Determining Electro-Mechanical Relay Power
How Much is Enough?
Electro-mechanical protective relays provide a wide range of protection for electric power systems. If it seems as though these types of relays have been around forever, they have! But aside from the longevity of these ancient devices, the greatest aspect, perhaps, has involved testing. Electro-mechanical relays are predominately tested using single-phase devices, despite the fact that the relays were constructed to protect complex three-phase power systems. Accordingly, testing presents a significant concern to these relay users. So what devices, then,have historically been used to test these electro-mechanical relays? The answer...a load box, phase shifter and/or variable transformer.
The resistive or transformer load box is a single phase device with the capability to deliver 100 amps at 1200VA. The limit of this test device was the down stream fuse, but it was not accurate since heating of the resistive coils or transformer core caused the current to become unstable. As such, the electro-mechanical relay reliability could not be accurately tested without a dependable, repetitive test.
A phase shifter had similar problems in testing, especially when inter-connected to the power system VTs. Depending on the time of day and the loading conditions of the line, the phase shifter became unstable, and an accurate phasor measurement could not be repeated.
The variable transformer also had similar problems in testing. Since it was interconnected to the VTs, the variable transformer experienced the same problem as the phase shifter.
Thus, the great testing dilemma remained. OMICRON responded, and the CMC Test System was created for test repeatability, accuracy, and stability. The power requirement was our next consideration.
An electro-mechanical induction disk time overcurrent relay can be provided by numerous manufacturers, but in general, the relay is either General Electric's IAC or ABB's (formerly Westinghouse) CO. And, they are significant in that almost every electric power system has at least one type of these protective relays within its system. Both offer the true single-phase test of any electronic amplifier requiring the most VA for periodic maintenance.
When beginning to test induction disk time overcurrent relays, it is best to know the maximum VA requirements in advance. The relay manufacturers will publish this information in the instruction leaflet for each type of relay. However, these directives may need to be extrapolated from the relay manufacturer's data. Some instruction leaflets provide a table of VA requirements for minimum tap setting from <1.0 to 6.0 VA, at 3 times the tap setting from 6.0 to 40VA and at 10 times the minimum tap setting these VA requirements may range from 60VA to 260VA. Testing at 20 times the minimum tap setting the VA requirement may approach 1.0 kVA.
At maximum tap setting the VA requirement may range from 2.0 to 10VA. As the multiple of tap increases, so does the VA requirement. For example, when multiples of the minimum tap are tested at 3 times the tap, the VA could range from 15.0 to 70VA depending upon the relay type.
Significant increases continue at 10 times the minimum tap setting, thus the VA requirements may range from 160VA to 600VA. Testing at 20 times the maximum tap setting the VA requirement may approach 2.1 kVA.
But what about the relay manufacturer's instruction leaflet that provides only the ohm burden of the relay to be tested? How is the VA requirement found for this type of induction disk time overcurrent relay? Ohm's Law of course!
The relay tap value (I) and relay ohm burden at the tap value (Z) are both known but the voltage (V) and the VA requirements are unknown. First, solve for V; where V = Z*I, then solve for VA, where VA = V*I.
As an example, a certain relay has a 20 ? * 0.5A tap = 10.0V. Then solve for VA, where; 10.0V * 0.5A= 5.0VA at the tap value. Now, solve for 3 times the tap value, where; V = 20 ? * 1.5A = 30.0V and VA = 30.0V * 1.5A = 45.0 VA.
These examples aid in understanding the necessary power required to test electro-mechanical relays. There are other types of electro-mechanical relays, but all will conform to the same methods in determining the power requirements. A listing of these relay types may include overcurrent plunger, over-voltage induction disk time and plunger, Differential current/voltage and Distance.
The OMICRON CMC 156 Test System will solve the power requirements by providing the VA required for electro-mechanical relay testing. By combining the CMC 156 test set with one or more of our three amplifier options (CMA 56,CMA 156 or CMS 156), a wide range of power requirements can be met. Providing this needed power in a light-weight, easy-to-use and highly accurate system are among the numerous key benefits for the protection specialist.
CMA 156 Solves Power Debate
How can the light-weight CMC 156 Test System provide enough VA to test electro-mechanical relays? By adding our OMICRON CMA 156 six-phase current amplifier, it has the most versatility in adapting to any environment, from numerical relays to electro-mechanical relays. Our CMA 156 can provide 6 x 70VA at 7.5A, 3 x 140VA at 15A, 1 x 420VA at 50A, or 1 x 420VA at 25A! (See diagram). And for testing an induction disk time overcurrent relay at 0.5A tap, we deliver the VA to accurately perform the test. With OMICRON, seeing is believing, and proof is in the performance!
Our light-weight, CMA 156 amplifier at 34.4 pounds, provides an amazing 12.2VA per pound - more VA per pound than our competition. But what does this mean to you? Light weight eliminates heavy lifting, loss of work and back problems. In addition to this comfortable weight, the CMA provides the highest accuracy in the electric power industry (<0.01% guar.), the least distortion (<0.1% typ. THD +Noise), full power output from dc to 1,000 Hz (when controlled by the CMC) a frequency accuracy of ±0.5 parts-per-million, frequency resolution of 5 µHz, current resolution of 1mA and phase resolution of 0.001º are achieved. And to further our commitment to the electric power industry, the CMC 156 test set and CMA 156 amplifier conform to CE, EN, FCC, IEC, TUV-GS, CUL and UL. No other test system in the world has our accreditations! We wear them proudly...you can too when you own a CMC 156 Test System.
M.V. Tacoma: The Jumbo Mark II
THE OMICRON CMC 56 test set was used to perform the initial testing of the protection system recently installed on this exciting new ferry. The light weight, easy to use software and outstanding flexibility allowed the testing to be completed in record time. The protection and control system has been designed by Siemens Electric Ltd. of Canada who is using Bussiere and Frechett to perform the field testing on the system.
The new Jumbo Mark II ferry, 'Tacoma', is being built by Todd Shipyards for service on the Seattle/Bainbridge route in Washington State. The new ferry is scheduled to be launched next August, followed by further construction and final service beginning in the Spring of 1997. A regularly updated display of the project's progress can be viewed in the waiting area at Colman Dock in Seattle.