Muhimman Kirki DC na Karamin Dabbobi na Aiki da Yawanci

Duk da ya faruwa aiki na kamar yadda ake faruwa wata mai sarrafa shi a tsakiyar karamin gaba:
Ake amfani da semiconductors a karamin gaba don in faruwa cashi a lokacin fault. Wani topo ta haka na solid-state DC breaker an nuna a Figure. 1. Duk da lafiya biro da IGCT suna cikin hanyar zama, sannan surge arrester an amfani da shi don in faruwa line inductance a lokacin fault. Idan ake faruwa DC breaker, IGCT an faruwa. Saboda energy da aka gudana a cikin inductance, voltage across semiconductors ya zama har zuwa, sannan surge arrester baka tafi yau da current. Don in faruwa line inductance, protection voltage of the surge arrester ya kamata aiko da nominal grid voltage. Kuma ya kamata a duba cewa semiconductors suna iya tabbatar da protection voltage of the surge arrester. Mafi kyau na solid-state DC breaker shine kadan ya faruwa da kuma babu abubuwa masu haruffa. Saboda semiconductors suna cikin hanyar zama, on-state losses za a faru.

Figure 1:Simple design of solid stat circuit breaker

Solid-state circuit breakers suna yi amfani da solid-state switch kawai don in zama da kuma faruwa current. Saboda electric arc an faruwa, ana bukata wani hali na musamman don in faruwa energy da aka gudana a cikin circuit inductance. Ana yi hakan a kan metal-oxide varistor (MOV) da ke cikin parallel. MOV na non-linear voltage/current characteristic.
Resistance ta zai zama mai yawa (effectively acting as an open circuit) har zuwa lokacin da voltage across it ya zama da adadin kasuwanci, inda resistance ta zai zama mai kafin allow current to conduct through the device. A lokacin da ake yi conducting, MOV ya zama tafi yau da current da kuma clamp voltage across it at a constant value.
Wani abu na haka ana amfani da shi a high voltage systems as a surge arrester and is also used as a protection device for voltage-sensitive components.
Biyo biyu na bi-directional solid-state circuit breaker topologies an nuna a Figure 2. Idan ake faruwa breaker, both semiconductor devices an faru, allowing current to flow in both directions. A lokacin da ake faruwa current, both devices an faru, forcing the voltage across the devices to rise until the MOV starts to conduct and clamp the voltage across the devices. Conducting MOV acts to dissipate the energy stored within the circuit inductance.
While IGCTs are shown in Figure 2 (a), GTOs have also been used in older designs based on the same circuit topology.

 
Figure 2   a) IGCT based simple bi-directional solid-state circuit breaker, (b) IGBT based simple bi-directional solid-state circuit breaker

Figure 3 shows a number of alternative designs which apply this concept to medium voltage systems. In these systems, multiple devices are connected in series to increase the total voltage withstand capability of the solid-state breaker. Diodes are also often connected in series with the main breaking switches to improve the reverse block voltage of the system, due to the limited reverse blocking capability of existing devices such as IGCT and GTO. The circuit is shown in Figure 3 (c) includes parallel-connected RC snubbers which are required for GTO-based systems to aid the turn-off of devices, and also contain two interesting features that might be applied to other solid-state circuit breakers. Firstly, it includes a parallel-connected resistor which is used to limit the fault current during the current interruption. During normal operation, this resistor is shorted out by the main semiconductor switches and therefore does not contribute to the on-state losses of the breaker. Secondly, a mechanical switch is connected in series to provide physical isolation.
While designs shown in this section are primarily designed for ac power systems, it should be possible to apply these designs to dc applications with minimal modifications.

 
Figure 3: a) IGCT based medium voltage bi-directional solid-state circuit breaker, (b) IGCT based medium voltage bi-directional solid-state circuit breaker, (c) GTO based bidirectional solid-state circuit breaker

A simplified block diagram of a solid-state circuit breaker is shown in Figure 4. The solid-state current interrupter is comprised of a series string of solid-state devices to safely handle the DC bus voltage. A fast coordinated inverse-time controller provides the gate drive signal for the switches in the interrupter which synchronously open and close. The fast inverse time controller receives commands from either a manual input, from other breakers in the network, or from fast sensors that detect local fault currents. The inverse-time controller provides inverse trip time control for overcurrent states, and a fast instantaneous trip if the overcurrent limit is reached. These operational parameters can be adjusted for each breaker depending on its location in the network, providing an orderly, sequenced response to fault conditions.

