HT Panel- Function, Components, Working, Features

Learn about HT Panel, its working, types, components, advantages, and applications. Discover how HT panels ensure efficient power distribution and safety.

Definition –: An HT panel is a metal enclosure equipped with HT circuit breakers, relays, and metering. It receives an 11KV/33KV supply from one or more sources and distributes the power to loads through its outgoing feeders. The number of outgoing feeders varies based on the load of the building. Electrical substations in commercial complexes, residential colonies, factories, schools, and hospitals use it to receive and distribute high voltage supply.

Table of Contents

Full form of HT Panel

The full form of HT panel stands for High Tension panel.

Functions of HT panel

The HT panel receives high-voltage power from various sources, like thermal, grid, and solar, through a transformer that controls the incoming power supply. They typically have switchgear such as circuit breakers, isolators, and fuses to regulate and protect the incoming power.

Incoming Power Control: The HT panel receives high-voltage power from various sources, like thermal, grid, and solar, through a transformer that controls the incoming power supply. They typically have switchgear such as circuit breakers, isolators, and fuses to regulate and protect the incoming power.
Distribution of Power: After receiving power at the incomer, the HT panel distributes it to various loads or electrical systems within the facility. It has busbars, which act as conductors to distribute electricity efficiently to different parts of the establishment.
Load Management: High-tension (HT) panels manage loads by providing the ability to connect and disconnect various loads as needed. Operators can control the flow of electricity to different equipment or areas by using switches or circuit breakers. This helps in the efficient use of electricity and prevents system overloading.
Protection: Protection is a crucial function of HT panels. They have protective devices like circuit breakers, relays, and fuses to prevent overloads, short circuits, and other electrical faults. These devices can detect abnormal conditions and interrupt the flow of electricity to prevent equipment damage and ensure the safety of personnel.
Metering and Monitoring: HT panels typically contain meters and monitoring equipment to measure voltage, current, power factor, and energy consumption. This data is crucial for monitoring electrical system performance, diagnosing issues, and optimizing energy usage.
Remote Control and Automation: HT panels may have remote control and automation capabilities in modern systems. This allows operators to control the panel remotely, make adjustments as needed, and receive alerts in case of abnormalities.

Major Components of an HT Panel

The HT panel has the following components.

Circuit Breakers: Circuit breakers are devices used to switch ON/OFF high tension (HT) supply, either manually, automatically through relays, locally or remotely. They also provide overcurrent protection and can interrupt the flow of electricity in case of short circuits or overloads. Different HT circuit breakers are available, including vacuum circuit breakers (VCB), SF6 (sulfur hexafluoride), and Oil-type circuit breakers. However, VCB and SF6 Circuit breakers are the most commonly used in HT panels. Depending on the electrical distribution system’s requirements, an HT panel may have one or more circuit breakers. The circuit breaker can be operated mechanically (through a push button) and electrically (through a switch). The circuit breakers are generally drawn-out types. The EDO-type breakers can be operated remotely.
Isolators (Disconnector Switches): Isolators, also known as disconnector switches, are used to separate a specific section of an electrical circuit for maintenance or repair purposes. They create a physical break in the electrical continuity and ensure no current flows through the isolated section.
Busbars: Busbars are conductive strips that distribute current to circuits and loads. They carry the current from the incoming power source to the outgoing circuits and loads. The busbar is made of copper or aluminum.
Relays: Relays are electromagnetic or electronic switches used to control the operation of various components within the HT panel. Various relays are used in a High Tension (HT) panel to safeguard the electrical system from faults like earth fault, overloading, and short circuits. Each type of relay is specifically designed to protect against a particular fault. The primary function of a relay is to detect the fault and send a tripping command to the circuit breaker of the panel. Winding Temperature Indicator (WTI) and Oil Temperature Indicator (OTI) relays protect transformers from overheating the winding and oil. The transformer feeder uses the differential relay to detect the fault in the transformer winding. The distance protection relay detects the fault in the overhead transmission line.
Meters and Instrumentation: HT panels may include meters and instrumentation to monitor various electrical parameters such as voltage, current, power factor, and active and reactive power. This data is essential for system monitoring, diagnostics, and optimization. The power monitoring data is used for load-shedding. The multi-function meter(MFM) measures all the electrical quantities, such as voltage, current, frequency, active power, reactive power, power factor, surges, sags, swells, transients, and harmonics.
Control Devices: The HT panel has control devices, such as switches, push buttons, and indicators, to control and operate the HT panel manually. These devices enable operators to switch circuits on or off, reset alarms, and perform other control functions.
Protection Devices: Besides circuit breakers and relays, HT panels include other protective devices such as fuses, surge arrestors, and voltage regulators. These devices safeguard the electrical system against overvoltages, voltage spikes, and other transient events.
CT & PT: Current transformers (CT) and Potential transformers (PT) are crucial components of a high-tension (HT) panel. The primary function of CT and PT is to reduce the high current and voltage values, respectively, to a very low value. For instance, a current of 4000A can be reduced to 5A or 1A. Similarly, a voltage of 11KV can be reduced to 110V. This facilitates the use of low values for metering and protection purposes. The metering and protection classes CT and PT are used in the HT panel.
Enclosure: The HT panel enclosure is made of metal to protect components from dust, moisture, and physical damage while meeting safety and regulatory standards. –

