By R. Tapia, M. Gutierrez and M. Infantado


Independent System Operators (ISO) constantly face the challenge of assessing the impact of facility additions to the power grid. They normally require a system impact study for any proposed interconnection of a large generating plant or transmission project. The purpose of this analytical study is to determine the potential adverse impacts of the interconnection of transmission facilities to a power system and whether it would cause any of the following:

  • Post-contingency thermal overloading on transmission lines and transformers,
  • Voltage criteria violations on substations,
  • Negative impact on the dynamic response of power system facilities,
  • Degradation on the transfer limit of transmission interfaces,
  • Increase in substation short circuit current that could possibly exceed the fault duty of existing circuit breakers.

The system impact study determines the impact of the proposed project by comparing simulation results of the case with the project in service against the case without the project. If adverse impacts were to be found, appropriate solutions to mitigate the violations would be required, except for the extreme contingency assessment which is performed for information purposes on issues such as avoidance of widespread load interruptions, uncontrolled cascading, and system blackouts among others.

The Study

Pterra recently conducted a system impact study on a proposed transmission interconnection project (the “Project”) to a 345 kV grid with the purpose of increasing the transmission capability in the region where the Project is located. As illustrated in Figure 1, six existing lines are interconnected to a new NEWBUS1 345 kV substation and a new line from NEWBUS1 to LOAD2. The analyses conducted in the study were as follows:

  • N-1 AC contingency analysis – Thermal and voltage
  • N-1-1 AC contingency analysis – Thermal and voltage
  • Transfer limit analysis – Thermal, voltage, and stability
  • Stability analysis – Includes critical clearing time
  • Short circuit analysis
  • Extreme contingency assessment – Steady state and stability

Figure 1: Project Single Line Diagram


This blog presents brief descriptions of the analyses performed for the study and includes some partial results obtained. Names have been changed for confidentiality.

Following are the descriptions of and results obtained from the analyses performed for the study:


1. Steady State N-1 AC Contingency Analysis

1.1 Thermal Assessment

The thermal analysis assessed branch overloads for both the pre- and post-contingency conditions, with and without the Project. The term N-1 refers to the contingency loss of one element among all the N elements of the grid. Thermal criteria require branch loadings to be less than 100% of applicable normal ratings for pre-contingency and emergency ratings for post-contingency conditions.

The analysis found no thermal criteria violations as impact of the Project except for overloads on two 115/34.5 kV transformers on the summer peak case which are shown in Table 1. The connecting transmission owner, however, indicated that they have existing local plans and appropriate post-contingency 34.5 kV switching actions that would address the overloads. Several other violations were found without and with the Project in service and they were considered as pre-existing violations and not an impact of the Project.

Table 1: N-1 Thermal Analysis Results
Monitored Facility Contingency Rating MVA Pre-Project Post-Project % Delta
MVA Flow % Loading MVA Flow % Loading
Substation A 115/34.5 kV Transformer 926-ER 60 73.2 122 81.7 136.1 14.1
Substation B 115/34.5 kV Transformer STEEL115 28 18.4 65.8 29.9 106.9 41.1

1.2 Voltage Assessment

The voltage analysis assessed violations on voltage criteria for both the pre- and post-contingency conditions, without and with the Project in service. The criteria were based on different high and low bus voltage limits specified by the transmission owners involved.

The analysis found high and low voltage impacts (see Vcont) due to the Project and some of them are shown in Table 2. All the voltage violations have been mitigated by adjustment of voltage settings of the tap-changer of nearby 345/115 kV and 230/115 kV transformers; the last column shows the post-contingency voltages after mitigation.

Table 2: N-1 Voltage Analysis Results
Bus Name kV Contingency Pre-Project Post-Project Delta Vcont (pu) Vcont After Mitigation
Vbase Vcont Vbase Vcont
VISTA115 115 GVNY115-67 1.0202 1.0351 1.0375 1.0706 0.0355 1.0257
BIRD115 115 GVNY115-67 1.0174 1.0194 1.0399 1.0563 0.0369 1.0092
PART115 115 SERD115-4 1.0049 0.9555 1.023 0.9424 -0.0131 0.962
IRENE115 115 SERD115-4 0.9989 0.9541 1.0149 0.9409 -0.0132 0.9606

2. Steady State N-1-1 Contingency Analysis

The steady state N-1-1 analysis evaluates the system performance with the non-simultaneous outage of two elements. The idea is to have a first contingency event applied and the system allowed to adjust within 30 minutes, then the second contingency is assumed to occur. One example of this scenario is when a contingency occurs while there is an outage of a transmission facility due to an ongoing scheduled maintenance. This analysis was performed for the summer peak cases.

