A protection coordination study needs the updated single-line diagram, the data of transformers, cables, lines and generation, the CT and VT ratios, the relays with their model and the utility's network equivalent at the connection point. It goes through modeling, short circuit, setting criteria, settings and a selectivity check, and delivers a report and the setting tables. For grid connections, the utility or the transmission owner validates the coordination at the connection point.
What it is and when it is needed
A protection coordination study defines the settings of each relay, recloser or breaker with a trip unit. Its purpose is that, in a fault, only the closest protection operates and the others stay as backup, in the shortest time and without damaging equipment. The concept is explained in coordinated and selective electrical protection.
It is needed in a new installation, when the network changes (a larger transformer, own generation, a new service entrance), when the utility reports a change in the short-circuit level, and when a trip showed that selectivity did not work. For generation connected to the grid it is usually mandatory: grid codes commonly require a protection setting and coordination study for generators above a small size threshold. See studies required to connect generation.
This guide is for anyone about to hire the study: what information to have ready, how it proceeds, what the schedule depends on and what to ask for at the end.
Input data
A study is only as good as its data. This is the information usually needed; a serious offer asks for it before quoting or at kick-off.
| Information | Why it matters |
|---|---|
| Updated, as-built single-line diagram | It is the map of the study; an outdated diagram produces settings for a network that does not exist |
| Transformer nameplate data (power, voltages, impedance, connection group, grounding) | They define the fault currents and the inrush currents |
| Cables and lines: size, length, configuration | They determine impedances and thermal limits |
| Own generation, large motors and capacitor banks | They contribute fault current or change the flows |
| Ratios and class of the current and voltage transformers | They limit the possible settings and the behavior under saturation |
| Relays: manufacturer, model and available functions | Each model implements curves and logic in its own way |
| Network equivalent and utility settings at the connection point | They fix the short circuit and the time step that cannot be invaded |
| Operating conditions: loads, scenarios, transfers | They define the maximum and minimum cases that must be covered |
Short circuit is the basis of everything; see short-circuit study.
Stages at a high level
- Collecting and validating the information. The data is reviewed, missing data is requested and the single-line diagram is confirmed.
- Network model in power system analysis software.
- Short circuit and load flow in the maximum and minimum scenarios.
- Setting criteria: what each function must protect, what it must be stable against (load, inrush, motor starting) and what time margin it needs against the neighboring protection. See criteria for setting electrical protection.
- Settings calculation and selectivity check with time-current curves and, on lines, with distance zones.
- Review with the utility or the transmission owner and handling of comments.
- Implementation: the approved settings are loaded into the relays and tested.
Coordination method, step by step
Coordination orders the settings so that, in a fault, the nearest protection trips first and the others act only as backup. The following steps follow the criteria that the manufacturer Siemens publishes in the SIPROTEC 5 manual for the overcurrent functions. They are the skeleton of a study, not the values of a case. The curves of relays, reclosers and fuses can be plotted together with our free relay coordination tool.
- Set the pickup above the maximum load. Overcurrent protection with short times acts as short-circuit protection, not overload protection, so normal load must not make it pick up. The manual indicates a pickup about 10 % above the maximum expected load on lines and 20 % on transformers and motors. Manual example: a 110 kV line with a maximum power of 120 MVA, that is, 630 A. With a safety factor of 1.1 the pickup is 693 A on the primary, about 5.8 A on the secondary of a 600/5 CT.
- Draw the grading plan. It is the diagram of the network with each protection and its time. Times rise against the direction of load flow: the protection farthest from the source is the fastest.
- Choose the time of each step. The delay comes from the plan, not from a fixed value. The interval between two protections must cover the breaker opening time with its spread, the reset of the protection and the spread of the timers. The same manual gives 0.2 s to 0.4 s as the usual interval in distance protection.
- Use the second (high-current) step where the impedance allows. It works on long lines with low source impedance or before high reactances, such as transformers. Its pickup is set so that the step does not operate for a short circuit at the end of the line. In this way it covers only the initial part of the section. Its delay is 0 or minimal. The manufacturer recommends calculating those pickups with a network analysis.
