In order to further analyze the cause of three-phaseunbalancedcurrent of 220kV Fenghuang - East Shihezi/West Shihezi – Mnasi power plant - third stage engineering of Manasi power plant, PSASP power system analysis synthesis program is applied to equivalent the loop network outside the network, and the corresponding equivalent parameters are obtained. The tower information and equivalent data are substituted into the ATPDraw model, and the three-phasecurrent value of the circuit is calculated. Finally, the current imbalance degree is calculated and compared with the measured data. The schematic diagram of the ring network simulation of Feng-Shi-Ma short line is shown in Figure 2.
inverter source systems, especially high power 400Hz inverter source systems, with finite output impedances, will experience voltage distortion in the form of phase voltage imbalance and phase shift due to unbalanced loading. At worst, unbalanced voltages can cause mal- function and even failure of power-consuming equipment. Thus, in inverter-fed power systems, it is the responsibil- ity of the inverter to ensure that certain tolerances on phase voltage imbalance at the load terminals are met under specified loading conditions. Reference  pro- posed a method of extending controller bandwidth. By transforming abc to dqo, the negative- and zero-sequence components can be view as a disturbance of 2ω and ω respectively. By utilizing controllers with bandwidths greater than the frequency of the disturbances, some at- tenuation will be achieved and the output voltage balance may be ensured. While this may be possible in low power 50/60Hz inverters, it would be very difficult, and likely impossible, to accomplish in high power inverters due to the difficulties of improving the switching frequency. The impossibility of this option is amplified for inverters with high output fundamental frequencies, such as the inverter under study. Reference  put forward a method of load current feedforward control. By using load cur- rent feedforward, the output voltage balance can be
In a regular three-stage three-wire converter structure, there are four current control flexibilities, and it might be insufficient to accomplish tasteful exhibitions under the unequal air conditioning source, in light of the fact that either altogether the wavered control or the over-burden current will be introduced. In the three-stage converter structure with the zero grouping current way, there are six current control opportunities. The additional two control opportunities originating from the zero grouping current can be used to expand the controllability of the converter and enhance the control execution under the uneven air conditioning source. By the proposed control techniques, it is conceivable to thoroughly cross out the swaying in both the dynamic and the responsive control, or diminished the swaying sufficiency in the receptive control. Then, the present adequacy of the flawed stage is altogether
In this strategy as unbalanced voltage supply is apply to the motor, current flowing throughout it is also unbalanced. To manage the unequal current require to apply firing position α with a remedial position Δα. The nonstop monotonic function between fixed starting and ending points is interpreted as a purpose of the load to be prohibited. The purpose is first empirically single-minded by reproduction process. In this system remedial position is additional with the firing position of phase which contains high current while corrective position is subtracted from the firing position of phase which contains a lesser amount of current. Corrective position can be finding out by equating voltage at motor load. By try and error method we are able to find out the range for the corrective position. The corrective position varies in range of 0˚ to 30˚. At firing angle (α) 0˚ to 60°, a corrective position Δα = 0˚ while at firing position (α) 150° to 180˚, Δα = 30˚. The graph for corrective position Δα is shown in Fig. 4.
To observe the system’s capability, the minimal value of active and reactive power flows through the grid, with the nonlinear load represented by a non-controlled three-phase power rectifier plus RC load that demands around 50 kVA from the DG. In this operation mode, the THD of the voltage imposed by the VSI rises to 3%, even with the resonant controller minimizing the 1st, 3rd, and 5th to 15th harmonics contents, however, the THD of the load current achieves more than 115%. To observe what happens with the DG network, a small time interval (1.96 to 2.04 s) before and after the connection with the grid is present while demonstrates the DG capability to supply nonlinear loads in the connected or isolated modes.
With the development of current society and the increase of people’s electricity consumption, the problem of power quality is becoming more and more ob- vious. Single-phase load in rural power network is relatively large, and it causes three-phase imbalance. Three-phase unbalance has great harm to power grid, including increasing line loss, power distribution system output reduction and generating zero sequence current .
