Clark G. Hochgraf

Statcom controls for operation with unbalanced voltages

Voltage sourced static VAr compensators such as the Statcom need to be able to handle unbalanced voltages. Mild imbalance can be caused by unbalanced loads while severe short-term imbalance can be caused by power system faults. A synchronous frame voltage regulator is presented that works even when three phase symmetry is lost. This regulator addresses voltage imbalance by using separate regulation loops for the positive and negative sequence components of the voltage. The proposed regulator allows the Statcom to ride through severe transient imbalance without disconnecting from the power system and, further, to assist in rebalancing voltages. The regulator maintains sufficient bandwidth to perform flicker compensation. The controllers performance is simulated for a Statcom in a model distribution system where it is subjected to a severe single line to ground fault and a rapidly varying three phase load.

ACT3: a high-speed, high-precision electrical impedance tomograph

Presents the design, implementation, and performance of Rensselaers third-generation adaptive current tomograph, ACT3. This system uses 32 current sources and 32 phase-sensitive voltmeters to make a 32-electrode system that is capable of applying arbitrary spatial patterns of current. The instrumentation provides 16 b precision on both the current values and the real and reactive voltage readings and can collect the data for a single image in 133 ms. Additionally, the instrument is able to automatically calibrate its voltmeters and current sources and adjust the current source output impedance under computer control. The major system components are discussed in detail and performance results are given. Images obtained using stationary agar targets and a moving pendulum in a phantom as well as in vivo resistivity profiles showing human respiration are shown.<<ETX>>

A transformer-less static synchronous compensator employing a multi-level inverter

This paper examines the application of a high voltage multi-level invertor in a 13.8 kV distribution system static synchronous compensator (SSC). Equations are developed for the component MVA of the multi-level inverter. Trends in component MVA as a function of the number of inverter voltage levels and the modulation strategy are identified. Control of the DC bus capacitor voltages during phase voltage imbalance is identified as a problem. A method is described whereby the multi-level inverters DC bus capacitor voltages are actively controlled without using additional power components. The operation of the capacitor voltage control loop is demonstrated through EMTP simulations of an SSC responding to single phase and three phase load variations in a model distribution system.

Unbundling power quality services: technical issues

The modern industrial facility depends on sensitive electronic equipment that can be shut down suddenly by severe power system disturbances. A large number of these disturbances on the power system are a result of line faults which can cause momentary voltage sags. This results in equipment malfunctioning and high cleanup cost. This papers describes some of the emerging opportunities for new services to users of electrical power. In particular the focus is on the need for improved quality of power and methods for providing this new service.

Analog Electronics For A High-speed High-precision Electrical Impedance Tomograph

Electronic components for each electrode of Rensselaers ACT 111 impedance imaging system occupy two circuit boards. One is devoted to digital and timing circuits and the other to analog and interface circuits. The analog board includes the sinusoid generator, the current source and its compensator, the voltmeter buffers and some switching and calibration components. Some details on the design considerations and implementation of this portion of the system will be presented here.

Current Sources For Impedance Imaging Systems

It can be seen from the calculations in (l) that impedance imaging systems that apply currents and measure voltages are less sensitive to errors than systems which apply voltages and measure currents. For some system designs (2), it is possible to use a single floating current source with an appropriate switching system to sequentially address all electrodes in pairs. Other system designs require an individual source for each electrode, with consequent increased costs (3). For in-vivo systems which operate above 10 kHz, high resolution images require a high internal impedance for the current sources in the presence of cables from the instrument to the electrodes. This paper explores some of the circuits available for such current sources, and indicates their disadvantages along with an approach to a circuit configuration that resolves most of the problems.