2.1 Thermal Assessment

The analysis did not find adverse thermal impacts caused by the Project. Thermal violations were found with and without the Project in service and were considered as pre-existing. Three of them are shown in Table 3.

Table 3: N-1-1 Thermal Analysis Results
Monitored Facility First Contingency Second Contingency Rating MVA Pre-Project Post-Project % Delta
MVA Flow % Loading MVA Flow % Loading
RELIC-DEPEN 115 KV LINE LD3TRANS 345/115 LN111_LP 119 176.3 148.1 177.5 149.1 1
ARCHER-HATH 115 KV LINE LD3TRANS 345/115 MEAD115 181 221.8 122.5 223.2 123.3 0.8
HERD-STN22 115 KV LINE LN_115_36 SHELL115 148 151.6 102.5 152 102.7 0.2

2.2 Voltage Assessment

No adverse voltage impacts were found from the analysis but only pre-existing violations. Three of them are shown in Table 4, with violations without and with the Project in service.

Table 4: N-1-1 Voltage Analysis Results
Bus Name kV First Contingency Second Contingency Pre-Project Post-Project Delta Vcont (pu)
Vinit Vcont Vinit Vcont
ROWE345 345 HILL 230/115 3TR CLARW-DUKE 345 1.0408 1.0545 1.0414 1.0609 0.0064
MAYN230 230 HILL 230/115 4TR FIELD-MAY 345 0.9935 0.9335 0.9971 0.9353 0.0018
DUKE230 230 DUKE 345/115 3TR DUKE345-32 1.0031 0.8832 1.0035 0.8939 0.0107

3. Transfer Limit Analysis

Transfer capability is the measure of the ability of interconnected electric systems to reliably transfer electric power from one designated area to another. The transfer limit assessment determined the incremental impact of the Project on the transfer limit of transmission interfaces close to the Project. The transfer limits were evaluated on the predominant west-to-east and north-to-south directions of the transmission system using summer peak loading conditions. The assessment involved three analyses: thermal, voltage, and stability. Whichever has the lowest transfer for each interface defines the transfer limit.

3.1 Thermal Transfer Limit

The thermal transfer limit is obtained by computing for the maximum MW flow across a transmission interface with no thermal violations. The thermal transfer limit analysis determined the incremental impact of the Project on the normal and emergency thermal transfer limits. Table 5 shows the positive impact of the Project on the transfer limit of the transmission interfaces adjacent to the Project. The other interfaces, not shown here, did not show significant impact. The transfer limits increased by at least 1,140 MW on the transmission interfaces shown. If the interface limit is decreased by a Project, some upgrades may be required in order to return the interface capability to the pre-Project value.

Table 5: Thermal Transfer Limits
Transfer Limit (MW)
Transfer Limit (MW)
Project Out of Service Project in Service Delta Project Out of Service Project in Service Delta
Fgate-One 910 2,050 1,140 1,570 2,750 1,180
Fgate-Two -950 220 1,170 -220 1,060 1,280
Fgate-Three 270 1,800 1,530 1,090 2,810 1,720

3.2 Voltage Transfer Limit

The PV analysis is performed by increasing in steps the power transfer across an interface until either the PV “nose of the curve” or the low voltage limit of one of the major buses close to the interface is reached, whichever is lower. The maximum power reached would be the voltage transfer limit.

The voltage transfer limits for the three interfaces in Table 5 are shown in Table 6. The determination of the limit was based on 95% of the MW transfer corresponding to the nose of the curve (see The In-Between Voltage State). The Project increased the limit on the interfaces shown by at least 130 MW.

