Clemson University
Electric Power
Research Association

Research Activity

The power industry is going through a revolution in integration of renewable energy with the system at both transmission and distribution levels. This, along with the increased computational power, better sensors, and better communication has been a driving force to achieve the vision of what has been called a smart grid, characterized by a more resilient and modular structure. However, the path to this vision has to be carefully crafted with changes in existing operational paradigm that support new technologies and integrate the renewables securely, in a cost-effective manner. This is where the academic research must be informed by industry practices and constraints to develop solutions for a smart grid.

In keeping with this goal, current research funded by CUEPRA has focused on 1) improved short circuit methods in presence on inverter based resources (IBRs), 2) comprehensive protection for microgrids fed by high levels of IBRs, 3) creating a test-bed on EMTP platform to test operation, stability, and protection of microgrids in real time, 4) real time communication platform to support testing of high speed induction motor drive (HSIM), 5) selective protection of meshed dc grids, and 6) grid impact study of battery energy storage system (BESS) in presence of PV.

Phani Harsha Gadde with advisor - Sukumar Brahma. Phani received second-best paper prize at the North American Power Symposium (NAPS) held at Wichita State University in October 2019 for his paper titled Realistic Microgrid Test Bed for Protection and Resiliency Studies. A total of 225 papers were in competition for the best-paper award. Congratulations Phani!

Current Projects

Phasor domain short circuit analysis in presence of inverter based resources

Austin Greenwood    Researcher- Austin Greenwood, MS student

    Advisor- Sukumar Brahma

    Funding- CUEPRA

The development of the synchronous generator in the late 19th century was a catalyst for the energy revolution we experienced in the 20th century. Charles Fortescue's paper demonstrating that unbalanced phasors could be expressed as a symmetrical set of balanced phasors was the match that lit the fire of this energy revolution. This paper is regarded as the one of the most important papers written in the 20th century and it has laid the foundation for how every single utility in the world performs fault analysis. The underlying assumptions in this analysis are 1) faulted system is linear, which means sources can be represented by a representative Thevenin model, 2) load currents can be neglected compared with fault currents. However, times are changing, and so must our methods of fault analysis.

            Over the past 30 years the price of fossil fuels, climate change awareness, and efficiency of non-conventional methods of generation such as wind and solar have all increased drastically. This, paired with progressive policy-making using tax breaks and renewable quotas, has begun another revolution in the power industry. Wind and solar are growing at an accelerating rate and this growth is causing waves in the utility industry. These resources use inverters to create AC waveforms on the grid. The primary problem with the proliferation of inverter-based resources is that almost all of them limit the amount of current they can output during a fault scenario to protect their internal components such as MOSFETs and IGBTs. In addition, most inverters connecting solar generators and Type IV wind turbine generators block negative sequence currents. This means an inverter-based resource (IBR) cannot be modeled as a linear source. Due to the low fault contribution the practice of neglecting load currents in fault analysis also comes under scrutiny.
            Thus, as IBRs reach higher rates of penetration (and in the case of certain microgrids, 100% penetration) traditional ways of carrying out fault analysis and standard protection schemes will prove to be incapable of achieving their performance objectives. This research will focus on developing new ways to perform fault analysis by using an iterative method to accommodate the behavior of nonlinear sources. The approach will be based on recommendations developed by Working Group C24 of the IEEE Power System Relaying and Control Committee (PSRCC) [1]. The approach uses the output characteristics of IBRs over a range of terminal voltages provided by the manufacturer, which allows for a control-agnostic modeling. By using a standard IEEE system and methodically replacing synchronous generators with IBRs the new approach to perform fault analysis will be demonstrated that can hopefully be scaled up to be used in field. Results will be validated with the same system simulated in electromagnetic transient program (EMTP), using PSCAD software.

[1] "Modification of Commercial Fault Calculation Programs for Wind Turbine Generators", draft report developed by Working Group C24, IEEE Power System Relaying and Control Committee.


grid impact study for customer owned and operated battery energy storage system in the presence of pv

Joshua Smith    Researcher- Joshua Smith, PHD student

    Advisor- Ramtin Hadidi, Randy Collins

    Funding- CUEPRA, CAPER

Technological advances, federal and state policy, and public opinion supported the exponential increase in PV penetration which has already changed the landscape of distribution planning. While the high PV penetration steepens the ramp rates for the generation and transmission planners, the power producing hours for PV do little to nothing to reduce the demand for the distribution planners. This is especially true for utilities that are winter peaking - as this peak likely occurs in the early morning before the sun has risen. While the high penetration of PV may not affect the capacity needs for distribution planners, utilities have already experienced: back feed through the substation transformers, excessive equipment operations, and difficulties with protection and control.

