Under over excitation synchronous phase modifier works as

 (A) shunt capacitor

 (B) series capacitor

 (C) shunt reactor

 (D) any of the above


Answer: (A) shunt capacitor

Explanation :


Synchronous phase modifier is basically a synchronous motor operating under no load and by adjusting the excitation it is used for controlling reactive power, so that power factor and voltage are controlled.

If synchronous phase modifier is used under over excitation it works similar to a shunt capacitor delivering reactive power and absorbing leading reactive power.

If synchronous phase modifier is used as under excitation it works similar to shunt reactor which absorbs lagging reactive power and delivers leading reactive power. This is used for ferranti effect.


Phase modifier is normally installed in the case of

(A) short transmission lines

(B) medium length lines

(C) long length lines

(D) all length lines

Answer: (C) long length lines

Full Explanation: 

Phase modifier is normally installed in the case of Long transmission lines.

Concept:

For constant voltage transmission, specially designed synchronous motors are called the synchronous phase modifiers.

Synchronous Phase modifier installed at the receiving end, which maintain the voltage drop along the line remain constant.

Synchronous phase modifiers differ from the ordinary synchronous motors in as much as they are built for the highest economical speeds and provided with smaller shafts and bearings and special attention is paid for securing a high overall efficiency.

A transmission line needs lagging VARs (reactive volt-amperes) at the receiving end during peak load conditions to avoid excessive voltage drop and these lagging VARs are supplied by the synchronous phase modifier when it is over-excited as it absorbs leading VARs when over-excited. During off-peak hours the transmission line needs leading VARs at the receiving end to prevent any voltage rise.

These leading VARs are supplied by the synchronous phase modifier when it is under-excited as it absorbs lagging VARs when under- excited. Thus synchronous phase modifier or compensator is a very flexible source of supply of reactive VARs (leading or lagging as per need) and is well suited for voltage regulation (or compensation).

 

Advantages:

Possibility of carrying increased power for a given conductor size in case of long distance heavy power transmission.

Availability of steady voltage at all loads at the line terminals.

Improvement of power factor at times of moderate and heavy loads.

Possibility of better protection for the line due to possible use of higher terminal reactance.

 

Disadvantages:

Increase risk of interruption of supply due to falling of synchronous motors this condition are called the synchronous phase modifiers out of synchronism.

Increase the short-circuit current of the system therefore increase in the ratings circuit breaker. 


 Under excited Synchronous phase modifier works as

 (A) shunt capacitor

 (B) series capacitor

 (C) shunt reactor

 (D) any of the above


Answer: (C) shunt reactor



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