Table 6: Voltage Transfer Limits
Transmission Interface Voltage Transfer Limit (MW)
Project Out of Service Project in Service Delta
Fgate-One 2,400 2,530 130
Fgate-Two 560 750 190
Fgate-Three 1,980 2,240 260

3.3 Stability Transfer Limit

Stability transfer limit analysis was conducted for the three interfaces, among others, to evaluate the dynamic performance of the system at testing transfer levels. The testing transfer level was above 11% of either the thermal emergency transfer limit or the voltage transfer limit, whichever is lower. All the simulation test results yielded stable response.

3.4 Transfer Limit Summary

With stable response on the transfer stability test, all the three interfaces in Table 5 and Table 6 were deemed thermally limited since the normal thermal transfer limits are lower than their respective voltage transfer limits.


4. Stability Analysis

Stability analysis was performed for the summer peak and light load conditions. Dynamic simulations are conducted with the Project in service using design contingencies. If stability issues were to be found with the Project in service, corresponding analysis would be performed on the pre-Project case to assess the Project’s stability impact on the power system. In some projects, adverse impact on the transmission system may be found in which case the issues are submitted to the client for satisfactory resolutions by all parties involved.

For all the tested contingency events in the analysis, the system was found to be stable. Since no stability issues were found with the Project in service, no further analysis on the pre-Project case was deemed necessary. A sample dynamics simulation plot of machine angles is shown in Figure 2.

Figure 2: Stability Simulation Plot


4.1 Critical Clearing Time

Critical clearing time is the maximum time duration for a fault on a line next to the tested plant where the power system remains stable. The determination of the clearing time was conducted on both the pre- and post-Project cases. Results showed that there was no change on the critical clearing time and the Project therefore does not cause adverse impact. Table 7 shows the summary of results.

Table 7: Critical Clearing Time
Bus Critical Clearing Time (cycles) Impact
Pre-Project Post-Project
COAL1 345 kV 11 11 0

5. Short Circuit Analysis

The short circuit analysis was performed to evaluate the impact of the Project on system protection and the adequacy of existing circuit breakers and other fault current interrupting devices within the study area. Two cases were used, one with the Project and the other without. Three-phase-to ground, two-phase-to-ground, and single-line-to-ground faults were calculated on buses rated 69 kV and above. The maximum current of these three faults for the two cases were compared.

Table 8 shows the buses with the highest increase in fault current. NEWBUS1 has the highest increase of 10 kA. The fault current level of several other buses close to the Project increased considerably but no breaker rating was exceeded by its fault current.

Table 8: Short Circuit Analysis Results
Bus Name Bus kV Short Circuit Current (kA) Fault Current Increase (kA) (Max On-Max Off) Lowest Breaker Rating (kA) Allowance (LBR-Max On), kA
Project Off Project On
3LG 2LG 1LG Max Off 3LG 2LG 1LG Max On
NEWBUS1 345 4.69 4.52 4.11 4.69 14.72 13.94 11.51 14.72 10.03 40 25.3
NEWBUS2 115 15.53 14.9 13.54 15.53 22.83 22.07 20.23 22.83 7.3 37 14.5
BUS_A 345 13.94 16.98 17.55 17.55 18.33 21.25 22.18 22.18 4.63 40 17.8
BUS_B 115 13.76 12.93 10.64 13.76 17.36 16.27 13.01 17.36 3.6 40 22.6


6. Extreme Contingency Assessment

Extreme contingency assessment is performed to test the robustness of the power system. The steady state power flow analysis was performed using extreme contingencies with the post-contingency thermal loading criteria based on the branches’ short-term emergency ratings. No adverse thermal impact was identified due to the Project. Thermal criteria violations were found on both the pre- and post-Project cases.

For the voltage contingency analysis, a new high voltage violation was found on a 115 kV bus as shown in Table 9.

Table 9: Extreme Contingency Voltage Analysis Results
Bus Name kV Contingency Pre-Project Post-Project Delta Vcont (pu)
Vbase Vcont Vbase Vcont
LOAD3 115 L/O_EAST-LESTER 1.0231 1.0247 1.049 1.0525 0.0278

Dynamic stability simulations using the same set of contingencies were also performed. The simulations yielded stable response.



The preceding reliability impact study example which are based on an actual study illustrates the various components that may be included in such a study. In this case, the proposed transmission interconnection does not cause any adverse impact on the reliability and stability of the transmission system.  Subsequently, the project was approved for interconnection and is now in construction.