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AUTOMATIC VOLTAGE REGULATION TECHNOLOGIES
 
Tap Changing Transformers :
Tap Changing Transformers
This technique is rather simple in concept and uses established thyristor technology. Electronic tap changing is achieved via the use of back-to-back thyristors with a tap-changing transformer as shown in diagram. This technique has a reasonable response time (1 cycle) and is popular for medium power applications (> 3kVA). However, high control resolution requires large number of thyristors (60 thyristors for +/- 3% regulation with +10/-20% input range).
 
As a result, the control for fast response becomes fairly complex. Another drawback of this scheme is its susceptibility to high transient current with motor loads upon tap changing and its poor transient voltage rejection.
 
 
Saturable Reactor Regulators :
Saturable Reactor Regulators

This scheme controls the output voltage by varying the impedance of a saturable reactor as shown in diagram. This scheme is simple in concept and has a good line transient rejection. The drawbacks of this technique include slow response (10 cycles), high output impedance which gives high distortion with non-linear loads such as adjustable speed drives, sensitive to load power factor,
will not handle surge currents such as motor starting and will not suppress transients generated inside plant. the saturable reactor. The saturable reactor is an electro-magnetic "switch". This switch controls current flow to the load by "switching" in and out of saturation during each half cycle. The resulting current waveform is rich in harmonics. Referring back to Fig. 1, the switch represents the AC coils and core of the saturable core reactor. The switch control represents the DC coil and DC drive circuitry used to saturate the core. The amount of current to the load is controlled by controlling the amount of DC current applied to the DC coil which in turn controls the amount of saturation in the core.
 
 
Saturable Reactor Control
Typical input voltage and current waveforms for a saturable core reactor power supply, with the core partially saturated, is shown in the oscilloscope picture of Fig. 2b. When the core is fully saturated the current waveform is near sinusoidal. The inherent series reactance of a saturable reactor causes a phase shift between the voltage and current. Fig. 2b shows the voltage leading the current. Due to the series reactance, the best power factor that can be achieved when the saturable core reactor core is fully saturated is a power factor of about 0.95.
 

The advantage of a saturable core reactor power supply is its reliability, no moving parts. Disadvantages are its poor power factor, higher losses, larger size, and higher cost. A typical saturable reactor costs more than a contactor or an SCR controller (yet to be discussed). A variation of saturable reactor control is the variable reactance transformer, which combines the saturable reactor and transformer functions onto a common core structure. Later in this paper the phrase saturable magnetics will be used to refer to both saturable reactors and variable reactance transformers.

 
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