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BERTA Virtual Grid

The On-Site BERTA application is used to simulate two virtual grids:

  • A virtual grid for test setup or training;
  • A virtual grid of AC generators and loads coupled with the tested generating unit (real or virtual) during simulations of an islanded grid operation.

Virtual Grid for Test Setup or Training

This grid simulates a real test.

It offers a variety of models used to simulate an AC generator synchronized with the main grid, such as for a real power plant test:

  • A synchronous AC generator corresponding to GENTRA (PSS/E) model; no saturation representation
  • Models for voltage and speed regulation systems (including stabilizer circuit, if required) of an AC generator, based on the type of turbine: hydraulic, thermal or gas
  • Simplified models for voltage and current transformers
  • Serial R+jX impedance representing the short-circuit capacity of the main grid
  • Total rotational inertia of the main grid (using the tested unit rated MVA as MVA base)
  • PI system for frequency control of the main grid






F0 (60 Hz / 50 Hz)

Rated frequency

50 or 60 Hz* 

Virtual tested unit AC generator (Sync. Gen.))


Rated voltage of the virtual tested unit

6.6 kV

VG MVA ref

Rated MVA of the virtual tested unit. Used as the reference for all the parameters of the virtual grid

100 MVA


Xd (saturated)

1.3 p.u.


Xq (saturated)

0.6 p.u.

VG X'd

X’d (saturated)

0.3 p.u.

VG T'do

Open circuit field time constant T’do

6.0 s

Not simulated

Stator resistance Ra

0.0 p.u.

VG H alt

Inertia constant H


VG Kd alt

Damping coefficient Kd to compensate for neglecting the dampers



Power set point of the virtual tested unit

81 MW


Voltage set point of the virtual tested unit

1 p.u.

Voltage transformer (PT)


PT ratio

115 V / 6600 V

VG Base V

PT secondary winding voltage corresponding to 1 p.u.

115 V

Current transformer (CT)


CT ratio

5 A / 12000 A

VG Base I

CT secondary winding current corresponding to 1 p.u.

3.645 A



Rated voltage of the loads, assumed same as the unit voltage

6.6 kV

Ch1 MW

Active power of load #1

81 MW **


Reactive power of load

0 Mvar

Source equivalent to main grid (Equivalent Machine)

X Thevenin

Series reactance of the equivalent Thevenin source

0.1 p.u.


X/R ratio of the equivalent Thevenin source


R Thevenin

Series resistance of the equivalent Thevenin source

0.014 p.u.

VG Main H

Equivalent total rotating inertia in the main virtual grid

400 MJ/MVA

Virtual Grids

VG Main Kp

Proportional gain of the equivalent machine frequency controller


VG Main Ki

Integral gain of the equivalent machine frequency controller


Ki frequency setting


* Set in field F0 of Tested Unit Parameters, Panel Controller, Tab Settings

** Set in field “Load amount (MW)”, Panel LOAD, Tab Load Parameters

Islanded Virtual Grid

The virtual islanded grid consists at least of a load opposing the mechanical power of the virtual or real tested AC generator and from 0 to 5 virtual AC generators coupled with the tested AC generator and from 0 to 5 specific virtual loads varying with the frequency and that can be shed following subfrequency thresholds. This grid is enabled only in the closed-loop operating mode (i.e. in the islanded grid operating mode).

The islanded virtual grid AC generators are simulated using the following models:

  • Hydraulic, thermal or gas turbine
  • Speed governor
  • Complete inertia of the rotating masses

Each virtual AC generator model enabled generates a mechanical power which adds to the mechanical powers of the other enabled virtual AC generators and the tested AC generator.

The virtual loads of the islanded grid are described using the following parameters:

  • Rated apparent power in MVA
  • Actual load in MW
  • Rotational inertia
  • Variation coefficient following the frequency, in p.u./p.u.
  • Up to five shedding scenarios by sub-frequency and model

The sum of the activated loads makes up the electrical power which opposes the total mechanical power when a disturbance is triggered in the islanded grid mode. Then, any imbalance between the total mechanical power and the total load (electrical power) causes a speed deviation calculated by the simulation algorithm of the islanded grid, taking into account the total inertia of the AC generators and the loads.

The frequency stability of the islanded grid depends on the following factors:

  • Total rotational inertia of the AC generators and the load
  • Self-regulation of the load (coefficient versus frequency)
  • Shedding method of the loads in critical situations
  • Physical characteristics of the turbine
  • Characteristics of speed governor and command settings

Virtual Grid Main Panel: Training

This panel allows accessing the control parameters of the virtual grid setup for tests or training. The initial display is the secondary grid panel that provides access to the modeling parameters of the virtual synchronous AC generator and the main grid.

The right section of the panel consists of command buttons for the simulation and selection of the type of AC generator simulated:

  • Hydraulic
  • Thermal
  • Gas

The left section of the panel shows the configuration diagrams and frequency setting options.

The parameters are the following:

Virtual Grid Main Panel



Default Value


Voltage at the terminals (p.u.)

1.0 p.u.


Initial power (MW)

81 MW


Reference power (MWA)

100 MWA

VG Base V

Secondary voltage at T.T. corresponding to 1 p.u. (V)

115 V

VG Base I

Secondary current at T.C. corresponding to 1 p.u. (A)

3.645 A


Direct axis synchronous impedance (saturated)

1.3 p.u.

VG X`d

Direct axis transient impedance (saturated)

0.3 p.u.


Quadrature axis synchronous impedance (satirated)

0.6 p.u.

VG T`do

Time constant of the field in open circuit (s)

6 s

VG H alt

H inertia constant of AC generator (MJ/MVA or s)

4 s

VG Kd alt

Electrical torque damping coefficient as a function of speed (p.u./p.u.)


R Thevenin

Resistance of Thevenin equivalent of the grid (p.u. on VG MVA ref.)

0.014 p.u.

X Thevenin

Reactance of Thevenin equivalent of the grid (p.u. on VG MVA ref.)

0.1 p.u.

VG Main H

Total inertia of equivalent synchronous machine of the main grid (MJ/MVA on VG MVA ref.)

400 s

VG Main Kp

Self-tuning proportional coefficient of mechanical power of the main grid as a function of its frequency (p.u./p.u.)


VG Main Ki

Fine tuning integral gain of mechanical power of the main grid as a function of its frequency


3 (1/s)

The secondary panels describing the models for stabilizer circuits, speed governors and turbines of the virtual grid are displayed with the following tabs:

  • Stab 1
  • Stab 2
  • Stab 3
  • PID Hydro V.G.
  • Classical Hydro V.G.
  • Hydro Turbine V.G.
  • Gas Turbine V.G.
  • Gas Turbine Model V.G.
  • Thermal V.G.
  • Thermal Turbine Model V.G.
  • Boiler V.G. (of the thermal turbine)

The panels show the functions available in the models with matching parameters described in the tables. To avoid any confusion with islanded grid models, identifications are preceded or followed by V.G. or Virtual Grid. The following figures show the operating panels of the speed governors and turbines for the training virtual grid.

OPAL-RT TECHNOLOGIES, Inc. | 1751, rue Richardson, bureau 1060 | Montréal, Québec Canada H3K 1G6 | | +1 514-935-2323