What are Power Problems

When power problems are discussed, brownouts, blackouts and transients are usually mentioned but there are more power problems that we must ad dress. Brownouts occur s when the steady state voltage of he of the power company is reduced from 5% to 8%.  This is very close to the limits of the capabilities of most sensitive equipment.

Transients are power line blips that dip the voltage below acceptable voltage levels for two to five cycles (one cycle equals 16.66 msec). The equipment may survive a brownout condition or a transient condition but the combination of both occurring simultaneously will cause sensitive equipment to malfunction causing errors in its output. Consider a piece of equipment that can tolerate a 15% power disturbance. If an 8% brownout condition and an 8% transient dip occur simultaneously, the total power dip will be 16% which is more than the equipment can tolerate.  Combined effects are very common.  The reason for the brownout condition is that the power company is under so much load stress that they had to reduce their voltage to meet load demands. In this stress condition, transients become more abundant on a weak power system, therefore brownouts and transient conditions often occur simultaneously.

EMI – Cause and effect

One of the last power problems is power pollution, caused by Electromagnetic Interference (EMI). EMI occurs when high frequency signals are superimposed upon power lines. When these signals are fed into sensitive equipment, they can cause malfunctions in the low level logic sections of equipment. EMI on power lines is caused generally by the use of SCR rectifiers and welders used in many production processes.

SCR rectifiers cause harmonic distortion when the SCR’s “gate on” and the SCR’s are in a phase back condition. In this case, the line current suddenly goes from zero current to hundreds of amperes. In most cases, this sharp leading edge of current will propagate down the power line several miles from the actual origin of the EMI problem. These sharp current pulses excite the many resonant circuits that occur in power lines and cause ringing. This ringing, when fed to the input of many pieces of sensitive equipment, will cause the internal logic to malfunction causing errors or burnout.

Lightning 

The last power problem is caused by nature and can be more disastrous than all the above LIGHTNING. Lightning is a major power problem causing transients, outages and EMI. Unlike other power problems, lightning itself can cause very high voltage transients and actually destroy the input stages of many pieces of sensitive equipment. This hardware damage may result in many hours of down time and high repair costs.

Solutions to Power Problems

There are basically four types of equipment available today that will protect sensitive equipment from power problems relating to power quality at the electrical service entrance:

  • Isolation Transformers (Shielded)
  • Ferroresonant & Automatic Tap Changing Transformers
  • Motor Generator Sets
  • Uninterruptible Power Supplies

Shielded Isolation Transformers

Shielded isolation transformers have no voltage regulation ability so they provide no protection against brownouts. They are equipped with a Faraday (electrostatic) shield between primary and secondary windings. Since high frequency transients are capacitively coupled through the windings as opposed to being coupled by transformer action. The shield is an effective barrier against some transients. They are often used in combination with other equipment.

Dynamic Transformers

The ferroresonant and automatic tap changing transformers are generally small with varied efficiencies, and cost effective. However, they have little or no stored energy and therefore cannot protect equipment from outages more than four to eight milliseconds (¼ to½ cycle). They cannot protect equipment from extremely high voltage transients because they are standard transformers.  However, they do an excellent job of protecting against brownout and transient conditions. Tap changers are generally very efficient throughout the load range, ferroresonant units have low part load efficiency because the core has to be saturated to cause oscillation, so losses are the same at full and part load.

Motor Generator Sets

Motor Generatots sets give 100% protection against brownout conditions, all transients, and outages up to one half second. Most alternator sets have their motor and alternator in separate frames which give inherent protection from lightning and EMI. The motor alternator set protects against lightning and EMI because the input and output windings are separated.  This is accomplished by thick, magnetic shield and by at least one foot of air gap.

Uninterruptible Power Systems

The UPS gives 100% protection for all power line problems, except long term blackouts. However, some configurations offer much more protection than others. The chapters that follow will go into detail on hybrid static and rotary UP S’s and their configurations.