD: Closed

SDFI [2a] shall switch lines and neutral (4P)
SRCSD [2b] switches the referencing conductor only (1P)

In some situation it may be relevant to use a separate LV/LV transformer together with a 3 Phase source without neutral. This LV/LV transformer is used to create a local neutral to earth and handle unbalanced loads.

Fig. K61 – Example of PEI TT or TN grid connected, TN-S islanded with local grid forming source 3P and LV/LV transformer
DB431211.svg

[a] PV local source
[b] Energy storage or other grid forming source 3P
[c] Distribution network in outage (or intentional islanding)
[d] Loss-of-mains protection
[e] Loads supplied in island mode
[f] Artificial neutral
[2a+2b] 4P Switching Device For Islanding (SDFI) combined with 4P switching device for system referencing conductor (SRCSD)

In Grid Connected mode:
[1] Main OCPD: Closed
[2a] SDFI: Closed
[2b] SRCSD: Open
[3a] ESS LV/LV Transformer: Open
[3b] Bypass LV/LV Tr: Closed

In Islanded mode:
[1] Main OCPD: Closed
[2a] SDFI: Open
[2b] SRCSD: Closed
[3a] ESS LV/LV Transformer: Closed
[3b] Bypass LV/LV Tr: Open

SDFI [2a] and SRCSD [2b] shall switch lines and neutral (4P)
[3a] and [3b] need to swicth only 3 lines (no Neutral distributed at this location)

Advantages :

  • Permanent neutral to earth connection at LV/LV transformer level
  • Standard 4P/4P and 3P/3P transfert switching equipment
  • Transfoless ESS.
  • Possibility to have one transformer for several ESS
  • No neutral interconnection between local sources and distribution network when grid connected. Some national regulation forbid 3P+N source to be connected to the LV Distribution Network, see for example NFC 15-400.

This architecture can even be optimized by the use of an artificial Neutral point in place of LV/LV transformer:

Fig. K62 – Example of PEI TT or TN grid connected, IT islanded with local grid forming source 3P and artificial neutral point
DB431212.svg

[a] PV local source
[b] Energy storage or other grid forming source 3P
[c] Distribution network in outage (or intentional islanding)
[d] Loss-of-mains protection
[e] Loads supplied in island mode
[f] Artificial neutral
[2a+2b] 4P Switching Device For Islanding (SDFI) combined with 4P switching device for system referencing conductor (SRCSD)

In Grid Connected mode:
[1] Main OCPD: Closed
[2a] SDFI: Closed
[2b] SRCSD: Open

In Islanded mode:
[1] Main OCPD: Closed
[2a] SDFI: Open
[2b] SRCSD: Closed

SDFI [2a] and SRCSD [2b] shall switch lines and neutral (4P)

Advantages:

  • Permanent neutral to earth connection at artificial neutral point level
  • Standard 4P/4P transfert switching equipment
  • Pure 3 phase without neutral ESS
  • Earth fault current not dependent on the number of connected local sources
  • No neutral interconnection between local sources and distribution network when grid connected. Some national regulation forbid 3P+N source to be connected to the LV Distribution networkk, see for example NFC 15-400

The artificial neutral point will ensure supply of unbalanced load. It shall be sized accordingly.

Such a system according to IEC 60364-4-41 is an "IT" system but protective measure against electric shock can still be disconnection at the first fault thanks to RCD protection.

Protection by automatic disconnection of the supply in islandable PEI with galvanic separation from the Distribution Network

When Prosumer installation is connected to the Distribution Network through an HV/LV transformer the transformer neutral point may be earthed as in a classical installation. The solution described in the previous paragraph can be applied, or a single permanent Neutral to earth connection can be done at the switchboard level as follows:

Fig. K63 – Example of PEI TN grid connected, TN islanded with local grid forming source 3P+N (capable of supplying unbalance loads)
DB431213.svg

[a] PV local source
[b] Energy storage or other grid forming source 3P+N
[c] Distribution network in outage (or intentional islanding)
[d] Loss-of-mains protection
[e] Loads supplied in island mode
[2] 4P Switching Device For Islanding (SDFI)
[4] Permanent Neutral to Earth connection

In Grid Connected mode:
[1] Main Overcurrent Protection Device (OCPD): Closed
[2] SDFI: Closed

In Islanded mode:
[1] Main OCPD: Closed
[2] SDFI: Open
If intentional islanded mode, with MV/LV transformer energized, is expected to last for significant periods of time, an IMD can be switched-on on the transformer side [3]

SDFI [2] shall switch lines and neutral (4P)

Advantages:

  • Simplicity
  • Single point of connection of the system to earth clearly identified and easy to check
  • No need for switching device for system referencing conductors
  • No neutral interconnection between local sources and distribution network when grid connected. Some national regulation forbid 3P+N source to be connected to the LV Distribution networkk, see for example NFC 15-400

Drawback:

  • In islanded mode the secondary winding of the MV/LV transformer is in "IT" system. If such an operating mode with the MV/LV transformer energized for a significant period of time is expected, an IMD can be switched-ON on the transformer side.

Location of overcurrent protection for the photovoltaic system

To clear electrical faults on the photovoltaic installation, an overcurrent protection device must be located at the point where the photovoltaic system is connected to the building’s electrical installation. It is essential to include the overcurrent protection in this exact location, to avoid tripping the main circuit breaker, in case of fault in the photovoltaic feeder.

Fig. K64 – An overcurrent protection device must be located at the point where the photovoltaic system is connected to the building’s electrical installation

To ensure that the electrical installation works properly, this overcurrent protection should be coordinated with the main circuit-breaker to guarantee selectivity.

Fig. K65 – The overcurrent protection of the PV Switchboard should be coordinated with the main circuit-breaker

This selectivity may not be so easy to achieve, in particular when the photovoltaic production capacity is equivalent to or higher than the building installed power. In this case, the circuit breaker at the photovoltaic incomer has the same rating or higher than the main circuit breaker. Thus, to ensure selectivity between the two breakers, possible options include:

  • oversize the main circuit breaker
  • split the photovoltaic system into smaller systems, or
  • consider time selectivity, for applications higher than 630 Amps

Isolation and switching of the photovoltaic power supply system

As described in the previous paragraph, an AC disconnection device must be located at the local sources switchboard, to isolate the photovoltaic system from the rest of the electrical installation during maintenance. Moreover, an isolation function must be provided for the photovoltaic inverters, by disconnection means, on both AC and DC sides.

Fig. K66 – Disconnection devices must be provided for the photovoltaic inverters on both the DC and the AC side, as well as for the PV Switchboard

Location of overcurrent protection for Energy storage system

The same rules apply to Energy Storage System (ESS) as for Solar Inverter.

Additionally, an overcurrent protective device is required between batteries and inverter.

Isolation and switching for Energy storage system

The same rules apply to Energy Storage System (ESS) as for Solar Inverter.

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