The following diagram provides details of HT panel Components.

The diagram above displays different indication lights, relays, meters, and switches utilized in HT panels. It is important to note that the types and number of relays, switches, meters, and indicating lights used vary depending on the type of HT panel feeder, such as incomer feeder, bus coupler, and outgoing feeder.

Let’s learn about the function of each component installed in an HT panel.

R, Y, B Indicatiion lights: The red, yellow, and blue indicating light is associated with the R, Y, and B-phase of the electrical power system. These indicator lights display the incoming HT supply in the panel. When all three lights glow, the incoming supply is healthy, and all three phases are available. However, all these lights will go off if the incoming supply is interrupted from the source end. If any lights are off, a particular phase is unavailable, and an investigation is required to identify the issue.
Breaker on/off Indications Lights: These indicators show the breaker’s on/off status. The red light indicates that the breaker is on, and the green light indicates that it is off. The green light glows when the breaker is off, and the red light will not glow. Conversely, when the breaker is switched on, the green light goes off, and the red light turns on.
Spring Charging indicator: To ensure the proper operation of the VCB, it is necessary to check whether the breaker’s spring is charged. The charging of the spring is a crucial part of VCB operation. Here’s how it works: if the breaker is switched off and the operator wants to switch it on, the spring must be charged first. The spring can be charged manually by using the handle or electrically with the help of the motor installed in the breaker itself. Once the spring is charged (indicated by the ON light), the breaker can be switched manually by pressing the button or electrically using the TNC switch. The spring is immediately discharged when the breaker is switched on (the indicator light will turn off). Therefore, it is essential to charge the spring again for the next operation of the breaker. The spring motor automatically charges the spring, allowing the VCB to turn on the breaker after being switched off.
Trip indicator: This light remains off until the breaker remains healthy and ON. However, when the breaker trips on fault, the trip indicator light will glow, indicating a trip status.
Trip circuit healthy indicator: To determine the health of the trip status, an indicator is necessary. If the trip circuit is faulty or not in good condition, the breaker will not trip at fault. The indicator will be on when the trip circuit is healthy. This indicates that the trip circuit is healthy. On the other hand, if the indicator is off, the trip circuit is not healthy.
DC fail indicator: The indicator displays the status of the DC supply that is necessary for the protection and metering of the HT panel. DC supply performs various purposes, such as tripping and controlling the relay. Usually, a 110V DC is used in the HT panel for metering and protection purposes. If the DC circuit is not functioning correctly, the necessary DC supply will not reach the meters and relays, and these devices will not work.
Overcurrent & Earth Fault Relay: A relay can be electronic or electromagnetic. Its purpose is to trip the circuit breaker in case of overcurrent, which can be due to overloading, a short circuit, or an earth fault. These relays are set to a specific value to trip the breakers if the fault current (overcurrent or earth fault) exceeds the set limit. The HT panel for the transformer feeder has protection relays like differential protection and overfluxing relay.
Master Trip Relay: This relay is installed in the HT panel, which integrates with all other protective relays. It is a manual reset type with a flag that drops on fault. This down flag prevents the breaker from switching on. Therefore, the purpose of this relay is to ensure that the operator checks and rectifies the fault before switching on the breaker. You can reset this relay by lifting the flag after opening the front cover of the relay. This relay prevents the breaker from switching on if a fault persists in the system.
