Description

This is a general model of a static excitation system representing a Silicon-Controlled Rectifier. The exciter may be bus fed or solid fed. The model was described in [1] as part of study for load rejection over-voltages. The SCRX component was developed in line with the references [1] [2].

Mask and Parameters

Exciter Parameters

Expanding the "SCRX diagram" displays the block diagram in the parameters window.

Name	Description	Unit	Variable = {Possible Values}
Ta_Tb	Gain reduction ratio Ta/Tb	-	-
Tb	Time constant	s	-
K	Exciter gain	-	-
Te	Time constant	s	-
Emax	Maximum field voltage output	pu	-
Emin	Minimum field voltage output	pu	-
Cswitch	Voltage source switch – Bus fed = 0, Solid fed = 1.	-	[0, 1]
Rc_Rfd	Ratio of the crowbar resistance to field winding resistance Rc/Rfd	-	-

The choice of the Cswitch parameters switches the voltage signal between bus fed, which is the generator terminal voltage, or solid fed, which is a 1.0 pu signal.

The negative current logic switches the operation of the exciter between allowing or blocking the negative field current, Ifd. When bidirectional Ifd is allowed, the exciter represents two rectifier bridges connected in reverse polarity to allow for Ifd to flow in both directions. On the other hand, if unidirectional Ifd is allowed, the excitation system represents a single bridge rectifier. In the uni-directional case, the bridge is protected by a crowbar circuit with a resistance Rd and a reverse bypassing thyristor [1]. The logic depends on the value of the ratio of the crowbar resistance to the field winding resistance, Rc_Rfd. If Rc_Rfd = 0, then negative Ifd is allowed. In this case, Efd = Ex in all cases. On the other hand, if Rc_Rfd > 0, then the logic depends on the sign of the Ifd. If Ifd is positive, then Efd = Ex. Otherwise, if Ifd is negative, then Efd = – XadIfd*Rc_Rfd.

Initial Value Parameters

Name	Description	Unit	Parameter Range
Efd0	Exciter output voltage initial value	pu	--

The parameters Efd0 can be set manually, by entering a numerical value. It can also be set automatically based on load flow calculations, by entering a referenced synchronous machine variable. For instance, if the name of the synchronous machine on which the excitation system is connected is “SM1”, then Efd0 shall be set as “=SM1.EfdInit” if a thermal machine or a hydraulic machine is used, and as "=SM1.EFD_Init" if a pu standard or pu fundamental machine is used.

The HYPERSIM simulation option “Set Initial Conditions” must be checked for the automatic initialization to work properly.

Inputs and Outputs and Additional Signals Available for Monitoring

Inputs

Name	Description	Unit
Voel*	Overexcitation limiter output	pu
Vc1	Signal proportional to compensated terminal voltage. If a load compensator block is not used upstream from the exciter block, then Vc1 is equal to Vt (main alternator terminal voltage)	pu
Vuel*	Underexcitation limiter output	pu
Vref	Voltage regulator reference voltage	pu
Vs	Output voltage of a Power System Stabilizer (PSS)	pu
Ifd**	Synchronous machine field current	pu
Vd	d-axis component of generator terminal voltage	pu
Vq	q-axis component of generator terminal voltage	pu

*Vuel and Voel are normally inputs to the excitation system but the user has the option to use a fixed constant value directly in the component mask by choosing “internal”.

**The observable field current Ifd from the synchronous machine in HYPERSIM needs to be multiplied by the machine’s Xad = Xd – Xl prior to its input to the exciter.

The terminal voltage calculation is performed in terms of Vd and Vq using the following equation:

Outputs

Name	Description	Unit
Efd	Exciter output voltage	pu

References

1. F.P. de Mello, L. M. Leuzinger and R. J. Mills, "Load rejection overvoltages as affected by excitation system control," in IEEE Transactions on Power Apparatus and Systems, vol. 94, no. 2, pp. 280-287, March 1975, doi: 10.1109/T-PAS.1975.31853.

2. PSS®E 34.2.0 Model Library. NY, USA: Siemens Industry, Inc., 2017.

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