Degradation of Insulation in Switchgear (What’s Really Happening)

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Partial Discharge

Partial Discharge (PD)

Electrical insulation is subjected to electrical and mechanical stress, elevated temperature and temperature variations, and environmental conditions especially for outdoor applications. In addition to normal operating conditions, there are a host of other factors that may trigger accelerated aging or deterioration of insulation.


Switching and lightning surges can start ionization in an already stressed area. Mechanical strikes during breaker operation can cause micro cracks and voids. Excessive moisture or chemical contamination of the surface can cause tracking. Any defects in design and manufacturing are also worth mentioning.
Partial Descharge




PD is a localized electrical discharge that does not completely bridge the electrodes. PD is a leading indicator of an insulation problem. Quickly accelerating PD activity can result in a complete insulation failure.

PD mechanism can be different depending on how and where the sparking occurs:
  • Voids and cavities are filled with air in poorly cast current transformers, voltage transformers and epoxy spacers. Since air has lower permittivity than insulation material, an enhanced electric field forces the voids to flash-over, causing PD. Energy dissipated during repetitive PD will carbonize and weaken the insulation.
  • Contaminants or moisture on the insulation induce the electrical tracking or surface PD. Continuous tracking will grow into a complete surface flash-over.
  • Corona discharge from sharp edge of a HV conductor is another type of PD. It produces ozone that aggressively attacks insulation and also facilitates flashover during periods of overvoltage.
Features of partial discharge activity, such as intensity, maximum magnitude, pulse rate, long-term trend, are important indications of the insulation’s condition.

Healthy switchgear has very little or no PD activity. If PD activity is significant, it will eventually deteriorate insulation to a complete failure. Higher voltages produce higher intensity partial discharges, thus PD detection in gear with higher voltages (13.8 kV and up) is more critical.

Possible locations of partial discharge in switchgear:
  1. Main bus insulation
  2. Circuit breaker insulation
  3. Current transformers
  4. Voltage transformers
  5. Cable terminations
  6. Support insulators
  7. Non-shielded cables in contact with other phases or ground


 

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Electrical Switchgear Protection

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Definition of Switchgear

A switchgear or electrical switchgear is a generic term which includes all the switching devices associated with mainly power system protection. It also includes all devices associated with control, metering and regulating of electrical power system. Assembly of such devices in a logical manner forms a switchgear. This is very basic definition of switchgear.

Switchgear and Protection

Switchgear
 We all familiar with low voltage switches and re-wirable fuses in our home. The switch is used to manually open and close the electrical circuit in our home and electrical fuse is used to protect our household electrical circuit from over  current  and short circuit faults. In same way every electrical circuit including high voltage electrical power system needs switching and protective devices. But in high voltage and extra high voltage system, these switching and protective scheme becomes complicated one for high fault  current  interruption in safe and secure way. In addition to that from commercial point of view every electrical power system needs measuring, control and regulating arrangement. Collectively the whole system is called switchgear and protection of power system. The electrical switchgear have been developing in various forms.

Switchgear protection plays a vital role in modern power system network, right from generation through transmission to distribution end. The  current  interruption device or switching device is called circuit breaker in switchgear protection system. The circuit breaker can be operated manually as when required and it is also operated during over  current  and short circuit or any other faults in the system by sensing the abnormality of system. The circuit breaker senses the faulty condition of system through protection relay and this relay is again actuated by faulty signal normally comes from current transformer or voltage transformer.

A switchgear has to perform the function of carrying, making and breaking the normal load current  like a switch and it has to perform the function of clearing the fault  in addition to that it also has provision of metering and regulating the various parameters of electrical power system. Thus the switchgear includes circuit breaker, current transformer, voltage transformer, protection relay, measuring instrument, electrical switch,electrical fuse, miniature circuit breaker, lightening arrestor or surge arrestor, electrical isolator and other associated equipment.
Switchgear Panels
 
 
Electric switchgear is necessary at every switching point in the electrical power system. There are various voltage levels and hence various fault levels between the generating stations and load centers. Therefore various types of switchgear assembly are required depending upon different voltage levels of the system.
 
Besides the power system network, electrical switchgear is  also required in industrial works, industrial projects, domestic and commercial buildings.

 

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PSIM Software

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PSIM Software

PSIM Software

PSIM is one of the fastest simulators for power electronics simulation. It achieves fast simulation while retaining excellent simulation accuracy. This makes it particularly efficient in simulating converter systems of any size, and performing multiple-cycle simulation.

PSIM is the engine of the simulation environment. PSIM uses a strong algorithm dedicated to electrical circuits (piecewise method, generic models and a fixed time-step). The fast simulation allows repetitive simulation runs and significantly shortens the design cycle.

PSIM can simulate control circuit in various forms: in analog circuit, s-domain transfer function block diagram, z-domain transfer function block diagram, custom C code, or in Matlab/Simulink®. PSIM’s control library provides a comprehensive list of components and function blocks, and makes it possible to build virtually any control scheme quickly and conveniently.

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The Switchboard Design Requirements

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Switchboard Panel

The Switchboard Design Requirements

Design Rules

The following rules of design have to be implement in the aim to facilitate the assembly and especially the maintenance of the installation.

 The switchboard must be designed the way to have a clearly visible separation between the 3 following zones:
  • One dedicated for the devices installation
  • One dedicated for the bus bars mounting
  • and one dedicated for the out-goers cables connections 

Switchboard Areas

The goal of that architecture is to separate the switchboard in different areas in function of each professional user.
  • Devices zone => panel builder and exploiter
  • Bus bars zone => panel builder
  • Cable connection zone => installer and maintenance

In order to facilitate the access within the switchboard for the maintenance, its covering panels must be dis-mountable on all surfaces for any IP degree.

All the devices must be installed onto dedicated mounting plate designed for one or several switchgear of the same type. The objective of that point is to regroup the protection equipment of the same nature each others and distinguish inside the switchboard the function of each device or group of devices.

Theses mounting plates will have an independent fixing system affording them to be transformed and moved anywhere in the switchboard and especially to make it easier the installation evolution.
To insure the maximum protection of people around the electrical installation, front plates must be installed in front of all control and protection equipment in order to avoid a direct access without a tool to the devices and consequently to the active parts.

For safety reasons and especially when the door will be opened during the switchboard working, all bus bars have to be covered by barriers onto the whole perimeter of the bus bars zone.

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