Download GENTPJ Recommendation White Paper_Approval Item

The Recommended Synchronous Generator
Model: GENTPJ
A WECC White Paper
June 14, 2016 (revised 11/17/16)
155 North 400 West, Suite 200
Salt Lake City, Utah 84103-1114
The Recommended Synchronous Generator Model: GENTPJ
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Abstract
This white paper provides a brief discussion of the synchronous generator models and recommends the
use of the GENTPJ model in lieu of all other synchronous generator models.
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Table of Contents
Introduction...................................................................................................................................... 1
The GENTPJ Model ............................................................................................................................ 1
Summary and Conclusions ................................................................................................................ 3
References ........................................................................................................................................ 3
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The Recommended Synchronous Generator Model: GENTPJ
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Introduction
This brief white-paper gives a quick review of the GENTPJ model, how it is different from the other
models, and why it is recommended for use moving forward. Routine planning and operation studies
performed in the Western Electricity Coordinating Council (WECC) use commercial power system
simulation tools that employ steady-state and dynamic models for power equipment. The most
commonly used models, for the past several decades, for simulating synchronous generators have been
the standard models in most commercial tools called GENROU (for round-rotor machine) and GENSAL
(for salient-pole machines). WECC has for well over a decade also used a model called GENTPF (for
round-rotor machines), originally developed in WECC and based on the model described in [1]. In recent
years, WECC adopted a slightly modified version of the GENTPF model, called the GENTPJ model [2] for
use in modeling salient-pole machines [3]. There is now clear evidence that the GENTPJ model is perhaps
the more suitable model not only for salient-pole hydro generators, but for all synchronous generators
[4], [5].
The GENTPJ Model
The equations, block diagram and theory behind the GENTPJ model may be found in the public
references [2] and [5]. Reference [2] is the official approved WECC model specification for the GENTPJ
model.
The essential differences between GENTPJ and the other commonly used models, GENTPF, GENROU and
GENSAL, are as follows [2], [5]:
o In GENROU and GENSAL subtransient saliency is ignored (X’’d = X’’q). This is not true in
GENTPF and GENTPJ.
o In GENROU, saturation is a simple term added to the d- and q-axis flux. In GENTPF and
GENTPJ, saturation is applied to all the inductive terms in the state-equations.
o In GENSAL, q-axis saturation is neglected. This is not true in GENTPJ and GENTPF.
o GENTPF, GENROU and GENSAL all neglect the change in saturation as a function of
machine loading. GENTPJ, however, models this effect by introducing a coefficient (Kis)
as a multiplier to the magnitude of the machine’s stator current, which then adds to the
total voltage entered into the saturation function. That is, 𝑆𝑑 = 𝑓𝑠 (𝐸𝑙 + 𝐾𝑖𝑠 𝐼) and 𝑆𝑞 =
𝑋𝑞
𝑓 (𝐸
𝑋𝑑 𝑠 𝑙
+ 𝐾𝑖𝑠 𝐼), where Sd is the saturation factor for the d-axis and Sq is the saturation
factor used in the q-axis, I is the magnitude of the stator current, El is the magnitude of
the internal machine voltage phasor, and fs is the (typically) quadratic saturation function.
Table 1 shows the comparison of the parameter list for the four models. Evidence from numerous large
and small synchronous generators for hydro (salient-pole), and steam-turbines and gas turbines (roundrotor) has shown that the GENTPJ model provides better performance in capturing both the transient
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and steady-state field current response of synchronous generators over the operating range of the
machine [3], [4], [5]. As shown in [5], due most likely to the combination of the way saturation is
modeled in GENTPJ (and GENTPF), and modeling subtransient saliency probably to a less extent, GENTPJ
(and also GENTPF) better captures the transient response of field current following an event. More
importantly, as shown in [3], [4] and [5], due to the Kis factor (i.e. modeling the change in saturation as
a function of machine loading) only the GENTPJ model can achieve reasonable match between the
simulated and measured steady-state response of the field current of the generator over the entire
operating range of the machine. The key reason why this is a significant result is that for studies were
the over-excitation limiter of a generator comes into play to limit the reactive output of a machine, if a
model other than GENTPJ is used there is a tendency to over-estimate the reactive output of the machine
at the field current limit, because of the error in the estimated steady-state value of the field current [5],
[6].
