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

Device	Parameters	Description	Values
Grid	F0 (60 Hz / 50 Hz)	Rated frequency	50 or 60 Hz*
Virtual tested unit AC generator (Sync. Gen.))	Unspecified	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
	VG Xd	Xd (saturated)	1.3 p.u.
	VG Xq	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	4 MJ/MVA
	VG Kd alt	Damping coefficient Kd to compensate for neglecting the dampers	12.0
	VG MW 0	Power set point of the virtual tested unit	81 MW
	VG V0	Voltage set point of the virtual tested unit	1 p.u.
Voltage transformer (PT)	Unspecified	PT ratio	115 V / 6600 V
	VG Base V	PT secondary winding voltage corresponding to 1 p.u.	115 V
Current transformer (CT)	Unspecified	CT ratio	5 A / 12000 A
	VG Base I	CT secondary winding current corresponding to 1 p.u.	3.645 A
Load	Unspecified	Rated voltage of the loads, assumed same as the unit voltage	6.6 kV
	Ch1 MW	Active power of load #1	81 MW **
	Unused	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.
	Unspecified	X/R ratio of the equivalent Thevenin source	15
	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

50

VG Main Ki

Integral gain of the equivalent machine frequency controller

3

Ki frequency setting

2

* 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

Parameter	Description	Default Value
VG VO	Voltage at the terminals (p.u.)	1.0 p.u.
VG MW0	Initial power (MW)	81 MW
VG MVA Ref	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
VG Xd	Direct axis synchronous impedance (saturated)	1.3 p.u.
VG X`d	Direct axis transient impedance (saturated)	0.3 p.u.
VG Xq	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.)	12
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.)	50
VG Main Ki	Fine tuning integral gain of mechanical power of the main grid as a function of its frequency (p.u./p.u./s)	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.

Specialized Solutions Documentation

BERTA Virtual Grid

Virtual Grid for Test Setup or Training

Virtual Grids

Islanded Virtual Grid

Virtual Grid Main Panel: Training