Rajendra Prasad Kandula

Mitigating distribution transformer lifetime degradation caused by grid-enabled vehicle (GEV) charging

Although 75% of the vehicle miles traveled in the US by 2040 could be electric, few studies have quantified their impact on the distribution network even at low GEV penetration levels. This paper presents a Monte Carlo simulation of transformer life degradation using a fundamental and harmonic transformer thermal model, historical distribution transformer load profiles, hourly temperature data, and surveyed vehicle data to determine transformer loss of life. A simple control strategy, based on transformer current, is proposed to mitigate lifetime degradation. Simulation results are presented for a fleet of distribution transformers in Phoenix, AZ and Seattle, WA under controlled and uncontrolled charging scenarios

Loss comparison between SiC, hybrid Si/SiC, and Si devices in direct AC/AC converters

Direct AC/AC topologies for AC-to-AC power conversion benefit from the absence of DC-link capacitors and therefore high reliability as compared to traditional VSI-based topologies. Moreover, it is shown in this paper that the direct AC/AC converters also promise to provide higher efficiency than their voltage source inverter (VSI) based back-to-back (BTB) counterparts due to a dramatic reduction in switching losses. These factors allow the direct AC/AC converter to switch faster, and maintain much smaller size and lower cost relative to their competition. This paper compares the performance of three different device types (SiC, hybrid Si/SiC and Si) for use in a direct AC/AC converter. It is conjectured that traditional datasheets lack the level of detail needed for designing highly efficient direct AC/AC converters. Therefore, comprehensive loss models for all the devices are formed through a rigorous device characterization under varying (V, I, T) operating conditions. Finally, a loss comparison is performed to identify the most suitable device (among those characterized) for a specific 13 kV / 1 MW highly efficient direct AC/AC power flow controller.

Design and testing of a medium voltage Controllable Network Transformer Prototype with an integrated hybrid active filter

Dynamic control over real and reactive power flow is one of the key areas of concern for the modern grid. Traditional FACTS based devices have proven to be too costly and complex to achieve any significant market penetration. Modifying existing transmission assets to make them controllable may be a cost-effective method of increasing the grid controllability. Controllable Network Transformers (CNTs) have been shown theoretically to be an effective real and reactive power flow controller. The various options for filter design of a CNT are presented in this paper. The paper also discusses the design and testing of a medium voltage CNT prototype with an integrated hybrid active filter, rated at 200 kVA and 2.4 kV.

Losses in Medium-Voltage Megawatt-Rated Direct AC/AC Power Electronics Converters

Direct ac/ac topologies for ac-to-ac power conversion benefit from the absence of dc-link capacitors, and therefore, are highly reliable and have low cost as compared to the traditional voltage-source inverter (VSI)-based topologies. This paper deals with one of the more important tradeoffs considered in designing highly efficient converters: Losses. It is shown in this paper that the direct ac/ac converters have an inherently higher efficiency than their VSI-based back-to-back counterparts due to a dramatic reduction in switching losses (nearly 60%). Further, this paper compares the performance of three different device types (SiC MOSFETs, hybrid Si IGBT/SiC diode, and Si IGBTs) using wide-range device characterization that help to create detailed loss models. It is conjectured that traditional datasheets lack the level of detail needed for computing losses in direct ac/ac converters, and the availability of a multivalue voltage, current, and temperature-based loss profile is advocated. Using the obtained loss models, a comparison is drawn between the considered devices through simulations when operated in a 13-kV/1-MW direct ac/ac power flow controller, the controllable network transformer (CNT). The same loss-models are also used to compute losses in an experimental prototype of a 720-V, 10-kVA CNT and the results are compared with direct efficiency measurements. A similar computation is carried out for another experimental prototype at a 6.7-kV, 400-kVA, three-level, paralleled CNT. These experimental tests are used to confirm the validity of the analytical results presented in this paper.

A Practical Directional Third Harmonic Hybrid Active Filter for Medium-Voltage Utility Applications

An increased level of harmonics due to the proliferation of single-phase non-linear loads is raising serious concerns among utilities. Historically, passive filters have been proposed to reduce harmonics in MV utility applications. However, due to their limitations utilities are turning their attention to alternative solutions. At the same time, active filters are prohibitively expensive and are unlikely to become a realistic solution in the near future. In this paper a practical directional third harmonic hybrid active filter is proposed. A novel feature that adjusts the level of compensation provided by the filter based on the loading conditions of its passive components is introduced. Simulation and experimental results are presented. Issues related to utilizing existing VAr support capacitors for retrofit applications thereby achieving cost reduction and fail normal feature for increased reliability are addressed. Cost vs. performance curves are developed using factual utility harmonic data. Finally, the impact of a distributed filtering solution based on the proposed filter is shown using a simplified MV distribution system.

