Medium Voltage Switchgear
Medium Voltage Switchgear
Introduction
What does Medium Voltage Switchgear do? According to Google AI:
"Medium voltage (MV) switchgear (typically 1–36 kV or up to 69 kV) manages and protects electrical power systems by controlling the flow of electricity, isolating equipment for safe maintenance, and interrupting faults like short circuits or overloads. It acts as a safety bridge between high-voltage transmission and low-voltage end-use in industrial plants, substations, and data centers.", Google Artificial Intelligence.
The switch gear must do the following:
Make or break full-load current,
Transfer the prospective fault current,
Make prospective fault current,
Break prospective fault current if protection devices are installed.
What is the difference between ’making’ and ‘breaking’ fault currents? According to Google AI:
"Making current is the peak, asymmetrical, transient current a circuit breaker handles upon closing onto a fault, typically reaching 2.55 times the symmetrical RMS value. Breaking current is the RMS value of the current the breaker interrupts at contact separation. Making current is usually higher because it includes the highest initial DC surge.", Google Artificial Intelligence.
In North America ANSI (American National Standards Institute) and IEEE (Institute of Electrical and Electronics Engineers) sets the standards.
IEC (International Electrotechnical Commission) sets the standards in Europe and many other 50 Hz countries.
Equipment cost is influenced by standards that the device meets.
Switchgear Layout
Double bus bars are more reliable than single bus bars. Multiple sections make the bus bar more reliable.
Ratings
3.1 Nominal Voltage
Nominal voltage rating is the average voltage at which the equipment operates. The switchgear system voltage rating is the minimum voltage rating in all the components in the system.
3.2 Rated Voltage
This is the voltage at which the system operates under normal conditions. This voltage influences the insulation properties of switchgear components that include:
Individual circuit beakers,
Busbars,
Or Cable terminations.
Typical rated voltage should be 10 % higher than nominal voltage.
3.3 Power Frequency Withstand Voltage
Power frequency withstand voltage is 50 Hz or 60 Hz AC voltage that the switchgear can tolerate for 1 minute. Equipment is tested for 1 minute during manufacturing and commissioning during installation. Such a test is called the “pressure test”.
3.4 Impulse Voltage
Impulse voltage is the maximum voltage that the equipment can tolerate for a short period of time (during lightning, switching or other transients). This voltage is defined by BIL standards. What is BIL? According to Google AI:
“Basic Insulation Level (BIL) standards define the minimum lightning impulse voltage (typically a 1.2/50 𝜇s wave) electrical equipment must withstand without failure. These standards are critical for insulation coordination, ensuring transformers and switchgear can survive transient surges (lightning or switching) and that protection devices, like surge arrestors, operate below this peak value.”, Google Artificial Intelligence.
BIL standards are defined by the IEC 60071 series.
3.5 Surge Suppressors and Surge Arrestors
Surge suppressors or surge arrestors are used to limit the maximum voltage. More on those devices is explained by Google AI:
"Surge arrestors and suppressors protect electrical systems from voltage spikes by grounding excess energy, with arrestors handling high-energy lightning/grid surges at service entrances and suppressors (or SPDs) defending sensitive electronics from lower-energy, internal spikes. arrestors offer a primary, heavy-duty line of defense, while suppressors provide secondary, fine-tuned protection closer to loads.", Google Artificial Intelligence.
What are SPDs? According to Google AI SPDs are:
"Surge Protective Devices (SPDs) are electrical components designed to protect equipment from transient overvoltages and surges caused by lightning or grid switching. Commonly installed in residential, commercial, and industrial systems, they prevent damage to sensitive electronics by diverting excess energy to the ground", Google Artificial Intelligence.
There are three types of surge arrestors:
Rod spark gapped.
Multiple gapped arrestors.
Zinc metal oxide surge arrestors.
3.6 Full Load Current
Full load current is continuous maximum load current in the switchgear. Unlike with voltage rating the current rating is not that of minimum rating among various components in the switchgear system but individual rating for each component that feeds current to the load.
Medium voltage circuit breakers are produced could have the following current ratings:
630 A,
1250 A,
1600 A,
2000 A,
Or 2500 A.
3.7 Fault Current
Fault current is usually the maximum current that is likely to pass through the switchgear during faults. This current is not influenced by the load but by the transformer that is feeding the system because transformers have limited output current that they can supply.
