How ETAP helps model & analyze generator stability during step loading

Our case study is based on an ETAP model prepared for a fertilizer plant in India. We evaluated transient stability of two 25-MW gas-turbine generators during step loading events, and ETAP allowed us to confirm stable operation under all scenarios.
By Mr. Vedant Sonar, Electrical Engineer, L&T-Sargent & Lundy Limited

This study analyzes the dynamic response of generator step loading for a chemical fertilizer plant in India. The captive power plant includes two 25-MW gas-turbine generators (GTGs). Using the ETAP Transient Stability module, engineers assessed generator electrical frequency and terminal voltage performance over multiple controlled load steps to confirm the plant’s operational resilience. The ETAP model was built using detailed OEM governor and excitation parameters (IEEE GGOV1 and IEEE AC8B), validated through power flow and short-circuit studies prior to running transient simulations.

L&T-Sargent & Lundy Limited is a joint venture between Larsen & Toubro, India, and Sargent & Lundy L.L.C., USA. Established in 1995, it provides consultancy and engineering design services in Power and affiliated sectors. With a professional staff of about 500 engineers and designers, it has extensive experience in power plant engineering and consulting assignments. A unique feature of L&T-S&L’s engineering proficiency is the PLADES (Plant Design Software) modeling, which not only facilitates smooth project execution but also gives it an edge with enhanced quality.

Location: Vadodara, Gujarat, India

Year: 2022

To perform transient stability studies for the step loading of the generator using the ETAP model

Challenges

  • Evaluating transient stability during sequential load steps on an islanded GTG using accurate GTG models.
  • Assessing generator dynamic response using OEM-verified IEEE GGOV1 turbine governor and IEEE AC8B excitation system data.
  • Modelling lump loads as 80% dynamic and 20% static per customer specification.
  • Simulating worst-case loading scenarios to determine voltage and frequency recovery.
  • Confirming that all transient dips remain within ±5% (frequency) and ±10% (voltage) stability criteria.
  • Ensuring no operational or protection changes were required after the step loading study.

Which solutions did they choose?

Selected applications

Engineers used:

  • ETAP Transient Stability → To simulate GTG response during eight sequential load steps.
  • ETAP Dynamic Models → To import IEEE-standard governor/exciter models with OEM parameters.
  • ETAP Power Flow & Short Circuit Analysis → To confirm network adequacy prior to stability simulations.

Why do they use ETAP?

Main customer benefits

  • Accurate GTG modelling using IEEE GGOV1 and AC8B models ensured realistic generator behavior
  • Clear visualization of system recovery across all eight steps for voltage, frequency, active/reactive power and mechanical torque
  • Verification of safe operational margins with minimum steady-state voltage of 94.69% and minimum frequency of 99.567%
  • Identification of no required corrective actions - all transient responses remained within limits
  • Improved system reliability through validated GTG response under conservative worst-case loading
  • Operational confidence for plant engineers thanks to ETAP’s detailed plots and time-domain simulations

What do they think about ETAP?

Customer perspective

ETAP enabled us to accurately model the GTG dynamic behavior and verify that all step-loading scenarios remained within acceptable limits. The results confirmed no changes to relay settings or operational sequences were necessary.
By Mr. Vedant Sonar, Electrical Engineer, L&T-Sargent & Lundy Limited


Videos

How to dynamically model & analyze generator stability during step loading using ETAP software

This study simulates the impact of step loading on the transient stability of a 2 x 25 MW GTG-based captive power plant at one of the Chemical Fertilizer Plants located at Trombay, India. The variation of electrical frequency and terminal voltage of the generator during the most conservative step loading of one generator unit is studied. The loads divided in steps are switched at certain time intervals to determine whether the frequency profile of the system is within the acceptable limit of ±5%. Load Flow and Short Circuit Studies are performed on the entire power plant distribution network of the fertilizer plant in advance to check the adequacy of equipment ratings during normal and short circuit conditions with all verified input data. The step load response study is carried out with only one GTG unit running in isolation, feeding only the critical emergency loads. Transient behavior of the GTG unit is simulated with IEEE transfer function dynamic models, viz. AC8B model of excitation system (AVR) and IEEE GGOV1 model for turbine governing system using the in-built standard library in ETAP software version 16.1.0. Various parameters of the generator like speed, active and reactive power, bus voltage, and frequency are plotted for determining the transient performance of the GTG unit.


Solutions


Packages/Products