Negative Phase Sequence Research Articles

A control strategy of frequency self-adaptation without phase-locked loop for VSC-HVDC

A control strategy of frequency self-adaptation without phase-locked loop (PLL) under αβ stationary reference frame (αβ-SRF) for a VSC-HVDC system is presented to improve the operational performance of the system under severe harmonic distortion conditions. The control strategy helps to eliminate the cross-coupling under dq synchronous reference frame (dq-SRF), and is achieved through two key technologies: 1) positive phase sequence (PPS) and negative phase sequence (NPS) fundamental components are extracted from the AC grid voltage with an improved multiple complex coefficient filter (IMCF), and 2) grid instantaneous frequency is rapidly and precisely tracked using a frequency self-adaptation tracking algorithm (FATA) without PLL. The proposed strategy is applied to a point-to-point VSCHVDC system and validated by means of simulations. The results are compared to those with the traditional vector control strategy under dq-SRF. Simulation results illustrate that the proposed strategy results in better system performance than that with the traditional strategy in terms of harmonic suppression under normal and severe operating conditions of the AC system.

Software for optimal selection of places for installation of balancing devices in 0,4 kV electric power systems loaded with electric motors

This publication considers the issues of development of the software program for designing of 0,4 kV power supply systems with motor-actuated load under voltage unsymmetry conditions (using the example of the Trans-Baikal Territory). Voltage unsymmetry is practically constant phenomenon in the electric power networks of different voltage types. Voltage unsymmetry effects significantly the electric power consumers, including the supply mains itself. It has especially negative impact on the electrical equipment operation process and its lifetime. The urgency of the problem is confirmed by multiple research on the same topic and by significant number of damages suffered by the electric power consumers staying in service (especially in the Trans-Baikal Territory and in the Far-East regions). Voltage unsymmetry causes economic loss and reduction of the electromagnetic interference value by the voltage unsymmetry coefficient in negative-phase sequence (K 2 U) gives inevitable economic effect accordingly. However, the payback period for the activities aimed at reduction of electromagnetic interference, will vary from some months to several years. The more accurate value of the payback period may be obtained using the developed software program. The developed software design program is implemented by means of the programming language C# in Microsoft Visual Studio environment, using the built-in cross-platform database SQLite. The software program shall allow making quick and accurate calculation of the power losses, to determine the economic feasibility of provision special measures for removal of the voltage unsymmetry, for determination of optimal application and location of the balancing devices. The software implementation in power systems loaded with electric motors will improve reliability and efficiency of asynchronous motors. The software is of interest for developers of projects on power supply systems for regions with non-linear loads.

The effect of fast changes in voltage unbalance on vibration characteristics of asynchronous motors

The effect of voltage unbalance in electrical networks that supply electrified traction loads on the operating regimes of large asynchronous motors (AMs) is considered. Based on the method of symmetrical components and analysis of hodographs of magnetic fields of AMs with a short squirrel-cage rotor, the influence of various types of voltage unbalance (continuous and intermittent) on its vibration characteristics is studied. It is shown that an increase in magnetic induction in the air gap of an AM causes an increase in radial vibration generating forces in the case of voltage unbalance. The obtained results indicate that alternating voltage unbalance can be riskier for AM operation than is constant voltage unbalance, in addition to having increased levels of AM vibrations. It is concluded that the use of new parameters of electric-energy quality allowing determination of instantaneous values of unbalance amplitude and the angle between symmetrical components of voltage of positive and negative phase-sequence is reasonable. The relevance of this research is confirmed by typical cases of AM shutoff at an oil-pumping station due to tripping of vibration-protection sensors.

Study Hybrid Compensation Cophase Traction Power Supply System Schemes for High-Speed and Heavy Haul Electrical Railway with V Connection Transformer

In order to solve negative phase sequence problem of V connection transformer in the high speed and heavy haul electrical railway of China, the hybrid compensative co-phase traction power supply system which based on passive and active compensation is proposed. Firstly, There construction and capacity distribution are analyzed, and the compensation current of active equipment is gave; Second, the feature of the hybrid compensative schemes are discussed. In the end, the related simulation results have confirmed the effectiveness of the compensation schemes in this paper.

Analysis the Reason of Head and Tail Pully Driving and Bidirectional Motor Negative Phase Sequence over Current Protection Malfunction

Analysis the wiring of head and tail pully driving & bidirectional 6KV motor，and found the reason of negative phase sequence over current protection malfunction did occur when the motor ran in the reverse direction , then compared the solution methods to make sure which one is better. At last use the method that change the sampling program to make the motor negative phase sequence over current protection wil in effect.