In the second section of the study, all the test cases for unbalanced voltages are rendered and simulated in software. The parameter values obtained in the first section are used to create a software model of a 3P-IM. The 3P-IM model is used to simulate and compare different variables and results when subjected to different unbalanced voltage situations. The simulation program is required to have certain features, such as it should be able to render displays and plot various selected variables like output current, speed and torque. These variables and their plots are used to illustrate and compare the impacts of balanced and unbalanced voltage situations on the 3P-IM. The rating specifications of the induction motor are required to determine the physical parameters and are used in different computational steps in the simulation. These specifications are provided by the vendor, as shown in Table 1 and remain unchanged for any purpose of the experiment. This study uses a squirrel-caged rotor 3P-IM in a wye configuration and the model is based upon a Toshiba 0.37kW three-phase motor. To facilitate the analysis of three-phase quantities in the simulation, the variables are transformed to a simplified reference frame. Otherwise, the equations to determine the performance of a 3P-IM are tedious to derive and apply due to the presence of time varying inductances. An alternate approach commonly known as the Park’s transformation is applied to eliminate time dependant inductance in the performance equations. The Park’s transformation transforms the conventional abc reference frame to a dq0 reference frame, which makes the performance equations free of time dependant inductances. The dq0 reference frame is adapted in this study as it simplifies the analysis.
Test results Automatic control system of three-phase electrical load balance, can be measured unbalanced and balanced current on the sensor input side (table 1) and the voltage at the output side of the phase sensor R, S, T (tables 2 and 3). From the results of testing the load and stress load is not balanced with the current and voltage after being balanced, the phase data is unbalanced, phase R = 4.43, phase S = 1.162 phase T = 3.18. After being balanced by the system to R = 2.78, S phase = 3.27 phase T = 3.27, From the results of measurements and calculations in theory the percentage error is obtained for the average for each test current in phase R, S, T = 0, 01 A. With the implementation of an automatic control system that immediately detects phases to overload, where if there is an overload of more than 10% in one phase, the system will balance the load by activating the phase switches R, S, T by moving more phase paths to the phase path that is lacking until a load balance is obtained.
report the grid code requirements during voltage sags which necessitates unbalancedcurrent injection, the necessity of unbalancedcurrent injection into grid can be achieved by controlling each phasecurrent of the threephase voltage source inverter (VSI) based on the voltage drop of the respective phases in which voltage sag or fault occurs in the Grid Connected Photovoltaic Power Plants (GCPPP).The Individual phasecurrent control of the Voltage Source Inverter needs the calculation of the grid voltage angle of each phase and the computation of current reference of each phase to feed the current control loop which will generate voltage reference to trigger the power devices of the Voltage Source Inverter. The current reference generation consists of limitation of active current component based on reactive current required further elimination of zero sequence currents is proposed and finally re-scaling of instantaneous current references to avoid over voltages of non-faulty phases or healthy phases while preventing Grid Connected Photovoltaic Power Plants (GCPPP) from over currents is proposed. In order to validate the proposed method, a simulation model was developed using MATLAB/SIMULINK software and tested the system, the obtained results were presented here for different type of power system faults.
The modular cascaded H-bridge multilevel inverter,  which requires an isolated dc source for every H-bridge, is one dc/ac cascaded inverter topology. The different dc links in the multilevel inverter make autonomous voltage control conceivable . As an outcome, individual MPPT control in each PV module can be accomplished, and the energy reaped from PV panels can be maximized. In the interim, the particularity and ease of multilevel converters would position them as a prime hopeful for the up and coming generation of efficient, robust, and reliable grid connected solar power electronics . A measured cascaded H-bridge multilevel inverter topology for single- or three-phase grid connected PV systems is exhibited in this paper. The panel losses issues are tended to demonstrate the need of individual MPPT control, and a control plan with circulated MPPT  control is then proposed. The distributed MPPT control plan can be connected to both single and threephase systems. What's more, for the introduced three-phase grid connected PV system, if each PV module is worked at its own particular MPP, PV losses may acquaint unequal power supplied with the three-phase multilevel inverter, prompting introduce unbalanced grid current . To adjust the three-phase grid current, modulation compensation is additionally added to the control system.
The 4-leg FCMI circuit diagram is as shown in Fig. 1(b), the experimental test rig is shown in Fig. 5(a) and specifications are listed in Tables III – V in the Appendix. This is a compact modular construction, consisting of eight identical cell modules with circuit schematics as shown in Fig. 5(b), each containing: two complementary Infineon 600 V / 30 A IGBT switches with anti-parallel fast recovery diode in a TO-247 package, optically-isolated gate drivers, a 400 V, 560 µF flying capacitor, and on-board measurement and device fault protection circuitry. Two such cell modules are connected together to form a 3-level phase leg, and then to a common DC-bus, across which is a total capacitance of 1.5 mF. All capacitance values are selected from simulation studies to give ripple voltage levels below 5% at steady state . The DC-bus contains two identical capacitors connected in series to form a central neutral-point for 3-leg operation, and is supplied by a variable DC Switch-Mode Power Supply. PWM signals are generated using a Texas Instruments TMS320F2812 DSP switching at 5 kHz. Testing uses a Y-connected balanced resistive load of 25 Ω/phase is connected through 8.2mH filter inductors for current smoothing. The ‘star’ point of the load is connected back to the neutral-leg of the converter.
