Ecmweb 17971 Led And Pq 0218 Pr
Ecmweb 17971 Led And Pq 0218 Pr
Ecmweb 17971 Led And Pq 0218 Pr
Ecmweb 17971 Led And Pq 0218 Pr
Ecmweb 17971 Led And Pq 0218 Pr

Ground Planes for LED Drivers — Part 2 of 3

Feb. 14, 2018
How grounding and bonding connections in facility power systems affect LED driver ground stability

In Part 1 of this three-part series, we took a brief look at the need for a consistent and frequency-independent ground plane for LED drivers. In Part 2, we take a look at how facility power systems should provide a consistent, reliable, and conductor-dependent grounding and bonding system from the electric utility service transformer to the LED drivers. We’ll also discuss a recommended design for grounding and bonding that helps ensure good power quality in facility electrical systems with disturbance-generating loads and electronic equipment.

With respect to power quality for customer facilities, especially industrial plants, and the need to support a growing number of electronic loads, older facilities — some dating back to the 1920s and 1930s — still lack a consistent, reliable, and conductor-dependent grounding and bonding system across the entire facility power system. What do we mean by this? How do facility power systems in older facilities increase the likelihood of malfunctions and failures to LED drivers and other electronic equipment?

With the use of electronic equipment — both low- and high-power devices — grounding and bonding for power quality is not the same as grounding and bonding to meet the requirements of the National Electrical Code (NEC). The NEC is concerned with electrical safety and shock prevention — a topic that is obviously extremely important to all of us. However, the reliable and sustainable operation of electronic equipment in industrial facilities requires more attention to grounding and bonding than the NEC offers.

Electricians are accustomed to using conduits, rigid pipe, etc., as an acceptable type of equipment grounding conductor (EGC) as long as the NEC requirements are met. In addition, electricians are accustomed to using more than one grounding bar to bond ground conductors together and to the frame (“can”) of an electrical panel — again, as long as all NEC requirements for grounding and bonding are met. Personal safety concerns are met through a number of NEC (and other safety codes) requirements, including the tripping of protective devices like circuit breakers and the blowing of fuses.

However, when trying to establish best engineering practices for power quality, the performance of facility grounding and bonding systems in the presence of operating hundreds of electronic devices — both low- and high-power — is very important. This would not be important if electrons always flowed at a consistent 50 or 60 Hz. The reality of today’s electrical environments is that current must flow at many frequencies other than 50 or 60 Hz. Answering the question, “Why do currents other than at frequencies of 50 and 60 Hz flow?” seems to be the difficult one to answer.

The laws of physics again play a vital role in helping us understand why. In simple terms, not all materials (e.g., metals, etc.) can support current flow the same way — hence, the word “semiconductor”. For example, aluminum conductors support current flow differently than copper conductors do. Most of us are comfortable with why this is the case. Different types of steel support current in a different manner too. The interesting part here is that we also know that when the frequency of the current changes, not only does current flow change, but how current flows in each conductor (copper and aluminum) also changes.

The same holds true when other materials (e.g., plastic, paint, etc.) are applied to metals that support current flow. In grounding and bonding in facility power systems, conductors (e.g., grounded conductors and grounding conductors) are used to support the flow of ground currents. Special metal hardware (e.g., ground bars and lugs) are used to establish bonds between these ground conductors and metal objects (e.g., electrical panels and equipment housings). These conductors and hardware make up an important part of the grounding and bonding system.

At low frequencies like 50 and 60 Hz (power frequencies), ground (and fault) currents flow through the grounding and bonding system with very little resistance, as long as the bonding hardware is tight and conductors are intact.

Figure 1 illustrates a typical facility electrical system. Here, a delta-wye, dry-type transformer and a set of 600V, 125A power distribution panels have been added to an existing 600V, 225A upstream panel. These modifications were made in the facility to support the installation of a new variable-frequency drive (VFD) and LED luminaires — both were added to the facility as a part of an incentivized energy-saving program.

