SVG compensation solution applied in Diesel Generator Microgrid Systems to High-Frequency Non-Motor inrush Loads

■ 1. Background and Operating Conditions Overview

■ 2. Core Technology Characteristics Analysis: Why Choose SVG?

■ 3. Comparison of Effects Before and After Compensation

■ 4. Conclusions and Industry Implications

1. Background and Operating Conditions Overview

This case study focuses on a microgrid at a non-neutral oil and gas exploration base on an offshore island . The system operates completely independently from the main power grid (island microgrid), with its core power source being two diesel generator sets connected in parallel (rated total capacity 1200kVA / 960kW ).

This microgrid contains no large power motors, but includes two typical non-electrical, inrush load and nonlinear loads :

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1. High-frequency pipe welding machine group : spot welding duration is only 80ms - 150ms , and a large amount of reactive power is instantly demanded during operation.

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2. Thyristor phase-controlled rectifier heater (300kW) : Temperature is controlled by rapidly adjusting the conduction angle, resulting in severe low power factor reactive power impact and 5th and 7th high-frequency harmonics.

Systemic headache before compensation

At the moment when the welding machines were operating concurrently, the microgrid was instantly flooded with an inductive reactive power surge of 450 kVar with a power factor of only 0.35 . Due to the mechanical and electrical transient response lag of more than 200 ms in the diesel generator voltage regulating system (AVR), the system encountered a severe transient voltage sag : the bus voltage dropped instantly from 400V to 310V (a drop of 22.5%), causing the precision measuring instruments at the base to frequently trip due to "low voltage protection," forcing the production line to stop.

2. Core Technology Characteristics Analysis: Why Choose SVG?

For this typical scenario, traditional smart capacitor compensation cabinets (TSCs) are ineffective (TSC switching response is usually >40ms, and is limited by thyristor zero-crossing triggering, making it impossible to catch up with lightning-fast impacts within 100ms, and it is prone to resonance with thyristor loads).

The final solution selected a 500kVar three-level static var generator (SVG) . It possesses the following four core technical characteristics, which are also the most convincing supporting points of this article:

Impact occurs at t=0 ────> DSP locks in reactive power characteristics within 50μs ────> IGBT outputs full capacity within 2ms ────> Bus voltage stabilizes completely within 5ms

1. Millisecond-level ultra-fast response speed (2ms-level tracking)

SVG adopts a 32-bit high-speed dual DSP + FPGA digital control architecture, and the reactive current detection algorithm adopts the FFT method based on instantaneous reactive power theory .

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Characteristic data : Command calculation and PLL alignment takes only 50μs ; full capacity current response time <2ms ; overall reactive power compensation reaches steady state in less than 5ms .

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The persuasive logic is as follows: The pulse width of the welding machine is 100ms. The SVG has already filled the missing reactive power within the first 5ms. The inrush current is "digested on the spot" at the load end. The diesel generator AVR does not even have time to sense the voltage drop before the impact ends.

2. Characteristics of a "voltage-independent" constantly controlled current source

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Characteristic analysis : The reactive power output of traditional capacitor compensation is proportional to the square of the voltage (Q = ωC U²). When the voltage drops to 310V, the compensation capability of traditional capacitors will plummet by about 40% , forming a vicious cycle of "the lower the voltage, the weaker the compensation".

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Persuasive logic : SVG is essentially a controlled current source (VSC). Even if the bus voltage drops briefly, it can still maintain 100% of the rated current output (721A) , demonstrating extremely strong "low voltage ride-through and hard withstand support" capabilities.

3. Multifunctional integrated management of reactive power and harmonics (SVG)

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Characteristic analysis : Due to the thyristor heater injecting a large amount of 5th (250Hz) and 7th (350Hz) total harmonic current (THDi up to 28%) into the microgrid at a specific conduction angle.

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Persuasive logic : This SVG has a comprehensive reactive power and harmonic compensation function. Its high-frequency IGBT (switching frequency up to 20kHz) uses a dynamic control algorithm to filter out the main harmonics within the 13th order during the reactive power compensation interval , strongly suppressing the total voltage distortion rate (THDu) of the system from 8.5% to less than 2.2%, ensuring that the diesel generator rotor will not overheat due to harmonics.

4. Completely eliminate microgrid system resonance

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Characteristic analysis : The system impedance of a diesel generator unit is several times higher than that of the main power grid. If a fixed capacitor is connected, it is very easy to trigger parallel resonance with the scattered inductance of the system (such as cables and transformers) in the 250Hz-350Hz frequency band , resulting in overvoltage and burning out the equipment.

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Persuasive logic : The SVG uses an LCL filter internally, and the control algorithm has active damping control, which prevents series and parallel resonance with any impedance in the microgrid system, thus ensuring absolute safety in weak grid systems.

3. Comparison of Effects Before and After Compensation

To provide the strongest data support for your article, the following is a comparison table of key power quality parameters before and after the introduction of SVG for this microgrid:

Monitoring parameters	Before Compensation (Diesel Generator)	After Compensation (diesel generator + SVG )	Improvement effect
Transient BUS voltage drop	It dropped to a low of 310V (-22.5%).	Stable at 388V (-3.0%)	Meets the IEEE Standard Class A power quality requirements
BUS power factor (cosphi)	It instantly dropped to 0.35	Dynamically maintained between 0.98 and 0.99	Diesel generators basically output no reactive power, and their capacity is fully utilized.
Total voltage distortion (THDu) of the system	Up to 8.5%	Reduced to 2.2%	Well below the 5% public grid limit standard
Voltage flicker	The light is visibly flickering violently,ΔU=2.5%	Completely eliminate flicker, U=0.45%	Improved online reliability of sensitive measuring instruments
Diesel Generator operation status	AVR frequently overheats due to strong excitation, causing abnormal vibration in the unit.	With stable excitation current, unit fuel consumption and temperature rise decreased.	Extend the lifespan of diesel generators and reduce maintenance costs

4. Conclusions and Industry Implications

This article shown that in microgrids with low inertia and high impedance diesel generators , traditional capacitor bank reactive power compensation equipment is no longer able to cope with non-motor transient inrush current such as modern electronics and high-frequency welding.

With its microsecond-level detection and computing power, current injection speed within 2 milliseconds, and low-voltage-resistant current source characteristics, SVG acts as a "high-frequency reactive power buffer" between the microgrid and the impact load . It successfully disguises a highly destructive "severe impact load" as a "stable, high-power-factor high-quality load" at the electrical level, and is a core technology for ensuring the voltage stability of islanded microgrids.