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Interconnection System Impact Study
Final Report
Generator Interconnection Request No. TI-15-0832
150 MW Solar Photovoltaic (PV) Energy Generating Facility
In Hidalgo County, New Mexico
Prepared By:
Jeffery L. Ellis
Utility System Efficiencies, Inc.
Reviewed By:
Mark Stout
Tri-State Generation and Transmission Association, Inc.
February 26, 2016
DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITY
THIS DOCUMENT WAS PREPARED FOR TRI-STATE GENERATION AND TRANSMISSION ASSOCIATION, INC., IN ITS
CAPACITY AS TRANSMISSION PROVIDER (TP), IN RESPONSE TO A LARGE GENERATOR INTERCONNECTION
REQUEST. NEITHER TP, NOR ANY PERSON ACTING ON BEHALF OF TP: (A) MAKES ANY REPRESENTATION OR
WARRANTY, EXPRESS OR IMPLIED, WITH RESPECT TO THE USE OF ANY INFORMATION, METHOD, PROCESS,
CONCLUSION, OR RESULT INCLUDING FITNESS FOR A PARTICULAR PURPOSE; OR (B) ASSUMES RESPONSIBILITY
FOR ANY DAMAGES OR OTHER LIABILITY, INCLUDING ANY CONSEQUENTIAL DAMAGES, RESULTING FROM USE
OF THIS DOCUMENT OR ANY INFORMATION CONTAINED HEREIN.
System Impact Study for TI-15-0832
Tri-State Generation and Transmission Association, Inc.
Page 2 of 26
TABLE OF CONTENTS
Page No.
1.0 EXECUTIVE SUMMARY ...............................................................................................3
2.0 BACKGROUND AND SCOPE ........................................................................................6
3.0 GF MODELING DATA ....................................................................................................8
4.0 STEADY-STATE POWER FLOW ANALYSIS ............................................................9 4.1 Criteria and Assumptions .......................................................................................9
4.2. Voltage Regulation and Reactive Power Criteria .................................................10
4.3 Steady-State Power Flow Results .........................................................................11
5.0 DYNAMIC STABILITY ANALYSIS ............................................................................14 5.1 Criteria and Assumptions .....................................................................................14
5.2 Base Case Model Assumptions ............................................................................16
5.3 Methodology ........................................................................................................17
5.4 Results ..................................................................................................................17
6.0 SHORT-CIRCUIT ANALYSIS ......................................................................................19 6.1 Assumptions and Methodology ............................................................................19
6.2 Results ..................................................................................................................19
7.0 SCOPE, COST AND SCHEDULE .................................................................................22
NOTE: Appendices are Tri-State Confidential, are available only to the IC and Affected
Systems upon request, and are not for posting on OASIS.
Appendix A: Steady State Power Flow Study – List of N-1 Contingencies
Appendix B: Steady State Power Flow Study – Plots
Appendix C: Transient Stability Switching Sequences
Appendix D: Transient Stability Plots
Appendix E: Generation Dispatch Summary Listing (BAs 10, 70, 73)
System Impact Study for TI-15-0832
Tri-State Generation and Transmission Association, Inc.
Page 3 of 26
1.0 EXECUTIVE SUMMARY
This System Impact Study (SIS) is for Generator Interconnection Request No. TI-15-0832, a
proposed 150 MW solar photovoltaic Generating Facility (GF) located in Hidalgo County,
New Mexico. This SIS was conducted for the Transmission Provider, Tri-State Generation and
Transmission Association, Inc., (Tri-State) in accordance with its Generator Interconnection
Procedures, and includes steady-state power flow, dynamic stability, short-circuit, cost and
schedule analyses for interconnection of the Project as either a Network Resource or a Non-
Network Resource.
In its application, the Interconnecting Customer proposed an energization in-service date of
September 2017 and a commercial operation date of September 2017. Cost and schedule
estimates are as provided by Tri-State, and are good faith estimates only (typically +/-30%
accuracy). Higher accuracy (+/- 20%) will be provided as part of an Interconnection Facilities
Study.
The proposed Project consists of 180 SMA 800CP 0.85 MW solar inverters with one 34.5-115
kV, 100/133/167 MVA transformer at the main solar energy Generating Facility. The Facility is
located approximately 1 mile west of the Pyramid 115 kV Substation which is the Point of
Interconnection (POI) to the Transmission Provider’s system (see Figures 1 and 3 for reference).
In addition, a sensitivity to determine the maximum generation output at this location was
simulated.
Steady-state power flow results:
Single contingency analysis was completed using 2017 heavy summer load and dispatch
conditions with and without the planned Project. To stress the system in the area local to
the proposed project, generation at Lordsburg, Pyramid and Afton was modeled at full
output. Study results indicate that loss of the Hidalgo - Pyramid 115kV line results in the
Hidalgo - Pyramid_T - Pyramid 115 kV line loading to 138.8% of its emergency thermal
limit (215 MVA) in the post-Project case. Therefore, the maximum Project output as a
Network Resource is 64 MW.
If loading on the Hidalgo - Pyramid 115 kV lines is mitigated, the next limiting element is
the Hidalgo 345/115 kV transformer for loss of the parallel Hidalgo 345/115 kV
transformer. Reducing the Project output to 125 MW will alleviate this thermal overload.
Under normal system conditions (all lines and transformers in-service) the Project
generation can be added with no thermal or voltage violations and may use the existing
firm or non-firm capacity of the Transmission System on an as available basis as a Non-
Network Resource.