Symmetrical faults include faults in all three phases. Asymmetrical faults, also known as unbalanced faults include phase-to-ground, phase-to-phase or phase-to-phase-to-ground. This is what Google AI states about symmetrical and asymmetrical faults:
“Asymmetric faults (e.g., single line-to-ground) are often considered more severe in operational terms, despite lower magnitudes than symmetrical (3-phase) faults, because they occur more frequently (~90–95% of all faults) and produce massive voltage imbalances. Their sudden, unbalanced nature creates high stress on equipment, causing severe electromagnetic forces and potential damage.”, Google Artificial Intelligence.
High power cables that are used in medium voltage (MV) systems, connected in parallel for greater reliability, would have low cable impedance. This is why cable impedance is often ignored in fault current calculations. If you ignore cable impedance then the fault current will equal to:
(Where: Vs = Transformer Output (secondary coil) Voltage Rating (V, volts),
Zb = Transformer Base Impedance (ohms),
Z% = Percentage of Base Value (%))
The transformer impedance is a percentage of base value. The base value (Zb) is equal to:
(Where: Vs = Transformer Output (secondary coil) Voltage Rating (V, volts),
S = Transformer Power Rating (VA, volts-amps))
We can combine the last two equations (insert second equation into first) to get:
Cable impedance needs to be assumed in calculations when the switchgear is far away from the transformer and connected via long cable. The additional impedance in the circuit (cable impedance) will reduce the fault current.
Fault current has two characteristics:
Amount of electrical energy:
E = Pt
Where: P = Power (W, watts)
t = time (s, seconds)
Also can be expressed as:
(Where: Z = Impedance (ohms))
or
(For unit impedance)
or
That means higher currents would have lower fault duration before the switchgear would fail.
Electromechanical stress is caused by forces from electromagnetic fields due to high currents flowing in wires, busbars or coils. This force is proportional to the square of instantaneous current flowing in the wires.
3.8 DC Offset
If the switchgear is installed next to reactive loads such as generators or large induction motors then the maximum current flowing in the system will have a decaying DC offset component. This DC offset will increase the maximum current, thus must be considered in fault current calculations. The problem can be fixed with power factor correction methods.
MV Switchgear Types
4.1 Switch
This is a general term for a switch gear device. A switch is a device that interrupts the flow of current via mechanical action. It can be a disconnector/isolator or circuit breaker.
4.2 Disconnector/Isolator
Disconnector/isolator is designed for manual disconnection of the circuit unlike circuit breaker or fuse that automatically disconnect the circuit. Manual disconnection can include a computer controlled command that must be initiated by a human.
What is the difference between on-load and off-load disconnector/Isolator? According to Google AI:
"The primary difference is that an on-load disconnector (or switch-disconnector) can safely make or break circuits while current is flowing, featuring arc-quenching mechanisms, whereas an off-load disconnector (or isolator) must only be operated when the circuit current is zero, usually to ensure safe maintenance isolation, as it lacks arc-breaking capabilities.", Google Artificial Intelligence.
4.3 Circuit Breaker
A circuit breaker (CB) automatically disconnects the circuit via mechanical action. When sufficiently high current is received a connected coil creates a sufficiently high magnetic field that is enough to trigger the mechanical switch via electromagnetic force.
Total Fault Clearing Time = Relay Pick Up Time + CB Opening Time + CB Arcing Time
4.4 Contactors
Contactors are control devices, not switchgear devices. They are used for:
Starting and stopping of motors,
Or connecting capacitor banks ON and OFF for power factor correction.
This is what Google AI states about contactors:
“A contactor is an electrically controlled, heavy-duty switch used to turn high-power electrical loads on or off, such as motors, pumps, or lighting systems. It utilizes an electromagnetic coil that, when energized by a low-voltage signal, creates a magnetic field, forcing contacts together to close the circuit and allow high current to flow.”, Google Artificial Intelligence.
4.5 Fuses
Similarly to contactors fuses are not switchgear. Fuses are protection devices. Fuses are made from conductors that melt when high current is applied. Fuses do not melt straight away. The higher the current the faster the fuse will melt. Fuses thus have inverse time melting characteristics.
4.6 Is-Limiters
Is limiter works as follows:
Current transformer measures and detects short circuit current.
The measuring device compares the current with a reference value and sends a signal to the pulse transformer if the current value is about the reference value.
Pulse transformer creates the appropriate signal for tripping.
The insert holder limits the output current.
Figure 1: Function Block Diagram of Is-Limiter.
