METHOD OF RESTORING FOR VOLTAGE SAG

1) TAP CHANGING TRANSFORMERS

Electronic tap changing is achieved via the use of back to back thyristors (SCR) with a tap changing transformer. Has a reasonable response time (1 cycle) and is popular for medium power applications (>3kVA). However, high control resolution requires large number of SCRs. 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.

2) SATURABLE REACTOR REGULATORS

This plan is simple and output voltage can be controlled by changing the impedance of saturater reactor. One of the problems is slow response (10 cycle) and high out impedance which makes distrurbance in non linear sensitive loads in co-efficent load power factor.

3) PHASE CONTROLLER REGULATORS

This technique uses phase controlled thyristors with LC filter to control output voltage. It has a slow response, high distortion especially with non-linear loads, over sized filters, very poor input line harmonics and will not handle surge currents such as motor starting.

4) ELECTRONIC VOLTAGE REGULATORS

They are a new class of automatic voltage regulators based on high frequency switching inverter technology. It can provide fast response (1-2 ms), sinusoidal voltages, and compact design.

5) STATIC VOLTAGE REGULATORS (SVR)

This device, through the use of static tap changers, simply regulates the voltage to equipment operational levels. The SVR is able to correct voltage sag conditions (a 55% of the normal voltage maximum depth) in a quarter of a cycle (4 ms), to allow even the most sensitive manufacturing equipment to ride through voltage sag conditions caused by faults in the utility distribution or transmission systems .

6) FERRORESONANT TRANSFORMERS (CVT)

Ferro-resonant transformers, also called constant-voltage transformers (CVT), can handle most voltage sag conditions (always beneath 20 kVA). The ferro-resonant regulator has a response time of about 25 ms or 1.5 cycles. More important are its high output impedance (again up to 30% of load impedance), sensitivity to both leading and lagging load power factors, and low efficiency at partial loads. In summary, the ferro-resonant regulator is useful in small systems that do not contain large motors.

7) DYNAMIC VOLTAGE RESTORER (DVR)

This method immediately restorer and clear the voltage sag, quick response (< 1 ms), low waste conductivity switching is the main specifications.