Relay Loadability Studies

Reliability Standard PRC-023, enacted by FERC in 2010, requires transmission owners, generator owners, and distribution providers to set their phase relays in such a way that the relays protect the electric network from short circuits, but do not limit transmission loadability or interfere with system operators’ ability to maintain system reliability.

The PRC-023 standard applies to phase relays that protect transmission lines and transformers 200 kV and above, plus some lines and transformer between 100 kV and 200 kV that the planners deemed critical to the reliability of the electrical network. NERC enforces these requirements though an auditing program. More information on PRC-023 can be found in NERC’s web site: www.nerc.com

Several years ago we implemented a Check Relay Loadability command in OneLiner to check the loadability of overcurrent and distance phase relays. With a single Check Relay Loadability command, you can direct the program to check all the phase relays in the system, or just the relays in a specific location, area, or zone that are within a certain nominal-kV range. Specifically, the Check Relay Loadability command performs the following tasks for each phase relay that is on a transmission line that has a non-zero current rating:

1. Computes a load current as a multiple of the line’s current rating. We call this the “calculated load current”
2. Simulate the relay’s response to the calculated load current, with the voltage magnitude and the power-factor angle at the prescribed values. The relay passes the test if it does not trip. Otherwise it is flagged.
3. Output the above results to a text report or a CSV-formatted report. The CSV report includes the key relay settings for auditing purposes.

We have improved this command since then to make the checking process more comprehensive. For example, in v11.9, instead of checking just zone 3 of distance relays, OneLiner checked all the tripping zones, in the forward and reverse direction, to see if they trip on load. The fastest zone that trips, if any, is indicated in the report.

In the latest version of OneLiner (v11.10), the program calculates and outputs an additional “load current threshold”, regardless of the relay location and whether a current rating is available. The load current threshold is the minimum load current that will cause the relay to trip. For a relay that passed the test in step (2), the difference between this threshold and the calculated load current gives you an idea how close the relay is to the tripping point.

In addition to the reports of the Check Relay Loadability command, you have the option of using graphics to document the loadability checks. In figure 1, the calculated load current in primary amps is shown as a vertical line in a plot of the overcurrent relay curve. This relay clearly passed the loadability test because its curve begins to the right of the calculated load current. A transformer damage curve in this plot, appearing to the right of the relay, shows that the relay is still sensitive enough to protect the transformer. You can store such plots in a Curve Collection files for later retrieval, in case of an audit.

Figure 1

Figure 2 shows a distance relay with a load region plotted as a wedge-shaped area to the right of the relay characteristics. This picture was generated by the Load Region command in the Distance Relay Window. This phase distance relay passed the loadability test because the load region is outside of all the protective zones. For the first time, in OneLiner v11.10, you can store this kind of plot within the Curve Collection file, also, for later retrieval.

The Check Relay Loadability reports and the Curve Collection files can all be used as evidence that your organization has checked the loadability of your phase relays according to the requirements of PRC-023.

Figure 2

OneLiner Relay Data Import

An “Import” button was added to the relay-group dialog box and to the relay dialog boxes in v11.7. These buttons let you import relay settings into OneLiner from a number of other sources, including relay manufacturers’ setting files:

• SEL-5010 database: Setting data in an SEL-5010 MDB database.
• SEL-5010 export database: Setting data in an MDB file exported by the SEL-5010 program.
• SEL-5030 acSELerator® export files: Setting data in text files exported by SEL-5030. These text files are in a single folder.
• GE UR and 469 relay setting export files: Setting data in text files with .CSV or .URS extension. The support for .URS files is new in v11.10.
• ASPEN Database text data file: Setting data exported from ASPEN Relay Database with .txt extension.
• ASPEN OneLiner relay data: Setting data exported from OneLiner with .rat extension.
• ASPEN Relay Database.
• ENOSERV® PowerBase. This is new in v11.10. Please call or write us if you are interested.

An improved user interface in v11.10 lets you import multiple relay elements into OneLiner at the same time. For example, you can create a phase distance relay, a phase overcurrent relay and a ground distance relay from a SEL-5030 folder, with a single push of the Import button. See Figure 3.

Figure 3

The relay-import feature works seamlessly with the manufacturer-specific distance relay models for SEL and GE relays. When you import the setting data for a SEL-421 phase distance relay from a SEL-5030 file, for example, OneLiner will create a new distance-relay object using the manufacturer-specific SEL-421P__ model which simulates SEL’s unique way of implementing positive-sequence polarized mho characteristic, compensator-distance mho characteristic, quadrilateral characteristic, negative- and zero-sequence voltage directional logic, and load encroachment settings.