It is noted that different applications may have different requirements for the control of the average power, e.g., in the power production application, the active power reference Pre finjected to the grid is normally set as positive, meanwhile the large amount of the reactive power Qre f may be needed in order to help to support the grid voltage , . As for the electric machine application, the Pre f is set as negative for the generator mode and positive for the motor mode, there may be no or just a few reactive power Qre f requirements for magnetizing of the electric machine. While in most power quality applications, e.g., STACOM, Pre f is normally set to be very small to provide the converter loss, and a large amount of Qre f is normally required. Consequently, for the three-phasethree-wire converter system, there are only two more current control freedoms left to achieve another two control targets besides (10). These two adding control targets may be utilized to further improve the performances of the converter under the unbalanced ac source, which have been generally investigated in  and –.However, this paper focuses more on the evaluation of control limits and the control possibilities under the whole voltage dipping range.
Traditionally threephase diode rectifier and thyristor rectifier are widely used in common AC-DC power conversion and rectifier-inverter based AC motor drives however such topology draw pulsed current from AC mains, causing low input power factor (PF)and large current harmonics pollution into utilities, resulting in increased distortion of supply voltage and losses contributing to inefficient use of electrical energy. In recent year various controlled technique has been proposed in order to reduce the voltage stress on power switches. The rectifier with variable switching frequency controller results in heavy interaction between phases when midpoint of DC link capacitor is not connected to the ground or source neutral of controlled rectifier. This results in acoustic noise, high switching losses and difficulty in designing input filter. C. Qiao. et al.  proposed constant frequency integration controller for threephase star connected switch three level rectifier (VIENNA) with continue conduction mode at unity power factor correction. The proposed controller can operate by sensing either inductor current or switching current. The operation of VIENNA rectifier at constant switching frequency not only reduce the harmonics at DC side of rectifier but also the voltage stress on power switch devices over two level converter. However when the neutral point potential operation imbalance, the capacitor voltage exceed half of DC link voltage which increase the voltage stress of power devices. LiGao He et al.  proposed a neutral point potential balancing controller for three level three switch VIENNA rectifier base on balance factor correction by measurement of input phasecurrent. The proposed method balance neutral point potential with small variation by generating duty cycle of PWM signal with the help of real time calculation. June-Seok Lee et al.  introduced carrier based discrete PWM for VIENNA rectifier. The proposed method has high efficiency compare to carrier based continuous PWM method.
ABSTRACT-In this paper proposes a control of threephase bidirectional current source converter with fuzzy controller is proposed to inject the balanced currents in different conditions. In this paper we are using the fuzzy controller compared to other controllers i.e. The fuzzy controller is the most suitable for the human decision-making mechanism, providing the operation of an electronic system with decisions of experts. In this topology, under a balanced grid voltage condition, the DC-link inductor current can be regulated over a wide range from zero to rated value while the AC-side current has low harmonic distortion. The AC-side currents will be unbalanced due to the presence of a negative sequence component. The FLC comprises of three parts: fuzzification, interference system and defuzzification. Various control loop structures for the operation of voltage source converter under unbalanced grid voltage conditions are reported in the literature. However, use of similar control loop structures for CSC may lead to unstable operation. Therefore, a control scheme to inject balanced three- phase currents into the AC grid under an unbalanced grid voltage condition is proposed in this paper. The stability of the proposed control scheme is studied using a small-signal model of the converter.