Insulation Methods
5.1 Oil Circuit Breakers
Circuit breaker contacts are submerged in oil. The oil cools the arc and gives insulation. During arcing that occurs during disconnection the energy splits the oil into four components:
70 % Hydrogen.
22 % Acetylene.
5 % Methane.
3 % Ethylene.
This process worsens the insulation characteristics of the oil. Thus the oil needs to be constantly replaced.
The following table shows advantages and disadvantages of oil circuit breakers.
Table 1: Advantages and disadvantages of oil circuit breakers.
Oil circuit breakers are not produced on a large scale due to the disadvantages.
5.2 Air Circuit Breakers (ACBs)
In air circuit breakers (ACBs) the contacts are in free air and not submerged in any liquid. Air purifiers are sometimes used to maintain cleanliness of electrical components. High pressure air is sometimes used to reduce the time of the arcing process.
The following table shows advantages and disadvantages of air circuit breakers.
Table 2: Advantages and disadvantages of air circuit breakers (ACBs).
5.3 Sulphur-Hexaflouride (SF6)
In SF6 circuit breakers the contacts are submerged in Sulphur-Hexafluoride is inert insulating gas under pressure. Those contacts are not designed to operate in air. Thus SF6 leaks will cause disasters. Sensors can be used to raise the alarm if the gas pressure falls. The SF6 gas is not toxic but heavier than air and can fall to the lowest part floors of the substation thus causing suffocation.
5.3.1 Gas Insulated Switchgear (GIS)
Gas Insulates Switchgear insulates not just the contacts but nearly all switchgear systems. Applications include high voltage levels of 66,000 volts and above as well as medium voltage levels.
The following table shows advantages and disadvantages of SF6 circuit breakers.
Table 3: Advantages and disadvantages of SF6 circuit breakers.
5.4 Vacuum Switchgear
In vacuum switch gear the contacts are in vacuum. Early designs of vacuum switchgear had many problems but those problems have been solved in modern designs. Vacuum allows fast operating speeds. However, vacuum switchgear has higher voltage peaks. This problem can be solved with surge arrestors and wave-slowing capacitors.
The following table shows advantages and disadvantages of vacuum circuit breakers.
Table 4: Advantages and disadvantages of vacuum circuit breakers.
The different insulation characteristics are compared in the following table:
Table 5: Comparing Insulation Characteristics.
Closing Mechanism Types
6.1 Hand Operated
What is hand operated switchgear? According to Google AI:
"Hand-operated switchgear refers to electrical switching devices that are directly controlled by human action—such as rotating a handle, pushing a button, or flipping a lever—to isolate, control, or protect electrical circuits. These devices are essential for manual intervention in industrial, commercial, and utility environments, often used to turn on/off power, changeover sources, or provide emergency stop capabilities.", Google Artificial Intelligence.
6.2 Hand Operated With Spring Assist
What is hand operated switchgear with spring assist? According to Google AI:
“Hand-operated switchgear with spring assist is a type of electrical switching mechanism where the operator manually charges a spring—storing mechanical energy—before releasing it to close or open the contacts at a high, consistent speed. This design ensures that the switching speed is independent of the operator's handling speed, which is crucial for reducing arcing and burning on contacts.”, Google Artificial Intelligence.
6.3 Quick Make
What is switchgear with “quick make”? According to Google AI:
“Switchgear with "quick make" (also known as quick-make, quick-break, or snap-action mechanism) is a feature in switching devices designed to ensure that the speed of the contacts making (closing) or breaking (opening) an electrical circuit is independent of how fast or slow the operator turns the handle or pushes the button. This rapid, independent operation is crucial for preventing electrical damage.”, Google Artificial Intelligence.
What is hand operated switchgear with “quick make”? According to Google AI:
“Hand-operated switchgear with "quick-make" is a type of electrical switching mechanism where the closing speed of the contacts is independent of the speed at which the operator moves the handle. This mechanism uses stored mechanical energy—typically a spring—to ensure that contacts close swiftly and firmly, regardless of human input speed.”, Google Artificial Intelligence.
6.4 Motor Wound Spring
What is switchgear with a motor wound spring? According to Google AI:
“Switchgear with a motor-wound spring mechanism uses an electric motor to automatically compress (charge) a spring, storing potential energy to rapidly open or close high-voltage circuit breakers. This system ensures reliable, high-speed switching for protection and control, allowing for remote operation and automatic recharging after every action.”, Google Artificial Intelligence.
