Controllable Network Transformers Research Articles

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.

Dynamic Grid Power Routing Using Controllable Network Transformers (CNTs) With Decoupled Closed-Loop Controller

Increases in system loads and in levels of penetration of renewable energy, together with limited investment in transmission infrastructure, are fostering the need for a smarter and more dynamically controllable grid. Flexible ac transmission systems devices can be used to dynamically control the grid and more efficiently route power and thus mitigate these stresses, but such devices are either too complicated and expensive for implementation or incapable of independently controlling active and reactive powers. A controllable network transformer (CNT) has a fractionally rated direct ac/ac converter and was introduced as a simpler and more cost-effective solution to realize dynamic power control between two areas. The CNT utilizes the dual virtual quadrature source (DVQS) technique to change both the line voltage amplitude and phase angle, thus enabling a dynamic power control; however, the control variables defined in this technique have a cross-coupling effect between active and reactive powers. In this paper, the CNT operating ranges with and without considering line resistance are analyzed; then, a decoupled closed-loop controller is designed to achieve independent active and reactive power control based on a reference power control command. To address the possibility of power overshoot in a CNT with DVQS, a hybrid open-loop/closed-loop proportional–integral controller is also proposed. Simulations and experimental results are given to verify the controller design.

An Integrated Controllable Network Transformer—Hybrid Active Filter System

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 theoretically shown to be an effective real and reactive power flow controller. However, the CNT is shown to produce unwanted third harmonic voltages and currents in the system, which must be handled. In this paper, the use of a hybrid active filter (HAF) is proposed for this purpose. This paper discusses the approach for design and control of such an integrated CNT-HAF system. It is shown that the integrated CNT-HAF system can also mitigate system unbalance, apart from providing four-quadrant control capability. Furthermore, this paper also presents experimental results of an integrated CNT-HAF system, rated at 200 kVA and 2400 V.

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.

Power flow control in networks using controllable network transformers

The drive for higher reliability has motivated many utilities to move toward a more meshed system. Two control areas are often connected together with tie-lines. Power flow through the tie-lines connecting two control areas is difficult to control. This lack of controllability of power flow is one of the major issues in the modern grid. It causes asymmetric stress on the grid assets. This makes some grid assets more vulnerable to failure than others, and therefore, decreases the overall system reliability. Presently utilities can achieve very limited power flow control using devices like load tap-changing transformers and phase-shifting transformers. Controllable network transformers (CNTs) were introduced as a simple, low-cost solution to the power flow problem. This paper develops a theoretical analysis for the operation of CNT in a meshed network. It also shows the various possible applications of the CNT. Experimental validation of the working principle of a small-scale prototype CNT is also provided.