ERCOT Dynamic Model Review Platform
Development
Yunzhi Cheng2, Shun-Hsien (Fred) Huang1, Yang Zhang2, and Jose Conto2
1
Member, IEEE 2Senior Member, IEEE
Abstract—This paper presents a dynamic model review platform
developed by ERCOT to evaluate the dynamic performance of
the dynamic model associated with every type of generation.
Based on a single power plant – infinity generator test system,
the platform can be utilized to test the voltage and frequency
responses for the purpose of dynamic model review and grid
code compliance studies. It can also be utilized to assess the
short circuit ratio (SCR) limit that the test generation resource
can survive in dynamic simulation under a large disturbance
and a small disturbance, respectively. ERCOT is currently
utilizing the platform as the screening tool to assess the dynamic
performance associated with generation resources with
proprietary dynamic models.
Index Terms—Dynamic model, voltage response, frequency
response, short circuit ratio
I.
INTRODUCTION
In the past decade, renewable power generation, especially
wind and solar, has exponentially grown across the world.
Numerous power electronics devices with various novel
control methods have been installed in power systems. At the
same time frame, power industry has been in the transition
toward the electricity market deregulation. In deregulated
regions, transmission & distribution (T&D) sector is split from
the traditional vertically integrated utilities which were
composed of generation, T&D, and load serving.
Consequently, transmission systems are increasingly exposed
to the operating limits, especially stability limits [1].
Currently, dynamic models play a critical role of determining
the System Operating Limits (SOL) and Interconnection
Reliability Operating Limits (IROL). Even more attention has
been paid to model testing and calibration as North American
Electric Reliability Corporation (NERC) proposed MOD-26,
MOD-27, and MOD-33 standards that are mandatory to
generator owners, transmission owners, and reliability
coordinators [2].
In ERCOT, more than 19,000 MW wind generation has
been built, and solar generation is currently booming at an
unprecedented rate. EROCT wind generation has set a record
of 54% penetration on October 27, 2017 [3]. Currently, there
Yunzhi Cheng (yunzhi.cheng@ercot.com), S-H Huang, Yang Zhang and
Jose Conto are with Electric Reliability Council of Texas (ERCOT Inc.),
Taylor, TX 76574, USA
is no requirement of using generic models for renewable
generation modeling in ERCOT. Instead, most of ERCOT
renewable generation resources are modeled with proprietary
user-defined dynamic models. As a result, the number of
proprietary dynamic generator models in ERCOT is more than
double of that of standard generator models although the
renewable generation capability accounts for less than 25% of
the total generation capacity. It is a challenge for ERCOT to
maintain so many kinds of dynamic models and build a whole
system dynamic dataset. ERCOT is planning to review the
dynamic model for each individual generation resource prior
to integrating them into the system dynamic dataset. To
authors’ knowledge, Siemens PTI PSSE’s ERUN (exciter
simulation test) and GRUN (governor simulation test) can
only be applied to synchronous generators with standard
models [4]. There is no commercial software available to test
the proprietary dynamic models. Many researchers have
worked on model calibration using the field recording data to
adjust the dynamic parameters such that the dynamic
simulation could align well with the recordings [5] [6] [7]. A
dynamic model review platform not only can be served as the
simulation tool for model calibration, but also can be utilized
as a screening tool to flag the suspect models to trigger the
model calibration.
Another challenge associated with renewable generation
integration in ERCOT is the grid strength issue. Most of
renewable generation resources in ERCOT are located at
remote areas which are distant from ERCOT load centers.
Currently, there are more than 3,000 MW wind generation
resources installed and more wind generation resources are
expected to come online in ERCOT Panhandle area. Without
synchronous generators or loads, the Panhandle is located at
the edge of the ERCOT system (300 miles from the ERCOT
load centers) so the Panhandle grid is relatively weak. ERCOT
proposed Weighted Short Circuit Ratio (WSCR) to measure
the system strength and dispatch the Panhandle wind
generation so that the real time WSCR value is no lower than
the WSCR limit to maintain the system stability [8]. Testing
the SCR limit for each individual model can help system
operators and planners to address the reliability concerns
associated with grid strength and manufacturers to improve
the control performance based on the particular SCR range as
observed at the specific project location.
To evaluate the dynamic performance associated with all
generation resources, irrespective of the generation type or
model type, ERCOT developed a dynamic model review
platform comprised of the following functions:

