LDHT Engineering, power system engineering consultancy

Study

Stability

We check that machines and inverters stay connected after a fault: critical clearing time, voltage dip ride-through, motor starting.

During a fault, a generator can no longer export its power and it accelerates. If the fault is not cleared in time, it loses synchronism. This critical time is compared directly with the operating times of the protection, so the stability study is linked to the protection plan.

On industrial sites, the question often arises the other way round. After a voltage dip, the motors slow down and then all re-accelerate at the same time. The current they draw can hold the voltage down and trip the whole site.

For inverter-based plants, stability depends mainly on the controls: dip ride-through, reactive current injection, active power recovery.

01What we do

Transient stability

  • Critical clearing time
  • Voltage dip ride-through
  • Post-fault recovery

Other dynamic studies

  • Motor starting and re-acceleration
  • Load shedding
  • Islanding and standby generators

02Examples

Generator rotor angle after a fault cleared before and after the critical time0.00.20.40.60.81.01.21.41.6-40°0°40°80°120°160°200°critical time ≈ 250 mscleared at 200 ms: stays stablecleared at 300 ms: loses syncTime (s)Rotor angle
Fig. 01Generator after a three-phase fault. It remains stable if the fault is cleared in 200 ms and loses synchronism if it is cleared in 300 ms. The critical time, about 250 ms, is to be compared with the protection times.Worked example
Voltage dip at the connection point and reactive current injection by the inverter01002003004005006000.00.20.40.60.81.01.2Time (ms)puvoltageinjected reactive currentactive currentsimplified model: k = 2, Imax = 1.1 pu
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.

  1. Package 1Dynamic models

    Machines, controllers, inverters, motor loads.

  2. Package 2Fault cases

    Location, duration, configuration, defined in the quotation.

  3. Package 3Simulations

    Angles, speeds, voltages, powers.

  4. Package 4Conclusions

    Critical times compared with protection times, settings or load shedding.

04How we work

  1. 01Dynamic models

    Machines, controllers, inverters, motor loads.

  2. 02Fault cases

    Location, duration, configuration.

  3. 03RMS simulations

    Angles, speeds, voltages, powers.

  4. 04Comparison with protection

    Critical times, settings, load shedding.

05References

Integrator, West Africa

Four PV plants

Load flow, short-circuit, neutral earthing, earthing, stability and protection plan for each plant.

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.

All references

06Frequently asked questions

Stability or EMT?

A stability study (RMS simulation) covers machine oscillations over a few seconds. EMT deals with phenomena lasting a few milliseconds.

Do you have the controller models?

We use the manufacturers' models where they exist, otherwise standard IEEE models, with the assumptions in writing.

07See also

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