As distribution planners continue to deal with these problems presented by the increasing penetration of PV, they must now also consider the difficulties that will be faced as battery energy storage system (BESS) penetration starts with unforeseeable adoption rates. While utility owned BESS might postpone distribution upgrades, customer owned and operated BESS might expedite the need for distribution upgrades. This research seeks to present the possible outcomes from a variety of penetration scenarios to guide utilities in their planning as they shift their distribution planning paradigm to address the inevitable adoption of behind the meter BESS.

The IEEE 123 bus system and EPRI Ckt5 systems were used for this analysis. Since historical data is unavailable for these systems, a study was performed to determine whether the random number generator used for assigning load shapes would negatively affect the results of this research. It was determined that the effect of the random number generator was negligible.

A dataset created by another student in conjunction with this research was used. There are 100 sets of curves, each set containing: (1) base load, (2) base load + PV, (3) base load + PV + Time of Use (TOU) BESS, and (4) base load + PV + Maximum Demand (MD) BESS. For each circuit, every load was assigned one of these curve sets. The base case was simulated by applying (1) to every load. The solar penetration level was increased by replacing the penetration percentage of (1) with (2). The battery penetration level was increased by replacing the penetration percentage of (2) with (3) or (4). That is, 50% PV penetration with 50% BESS penetration represents 50% (1), 25% (2), and 25% (3) or (4).

As is expected, the increasing solar penetration produces the duck curve. Due to the choice of intervals used for the TOU pricing scheme, the increasing battery penetration worsens the duck curve. With minimal effect on reducing the peak, the extreme case of 100% battery penetration with 100% solar penetration has the worst back feed and the most variability in the head of feeder demand. For the MD pricing scheme, the increasing battery penetration level flattens the demand curve. The morning and evening peaks are reduced and the ramps are smoothed out. The valley in the middle of the solar day does not disappear because there is not sufficient capacity in the battery to charge extra in hopes of reducing the peaks further.

The increasing solar penetration injects more variability in the voltage from hour to hour while reducing the voltage variability throughout the circuit for a given hour. The hour to hour variability is seen in the increased rigidness of the voltage profiles. The reduced voltage variability throughout the circuit for a given hour is seen by the tightened bandwidth - that is, the difference between the minimum and maximum voltage during the solar hours is smaller for high solar penetration levels. The introduction of the BESS worsens the rigidness but counters the tightening of the bandwidth. Both the increases in the solar penetration and battery penetration work to push the voltage outside of the acceptable limits.


Communication platform to enable real-time testing of high speed induction machine (hsim) and drive

Michaela Loar    Researcher- Michaela Loar, PHD student

    Advisor- Ramtin Hadidi

    Funding- CUEPRA

Clemson, in conjunction with TECO Westinghouse, has been awarded funding by the DOE to create and test a high speed induction machine (HSIM) and drive. This machine runs at 15,000 rpm. This system is replacing a low speed induction machine and gearbox that typically steps up to a higher speed. Efficiency increases by taking out the gearbox. For testing purposes, the HSIM is coupled with a gearbox to a low speed induction machine (LSIM) to act as a varying load. There is a power amplifying unit (PAU) that controls the LSIM.


   The Speedgoat is a real-time target machine that is being utilized for communication between the both of the machines and their drives. Previously, the Speedgoat had not been used at the facility. Therefore, research has been done on communication protocols and how to implement on the Speedgoat. Initially, loopback test was conducted on digital inputs/outputs, IP/TCP, UDP to determine capabilities. Modbus communication will be the next topic as the PAUs use this protocol. Modbus will be used to send commands to both drives to stop testing due to faults. Currently, models in MATLAB/Simulink are being created to do HIL testing to simulate the faults and response of the Speedgoat.

protection strategy based on local measurements for multi-terminal dc grids

Munim Bin Gani    Researcher- Munim Bin Gani, PHD student

    Advisor - Sukumar Brahma

    Funding- Idaho National Laboratories, Department of Energy

Recent advancement in renewable energy generation technology is enabling the conventional ac grid system to shift towards dc, or hybrid grid. Since majority of renewable technologies produces dc output, which is being converted to ac through inverters, it is being conceived that such dc sources be connected to a dc grid at low voltage levels to feed various electronic loads that inherently operate on dc, and chargers for rechargeable devices including electric vehicles (EVs). In addition to reducing conversion losses, this approach can benefit from the known operational simplicity of dc circuits.

