Technical Presentations

Dynamic Overvoltage Conditions Experienced at a PV Solar Power Plant838

Abstract
We present the results from an investigation of the cause of dynamic overvoltage conditions experienced at a solar power plant, that resulted from energizing the PV solar plant 34.5 kV collection syst... see moreem feeders using a back-up diesel generator. The diesel generator unit was installed at the solar plant for emergency back-up power to the project, in case of power outage from the connecting transmission utility system.
The back-up power source was provided to allow the operation of the solar power tracking system during emergency conditions. A subsequent study was performed using EMTP software program, to investigate the cause of these dynamic overvoltage conditions at the solar power plant. A detailed transient model of the solar plant and the diesel generator were developed using EMTP.


The EMTP simulations were useful in replicating the overvoltage event measurement recordings from the site, and the simulation results from the study, concluded that the diesel generator did not have sufficient capability to pick up capacitive loads and therefore, the unit was unable to control the overvoltage conditions which led to the overvoltage event that caused equipment damage to the PV inverters and their auxiliary systems. The study evaluated various solution alternatives and mitigation strategies to reduce these dynamic overvoltages to acceptable conditions.

Tag(s): pv park, tov, diesel generator, temporary overvoltage

Ground Fault Overvoltage with Inverter-Based Distributed Energy Resources841

Abstract
A Ground Fault Overvoltage (GFO) can occur in situations where an ungrounded transmission line and associated ungrounded transmission equipment is energized from distribution connected Distributed En... see moreergy Resource (DER) during a transmission single-phase-to-ground (SLG) fault. The condition could result in 1.73 pu overvoltage on phase to ground connected equipment which will be sustained until the DER ceases to energize the substation distribution transformer. Transmission equipment on the isolated circuit will be subjected to the above phase to ground overvoltage, also if there is frequency shift, the isolating transmission circuit breakers could potentially see up to 2.73 pu phase to ground overvoltage. Such an overvoltage can cause damage to customer or utility equipment and must be avoided. GFO concerns are already becoming a barrier to large-scale DER deployment either by posing low hosting capacity or by necessitating installation of costly mitigation equipment. Utilities need to study GFO risks and identify low-cost mitigation options to enable large-scale DER deployment. Given the fundamental differences in mechanisms and phenomena driving a GFO under DERs, classical GFO analysis methods do not apply to inverters, and advanced simulation methods and tools are required.


This presentation highlights EPRI’s work on GFO analysis, concerns, and evaluation of mitigation options using EMTP. The objective is to determine if or when high DER penetration presents a GFO problem and how to prioritize the mitigation and equipment evaluation. This includes evaluating the impact lightning arresters can have in reducing or clamping the overvoltage, the application and performance of high voltage substation overvoltage protection, and DER on-board voltage/frequency protection. The presentation further provides modeling recommendations for such a study using EMTP. As DER deployment increases, utilities will need to perform similar analysis to identify potential overvoltage risks and develop low-cost mitigation methods. This work provides a basis for such a study.

Tag(s): gfov, tov, ibr, DER, inverter, islanding, load rejection, inverter based resources

Measurement and EMTP Simulation of Ferroresonance Phenomenon849

Abstract
Ferroresonance is difficult to predict because its occurrence in the power system depends on several parameters, some of which are unknown (parasitic capacities). In addition, measuring electrical con... see moreditions during high voltage ferroresonance is very difficult to achieve. In order to check the possibilities of numerical calculations of ferroresonance, the ferroresonant circuit was made as a low voltage circuit in which all parameters were known.

Measurement of ferroresonant current and voltages in a single-phase low voltage circuit will be described. The system consists of an AC voltage source, a capacitor, a switch and the single-phase transformer. These electrical elements are connected in series. The magnetic flux-current points of the magnetization curve of the transformer was experimentally obtained and used in the simulation. When the switch closes, ferroresonance takes place since all conditions were met for its occurrence. The waveforms of the transformer current and the voltage at the transformer terminals were measured after switch closing.

During the ferroresonance, transformer voltage and current reach relatively high values. At the beginning of the ferroresonance (during its transient part), the voltage of the transformer reaches values that are about two times higher in amplitude than the amplitude of the AC source. The primary side of the transformer was modelled with a series connection of the ohmic resistance and leakage inductance. The core was modelled with a parallel connection of the ohmic resistance and nonlinear inductance. The ohmic resistance represents the core losses while the nonlinear inductance models the magnetizing characteristic of the transformer.

A comparison between simulated and experimentally obtained results shows that EMTP successfully simulates the ferroresonance phenomenon for this specific electrical system as simulated results are in a good agreement with the measured results.

Tag(s): ferroresonance, tov, saturation

TOV for renewable, evaluating three different grounding technologies783

Abstract
Temporary overvoltages (TOV) are oscillatory overvoltages mainly caused by switching or faults, which are of relatively long duration and undamped or slightly damped. Maximum overvoltages under faulte... see mored conditions can require considerable energy magnitudes absorbed by surge arresters.
These energies may overpass maximum limits and lead to equipment damage.

This project presented a study to identify the maximum TOV at the MV equipment of a PV power plant during fault-clearing events. Two methods of TOV suppression are evaluated, including grounding transformers and fast grounding switches under two different configuraions. An electrical model was elaborated in EMTP to perform the simulation of the transient overvoltage due to the un-balanced fault clearance, considering each inverter of the PV plant. The results show that considering no grounding technologies at the feeders, the energy absorption limit is overpassed in the islanded system.

Therefore, a fast grounding switch or a system with grounding technology at each feeder is needed in order to protect the substation equipment.