Study

EMT studies

Phenomena lasting a few milliseconds, such as inverter response, energisation or switching overvoltages, require a time-domain simulation.

A load flow or a stability study works on averaged quantities. The peak of an inrush current, the response of an inverter to a voltage dip or a resonance between two plants do not appear in them.

EMT solves the network equations step by step, at microsecond scale, representing inductances, capacitances, transformer saturation and inverter control. The result depends directly on the quality of the models, so we always start by checking the models supplied by the manufacturers.

European network operators are preparing requirements for grid-forming inverters. Demand for this type of study will grow, especially for storage.

The calculated overvoltages are also used for insulation coordination to IEC 60071, to select and locate surge arresters and to check the margins between the protection level and the equipment withstand.

01What we do

Inverters and storage

Check of manufacturer models
Fault ride-through
Grid-following and grid-forming inverters
Interactions between inverters

Switching

Transformer energisation
Capacitor bank energisation
Switching overvoltages, reactor de-energisation
IEC 60071 insulation coordination

02Examples

Fig. 01Energisation of a 5 Mvar capacitor step onto a step already energised. The current reaches a peak of about 10 kA at 5 kHz, which requires a current-limiting reactor.Worked example

Fig. 02Voltage dip to 0.3 pu for 150 ms. The inverter injects reactive current and reduces its active current, then recovers.Simplified model

03Our offer

Usual content of an offer for this type of assignment. The exact scope, deliverables and price are set in the quotation.

Package 1Method note
Phenomena studied, cases, criteria, configurations.
Package 2Model check
Simple tests of the manufacturer models before use, check report.
Package 3Simulations
Cases defined in the quotation, sensitivities on uncertain parameters.
Package 4Report
Annotated results, comparison with criteria, recommendations.

04How we work

01List of cases
Phenomena to study, criteria, configurations.
02Model check
Simple tests of the manufacturer models before any use.
03Simulations
Defined cases, sensitivities on uncertain parameters.
04Recommendations
Settings, equipment (reactors, pre-insertion resistors, surge arresters).

05References

Storage plant connected at 225 kV

EMT study method in PowerFactory

Grid-following and grid-forming inverter models, study cases defined according to CIGRE publications.

Wind farm owner

Dispute over an MV cable network

Technical notes on cable joint failures and switching overvoltages, responses to the opposing party's submissions (dires).

Generating substation

Capacitor bank

Analysis of SCADA logs, identification of control hunting and projection of circuit-breaker wear.

All references

06Frequently asked questions

Does EMT replace the stability study?

No, it complements it for fast phenomena.

What if the manufacturer's model is unusable?

We report it in writing and propose an alternative method, or a request to the manufacturer for additional information.