Rectifier/transformer harmonics

The inverter bridge switches the DC voltage across the lines of the motor. The waveform appearing across the hnes is a 120° rectangle, similar to the current waveform described in subsection 15.3.2 for the star-star rectifier transformer, but with vertical sides. Hence the voltage is switched and the load current responds. The Fourier coefficients for this waveform are the same as for (15.17) except that V replaces 1 and V iax replaces /max- The harmonic content is,... 

When installing multiple transformers in parallel, one must ensure that their impedances are equal. If this is not so, sharing of the load is unequal and there will be circulating currents between adjacent transformers. These currents directly result in a loss of efficiency, which can lead to early failure, faulty power metering, and a loss in reliability. In the special case of rectifier transformers, the current in each is dictated by the rectifier control systems, but if they have been given different phase shifts to minimize harmonic distortion (Section 8.3.1.3), it is again necessary to ensure that their impedances are equal. 

Pinellia and Poria dispel dampness and transform phlegm. Citrus and Orange Peel rectify the qi and loosen the chest. Fresh Ginger, Red Dates, and Licorice regulate the constructive and defensive and harmonize all the other medicinals. Taken as a whole, this formula emits the exterior, diffuses the lungs, and cools and resolves dryness. It also disinhibits the qi, transforms phlegm, and stops cough. 

Functions Dries dampness and transforms phlegm, rectifies the qi and harmonizes the center... 

The rectifier (1) may be of the 6 or 12-pnlse type. The choice mainly depends npon the tolerance that is available for the harmonics, which will be injected into the upstream power system. The rectifier will be designed to provide a sonrce of variable voltage to the DC link. The 12-pulse type will usually be necessary for the highly rated motors within their voltage level. Inside the rectifier compartment will be a set of voltage transformers (9) which will be nsed to derive a set of firing pulses (10) for the rectifier elements. These pnlses will be in synchronism with the power supply. 

The electrical supply equipment is the second biggest equipment package after the mechanical equipment. The package includes the transformer/rectifier (T/R) sets and bus bars for providing DC power to the cells, the associated power factor correction equipment and harmonics filtration equipment, and the service transformers and circuitry. This equipment will typically comprise about 15 - 25 % of the cellhouse cost. The cost is heavily influenced by whether or not an electrical substation is included, and by the redundancy requirements for the T/R sets. A plant can choose to have no spare T/R capacity, partial redundancy, or full redundancy. Such cost savings must be weighed against the cost of lost production when a T/R set fails. 

A. Current Variability and Harmonics. The function of a rectifier system is to convert a power supply received as alternating current into the direct current required by the cells. There may be a reduction in voltage through transformers prior to the rectifiers, but this is outside the scope of the present discussion. Within the rectifier system, there will be some reduction in voltage to match the varying demands of the... 

Again, the above is a simplified andidealized picture. In a real system with imperfections in the supply voltages, the transformers, and the rectifiers, more complex rectifiers still produce lower-order nontheoretical harmonics at certain levels. Table 8.2 gives some typical values, as listed by Cameron [13]. Regardless of the number of pulses, some low-level fifth and seventh harmonics remain and in fact become the dominant form in the more complex units. 

If a delta/wye connected power transformer is installed between the power source and the load, the power factor at the transformer input generally will reflect the average PF of the loads on the secondary. This conclusion works on the assumption that the low PF is caused by inductive and capacitive reactances in the loads. However, if the load current is rich in harmonics from rectifiers and switching regulators, some of the harmonic currents will flow no farther toward the power source than the transformer delta winding. The third harmonic and multiples of three will flow in the delta winding and will be significantly reduced in amplitude. By this means, the transformer will provide some improvement in the PF of the total load. 

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