The following points highlight the four main methods used for controlling harmonic distortion in power system. The methods are: 1. Constant Frequency Control 2. Constant Tolerance Band Control 3. Variable Tolerance Band Control 4. Discontinuous Current Control.
Method # 1. Constant Frequency Control:
A usage of frequency controlled drives with AC motors (like asynchronous or synchronous) increased in last few years. Power range of the drives is up to few M W. Power inverters use switching devices like IGBT (Insulated Gate Bipolar Transistor) or GTO thyristors (Gate Tom-off). Switching frequency range is from hundreds of Hz. up to tens of kHz. High switching frequency causes high range of voltage and current rise. Due to this the problem with EMC of the drive rises.
Analytical methods of solution are ineffective for EMC of drives even in laboratory conditions. They are completely ineffective in industrial plants. To reach satisfactorily EMO of drives, installation and operation of the drives. However, the observing of the rules does not always ensure right behavior of the drive from EMC point of view.
For example, a frequency controlled drive with 3 kW asynchronous motor was installed during reconstruction of river weir control technology. A sinus filter was used because cable length was 150 m. A non-shielded cable was used between filter and motor. It was standard solution of a drive. However, during start up a converter current protection was triggered.
The problem was solved after consultation with the producer of the used converter. The producer did not suppose sinus filter connected on the converter output. A sinus filter with lower capacity has been chain. Although the drive worked satisfactorily, another problem with security camera system raised. The second problem has been fixed by reaction connected on the sinus filter output.
Method # 2. Constant Tolerance Band Control:
Constant tolerance band control, that operate and perform a cost effective ac-dc current- controlled boast-type converter that provides input power factor correction and a near- sinusoidal input current waveform. The power factor correction converter can reduce harmonic pollution and disturbance on the supply mains. The current mode controller essentially forces the envelope of the input current to vary sinusoidally and a feedback circuit adjusts the amplitude. As a result, the current distortion factor approaches unity, and the harmonic pollution it also reduced. The size of the capacitor is reduced to almost 1/3rd and output dc voltage is stabilized to a nearby constant value for large variations in the line voltage.
Many emitting power converters and motor drive systems draw non-sinusoidal input current from the AC supply mains. For example, a boast converter load draws a non-sinusoidal current from source, causing a reactive power at the source, i.e., the source voltage and current are out of phase.
The classical ac-dc rectification approach of using a full-wave diode bridge followed by a bulk capacitor is unsuitable because of the undesirable input current harmonic content.
Normally, a large capacitor bank of thousands of micro-farad is required. The large capacitor bank not only increases the size and weight of the converter equipment, but also the equipment cost.
In enhance, ac-dc converter in the power factor correction emulates a resistor. The resistor emulator, also called the power factor pre regulator is basically a dc-dc converter.
Thus two separate control loops are required. The output voltage is schemed with a dc reference, and the error signal is used to modify the pulse rate of the switching device in the PWM converter. In the other loop, the output current in sensed, component with the sinusoidal reference, and the error is used to control the pulse rate. These two loops are merged into one and called the current mode control.
Because the input current to the step-up converter is to be shaped, the step-up converter is operated in a current-regulated mode. The feedback control is shown in a black diagram form in Fig. 7.20 (c) and (d), where iL* is the reference or the desired value of the current iL* in Fig. 7.20 (a). Here iL* has the same waveform as list by Fig. 7.20 (a) a resistive potential divider and multiplying it with the amplified error between the reference value vd* and the actual measured value of vd. The status of the switch in the step-up converter is controlled by comparing the actual current iL with iL* using. Constant tolerance band control scheme.
Since the switching frequency is very high in comparison to the line frequency, the input and output voltages of the power factor pre regulator may be considered to be constant throughout the switching period.
Method # 3. Variable Tolerance Band Control:
Electro-magnetic compatibility is the property of a system. Its used for designed when this system is used. PWM (Pulse width modulation). We can say the system is running variable tolerance band control it means EMC is not working in constant frequency.
Method # 4. Discontinuous Current Control:
The discontinuous consumption is a genetic topic, it requires a reference frame linked with wireless systems. This considers two types of discontinuous consumption in wireless devices a random one not directly involved in the communication process, for example, the activation of the backlights, the speaker, servos and the like, and a periodic one that will be addressed as discontinues which is the subject of the research.
This periodic consumption is linked with the access technology employ in the wireless system and leads to the transmission and reception time periods. In spite of such classification, it is interesting to highlight that almost all tasks performed by a wireless systems processor are controlled and previously programmed, therefore the magnitude of the current consumptions, demanded by a particular event, it is predefined.
From the power supply perspective, one of the main attributes of a wireless system with TDD access scheme is its periodic consumption pattern.
The characteristics parameters of the consumption are represented in the picture and are the following:
i. Period and duty cycle of the consumption (t1, t2).
ii. Magnitude of the consumption (IPeak, ILoad, IStandby).
iii. Time mask and slopes of the communication burst.
These parameters are the tools to determine or dimension the power supply system of a wireless system. Period, duty cycle and magnitude set the energy demands place upon the power supply. Meanwhile, the time mask and slopes of the communication burst are relevant to control the switching harmonics of the signal and, at the same time, maintain the signal spectrum within its assigned bandwidth. Fast transitions mean switching harmonics of high frequency difficult to be restrained within regulation specifications, particularly at extreme conditions of temperature and voltage.
The noticeable effects of discontinues consumption in wireless systems are fluctuations and drops in the supply voltage, applied to the terminals of the load, around the nominal value; this fluctuation follows the consumption pattern. Voltage drop is ruled by the Ohm law, but not only must be considered the distributed resistive component of electric path between load and source, but also its reactive part. The resistive component conditions or determines the magnitude of voltage drop, meanwhile; the reactive be defines the shape and damping of consumption rise and fall slopes.
In wireless systems, the direct outcomes of voltage ripple are two; switching harmonics, and voltage level out of operational ranges.
The frequency bandwidth available for a wireless system is a scarce resource and must be optimized to allocate as many communication channels as possible. The TDD strategy to achieve this goal is multiplex in time a number of channels at the same frequency within a specific bandwidth. To make the communication systems work it is required that the transmission is produced in a specific timing.
Transceiver activation, on its assigned time slot, is not produced instantaneously, which implies, before the information is received or transmitted, that there are two periods of time for conditioning the signal. These two time periods constitute the rise and fall ramp time. To this extent there are two situations to be considered.
If ramps are too fast implies high-frequency interferences, switching harmonics. Switching harmonics reduce the amount of channel spectral density energy available for communication, consequently, they degrade the link traffic capacity and its overall performance, in other words, it means that could be set less communication links. If slopes are too slow, they widen the bandwidth and corrupt the spectral modulation mask, which occupy the adjacent channel reducing the traffic maximum rate and the sensitivity of adjacent receivers as their SINAD, (signal to noise ratio), is diminish.
The voltage level apply to the load varies between two values that correspond to minimum a maximum load. It is likely that the voltage operative range of the wireless device is exceeded in certain situations, particularly at extreme conditions of temperature.
Moreover, whenever wireless systems are battery powered, voltage drift increases as the power source voltage varies, between maximum and minimum load, due to the battery internal resistance. This is also applicable, to a certain extent, if a converter is placed between the power source and the load, as voltage drift could set the converter out of its regulation input voltage range.
Discontinuous Current and Electromagnetic Compatibility:
Seemingly, discontinuous consumption and voltage drops imply that the current is also variable. On the other hand, the discontinuous current drain from the power source has a direct impact on it, particularly for battery powered devices, which means energy losses in the internal battery resistance that are not uniform, as the load impedance presented varies.