Managing Power Quality for LED Lighting Systems

Figure 1 is an example of an EPS featuring dual electric utility feeders, two ATSs, a two-sided bus with a tie breaker, and three generators — all powering an emergency bus and two normal buses. Critical branch circuits and other loads on normal circuits can be powered from either electric utility feeder or the emergency generator bus.

C&I facilities use some type of lighting system powered from all three buses. Emergency lighting circuits may be derived from the critical branch circuit. Many facilities that use EPSs are planning or have already upgraded their lighting systems to include LEDs. It is a known fact that LED lighting is sensitive to power quality disturbances, especially disturbances that contain voltage transients.

Normal and EPS buses are subject to different types of power quality disturbances. Normal buses are subject to disturbances generated by everyday electric utility operations and internally generated disturbances caused by breaker switching and load switching. EPS buses are not subject to electric utility power quality problems, but they are subject to EPS power quality issues and disturbances generated by breaker operations and load switching occurring on EPS buses. EPS power quality includes the disturbances generated when any ATS operates.

Disturbances from ATS operation

Figure 2 illustrates the automatic transfer from the electric utility voltage to the generator voltage. You can see that the electric utility voltage waveform is fairly clean. However, the transfer of power from the electric utility feed to the generator at the 0.00 time point on the graph created a high-frequency voltage transient. Following the transient is high-frequency voltage noise combined with additional transients. Thus, LED lighting on the emergency bus has been subjected to the transients created by the ATS and the transients present on the emergency bus after the transfer.

Figure 3 shows another example of transfer of power from the electric utility to the generator, and then back to utility. You can see that the electric utility power did contain some voltage imbalance plus high-frequency transients on all three phases, especially on Phase C (blue). However, a transfer to generator power introduced voltage sags on Phase B (green) and Phase C (blue), plus additional transients during the initial transfer and subsequent transfer back to the electric utility feed. These disturbances present a significant amount of disturbance energy to the LED lighting system.

ATS characteristics to help ensure LED lighting reliability

When you upgrade to LED lighting, especially if the lighting is powered from a 480V or 277V bus, you should consider the effects of electrical disturbances caused by ATS operations before the upgrade starts. There are many ATS characteristics that should be considered, including switching time, solidly grounded systems, and the effects of ungrounded systems and high-resistance ground systems. Switching time should be considered carefully based on the type of facility. Is the facility a mission-critical one like a hospital? Inexpensive ATSs use break-before-make switches, which have longer switching times and are likely to generate more severe disturbances when they operate. The more expensive paralleling and synchronized ATSs have shorter switching times, and generate less severe disturbances.

Protection against ground faults must be provided in EPSs with line-to-neutral loads and solidly grounded systems. This is to ensure that ground-fault protection devices and relays sense the full fault level and can act quickly. On the other hand, ungrounded systems have no intentional connection between conductors and earth ground. Ungrounded systems do not limit phase-to-ground-fault currents but can produce high transient overvoltages during ground faults. High-resistance ground (HRG) systems are popular for line-to-line loads and use a resistor between neutral and ground to limit ground-fault currents to low levels. Detection of low-level ground currents is important. Thus, neutral bonds and grounding mistakes must be avoided so the ground-fault protectors can sense small ground currents accurately.

When selecting a transfer switch type, you should consider:

1) Switching delay: open versus closed transition and synchronization;

2) Correct grounding and neutral bonding for proper ground-fault protection;

3) Contact rating and mechanical structure; and

4) Maintenance and testing with minimal power system disruption.

Solid-state automatic transfer switches (SSATSs) are also available today. However, they do not eliminate all of the problems mentioned above. Operation of SSATSs still generate disturbances with some high-frequency content when they are switched. If switching the neutral conductor using a 4-pole ATS versus a 3-pole ATS, the timing associated with switching the neutral must be carefully considered.

Lastly, you should use a power quality monitor to characterize the quality of power on the input versus the output of any ATS before LED lighting is installed. Disturbances will be recorded when the switch operates. The question here is how much high-frequency voltage transient content do these disturbances contain. One may find that some type of surge protective device (SPD) is required to mitigate disturbances generated by the ATS when it operates. In addition, you’ll want to install the right SPD on the emergency lighting panels powering the LED lighting system. This will ensure that the EPS is properly managed for surge-related power quality disturbances. The use of SPDs will not mitigate against voltage sags, which should not cause damage to LED lighting equipment when they occur.                                                      

Keebler is a senior power quality engineer and power systems consultant at Electrotek Concepts, Inc. in Knoxville, Tenn. He can be reached at [email protected].