Proposed Design and the Operation of a Disconnecting Device for Small EG Installations Prepared by Hendri Geldenhuys

“Rooftop PV” Minimum Safety Requirements DB POS POC Locked in Grid-Tied-Mode Subject of this set of slides

Cost of an External DD Fully Compliant to VDE 4105 For small systems the cost of fully VDE 4105 compliant external DD is between 6% and 25% of the installation cost. For larger systems this cost is a small fraction of the overall cost.

Purpose The Purpose of this device and the set of slides is to explain the operation of a proposed Automated Disconnecting Device aimed at ensuring that power cannot flow back from a EG once a utility worker has declared this part of the LV network as safe to work on. It relies on the EG to power down. If this does not happen, the safe work procedure will detect the condition and further steps is to be taken to make the installation safe. The sole purpose of this device is to secure that once the network is thought to be dead- that it remains so, regardless of any actions or failure of equipment during this time on the EG’s side. The device comprises of sub-components that are currently regulated in SA and are available on the market and that can meet the “robustness” levels that Eskom would consider adequate for ensuring safety of staff. This design make the Proof of Compliance and Assurance to Minimum Safety requirements easy to achieve. At reasonable cost for small systems.

Disconnecting Device Layout Contactor 1 Contactor 2 Auxiliary Relay The DD comprises of:  Auxiliary Relay: Double pole NO contacts  Contactor 1 and 2: NO contacts to break all phases and Neutral. Single auxiliary NC contact

When 0V is measured on the gird the DD lapses to its OFF State Contactor 1 Contactor 2 Auxiliary Relay 0 V Open

Once in the Off State, power from the EG cannot power up the DD. The EG is isolated from the grid. Contactor 1 Contactor 2 Auxiliary Relay 0 V 230V Open

On State: Auxiliary relay hold itself in via supply Contactor 1 Contactor 2 Auxiliary Relay 230V Closed

On State: Auxiliary relay hold itself in via supply And hold in Contactors 1 and 2 Contactor 1 Contactor 2 Auxiliary Relay 230V Closed

Switch on Transition- Step 1 Contactor 1 Contactor 2 Auxiliary Relay 230V Switch on The relay contact is still open. It is in the process of being closed by the 230V switch on, and close auxiliary contacts 1&2 Only if both contactor 1 and contactor 2 are open, would their auxiliary NC contact be closed. If this is the case power to pull the auxiliary relay in, is provided through this path. Open

Switch on Transition- Step 2A Contactor 1 Contactor 2 Auxiliary Relay 230V Relay contact closes and provide power to hold itself in the on state Open Closed

Switch on Transition- Step 2B Contactor 1 Contactor 2 Auxiliary Relay 230V Relay contact closes and provide power to hold itself in the on state AND it power up the coils of Contactor 1 and 2 to pull the contactors close. Open Closed

Switch on Transition- Step 3 Contactor 1 Contactor 2 Auxiliary Relay 230V Power to the Auxiliary Relay via the contactors are now lost because they have closed, but the AR is held in by its own contact Closed Contactor 1&2 are now closed and power can flow through the primary circuit.

On State: Auxiliary relay hold itself in via supply And hold in Contactors 1 and 2 Contactor 1 Contactor 2 Auxiliary Relay 230V Closed

Contactor 1or2 burned closed: DD Lock Out - cannot switch on. Contactor 1 Contactor 2 Auxiliary Relay 230V Relay cannot be closed because one of the contactor auxiliary contacts are not closed Open 230V

Auxiliary Relay burned closed: Primary protection function remains intact. Contactor 1 Contactor 2 Auxiliary Relay Open 0 V 230V

Auxiliary relay and one Contactor burned closed: LO function lost. Primary protective function remains. Contactor 1 Contactor 2 Auxiliary Relay Relay cannot be closed because one of the contactor auxiliary contacts are not closed Open Burned closed 0 V 230V

Contactor 1 Contactor 2 Auxiliary Relay Burned closed Auxiliary Relay burned closed followed by Contactor 1 and 2 BOTH burned closed: Primary Protection Function is Lost 230V

Failure Outcomes

Same Cause Failure Minimization Fault current: Co-ordinate power system protection not to allow damage to the contactors when faults occur. Lighting : Suitable lighting protection on both sides ( AC and DC side of inverter ) as well as good bonding of PE and grounding conductors and DC earths is required to minimize this risk.

Disconnecting Device for Σ P < 13.8 kVA Contactor 1 Contactor 2 Auxiliary Relay Design application limit: If aligned to typical shared feeder criteria it would be 13.8 kW. Grey area- but it could be up to 30A or 50A?

Design Principles Continue…. Rating of Contactors not less than 140% of the full load rating of the EG maximum output SANS/ IEC compliant (Which spec) Contactors rated for operations at this level. 230/ 400V rated ……. All devices. Should withstand 1.5 Vn ?? (Especially the coils of the relay and contactors.) Relay must have current rating of 300% of maximum operating current for operations?? Standing losses of the DD??? Impulse withstand of the devices should at least be 6 kV 1.2/50us Insulation withstand should be 2 kVrms 60 sec. at least. Housing IP ?? Separate DB for all of this and the other stuff for the EG etc… Terminal connections should be ? Interconnecting wires should be ? SANS ??? Interconnecting wires for auxiliary circuits should be?? Pull in and drop out voltage for contactors and relay? Response to DC?? External closing ability………. Disallowed such as normal contactor push in with screwdriver etc..?? General requirements such as 50 Hz, -5 degrees to + 55 degrees Celsius, altitude 0m – 2000 m etc. What else???