ST4C

Exc IEEE ST4C 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 ST4B. Compared to ST4B, ST4C has additional options for connecting OEL and UEL inputs, an additional block with time constant \(t_A\) and an additional time constant \(t_G\) in the feedback path with gain \(K_g\).

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 \(S N_{om}\), user-selected base voltage)
\(i_t P_u\)	complex current at the terminal	\({pu}\) (base \(S N_{om}\), \(U N_{om}\))
\(U_s P_u\)	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 \(U N_{om}\))
\(U S_{cl} O_{el} P_u\) (optional)	output voltage of stator current overexcitation limiter	\({pu}\) (base \(U N_{om}\))
\(U S_{cl} U_{el} P_u\) (optional)	output voltage of stator current underexcitation limiter	\({pu}\) (base \(U N_{om}\))
\(U U_{el} P_u\) (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
\(K_c\)	Rectifier loading factor proportional to commutating reactance	\({pu}\)
\(K_g\)	Feedback gain of inner loop field regulator	\({pu}\)
\(K_i\)	Potential circuit (current) gain coefficient	\({pu}\)
\(K_{im}\)	Integral gain of second PI	\({pu}\)
\(K_{ir}\)	Integral gain of first PI	\({pu}\)
\(K_p\)	Potential circuit gain	\({pu}\)
\(K_{pm}\)	Proportional gain of second PI	\({pu}\)
\(K_{pr}\)	Proportional gain of first PI	\({pu}\)
\({PositionOel}\)	Input location : (0) none, (1) voltage error summation, (2) take-over at AVR input, (3) take-over at inner loop output	\(-\)
\({PositionPss}\)	Input location : (0) none, (1) voltage error summation, (2) after take-over UEL	\(-\)
\({PositionScl}\)	Input location : (0) none, (1) voltage error summation, (2) take-over at AVR input, (3) take-over at inner loop output	\(-\)
\({PositionUel}\)	Input location : (0) none, (1) voltage error summation, (2) take-over at AVR input, (3) take-over at inner loop 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 \(U N_{om}\))
\(V_{g} Max P_u\)	Maximum feedback voltage of inner loop field regulator	\({pu}\) (user-selected base voltage)
\(V_{m} Max P_u\)	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: ST4C block diagram

Model variant

In the ST4B model :

• the overexcitation limiter voltage is applied in the inner loop field regulator
• the voltages from the underexcitation limiter and the power system stabilizer are added to the voltage error
• there is no stator current limiter
• the power source is derived from terminal voltage
• there are no first order filter and no upper limit for 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