WHAT IS THE ACTUAL SWITCHING TIME AND HOW DO THEY ACHIEVE THIS PERFORMANCE WHILE MAINTAINING A HIGH LEVEL OF RELIABILITY?
The operating time of an electromechanical relay is a fundamental parameter when selecting the component that best suits a specific application.
A fast relay, also called a high-speed relay or trip relay, is generally used downstream of high-voltage line protection system, where it is necessary to minimize the overall operating time of the control chain of substation equipment.
From the point of view of operating speed and the consequent switching of the contacts, we can distinguish between two main categories:
- Standard electromechanical relays
- Fast relays (high-speed or trip relays)
Although there is no single overarching standard that defines a universal time limit in milliseconds for distinguishing a conventional relay from a fast relay, industry practice and the technical specifications of major users generally consider a relay to be "fast" when it can switch its contacts in less than 10ms.
THE OPERATING MODE AND CONTACT CONFIGURATION AFFECT SWITCHING SPEED
Definitions:
Make contacts (normally open)
contact that closes when the relay is in the operated state and is open when the relay is in the released state.
Break contact (normally closed)
a contact that opens when the relay is in the operated state and is closed when the relay is in the released state.
Changeover contact
a combination of two contact circuits with three contact elements, one of which is common to the two contact circuits, such that when one of these contact circuits is open, the other is closed.
When trying to increase the switching speed of a relay, the first step is to define exactly what is meant by “switching”. Defining when the operation of a relay can be considered complete allows the designer to understand the actual operating time of the relay and to define more precisely the timing of subsequent operations and control logic.
The first point to establish, therefore, is whether the fast relay is intended to operate during energization (pick-up) or during de-energization (drop-out).
There are fast relay models designed to ensure high contact-switching speed during the energization phase, while other models are designed to ensure high contact-switching speed during the release phase.
Another important difference concerns the type of contacts.
A fast relay or trip relay can have normally closed (NC), normally open (NO), or changeover (SPDT) contacts.
In the absence of electrical arcing, for example, the opening of NC contacts is complete as soon as the moving contact pole has separated from the fixed pole by just a few fractions of a millimetre.
This type of operation makes it possible to achieve the fastest operating times, also because it is not affected by the phenomenon of contact bounce.
Switching is longer in the case of NO contacts, where an operation can be considered complete when the moving contact pole has travelled the entire “stroke” separating it from the fixed pole.
The closing of an NO contact is also subject to an increase in operating time due to the phenomenon known as contact bounce.
When a fast relay is equipped with changeover (SPDT) contacts, everything depends on how the designer chooses to use the NO and NC poles of the contacts and how the system logic is managed.
CONTACT BOUNCE TIMES: WHY ARE THEY IMPORTANT?
In an instantaneous monostable relay, the closing of an NO contact normally occurs between 15 and 40ms, depending on the characteristics of the product.
A fast relay, on the other hand, can complete the operation in between 2.5 and 10ms.
The switching time is measured from the instant the coil is energized or de-energized to the stabilization of the contact state change, therefore including contact bounce.
Contact bounce is an intermediate contact position characterized by significant dynamic movement that gradually decreases until the contact stabilizes in its final position.
Considering contact bounce times when analyzing the speed performance of a trip relay is essential for determining the ACTUAL command execution time.
Generally expressed in milliseconds (ms), the contact bounce time of a fast relay refers to the intermittent contact condition that occurs due to the collision between the moving metal parts (or moving contact poles) when the relay is energized or de-energized.
It is essential to consider this phenomenon because, in most cases, the contact bounce time doubles the number of milliseconds required to complete an operation.
A relay whose operating time is specified as 8ms, if contact bounce times are not included, implicitly has an actual switching time of approximately 16ms.
This is certainly not a negligible value, as it represents a 100% increase in command execution time.
Unless otherwise specified, the switching times stated in the technical product documentation for AMRA fast relays ALWAYS include contact bounce times.
WHAT DETERMINES THE SPEED OF FAST RELAYS AND HOW ARE THEY MANUFACTURED?
AMRA fast relays are assembled with specially designed coils sized to generate a very high magnetic flux when energized.
The optimization of the ferromagnetic circuit therefore enables very fast contact switching.
The manufacturing processes used for AMRA fast relays and the careful selection of materials give this range of fast relays excellent longevity and robustness.
These characteristics make AMRA fast relays suitable for use in the most demanding sectors, such as control and signalling functions in power generation plants and electrical substations.
AMRA fast relays are also immune to the high levels of electromagnetic interference typical of high-voltage power transmission substations.
In these applications, the relays are generally connected to digital protection systems and used to protect high-voltage lines in the event of a fault.
Switching speed in these critical applications is an essential characteristic for electrical designers and for the end users of the installation.
Finally, their ability to efficiently handle the opening of DC loads, together with the other electrical and mechanical parameters that determine their reliability, has enabled their approval by TERNA, ENEL, and other major TSOs and DSOs at national and international level.
Like all AMRA relays, fast relays are assembled within a controlled manufacturing process in which each manufacturing stage is verified by the subsequent stage.
Each relay is individually calibrated and tested manually to ensure maximum reliability.
FAST RELAY WITH CONVENTIONAL CONTACTS OR REED CONTACTS?
To understand which trip relay is best suited to the task it must perform in an installation, it is necessary to understand the advantages of a conventional fast relay and those of a fast relay with REED contacts.
The two technologies offer different performance characteristics in terms of speed, immunity to interference, and interrupting capacity.
A REED-type fast relay generally operates in less time because the contact poles are much closer together, only a few fractions of a millimetre apart.
A conventional fast relay, on the other hand, can have a distance of several millimetres between the poles of a single contact, which is tens of times greater than the distance in a relay with REED contacts.
On the other hand, a fast relay with REED contacts, which can switch in 2–3ms, has a significantly lower interrupting capacity, handling a maximum of 250mA.
The REED relay is therefore suitable only for switching signals in non-demanding environments with low levels of interference.
This is because the overall mass of the REED contacts is many times lower than the mass of an electromechanical contact assembly, and this negatively affects immunity to the electromagnetic interference typical of electrical substations.
PROTECTION DEVICES SLOW DOWN RELAY SWITCHING
Choosing to add overvoltage protection devices to a fast relay can literally eliminate its speed performance, bringing the switching time of a trip relay back to the switching time of a conventional relay.
A diode, for example, can increase the switching time to as much as 20ms when the relay changes from the operated state to the released state.ù
For this reason, fast electromechanical relays generally do not provide for the installation of this type of component.
THE MODELS
The fast relay range includes several models specifically designed for use as lock-out relays, trip relays, or high-speed relays.
On this page we have collected all AMRA fast-actuating relay models to provide a quick overview of their main characteristics and an overall view of our range of fast relays.
If you need support in selecting the right model, you can contact us through this webform dedicated.