- Add direction where there are two sources. On parallel lines, in rings and in sections fed from both sides, overcurrent needs a directional element. Grading is set with times rising against the direction of load flow, and in sections fed from both sides it is set from both sides.
- Decide whether there is a fast protection by directional comparison. If the relays at the two ends of a section communicate (protection channel, pilot wire or GOOSE), they can clear the faulted section without a long time grading.
- Coordinate with reclosing. Before a reclose, fast clearing matters more than selectivity, so the first trip can be set to 0 s or to a minimum value. Final lockout uses the time from the grading plan.
- Check the emergency mode. If overcurrent acts only as backup when the main protection fails, the manual allows shorter times.
After calculating, time-current curves are used to verify that each pair of protections keeps its margin for the maximum and minimum fault currents. Criteria by function are in protection setting criteria, the directional function in directional overcurrent 67 and reclosing in automatic reclosing 79.
What the schedule depends on
There is no standard published time for this study, so be wary of anyone who promises one without knowing your network. What matters most:
- Size and complexity: number of buses, voltage levels, generation and special schemes such as teleprotection or automatic transfers.
- Quality of the input data. Missing nameplates or an outdated diagram force data collection on site.
- Utility response times to deliver the network equivalent and its settings, and to review the study.
- Rounds of comments. Each utility comment opens a new review.
- Project deadlines set by the grid code. Some connection codes ask for the coordination study several months before commissioning. Plan the study by counting backwards from that date.
Utility approval
- Distribution system: the utility validates the coordination at the connection point. For generation, the developer reports the final settings before start-up, and any later change needs the utility’s authorization.
- Transmission system: the user submits the study for the transmission owner’s approval, and neither party can change the settings unilaterally.
- Content for generation: the study follows the grid operator’s or utility’s guidelines and includes all enabled functions and logic.
Internal coordination of the plant or industrial site is the owner’s responsibility; coordination at the connection point is agreed with the utility.
Warning signs when comparing offers
- It does not ask for information before quoting or does not mention the utility’s network equivalent.
- Overcurrent only, when the installation has differential, distance, voltage and frequency functions or anti-islanding.
- It does not include the review with the utility or the handling of comments.
- It delivers curves but not setting tables ready to load into each relay.
- It does not consider the minimum scenarios: the protection must also see the weakest fault.
- It does not include implementing and testing the settings. A study that nobody loads or tests protects nothing.
What the deliverables should contain
- Calculation report: data used and its source, assumptions, scenarios, short-circuit results and setting criteria.
- Setting tables per relay, with all enabled functions and their values, in a format that can be moved to the manufacturer’s software.
- Coordination curves (time-current) and, if applicable, R-X distance diagrams.
- Selectivity check by section, with the margins obtained.
- Recommendations: CT ratio changes, functions to enable, or equipment that cannot reach coordination.
- For generation, the content that the grid guidelines and the utility ask for.
If your project is self-generation, see also protection study for self-generation.
We do your study
Our team does protection coordination studies for medium, high and extra-high voltage substations, generation plants and industry. We handle the utility’s review remotely, worldwide, and can load the approved settings on site in Colombia, or elsewhere on request. See the protection coordination service within our electrical studies and send us your single-line diagram through contact.
Related guides
- Studies required to connect generation
- Short-circuit study: IEC 60909 versus ANSI/IEEE
- Criteria for setting electrical protection
- Coordinated and selective electrical protection
- Protection study for self-generation
References
- The grid code, connection code and utility guidelines that apply to the project.
- IEEE Std 242, Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Buff Book).
Frequently asked questions
How long does a protection coordination study take?
There is no standard published time. It depends on the size of the system, on the input data being complete, on the utility delivering the network equivalent and its settings on time, and on the rounds of comments in the review.
Who approves the protection coordination study?
On distribution systems, the utility validates the coordination at the connection point. On transmission systems, grid codes usually say the user submits the study for the transmission owner's approval, and neither party can change the settings unilaterally.
Why does the utility have to provide information for the study?
Because coordination at the connection point depends on its network: the short-circuit level, the equivalent impedance and the settings of its upstream protections. Without that data, selectivity cannot be demonstrated.