Short-time withstand

Short-Time Withstand - Electrical Parameters in Low and Medium Voltage Components

Many electrical parameters in low and medium voltage components share similarities, one such parameter is the short-time withstand current, denoted as Icw. This parameter is essential for switchgear to handle short-circuit faults. It ensures the switchgear can withstand a specific short-circuit current for a predetermined time, either in cases where a circuit breaker trips due to a fault or when sectional protection necessitates the transfer to withstand short-circuit current.

Icw consists of two main aspects. The first is the ability to withstand the rated short-circuit current in root mean square (rms). This requirement varies, such as medium voltage Icw values like 31.5, 40, 50kA and low-voltage Icw values such as 50, 63, 80, 100, and 150kA. The second aspect is the duration it can withstand this current. Low-voltage applications typically require 1 second, while medium-voltage applications need 3/4 seconds.

These requirements are tailored to protection needs. In low voltage, close to the end of a line, the short-circuit current is higher, and protection logic demands rapid disconnection. Thus, the time for Icw is short. In contrast, medium voltage scenarios are often further from the fault location. In such cases, protection circuit breakers are situated upfront, allowing for longer Icw times. Moreover, as short-circuit current rapidly decays at medium voltage levels, a lower Icw value is acceptable. Some specialized applications, like offshore oil platforms or scenarios with short cable lengths, necessitate higher short-circuit currents and require shorter disconnection times for equipment safety.

The Icw parameter primarily considers the thermal effect on conductors, such as conductor cross-sectional area and the contact pressure's contact size. Minimum cross-sectional areas can be calculated using GB/T3906, and contact resistance can be calculated based on a company's specifications. An increase in contact resistance leads to a voltage drop across the contact point. If this voltage drop exceeds 0.41V, it can result in the contact melting or welding, rendering it unable to withstand the short-time withstand current.

Short-circuit current comprises three components. The first and most significant component is the frequency component, which remains constant if the power grid is considered infinite. The second component is the harmonic component, which contributes non-50Hz frequency components but is generally negligible. The third component is the DC component, which occurs due to instantaneous short circuits, causing the short-circuit current to peak and gradually decay. Industrial frequency short-circuit currents include a superimposed DC component. The DC component can be calculated by finding the midpoint between the current peak and trough, and it continuously attenuates. Once the DC component's attenuation is complete, the short-circuit current returns to its steady-state value, often referred to as the cyclic component.

As a result, short-time withstand current corresponds to the peak withstand current, or Ipk, which represents the maximum value of current on one side of the bias due to short-circuit conditions. For low-voltage systems, the peak withstand current is 2.2 times the short-time withstand current. For example, if the short-time withstand current is 100kA rms, the corresponding peak withstand current is 220kA. In medium-voltage systems, the peak current varies with frequency. In a 50Hz environment, it's 2.5 times the short-time withstand current, while in a 60Hz environment, it's 2.6 times. Therefore, for a 50kA short-time withstand current in a 50Hz setting, the peak withstand current is 125kA, whereas in a 60Hz system, it reaches 130kA. These requirements pose significant mechanical endurance challenges for switchgear, especially in specialized applications such as generator outlets, where a peak withstand current requirement of 2.74 times exists, translating to 137kA for a 50kA short-time withstand current. This results from the immense peak short-circuit current generated during power supply faults.

Peak withstand current, known as dynamic stability, involves mechanical stress due to electrical forces, impacting components such as copper bars and contacts. These components must withstand these forces without collapsing.