Although this idea is conceived at microgrid-level at low voltages, dc is also gaining popularity in high voltage transmission systems. With the advent of voltage source converter (VSC), HVDC systems have achieved much better controllability compared to the old line-commuted systems. Theoretically, there's no stability constraints or length-restrictions on power transfer over dc lines. These technological advances and operational advantages have prompted huge investments in HVDC grid, with China leading the field, having installed unconventionally long dc lines and formed a multi-terminal dc grid [1]. However, successful implementation of multi-terminal DC grids is slow due to lack of proper protection strategies. The difficulty of breaking dc currents is well-known but is being addressed by power-electronic devices [2]. The still unresolved issues in protection of dc systems are speed and selectivity. With the inception of fault, the current in dc system rapidly spikes, and goes on increasing until it reaches a steady state value which is generally much higher than the tolerance limit of the inverter circuitry, or the breaking capacity of the associated circuit breaker. Thus, fault current has to be interrupted in the transient stage, before it exceeds these limiting values. The required breaking time is shown to be a couple of milliseconds in HVDC [1] and a fraction of a millisecond in LVDC grids [3]. Such requirements are unprecedented. The protection system must reliably detect the fault (reliability), locate the faulted section (selectivity) and isolate the fault in as low as 50-100 microseconds (speed), depending on the system topology [3]. For this reason, main protection should be based on local measurements rather than remote communication, as communication latencies may not be able to match the speed requirements of protection for all topologies. In the existing HVDC systems, whenever a fault is detected, the whole system is de-energized due to lack of selectivity, which is a major drawback.

Due to speed requirements, the proposed methods of fault detection in different scholarly articles are based on traveling wave for HVDC systems. For LVDC grids, the methods are still based on overcurrent or undervoltage. However, selectivity is yet to be achieved for both LVDC and HVDC systems. With traveling wave based protection, the threshold values that have to be set for selective detection are highly system-dependent. This makes the settings vulnerable to temporary and permanent changes in system topology and creates obstacles towards further expansion of the system. A lot of simulations are required to determine the thresholds, which is time-consuming, and carries a risk of error under field conditions.

This project aims to completely resolve these issues and develop a protection scheme based on local measurements that lends itself to all topologies at high and low voltages, using the physics underpinning the system transients. An ambitious project, if successful, it will contribute towards the successful realization of multi-terminal and meshed dc grids.

 [1] D. Jovcic, G. Tang and H. Pang, "Adopting Circuit Breakers for High-Voltage dc Networks: Appropriating the Vast Advantages of dc Transmission Grids," in IEEE Power and Energy Magazine, vol. 17, no. 3, pp. 82-93, May-June 2019.
doi: 10.1109/MPE.2019.2897408

 [2] S. Augustine, J. E. Quiroz, M. J. Reno, and S. Brahma, "DC microgrid protection: Review and challenges," in Sandia National Laboratories, SAND2018-8853, 2018

 [3] S. Augustine, S. Brahma, and M. J. Reno, "Fault Current Control for DC Microgrid Protection Using an Adaptive Droop," in IEEE International Symposium on Industrial Electronics, June 2019.


emtp test bed for microgrids with inverter based resources

Phani Harsha Gadde    Researcher- Phani Harsha Gadde, PHD student

    Advisor- Sukumar Brahma

    Funding- Sandia National Laboratories, Department of Energy

Momentum towards realization of smart grid will continue to result in high penetration of renewable fed Distributed Energy Resources (DERs) in the Electric Power System (EPS). The drive towards resiliency will enable a modular topology where several microgrids are tied to-gather, operating synchronously to form the future EPS. These microgrids may very well evolve to be fed by 100% Inverter Based Resources (IBRs) and required to operate reliably in both grid-connected and islanded modes.

Since microgrids will evolve from existing distribution feeders, there will be unbalance in terms of load, phases, and feeder-impedances. Conventional inverter-controls that block negative-sequence currents may conflict with the unbalanced topologies, especially in islanded modes. Protection and control of such microgrids, spanning over grid-connected mode, islanded mode, and transition mode need urgent attention [1,2]. However, it is required to first design microgrid systems that are stable with up to 100% IBRs in all operating modes. Thus, as a start of this DoE-funded project, a detailed EMTP model of a testbed using the IEEE 13-bus distribution system is created in PSCAD, with multiple three-phase inverters connected, as it can happen in a realistic microgrid. Controls of such inverters are designed, without the need for communication, such that they operate harmoniously and support the unbalanced nature of the distribution systems in all operating modes. Grid-forming and grid-following operation philosophies are adopted for this purpose. Since inverters are not capable of providing high fault currents, current limiting function will also be implemented in the control-design. A series of time-domain simulation studies will be conducted to investigate the Fault Ride Through (FRT) characteristics of the fault limiter and the control strategy against both symmetrical and asymmetrical faults across the testbed. Also, based on the studies conducted the design of protection schemes suitable for unbalanced microgrids in all modes of operation (grid connected, transition, islanded) will be performed.

 [1] Sukumar Brahma, "Protection of Distribution System Islands Fed by Inverter-Interfaced Sources", Proc. IEEE PES PowerTech 2019, Milan, Italy.