No-disturbance flat start test

Voltage response test

Frequency response test

SCR limit test
While the platform can be utilized to facilitate grid code
compliance studies, NERC compliance studies and any other
voltage & frequency tests that users specify such as helping
the manufacturer to optimize the control design, ERCOT is
currently utilizing the platform as the screening tool to assess
the dynamic performance associated with the generation
resources with proprietary dynamic models. For any model
with suspect response in the tests, ERCOT will communicate
with the generation resource so the model can be re-visited,
tuned and updated.
The subsequent sections demonstrate the platform design
and illustrate selected review experiences with wind and solar
dynamic models that were provided to ERCOT.
II.
DYNAMIC MODEL REVIEW PLATFORM
The dynamic model review platform is based on single
power plant – infinity bus system to test each dynamic model
individually. The power plant model includes every element in
the power plant such as generators, pad-mount (PM)
transformers, station transformers, medium voltage (MV)
collection system, shunt devices, and tie line connecting the
power plant to POI (Point of Interconnection) bus. Except for
the SCR limit test, the line connecting the POI to the infinity
bus is modeled as zero impedance branch.
Fig. 1 demonstrates the scheme of the platform. The
platform is developed in python with PSSE API functions.
Users can build the test generation resource model in
steady state and dynamics to evaluate the dynamic response of
the actual generation resource. Alternatively, users can also
take advantage of a prepared 200MW renewable resource
power flow template case and plug into the dynamic model
with proper parameters associated with the actual generation
resource and then evaluate the response trend under test. In the
latter case, the test results cannot be utilized as the grid code
compliance study. However, the response trend can be utilized
to check if the model is usable and has a reasonable response.
Fig. 1 Dynamic Model Review Platform Scheme
A. No-disturbance Flat Start Test
A 10-second no-disturbance flat start (FS) test is firstly
applied in the model review platform to identify if there is
any initialization error or numerical stability issue which is
mainly related to integration errors. Once the model passes
the FS test, the following tests can be performed.
B. Voltage & Frequency Response Test
A time-domain voltage & frequency table (Table I as an
example) is designed to model the point of interconnection
(POI) profile for voltage and frequency response tests. The
platform converts the table to dynamic parameters associated
with the PSSE playback model (“PLBFVU1”) which is
utilized to accurately controlling the POI voltage during the
dynamic simulation tests.
The voltage & frequency response tests include the
following dynamic response tests:

Generic ERUN test: to test the reactive power
response of the dynamic model under the 0.1pu
voltage deviation at the POI bus. To fulfill the grid
code requirements, the reactive power is supposed to
increase/decrease when the POI bus voltage goes
down/up.

Generic GRUN test: to test the active power response
of the dynamic model under the 0.05Hz frequency
deviation at the POI bus. To fulfill the grid code
requirements, the active power is supposed to
decrease when the POI bus frequency goes up.

HVRT test: to test if the Intermittent Renewable
Resources (IRRs) can remain interconnected to the
system during the following high-voltage conditions
as illustrated in Fig. 3. The generation resource shall
not cease providing real or reactive power except to
the extent needed to provide frequency support or aid
in voltage recovery [9].

LVRT test: similar to HVRT test, the low voltage
profile under LVRT test is illustrated in Fig 3.

Multi-LVRT test: every two seconds, POI bus is
subjected to a six-cycle (0.1 second) three phase fault.
Ten (10) times of such consecutive faults are
simulated in the test. The purpose of the test is to
identify the limits of time duration or times associated
with the generator modeled LVRT capability. MultiLVRT issue was brought to industry’s attention since
a large amount of wind generation significantly
reduced power output reacting to multiple voltage
dips in South Australia (SA) system and caused
transmission line overload and finally blackout
occurred in SA on September 28, 2016 [10].

