Overexcitation Limiter (OEL2C)

IEEE 421.5 Type OEL2C overexcitation limiter: protects the generator field from thermal overload. The measured field current Ifd is scaled and filtered to Ipu; an inverse-time timer integrates K2*((Ipu/ITFpu)^c2 - 1) while the field is overloaded, and once it reaches TFCL the current reference Iref ramps from the instantaneous limit Iinst down to the thermal limit Ilim. The error Ibias + Iref - Iact drives a PID controller and two lead-lags whose limits make it a summation-point limiter (VOELmax = 0, lowers excitation only) or a take-over limiter (for an LV gate). Activation logic with pickup / reset delays keeps the limiter out of service in normal operation (Ibias = Ireset). Wire Ifd from the synchronous machine and VOEL to the exciter's VOEL input. Pairs with the machine for load-flow initialization.

Category: Control / Excitation

Overview

The Type OEL2C overexcitation limiter (IEEE Std 421.5-2016, clause 10.5) protects the generator field winding from thermal overload. A field winding can carry a high current for a short time (field forcing during a fault keeps the machine in step), but not indefinitely. OEL2C lets the voltage regulator force the field freely for a while, then — once an inverse-time "heating" timer expires — pulls the field current back down to its continuous rating.

The limiter outputs VOEL, a signal for the exciter's VOEL input. It does nothing in normal operation: VOEL sits at its resting value (0 for a summation-point limiter) until the field has been overloaded long enough.

Block diagram

The model has four parts.

1. Measurement. The field current is scaled and filtered, $I_{pu}=K_{SCALE}{I}_{fd}/(1+s{T}_{Roel})$, and $I_{act}=K_{ACT}{I}_{pu}$ is what the limiter compares against its reference.

2. Inverse-time timer. While the field is overloaded ($I_{pu}>I_{TFpu}$, the continuous rating) the timer $T_{lim}$ integrates

   $$W={\mathit{Fixed}}_{rd}+K_2[{\left(\frac{I_{pu}}{I_{TFpu}}\right)}^{c_2}-1]$$

   (with ${\mathit{Fixed}}_{ru}$ in place of ${\mathit{Fixed}}_{rd}$ at or below the rating). With $c_2=2$ this is the classic $I^{2}t$ heating model: twice the overload heats four times faster. The timer is limited to $[T_{min},T_{max}]$ and, with $K_{FB}>0$, decays back on its own.

3. Reference ramp. The current reference $I_{ref}$ rests at the instantaneous limit $I_{inst}$. Once the timer error $T_{err}=T_{FCL}-T_{lim}$ reaches zero the reference ramps down to the thermal limit $I_{lim}$; once the timer has recovered ($T_{err}\ge K_{ZRU}{T}_{FCL}$) it ramps back up. The ramp rates are fixed ($K_{RU}$, $K_{RD}$) or follow the inverse-time term $K_1[(I_{pu}/I_{TFpu}{)}^{c_1}-1]$.

4. Controller. The error $I_{err}=I_{bias}+I_{ref}/(1+s{T}_{Aoel})-I_{act}$ drives a PID controller and two lead-lag stages, each with its own output limits, producing VOEL.

Activation logic

The bias $I_{bias}$ keeps the limiter out of service in normal operation. While the limiter is reset, $I_{bias}=I_{reset}$ (a large number), so the error is strongly positive and the PID rests on its upper limit. The limiter becomes active ($I_{bias}=0$) when the timer expires, or when $I_{act}>I_{ref}$ for longer than $T_{en}$ (set $T_{en}=0$ to keep it always active). It resets again once the reference is back at $I_{inst}$ and the current has stayed at least $I_{THoff}$ below it for $T_{off}$.

Summation point or take-over

The standard sets the limiter's action through the output limits alone, and so does this component:

• Summation point (the default data): $V_{OELmax1}=0$ and $V_{OELmin1}<0$. VOEL is zero at rest and goes negative to lower excitation. Wire it to an exciter whose OEL input adds into the voltage error.
• Take-over: a large $V_{OELmax1}$ (e.g. 10). VOEL rests high and an LV gate in the exciter lets the limiter take control once VOEL drops below the regulator's own signal. Choose an exciter OEL location that uses a take-over gate.