Negative Phase-Sequence Voltages on E&W Transmission System

In the absence of phase transposition, overhead lines are a major source of unbalance in both currents and voltages on the transmission system. In the England and Wales (E&W) transmission system, the individual circuits are phased such that the average unbalance produced is minimized. The main negative phase-sequence (NPS) voltages on the transmission system arise at loop-in points for generation and load connection under an outage condition of one of the connection circuits and where there is a large NPS voltage induction from the parallel circuit. If not limited, excessive unbalance can cause system incidents and lead to damage of power system equipment.

Design of Harmonics Extraction Filter for Three-Phase System

This paper presents a filter design method for harmonics extraction of positive- and negative-phase sequence in a three-phase system. Using the complex transformation of the three-phase voltage, the filter can be designed in the frequency domain. The design example and the experimental results have been shown.

A wavelet-based approach for protection of generators against unbalanced currents

Abstract This paper presents a new wavelet-based negative-phase sequence (unbalanced currents) protection scheme for synchronous generators. The scheme utilizes the three line voltages and the three line currents available at the generator terminals. Wavelet transform concept is presented. The principle of operation of the algorithm is introduced. Feature extraction technique is derived. Data window necessary for the algorithm is ten samples based on 1200 Hz sampling frequency. Generated data from the simulation of a model power system using emtp were used in the matlab software to test the performance of the technique. The performance under different system conditions is discussed. The results show that the new approach provides robust and accurate classification for a variety of power system conditions.

不平衡変圧器漏れインダクタンスを考慮した自励式変換器の制御方式

A voltage source type converter must be linked to a power line through some amount of inductance to restrict the short circuit current. A leakage inductance of a transformer is usually used as that current limiting inductance. The conventional converter control is designed with assuming that the transformer has three-phase balanced equal reactance. This paper proposes novel control system of voltage source type converter, which can manage converter output even if the converter transformer has three phase unbalanced leakage inductance. The proposing control can apply 3 legs type three-phase transformer having three-phase unbalanced leakage inductance to a voltage source type converter, which is cheaper and smaller than three single-phase transformers or precisely well turned 5 legs type three phase transformer having three phase balanced inductance. The proposing control is based on the phase sequence description of the converter system, and the formulization of the system in phase sequence domain is performed in the paper. The control utilizes real time DFT as pre-processing of the control to obtain the state variables in phase sequence domain. The controller indicates the converter to output positive and negative phase sequence voltage in accordance with the transformer impedance, output current reference and AC bus voltage. The theoretical qualitative discussion of the proposing control concept and quantitative evaluation of the proposing control with using computer simulation are presented in the paper.

Static relays permit enhanced protection

Multi-function multi-phase static overcurrent relays, using digital technology available today, provide basic time, (51) (51G) (51N) and instantaneous (50) (50G) (50N) three phase and ground overcurrent protection, and may offer some or all of the following additional functions: undervoltage (27); overvoltage (59); reverse phase sequence (47); negative phase sequence voltage (47N); directional overcurrent (67) (67N); frequency (81O/U); reverse interlocking trip coil monitoring; and battery monitoring. This article describes two applications that use some of the additional functions in such a relay: reverse interlocking, also using the directional overcurrent (67) function; and ground fault detection for systems utilizing high resistance grounding. The first application was installed on a 4160 volt power system which was supplied by two 5000 kVA transformers.

Unbalance Load Compensation for a Three Phase Electrolytic Capacitor-Less Converter /Inverter

Unbalance load compensation for an electrolytic capacitor-less converter/inverter of the flywheel long life uninterruptible power supply which rating of 3 phase, 200V and 5kW-1min is described. The compensation has following characteristics.(1) Automatic output voltage balancer to cancel output voltage offset and output unbalance of 3 phase inverter.(2) An input negative phase sequence current controller to compensate input current harmonic distortion at unbalanced loads.(3) Automatic gain control on converter to improve input currents and transient states.The UPS has excellence test results which are obtained such as total harmonic distortion THD of input current of 1.2% and 2.0% at 2.6kW single phase load and single rectifier load, respectively. Output voltage offset and unbalance are compensated by automatic output voltage balancer under 0.5s.

DPLFS (Digital Phase Locked Frequency Synthesizer) Used in the Individual Phase Voltage Control of ±300 KVAR ASVG

ASVG is often expected to operate while the asymmetrical system fault occurs at the ASVG export with some electrical distance. A new firing control method is put forward . This paper describes the realization of DPLFS(Digital Phase Locked Frequency Synthesizer) and its use in the individual phase voltage control of ASVG. The field test reveal that the ASVG can generate a controlled negative phase sequence voltage through 3-phase individual phase voltage control. The computer simulation results show that the ASVG can work continuously owing to this control method even when asymmetrical fault occurs.

