Monday, September 03, 2007

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)