Jorge E. Hernandez

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

Ubiquitous power flow control in meshed grids

A new method of power flow control in a meshed network has been presented earlier, to enable power dispatch along a defined path without impacting flows on branches adjacent to the path. This paper explores the type of power converter topology that could optimally provide the desired control function. This includes well known FACTS devices such as Back-to-Back Converters (BTB) and Unified Power Flow Controllers (UPFC), as well as newer devices such as Controllable Network Transformers (CNT), and Distributed Series Impedance (DSI) devices. Equipment ratings are calculated for the BTB, UPFC, CNT and CNT-DSI hybrid solutions. Scalability to grid voltages and power levels is also discussed. Power flow capability, implementing the proposed solution with ideal voltage sources, is shown via simulation for the IEEE 39 bus system. Finally, functionality is simulated in a meshed, four-bus system using switching devices. Proposed functionality would allow more efficient market operation, incentivize transmission investment and ensure physical delivery of contracted energy, thus enabling real-time markets for lowcarbon (green) sources.

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.

Flexible electric vehicle (EV) charging to meet renewable portfolio standard (RPS) mandates and minimize green house Gas emissions

Meeting RPS mandates and accommodating EVs poses a difficult problem for utilities. This paper proposes coupling EV charging with the availability of renewable energy at the point of use to realize dramatic system level benefits. A realistic set of scenarios are simulated, using real-time grid data from NYISO and wind farm data from NREL, to prove that with a small percentage of wind energy penetration and flexible EV charging, GHG emissions of the light-duty transportation sector can be dramatically reduced. This is achieved without any additional generation requirements from the existing power grid, while providing the additional benefit of reducing the spinning reserve requirements needed to cope with the intermittency problem of the new wind resource. The concept of a parallel market structure as a means to practically implement the concepts in this paper is also introduced.

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.

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.

Power flow router sensititvities for post-contingency corrective control

Power electronics based power flow routers are powerful tools that provide easy and cost-effective control of power flows in large-scale electricity networks. By adding additional controls, fast power flow routing can increase the secure operational range of bulk power systems both during normal operations and contingencies. Increased integration of power flow routers could allow the bulk power system to operate at a lower cost while maintaining, and possibly increasing, the level of security. In this paper, we develop power flow router sensitivities that enable corrective flow routing capabilities during post-contingency conditions.

Validation of the plug-and-play AC/AC power electronics building block (AC-PEBB) for medium voltage grid control applications

As ac-to-ac power conversion becomes increasingly important in applications such as grid power control and motor control, demand for a plug-and-play converter has increased. Traditional voltage-source-inverter-based ac-to-ac power converters such as the back-to-back converter require dc-link capacitors that limit their reliability or significantly increase their size and cost. This paper examines a plug-and-play concept for achieving ac-to-ac power conversion via the direct-ac/ac power electronic building block (AC-PEBB). The AC-PEBB is a compact self-contained cell requiring no intermediate energy storage. It can be used in a variety of ac/ac power electronic applications such as matrix converters and controllable network transformers. This paper details the design of an 800-V 100-A AC-PEBB prototype to be used in a 13-kV 1-MVA application. Successful test results using several AC-PEBBs in a variety of configurations are demonstrated at up to 11 kV and 600 kVA.

A 7.2 kV experimental setup of a third harmonic hybrid active filter for medium voltage utility applications

A practical third harmonic hybrid active filter capable of cost-effectively suppressing third harmonic currents in medium voltage distribution systems was proposed in [1]. The design of the proposed filter was driven by strict utility requirements - reliability, cost, and system impact. This paper extends the previous work by designing, building, and testing a 7.2 kV medium voltage experimental implementation of the proposed filter. Extensive simulations were performed to ensure safe operation of the experimental setup under normal operating conditions and during faults/shutdown of the filter. Experimental results are presented which demonstrate the performance of the proposed filter. The impact of a distributed filtering solution based on the proposed filter on a typical residential/small commercial distribution system was analyzed using actual utility data. The simulation results show considerable third harmonic neutral current and neutral-to-earth voltage reduction by deploying the proposed filter.