Power System Stabilizers and Discontinuous Excitation Control Transfer Functions (2016)

Single Input Power System Stabilizer (PSS1A)

IEEE Power System Stabilizer Type PSS2C
IEEE Power System Stabilizer Type PSS3C
IEEE Power System Stabilizer Type PSS4C
IEEE Power System Stabilizer Type PSS5C
IEEE Power System Stabilizer Type PSS6C
IEEE Power System Stabilizer Type PSS7C
IEEE Discontinuous Excitation Control Type DEC1A
IEEE Discontinuous Excitation Control Type DEC2A
IEEE Discontinuous Excitation Control Type DEC3A

As indicated, each of the power system stabilizer models and discontinuous excitation control models have a unique transfer function. The schematic diagram of each is given below.

For more details, see IEEE Standard 421.5-2016.

Single Input Power System Stabilizer (PSS1A)

Where,

IEEE Power System Stabilizer Type PSS2C

PSS2C is an extended version of PSS2A from IEEE Standard 421.5-1992 and PSS2B from the IEEE Standard 421.5-2005. The PSS2C model shown in the figure below can be used to represent a variety of dual-input stabilizers, which normally use combinations of power and speed (or frequency or compensated frequency) to derive the stabilizing signal. Unlike PSS2B, it allows the representation of the PSS output logic associated with the generator active power output PT, i.e., the PSS output depends on the generator active power output as compared to the thresholds PPSSon and PPSSoff. These threshold values are used to define a hysteresis.
 

The washout block should be bypassed if the associated time constant is set to zero:

IF TW4=0 THEN
  y=u1
ELSE
  y=u2
ENDIF

The PSS output logic uses user-selected parameters PPSSon and PPSSoff. It also uses signal VPSS, shown in the block diagram, and the generator electrical power output PT. The output logic implements the following hysteresis to define the output signal VST:
 

As PSS2C is an extended version of PSS2A as well as PSS2B:

1. Any PSS represented by the PSS2A model could be represented by the PSS2C model, with the time constants T10 to T13 set equal to each other, in order to bypass the third and fourth lead-lag blocks in the PSS2C model, and the threshold values for the output logic PPSSon and PPSSoff set equal to zero.
2. Any PSS represented by the PSS2B model could be represented by the PSS2C model, with the time constants T12 and T13 set equal to each other and the threshold values for the output logic PPSSon and PPSSoff set equal to zero.

The tables below give the sample data for PSS2C for some specific excitation system models:

For the AC6C & ST1C model:

Notes: 

PSS settings depend not only on the excitation system model and parameters, but also on the generator model. These PSS parameters might not work properly for different generator models, even if the excitation system model remains the same.

1. The gain KS2 should be calculated as T7/(2H), where H is the total shaft inertia of all mechanically connected rotating components of the unit (MW-s/MVA).
2. The time constant T7 should be equal to Tw2.
3. The washout block with time constant Tw4 should be bypassed. Set Tw4 as necessary to bypass this block, based on the documentation of the software being used and the description in IEEE Standard 421.5-2016 (Section E.7).
4. The third lead-lag block is not used in this example. Set T10 = T11 or follow the instructions in the documentation of the software being used.
5. The fourth lead-lag block is not used in this example. Set T12 = T13 or follow the instructions in the documentation of the software being used.

For the AC7C model:

Notes: 

PSS settings depend not only on the excitation system model and parameters, but also on the generator model. These PSS parameters might not work properly for different generator models, even if the excitation system model remains the same.

1. The gain KS2 should be calculated as T7/(2H), where H is the inertia constant of the generator (MW-s/MVA).
2. The time constant T7 should be equal to Tw2.
3. The washout block with time constant Tw4 should be bypassed. Set Tw4 as necessary to bypass this block, based on the documentation of the software being used and the description in IEEE Standard 421.5-2016 (Section E.7).
4. The fourth lead-lag block is not used in this example. Set T12 = T13 or follow the instructions in the documentation of the software being used.