Reactive power / voltage regulation:
The collector system model provided by the Interconnecting Customer shows that this GF
can meet Tri-State's 0.95 p.f. lag (producing) criteria at the POI most of the time, however
is deficient for Project output greater than 112.5 MW. The study determined that the GF
cannot meet Tri-State's 0.95 p.f. lead (absorbing) criteria at the POI, being deficient at
output levels below 75 MW. See Table 5. Therefore, supplemental reactive power
equipment of approximately 6 MVAR of switched shunt reactors will be required at the full
150 MW GF output to meet the net 0.95 p.f. lag (producing VAR) criteria at the POI, and
the same amount of reactive power equipment will be required on the 34.5 kV bus to offset
the collector system VARs and meet Tri-State’s VAR neutral requirement.
System Impact Study for TI-15-0832
Tri-State Generation and Transmission Association, Inc.
Page 4 of 26
Transient stability results:
The transient stability analysis studied the Project as both an Energy Resource and a
Network Resource.
As an Energy Resource, the following local generation dispatch was modeled:
Pyramid generation: Unit 1 - 40 MW, Unit 2 - 35 MW, Units 3 and 4 - Off
Project generation: 150 MW equivalent unit
The above totals 225 MW and is the maximum generation that can be interconnected to
the Pyramid 115kV substation. It is limited by the rating of the Hidalgo - Pyramid No.2
115kV line for loss of the Hidalgo - Pyramid No.1 115kV line.
As a Network Resource, the case modeled a Hidalgo - Pyramid No.3 115kV transmission
line (16 miles) and the following local generation dispatch:
Pyramid generation: Units 1, 2, 3 and 4 - 40 MW each
Project generation: 125 MW equivalent unit
The above totals 285 MW and is the maximum generation that can be interconnected to
the Pyramid 115kV substation with the Network Upgrade noted above. It is limited by
the Hidalgo 345/115kV No.2 transformer for loss of the Hidalgo 345/115kV No.1
transformer.
Transient stability results were similar for the Project studied as an Energy Resource or a
Network Resource. Results from the study follow:
1. With the SMA photovoltaic inverters, the Project did not trip during any
contingencies and had acceptable voltage levels.
2. The local area system experienced transient voltage levels at or above 1.15 per unit
for loss of the Hidalgo - Pyramid 115kV line. As a result, the Project should be
designed to compensate for high transient voltage levels to ensure that the inverters
do not trip off-line due to overvoltage conditions during local area contingency
events.
3. Pyramid generators experienced rotor angle instability for loss of the Project’s one
mile transmission line when line clearing was greater than 10 cycles. A longer
clearing time will result in the Pyramid units tripping off-line due to rotor angle
instability. As a result, it is important that the near and far end clearing time be less
than 10 cycles.
4. Acceptable damping and voltage recovery was observed.
Short - Circuit analysis:
Results indicate that the GF increases the fault duty by approximately 1564 Amperes at the
Pyramid 115 kV POI bus. The resultant total fault currents are within Tri-State’s substation
planned equipment ratings.
System Impact Study for TI-15-0832
Tri-State Generation and Transmission Association, Inc.
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The estimated cost for interconnecting the proposed Project at the 115 kV POI is as follows
(refer to Figure 3). Note that this estimate does not include costs for Network Upgrades beyond
the POI.
Network Upgrade Costs (Reimbursable): $ 0.7 M
Interconnection Facilities Costs (Non-Reimbursable): $ 1.0 M
TOTAL Cost for Interconnection: $ 1.7 M
The in-service date for this GF will depend on construction of the Interconnection Facilities and
Network Upgrades. The Interconnection Facilities will require a minimum of 24 months after
execution of a Generator Interconnection Agreement or Engineering and Procurement contract.
Additional Network Upgrades (e.g., a third Hidalgo - Pyramid 115kV transmission line) will
require a minimum of 48 months.
NOTE: Pursuant to Section 3.2.2.4 of the Tri-State’s GIP, “Interconnection Service does not
convey the right to deliver electricity to any customer or point of delivery. In order for an
Interconnection Customer to obtain the right to deliver or inject energy beyond the Generating
Facility Point of Interconnection or to improve its ability to do so, transmission service must be
obtained pursuant to the provisions of Transmission Provider’s Tariff by either Interconnection
Customer or the purchaser(s) of the output of the Generating facility.” See Tri-State’s Open
Access Same Time Information System (OASIS) web site for information regarding requests for
transmission service, related requirements and contact information.
System Impact Study for TI-15-0832
Tri-State Generation and Transmission Association, Inc.
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2.0 BACKGROUND AND SCOPE
On August 31, 2015 the Interconnecting Customer submitted a Generator Interconnection
Request for a 150 MW solar energy GF to be located approximately 1 mile west of the existing
Pyramid 115 kV Substation. An Interconnection System Impact Study Agreement was
executed on October 7, 2015. The inverter model data used in this study is that which was
provided by the Customer in its Generator Interconnection Request.
This System Impact Study was prepared in accordance with Tri-State’s Generator
Interconnection Procedures and relevant FERC, NERC, WECC and Tri-State guidelines. The
objectives are: 1) to evaluate the steady state performance of the system with the proposed
project, 2) identify Interconnection Facilities and Network Upgrades, 3) check the GF’s ability
to meet Tri-State’s voltage regulation and reactive power criteria, 4) assess the dynamic
performance of the transmission system under specified stability contingencies, 5) perform a
basic short circuit analysis to provide the estimated maximum (N-0) and minimum (N-1) short
circuit currents, and 6) provide a preliminary estimate of the costs and schedule for all
necessary Interconnection Facilities and Network Upgrades, subject to refinement in a
Facilities Study.
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Tri-State Generation and Transmission Association, Inc.
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Figure 1: Area Map One-Line Diagram Of Study Area And Location of GF
TI-15-0832
150 MW
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Tri-State Generation and Transmission Association, Inc.
