What is the function of a magnetically controlled reactor?

Basic Principles
 
The Magnetic‑Valve Controlled Reactor, abbreviated as Magnetically Controlled Reactor (MCR), works on the magnetic amplifier principle. It is an iron‑core reactor with controllable saturation magnetized by both alternating current and direct current. During operation, an extremely small DC power (approximately 0.1%–0.5% of the reactor’s rated power) is used to adjust the operating point of the iron core (i.e., the core saturation level or magnetic permeability μ), so as to change its inductive reactance value. This enables regulation of reactive current magnitude and smooth adjustment of reactive power. Its outstanding advantages include high stability, reliability, compact size, low cost, flexible control and easy maintenance.
 
What are the Functions of Magnetically Controlled Reactors?
 
Figure 2 Working Principle Diagram of MCR 
As shown in the above figure, the main iron core of the MCR is split into two halves (Core 1 and Core 2) with a cross‑sectional area of A. Each half‑core has a section with reduced cross‑sectional area. Four coils each with N/2 turns are symmetrically wound on the two half‑core limbs (the total turns of coils on each half‑core limb equal N). Each upper and lower winding on every half‑core limb is equipped with a tap with a tap ratio of δ=N₂/N, with thyristors K₁ and K₂ connected between these taps. After cross‑connecting the upper and lower windings on different iron cores, the assembly is connected in parallel to the grid power supply, while a free‑wheeling diode is bridged across the cross‑connection terminals.
 
Within the full capacity regulation range, only the magnetic circuit of the small‑cross‑section section is saturated, and all other sections remain in an unsaturated linear state. The reactor capacity is adjusted by changing the saturation degree of the magnetic circuit in the small‑cross‑section section. Featuring simple manufacturing techniques and stable structure, MCRs have great application potential in improving grid transmission capacity, regulating grid voltage, compensating reactive power and limiting overvoltage.
 
What are the Functions of Magnetically Controlled Reactors?
 
Figure 3 Circuit Structure Diagram of MCR 
As can be seen from the above figure, when K₁ and K₂ are non‑conducting, the MCR functions as an unloaded transformer due to the symmetry of the winding structure. Assume the power supply e is in its positive half‑cycle: thyristor K₁ bears forward voltage while K₂ bears reverse voltage. When K₁ is triggered into conduction (points a and b are at equal potential), the power supply e provides DC control voltage (δEₘ sinωt) and currents iᵧ′, iᵧ″ to the circuit via the N₂‑turn coil after auto‑transformation by coils with a transformation ratio of δ. The equivalent circuit when K₁ conducts is shown in Figure (a) below. Similarly, when K₂ conducts during the negative half‑cycle of the power supply (points c and d are at equal potential), the corresponding equivalent circuit is shown in Figure (b) below.
 
Figure 4 Equivalent Circuit Diagrams under Thyristor Conduction 
It can be seen from the diagrams that the control currents iᵧ′ and iᵧ″ generated by K₂ conduction flow in the same direction as those generated by K₁ conduction. That is to say, within one power‑frequency cycle of the power supply, the alternate conduction of thyristors K₁ and K₂ achieves full‑wave rectification, and the diode acts as a free‑wheeling component. Adjusting the firing angles of K₁ and K₂ changes the magnitude of control current, which further modulates the saturation level of the reactor iron core, realizing smooth and continuous regulation of reactor capacity.