IEEE Power System Stabilizer Type PSS3C

PSS3C is an extended version of PSS3B from the IEEE Standard 421.5-2005. The PSS3C model shown in the figure below has dual inputs, usually generator electrical power output (VSI1 = PT) and rotor angular speed deviation (VSI2  = Δω). The signals are used to derive an equivalent mechanical power signal. By properly combining this signal with electrical power a signal proportional to accelerating power is produced.

The PSS output logic uses user-selected parameters PPSSon and PPSSoff. It also uses signal VPSS, shown in the block diagram, and the generator electrical power output PT. The output logic implements the following hysteresis to define the output signal VST:

As PSS3C is an extended version of PSS3B, any PSS represented by the PSS3B model could also be represented by the PSS3C model, by setting the PSS3C parameters associated with the PSS output logic (PPSSon and PPSSoff) to zero.

The table below gives the sample data for PSS3C:

IEEE Power System Stabilizer Type PSS4C

PSS4C is an extended version of PSS4B from the IEEE Standard 421.5-2005. The PSS4C model’s structure is based on multiple working frequency bands as shown in the figure below. Four separate bands respectively dedicated to the very low, low, intermediate, and high-frequency modes of oscillations are used in this delta-omega (speed input) PSS.

The PSS4C measures the rotor speed deviation in two different ways. The input signal Δω(L-I) feeds the very low, low, and intermediate bands while the input signal ΔωH is dedicated to the high-frequency band. The equivalent model of these two-speed transducers is shown in the figure below. Tuneable notch filters Ni(s), can be used for turbo-generators with well-tuned notch filters attenuating PSS gain at torsional mode  frequencies, generally above 10 Hz, as shown below:

Where,

ωni is the filter frequency
Bωi is the filter 3 dB bandwidth  
 

As PSS4C is an extended version of PSS4B, any PSS represented by the PSS4B model could also be represented by the PSS4C model, by setting the PSS4C parameters in order to ignore the very low-frequency band. This is easily done by setting the gain KVL equal to zero. 

The table below gives the sample data for PSS4C:
 

NOTE 1: PSS settings depend not only on the excitation system model and parameters, but also on the generator model. These PSS parameters might not work properly for different generator models, even if the excitation system model remains the same.

NOTE 2: Refer to the figure above regarding the input signals for the PSS4C model. This is a dual-input model using rotor speed and generator electrical power output as inputs.

IEEE Power System Stabilizer Type PSS5C

PSS5C represents a simplifying model of the PSS4C. Like the PSS4C, the PSS5C model represents a structure based on multiple working frequency bands as shown in the figure below, but this model uses only four gains and four central frequencies, and the ten limits associated for a total of eighteen parameters. The principal difference is the transducer for which only one input is used as shown in the figure below. Compared with the PSS4C model, this model is easier for tuning studies, but it has a limitation as it cannot represent the rate of change of electrical power (MW/minute) which affects the output of the on-site stabilizer. For studying and stability software where f ≤ 3 Hz, the notch filters could be omitted.

The table below gives the sample data for PSS5C:

NOTE 1: PSS settings depend not only on the excitation system model and parameters, but also on the generator model. These PSS parameters might not work properly for different generator models, even if the excitation system model remains the same.

NOTE 2: Refer to the figure above regarding the input signals for the PSS5C model. This is a single-input model using rotor speed as the input signal.
 

IEEE Power System Stabilizer Type PSS6C

PSS6C shown in the figure below is related to the PSS3C. The PSS6C model also has dual inputs, usually generator electrical power output (VSI1 = PT) and rotor angular speed deviation (VSI2  = Δω). The signals are used to derive an equivalent mechanical power signal. By properly combining this signal with electrical power a signal proportional to accelerating power is produced.