 
Figure 4: Simplified system diagram of a typical MVDC solid-state circuit breaker

he solid-state interrupter provides the primary functionality of a complete circuit breaker assembly fast fault protection and isolation. The complete circuit breaker assembly must also provide a means of safely disconnecting the interrupter from the power network when maintenance or service is required.
A preliminary layout for an 8 MW load-level circuit interrupter is shown in photo 1. This interrupter
consists of six 4,500 V IGBTs (CM900HB-66H) connected in series. The 8 MW interrupter is
approximately 23” wide x 9” high  11” deep and weighs approximately 60 lb. The IGBTs are mounted
on water-cooled aluminum cold plates, which are, in turn, mounted on an electrically insulating mechanical
frame. The non-metallic water lines are sufficiently resistive to limit the current leakage down the lines.
This will require a small, closed-loop cooling system and a long-lasting ion-exchange cartridge to maintain
the resistivity of the cooling water.
This will require a small, closed-loop cooling system and a long-lasting ion-exchange cartridge to maintain the resistivity of the cooling water.
In photo 1 shows the preliminary mechanical layout of a 10 kV, 8 MW (800 A) IGBT interrupter. The IGBTs are mounted on water-cooled cold plates. Non-metallic cooling lines between adjacent cold plates are designed to stand off the full switch voltage when the switch is open.
Parallel arrays of these assemblies are used to meet the overall current requirements for the load.

 
Photo 1: Preliminary mechanical layout of a 10 kV, 8 MW (800 A) IGBT interrupter. The IGBTs are mounted on water-cooled cold plates

Compare the advantages and disadvantages of Solid-State Circuit breakers with other circuit breakers briefly:
While solid-state circuit breakers can achieve substantially faster interruption speed compared to conventional electro-mechanical-based circuit breakers, one major drawback of solid-state breakers is their high on-state losses. With contact resistance as small as a few micro-ohms, electro-mechanical contacts in classical circuit breakers introduce negligible on-state losses. In contrast, most solid-state devices introduce a voltage drop of at least two volts, therefore as a large current flows through the breaker, the on-state losses of a solid-state circuit
breaker can be significantly higher than those of a classical circuit breaker. The increased energy loss also leads to increased requirements for cooling. Traditionally large metallic heatsinks are used to passively cool power semiconductor devices, however, they can contribute to substantial portions of the systems’ overall size and weight. While the installation of active cooling systems such as forced air (fan) or liquid cooling might help to reduce the size and weight of the overall system, they introduce additional complexities such as increased acoustic signature, energy losses, and maintenance issues.
According to figure 5, the values are given in relation to the highest value per group.
For every criterion, small values are considered to be preferable. A small area, therefore, indicates an overall good performance of a switching concept.
Based on the findings, the solid-state circuit-breaker shows a good overall performance. Due to its fast switching capabilities, the turn-off time is small and only low current amplitudes occur. Also, the reliability and the complexity of the switching process can be considered to be good. However, the solid-state breaker suffers from high losses, compared to mechanical or hybrid switches.
An alternative concept with low losses, medium relative costs, and good reliability is the snubber mechanical breaker. Also, the conventional hybrid breaker shows an overall medium performance. It suffers from high peak currents due to the mechanical switch. The concepts taken from HVDC systems do not have a good performance within the voltage and power levels investigated. However, for higher voltages and powers, this might change. Finally, the concept of a pure mechanical-breaker is still interesting for low and low-medium voltage applications since it’s the only well-proven one.

Figure 5: Overview of all switching concepts for DC circuit breakers

Table 1 summarizes the characteristics of the four circuit breaker technologies:
It should be noted that the time of preparation of this table is 2012.

 
Table 1: Summary of circuit breakers technologies for low power DC applications