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3.0 GF MODELING DATA
The model consists of a 150 MW equivalent solar inverter with one 34.5-115 kV transformer to
be located approximately 1 mile west of the Pyramid 115 kV Substation. Model data is based
upon information provided by the Interconnecting Customer (IC). The IC must provide actual
data and confirm actual reactive power operating capabilities prior to interconnecting this
project, and ultimately prior to the IC’s GF being deemed by Tri-State as suitable for commercial
operation.
Generator Data: The study modeled an equivalent generator with a Pmax of 150 MW and
reactive capability of 0.95 lag and 0.95 lead, 49.3 and -49.3 MVAR, respectively.
Table 1: Generator Data for Steady-State Power Flow Analyses
Unit Description
Pmax Name plate rating (lumped equivalent
generator model) 150 MW
Qmin, Qmax Reactive capability 0.95 lag to 0.95 lead
Et Terminal voltage 0.357 kV
RSORCE Synchronous resistance 0.0000 p.u.
XSORCE Synchronous reactance 9999 p.u.
Table 2: Inverter Trip Settings
CON Voltage Limit (per unit) Delay (sec)
J+28 1.2 0.16
J+29 1.2 0.16
J+30 1.1 1
J+31 0.88 2
J+32 0.5 0.16
J+33 0.5 0.16
34.5 kV Collector System: The medium voltage collector system was modeled with data
provided by the IC. The solar inverters interconnect to the POI via one 34.5-115 kV transformer
and an equivalent feeder circuit. In addition, a 12 MVAR switchable shunt capacitor was
modeled as described in the Interconnection Request.
Main GF Substation Transformer: The substation transformer was modeled with ratings of
100/133/167 MVA and a voltage ratio of 34.5 kV (gnd-wye) - 115 kV (gnd-wye). The
transformer impedance is 8.5% on the 100 MVA base FA rating with X/R of 31.1.
System Impact Study for TI-15-0832
Tri-State Generation and Transmission Association, Inc.
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4.0 STEADY-STATE POWER FLOW ANALYSIS
The Customer requested that the Project be studied as a Network Resource and as a Non-
Network Resource.
Network Resource Interconnection integrates a Generating Facility with the Transmission
Provider’s Transmission System in a manner comparable to the way the Transmission Provider
integrates its own generating facilities to serve its native load customers. Therefore, Network
Resources in the local area are studied at full output to determine if the aggregate of the existing
and proposed generation can be delivered to load consistent with the Transmission Provider’s
reliability criteria and procedures.
Non-Network Resource Interconnection delivers the Project output to load using the existing
firm or non-firm capacity of the Transmission System on an as available basis. Studies
interconnecting a Generating Facility as a Non-Network Resource identify the required Network
Upgrades at full project output and identify the maximum output of the Generating Facility
without Network Upgrades.
4.1 Criteria and Assumptions
Siemens-PTI PSS/E version 33.5.0 software was used for performing the steady-state power
flow analysis, with the following study criteria:
1. Tri-State’s GIP 2017 HS (PSS/E-v33) base case was developed from WECC approved
seed cases (17HS1A_v33, for 2013 CCPG Compliance Study), with updates from the
latest loads and resources data, topology (line and transformer ratings, planned and
budgeted projects, etc.), and updates from Affected Systems. This GIP base case was
further updated by Tri-State for this SIS to reflect generation re-dispatching to further
stress the local transmission system. The 2017 HS case was analyzed with and without
the new GF Project.
2. The output from TI-14-0832 was accommodated by displacing generation resources to
the west in the PG&E system.
3. The GF was modeled according to data provided by the IC. The Project was adequately
represented both for assessing power flow and dynamic performance.
4. The following local area generation was modeled at full output: Lordsburg generation
80MW, Pyramid generation 160MW and the Alta Luna (TI-15-0227) generation
25MW.
5. Power flow (N-0) solution parameters were as follows: Transformer LTC Taps –
stepping; Area Interchange Control – tie lines and loads; Phase Shifters and DC Taps –
adjusting; and Switched Shunts - enabled.
6. Power flow contingencies (N-1) utilized the following solution settings: Transformer
LTC Taps – locked taps; Area Interchange Control – disabled; Phase Shifters and DC
Taps – non-adjusting; and Switched Shunts – locked all. (Not allowing these voltage
regulating solution parameters to adjust provides worst case results.)
7. All buses, lines and transformers with nominal voltages greater than or equal to 69 kV
in the Tri-State and surrounding areas were monitored in all study cases for N-0 and
N-1 system conditions.
System Impact Study for TI-15-0832
Tri-State Generation and Transmission Association, Inc.
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8. Nearby study areas (PNM, EPE and Tri-State) were investigated using the same
overload criteria. Any thermal loading greater than 95% of the branch rating with a
thermal overload increase of 2% or more were documented.
9. This analysis assumes that the GF controls the high voltage bus at the POI and should
not negatively impact any controlled voltage buses on the transmission system.
10. Post-contingency power transfer capability is subject to voltage constraints as well as
equipment ratings. The Project was tested against NERC/WECC reliability criteria and
additions/exceptions are as listed in the following table.
Table 3: Voltage Criteria
Tri - State Voltage Criteria for Steady State Power Flow Analysis
Conditions Operating
Voltages Delta-V Areas
Normal N-0 0.95 - 1.05 All
Contingency N-1 0.90 - 1.10 7% Northeastern New Mexico
Contingency N-1 0.90 - 1.10 7% Southern New Mexico
Contingency N-1 0.90 - 1.10 6% Other buses in PNM area
Contingency N-1 0.90 - 1.10 7% Western Colorado
Contingency N-1 0.90 - 1.10 7% Southern Colorado
Contingency N-1 0.90 - 1.10 6% Other Tri-State areas
4.2. Voltage Regulation and Reactive Power Criteria
1. The GF must be capable of either producing or absorbing VAR as measured at the high
voltage POI bus at a 0.95 power factor (p.f.), across the range of near 0% to 100% of
facility MW rating, as calculated on the basis of nominal POI voltage (1.0 p.u. V).
