If you thought medium voltage (MV) busbars were just simple copper bars, think again. They are part of a complex power distribution system that comes in many forms and operates under challenging conditions.
What Are Busbars?
There are three main types:
- Internal busbars: used inside the switchgear, they link cable termination bars to switching devices to inter-switchgear connections. These busbars often have intricate forms and follow tight and twisting paths, allowing designers to create high-performance, compact switchgear.
- Inter-switchgear busbars: these are generally simpler in form, often straight, however don’t let this deceive you, they are part of the complex system as we will see.
- Earth bars: Yes, these are also considered a type of busbar and perform the critical job of protecting people and equipment.
What Do Busbars Do?
In short, busbars move electrical power efficiently from one point to another, often using copper as a conductor. However, it’s not that simple when they are subjected to dielectric, mechanical and thermal stresses.
Medium voltage (MV) switchgear is characterized by the International Electrotechnical Commission or IEC for short. While IEC62771-1 defines the rating and characteristics, IEC62771-200 defines how this is applied to switchgear. Let’s look at four main characteristics:
- Rated Voltage (Ur): Typically ranges from 12kV to 36kV.
- Rated Continuous Current (Ir): Usually between 100A and 4,000A.
- Rated Short-Time Withstand Current (Ik): Ranges from 16kA to 50kA.
- Rated Peak Withstand Current (Ip): Can go up to 150kA.
While there are many other characteristics, these four are essential for understanding MV switchgear. Let’s keep it simple. Now, Let’s dive into what each characteristic means and how it impacts the busbar system:
Rated Voltage (Ur):
Determines distances: The clearances between phases and between conductors and earthed parts and the height of insulating supports.
Example: A 24kV system will have a Rated Lightning Impulse Withstand Voltage of 125kV so we need large distances but with clever design we can reduce these dimensions by optimizing the shape of the busbars and connections to reduce the dielectric stresses and improve ageing.
Rated Continuous Current (Ir)
Determines how hot things get: IEC standards require compliance with specific temperature rise values at different points in the switchgear, the Rated Continuous Current defines the conductor’s cross-sectional area and material, optimization in design allows us to reduce the amount of material used.
Example: A domestic electric kettle uses around 12A to boil water in less than 5 minutes, while a busbar carries 4000A for 40 years.
Rated Short-Time Withstand Current (Ik) & Rated Peak Withstand Current (Ip)
Determines forces and additional thermal impact during a short circuit: Therefore defining the mechanical strength required and thermal withstand.
Example: A 24kV air-insulated busbar during a short-circuit rated at 31.5kA experiences forces equivalent to a large bear jumping up and down on it (483 daN).
Other Influences On Busbars:
The above factors are defined by switchgear characteristics, but busbar systems must also consider external or environmental factors such as:
- Pollution
- Condensation
- Pressure
- Humidity
- Ambient Temperature
These factors are difficult to control, this is why we specify limits. For example the operating temperature is between -25°C and 40°C. Derating that needs to be applied when we have installations above 1000m altitude, or heaters that are used to limit condensation.
By understanding and mastering these characteristics, we can design busbar systems that are efficient, safe, and reliable, and not just for one day but for 40 plus years.
We can conclude:
There is more to busbars than copper, busbars are fascinating and essential components in electrical systems, ensuring power is safely and efficiently distributed in challenging environments. Here at Schneider Electric we use type tests and international standards to validate the performance of our busbar systems both within products and also in wider installations.
Want to discover more about MV switchgear design? Learn more here.
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