T-N-C switch: This is a switch that operates the breaker. It’s an electrically powered switch with three stages: T, N, and C. T stands for Trip, which means you can switch off the breaker manually by rotating the knob anti-clockwise. N is the neutral position where the knob remains vertical, and C means you can close the breaker by rotating the knob clockwise.
Annunciator Panel: The role of the annunciator panel is to indicate the type of fault in a specific location. Each panel includes several windows, each assigned to a different type of fault. A hooter is placed on top of the HT panel to alert the operator who may not be in close proximity to the HT panel. The annunciator panel is connected to three push buttons: Test, Accept, and Reset. When a fault occurs in the system, it is displayed on one of the annunciator panel windows, and the hooter starts sounding.
Test, Accept, & Reset buttons: These buttons are a part of the annunciator panel. The test button is used to check the health of the panel. When you press the test button, all the windows on the annunciator panel will glow, and the hooter will sound. If this doesn’t happen, there’s a fault in the system that needs to be checked. The accept button is used when a fault occurs and the electrical supply is disconnected because the breaker trips on the fault. The hooter will start sounding in this case, and one window will show the fault. When the operator presses the accept button, the hooter stops sounding, but the fault window will continue to glow. The operator can then investigate the fault. The reset button is used when the system has been normalized after clearing the fault. When you press this button, the fault window stops glowing, and the electrical supply can be switched on again for normal operation.
Voltage Selector Switch: To check the incoming supply voltage, this selector switch routes the voltage between the R and Y phases, the Y and B phases, and the B and R phases to the meter. The voltage can be selected using the knob, and there are three stages available to view different voltage levels between these phases. Nowadays, HT panels have a multi-function meter that allows users to select parameters through the meter’s keypad.
Emergency switch: This is a push-and-lock button used to manually trip or switch off the breaker in an emergency. To do this, you must press the button and lock it in its pressed position. This ensures that no other person can operate the breaker.
Energy Meter: This meter measures and shows the overall energy usage of the building or facility’s load.
Multifunction Meter: This meter displays real-time measurements of electrical parameters such as voltage, KW, KVA, KVAh, power factor, frequency, and line current (R-phase, Y-phase, and B-phase).

How Does an HT Panel Work?

An HT panel in an electrical system acts as a switch that connects or disconnects the HT supply with the load. The HT cables are connected to circuit breakers from both the incoming and outgoing sides. You can switch the HT supply on or off using an electrically operated switch or the push button on the breaker. When the supply is on, the metering system starts measuring and recording electrical parameters based on the panel’s design.

The HT panel protects against various faults, depending on the type of protection relays installed in the panel. The HT panel is automatically switched off (tripping) when a fault occurs. Each relay gives an indication of the fault when it operates. Before switching on the HT supply again, it is crucial to identify and rectify the fault. If the fault persists in the system, the relay will not reset, and the HT panel cannot be switched on because of the interlock between the relay and the panel. Therefore, clearing the fault first and then switching on the HT supply again is crucial. It’s worth noting that the relay should not be bypassed for any reason.

