ST6C

Exc IEEE ST6C model

This article is incomplete, some sections must be written.

Context

This voltage regulator model first appeared in the IEEE Std 421.5-2016 [1]. It has been reproduced identically in the IEC 61970-302:2024 version [2]. In the previous standard version (2005), its predecessor model was called ST6B. Compared to ST6B, ST6C has additional options for connecting OEL and UEL inputs and an additional block with time constant \(t_A\).

Model use, assumptions, validity domain and limitations

To be completed

Model inputs and output

The input variables are :

Variable	Description	Units
\(I_r P_u\)	rotor current	\({pu}\) (base \(S_{Nom}\), user-selected base voltage)
\(i_t P_u\)	complex current at the terminal	\({pu}\) (base \(S_{Nom}\), \(U_{Nom}\))
\(U_s P_u\)	measured stator voltage	\({pu}\) (base \(U N_{om}\))
\(U_{s} R_{ef} P_u\)	reference stator voltage	\({pu}\) (base \(U N_{om}\))
\(u_t P_u\)	complex voltage at the terminal	\({pu}\) (base \(U N_{om}\))
\(U_{Oel} P_u\) (optional)	output voltage of overexcitation limiter	\({pu}\) (base \(U N_{om}\))
\(U_{Pss} P_u\) (optional)	output voltage of power system stabilizer	\({pu}\) (base \(U N_{om}\))
\(U_{Scl} O_{el} P_u\) (optional)	output voltage of stator current overexcitation limiter	\({pu}\) (base \(U N_{om}\))
\(U_{Scl} U_{el} P_u\) (optional)	output voltage of stator current underexcitation limiter	\({pu}\) (base \(U N_{om}\))
\(U_{Uel} P_u\) (optional)	output voltage of underexcitation limiter	\({pu}\) (base \(U N_{om}\))

The output signal is \(E_{fd} P_u\), the excitation voltage in \({pu}\) (user-selected base voltage).

Model parameters

Parameter	Description	Units
\(I_{lr} P_u\)	Exciter output current limit reference	\({pu}\) (base \(S_{Nom}\), user-selected base voltage)
\(K_c\)	Rectifier loading factor proportional to commutating reactance	\({pu}\)
\(K_{cl}\)	Field current limiter conversion factor	\({pu}\)
\(K_{ff}\)	Feedforward gain of inner loop field regulator	\({pu}\)
\(K_g\)	Feedback gain constant of inner loop field regulator	\({pu}\)
\(K_i\)	Potential circuit (current) gain coefficient	\({pu}\)
\(K_{ia}\)	Integral gain of PI	\({pu}\)
\(K_{lr}\)	Gain of field current limiter	\({pu}\)
\(K_m\)	Gain of error of inner loop field regulator	\({pu}\)
\(K_p\)	Potential circuit gain	\({pu}\)
\(K_{pa}\)	Proportional gain of PI	\({pu}\)
\({PositionOel}\)	Input location : (0) none, (1) voltage error summation, (2) take-over at AVR input, (3) AVR input summation, (4) take-over at AVR output	\(-\)
\({PositionScl}\)	Input location : (0) none, (1) voltage error summation, (2) take-over at AVR input, (3) AVR input summation, (4) take-over at AVR output	\(-\)
\({PositionUel}\)	Input location : (0) none, (1) voltage error summation, (2) take-over at AVR input, (3) AVR input summation, (4) take-over at AVR output	\(-\)
\({Sw1}\)	If true, power source derived from terminal voltage, if false, independent from terminal voltage	\(-\)
\(t_A\)	Voltage regulator time constant	\({s}\)
\(t_G\)	Feedback time constant of inner loop field regulator	\({s}\)
\(\Theta_p\)	Potential circuit phase angle	\({rad}\)
\(t_R\)	Stator voltage filter time constant	\({s}\)
\(V_{a} Max P_u\)	Maximum output voltage of limited first order	\({pu}\) (user-selected base voltage)
\(V_{a} Min P_u\)	Minimum output voltage of limited first order	\({pu}\) (user-selected base voltage)
\(V_{b} Max P_u\)	Maximum available exciter field voltage	\({pu}\) (base \(U_{Nom}\))
\(V_{m} Max P_u\)	Maximum output voltage of second PI	\({pu}\) (user-selected base voltage)
\(V_{m} Min P_u\)	Minimum output voltage of second PI	\({pu}\) (user-selected base voltage)
\(V_{r} Max P_u\)	Maximum output voltage of first PI	\({pu}\) (user-selected base voltage)
\(V_{r} Min P_u\)	Minimum output voltage of first PI	\({pu}\) (user-selected base voltage)
\(X_l P_u\)	Reactance associated with potential source	\({pu}\) (base \(S N_{om}\), \(U N_{om}\))

Model diagram

Figure 1: ST6C block diagram

Model variant

In the ST6B model :

• the voltage from the underexcitation limiter is applied at the AVR input
• there is no stator current limiter
• the power source is derived from terminal voltage, with no reactance
• there is no first order filter on the feedback signal of the inner loop field regulator

Open source implementations

This model has been successfully implemented in :

Software	URL	Language	Open-Source License	Last consulted date	Comments
Dynawo	Link	Modelica	MPL v2.0	24/05/2024

References

1.

Electrical TI of, Engineers E (2016) IEEE recommended practice for excitation system models for power system stability studies. IEEE Std 4215-2016

2.

Commission IE (2024) Energy management system application program interface (EMS-API) part 302: Common information model (CIM) dynamics. IEC 61970-302