2. The GF may be required to produce VAR from 0.90 p.u. V to 1.04 p.u. V at the POI. In
this range the GF helps to support or raise the POI bus voltage.
3. The GF may be required to absorb VAR from 1.02 p.u. V to 1.10 p.u. V at the POI. In
this range the GF helps to reduce the POI bus voltage.
4. The GF may be required to either produce VAR or absorb VAR from 1.02 p.u. V to
1.04 p.u. V at the POI, with typical target regulating voltage being 1.03 p.u. V.
5. The GF may utilize switched capacitors or reactors as long as the individual step size
results in a step-change voltage of less than 3% at the POI operating bus voltage. This
step change voltage magnitude shall be calculated based on the minimum system (N-1)
short circuit POI bus MVA level as supplied by Tri-State.
System Impact Study for TI-15-0832
Tri-State Generation and Transmission Association, Inc.
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The GF is required to supply a portion of the VAR on a continuously adjustable or
dynamic basis. The amount of continuously adjustable VAR shall be equivalent to a
minimum of 0.95 p.f. produced or absorbed at the invertor collector system medium
voltage bus, across the full range (0 to 100%) of rated MW output. The remaining
VARs required to meet the 0.95 p.f. net criteria at the high voltage POI bus may be
achieved with switched reactive devices.
6. When the GF is not producing any real power (near 0 MW), the VAR exchange at the
POI should be near 0 MVAR, i.e., VAR neutral.
4.3 Steady-State Power Flow Results
1. N-0 (System Intact, Category A) Study Results:
The proposed Project generation can be added with no thermal or voltage violations
with all lines and transformers in-service.
2. N-1 (Single Contingency Category B) Study Results:
Study results for N-1 studies of the 2017 HS case identify that there are thermal
violations with the proposed Project.
Loss of the Hidalgo - Pyramid 115kV line results in the Hidalgo - Pyramid_T -
Pyramid 115 kV line loading to 138.8% of its emergency thermal limit (215 MVA)
in the post-Project case. Therefore, the maximum Project output as a Network
Resource is 64 MW.
If loading on the Hidalgo - Pyramid 115 kV lines is mitigated, the next limiting
element is the Hidalgo 345/115 kV transformer for loss of the parallel Hidalgo
345/115 kV transformer. Reducing the Project output to 125 MW will alleviate this
thermal overload.
Table 4 identifies thermal overloads in the pre-Project base case for two (2)
elements. These elements are being addressed by PNM, and as a result, are not
being identified as limiting elements here. However, it can be seen from the post-
Project analysis that the Project does increase flow on these elements.
3. Steady-state voltage violations:
With Tri-State’s operating voltage criteria range of 0.90 p.u. to 1.10 p.u., under
single contingency outage conditions there are no voltage violations with the GF at
full output.
4. Steady-state contingency voltage deviation:
Each Balancing Authority Area’s V requirement was applied as per Table 3.
There were no V violations at any of the monitored buses.
5. Reactive power required at the POI:
At full 150 MW output, the VAR capability required at the POI ranges from 49.3
MVAR produced (0.95 p.f. lag) to 49.3 MVAR absorbed (0.95 p.f. lead). This is
the net MVAR to be produced or absorbed by the GF, depending upon the
System Impact Study for TI-15-0832
Tri-State Generation and Transmission Association, Inc.
Page 12 of 26
applicable range of voltage conditions at the POI. The 12 MVAR switched shunt
capacitor was in-service when producing reactive power.
The collector system model provided by the Interconnecting Customer shows that
this GF can meet Tri-State's 0.95 p.f. lag (producing) criteria most of the time but is
deficient for Project output greater than 112.5 MW. The study also determined that
the GF cannot meet Tri-State's 0.95 p.f. lead (absorbing) criteria at the POI, being
deficient at output levels below 75 MW. See Table 5.
Therefore, supplemental reactive power equipment of approximately 6 MVAR of
switched shunt reactors will be required at the full 150 MW GF output to meet the
net 0.95 p.f. lag (producing VAR) criteria at the POI, and the same amount of
reactive power equipment will be required on the 34.5 kV bus to offset the collector
system VARs and meet Tri-State’s VAR neutral requirement (less than 2 MVAR
flow at 0 MW output at the POI).
The Interconnecting Customer is responsible for equipment that will be installed to
ensure that the GF can achieve the net 0.95 p.f. lag and lead capability across the
0 to 150 MW net generation output rating as measured at the POI. Prior to entering
into a Generator Interconnection Agreement, the Interconnecting Customer will be
required to provide data that demonstrates compliance with Tri-State's reactive
criteria.
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Tri-State Generation and Transmission Association, Inc.
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Table 4: 2017 Heavy Summer
AFFECTED ELEMENT CONTINGENCY
Emergency
Thermal
Rating
(MVA)
Pre-
Project
Loading
(%)
Post-
150
MW
Project
Loading
(%)
Delta
(%)
Maximum
Project
Output
w/out
Additional
NU (MW)
Owner Notes
Hidalgo(12093)-Pyramid(13007) 115kV Line Hidalgo(12093)-Pyrmid_T(12092)-Pyramid(13007) 115kV 215 41.5 136.8 95.3 66 TSGT
Hidalgo(12093)-Pyrmid_T(12092)-Pyramid(13007) 115kV Hidalgo(12093)-Pyramid(13007) 115kV Line 212 37.8 138.8 101.0 64 TSGT
Hidalgo(11080)-Hidalgo(13007)
345-115kV Tran No.1/2
Hidalgo(11080)-Hidalgo(13007)
345-115kV Tran No.1/2 224 50.7 109.5 58.8 125 PNM
Lordbrg(13009)-Lordbrg(13020) 69-115kV Tran Hidalgo(13007)-Turquois(13014) 115kV Line 27 87.2 98.2 11.0 NA PNM
Silver_C(13012)-Tyrone(13015) 115kV Line MD(13003)-Turquois(13014) 115kV Line 29 100.2 111.7 11.5 NA PNM
Tyrone(13015)-TurqTap(13019) 115kV Line MD(13003)-Turquois(13014) 115kV Line 29 106.0 117.5 11.5 NA PNM
Highlighted elements are being addressed by PNM outside the scope of this study.
