By Ric Austria, Francis Luces, Dr. Moises Gutierrez

(This topic was presented at the Conference of Electric Power Supply Industry, CEPSI held in Kuala Lumpur, Malaysia on September 18-20, 2018. For the full presentation, please see this link.)

One of the tasks transmission planners, analysts, and system operation engineers to evaluate stability and resiliency of electric power system is to perform computer simulation of dynamic models and representation of power system components. The computer simulation may be part of an overall transmission planning process (5-10 years), evaluation of new generator interconnections, and assessment of system performance. Typically, the study interest for transient and voltage stability simulations falls on the range of 0.1-10 Hz where the said range of phenomena is sufficient to model and evaluate the dynamic response of power system components. With the influx of renewable energy sources on electric power system, the way traditional stability studies are performed would be affected physically (in terms of assessing the performance of the power system with renewables) and mathematically (in terms of algorithms and dynamic models being implemented).

By Ketut Dartawan

(This topic was presented at the PSCAD Users group Meeting held in Atlanta, GA on Sept. 20-21, 2018. For the full presentation, please see this link.)

Many interconnection challenges exist when connecting photovoltaic (PV) resources to the electrical distribution grid. Various challenges on the distribution feeders are covered in some technical papers; however, one of the urgent topics – as recently mentioned by utilities and recognized by inverter manufacturers as well as the developers – is the potential for ground fault overvoltage (GFO) on sub-transmission systems feeding distribution feeders via a delta-wye transformer (see Figure 1).

Figure 1: Ground fault overvoltage occurrence for a distribution feeder fed through a delta-wye transformer.

GFO can arise on the following sequence of events:

1. A solid or low impedance single-line-to-ground fault occurs on the transmission side of the service.
2. The fault is detected by the transmission protection which then disconnects the transmission source. This islands the transmission line and connected distribution feeders.  Normally, the island will de-energize if all the connected facilities are typical customer loads.
3. With sufficient amount of PV on the island, the island may stay energized with the fault still present.
4. GFO then arises on the sub-transmission segment of the island. The duration that the GFO remains can potentially be long enough to pose a safety risk to personnel and/or damage electrical devices and equipment.

Utilities have advocated the implementation of a protection scheme based on measurement of zero sequence voltage (referred to simply as a “3V0” scheme). This scheme requires that potential transformers (PT) be installed on the high side of the substation transformer as shown in Figure 2.  The scheme can be costly, especially for single PVs connecting to distribution feeder that does not yet have any PVs.

Figure 2

Pterra conducted a research project (funded by the New York State Energy Research and Development Authority, NYSERDA) to identify an alternative means of detection and protection.  The initial phase of the research used PSCAD as the simulation tool.

The focus of the investigation was on:

• Detection on low-voltage side of the substation transformer.
• Do not require extensive amounts of additional equipment, material or construction.
• Monitor parameters that distinctly identify a potential GFO condition without being overly subject to over sensitivity (such as failing to detect the onset or presence of GFO).

In line with the above, the research looked at electrical parameters on the low-voltage side of the substation.  These included such measurements as voltage imbalance, transient voltage rate of rise, and negative sequence current.  However, none of these parameters provided sufficient sensitivity to meet the objectives of an alternative protection scheme.  The parameters that proved most promising were the secondary (or “low-side”) positive and negative sequence voltages.  Based on this, the Negative Sequence Voltage (NSV) protection scheme was developed.

Figure 3 shows the PSCAD plot of the three phase voltages on the high side of the substation transformer.  At t=0.6 sec, the SLG fault is applied.  After 5 cycles, the transmission side breaker opens, islanding the distribution feeder with PV.  After this, GFO forms on the high-side voltage.

Figure 3: PSCAD plot of high-side voltages.

With the NSV logic, the GFO condition is detected and trips the distribution side breaker.  The overvoltage is dissipated before it has a chance to reach its maximum value.  This is shown in Figure 4.

Figure 4: NSV applied to detect GFO and open the distribution side breaker.

PSCAD provided a working platform that allowed the Pterra researchers to identify potential alternatives to the 3V0 protection scheme.  Following this software-based approach, Pterra is testing the concept of NSV protection using actual hardware.

by  Ketut Dartawan, Amin M. Najafabadi

Pterra is presenting a paper on the above subject at the IEEE General Meeting 2017- Chicago 16~20 July.  Abstract of the paper follows:

IEEE updated its recommended practice and requirement for harmonic control in electric power system after more than two decades. The most updated version of the standard (IEEE Std. 519-2014) revised the 1992 version and its static harmonic voltage and current limits. Unlike the 1992 and the older versions of the standard, the 2014 version introduces a newer approach which considers the stochastic nature of harmonic distortions.  Furthermore, it recommends limits based on the number of times distortions may occur. For example, for the harmonic current distortion, it recommends three limits: daily 99th percentile, weekly 99th percentile, and weekly 95th percentile values. Applying the IEEE Std. 519-2014 for planning studies and for harmonic assessment of proposed projects can be very challenging because presently there is no known commercial tool which fully considers the stochastic simulations and limits required in the standard. This paper demonstrates the approach used by the authors in applying IEEE Std. 519-2014 to a harmonic study recently performed for a 180 MW solar farm.

