FLICKERING LIGHTS – POWER QUALITY CASE STUDY AND SOLUTION



This case study concerns a residential electrical system. The homeowners were experiencing light flicker when loads were energized and deenergized in their homes.

Background
Residential systems are served from single-phase transformers employing a spilt secondary winding, often referred to as a single-phase three-wire system. This type of transformer is used to deliver both 120-volt and 240-volt single-phase power to the residential loads.

The primary of the transformer is often served from a 12 to 15 kV distribution system by the local utility. Figure 29.14 illustrates the concept of a split-phase system.

FIGURE 29.14 Split-phase system serving a residential customer.

When this type of service is operating properly, 120 volts can be measured from either leg to the neutral conductor. Due to the polarity of the secondary windings in the transformer, the polarity of each 120-volt leg is opposite the other, thus allowing a total of 240 volts between the legs as illustrated.

The proper operation of this type of system is dependent on the physical connection of the neutral conductor or center tap of the secondary winding. If the neutral connection is removed, 240 volts will remain across the two legs, but the line-to-neutral voltage for either phase can be shifted, causing either a low or high voltage from line to neutral.

Most loads in a residential dwelling, i.e., lighting, televisions, microwaves, home electronics, etc., are operated from 120 volts. However, there are a few major loads that incorporate the use of the 240 volts available. These loads include electric water heaters, electric stoves and ovens, heat pumps, etc.

The Problem
In this case, there were problems in the residence that caused the homeowner to question the integrity of the power system serving his home. On occasion, the lights would flicker erratically when the washing machine and dryer were operating at the same time. When large single-phase loads were operated, low power incandescent light bulb intensity would flicker.

Measurements were performed at several 120-volt outlets throughout the house. When the microwave was operated, the voltage at several of the 120-volt outlets would increase from 120 volts nominal to 128 volts.

The voltage would return to normal after the microwave was turned off. The voltage would also increase when a 1500-Watt space heater was operated. It was determined that the voltage would decrease to approximately 112 volts on the leg from which the large load was served. After the measurements confirmed suspicions of high and low voltages during heavy load operation, finding the source of the problem was the next task at hand.

The hunt began at the service entrance to the house. A visual inspection was made of the meter base and socket after the meter was removed by the local utility. It was discovered that one of the neutral connectors was loose. While attempting to tighten this connector, the connector fell off of the meter socket into the bottom of the meter base (see Fig. 29.15).

  FIGURE 29.15 Actual residential meter base. Notice the missing neutral clamp on load side of meter.

Could this loose connector have been the cause of the flickering voltage? Let’s examine the effects of the loose neutral connection. Figures 29.16 and 29.17 will be referred to several times during this discussion.

Under normal conditions with a solid neutral connection (Fig. 29.16), load current flows through each leg and is returned to the source through the neutral conductor. There is very little impedance in either the hot or the neutral conductor; therefore, no appreciable voltage drop exists.

FIGURE 29.16 The effects of a solid neutral connection in the meter base.

When the neutral is loose or missing, a significant voltage can develop across the neutral connection in the meter base, as illustrated in Fig. 29.17. When a large load is connected across Leg 1 to N and the other leg is lightly loaded (i.e., Leg 1 to N is approximately 10 times the load on Leg 2 to N), the current flowing through the neutral will develop a voltage across the loose connection.

This voltage is in phase with the voltage from Leg 1 to N0 (see Fig. 29.17) and the total voltage from Leg 1 to N will be 120 volts.

FIGURE 29.17 The effects of a loose neutral connection in the meter base.

However, the voltage supplied to any loads connected from Leg 2 to N0 will rise to 128 volts, as illustrated in Fig. 29.17. The total voltage across the Leg 1 and Leg 2 must remain constant at 240 volts. It should be noted that the voltage from Leg 2 to N will be 120 volts since the voltage across the loose connection is 1808 out of phase with the Leg 2 to N0 voltage.

Therefore, with the missing neutral connection, the voltage from Leg 2 to N0 would rise, causing the light flicker. This explains the rise in voltage when a large load was energized on the system.

The Solution
The solution in this case was simple—replace the failed connector.

Conclusions
Over time, the neutral connector had become loose. This loose connection caused heating, which in turn caused the threads on the connector to become worn, and the connector failed.

After replacing the connector in the meter base, the flickering light phenomena disappeared. On systems of this type, if a voltage rise occurs when loads are energized, it is a good indication that the neutral connection may be loose or missing.



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