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Tri-State Generation and Transmission Association, Inc.
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Table 5: Reactive Power Delivered to the Solar Bus and at POI Bus
Project Size: 150 MW, SMA 800CP 0.85 MW-180 Units
Base
Case
Fixed P.F.
at MV
Gen
Equiv
Collector
Bus
P, Q, V At Gen Equiv MV Net P, Q, V, PF At HV POI Bus
Pgen
(MW)
Qgen
(MVAR)
Voltage
(p.u.)
P
(MW)
Q
(MVAR)
PF at
POI
Voltage
(p.u.)
MVAR
to meet
PF Reqd
at POI
of 0.95
MVAR
Short(+)
or
Excess(-)
HS Base Case – 0.95 power factor lag (producing MVAR)
0.95 0 0.0 0.951 0 6.2 0 0.95 0 -6.2
0.95 37.5 12.3 0.982 37.3 27.4 0.806 0.95 12.3 -15.1
0.95 75 24.7 0.999 74.2 33.8 0.910 0.95 24.4 -9.4
0.95 112.5 37.0 1.013 110.8 37 0.949 0.95 36.4 -0.6
0.95 150 49.3 1.025 147 35.8 0.972 0.95 48.3 12.5
LA Base Case – 0.95 power factor lead (absorbing MVAR)
-0.95 0 0 1.052 0 7.6 0 1.05 0 -7.6
-0.95 37.5 -12.3 1.04 37.3 -6.6 0.985 1.05 -12.3 5.7
-0.95 75 -24.7 1.025 74.3 -24.8 0.949 1.05 -24.4 -0.4
-0.95 112.5 -37.0 1.007 110.8 -47.2 0.920 1.05 -36.4 -10.8
-0.95 150 -49.3 0.986 146.9 -79.5 0.879 1.05 -48.3 -31.2
5.0 DYNAMIC STABILITY ANALYSIS
5.1 Criteria and Assumptions
5.1.1 NERC/WECC Dynamic Criteria
PSSE version 33.5.0 was used for dynamic stability analysis. Dynamic stability
analysis was performed in accordance with the dynamic performance criteria shown in
Table W-1 and Figure W-1 from the NERC and WECC TPL-001 through 004 System
Performance Criteria. These criteria are shown below.
In addition, the NERC/WECC standard states that “[r]elay action, fault clearing time,
and reclosing practice should be represented in simulations according to the planning
and operation of the actual or planned systems. When simulating post transient
conditions, actions are limited to automatic devices and no manual action is to be
assumed.”
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5.1.2 Voltage Ride-Through Requirements
1. The GF shall be able to meet the dynamic response low voltage ride through (LVRT)
requirements consistent with the latest WECC / NERC criteria, as stated in Tri-State’s
Generation Interconnection Procedures Appendix G and FERC Order 661a.
2. Generating plants are required to remain in service during faults (three-phase or
single line-to-ground (SLG) whichever is worse) with normal clearing times of
approximately 4 to 9 cycles or SLG faults with delayed clearing, and subsequent post-
fault voltage recovery to pre-fault voltage unless clearing the fault effectively
disconnects the generator from the system. The clearing time requirement for a three-
phase fault will be specific to the circuit breaker clearing times of the affected system
to which the Customer facilities are interconnecting. The maximum clearing time the
PV inverter generating plant shall be required to withstand for a fault shall be 9
cycles. After which, if the fault remains following the location-specific normal
clearing time for faults, the PV generating plant may disconnect from the
transmission system. A PV generating plant shall remain interconnected during such
a fault on the transmission system for a voltage level as low as zero volts as measured
at the POI. The Customer may not disable LVRT equipment while the plant is in-
service.
3. This requirement does not apply to faults that may occur between the PV inverter
generator terminals and the POI.
4. PV generating plants may meet the LVRT requirements by the performance of the
generators or by installing additional equipment, e.g., Static VAR Compensator, or by
a combination of generator performance and additional equipment.
5.2 Base Case Model Assumptions
1. A SMA user written model was used for the simulations. Transient stability analysis
was completed with the SMA 800CP kVA Inverter model
(SMASC_B14_33_IVF111.obj).
2. The Energy Resource base case backed down Pyramid generation to accommodate
Project generation. Pyramid generation output was modeled at 75 MW and the
Project generation output at 150 MW.
3. The Network Resource base case modeled Pyramid generation at its full output. In
addition, a third Hidalgo - Pyramid 115kV line and a Project output of 125 MW was
modeled. The full Project output of 150 MW was not modeled since additional
345/230kV transformer capacity at Hidalgo would be required.
4. The collector system was modeled with an equivalent collector system and one
115/34.5 kV substation transformer.
5. The Rosebud 13.8kV (bus 12063) load was netted since the transient stability models
did not yield stable conditions during faulted conditions. This instability is not related
to the Project.
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Tri-State Generation and Transmission Association, Inc.
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5.3 Methodology
Dynamic stability was evaluated as follows:
1. The 2017 HS base case was utilized with the GF in service.
2. System stability was observed by monitoring the voltage, frequency and relative rotor
angles of local machines and system damping.
