Open-delta Connections Research Articles

A method for measuring capacitive current in distribution networks based on injection signal method

Abstract In view of the problems that exist in the current distribution network, such as the narrow range of measurement capacitance to the ground and the difficulty of selecting the appropriate signal frequency. Based on the phasor method, this paper proposes a method for measuring capacitance current in a distribution network based on injected signal and analyzes and calculates the error of the method. The measurement technique involves inputting two constant-amplitude signals—one high-frequency and one low-frequency—into a zero-sequence PT (Potential Transformer) configured with an open delta connection. Based on the equivalent circuit models for both high and low-frequency signals, the high-frequency signal’s increased frequency allows the magnitude to be disregarded, simplifying the calculations. Using a simplified equation for the low-frequency signal component, this method enables rapid calculation of the capacitor current in the distribution network. Based on MATLAB software, the simulation calculation model is established, and the simulation results show that the measurement method is simple to calculate and has high accuracy, which can be used to measure the large-capacity grounding capacitance current test, so as to improve the measurement accuracy of the capacitance current of medium and low voltage distribution network.

PERFORMANCE STUDY OF UNBALANCED LOAD ON OPEN DELTA TRANSFORMERS

A transformer is a device that can transfer (transmit) and change electrical energy from one electrical circuit to another with the same frequency based on the principle of electromagnetic induction. The transformer that will be used in this research is a three-phase open delta connection transformer, which is connected to a delta connected load. If one of the three phase windings in the delta connection transformer is damaged, then the remaining two windings can be implemented or designed into three phase open delta connections, which can be used to distribute three phase electrical power to the load. The performance of the unbalanced load influence on the open delta transformer, when it is loaded with an unbalanced load that is not the same size on each phase, causes an increase in current on each phase, resulting in an increase in the input power absorbed by the unbalanced load resulting in power loss. larger (losses) with reduced efficiency. The aim of the research is to study the performance of unbalanced loads on open delta transformers and the research method is an experimental method in the laboratory, which includes testing on three-phase open delta transformers in unbalanced load conditions. The measurement results show that there are differences in performance (output results) in the influence of unbalanced loads between the average current and losses and the efficiency of the open delta transformer.

4-Node Test Feeder with Step Voltage Regulators

This work has two main contributions; First, the development of a general, exact and standardized Step Voltage Regulator model considering all possible configurations and second, the proposal of a 4-Node Test System for testing and evaluation of three-phase Step Voltage Regulator connections. Although the 4-Node Test Feeder for testing three phase transformer configurations is already available in the literature, there is not such model for the inclusion, testing and validation of Step Voltage Regulators in a test feeder. With the work presented in this paper, a new test system will be available to evaluate and benchmark programs or algorithms that attempt to include different configurations of Step Voltage Regulators. The formulation is stated for all three phase Step Voltage Regulators; i.e. wye, close-delta and open-delta connections, both type A and B regulators, in raise or lower positions. Then, all these models are included in a 4-Node Test Feeder to obtain several power flow solutions. All obtained results will be available for power flow software developers on-line.

A Three-Phase AC–AC Converter in Open-Delta Connection Based on Switched Capacitor Principle

This paper proposes a new topology for a three-phase open-delta connection ac–ac converter based on the switched capacitor principle. The topology employs only capacitors and switches in the power stage, which can provide high efficiency and power density. It operates in open loop with constant values for the gain, duty cycle, and frequency. The converter is bidirectional and thus it can step-down or step-up the output voltages. A 3.5-kW prototype was built, tested and used to verify the theoretical analysis for resistive, inductive, and nonlinear loads. The efficiency of the topology is around 93% in nominal load and the power density is 1.178 kW/kg.

