What are voltage fluctuations and what is the solution to reduce it ?

What are voltage fluctuations?

Voltage fluctuations can be described as repetitive or random variations of the voltage envelope due to sudden changes in the real and reactive power drawn by a load. The characteristics of voltage fluctuations depend on the load type and size and the power system capacity.





Causes of voltage fluctuations

Voltage fluctuations are caused when loads draw currents having significant sudden or periodic variations. The fluctuating current that is drawn from the supply causes additional voltage drops in the power system leading to fluctuations in the supply voltage. Loads that exhibit continuous rapid variations are thus the most likely cause of voltage fluctuations.

Examples of loads that may produce voltage fluctuations in the supply include arc furnaces ,arc welders ,installations with frequent motor starts (air conditioner units, fans),motor drives with cyclic operation (mine hoists, rolling mills) ,equipment with excessive motor speed changes (wood chippers, car shredders).

Often rapid fluctuations in load currents are attributed to motor starting operations where the motor current is usually between 3-5 times the rated current for a short period of time. If a number of motors are starting at similar times, or the same motor repeatedly starts and stops, the frequency of the voltage changes may produce flicker in lighting installations that is perceivable by the human eye.


Reducing the effects of voltage fluctuations

To allow equipment connected to the power system to operate correctly it is important for both the utility and their customers to ensure that the operating voltage of the system remains within the boundaries set by the appropriate standards. As mentioned previously power system equipment does not usually provide adequate response time for mitigation of rapid voltage changes. It is inherent that complete compensation of flicker is not possible. However, the magnitude of the voltage fluctuations may be reduced using one of the following network strategies.

Increasing the fault level at the point of connection. Strengthening the system or reconnecting the offending load at a higher voltage level can achieve this.

Decrease the reactive power flow through the network due to the load. This may be achieved through the use of a Static VAr Compensator (SVC) and will help reduce voltage sags.

Strengthening the network reactive power compensation. A larger number of smaller capacitor banks distributed throughout a system will allow finer tuning of reactive power requirements

Frequent motor starting has been highlighted as a significant cause of flicker. This is especially significant for larger single-phase air conditioner compressor motors connected to weak low voltage distribution systems. In order reduce the magnitude of voltage fluctuations a reduction in the starting current of a motor must be accomplished.

This can be achieved through the use of various starting techniques like  suggests the use of the following motor starting techniques , inclusion of an intermediate star-delta resistance-delta starting configuration for three-phase motor applications, installing a series resistance or inductance with the motor stator to effectively apply reduced voltage starting.

Use of an exclusive autotransformer matched to the design of the motor and make a soft start using power electronic soft starters.

Full inverter control of motor. This has the advantage of controllable speed and torque providing efficient motor operation.

As frequency is also an important parameter of voltage variations a reduction in the number of motor starts may also lessen the effects of flicker. This may be achieved through coordinated control of motors or by providing sufficient storage of heat for the case of air conditioners and heat pumps