Design voltage transformers

General specifications and design considerations for voltage transformers (VTs) 15/457... 

The instmments discussed above (and many of the others which follow) act in response to the system voltage and/or current. In most cases, the values of these two parameters are very high, which presents problems in the design of the insulation and current-carrying capabilities of the instmment. In these instances, the instmment is supplied with a known fraction of the measured quantity using a voltage transformer or a current transformer, as appropriate. 

Instead of the ignition circuit shown in the control box of Fig. 3, a low-voltage transformer can be used. Transformer ignition circuits are available commercially as accessories from calorimeter manufacturers or can be easily constracted. A suitable design is shown in Fig. 4. 

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. 

As the alternating current input of a transformer increases, the output also increases, but it reaches a maximum value when the core becomes "saturated" and can no longer increase its magnetization. If the transformer is purposely operated under these conditions, the output will be constant, even if the input varies a small amount. This is sometimes used to provide a dependable, constant voltage to operate sensitive instruments, heaters, and other devices, and it is called a "saturable core reactor" or a "constant voltage transformer." In newer equipment designs, it has mostly been replaced by the semiconductor devices that will be described in Chapter 15 (Fig. 15.6). 

This section discusses the cable system design, except for overvoltage analysis and insulation coordination. The cable system design includes the selection and specification of the cable itself and the related equipment such as CBs and voltage transformers (VTs). The cable system design related to the sheath is discussed in Section 3.2. 

Iligh-voltage controllers which regulate prirnaiv input voltage to the rectifier and wiper transformer and house primary current-limiting protection, meters, and instrumentation are designed for local or remote operation. 

Tappings are generally not necessary, as a transformer is designed for a particular voltage system. If and when such a need arises (as in a control transformer (Section 15.4.5)) they can be provided on the primary side of the transformer. 

These are protection CTs lor special applications such as biased differential protection, restricted ground fault protection and distance protection schemes, where it is not possible to easily identify the elass of accuracy, the accuracy limit factor and the rated burden of the CTs and where a full primary fault current is required to be transformed to the secondary without saturation, to accurately monitor the level of fault and/or unbalance. The type of application tind the relay being used determine the knee point voltage. The knee point voltage and the excitation current of the CTs now form the basic design parameters for such CTs. They are classified as class PS CTs and can be identified by the following characteristics ... 

Type tests These are conducted on a finished voltage or current transformer, one of each design and type, to verify their compliance with the design data and relevant Standards. 

Solve for the highest input average input eurrent that oeeurs at the lowest speeified input voltage. This value allows the designer to determine the size of the wire used in the primary winding of the transformer or induetor. 

There are two main functions the forward-mode transformer performs the first is to provide a dielectric isolation barrier from the input to the output and the second is to step up or step down the pulsewidth modulated ac input voltage signal. The design flow becomes a step-by-step design procedure. 

The physical design of the transformer is critical in flyback converters. If they are not properly designed physically, excessive voltage spikes could be generated that would adversely affect the reliable operation of the semiconductor components (refer to Section 3.5.8). 

The design philosophy behind the fixed-base drive circuit is to draw current from a relatively low voltage source (3 to 5 V) which is usually provided by an auxiliary winding on the power transformer. The resistor directly in series with the base (R2 as shown in Figure 3-33) should be on the order of 100ohms. Its... 

The topology is going to be an isolated, multiple output flyback converter that must meet the safety requirements of UT, CSA, and VDE. These considerations affect the design of the final packaging, transformer, and voltage feedback designs. 

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