 [2] Sukumar Brahma, Nataraj Pragallapati, and Mukesh Nagpal, "Protection of Islanded Microgrid Fed by Inverters", Proc. IEEE PES General Meeting 2018, August 2018, Portland, USA.

flexible protection scheme for microgrids

    Researcher- Trupal Patel, MS student

     Advisor- Sukumar Brahma

    Funding- Sandia National Laboratories, CUEPRA

The penetration of renewable energy resources has been increasing over the last decade leading to a subset of the power grid known as a microgrid. Microgrids are a combination of generation resources and load, forming an electrically sustainable grid that can function either connected to the larger grid or in an islanded mode. Microgrids tend to have a significant amount of their energy generation as rentable sources such as solar photovoltaic or wind-based generation. As the penetration of these renewable energy resources increases the protection schemes used to protect these grids and the generation devices must also evolve to accommodate the microgrid structure. A microgrid can operate in one of two modes grid-tied or islanded, in grid-tied mode the microgrid is connected to the macro grid and sharing its load and generation resources with the macro grid, in islanded mode the microgrid is disconnected from the macro grid and is self-sufficient, supplying energy to its local loads from the generation present in the microgrid. This ability to switch from grid-connected to an islanded mode of operation poses several challenges from a protection perspective.

There has been a significant push for the world to start using more renewable energy resources due to rising concerns regarding the environmental effects of energy production and as the cost associated with renewable energy decreases more and more renewable energy generation is expected to be connected to the grid. Several states in the US have also enacted legislation to increase the share of renewables in their energy mix, for example, Washington DC, California and Hawaii have increased its (Renewable Portfolio Standards) RPS target to 100% renewable by 2040, 2045 and 2045 respectively. More than half of the states in the US have set some RPS targets to increase the amount of renewable energy generation and reduce the carbon footprint of energy production. Many of the RPS targets also include specific requirements for a solar photovoltaic based generation to ensure variety in their energy generation [1, 2].

One of the problems arises due to the significant portion of the generation being renewable is that the most common renewable resources connecting at distribution level are photovoltaic solar generation units which connect through inverters whose output during system changes are dependent on the controls schemes. Inverter based sources limit the amount of current supplied during a fault or other disturbances to protect their power electronics. The limited current poses challenges as it drastically reduces the amount of fault current observed by traditional protective devices, which heavily rely on current magnitude for fault detection, creating difficulties in identifying and isolating faults in the microgrid. Due to low fault currents, most of the conventional protection principles fail to provide reliable protection [3]. The low fault current during islanded mode compared to grid-connected mode also pose challenges in terms of protective device coordination, as the coordination ranges grow large due to the low fault current during islanded mode and high fault current during the grid-tied mode.  In many cases depending on the microgrid composition, a single protection scheme and settings for both grid-tied and islanded modes is not possible. A protection scheme designed for the grid-tied mode is not guaranteed to work in islanded mode and vice versa.

Considering these problems, a possible solution is to create a protection scheme that can switch modes as the microgrid switches modes to and from grid-tied to island. A protection scheme where the setting for the protective devices can change when the Microgrid switches modes could be created to adequately protect a microgrid with a significant amount of inverter-based energy sources. The problem of limited fault current being present during islanded mode operation of a microgrid with high inverter-based generation is the topic of study actively being explored with proposed solution ranging from differential protection which is effective but expensive to using a combination of under-voltage, transient based and zero sequence-based protection devices [3]. A combination of a Flexible protection scheme that can switch modes as necessary alongside existing protective devices could serve as a possible solution to the problem of both low fault current and the difficulties in coordination of protective devices in the different operational modes of a microgrid.

            The primary scope of this Sandia National Lab (SNL) and CUEPRA funded project is to analyze an existing feeder that is being converted to microgrid with the addition of a significant amount of inverter-based solar generation, and design a protection scheme for this feeder. This protection scheme should be able to adequately protect the microgrid system in both grid-tied and islanded mode by seamlessly switching the protective device setting as the microgrid switches modes. The goal of the project is to create a microgrid protection system that properly protects the system and functions using readily available devices available today without the need for a new type of protective device. The feeder is modeled and analyzed using MATLAB - SIMULINK.

[1]    Bowers, R. "Updated renewable portfolio standards will lead to more renewable electricity generation" Available at:

[2] National Conference of State Legislatures. "State Renewable Portfolio Standards and Goals" Available at:

[3] T. Alexopoulos, M. Biswal, S. M. Brahma, and M. E. Khatib, "Detection of fault using local measurements at inverter interfaced distributed energy resources," 2017 IEEE Manchester PowerTech, 2017.

[4] S. M. Brahma, J. Trejo, and J. Stamp, "Insight into microgrid protection," IEEE PES Innovative Smart Grid Technologies, Europe, 2014.