USING RECLOSERS ON MV LINES TO CONNECT INDEPENDENT POWER PRODUCERS / SMALL SCALE EMBEDDED GENERATORS (SSEG)

Article by Jeremy Wood (RWW Engineering) and Noja Power.

Introduction

Connecting renewable generation to the distribution grid has its own challenges. Most electricity distribution networks were originally designed to take electricity from generators via the transmission grid (the big towers), and distribute it to all the lower voltage customers, where it would be stepped down to residential and commercial voltages for consumption.

Today, the electricity industry is transitioning at a very high rate to using large volumes of renewable and distributed generation. Mostly small scale PV plants up to 10MW are being installed in businesses and farm applications.

The challenge with renewables is they behave differently to centralized power plants, and the projects are commonly connected at the distribution network, instead of the transmission network. Accordingly, these connections need special protection, control and metering. NOJA Power OSM Reclosers are commonly used as the grid connection point for renewable energy such as solar & wind, as the product includes all of the technical control requirements for this distributed generation connection.

The reason NOJA Power OSM Reclosers are used for this application is that the standard product includes Synch check, Live load Blocking, ROCOF & VVS protection. Therefore, most engineering businesses that are building these projects can be compliant with utility requirements to connect the renewable energy to the distribution grid. Also, with all connection requirements in a single switchgear device, this saves a fortune in site engineering works.

ESKOM Requirements

The Eskom Interconnection Standard (document 240-111703934) defines the requirements for using Reclosers for SSEG applications.

The Reclosers must have the following:

• Synchronisation check (ANSI 25)
• Live Close blocking
• 5 x Over Voltage requirement for the Point of Connection (POC) breaker
• An I/O module for inter-tripping between the Point of Utility Connection (PUC) and the POC Recloser.

Regarding the above 2.5 x voltage requirements the out of phase voltage on the generation side will result in high voltage discrepancies between the line and generated voltages when out of phase, therefore the following applies:

• 11kV system uses the 22kV Recloser (OSM27)
• 22kV system uses the 33kV Recloser (OSM38)
• 33kV system uses the 33kV Recloser (OSM38-16kA) – out of phase switching type tests have been conducted.

ANSI 25 Synchronism description

The synchronisation functionality can be used for the following applications:

• connection of incoming generators to the grid
• re-establishment of a connection between two interconnected parts of the network
• re-establishment of a connection between islanded systems.

Synchronizing is based on voltage magnitude, phase angle and frequency.

The ABC and RST sides of the recloser can be designated as either the Bus side or the line side of the recloser.

This allows the OSM to work with synchronism functionality, regardless of the orientation of the OSM.

First of all, the general synchronism settings define whether it is safe to close when both the bus and the line are considered dead (below the LSD level).

The live/dead conditions are also defined. For example, if the bus side is live and the line side is dead, do you want to close manually and/or autoreclose in that case. This is known as Dead Line Live Bus (DLLB) conditions.

Similar options are available for LLDB conditions.

• Sync-Check “supervises” the closing of a device and only allows a close when synchronising conditions are met.
• Auto-Synchronisation can be initiated via the panel or CMS and it will wait for the auto-synchroniser conditions to be met prior to requesting a close of the device.

Should the Eskom supply fail, the Recloser will prevent closing until the supply is restored and then will close once the synch check requirements are fulfilled.

For further details contact RWW Engineering www.rww.co.za or Noja Power https://www.nojapower.com.au/