As non-linear loads proliferate in the industrial and commercial sectors, problems due to harmonic currents caused by these loads are becoming more common. However, the presence of harmonic currents is often not, in itself, a problem. The real problem may be some secondary effect they cause. Let’s look at some examples.
Industrial non-linear loads using three-phase power, such as adjustable speed drives and rectifiers, all cause fifth and seventh harmonic currents to flow. These currents use up system capacity and can contribute to transformer overheating. However, in many facilities, these currents go unnoticed until a larger event signals their importance.
This event might be the imposition of a utility power factor penalty. Capacitors installed to improve the power factor cause an electrical resonance to occur and the capacitors to fail or even worse, some part of the facility to be electrically damaged. Now the harmonics are interfering with the power factor improvement plan. The solution is to add capacitors so as not to cause resonance. Manufacturers can provide such a solution under names such as “detuned capacitors” or “protected capacitors.” These devices are primarily designed to add capacitance and provide minimal harmonic filtering.
Second, the utility might decide to impose harmonic regulations, such as IEEE 519 on the facility and, once again, the harmonic currents surface as a problem. They cause the facility to exceed allowable distortion limits at the utility interface. The solution is to add filters to remove the harmonic currents, and again a number of manufacturers can provide these devices. Such filters are often placed at the transformer, where they intercept harmonic currents before they can reach the utility.
Third, more equipment may needed. However, because the electrical system is carrying harmonic currents, no useable capacity is available to power this equipment. Again, filters can be used to remove the harmonic currents, but now they must be placed near the harmonic generating loads to reduce loading on the distribution buses.
Most commercial non-linear loads are personal computers. Commercial facilities usually are powered by a third-phase, four-wire, 208/120-volt wye system with the loads connected phase-to-neutral to 120 volts. Such loads cause large amounts of all odd harmonic currents to flow in the system, with the third harmonic often equal in magnitude to the fundamental current. Valuable system capacity is wasted carrying harmonic currents. And third harmonic currents from all three phases combine in the neutral wire, often leading to a neutral current higher than the phase currents. The extra currents—if left unchecked—can lead to overheating of transformers, switchgear, and neutral wires.
When computer loads are moderate in relation to other system loads, the harmonic currents are not a problem. However, problems arise when computer loads are added. For example, a small call center in a leased office space might wish to add personnel. As soon as more computers are plugged in, the breakers start to trip; there is no additional useable capacity and the building owner is unwilling to add capacity. The only way to release capacity here is to plug each computer into a product that removes the harmonics.
On a larger scale, major computer users may suddenly occupy a facility built with an electrical system that is not designed to handle large amounts of harmonic currents. If transformer overheating is the problem, they can be replaced with k-rated or oversized units designed to withstand the extra heat, or zig-zag filters can remove harmonic currents. If neutral wire overheating is the problem, new conduit and double wires can prevent its consequences.
The most common problem caused by harmonic currents is lack of useable system capacity. To add useable capacity, a harmonic suppression system is available that can prevent formation of harmonic currents, releasing previously wasted capacity and enabling full use of system capacity for the powering of multiple computer loads—without upsizing the system.
When a facility is to be built and it is known that many computers will be powered, the optimal solution appears to be to ensure that a harmonic suppression system is specified and installed. At a minimum, the electrical system should be designed to withstand harmonic currents using oversized k-rated transformers, double neutral wires, and additional cooling equipment.
While small amounts of harmonic currents do not trouble electrical systems, harmonic-producing loads are increasing. Reduced useable capacity, overheating, and power factor improvement are becoming the norm. Installing equipment to avoid or eliminate problems adds up-front costs and may be unpopular with facility owners. However, it can be more cost effective to avoid or eliminate harmonic-caused problems in the design stage, rather than using an expensive retrofit solution.
LOWENSTEIN is president and chief technical officer (CTO) of Harmonics Limited. He can be reached at email@example.com.