Once faults are detected, the current reference generationshould be prioritized in the LVRT operation, as it also contributes to the current limitation. Different methods for current reference generation during grid faults have been presented in literature. In , the LVRT capability of the single-phase PV inverters is thoroughly discussed. In , a review on current reference generation of three-phase PV inverters during grid faults is performed. A few methods such as  have discussed the operation of a Pulse Width Modulated (PWM) grid-connected rectifier under grid faults. However, the LVRT strategy in grid- connected PV inverters is challenging, since the dynamics of the PV panels, dc power processing stage, and the capacitive dc-link can affect the operation of the entire system. In [16, 23], an Instantaneous Active Reactive Control (IARC) was proposed, which leads to non- sinusoidal output currents under unbalanced faults. A current reference generation method dealing with both PS and NS aiming at reducing the NS of the grid voltage has been proposed in . However, the active and reactive power waveforms include oscillatory components under unbalanced grid faults. In , the LVRT strategy controls both NS and PS to eliminate the active power oscillations under grid faults. In , a transformer less three-level PV inverter is introduced and the effects of the unbalanced faults on the neutral point in this inverter are analyzed for LVRT operation. The focus of  is on proposing new control strategies to further balance the voltage fluctuations on the neutral point under unbalanced faults.  Has proposed a LVRT control strategy in the d-reference frame for the grid-connected converters without considering the characteristics of a renewable energy source, either PV or wind. In , a three-phase system has been investigated, which offers six current control freedoms with a zero- sequence current path to mitigate both active and reactive power oscillations and also inject sinusoidal currents. However,in , a constant dc source has been used; the negative effects of the unbalanced faults on the capacitive dc-link have not beenexplored. Also, in , the dc-link voltage is assumed to be constant. This assumption is not proper in case of an unbalanced fault as total power would not be zero and ripple would be induced to dc link voltage. Although in  a PV source is modeled at the input side, the performance of the Fuzzy controllermethod is only
The 3DSVM system was analysed for various load conditions and THD was obtained. Fig. 10 and Fig. 11 shows the threephase output waveforms of voltage and current for various load conditions. For unbalanced load condition the tetrahedron will be found according to the voltage vector reference and it resides in tetrahedron values of 13 to 24 (i.e.) the lower half of the polyhedron. Requirement of DC link capacitor value is also minimum due to the use of four leg inverter and 3DSVM technique has an inherent property of having high DC link voltage utilization. As they contain only a single capacitor, handling it for circulating energy is not much complicated. THD analysis was performed for all the modulation strategies and the values are tabulated in TABLE VI.
This Paper has proposed Distributed MPPT based cascaded H-bridge multilevel inverter. The multilevel inverter topology will help to improve the utilization of connected PV modules if the voltages of the separate dc links are controlled independently. Thus a distributed MPPT control scheme for three-phase PV systems has been applied to increase the overall efficiency of PV systems. For the three-phase grid-connected PV system, PV mismatches may introduce unbalanced supplied power, resulting in unbalanced injected grid current. A modulation compensation scheme, which will not increase the complexity of the control system or cause extra power loss, is added to balance the grid current. A modular three-phase five-level cascaded H-bridge inverter has been built in the laboratory and tested with PV panels under different partial shading conditions. With the proposed control scheme, each PV module can be operated at its own MPP to maximize the solar energy extraction, and the three-phase grid current is balanced even with the unbalanced supplied solar power.
In a typical three-phasethree-wire converter structure, there are four current control freedoms, and it may be not enough to achieve satisfactory performances under the unbalanced ac source, because either significantly the oscillated power or the overloaded current will be presented. In the three-phase converter structure with the zero sequence current path, there are six current control freedoms. The extra two control freedoms coming from the zero sequence current can be utilized to extend the controllability of the converter and improve the control performance under the unbalanced ac source. By the proposed control strategies, it is possible to totally cancel the oscillation in both the active and the reactive power, or reduced the oscillation amplitude in the reactive power. Meanwhile, the current amplitude of the faulty phase is significantly relieved without further increasing the current amplitude in the normal phases. The advantage and features of the proposed controls can be still maintained under various conditions when delivering the reactive power. The analysis and proposed control methods are well agreed by experimentalvalidations.
Power electronic equipment usually introduces current harmonics. These current harmonics result in problems such as a low power factor, low efficiency, power system voltage fluctuations and communications interference. Traditional solutions for these problems are based on passive filters due to their easy design, simple structure, low cost and high efficiency. These usually consist of a bank of tuned LC filters to suppress current harmonics generated by nonlinear loads. Passive filters have many disadvantages, such as resonance, large size, fixed compensation character and possible overload. To overcome these disad-vantages, active power filters have been presented as a current-harmonic compensator for reducing the total harmonic distortion of the current and correcting the power factor of the input source .Fig. 3.1 shows the configuration of a three-phase active power filter.
This paper has proposed a fuzzy based control scheme for the three-wirethree-phase two-stage PV converter to improve the power quality under abnormal conditions. Among the major contributions of the fuzzy controller method is the mitigation of the double grid frequency oscillations in the dc-link voltage and the active power under unbalanced faults. Using the proposed current reference generation, the injected currents are sinusoidal with the THD value of lower than 5% in the experiments. More importantly, the control structure benefits from two operation modes, MPPT and Non-MPPT, both of which can operate under abnormal conditions. One of the main contributions of the paper is that a Non-MPPT operation mode for the dc-dc converter is