3. Three-phase faults were simulated for all contingencies. Two contingencies were
simulated for each line: a fault was applied at the near end and then applied at the far
end of the transmission line. Contingencies used to evaluate compliance with
NERC/WECC criteria for dynamic stability are listed in the following table.
Table 6: List of Dynamic Stability Contingencies
Dynamic Stability Contingencies
Bus Numbers No. Description
1 28-cycle 3-phase fault at POI 15-0832 115 kV, trip Pyramid - POI 15-0832 115 kV line 12093 - 12300
2 28-cycle 3-phase fault at Hidalgo 115 kV, trip Hidalgo - Pyramid 115 kV line 13007 - 12093
3 28-cycle 3-phase fault at Hidalgo 115 kV, trip Hidalgo - Turquois 115 kV line 13007 - 13014
4 3-cycle 3-phase fault at Hidalgo 345 kV, trip Hidalgo - Greenlee 345 kV line 11080 - 16101
5 3-cycle 3-phase fault at Hidalgo 345 kV, trip Hidalgo - Luna 345 kV line 11080 - 11093
6 3-cycle 3-phase fault at Luna 345 kV, trip Luna - Macho Springs 345 kV line 11093 - 11047
7 3-cycle 3-phase fault at Hidalgo 345 kV, trip Luna 345/115 kV No.1 Transformer 11080-13007
5.4 Results
The transient stability analysis studied the Project as both an Energy Resource and a
Network Resource.
As an Energy Resource, the following local generation dispatch was modeled:
Pyramid generation: Unit 1 - 40 MW, Unit 2 - 35 MW, Units 3 and 4 - Off
Project generation: 150 MW equivalent unit
The above totals 225 MW and is the maximum generation that can be interconnected to
the Pyramid 115kV substation. It is limited by the rating of the Hidalgo - Pyramid No.2
115kV line for loss of the Hidalgo - Pyramid No.1 115kV line.
Simulation results for summer system conditions show that:
1. With the SMA Photovoltaic Inverters (150 MW), the Project did not trip during any
contingencies and had acceptable voltage levels.
2. The local area system experienced transient voltage levels at or above 1.15 per unit
for loss of the Hidalgo - Pyramid 115kV line. As a result, the Project should be
designed to compensate for high transient voltage levels to ensure that the inverters
do not trip off-line due to overvoltage conditions during local area contingency
events.
3. Pyramid generators experienced rotor angle instability for loss of the Project’s one
mile transmission line when line clearing was greater than 12 cycles. A longer
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Tri-State Generation and Transmission Association, Inc.
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clearing time will result in the Pyramid units tripping off-line due to rotor angle
instability. As a result, it is important that the near and far end clearing time be
less than 12 cycles.
4. Acceptable damping and voltage recovery was observed.
Table 7: Dynamic Stability Results, Energy Resource - 2017 Heavy Summer
Dynamic Stability Contingencies 2017 Heavy Summer
No. Description
1 28-cycle 3-phase fault at POI 15-0832 115 kV, trip Pyramid - POI 15-0832 115 kV line
Unstable for clearing
times > 12 cycles
2 28-cycle 3-phase fault at Hidalgo 115 kV, trip Hidalgo - Pyramid 115 kV line Stable
3 28-cycle 3-phase fault at Hidalgo 115 kV, trip Hidalgo - Turquois 115 kV line Stable
4 3-cycle 3-phase fault at Hidalgo 345 kV, trip Hidalgo - Greenlee 345 kV line Stable
5 3-cycle 3-phase fault at Hidalgo 345 kV, trip Hidalgo - Luna 345 kV line Stable
6 3-cycle 3-phase fault at Luna 345 kV, trip Luna - Macho Springs 345 kV line Stable
7 3-cycle 3-phase fault at Hidalgo 345 kV, trip Luna 345/115 kV No.1 Transformer Stable
As a Network Resource, the case modeled a Hidalgo - Pyramid No.3 115kV transmission
line (16 miles) and the following local generation dispatch:
Pyramid generation: Units 1, 2, 3 and 4 - 40 MW each
Project generation: 125 MW equivalent unit
The above totals 285 MW and is the maximum generation that can be interconnected to
the Pyramid 115kV substation with the Network Upgrade noted above. It is limited by
the Hidalgo 345/115kV No.2 transformer for loss of the Hidalgo 345/115kV No.1
transformer.
As noted above, the Project generation was limited to 125 MW. Full Project output
(150 MW) was not modeled since additional 345/230kV transformer capacity at
Hidalgo would be required. To replace/upgrade both 345kV transformers at Hidalgo or
add a third transformer is not practical and the cost is prohibitive to gain an additional
25 MW in Project output.
Simulation results for summer system conditions show that:
1. With the SMA Photovoltaic Inverters (125 MW), the Project did not trip during any
contingencies and had acceptable voltage levels.
2. The local area system experienced transient voltage levels at or above 1.15 per unit
for loss of the Hidalgo - Pyramid 115kV line. As a result, the Project should be
designed to compensate for high transient voltage levels to ensure that the inverters
do not trip off-line due to overvoltage conditions during local area contingency
events.
3. Pyramid generators experienced rotor angle instability for loss of the Project’s one
mile transmission line when line clearing was greater than 10 cycles. A longer
clearing time will result in the Pyramid units tripping off-line due to rotor angle
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Tri-State Generation and Transmission Association, Inc.
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instability. As a result, it is important that the near and far end clearing time must
be less than 10 cycles.