Initialization

At the load-flow operating point the limiter is inactive: the reference rests at $I_{inst}$, the timer at rest, and VOEL at its resting value. The only seed it needs is the machine's field current at $t=0$, Ifd(0), which the load-flow auto-initialization fills in from the machine that feeds the Ifd input. The first simulation step uses that seed in place of the input, so the limiter starts without a step.

Wiring

• Ifd ← the synchronous machine's Ifd output.
• VOEL → the exciter's VOEL input (enable OEL input on the exciter's Aux Inputs tab and pick the location that matches the limiter's action).

Enable the monitors on the Monitoring tab to record VOEL, the reference Iref, the timer Tlim and the error Ierr.

Ports

Port	Kind	Signal	Notes
Ifd	Control input	Real
VOEL	Control output	Real

Parameters

Pickup & Timer

Parameter	Type	Default	Units	Description	ID
KSCALE	Number	1	—	Scaling factor applied to the measured field current before the transducer (pu). Use it to express Ifd in the limiter's own base.	KSCALE
TRoel	Number	0.01	s (also ms)	Field-current transducer time constant of KSCALE/(1+sTRoel). 0 senses directly.	TRoel
KACT	Number	1	—	Gain from the scaled current Ipu to the actual-current signal Iact compared against the reference.	KACT
ITFpu	Number	3	—	Continuous (thermal) field-current rating ITF in the scaled per-unit. Above it the inverse-time timer starts to integrate.	ITFpu
K1	Number	0	—	Inverse-time gain of IERRinv1 = K1*((Ipu/ITFpu)^c1 - 1), the ramp rate used when the inverse-time ramp is selected.	K1
c1	Number	0	—	Exponent of the IERRinv1 inverse-time characteristic.	c1
K2	Number	0.0296	—	Inverse-time gain of IERRinv2 = K2*((Ipu/ITFpu)^c2 - 1), the rate at which the timer integrates during overexcitation.	K2
c2	Number	2	—	Exponent of the IERRinv2 inverse-time characteristic (2 gives the classic I^2 t heating model).	c2
VINVmax	Number	100	—	Upper limit on IERRinv2.	VINVmax
VINVmin	Number	0	—	Lower limit on IERRinv2. 0 stops the timer from running backwards through this term.	VINVmin
Fixedru	Number	0	—	Fixed rate added to IERRinv2 while Ipu is at or below ITFpu (cooling side).	Fixedru
Fixedrd	Number	-0.001	—	Fixed rate added to IERRinv2 while Ipu is above ITFpu (heating side).	Fixedrd
TFCL	Number	10	—	Timer threshold. Terr = TFCL - Tlim; the limiter acts once the timer Tlim reaches TFCL.	TFCL
Tmax	Number	10	—	Upper limit on the timer Tlim.	Tmax
Tmin	Number	0	—	Lower limit on the timer Tlim.	Tmin
KFB	Number	0	—	Timer self-reset feedback gain: the timer integrates W - KFB*Tlim. 0 makes the timer a pure integrator.	KFB

Reference

Parameter	Type	Default	Units	Description	ID
Iinst	Number	6	—	Instantaneous field-current limit: the reference Iref rests here while the timer has not expired (scaled pu).	Iinst
Ilim	Number	3	—	Thermal field-current limit: the reference ramps down to this level once the timer expires (scaled pu).	Ilim
TAoel	Number	0.04	s (also ms)	Reference filter time constant of 1/(1+sTAoel). 0 passes Iref straight through.	TAoel
SW1	Choice: Fixed rates (KRU / KRD) / Inverse time (IERRinv1)	Fixed rates (KRU / KRD)	—	Ramp-rate selection: fixed rates KRU / KRD, or the inverse-time rate IERRinv1 for both directions.	SW1
KZRU	Number	0.99	—	Ramp-up threshold factor: Iref ramps up once Terr >= KZRU*TFCL, ramps down once Terr <= 0, and holds in between.	KZRU
KRU	Number	1000	—	Fixed ramp-up rate of Iref (pu/s).	KRU
KRD	Number	-1000	—	Fixed ramp-down rate of Iref (pu/s, negative).	KRD

