Basics of High Voltage Switchboards

Introduction:

High voltage switchboards are specialized electrical distribution equipment designed to handle higher voltage levels than standard switchboards. They are commonly used in industrial & commercial facilities, power plants, substations, and other applications where high-voltage electrical power connections are sanctioned by the local utility.

Let us start with what is the relevant national and international standards for High voltage switchboards. The relevant standard for High Voltage Switchboards is 62271-200. This standard covers High Voltage Switchboards with voltage levels above 1kV and up to 52kV. In normal discourse, these are also termed Medium Voltage Switchboards with voltage ratings from 1kV to 36kV. In this category standard equipment operational voltages are 3.3kV, 6.6kV, 11kV, 22kV and 33kV. 22kV operational voltage is available in only a few states in India whereas most of the utilities offer primary power to industrial and commercial users at 11kV or 33kV.

All utility-scale users like commercial buildings or industries need power connections sanctioned by the distribution utility. These power connections are being given at 11kV or 33kV depending on the concerned state policy and load sanctioned. To receive these powers for further distribution and management, we need a High Voltage Switchboards. 

Major Components of High Voltage Switchboards:

It comprises of following components:

• Switching Device – Vacuum Circuit Breaker (VCB), Isolators or Vacuum Contactors (VC)
• Sensing Devices – Current Transformers (CT) and Potential Transformers (PT)
• Busbars
• Measuring Devices – Meters
• Protection Devices – Relays or Fuses
• Surge Protection Devices – Surge Arrestors
• Indicating Devices – Indication Lamps and Meters
• Earthing Switch

Functionality of Components:

VCB: VCB or Vacuum Circuit Breaker is used to make or break the circuit in “ON Load” and “Off Load” conditions. The major subcomponent is the interrupter or vacuum bottle. The making and breaking happens inside the bottle under a vacuum as an arc quenching medium.

• Current Transformer: It is a sensing device used for the conversion of high voltage current to a current at low voltage suitable for measuring and protection devices.

• Potential Transformer: It is another sensing and conversion device to convert high voltage to measurable low voltage at 110V AC.

• Bus Bar: Current-carrying conductors connecting various high-voltage components in a designed sequence. They are characterised by current rating, fault level, creepage & clearance and arrangement of supports.

• Measuring Devices: These are the devices that indicate current, voltage or other derived parameters like power and energy. They take current and voltage as input for the measurement of electrical parameters. A few other devices that measure non-electrical parameters like temperature take input from thermal sensors.

• Relay: A protection device used to protect the system against high currents and voltages arising due to faults in the system itself or the downstream network.

• Fuse: A protection device to protect the system against high current due to a short circuit.

• Surge Arrestor: Used to divert any external voltage surge, that crept into the system, to the ground thereby protecting the system.

• Earthing Switch: Used to ground any floating voltage after “switching off” the main switching device during maintenance. They are interlocked with switching devices so that only one of them can be switched “ON” at a time.

• Indication: They are the coloured lamps indicating the different states of switching devices like “ON”, “Off”, and “Trip”. They are also used to state the healthiness of a circuit.

Basic Structure of High Voltage Switchboards

These Switchboards are normally divided into three sections:

• VCB compartment
• Bus Bar Compartment and
• Low Voltage Compartment

Based on the medium of insulation around the high-voltage components, the High Voltage Switchboard can be classified as

•  Air Insulated Switchgear or AIS
• Gas Insulated Switchgear or GIS
• Solid Insulated Switchgear or SIS

Air Insulated Switchgear (AIS): These are the conventional High Voltage Switchboards where all high voltage components like VCB’s, CT/PT and Bus bars are immersed in natural Air. They are comparatively bigger because of the low dielectric strength of Air. See the picture below

Gas Insulated Switchgear (GIS): These are the High Voltage Switchboards where all high voltage components like VCB’s, CT/PT and Bus bars are immersed in SF6 gas. They are comparatively smaller because of the higher dielectric strength of SF6. See the picture below:

Solid Insulated Switchgear (SIS): These are the High Voltage Switchboards where all high voltage components like VCB’s, CT/PT and Bus bars are encapsulated in Epoxy. They are comparatively smaller because of the higher dielectric strength of Epoxy. Solid Insulated Switchgear is also considered to be a green product as it does not have harmful SF6 gas. See the picture below

Basic Characteristics of High Voltage Switchboards

These switchboards are characterised by the following parameters:

• Rated Current Ir
• Rated Voltage Ur
• Rated Service Voltage U
• Rated Insulation Level
  • Power Frequency withstand Voltage Ud
  • Lightening Impulse withstand Voltage Up
• Rated Short Circuit Current
  • Rated Short breaking Current Isc
  • Rated Short Circuit making (Ima) and peak current (Ip)
• Internal Arc Classification (__kA, __Sec)

Rated Current Ir:

This is the current that can flow continuously in the circuit at given ambient conditions and for which all high-voltage components are designed to carry the rated current.

Rated Voltage Ur:

The rated voltage is the maximum rms value of operating voltage that equipment can withstand. The rated voltage is greater than the service voltage and is associated with an Insulation Level.

Rated Service Voltage U:

This is the voltage effectively applied to the terminals of high-voltage equipment during normal service conditions. It will be equal to or less than the Rated Operating voltage.

Rated Insulation Level:

It defines the dielectric strength of the high-voltage circuit in the switchboard. It defines the withstand voltage of the high voltage circuit during normal service conditions or arcing between phases or phase to earth. In other words, this is the circuit’s strength against switching over voltages and lightning surges.

It has two subcategories:

1. Power Frequency withstand Voltage, Ud: During normal switching, ON & OFF produces overvoltage that is internal to the network. The equipment’s ability to withstand these overvoltages for 1 min at nominal frequency is termed as Power Frequency withstand Voltage.

• Lightning Impulse withstand Voltage, Up: This defines the withstand capacity of equipment against external or atmospheric overvoltage that arise due to switching disturbance in the external network or lightning surge travelling to the internalnetwork. It is against the peak value of overvoltage which arrives at peak in 1.2μs and falls to 50% in 50 μs.

Rated Short Circuit Current:

1. Short circuit making and breaking capacities are the switching capacity while short time current and peak current are the withstand capacities of the switching device.

• The IEC defines a fixed ratio of peak current and short-time current as Ip/Ik=2.5

Internal Arc Classification:

Internal Arc Classification is defined in IEC 62217-200 as A-F, A-FL and A-FLR and helps to define the internal arc performance of high-voltage switchgear. This defines the accessibility of high-voltage switchgear for operating personnel while it is live.

The AFLR definitions are described as follows: 

• A   switchgear installation in rooms with access for authorized personnel, closed electrical service location.
• A-F Access to switchgear front (F-front) only as safe for users
• A-FL Access to switchgear front and side (F-front, L-lateral) safe for users
• A-FLR Access to switchgear from all sides (F-front, L-lateral, R-rear) safe for users

External References

https://new.abb.com/medium-voltage/switchgear/3d-ecatalogue/mv-gas-insulated