Protection Coordination Study

What it is and when you need it

A protection coordination study defines how the protective relays, reclosers and fuses of an installation must respond so that, when a fault occurs, only the nearest breaker opens and the rest of the system stays in service. To get there, we calculate the short-circuit current at every bus, set the protection functions and check on time-current curves that each device backs up the next one without operating ahead of it.

We do it for medium, high and extra-high voltage (EHV) substations and for industrial plants. It is needed above all for:

New projects, as part of detailed design and before commissioning.
Grid interconnection: the utility asks for the settings table of the protection at the point of interconnection.
Expansions or topology changes: a new transformer, another bay, solar generation or large motors.
Non-selective trips, when an upstream breaker opens that should not have.
A change in the short-circuit level reported by the utility.

The study is part of our engineering design and studies service.

What is included

Data collection and system model: utility equivalent source, transformers, lines, cables, generators, motors and loads.
Short-circuit study per IEC 60909 (or IEEE 551 practices for US-based projects): three-phase, phase-to-phase, phase-to-phase-to-ground and single-line-to-ground faults, with maximum and minimum currents at every bus.
Equipment verification: interrupting and making capacity of breakers against the calculated short-circuit current.
Protection function settings: phase and ground overcurrent (50/51, 50N/51N), directional (67, 67N), distance (21) and differential (87T, 87B, 87L), using IEEE C37.2 device numbers, depending on what your installation has.
Time-current coordination (TCC) plots for each protection path, with transformer and cable damage curves.
Selectivity analysis and a list of the cases that do not coordinate, with their cause and proposed solution.
Settings tables per relay, ready for the utility and for loading into the device.
Review of existing settings, if the installation is already in operation.

How we work

Planning. We define with you the scope, the operating scenarios (for example, with and without generation, or with one transformer out) and the utility’s requirements.
Data gathering. We collect nameplate data, CT/VT ratios, the rated system frequency (50 Hz or 60 Hz) and the relay models. When information is missing, your team can collect it following our checklist, or our team can survey it on site in Colombia (elsewhere on request).
Model and short circuit. We build the model in simulation tools such as DIgSILENT PowerFactory or ETAP and calculate the short circuit with the IEC 60909 method. Maximum currents are used to verify equipment and set instantaneous elements; minimum currents, to confirm that the protection detects the weakest fault.
Settings and coordination. We calculate pickups, curves, time dials and delays, and check them on TCC plots from the load up to the point of interconnection. We also check CT saturation under fault current.
Review with you. We explain the cases that do not coordinate and the trade-offs, for example between clearing speed and selectivity.
Report and settings tables. We deliver the study report and the per-relay tables. If you want, we then load the settings remotely through our remote relay configuration service.

Standards and criteria

Short circuit is calculated with IEC 60909-0, which defines the voltage factor c for maximum and minimum cases, the initial symmetrical current Ik” and the peak current ip, among other quantities. That makes the results comparable with the utility’s own figures. For projects following US practice, we apply IEEE 551.

Inverse-time curves follow IEC 60255-151 (standard, very and extremely inverse) or IEEE C37.112 (moderately, very and extremely inverse), depending on the relay and the network criteria. Between two protective devices in series we leave a coordination time interval that accounts for breaker opening time, relay error and a safety margin. The final value depends on the relay type and the project criteria. The general coordination philosophy follows IEEE 242, now carried forward in the IEEE 3004 series.

Transformers are protected within their through-fault withstand curve per IEEE C57.109, and settings must not operate on inrush current or motor starting. To check CT performance we use IEEE C37.110.

Settings at the point of interconnection are also adapted to the local code and the utility requirements that you give us.

What you receive

Study report with assumptions, input data, scenarios and short-circuit results.
TCC plots for each protection path.
Settings tables per relay, in the utility’s format when required.
List of findings: uncoordinated cases, undersized equipment and recommendations.
Settings files ready to load into the relays, if you also hire the configuration.
A complete engineering package for your review.

Why Tiza Group

One team from start to finish: the people who calculate the settings are the same who stand behind them with you and with the utility.
Field-proven criteria: our team has tested and commissioned the kind of protection we now calculate, from design to energization, in medium, high and extra-high voltage.
Settings that reach the relay: we configure the main relay brands remotely, from SEL and Siemens SIPROTEC to reclosers.
Not sure your current settings are right? Start with a relay trip analysis or contact us.

Frequently asked questions

What information do you need to start the study?

The single-line diagram, nameplate data of transformers, generators and motors, lengths and sizes of cables and lines, the installed protective devices and the short-circuit level at the point of interconnection provided by the utility. If there are existing settings, we review them too.

Can the study be submitted to the utility?

Yes. We deliver the study report and the settings table in the format your utility or grid operator asks for, adapted to the local requirements you give us. If the utility comes back with comments, we address them within the same study.

How often should the coordination be updated?

Every time the network changes: an expansion, a new transformer, connected generation or a change in the utility's short-circuit level. It is also worth reviewing after a non-selective trip.

Can you review a study done by another company?

Yes. We review the model, the criteria and the proposed settings, and deliver our comments and, if needed, corrected settings.

Who loads the settings into the relays?

We can, remotely through our remote relay configuration service, or on site in Colombia (elsewhere on request), with verification by secondary injection. That way calculation, loading and testing stay with one team.

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