Power flow controller for meshed systems with a fractionally rated BTB converter

The increasing load demand, increasing level of penetration of renewable energy and limited transmission infrastructure investments have significantly increased the need for a smart dynamically controllable grid. Existing solutions based on FACTS devices, are complex and expensive to implement at transmission level or even sub-transmission level voltages. This paper proposes a novel power flow controller for dynamic control of active/reactive power in a meshed network. The proposed controller is realized by augmenting a transformer with a fractionally rated bi-directional Back to Back (BTB) converter. The main advantages of the proposed converter are the fractional converter rating, reliability and scalability.

Plug-and-play AC/AC power electronics building blocks (AC-PEBBs) for grid control

As ac-to-ac power conversion becomes increasingly importantly in applications such as grid power flow control and motor control, the demand for a plug-and-play converter is increased. Traditional VSI-based ac-to-ac power converters such as the back-to-back converter require bulky dc-link capacitors that limit their reliability. Therefore, approaches that offer direct ac-to-ac power conversion are gaining popularity. This paper proposes a plug-and-play concept for achieving ac-to-ac power conversion via the ac/ac power electronic building block (AC-PEBB). The AC-PEBB is a compact, self-contained cell requiring no energy storage. It can be used in a variety of direct ac/ac converters including matrix converters, controllable network transformers (CNT), etc. This paper details the construction of a prototype AC-PEBB to be used in a 13 kV, 1 MVA application. The effectiveness of the AC-PEBB is demonstrated experimentally in an example application of controlling grid power flows via a 10 kVA CNT prototype built by augmenting a standard transformer with a single AC-PEBB cell. Series and parallel connection of multiple AC-PEBBs to increase maximum voltage and current handling capability is also demonstrated.

Experimental validation of active snubber circuit for direct AC/AC converters

Although direct ac/ac converters such as the Matrix converter have been in existence for a long time, scaling of these converters to higher voltage levels has not been addressed widely in the literature. Tremendous challenges in terms of device voltage sharing, safe commutation, and fault protection inhibit the application of these converters to high voltage applications such as dynamic grid control. The novel active snubber concept remedies this problem by placing a half-wave rectified envelope around every device, absorbing excess energy and ensuring proper voltage sharing. This paper examines a high frequency active snubber design for direct ac/ac converters. The performance of the snubber is analyzed at 13 kV / 1 MVA via simulations, and a proof-of-concept is implemented in a 10 kVA lab prototype.

Compact dynamic phase angle regulator for power flow control

The need for a low-cost, high reliability power flow control technology is becoming exceedingly clear in the face of increased renewable penetration and electrification of automobiles. This paper proposes a power routing technology called the compact dynamic phase angle regulator (CD-PAR) for dynamic control of active/reactive power. The proposed device is a hybrid solution realized by pairing a fractionally-rated low-frequency transformer with a fractionally-rated power converter for a solution that is of lower cost and of higher reliability compared to traditional power electronics technologies for grid applications. The paper provides discussion of the technology and its implementation, followed by simulation and experimental measurements at 13 kV / 1 MVA. Key design challenges and their potential resolutions are also discussed.

Directional Triplen Hybrid Active Filter for radial systems

Increasing use of non linear loads in residential distribution systems can cause significant growth in neutral to earth voltages and false ground relay trips. Since it is impractical to enforce the residential and other small commercial customers to limit harmonics, utilities need an effective low cost harmonic filtering solution to limit 3rd harmonic propagation. This paper presents a Directional Triplen Hybrid Active Filter (DTHAF) for 3-ph 4-wire multi grounded radial distribution systems. The directionality feature enables isolating downstream triplen harmonic current from the system and blocking upstream triplen harmonic currents even with source voltage distortion. This feature restricts filter size to that of the harmonic load downstream. Hybrid active filter approach enables using a fractionally rated active filter and thereby reducing the cost of the filter. Existing power factor correction capacitors (PFC) on the system can be utilized for retrofit applications.