Index Terms- harmonic analysis, harmonic filters, solar power generation, statistical analysis, time series analysis

The undesired outages of generating units during the July 1996 Outages in the Western Interconnection and the August 2003 blackout in the Eastern Interconnection have resulted in updates to reliability standards which secure, improve, and optimize generator response during power system disturbances. The North American Electric Reliability Corporation (NERC) has recently issued Standard PRC-019-2 which specifies reporting and review standards for generator protection coordination.  Because the skill requirements to conduct the review are not normally included in plant operations, outside experts are brought in that have a knowledge of what may be available in terms of information and data at the plant, the technical knowledge to conduct the coordination assessment and the experience to identify needs and deficiencies that are critical to presenting a credible review report.

In recent work, Pterra, acting as an external resource, developed approaches to conducting the review for compliance with PRC-019-2 for several legacy power plants.  Such power plants have been in operation for many years, but may have changed ownership at least once, and where test results and data may not be readily available.  This article discusses the general review approach, and applies this to a sample a 230-MVA Steam Turbine Generator Unit in a combined-cycle power station.

In recent work performed by Pterra, the issue of ground fault overvoltage (GFOV) was raised in relation to integration of distributed generation (DG).  In particular, can inverter-based photovoltaic systems, connected in distribution feeders, induce GFOV on the high -side of the substation transformer?  And if so, under what conditions could this occur?  Pterra was engaged to conduct a research study by NYSERDA (the New York State Energy Research and Development Authority) to answer these very questions.  The resulting study and findings are documented and presented in the attached PowerPoint presentation.

Pterra ITWG – Phase I GFOV Study – 011717

For those who want to skip right to the answers: Yes, they can.  Subject to DG/load ratios, performance of surge arresters and interaction of inverter controls from different manufacturers.

This topic was presented at the PSCAD Conference held October 6-7, 2016 in Houston, Texas.

Overview: A 200 MW DFIG wind farm is experiencing nuisance tripping.  These occur during switching of power factor correction cap banks comprising of 4×12 MVAR connected to the farm’s 34.5 KV collector buses.  Trip signals recorded by WTGs indicate power quality issues.  Harmonic distortion study of the Project did not indicate potential violations.

A more detailed analysis indicated presence of interharmonics.  The pdf file below presents the modeling, analysis and findings of the assessment.  Simulations were conducted using the PSCAD/EMTDC software, commercial product of Manitoba Hydro.

Introduction                                                               

For developers of power plants, one of the important factors to consider is where and how to interconnect a plant to an existing transmission network in order to reliably deliver its full output. For conventional power plants (i.e. coal, oil, natural gas, etc.), the availability of fuel supply and environmental permitting are the main considerations for siting. In the case of solar photovoltaic (PV) projects, given the availability of land area for mounting solar panels and sufficient solar irradiance, the point of interconnection (POI) to the grid can be the determining factor for siting. An assessment of the thermal capacity at potential POIs provides an effective screen for potential sites. Using transmission capacity injection analysis, developers can swiftly determine the capability of the existing network to support additional power from a new source such as a PV project. With this type of analysis, solar power project developers can know fairly early in the development process if the selected site and POI can support the plant’s output.

Pterra has been a fan of the General Electric PSLF transmission planning software product almost ever since our little boutique consulting company was established in 2004. Aside from its robust analytical engines that improved on PSSE’s capabilities, PSLF also had a very dedicated and responsive development team that was willing to work with us customize the product to our own unique client-driven applications. So we are more than glad to support the next development step for PSLF, as evidenced by our participation in this promotional video for the new PSLF. We will note that neither Pterra nor Ric Austria received remuneration in exchange for our participation.

Circuit breaker nameplates sometimes indicate only rating on symmetrical short circuit current. In such cases, the rating only reflects the AC component of the short circuit current. A common misinterpretation occurs when one compares the symmetrical short circuit current against the symmetrical short circuit current rating of the circuit breaker for the purposes of circuit breaker duty evaluation. This article provides pointers to avoid making the mistake.

Why is X/R Ratio Important?

Short circuit analysis is a critical piece of the engineering study for a power system. This analysis determines the maximum available fault current in the system, and hence the maximum level that the electrical equipment should be able to withstand.

When a short circuit occurs, the total short circuit current consists of:

• ·        AC component (varies sinusoidally with time), also known as symmetrical current
• ·        DC component (non periodic and decays exponentially with a time constant L/R;  L/R is proportional to X/R)
• ·        The DC component makes the symmetrical current become asymmetrical.

The X/R ratio affects the dc component, and therefore, also the total current. The higher the X/R ratio of a circuit, the longer the dc component will take to decay (longer time constant).

How to integrate a long-term view to the development of transmission systems? This is the challenge of transmission planning: to take into account future uncertainty, new power technologies and economic and operating realities in order to come up with a plan for transmission development.

In the new and emerging competitive markets, transmission planning needs to demonstrate relevance by providing applicable solutions to anticipated problems. In doing so, planners must provide answers to questions that have not yet been asked.

We will cover the various methodologies that have been tried, and point out the ones that work, and don’t work in the new competitive environment. We will discuss the methodologies for developing the strategic plan, and software-based approach to developing the implementation plan. We address the least-cost methodology, and how this is applicable to modern power systems. Finally, the course will discuss the possible future impacts in increasing penetration of renewable energies into the bulk power system, how to address them and how to plan for them.