4. Acceptable damping and voltage recovery was observed.
Table 8: Dynamic Stability Results, Network Resource - 2017 Heavy Summer
Dynamic Stability Contingencies 2017 Heavy Summer
No. Description
1 28-cycle 3-phase fault at POI 15-0832 115 kV, trip Pyramid - POI 15-0832 115 kV line
Unstable for clearing
times > 10 cycles
2 28-cycle 3-phase fault at Hidalgo 115 kV, trip Hidalgo - Pyramid 115 kV line Stable
3 28-cycle 3-phase fault at Hidalgo 115 kV, trip Hidalgo - Turquois 115 kV line Stable
4 3-cycle 3-phase fault at Hidalgo 345 kV, trip Hidalgo - Greenlee 345 kV line Stable
5 3-cycle 3-phase fault at Hidalgo 345 kV, trip Hidalgo - Luna 345 kV line Stable
6 3-cycle 3-phase fault at Luna 345 kV, trip Luna - Macho Springs 345 kV line Stable
7 3-cycle 3-phase fault at Hidalgo 345 kV, trip Luna 345/115 kV No.1 Transformer Stable
6.0 SHORT-CIRCUIT ANALYSIS
Short-circuit analysis was performed for 3-phase-to-ground and single-line-to-ground faults
at the 115 kV Pyramid Switching Station POI bus, using the Aspen OneLiner model. Faults
were applied with and without the generation project. Model assumptions are as follows.
6.1 Assumptions and Methodology
1. The model used is shown in Figure 2 below.
2. The generation tie line, transformers, and generators were modeled as supplied by the
IC (reference Attachment B to Appendix 1 of Interconnection Request):
a. Zero sequence impedance of the 115-34.5 kV transformer was modeled as specified
by the IC.
b. The transformer delta windings were all modeled to lag the high side phase angles.
c. The zero sequence impedance of the 115 kV tie line and the 34.5 kV collector
system was modeled as three times the positive sequence impedance.
6.2 Results
Table 9 lists results for the 115 kV bus faults and contributions from each of the 115 kV
sources into the bus faults. The system impedances for the faulted buses for each
configuration is also included (see Figure 2 below). The results indicate that the GF
increases the fault duty by approximately 1564 Amperes at the 115 kV POI bus. The
resultant total fault currents are within Tri-State’s planned equipment ratings.
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Tri-State Generation and Transmission Association, Inc.
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Figure 2: Short-Circuit Model One-Line
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Tri-State Generation and Transmission Association, Inc.
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Table 9: Short Circuit Results
System Condition*
POI Bus
Total 3-
Ph Fault
(Amps)
Pyramid
T#1 to
POI 3-Ph
Fault
(Amps)
Pyramid
T#2 to
POI 3-
Ph Fault
(Amps)
Hildago 1
to POI 3-
Ph Fault
(Amps)
Hildago 2
to POI 3-
Ph Fault
(Amps)
Gen HV
to POI
3-Ph
Fault
(Amps)
POI Bus
Total
SLG
Fault
(Amps)
Pyramid
T#1 to
POI SLG
Fault
(Amps)
Pyramid
T#2 to
POI SLG
Fault
(Amps)
Hildago 1
to POI
SLG
Fault
(Amps)
Hildago 2
to POI
SLG
Fault
(Amps)
Gen HV
to POI
SLG
Fault
(Amps)
Thevinin Equivalent
Impedance
(R + jX p.u.)
115 kV POI Bus Fault
(w/o IC generation, N-0) 10631 2248 2240 3148 2974 11152 2998 2988 2708 2458
Z1(pos) = 0.69192+j6.20719
Z0(zero) = 0.75488+j5.33935
115 kV POI Bus Fault
(w/o IC generation, N-1
Pyramid T#1 OUT)
8395 2240 3148 2974 8124 3119 2630 2367
Z1(pos) = 1.06817+j7.83674
Z0(zero) = 1.50954+j8.58587
115 kV POI Bus Fault
(w/o IC generation, N-1
Pyramid T#2 OUT)
8403 2248 3148 2974 8134 3129 2630 2367
Z1(pos) = 1.06546+j7.82933
Z0(zero) = 1.49375+j8.57432
115 kV POI Bus Fault
(w/o IC generation, N-1
Hildago-Pyramid 1
115kV OUT)
8389 2248 2240 3891 9107 2937 2927 3240
Z1(pos) = 0.92536+j7.86027
Z0(zero) = 0.66470+j6.03810
115 kV POI Bus Fault
(w/o IC generation, N-1
Hildago-Pyramid 2
115kV OUT)
8554 2248 2240 4042 9328 2931 2921 3471
Z1(pos) = 0.78135+j7.72236
Z0(zero) = 0.76317+j5.81473
115 kV POI Bus Fault
(w/ IC generation, N-0) 11369 2248 2240 3148 2974 741 12716 2884 2874 2774 2537 1646
Z1(pos) = 0.61181+j5.80798
Z0(zero) = 0.47668+j3.97509
115 kV POI Bus Fault (w/
IC generation, N-1
Pyramid T#1 OUT)
9131 2240 3148 2974 741 9900 2928 2741 2499 1725
Z1(pos) = 0.91339+j7.21370
Z0(zero) = 0.73848+j5.53981
115 kV POI Bus Fault (w/
IC generation, N-1
Pyramid T#2 OUT)
9139 2248 3148 2974 741 9910 2937 2741 2499 1724
Z1(pos) = 0.91122+j7.20740
Z0(zero) = 0.73239+j5.53454
115 kV POI Bus Fault (w/
IC generation, N-1
Hildago-Pyramid 1
115kV OUT)
9127 2248 2240 3891 741 10546 2823 2813 3349 1559
Z1(pos) = 0.79238+j7.23147
Z0(zero) = 0.41537+j4.34600
115 kV POI Bus Fault (w/
IC generation, N-1
Hildago-Pyramid 2
115kV OUT)
9293 2248 2240 4042 741 10756 2821 2811 3563 1555
Z1(pos) = 0.67226+j7.11301
Z0(zero) = 0.47068+j4.23128
* PYRAMID, LORDSBURG AND AFTON GENERATION ENABLED
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Tri-State Generation and Transmission Association, Inc.
