Part 2 - Protection Relays

This post is the second in a series related to protection relays, and builds on the information previously presented. If you have not read the previous post, Part 1 may be found here.

Protection relay programming is typically performed by protection engineers, based on the specific protection element settings defined through a time current curve (TCC) study. I will not cover the specifics of TCC here; however, there is an excellent article written by David Paul P.E. that describes these studies in great detail.

 For project managers and developers, the process of establishing protection relay settings can be confusing and often overlooked as part of the project planning process. This post is intended to clarify the process and persons involved to support better planning.

Any grid-connected solar or storage development project will include an interconnection agreement (IA). The IA will define a set of protection settings for the inverters (not the protection relay) to provide safe operation per the utility's operating requirements.

In addition to the inverter's protection settings, the utility will program its own protection relay, whether located at the point of interconnection (POI) or somewhere along the distribution feeder the new facility is tied into.

The design of protection settings is to minimize the impact of any fault as close to the fault as possible and support the reliability of the grid for as many customers as possible. With this in mind, any inverter-level electrical fault should trigger the inverter's protection settings to trip first, leaving the distribution feeder operating normally. In the event the fault is somewhere between the inverter and the utility's protection relay, the distribution feeder could go offline. This is generally unacceptable; hence, the project is usually required to have a dedicated protection relay that sits between the inverters and the utility protection relay to provide another point of protection to protect the distribution feeder.

The customer's protection relay settings need to be calculated by comparing the inverter protection settings and the utility protection relay settings, to ensure the new protection relay operates in the proper sequence. The TCC, often referred to as a "coordination study," is performed by a professional engineer. The result of the coordination study will define the protection elements and settings that will need to be programmed into the protection relay.

The protection element settings programming may be performed by anyone with knowledge of the protection relay make/model software but is most often performed by a protection engineer.

For Aderis customers who use ClearSky or ClearSky PLUS equipment, the protection relay and customer's MV VFI are located inside the ClearSky pad-mount gear. Once the protection settings have been calculated, Aderis will program the protection relay to the settings provided.

The protection relay's program is often referred to as an RDB file. RDB, or relay database, is a manufacturer-specific file format supported by Schweitzer Engineering Laboratories (SEL) products. Other relay manufacturers (GE, Eaton, etc.) may use other file formats. The RDB file is typically required by the utility for review prior to scheduling a witness test or granting commercial operation. By reviewing the settings and logic programmed in the RDB file, the utility can ensure the TCC and calculated settings operate in a manner that supports grid reliability and customer safety.