Evaluation of load division among transformers of different capacities in the grounded wye–delta and open wye–open delta banks under balanced loading and various power factor conditions

This paper evaluates the load division among transformers of different capacities in the grounded wye–delta and open wye–open delta banks under balanced loading and various power factor conditions. In some contingency cases two or three single-phase transformers of different kVA ratings are connected as a three-phase transformer bank to provide three-phase service for critical customers. In these cases, the load division among transformers becomes unequal and results in an unbalanced distribution system. The results of this study corroborate power company experience. Even when both three-phase sources and loads of the bank are balanced, the load division among transformers is unequal if transformers of different capacities are used in a bank, or an open wye–open delta connection is adopted.

Performance analysis of an asymmetrical phase-converter-fed induction motor

Of the various types of single-phase-to-three-phase static power converters used to interface a three-phase motor to single-phase supply, one simple type uses an open delta connection of a sinusoidal supply and a pulsewidth modulated (PWM) waveform to feed the motor. This paper presents an analysis of the performance of the three-phase motor when fed from such an asymmetrical supply. The analysis is based on the steady-state approximate-equivalent circuit of the motor, using the method of symmetrical components. The analysis is directed toward prediction of motor phase and line currents, along with the nature of generated torque. The computer-simulated waveforms are presented and compared with results from tests on a laboratory setup.

Models of grounded mid-tap open-wye and open-delta connected transformers for rigorous analysis of a distribution system

The paper presents detailed mathematical models of grounded mid-tap open-wye and open-delta connected transformers for rigorous analysis of a distribution system. The transformers are accurately modelled in phase coordinates. The proposed transformer models consider the copper and core losses, winding connections, the phase angular displacement between primary and secondary windings, and off-nominal tapping. Open-wye and open-delta connections with mid-tap grounded on the secondary side of the transformer banks are quite often used to supply the single-phase lighting load and three-phase power load simultaneously. The detailed models of these transformers are indispensable for analysis of the inherent imbalance of a distribution system.

Substation transformer connection effects on sensitive industrial loads

In electrical power distribution, it is both convenient and economical to use the open delta transformer connection to supply three-phase power to small consumers. Recently, however, problems such as rectifier failure in equipment power supplies have been attributed to this type of transformer connection. This paper presents the results of an investigation into the existence and severity of consumer problems arising from the open delta connection. It describes both theoretical and laboratory analyses of the presence of distortion in magnitude, phase angle and waveshape of the voltage and current waveforms. The analyses show that, although there are some conditions under which the open delta connection should not be used, in general the connection is suitable for supplying consumer loads.

Improved Starting Process Alternative to Conventional Star-Delta Switching of A-C Meters

For the purpose of reducing mechanical shocks caused by the sudden increase of torque, and to avoid high current transients as a result of the interruption of supply in the conventional stardelta switching, an alternative starting process is presented here. It is proposed to start the motor on the unbalanced-delta connection in which a resistance is inserted in one of the lines feeding its delta-connected stator. When the running speed has been reached, the resistance is bridged to permit normal balanced running. It is also proposed to use the unbalanced-delta connection as an intermediate step in star-delta switching. In this respect, the once suggested open-delta connection is proved unsuitable for this purpose.

Motor-Starting Lamp Flicker on Open-Delta Transformer Banks

When a motor is started through a transformer bank that serves both power and lighting loads, there is a dip in voltage on the lighting circuits. A digital computer study has been made of the voltage on the lighting phase during motor starting for the open-delta connection of transformer secondary windings. Equivalent circuits have been developed for the general case of the open-delta transformer connections having legs of different kilovolt-ampere ratings.

A Field Method for Determining the Leading and Lagging Regulator in an Open-Delta Connection [includes discussion

To make correct line-drop compensator settings on single-phase regulators connected open delta on a 3-phase 3-wire system, it is necessary to identify the units, commonly called leading or lagging units, in which the regulator current has been advanced or retarded by the open-delta connection. This paper describes a field method for making this identification, using only the standard regulator controls without the knowledge of phase identity or sequence and also without additional measuring or phasing equipment. The discussion includes mathematical analysis, curves, sample calculations, and laboratory analysis used in developing this method.