To determine which strategy reduces electricity charges under a power factor tariff system for a manufacturing plant with predominantly inductive loads, we need to understand the concept of power factor and how it affects electricity costs.

Understanding Power Factor

The power factor is a measure of how effectively electrical power is being used. In simple terms, it's the ratio of the real power that is used to do work and the apparent power that is supplied to the circuit. It is defined as:

Power\ Factor = \frac{Real\ Power\ (kW)}{Apparent\ Power\ (kVA)}

For inductive loads, the power factor is typically less than one, which indicates that there is a lagging power factor due to the phase difference between voltage and current.

Impact of Low Power Factor

A low power factor means more apparent power is required for a given amount of real power, which can lead to increased demand charges on electricity bills. Power companies often charge a higher rate for lower power factors because they need to supply more apparent power to the users.

Solution: Installing Capacitors

The correct solution to improve power factor and reduce electricity charges is:

• Option: Installing capacitors to improve the power factor of inductive loads.

Capacitors can provide the necessary leading reactive power, which cancels out some of the lagging reactive power from inductive loads, thereby improving the power factor closer to one. This reduction in the phase difference means the apparent power is reduced for the same real power, thus lowering electricity charges related to demand and improving overall efficiency.

Ruling Out Other Options

1. Switching to higher voltage transformers without changing load characteristics: This will not improve the power factor of the facility's load.
2. Operating all equipment at full load constantly: This can lead to inefficiencies and increase power consumption rather than reduce charges.
3. Increasing lighting hours during low demand periods: This has no direct impact on power factor correction and might increase energy consumption.

Thus, the most effective and efficient method to reduce electricity charges in this scenario is by installing capacitors.