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Technical Assessment of the Impact of a Proposed HVDC Line in New York State on System Reliability and Congestion

Client: Gilberti Stinziano Heintz & Smith, P.C.

Pterra was asked to conduct a technical assessment of the impact of a proposed 1,200 MW Marcy-Rock Tavern High Voltage Direct Current (“HVDC”) project (the “Project”) on reliability in the New York State Bulk Power System (“NYSBPS”)1 and on transmission congestion between Marcy and the Lower Hudson Valley, especially the New York State load pockets in New York City and Long Island.

The specific topics of the assessment are: (a) Impact Electric System Reliability and (b) Impact on Transmission Congestion. The specific interfaces in NYSBPS addressed by the analysis are shown in Figure 1.

Figure 1

Table 1 shows the values of interface limits determined by the assessment. The conditions tested differ as to which generating plants are available in the future. All the cases meet performance requirements for thermal and voltage performance based on contingency analysis. The transfer limits show a slightly increasing trend in the upstate interfaces, and a slightly decreasing trend in the downstate interfaces. In all assessments, the Reliability Rules for operation were met.

 Table 1: Interface limits in MW for NYSBPS at various conditions.

Condition A

Condition B

Condition C








Central East (with Fraser-Gilboa ckt)







Total East





















Millwood South














Historical data on congestion in New York points to frequent congestion on the following interfaces: Central East, UPNY-SENY, UPNY-Coned and Dunwoodie South. With Project in service, the change in average hourly prices over time based on the simulations is shown in Figure 2.

Figure 2: Change in average hourly prices over time

The project reduces the average hourly prices between 2012 and 2015. Prices are highest at the load zones J (New York City) and K (Long Island)

1 The NYS Bulk Power System is “the portion of the bulk power system within the New York Control Area, generally comprising generating units 300 MW and larger and generally comprising transmission facilities 230 kV and above. However, smaller generating units and lower voltage transmission facilities on which faults and disturbances can have a significant adverse impact outside of the local area are also part of the NYS Bulk Power System”.

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Development of Dynamic Models for Submission to Regional System Operator

Client: Pacificorp

Pterra was contracted by PacifiCorp to compile, document and check system dynamic model data for submission to the Western Electricity Coordinating Council (“WECC”). In coordination with PacifiCorp Transmission Planning, Pterra reviewed existing data provided to PacifiCorp by generator owners and compiled appropriate synchronous generator model data. Synchronous generator, exciter, governor/turbine and power system stabilizer model data were developed consistent with NERC MOD-012 requirements.

PacifiCorp compiled dynamics data for the western control area power plants. For the generators owned by PacifiCorp, the data comprised of generator test reports from various engineering test firms. Model parameters for each generation unit were taken from the corresponding test reports. Data were adjusted as required for tuning or software compatibility purposes. For other power plants, the data available ranged from fairly complete data to little more than nameplate values.

Pterra conducted several tests of the models for each generating unit. These tests included comparison of typical range of values, step response tests on the excitation system and governor/turbine models, and disturbance response tests on a sample system. Pterra adjusted the model parameters to represent field conditions and to improve the numerical performance. A total 59 generating units were evaluated.

Pacificorp submitted to the models to the WECC.

Figure: Block diagram for an IEEE excitation system.

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Load Rejection Overvoltage Issues on Distributed Generation Projects

Client: Confidential

Increased penetration of solar photovoltaic (“PV”) generation in distribution circuits raises a potential issue with load rejection overvoltages. The condition occurs with the lightly loaded feeders. If the feeder load is much smaller than the total isolated generation then the load rejection overvoltage could pose a threat of damage to equipment insulation and surge arresters. In addition, the inverter characteristics used for PV installations present their own unique impacts. Different inverter designs respond differently to the phenomenon. Pterra used a time domain simulation tool to quantify the impact of load rejection on several different circuit configurations and several different types of inverters.

The study system is shown in Figure 1.

Figure 1.

The maximum magnitude of transient overvoltage (“TOV”) could reach 240% of the nominal voltage if the generation is 6 times the load on the islanded feeder. For a criterion setting, 120% of minimum load as the limit for PV penetration is conservative. In some utilities, this criterion is set at 150%. However, when in doubt, it is always preferable to run a simulation using the feeder and load electrical characteristics and the manufacturer model for their inverters.

Mitigation options to allow for increased PV penetration include implementation of a direct transfer-trip scheme, usually an expensive and complicated choice.

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Distributed Generation Impacts on the Island of Lanai in Hawaii

Client: National Renewable Energy Laboratory

National Renewable Energy Laboratory (“NREL”) contracted Pterra to conduct an island-wide system-level integration study to determine the impact of additional photovoltaic (“PV”) systems of varying size and locations being installed on Lanai over the next few years.

Pterra conducted several technical studies including power flow, short circuit, protection coordination, angular and voltage stability, frequency regulation, system operation and limitations, harmonics and power quality, transient overvoltage and ground-fault overvoltage. Each of these studies provided a unique look at the levels of PV penetration that may lead to undesirable reliability impacts and costly system reinforcements.

The key finding of the study is the critical impact on frequency response of new PV generation. The study system is shown in Figure 1. The existing electrical supply comes from: L7, L8 and CHP – conventional diesel units, and LSRPV – PV units. The existing PV includes a battery to provide support during low-frequency conditions. The generators provide power to a peak load of about 4,200 kilo-watts, with a day-time minimum of 2,300 kilo-watts. An additional 800 kilo-watts of new PV was considered for the study.

Figure 1. Existing Generator Locations

With the new PV in service, the option to operate with fewer diesel units is available. However, this dispatch mode results in a reduced spinning reserve. The study simulations show that the system voltages collapse after applying a contingency. The voltage plot is shown in Figure 2.

The result of the analysis is an operating requirement to continue to operate an extra diesel unit for spinning reserve purposes. This had an immediate consequence on the economic benefit of the new PV, as the non-power related costs of the extra diesel unit offset the savings from use of solar energy.

The other technical tests did not identify any other constraints on the 800 kilo-watts of new PV.

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