how_to:make_inverter_s_grid_connectivity_toggle_automatically [Sinexcel ESS knowledge base]

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====== How to make inverter's grid connectivity toggle automatically ======

Required devices:
  * Sinexcel PWS2-30K PCS
  * Remote controllable switch with auxiliary trip mechanism (it shall also have ability to remote close)
  * A digital protective relay with 
    * synchronism check functionality
  * If the protective relay doesn't have a synchro check function, then another sync check relay is required
  * Instead of these relays, the designer can use their own sensors to measure the phase angles, amplitudes and frequencies of the grid voltage and PCS voltage.

Available remote controllable switch

https://www.socomec.com/files/live/sites/systemsite/files/SCP/2_commutateurs/atys_ul/542564c.pdf

it is a transfer switch but it's perfect for this application since it's UL listed and in this case you need to use it as a SPST switch.

Protective relay is a relay device designed to trip a circuit breaker when a fault is detected. 
Abnormal operating conditions are as over-current, over-voltage, reverse power flow, over-frequency, and under-frequency.
In modern times the digital relay is widely used, usually these relays will have the ability to trip the breaker and output a signal, and can receive the command to close the breaker if the breaker has a motor drive. 

Available protective relay

https://search-ext.abb.com/library/Download.aspx?DocumentID=2CDC112271D0201&LanguageCode=en&DocumentPartId=&Action=Launch


A synchronism checking relay provides a contact closure when the frequency and phase of two sources are similar to within some tolerance margin. A "synch check" relay is often applied where two power systems are interconnected, such as at a switchyard connecting two power grids, or at a generator circuit breaker to ensure the generator is synchronized to the system before connecting it.

Available sync relays

https://cdn.selinc.com/assets/Literature/Product%20Literature/Flyers/547_PF00036.pdf?v=20181107-140149

https://www.pc-s.com/pdf/multitek-synchronising-check-relay-m200.pdf

https://thiim.com/datasheets/synd.pdf

https://cromptoncanada.com/wp-content/uploads/2014/11/Protector-256-PLLU.pdf

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The editor did **NOT** test these devices above. 

Consult these device manufacturers if they are suitable for you application before design! 
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===== How to work =====
==== Diagram ====

{{:how_to:switching_diagram.png|}}

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Though the transformer in this diagram is not always necessary.

PWS2-30K-NA has NO neutral current output ability in grid-forming mode. 

In case of 277V loads, typically industrial illumination, an isolation transformer is required for providing neutral current caused by unbalanced single phase loads.
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==== EMS logic: Grid outage ====
  - The grid goes into abnormal
  - The relay will trip the circuit breaker and send a signal to the EMS
  - The PCS will activate anti-islanding mechanism and trip\\ The load will lose power
  - The EMS send an off-grid command to the PCS
  - The PCS will clear the fault and operate in grid forming mode.  

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After firmware version D142, the grid-tied to off-grid toggling is optimized, the last two steps are combined.

When the grid outage occurs, just turn off the disconnect. And set “off-grid” register, the PCS will automatically clear the fault flags and start in grid forming mode. No need to send **clear fault** and **start** command, this will make the starting time 2~3 seconds.
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==== EMS logic: Tie back to grid====
  - The grid is back to service
  - The sync check relay compares phase differ between the two sides of the breaker/switch.
  - Once the PCS and the grid phase differ is within tolerance, such as in 1°, the sync check relay closes the breaker.
  - The PCS is operating in grid-forming mode but physically grid tied, the phase differ will still trend to vary.
  - The EMS shall command PCS to shut down within 300ms in case of phase differ trend to grow,\\ the loads are still powered.
  - The EMS commands PCS to toggle to grid-tied mode.
  - The EMS commands PCS to start. Everything is back to the status before outage.