The PSS output logic uses user-selected parameters PPSSon and PPSSoff. It also uses signal VPSS, shown in the block diagram, and the generator electrical power output PT. The output logic implements the following hysteresis to define the output signal VST:

 
It is possible to convert parameters from a PSS3B model into a PSS6C model and vice-versa, but this is not a trivial task, as it might require solving polynomials equations of up to fourth order. For details, see IEEE Standard 421.5-2016.

The table below gives the sample data for PSS6C:

Notes: 

PSS settings depend not only on the excitation system model and parameters, but also on the generator model. These PSS parameters might not work properly for different generator models, even if the excitation system model remains the same.

1. Third block is used in this example, Ki3 = 1.
2. Fourth block is not used in this example, Ki4 = 0.

IEEE Power System Stabilizer Type PSS7C

The PSS7C model shown in the figure below is a hybrid of the PSS2C and the PSS6C. The PSS7C model has exactly the same structure of the PSS2C from the dual inputs up to the main PSS gain KS1. The phase compensation, however, is provided by a canonical state equation, similar to what is applied in the PSS6C.

The PSS output logic uses user-selected parameters PPSSon and PPSSoff. It also uses signal VPSS, shown in the block diagram, and the generator electrical power output PT. The output logic implements the following hysteresis to define the output signal VST:

 
It is possible to convert the parameters of a PSS2C into the canonical form of the PSS7C. On the other hand, it might not be possible to convert the parameters from the PSS7C back to a PSS2C, as the canonical form might result in complex poles or zeros that cannot be represented by the lead-lag blocks in the PSS2C. For details, see IEEE Standard 421.5-2016.

The table below gives the sample data for PSS7C:

Notes: 

PSS settings depend not only on the excitation system model and parameters, but also on the generator model. These PSS parameters might not work properly for different generator models, even if the excitation system model remains the same.

1. The gain KS2 should be calculated as T7/(2H), where H is the inertia constant of the generator (MW-s/MVA).
2. The time constant T7 should be equal to Tw2.
3. The washout block with time constant Tw4 should be bypassed. Set Tw4 as necessary to bypass this block, based on the documentation of the software being used.
4. Third block is used in this example, Ki3 = 1.
5. Fourth block is not used in this example, Ki4 = 0.

IEEE Discontinuous Excitation Control Type DEC1A

DEC1A discontinuous excitation control, shown in the figure below, is used to represent a scheme that boosts generator excitation to a level higher than that demanded by the voltage regulator and stabilizer immediately following a system fault. For details, see IEEE Standard 421.5-2016.

• Voltage VF logic uses user-selected parameters VTM, VTN and VOmax. It also uses the signal VT shown in the block diagram.

IF VT>VTM THEN

  VP=VOmax

ELSEIF  VT<VTN THEN

  VP=0

ELSE

  VP is unchanged (retains previous value)

ENDIF

• Signal VA is defined in the block diagram for the ST1C exciter model.
• Signal VST is the output of the PSS model.
• Switch S1 is closed when the output of the logical block is true.
• The output of the decision block is 1 if the logic operation indicated in the block is true.

IEEE Discontinuous Excitation Control Type DEC2A

DEC2A discontinuous excitation control is shown in the figure below. This system provides transient excitation boosting via an open loop control as initiated by a trigger signal generated remotely. For details, see IEEE Standard 421.5-2016.

• Signal VK is a user-defined step signal to boost excitation.
• Signal VST is the output of the PSS model.

IEEE Discontinuous Excitation Control Type DEC3A

DEC3A discontinuous excitation control is shown in the figure below. In some systems, the PSS output is disconnected from the regulator immediately following a severe fault to prevent the stabilizer from competing with action of voltage regulator during the first swing. This is accomplished in the DEC3A.

• PSS switch logic uses user-selected parameters VTmin and TDR. It also uses the signal VT shown in the block diagram.

IF VT>VTmin THEN

  Open switch S1 and keep it open for the given time TDR

ELSE

  Switch S1 is closed, VS=VST

ENDIF

• Signal VST is the output of the PSS model.