Emergency Back-up Generation Systems for Buildings

The first Primer Hal Finkelstein wrote on Back-up emergency Generation was in January 2002 right after 9/11/2001. I am often asked why should any building use back-up generation if they are not forced to by local codes? I think 9/11 proved how important back-up generation is to many types of facilities. But remember if you go with back-up generation there are many important points you must consider to avoid it becoming a major building construction problem..

After September 11, 2001 many organizations directly affected by that calamity were shocked to find that their back-up generation did not function. Many systems that did not operate were systems that were never properly tested as normally required. Most code required systems are tested on a weekly basses in accordance with local code requirements.

A few days after September 11, 2001 many facilities were reporting that their backup generation systems were not operative. Studies indicated that the problems were due to many causes. Contaminated fuel, lack of fuel, poor maintenance and lack of testing and poor design were the basic causes of most of the failures.

Of the cases I evaluated several failed due to improper care of long term storage of diesel fuel and natural gas engines failed due to improper gas pressure. One system failed due to the total harmonic distortion exceeding 30% of the total load in trying to operate three elevators. The generator failed and several people became trapped in the buildings elevators. It should be pointed out that this particular installation was only several months old prior to September 11, 2001.

Many facility managers make the big mistake of installing a new generator with out a proper design. They fail to understand what is involved in selecting a generator that can operate all loads including any motors; with out failing do to increased Harmonic Distortion.

In designing any back-up generator system for a commercial and Industrial building the designer must understand the purpose of the back-up system. The majority of such systems installed are normally legally required emergency generator back up systems. Such systems are primarily utilized for short duration power outages caused by a variety of power interrupting occurrences that are considered non standard as related to the reliability of the power utility system.

These short duration power disruptions are normally in the magnitude of minutes to many minutes but not normally exceeding 2 hours. For this reason most life safety codes require the system to be able to stay on line for back up purposes at least 2 hours. If the owner desires a system that will stay activated for longer than two hours than the designer must coordinate this requirement carefully with the owner so they are aware of the increase in cost that can result from designing a system for longer durations.

When power disruptions are caused by disasters it is essential that the power from back-up generation not be activated until a complete damage assessment is completed. Deaths have occurred after disasters have passed due to electrocution caused by the power restored by back-up generation before a complete building damage assessment is done and before damage is repaired. Fires, grounding problems and back feeding caused by back up generation is known to have occurred because of faulty damage repairs or the lack of proper repairs. NEVER activate your back-up generation system after a disaster with out first doing a complete damage assessment.

Big or small plants we can solve your back up problems or prevent new ones.

Hal

Elevators and other Non-Linear Load problems for Generators

For emergency generator systems utilized for life safety the elevators may be the largest non-linear load. As important as these are the designer must make sure that these non-linear loads are not going to create a great deal of stress for the generators. In some cases if elevators are left to operate off the generator for a long duration they may cause the generators to burn out.

When designed to operate for standby power, the emergency generator must be capable of operating the elevators safely and with a great deal of reliability. If the elevators do not operate properly while on emergency power or if the generator can not handle the elevator load for a long duration serious problems may occur.

The most commonly ignored operation of an emergency generator is its ability to handle the elevator loads safely and over a long duration. Especially when the emergency generator system is designed for short duration power outages, let’s say, a class two system. If in fact the generator capacity was selected on a peak load expected to be occurring for short durations, say two hours, but instead the owner try’s to operate the emergency generator for a long period of time say, 48 hours at peek load, the generator may burn out.

Additionally, the owner must check to see if the local codes require elevator standby power testing annually. If more than one elevator is capable of running simultaneously, the local codes may require them to all be tested simultaneously.
The owner operator must also realize that with the advent of SCR and VFD drives for elevators and other devices, existing emergency generators may not be capable of providing the proper power to operate the elevators in an emergency over a long period of time. Additionally, older emergency generators may have a difficult time with the current demand changes that solid-state drives require.

With emergency generator systems current and voltage harmonics differ greatly from that produced by utility power. Improper grounding methods and increased impedance of the generator system can cause additional problems with sensitive devices on the emergency feeder system, due to increased harmonics and RFI. Studies and testing have shown that problems as follows can develop while operating elevators on emergency power:

The Total Harmonic distortion can increase substantially over that expected from utility power.

Grounding can be found to be insufficient.

Voltage regulation as the elevators operate can be found to vary by up to 15 to 25%.

Voltage may dip to values not tolerated by solid state drives.

It is very important that when upgrading to a modern solid state elevator drive that the owner also upgrade older emergency generators that they may be considering to operate all or any of the elevators in the event of a power disruption. If the emergency generator is not to be upgraded than an alternate type of motor drive may have to be specified.

In evaluating the emergency generator-elevator relationship other items effecting the generator operation may also have to be evaluated. Such items that could effect the operation of the generator and how it responds to the elevators are as follows:

What else is operating off the emergency system? Variable frequency drive motors?

Are any sensitive UPS systems expected to be served by the generators?

Are radio and emergency telephone services to be operated from the generator and if so how are they shielded and grounded?

Is the existing generator able to handle the regenerative power from the elevator SR and not have its performance adversely affected?

How will the emergency generators voltage regulator be effected and can it handle the major voltage variation it may be subject to?

Which type of elevator drive is to be installed and how will it effect the generator in actual emergency operation situations. The various types of common drives that may be utilized are as follows:

12 pulse SCR drives with out filters.
Variable Frequency-variable Voltage AC type Non Regenerative.
Motor Generator Set.
Six Pulse SCR drive with filters.
Hydraulic(Across the line)