What is hand operated switchgear with a motor wound spring? According to Google AI:
“Hand-operated switchgear with a motor-wound spring is a medium-voltage circuit breaker mechanism where a spring provides the energy to open/close contacts, with that spring charged either automatically by a motor or manually via a handle. This "stored energy" mechanism ensures fast, consistent operation, independent of the user's manual speed, allowing for remote operation and safe, automatic reclosing.”, Google Artificial Intelligence.
6.5 Solenoid
What is switchgear with solenoid? According to Google AI:
“Switchgear with a solenoid is an electrical protection and control system that uses an electromagnetic coil (solenoid) as part of its operating mechanism to actuate circuit breakers, enabling automatic and remote switching operations. In this configuration, the solenoid converts electrical energy into mechanical energy to open (trip) or close the high-power contacts of a switch or circuit breaker.”, Google Artificial Intelligence.
What is hand operated switchgear with solenoid? According to Google AI:
"A hand-operated switchgear with a solenoid is an electrical safety or control device that combines manual operation (via a knob, lever, or handle) with electromagnetic locking or actuation. The solenoid—an electromagnetic coil—is used to either lock the switch in a specific position (preventing unauthorized or dangerous access) or to automatically activate the switchgear based on a control signal, such as from a safety PLC.", Google Artificial Intelligence.
What is PLC?
“A Programmable Logic Controller (PLC) is a ruggedized, specialized industrial computer used to automate electromechanical processes, such as machinery control on factory assembly lines, robotic devices, or amusement rides. Acting as the "brain" of industrial systems, it uses custom, programmable logic to receive sensor inputs, process data, and control outputs to manage machinery efficiently.”, Google Artificial Intelligence.
6.6 Pneumatic
What is switchgear with pneumatic operation? According to Google AI:
“Switchgear with pneumatic operation, often known as pneumatic switchgear or an air-blast circuit breaker, is a type of electrical equipment that uses compressed air to open or close contacts and extinguish electrical arcs during high-voltage switching operations. This system combines high-voltage power interruption with pneumatic control mechanisms to manage fault currents, offering a fast and efficient solution for controlling high-voltage energy, particularly in applications where electromagnetic forces are not sufficient for rapid operation.”, Google Artificial Intelligence.
What is hand operated switchgear with pneumatic operation? According to Google AI:
“Hand-operated switchgear with pneumatic assistance refers to an electrical or mechanical control system where the initial action—switching on/off—is performed manually (by hand), but the heavy lifting required to move the contacts or components is powered by compressed air. This system combines the reliability of manual control with the efficiency and power of pneumatics, often used for isolating or switching high-voltage electrical circuits.”, Google Artificial Intelligence.
6.7 Hydraulic
What is switchgear with a hydraulic operation system? According to Google AI:
“Switchgear with a hydraulic operation system uses pressurized hydraulic fluid (typically oil) to power the mechanical opening and closing of circuit breaker contacts. This technology is primarily used in high-voltage (HV) and extra-high-voltage (EHV) systems (36 kV up to 1,100 kV) where significant, fast, and precise force is required to break heavy fault currents.“, Google Artificial Intelligence.
What is hand operated switchgear with a hydraulic operation system? According to Google AI:
“Hand-operated switchgear with a hydraulic operation system is a type of electrical switchgear that uses manual, hand-powered hydraulic pumps to generate the high pressure required to close or open heavy-duty circuit breaker contacts. This setup acts as a completely self-contained, independent mechanism designed for environments without access to electricity, such as remote substations, emergency shutdowns, or in critical backup scenarios.”, Google Artificial Intelligence.
Proofing of Internal Arc
What is internal arc proofing? According to Google AI:
“Internal arc proofing (or arc-resistant switchgear) is a safety design for electrical equipment that contains and redirects dangerous energy, toxic gases, and high pressure resulting from an internal arc fault. It protects personnel from blinding light, immense heat (up to 20,000 degrees Celsius), and flying debris by using reinforced doors, pressure relief flaps, and venting systems.”, Google Artificial Intelligence.
Internal arc proofing for a 200 ms fault include:
3 mm thickness steel panel,
bolted front doors,
hard access to busbar chambers,
arc energy directed away from front doors.
References
1. Practical Power Distribution for Industry, Jan de Kock, Cobus Strauss, Elsevier 2004.
2. Google Artificial Intelligence.
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