Synchronization at the Receiving End or Substation From a Generating End

All feeders need to be synchronized...the phases must match to be joined...

In general, one "backs in power," which produces 33kV on the common bus from the national grid, first. (Transformers work in both directions.) Of course, this is synchronization into a "dead bus," which carries no danger of huge currents. Then the individual units will be synchronized into this lower voltage side of the transformers. By then matching voltage and phase, the current flow across that breaker is minimal.

Cannot see your system, distances, type of line - so have to speculate - assume you mean by "Synchrocheck" an automatic synchroniser and check-sync relay combination or combined ASU/check sync unit.

1. Subject 2 gen hydro station "X" has local 6.6/33 transfo.
2. Line to 33 kV bus from "X" is open wire, vulnerable to lightning. Strike puts line out. Strike in well designed system does no damage if line is quickly de-energised. Since major lines have automatic single phase open-wait-close recovery which is fast enough to avoid loss-of-sync; one has had a [less probable] 2 phase fault disconnecting whole line.
3. The 2 generators should be able to load-reject and stay on-line when 33 kV breaker opens. This keeps the whole station powering its own ancillaries and ready to reconnect in seconds.
4. Without 33kV sync equipment, the 2 hydro-gen would have to be disconnected at 6kV; the 33 kV line & transfo re-energised by rest of system then 2 local gen re-synchronised. This would delay re-connection & hazard the system with surge at 33kV when it has just lost 2 gensets. Note that without auxiliary power from 6 kV the two local hydro-gen would trip & be dead until manual reset of trip.
5. A vital part of power Grids is a "reboot" method if whole system goes down. This needs gensets which can "black start" (a wording coming from no- lights: "black out"). Hydro sets are valuable for "black start" because they have no fuel requirements & potentially can be manually got to electrical life with a water driven lubricating oil pump or a small auxiliary hydro-gen.
6. Ref. 5. Hydro sets can run-up or load faster than other types. One minute from dry to full-load or 10 sec from spinning reserve to full load is possible. Open Cycle gas turbines manage 2 minutes stationary to full-load for nuclear station standby.
7. Losing the revenue from output of a 50 MW station for 15 minutes would pay for an ASU, without considering all the "post-mortem" trouble

Synchronising only needs to be done at the generators to connect them by the generator breakers to the national grid. The generators are the only devices which can have their speed varied to bring their frequency and voltage into synchronisation with the grid frequency.

The incoming generators are always synchronised to the grid at the power station as a unit is brought online.

I have met a situation where the sync is done only at the generator. The substation cannot connect on live feeder.

The case is for a 20kV/110kV substation which receives power from 4 photovoltaic stations (4MW each) and sends it to the grid. The small generators (inverters in this case) are not allowed to run without the grid (the generated power is higher than the 20kV grid consumption) so as long as the grid (or the 20/110kV transformer) is down the generation does not happen.

With the grid up and transformer online, the inverters have to sync themselves to the grid. These are not for cold startup so it is a particular case for non-island generation.

If the substation interconnects two or more networks, the syncrocheck is mandatory.