Any other voltage & frequency response: Users can
create a voltage & frequency profile in time-domain to
test the model response. For example, users can create
a voltage & frequency profile derived from the
recording data in a real disturbance event to run the
model validation such as NERC MOD-26, MOD-27
and MOD-33 analysis.
Where:
X: reactance of the POI Bus – Infinity Bus line (pu)
SCR: short-circuit ratio number
P: active power output of the power plant under test (pu)
Prior to the SCR limit test, the initial power flow solution
is based on the zero impedance line model in FS test which
means the infinity bus voltage and angle are the same as the
POI bus. However, the infinity bus voltage and angle will be
greatly deviated from POI bus when SCR is very low and
sometimes the platform experiences a power flow
convergence issue with the Newton-Raphson method under
extremely low SCR. To overcome the convergence issue, the
infinity bus voltage and angle are set up based on:
∗
sin
∗
∗
∗
Where:
Fig. 2 System Configuration for Voltage and Frequency Test
: Infinity bus voltage magnitude (pu)
: Infinity bus voltage angle (radian)
P: active power output of the power plant (pu)
Q: reactive power output of the power plant (pu)
VPOI: initial POI bus voltage (pu)
X: reactance of the POI Bus – Infinity Bus line (pu)
With the aforementioned setup, the power flow under any
SCR value can reach to convergence within one step.
Fig. 3 ERCOT Voltage Ride-Through Boundaries for IRRs
Table I. Voltage & Frequency Profile table for HVRT test
Time (sec)
5
5.2
5.2
5.5
5.5
6
6
25
Voltage (pu)
1.2
1.2
1.175
1.175
1.15
1.15
1.1
1.1
Frequency (Hz)
60
60
60
60
60
60
60
60
C. SCR Limit Test
Instead of using zero impedance line, the platform set up
the reactance (in per unit) associated with the line of POI Bus
– Infinity Bus:
∗
Fig. 4 System Configuration for SCR limit Test
III.
MODEL REVIEW EXPERIENCES
A. HVRT test
As shown in Fig. 5 and 6, a solar generation model (Model
A_01) initially failed in the HVRT test since the generator
ceased the active power during the HVRT test, which is not
allowed based on the ERCOT protocol. Based on the
platform’s feedback, the manufacturer tuned the control
parameters and the updated model (Model A_02) passed the
HVRT test.
Another HVRT test associated with a wind generation
model (Model B_01) indicates the reactive power increases
when the POI voltage is very high during HVRT test, as
shown in Fig. 7. It is unreasonable and against the voltage
control purpose. Based on the platform’s feedback, the
manufacturer tuned the control parameters and the reactive
power response with the updated model (Model B_02) is
reasonable.
1.225
in 20 seconds. As shown in Fig. 8 and 9, the generator active
power successfully recovered even after 10 times of 3 Phase
bolted fault in 20 seconds. However, it may indicate that the
model has not included the limit associated with the
generator’s real VRT capability or the actual protection
settings with the generator. ERCOT will confirm it with the
manufacturer.
1.1
1.2
1
1.175
0.9
1.15
0.8
1.125
0.7
0.6
1.1
0.5
1.075
0.4
1.05
0.3
1.025
0.2
0.1
1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0
Time (seconds)
0
5
10
15
20
25
30
35
Time (seconds)
Fig. 5 POI voltage profile for HVT test (Model A and B)
Fig. 8 POI Voltage Profile for Multi-LVRT test (Model C)
2.5
2.25
2.5
2
2.25
1.75
2
1.5
1.75
1.5
1.25
1.25
1
1
0.75
0.75
0.5
0.5
0.25
0.25
0
0
-0.25
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
-0.25
0
Time (seconds)
5
10
15
20
25
30
35
Time (seconds)
Fig. 9 Active Power Response during Multi-LVRT test (Model C)
Fig. 6 Active Power Response during HVT test (Model A)
C. SCR test
A wind generation model (Model D) survived with a large
disturbance (3 Phase bolted fault) under extreme SCR value
(0.7). ERCOT will investigate it in detailed PSCAD
simulation.
0.5
0.4
0.3
0.2
0.1
1.3
0
1.2
1.1
-0.1
1
-0.2
0.9
0.8
-0.3
0.7
-0.4
0.6
-0.5
0.5
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Time (seconds)
0.4
0.3
Fig. 7 Reactive Power Response during HVT test (Model B)
0.2
0.1
0
0
1
2
3
4
5
Time (seconds)
B. Multi-LVRT test
A wind generation model (Model C) passed the MultiLVRT test with 10 times of 3 Phase bolted fault at the POI bus
Fig. 10 Voltage Response during SCR test (SCR = 0.7, Model D)
Channel Plot
em_Incident_Reports/2017/Integrated-Final-Report-SA-Black-System28-September-2016.pdf, Mar. 2017
3
2.5
2
1.5
1
0.5
0
-0.5
-1
-1.5
0
1
2
3
4
5
Time (seconds)
Fig. 11 Power Response during SCR test (SCR = 0.7, Model D)
IV.
CONCLUSIONS
ERCOT developed a dynamic model review platform to
evaluate the dynamic performance for all the generation
resources. The tests include voltage and frequency response
tests and SCR limit tests. The test results can be utilized for
grid code compliance studies, NERC compliance studies, and
help manufacturers to optimize the control behavior. ERCOT
is currently utilizing the platform as the screening tool to
assess the dynamic performance associated with the
generation resources with proprietary dynamic models.
In the future, ERCOT is considering to add optimization
module to the platform so it could directly facilitate NERC
MOD-33 studies.
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