Activation

Parameter	Type	Default	Units	Description	ID
Ireset	Number	100	—	Bias Ibias added to the error while the limiter is reset (inactive), so it cannot act. Ibias = 0 once active.	Ireset
Ten	Number	0.2	s (also ms)	Pickup delay: the limiter activates once Iact > Iref has held for Ten, or as soon as the timer expires. 0 keeps it always active.	Ten
Toff	Number	5	s (also ms)	Reset delay: the limiter resets once Iref is back at Iinst with Iref - Iact > ITHoff for Toff.	Toff
ITHoff	Number	0.05	—	Current margin below the reference required for the reset timer (scaled pu).	ITHoff

Controller

Parameter	Type	Default	Units	Description	ID
KPoel	Number	0.3	—	PID proportional gain.	KPoel
KIoel	Number	1	—	PID integral gain (non-windup integral).	KIoel
KDoel	Number	0	—	PID derivative gain of sKD/(1+sTD).	KDoel
TDoel	Number	0.1	s (also ms)	PID derivative filter time constant.	TDoel
VOELmax3	Number	0	—	Upper limit of the PID output. 0 with a negative lower limit makes a summation-point limiter that can only lower excitation.	VOELmax3
VOELmin3	Number	-100	—	Lower limit of the PID output.	VOELmin3
TC2oel	Number	0.1	s (also ms)	Lead time constant of the second-stage lead-lag (1+sTC2oel)/(1+sTB2oel). Equal to TB2oel bypasses it.	TC2oel
TB2oel	Number	0.1	s (also ms)	Lag time constant of the second-stage lead-lag.	TB2oel
VOELmax2	Number	0	—	Upper limit after the second-stage lead-lag.	VOELmax2
VOELmin2	Number	-100	—	Lower limit after the second-stage lead-lag.	VOELmin2
TC1oel	Number	0.1	s (also ms)	Lead time constant of the output lead-lag (1+sTC1oel)/(1+sTB1oel). Equal to TB1oel bypasses it.	TC1oel
TB1oel	Number	0.1	s (also ms)	Lag time constant of the output lead-lag.	TB1oel
VOELmax1	Number	0	—	Upper limit of the output VOEL. 0 for a summation-point limiter; a large value (e.g. 10) for a take-over limiter feeding an LV gate.	VOELmax1
VOELmin1	Number	-10	—	Lower limit of the output VOEL.	VOELmin1

Monitoring

Parameter	Type	Default	Units	Description	ID
Monitor VOEL	Choice: Off / On	Off	—	Record the limiter output VOEL (pu) sent to the exciter's VOEL input as a named observable.	measure_voel
Monitor Iref	Choice: Off / On	Off	—	Record the field-current reference Iref (scaled pu), between Ilim and Iinst as a named observable.	measure_iref
Monitor Tlim	Choice: Off / On	Off	—	Record the inverse-time timer output Tlim; the limiter acts when it reaches TFCL as a named observable.	measure_tlim
Monitor Ierr	Choice: Off / On	Off	—	Record the error Ierr = Ibias + Iref - Iact (scaled pu) entering the PID as a named observable.	measure_ierr

Signal Names

Parameter	Type	Default	Units	Description	ID
VOEL name	Text	(empty)	—	Observable name for VOEL. Blank = skip.	voel_name
Iref name	Text	(empty)	—	Observable name for Iref. Blank = skip.	iref_name
Tlim name	Text	(empty)	—	Observable name for Tlim. Blank = skip.	tlim_name
Ierr name	Text	(empty)	—	Observable name for Ierr. Blank = skip.	ierr_name

Initialization

Parameter	Type	Default	Units	Description	ID
Ifd(0)	Number	1	—	Initial field current at t = 0 (pu). Seeds the transducer and stands in for Ifd on the first step. Normally set by the load-flow auto-initialization from the machine's field current.	ifd_init

Observables

Default name	Kind	Recorded when	Description	ID
set by VOEL name	Signal	Monitor VOEL is on	The limiter output VOEL (pu) sent to the exciter's VOEL input.	VOEL
set by Iref name	Signal	Monitor Iref is on	The field-current reference Iref (scaled pu), between Ilim and Iinst.	iref
set by Tlim name	Signal	Monitor Tlim is on	The inverse-time timer output Tlim; the limiter acts when it reaches TFCL.	tlim
set by Ierr name	Signal	Monitor Ierr is on	The error Ierr = Ibias + Iref - Iact (scaled pu) entering the PID.	ierr