Features of HT Panel

HT panel receives an electrical supply of 11KV or 33KV at the income breaker from the transformer via a 3-core HT cable or bus duct. It then distributes power through one or more outgoing feeders.
These outgoing feeders are connected to distribution transformers and HT motors. The distribution transformer converts high voltage supply into 415V, 3-phase, 4-wire AC supply.
An Energy Meter installed on the HT panel measures the electrical power consumption.
A multifunction meter installed on the panel measures voltage, current, frequency, power factor, etc.
Feeder and transformer protection relays are installed on the panel. A winding temperature indicator(WTI) is installed in a dry-type transformer. A winding temperature indicator(WTI), Oil temperature indicator(OTI), and Buchholtz relay are installed in an oil-type transformer.
VCB is the market’s most commonly used HT circuit breaker due to its low maintenance, although other types, such as SF6, are also available.
Short circuit, overload, and earth fault protection relays are installed for fault protection.
High-tension (HT) circuit breakers can be turned on or off either manually or electrically.
For various purposes such as metering, relay operations, and indicating lights, an external source to the HT panel is used, which can be a 24V or 110 VDC supply with a battery charger.
16A power sockets, panel lights, and space heater in the panel use a 240V AC supply.
A space heater is installed in the panel to prevent the accumulation of moisture inside it.
To measure the voltage of the incoming supply, a Potential Transformer (PT) is used to convert 11kV or 33 kV into 110V. The voltage is then measured using a 110V voltmeter.
To determine the electrical load of a building in terms of current, the total current of the building is first converted into 5A or 1A using a Current Transformer (CT). CTs come in different ranges such as 600/5A, 400/5A, 300/5A, or 600/1A, 400/1A, 300/1A.

A picture of the HT panel is given below.

Safety Requirement

Single Line Diagram (SLD): One is required to install one SLD in the HT room, which should contain the details of the incoming and outgoing feeders of the building.
Rescue Hook: One rescue hook must be easily accessible in the HT panel room in case of an electrical emergency or shock to safely remove a person from danger.
Shock Treatment Chart: Display instructions for first aid treatment after electric shock in the HT room.
Hand Gloves: It is essential to ensure that hand gloves with a suitable voltage rating (11kV or 33kV) are available in the HT panel.
Discharge Rod: A discharge rod should be available in the HT panel room. The purpose of the discharge rod is to discharge the HT cable ends and HT cable terminals when the cable is disconnected from the HT panel due to any fault or maintenance work.
Do not touch any uninsulated parts of the HT panel: Before discharging, do not touch uninsulated parts of the HT panel as HT busbars and cables may store charge during continuous operation. The stored energy can be harmful and may cause a shock.
Panel Keys: Panel keys should be easily accessible in the HT panel room, located in close proximity to the HT panel.
Door Lock: It is important to close and lock the panel door to prevent foreign objects or insects from entering the HT panel. Insects can cause a short circuit in the panel, so keeping them out is essential.
Holes/Openings: All openings must be adequately sealed to prevent insects and reptiles from entering the HT panel, which can cause a short circuit.
Identification of Feeders: All feeders of the HT panel should be numbered from both the front and back sides to prevent accidents during maintenance. The exact number should be used on both sides of each feeder.
Lock HT Room: It is recommended to lock the HT panel room when no manpower is available and keep the key at a suitable location.
Danger plate: It is essential to install an appropriate danger rating plate (11KV or 33KV) on the HT panel.
Entry of authorized person: A list of certified individuals authorized to perform tasks on the High Tension (HT) system should be prominently displayed at the entrance of the HT room.
Maintenance: The HT panel should be regularly maintained according to the schedule recommended by the Original Equipment Manufacturer (OEM) or by adequately trained in-house personnel.
Testing of relays – Relays installed in the HT panel should undergo regular checks to verify that the breaker will trip effectively in the event of a fault.

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