97/02881 Performance of a model treble-tripler saturated reactor

Abstract Results are given for a small laboratory model of a treble-tripler saturated reactor. Computed and experimental values are in reasonable agreement and illustrate the large change of current for a small voltage change together with low harmonic generation. Unbalanced voltage energisation produces a corresponding negative-phase sequence current in the supply with a significant third harmonic component.

Performance of a model treble-tripler saturated reactor

Results are given for a small laboratory model of a treble-tripler saturated reactor. Computed and experimental values are in reasonable agreement and illustrate the large change of current for a small voltage change together with low harmonic generation. Unbalanced voltage energisation produces a corresponding negative-phase sequence current in the supply with a significant third harmonic component.

Development and performance testing of a 200 kVA damperless superconducting generator

A 200 kVA, 3000 RPM superconducting generator has been developed and tested. The rotor has been wound with superconducting wire of Nb-Ti alloy. A closed-circuit liquid helium system has been designed and installed for cooling the superconducting windings. The stator carries the air-gap type armature windings and a laminated-iron flux-shield. A new concept in the design of superconducting generators with high short-circuit ratio (more than 5) has been introduced. This eliminates the requirement of an electromagnetic damper and quick response excitation system. The generator has been comprehensively tested in the superconducting state. Open-circuit and sustained short-circuit tests, three-phase sudden short-circuit tests, synchronization with the grid and parallel operation with power systems have been conducted. The synchronous machine was operated up to its rated kVA in the four quadrants-as a generator and as a condenser with leading and lagging power factors. A few special tests on superconducting generators, which were not reported earlier, such as direct-online starting of a 20 hp squirrel-cage induction motor and negative phase sequence tests have also been performed successfully. Test results and conclusions are given.

Novel microprocessor based negative phase sequence relay and meter

Microprocessor application in relaying and condition monitoring technology is becoming increasingly popular. In this paper, a novel microprocessor based negative phase sequence (NPS) current detector for motor condition monitoring and relaying for motor/generator protection is developed. The proposed meter eliminates the need of a negative phase sequence filter and does not require complicated digital signal processing techniques. Instead it uses a phase sensitive rectifier. A prototype based on Intel's 8085 A microprocessor was built and tested. Complete circuitry, main and interrupt software flowcharts are provided. Results show the proposed NPS relay and meter can provide a high degree of accuracy and satisfactory results.

Computation of hysteresis motor performance including negative sequence excitation

Torque/slip characteristics for a hysteresis motor with a cobalt-steel rotor are computed using a multilayer electrical circuit concentrated-parameter analogue to represent the electromagnetic field equations. Hysteresis in the material is represented using a digital simulation technique together with an empirical method for the inclusion of the rotational hysteresis component. Reverse acting torques due to the backward rotating fields produced by negative phase sequence excitation are examined. It is shown that the forward torque is altered by the backward rotating field owing to the nonlinearity of the rotor material.

Probabilistic modeling of voltage asymmetry

A new formulation is proposed regarding power network imbalance. From symmetrical component theory, a random vector of imbalance is defined having two dimensions. The probability distribution of negative phase sequence voltage is calculated and used to derive a probability distribution for current harmonics generated by a six pulse power converter. Three applications of the theory are considered, the first concerns the validation of the model and the characterisation of imbalance observed on two networks, the second concerns the interaction between voltage imbalance and harmonic current generated by a power converter and the third application concerns the overheating of an induction motor subject to a random negative phase sequence voltage. The analytical nature of the model forms a compromise between simplicity of utilisation and the representation of complex phenomena. >

Negative-phase-sequence reduction with adjacent static reactive-power compensators

Static reactive power (VAr) compensators (SVCs) can be used to reduce negative phase sequence (NPS) voltages by balancing, for example, phase-to-phase-connected loads. In such cases, they are effective until their controlled elements, usually thyristor-controlled reactors, reach their operating limits. Once this happens, NPS currents flow in the system and produce NPS voltages. If several load balancing SVCs are located in close electrical proximity, adjacent load balancing SVCs are able to absorb some of this uncompensated NPS current, reducing the overall NPS voltage profile. The mutual support thus provided should be considered when designing the ranges of such load balancing SVCs. The paper describes how to quantify the mutual support in a system with multiple load balancing SVCs.

Using phasors to analyze power system negative phase sequence voltages caused by unbalanced loads

An analytical method is demonstrated which allows the level of negative phase sequence (NPS) voltage at a busbar to be expressed as a sum of phasors representing independent sources. The method is extended to enable the balancing capability of Static VAr Compensators (SVCs) with individual phase voltage control to be assessed. The capability of such SVCs and the allowable levels of NPS voltage on the system, including any short term limits, can be combined in a capability chart showing the unbalanced loads which can be supplied from a substation. The approach facilitates the treatment of fixed unbalances due to filters or intentional offsets designed to maximise the SVC balancing range for specific loads. Field test results are presented which validate the analytical methods used. >