High voltage direct current (HVDC) emergency control can significantly improve the transient stability of an AC/DC interconnected power grid, and is an important measure to reduce the amount of generator and load shedding when the system fails. For the AC/DC interconnected power grid, according to the location of failure, disturbances can be classified into two categories: 1) interconnected system tie-line faults, which will cause the power unbalance at both ends of the AC system, as a result of the generator rotor acceleration at the sending-end grid and the generator rotor deceleration at the receiving-end grid; 2) AC system internal faults, due to the isolation effect of the DC system, only the rotor of the generator in the disturbed area changes, which has little impact on the other end of the grid. In view of the above two different locations of disturbance, auxiliary power and frequency combination control as well as a switch strategy, are proposed in this paper. A four-machine two-area transmission system and a multi-machine AC/DC parallel transmission system were built on the PSCAD platform. The simulation results verify the effectiveness of the proposed control strategy.
The Variable Frequency Drive (VFD) is used to control the speed of the pumpmotor to attain the desired flow rate and fluid level in a fluid system. An AC drive provides efficient flow control by varying the pump-motor speed. The comparison of energy requirements and costs in a system where a throttling device is used for flow control on a centrifugal pump with the power used when an variable frequency drive (VFD) is used to control the same flow, evidently shows potential savings. In this system, AC Motor Frequency drive and static pressure transmitter, turbine type flowmeter and Analog/Digital cards, micro-control unit and computer connection are designed specially to control flow rate, fluid flow type (turbulence or laminar) and water level at the different conditions with different PID parameters.
This paper presents a development of a model of a set of multistage centrifugal electro pumps including two 4 stage stainless steel centrifugal pumps, each coupled to a 4 kW three-phase induction motor, connected to a hydraulic application running under two control strategies including constant speed and variable speed methods. Each pump provides 16 m3/hr flow rate and 58mwaterhead at BEP (Best Efficiency Point). Dynamicity of the model causes variations in all operational parameters of pumping system in any variation on consuming flow rate. Each electro pump has been driven with a variable frequency drive utilizing frequency control method for adjusting the rotational speed under a PID control regarding to match of pumping system operational point with the consumption point to save the energy. 83% energy saving is achieved by model in variable speed control strategy comparing to constant speed control strategy. MATLAB/SIMULINK software using ode45 solver and variable step size simulates this model.
A lot of methods for sensorless drive control have been published last years for synchronous and asynchronous machines. One of the approaches uses high frequency carrier injection for position control. The injected high frequency signal is controlled to remain in alignment with the saliency produced by the saturation of the main flux. Due to the fact that it does not use the fundamental machine model which fails at standstill of the magnetic field it is possible to control the drive even at zero speed. In spite of this obvious advantage industry does not apply sensorless control in their products. This is due to the dependency of many published methods on physical parameters of the machine. The high frequency carrier injection method, presented in this paper, does not need to have exact machine parameters and it can be used for machines where there is only a very small rotor anisotropy like in Surface Mounted Permanent Magnet Synchronous Machines (SMPMSM) . Standard drives usually are supplied by a 6-pulse diode rectifier. Due to new European directives concerning the harmonic content in the mains it is expected that the use of controlled pulse-width modulated PWM rectifiers will be enforced in the future . An important advantage of this type of rectifiers is the regeneration of the energy back to the grid. Another benefit are low harmonics in comparison to diode rectifiers. Using one of many control methods published so far it is also possible to achieve almost unity power factor. However, in these methods voltage sensors are necessary to synchronize PWM rectifiers with the mains. Therefore they are not very popular in the industry with respect to the cost and the lack of reliability. Recently a control method was proposed which is based on a tracking scheme. It does not need any voltage sensor on the ac-side of the rectifier and it does not need to know accurate parameters of the system. This paper presents the control solution for a cheap, industry friendly (no additional hardware and installation effort) drive system. The phase tracking method for control of electrical drive and PWM rectifier is described. Encouraging experimental results are shown.
Fast and accurate grid signal frequency estimation is a very important issue in the control of renewable energy systems. Important factors that influence the estimation accuracy include the A/D converter parameters in the inverter control system. This paper presents the influence of the number of A/D converter bits b, the phase shift of the grid signal relative to the time window, the width of the time window relative to the grid signal period (expressed as a cycle in range (CiR) parameter) and the number of N samples obtained in this window with the A/D converter on the developed estimation method results. An increase in the number b by 8 decreases the estimation error by approximately 256 times. The largest estimation error occurs when the signal module maximum is in the time window center (for small values of CiR) or when the signal value is zero in the time window center (for large values of CiR). In practical applications, the dominant component of the frequency estimation error is the error caused by the quantization noise, and its range is from approximately 8×10-10 to 6×10-4.