Interconnection System Impact Study Final Report · Table 1: Generator Data for Steady-State Power Flow Analyses Unit Description Pmax Name plate rating (lumped equivalent generator

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


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)



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.



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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



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.



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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.



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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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Figure 1: Area Map One-Line Diagram Of Study Area And Location of GF

TI-15-0832

150 MW



Page 8 of 26

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.



Page 9 of 26

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.



Page 10 of 26

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.



Page 11 of 26

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



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.



Page 13 of 26

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.



Page 14 of 26

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.”



Page 15 of 26



Page 16 of 26

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.



Page 17 of 26

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



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






events.





Page 18 of 26


instability. As a result, it is important that the near and far end clearing time be

less than 12 cycles.


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



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






events.






Page 19 of 26

instability. As a result, it is important that the near and far end clearing time must

be less than 10 cycles.


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.



Page 20 of 26

Figure 2: Short-Circuit Model One-Line



Page 21 of 26

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


(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



Pyramid T#2 OUT)

8403 2248 3148 2974 8134 3129 2630 2367

Z1(pos) = 1.06546+j7.82933

Z0(zero) = 1.49375+j8.57432



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



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


(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


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


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


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



Page 22 of 26

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.



Page 23 of 26

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



Page 24 of 26

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



Page 25 of 26

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)



Page 26 of 26

Documents

Interconnection System Impact Study Final Report · Table 1: Generator Data for Steady-State Power Flow Analyses Unit Description Pmax Name plate rating (lumped equivalent generator