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7.0 SCOPE, COST AND SCHEDULE
The cost estimate is broken out into two categories (refer to Figure 3): 1) Interconnection
Facilities which include all equipment installed between the POI on the main 115 kV bus and
the Point of Change of Ownership (PCO) at the line dead-end structure, and 2) Network
Upgrades consisting of the rest of the facilities installed in the Pyramid Substation to
accommodate the interconnection.
Note that the Interconnecting Customer will be responsible for constructing the radial 115 kV
tie line to the GF site and for providing the primary protection (relaying and interrupting
device) for the Customer’s step-up transformer located in its 115-34.5 kV substation yard.
Equipment at the Pyramid 115 kV Substation will only provide backup protection for the
Customer’s 115-34.5 kV main transformer in the event of equipment failure or malfunction at
the Customer’s facility.
The Interconnecting Customer is responsible for providing a communication channel, such as
fiber optic cable (OPGW) on its radial 115 kV transmission line to provide for SCADA,
metering, and protective relaying. The Interconnecting Customer must also provide access to
analog, indicating, control and data circuits, as required to integrate the Project into the design
and operation of the Tri-State control system.
To mitigate transmission line thermal overloads for operation of the Project as a Network
Resource, the study assumed construction of a third Hidalgo - Pyramid 115kV transmission
line. Typical transmission line costs for 115kV construction are estimated at $300,000 per
mile. However, a detailed cost estimate for this Network Upgrade has not been included in this
report since the cost to terminate the line (at both substations) could be considerably more than
the cost of the line. This is based on a preliminary look at the Hidalgo and Pyramid 115kV bus
arrangements which suggests that an expansion to a breaker-and-a-half arrangement at both
substations would be required.
All costs are good faith estimates based on assumptions as stated in this SIS report. All
estimates are in 2016 dollars.
The estimated cost for interconnecting the proposed Project to the Pyramid 115 kV POI is as
follows (refer to Figure 3). This estimate does not include costs for Network Upgrades beyond
the POI.
Network Upgrade Costs (Reimbursable): $ 0.7 M
Interconnection Facilities Costs (Non-Reimbursable): $ 1.0 M
TOTAL Cost for Interconnection: $ 1.7 M
It is estimated that it will take approximately 24 months after receiving authorization to
proceed for Tri-State to complete the engineering, design, procurement, construction, and
testing activities identified in the scope of work for this Project. Additional Network Upgrades
(e.g., a third Hidalgo - Pyramid 115kV transmission line) will require a minimum of 48
months.
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Tri-State Generation and Transmission Association, Inc.
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Figure 3: One-Line Diagram Showing 115 kV POI Interconnection
Pyramid 115kV Switching
Station One-Line Diagram
PRELIMINARY
M
VTs
PCO(At DE
Structure)
POI
Red – Network Upgrades
Blue – Interconnection Facilities
Green – Interconnecting Customer’s Line
POI – Point of Interconnection
PCO – Point of Change of Ownership
TSGT 115kV Line to Playas
Substation
CTs
TI-15-0832 Line
Pyramid Generation
TSGT 115kV Line toHidalgo #1
Substation
TSGT 115kV Line toHidalgo #2
Substation
Pyramid Generation
New Breaker
and bay
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Table 10: Summary Cost Estimate Details – Interconnection Facilities
(Non-Reimbursable)
Element Description Cost Est.
Millions
Pyramid
115kV
Substation
115 kV line
termination
equipment
between PCO
and POI
Engineer, purchase, construct / install and test all
equipment installed at the Pyramid Substation that is
located between the PCO (Interconnecting Customer
line termination dead-end) and the POI (main bus tap
point), consisting primarily of the following
equipment:
One (1) 115 kV line dead-end structure
115 kV slack span from monopole to existing
structure at Pyramid Station
One (1) 115 kV 3-ph gang line end disconnect
switch and associated structure
*Three (3) 115 kV metering current transformers,
high accuracy class, extended range
*Three (3) 115 kV metering voltage transformers,
high accuracy class. *Or alternative CT/VT
combination metering units
PQ metering panel including SEL-735 Rev/PQ
meter, line meters, testing, checkout and
commissioning
Relaying for the Interconnecting Customer’s
radial 115 kV line protection (SEL-311C
primary, secondary and SEL-501 breaker-failure)
Three (3) 115 kV surge arresters
Line termination SCADA and telemetry
communication equipment additions to substation
RTU
Other associated substation equipment including,
but not limited to, grounding conductor, conduit,
cable, insulators, foundations, support steel, bus,
trenches, site prep, yard work, fencing, etc.
$1.0 M
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Tri-State Generation and Transmission Association, Inc.
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Table 11: Summary Cost Estimate Details – Network Upgrades (Reimbursable)
Element Description Cost Est.
Millions
Pyramid
115 kV
Substation
Expand Pyramid 115 kV five breaker bus arrangement to
include an additional circuit breaker and bay position (see
Figure 3, One-line Diagram). Scope includes typical
testing, checkout and commissioning.
One (1) 115 kV power circuit breaker
Two (2) 115 kV 3-ph gang operated disconnect
switches and associated structures (for breaker and
bus)
Circuit breaker station control panel
Other associated required substation equipment
including but not limited to grounding conductor,
conduit and cable, insulators, foundations, support
steel and tubular and cable bus
$0.7 M
Pyramid
Substation–
Relaying Mods
Relay settings changes (labor) for new POI line
termination protection. (Minimal)
Hidalgo
Substation–
Relaying Mods
Relay settings changes (labor) for new POI line
termination protection. (Minimal)
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