Table 1: Parameter list for synchronous generator models.
genrou
Xd
Xq
X'd
X'q
X''d
X''q = X''d
Xl
T'do
T'qo
T''do
T''qo
S10
S12
gensal
Xd
Xq
X'd
X'q
X''d
X''q = X''d
Xl
T'do
T'qo
T''do
T''qo
S10
S12
gentpf
Xd
Xq
X'd
X'q
X''d
X''q
Xl
T'do
T'qo
T''do
T''qo
S10
S12
gentpj
Xd
Xq
X'd
X'q
X''d
X''q
Xl
T'do
T'qo
T''do
T''qo
S10
S12
Kis
Note: in most commercial software tools, the models internally impose the X’’q=X’’d for GENROU and GENSAL.
To convert from GENTPF to GENTPJ is a simple process of translating the model parameters over one to
one. Experience has shown [5] that if reliable original equipment manufacturer (OEM) provided
parameters are used in the GENTPJ model, and the Kis parameter is estimated using measured V-curve
data, then a good fit of the generator’s performance, over a wide range of operating conditions, can be
achieved. In all cases, measurements should be compared with simulation to verify the model. The
simplest method to estimate Kis is to measure steady-state V-curve data on the generator and to then
plot simulated (using GENTPJ) versus measured data and thus estimate Kis in order to get a good match
across a wide range of operating conditions. If using reliable OEM data, then to go from GENROU to
GENTPJ (or GENTPF) all values translate one to one. In GENROU, however, there is no X’’q, so the X’’q
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from the OEM data sheet should be used in GENTPJ, if it is different from X’’d. In GENTPJ X’’q can be set
to equal to X’’d, if a value for X’’q is not available.
To use GENTPJ to model salient pole hydro generators, simply set X’q equal to Xq and set T’qo = 0 in
order to effect the desired result in the software tools. When using reliable OEM data again, all other
parameters translate one-to-one going from one model to the other. Again, Kis will need to be estimated
based on V-curve tests.
In the end in all cases the simulated performance of a model must be compared to measured response
to verify and validate the model being used. Models should not be converted arbitrarily.
Summary and Conclusions
It is recommended that moving forward the use of the GENROU, GENTPF and GENSAL models should be
discontinued for all synchronous generators, and the GENTPJ model should be used for all synchronous
generators because evidence shows that GENTPJ is a better model. The GENTPF and GENTPJ are
identical except for the Kis parameter. If Kis is not known, it can be initially set to zero. Where actual
on-line measured V-curve data is available, Kis should be estimated. The parameter Kis is constant
throughout the range of the generator's operation, since the change in the saturation characteristics is
nearly proportional to the loading of the machine (i.e. magnitude of stator current). Obtaining measured
on-line V-curve data may not necessarily always be a simple task for all units (e.g. nuclear units, or
peaking units, etc.) and so this should neither be mandated nor an absolute requirement, but a goal to
be achieved when possible. Equipment manufacturers should consider estimating and providing the Kis
parameter for all new generating facilities in the future.
References
[1] D. W. Olive, “Digital simulation of synchronous machine transients”, IEEE Trans. PAS, Vol PAS-87, pp
1669-1675, 1968.
[2] J. M. Undrill, “The GENTPJ model”, WECC approved model specification for the GENTPJ model,
November 19, 2007 (revised June 19, 2012). https://www.wecc.biz/Reliability/gentpj-typej-modelspecification.pdf
[3] S. Patterson, “GENTPJ Validation”, Presentation made to the WECC Modeling and Validation
Working Group, November 19, 2010. https://www.wecc.biz/Reliability/gentpj%20and%20gensal.pdf
[4] P. Pourbeik, “Generator Modeling”, Presentation made to the WECC Modeling and Validation
Working Group, March, 2016. https://www.wecc.biz/Reliability/GEN_MODEL_0316.pdf
[5] P. Pourbeik, B. Agrawal, S. Patterson, R. Rhiner, “Modeling of Synchronous Generators in Power
System Studies,” CIGRE Science and Engineering, Volume 6, October 2016, pages 21-31.
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[6] B. Agrawal and D. Kosterev, “Model Validation Studies for a Disturbance Event That Occurred on
June 14 2004 in the Western Interconnection”, Proceedings of the IEEE PES GM, 2007.
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