One can see that the switchgear is grounded through multiple grounds (e.g., ground rod, cold water pipe, Ufer concrete-encased grounding electrode, etc.). The 225A panel is grounded and bonded to the switchgear through the conduit that encloses the feeder conductors. Branch circuits (not shown) utilize equipment grounding conductors (EGCs) from grounding bars 1 and 2. Another feeder from this panel was installed to power the delta-wye transformer — again, grounded and bonded to the 225A panel through the conduit. The neutral-to-ground (N-G) bond is made in the transformer and bonded to the transformer’s metal frame. A separate grounding bar is installed in the front corner of the transformer to provide bonding of the grounding conductor to the new 600V, 125A power distribution panel (Section 1 area in the figure). A VFD is installed with its EGC bonded to this panel’s grounding bar with the bar bonded to the panel frame. Section 2 is bonded to Section 1 through the conduit between the two panels. Each of the lighting branch circuits also contain an EGC, which are bonded to the grounding bar to establish grounding for the LED luminaires. The grounding bar is bonded to the frame of Section 2 to
establish grounding for the bar. And, each luminaire is bonded to the building’s structural steel. All of these grounding and bonding practices are commonplace in facility electrical systems.

With reference to power quality, what’s wrong with this installation?

If the new transformer fails (e.g., a fault occurs between Phase C and ground), a fault current will flow through the transformer’s grounding conductor through the conduits, panel, and switchgear to earth. However, if a 3-phase circuit breaker in another part of the switchgear arrangement (not shown in supporting article graphics) activates — and an oscillatory transient is generated by opening / closing of the breaker contacts — a transient current developed in one or all of the phases will try to flow through the facility’s power system, including the grounding and bonding system. Part of this transient current will normally seek earth through the ground rod. The other part will seek the building steel through the grounding and bonding system at the 225A panel, through the transformer, through the 125A panels and into the LED luminaires connected to the building steel. Still another part of the transient current will flow through the building steel and back through the luminaires seeking earth from the downstream side of the system. The point here is that ground disturbance currents, which can originate from disturbances involving the phase conductors, much higher than 60 Hz will flow all throughout the grounding and bonding system, including the metal frames of the panels, transformer and switchgear with various (much higher than 60 Hz) frequencies, all seeking earth through the lowest impedance at those frequencies.

In the system of Fig. 1, the disturbance currents, trying to flow through the grounding and bonding system, will encounter parasitic distributed capacitances and inductances other than those already inherent to the grounding and bonding conductors and mechanical structures of the bonding hardware. These distributed capacitances and inductances are shown as red dotted lines in Fig. 2.

These capacitances and inductances, shown as Z1(f) through Z11(f) in Fig. 2, also contain a resistive characteristic. Just as the variable Z indicates, these impedances are dependent on frequency — the frequencies that make up the disturbance current. This “added” impedance in the grounding and bonding system causes an additional undesired voltage drop to occur throughout the grounding system. This voltage drop(s) shows back up across the VFD and the LED drivers. Depending on the magnitude and frequency content of the disturbance current, one or more electronic components (and possibly surge protection devices in the AC network and embedded in the digital electronics), either in the VFD or in the LED drivers, will fail. The higher the disturbance current, the higher the voltage drop(s). In fact, the presence of these distributed capacitances and inductances typically increases the impact of the disturbances on the electronic equipment. Minimizing the effects of the Z(f) impedances will minimize the voltage drops. The disturbance currents will still flow, but the objective is to aim them toward the grounding electrodes so they are not dissipated in the electronic equipment.

Figure 3 illustrates a recommended method for grounding and bonding to achieve good power quality performance for electronic equipment. One should note that a continuous grounded conductor is included in the switchgear, through the conduit, in the 225A panel, through the conduit to the delta-wye transformer and into the 125A two-section panel. Implementing a continuous conductor-based ground will provide the best performance. Because materials cost is often an issue, another option is to completely remove any paint, rust, etc. beneath grounding and bonding hardware and ensure bolts attaching this hardware to the metal panels and frames of transformers are properly tightened. With this method, the impedance uncertainties, which are depending upon disturbance frequencies, are no longer dominant in the grounding and bonding system. This method of grounding and bonding has a higher performance for managing ground disturbance currents and directing them toward the building infrastructure instead of developing undesired high-frequency voltage drops across electronic equipment.   

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].

About the Author

Philip Keebler | Senior Power Quality Engineer

Keebler — formerly with the Electric Power Research Institute since 1995 in Knoxville, Tenn., and principal investigator for his own consulting engineering firm since 2012 — is a power quality and monitoring applications engineer with Electrotek Concepts in Knoxville. A graduate of the University of Tennessee’s electrical engineering program, he brings a broad background focused on the power quality industry. His experience includes product testing, field investigations, standards development, training, and laboratory development. His customer focus includes commercial, industrial, residential, education and health care. He has authored more than 150 publications, including reference publications on voltage sags, surges, flicker, power quality monitoring, electromagnetic compatibility (and interference), grounding, appliances and safety related to power quality.

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