Shunt and Line Reactors. Part.I

It was an intriguing moment when I took the reins of my latest EHV 380kV Substation project in Al Baha, and found, at that time, the presence of an element in the Single line diagram, I had not encountered before in my previously accomplished heavy industry EHV Substations. As shown below

Inductors and capacitors play crucial roles in enhancing the performance of medium-length and long transmission lines. They are utilized to boost line load capacity and maintain voltages close to their rated values. And now, one might wonder and ask. Is it possible for the receiving voltage to be higher than the sending voltage? Does this phenomenon cause problems for the electrical system? What is the solution to this phenomenon?

Shunt reactors (Inductors), are frequently positioned at specific locations along Extra High Voltage (EHV) lines, connecting each phase to the neutral point. These reactors absorb reactive power and diminish overvoltages during periods of light load conditions. They also mitigate transient overvoltages stemming from switching events and lightning surges. However, shunt reactors can reduce line loadability if they are not removed under full-load conditions. In addition to shunt reactors, shunt capacitors are sometimes used to deliver reactive power and increase transmission voltages during heavy load conditions.

As shown above, the Shunt reactor compensates for the shunt capacitance and the charging current drawn by a transmission line. In the absence of compensation for charging current, the voltage at the receiving end of a long transmission line can exceed the voltage at the sending end by as much as 50%.

When reactive power transfer is minimized, indicating a balance of reactive power across different network segments, A higher level of Active Power can be transferred within the network. Consequently, the demand for reactors to control overvoltage is highest in weaker power system zones, particularly when the network's short-circuit power is relatively low.

Voltage increase in a system due to the capacitive generation can be expressed as follows:

As the short-circuit power of the network increases, the voltage rise diminishes, reducing the necessity for compensation to restrict overvoltage. Reactors necessary to achieve a balance of reactive power across different network segments are primarily required in heavily loaded networks

Considering the below equivalent circuit.

• Series capacitors reduce the net series impedance of a line, lowering line-voltage drops and increasing steady-state stability.
• Disadvantages include the need for automatic protection devices to manage high currents during faults and to reengage capacitor banks post-fault.
• The addition of series capacitors can induce subsynchronous resonance, potentially damaging turbine-generator shafts.
• Despite drawbacks, studies show series capacitive compensation can increase long-line loadability at a fraction of the cost of new transmission.
• In a compensated line section, Nc represents the amount of series capacitive compensation as a percentage of the positive-sequence line impedance, while NL represents the amount of shunt reactive compensation as a percentage of the positive-sequence line admittance.

Basic operating principle

The electromagnetic principle of a shunt reactor can be explained as follows:

• Initial Charging Current (i): When a shunt reactor is connected to a power system and voltage is applied, an initial surge of current flows into the reactor. This initial current is called the charging current. It creates a magnetic field around the reactor coil.
• Induced Voltage (e): The changing current in the reactor coil (di/dt) generates a changing magnetic field. According to Faraday's law of electromagnetic induction, this changing magnetic field induces a voltage in the coil. The induced voltage (e) is proportional to the rate of change of current and is in the opposite direction to the applied voltage.
• Resultant Current: The total current flowing through the reactor is determined by the difference between the applied voltage (V) and the induced voltage (e), divided by the resistance (R) of the coil. This relationship is described by Ohm's law

• Therefore, the initial charging current in a shunt reactor creates a pulsating magnetic field, which induces a voltage in the coil. The resultant current flowing through the coil is determined by the difference between the applied voltage and the induced voltage, divided by the resistance of the coil. This principle is fundamental to the operation of shunt reactors in power systems, where they are used to control system voltage and improve power factor

The operating principle of a shunt reactor can be summarized as follows:

• Inductive Load: When connected to a bus or line, a shunt reactor behaves as an inductive load, drawing current to supply both active and reactive loads.
• Reactive Power (KVAR): The reactive component of the current creates a pulsating magnetic flux in the reactor's core. The power required to generate this magnetic flux is known as reactive power, measured in kilovolt-amperes reactive (KVAR). It is calculated as the product of current, system voltage, and the sine of the phase angle (Φ) between voltage and current.
• Active Power (KW): The active component of the current results in resistive losses (I^2R losses) in the reactor, causing heating. The power loss due to this heating is known as active power, measured in kilowatts (KW). It is calculated as the product of current, system voltage, and the cosine of the phase angle (Φ) between voltage and current.

In summary, a shunt reactor consumes reactive power to establish a magnetic field and draws active power for resistive losses, operating as an inductive load in the system.

Shunt reactors can be classified based on several factors, including their purpose, design, and construction. Here are some common classifications:

1. Based on Purpose:Compensation Reactors: These are used to compensate for capacitive reactive power in the system, thereby reducing voltage levels.Filter Reactors: Used in harmonic filtering applications to mitigate harmonics in the system.Neutral Grounding Reactors: Connected to the neutral of a transformer or generator to limit ground fault currents.
2. Based on Design:Air-Core Reactors: These have coils wound on a non-magnetic core and are typically used for low voltage applications.Iron-Core Reactors: These have coils wound on a laminated iron core, providing higher inductance and are used for higher voltage applications.
3. Based on Construction:Single Phase Reactors: Consist of a single coil wound on a core and are used in single-phase systems or as part of a three-phase system.Three-Phase Reactors: Consists of three coils connected in delta or star configuration and are used in three-phase systems.
4. Based on Impedance:Fixed Reactors: Have a fixed impedance and are used for steady-state applications.Variable Reactors: : Some oil-immersed reactors are designed to have variable impedance. This allows for adjusting the reactor's impedance to meet varying system requirements, providing flexibility in power system operation.
5. Based on the Cooling Method: - Dry-Type Reactors: Air-cooled and do not use any liquid cooling. (system voltage Below 72.5 KV) ? Delta connected ? Range below 30 MVAR ?Connected at the tertiary winding of the Power transformer- Oil-Immersed Reactors: Use oil as a coolant for better heat dissipation.(system voltage 72.5 KV & above) ? Star connected with neutral grounding ? Range 30 to 300 MVAR ? Connected at the terminals of the transmission line

These classifications help in selecting the right type of shunt reactor for specific applications, ensuring efficient and reliable operation of power systems.

Reactor Earthing

1. Neutral Earthing: The neutral of the reactor is grounded to provide a return path for fault or unbalanced currents. This grounding is typically done using two separately treated earth pits to ensure reliable operation.
2. Tank Earthing: To prevent the heating of the tank due to eddy currents caused by voltage buildup, the tank's potential is kept at zero by connecting it to the earth grid. This connection helps in dissipating any induced currents effectively.
3. Steel Structure Earthing: To protect the steel structure from damage due to lightning strikes, the entire steel structure is grounded through an earth grid. This grounding ensures that any lightning-induced currents are safely dissipated into the ground, protecting the structure from harm.

voltage compensation methods in power system

Whenever the bus voltage raises to 4% to 5% more than the rated voltage, shunt reactor is to be kept in service and 2% to 3% less than the rated voltage this may be kept out of service

Case Study

• Percentage voltage regulation of the uncompensated line

• Percentage voltage regulation with shunt compensation

Base Design of Shunt Reactor

• FAT Overview