Comparison of Applicability Thresholds
The report written before the formation of the drafting team contained a
comparison of the estimation of the number of units in several 2013 base cases
which was summarized in the graphs below: (Refer to the report “WECC-0101
White Paper with Recommendations” for details)
Discussion during the variance drafting process raised the question the applicability
of units which are not considered “BES units”, which includes all units greater than
20 MVA, or plants with a total capacity greater than 75 MVA, which are connected to
the BES at a voltage greater than 100 kV.
After considerable effort, a comparison of applicable units for a 2013 base case base
was made, with the following results. A more rigorous determination of connection
voltage levels for each applicability class was used than the initial studies.
With the above case and one of the disturbances examined during the white paper
studies, the relative sensitivity of the affected units was explored by comparing the
simulations with the controllers of the exempted units removed from the case. The
following figure shows the results of the relative sensitivities. The effect on the
system damping correlates as a function of the combination of both the number of
control systems in the cases as well as the sum of the MVA capacity of affective units.
Analog vs. Digital Generator Controls in the WECC System
An estimation of the percentage of excitation systems and governors that are
digitally controlled was made by some detailed examination of the models in the
current WECC database. Some model structures are specifically designed to model
mechanically controlled (analog) equipment, some are designed to model digital
control systems, while others could be used to represent either a digital control
system or an analog electronic system. For the models that could be used for either
analog or digital controllers, the assignment of model parameters can provide
indication either of an analog or digital system.
This inspection of all generator models in the case led to the estimation that
approximately one third of the excitation system models in the database represent
analog control systems, and more than one half of all governor models represent
analog controls.
Based on the WECC database representation of generator control systems, the
percentage of analog systems remaining in the WECC system cannot be considered
small.
Furthermore regarding governor control systems, even when incorporating a digital
controller, a great deal of the control system will always consist of mechanical,
moving parts, which are continually wearing and possibly being adjusted. Therefore,
the justification of digitally controlled governors not requiring as frequent model
validation cannot be made as strongly as the case for excitation systems.
Generator Controller Behavior Over Time
A preliminary evaluation of the necessity to update generator models was made by
again examining the WECC model database. For generators that have existed in the
database since 2003, a comparison was made between the 2003, 2008, and 2013
database models.
Years
Common Models
Replaced
Modified
2003 - 2008
2008 - 2013
3843
4893
946
2086
856
2608
From 2003 to 2008, of the models types that were the same for each generator
between years (e.g., exst1 used in both cases), more than 22% were modified in that
time frame. From 2008 to 2013, about 36% were modified. Some of these
modifications may represent modification to the equipment, although in general for
equipment replacement, the model type will usually change, or at least be updated
by a newer model version. For the most part, these percentages can significantly
underestimate the actual model modifications made by generator owners to
represent the same equipment for the following reasons:

Many of the models that represent the same piece of equipment in the
compared cases were replaced with newer available models, e.g., exst1
replaced with esst1a. Considering only mechanical hydro governors and
salient pole machines models that have been “refreshed” to newer versions
(ieeeg3 to hygov4, gensal to gentpj) from 2008 to 2013 adds another 865
models to the modified count, increasing the total to over 50% of common
models between the cases being modified in this 5 year period.

At least 25 percent of all the models subject to the WECC validation policy in
the case have never been validated, therefore are largely unchanged.

At least 10 percent of all the models in the case are not subject to the WECC
validation policy, and are not likely to be modified.
“Drift” of Equipment Behavior Over Time
It is a well-known phenomenon with mechanical or analog electronic equipment
that the dynamic characteristics of a control system can change over time due to a
number of possible reasons such as wear, vibration, material aging, or component
failure. With this fact in mind, generator equipment owners typically test,
recalibrate, and retune these control systems as necessary, usually more frequently
than every ten years.
It is generally believed that the use of digitally controlled equipment drastically
reduces the variation in dynamic performance of generator control systems as they
age. However, experience has shown that a high degree of faith in digital generator
controllers is not entirely warranted. The following figure shows the behavior of a
digital excitation system as it was commissioned, as well as the response to the
same test input several years later. A malfunctioning firing circuit control module
had completely changed the behavior of the voltage regulator without giving any
indication to plant personnel that there was a problem. Such a problem can persist
indefinitely, and impacts the control system more severely than most analog system
component failures.
The following figures show frequency response measurements of a small sampling
of units over a period of time. Most of the drafting team members are quite
experienced with seeing different response results over a period of time, however, it
was generally acknowledged that normal procedures were that if significant
differences are noticed from previous test data, the controllers are retuned to match
the previous response. The data indicating the altered behavior is typically not
retained in favor of the record of the final settings. It was not possible to conduct a
more thorough search and comparison of archived data at this time, but further
efforts will be made. The results are typical only in that any differences from none to
very significant can be expected.
The differences in response between the two measurements in each figure can
range from none to several percent (in dB or degrees of phase lag) depending on the
frequency or frequency range of interest, making the dynamic performance error
variable depending on what the event the model is simulating.
As a demonstration of the sensitivity to this type of parameter variance, the case and
event used previously was altered by randomly modifying either an exciter or
stabilizer model parameter by alternating +/- 10 percent for a total of about 11
percent of all generator units, and then for a total of about 17 percent of all
generator models.
The Net Effect of Cumulative Drift
While it may be possible that altering two different elements in the same control
system, or two different elements between two different systems may result in one
change compensating for the other, effectively cancelling any net effects of the
modifications, in general this is not true. Basic error analysis dictates that all error
in variables that are combined arithmetically, whether positive or negative, must
accumulate positively [1]. That is, all error accumulates and adds into the overall
error margin of the net results. Therefore, the overall drift of variables cannot be
assumed to all cancel each other out. This can be easily demonstrated by again using
the same example as before, but comparing the result to a case that was modified as
above, and then the result of the same simulation, but with changes made oppositely
to the first case, i.e., those variables that were changed +10 percent changed -10
percent, and vice versa. Clearly the effects do not cancel.
The Value of a Lower Validation Threshold
The fact that accurate simulation results are valuable is generally accepted, however
the exact value is never likely to be expressed in dollars, since the benefits largely
consist of avoided costs, many unknown. By experience, the cost of validating a set
of generator models is not too difficult to estimate. However, most of the effort in
validating models is expended during normal equipment commissioning,
maintenance, and in developing models, which is required. If the real value is to be
determined, other significant costs which must be taken into account such as the
impact of erroneous database models which increase engineering costs. Errors in
the database must systematically be dealt with when using these models for studies,
which can easily dominate the total amount of time and effort an engineer spends
on a particular project. The cost of the lack of proper model development and
validation will paid eventually by those that use the models.
[1] “An Introduction to Error Analysis “, John R. Taylor, University Science Books,
1982.