What’s the difference between SVC and SVG?

Reactive power compensation devices are essential in power systems. Their primary role is to enhance the power factor of supply and distribution systems, thus improving the utilization of transmission and substation equipment, increasing electrical efficiency, and reducing electricity costs. Additionally, installing dynamic reactive power compensation devices at strategic locations along long-distance transmission lines can bolster the system’s stability, augment transmission capacity, and stabilize voltage at the receiving end and throughout the grid.

Reactive power compensation equipment has evolved through several developmental stages. The early exemplar, the synchronous condenser, was bulky and costly and has gradually been phased out. The second method, using shunt capacitors, offers the advantages of low cost and ease of installation and use. However, due to potential harmonics and other power quality issues in the system, the use of pure capacitors has become less common.

The current method of series reactor capacitor compensation is widely adopted to enhance power factor. For user systems with continuous production and low load variability, fixed compensation with Fixed Capacitors (FC) is generally recommended. Alternatively, automatic compensation controlled by contactors and implemented in steps is suitable for both medium and low voltage supply and distribution systems.

Rapid compensation is necessary when load changes are swift or in the presence of shock loads, such as in the rubber industry’s mixers, where the system’s reactive power needs fluctuate rapidly. However, capacitors used in standard reactive power automatic compensation systems retain a residual voltage after disconnection and removal from the grid. The magnitude of this residual voltage is unpredictable and requires 1-3 minutes to discharge. Therefore, reconnection to the grid must wait until the residual voltage is reduced to below 50V by the capacitor’s internal discharge resistor, precluding rapid response. Moreover, the presence of significant harmonics in the system means that LC tuned filter compensation devices, comprising capacitors and reactors in series, require substantial capacity to ensure capacitor safety. This can also lead to system over-compensation, resulting in a capacitive system.

The Static Var Compensator (SVC), a type of static reactive power compensation device, was thus developed. Its typical configuration consists of a Thyristor Controlled Reactor (TCR) combined with a Fixed Capacitor (FC) bank, often requiring series connection with a certain proportion of reactors. The significance of the SVC lies in its ability to continuously adjust reactive power by modulating the triggering delay angle of the thyristors within the TCR. SVCs are primarily used in medium and high-voltage power distribution systems and are particularly suited for scenarios with large load capacities, severe harmonic issues, shock loads, and high rates of load variation, such as in steel mills, the rubber industry, non-ferrous metallurgy, metal processing, and high-speed railways.

With the advancement of power electronics technology, particularly with the advent of IGBT devices and enhanced control techniques, a new type of reactive power compensation equipment has emerged, distinct from the traditional designs based on capacitors and reactors. This equipment is the Static Var Generator (SVG), which utilizes PWM pulse-width modulation control technology to either generate capacitive reactive power or absorb inductive reactive power. Unlike traditional systems, SVGs do not rely heavily on capacitors but on bridge-type converter circuits employing multilevel technology or PWM technology, eliminating the need for system impedance calculations during use. Moreover, SVGs offer the benefits of a smaller footprint and the ability to rapidly and smoothly adjust reactive power on a continuous dynamic basis, providing bidirectional capacitive and inductive compensation.

Comparative Analysis of SVG and SVC Reactive Power Compensation Devices

1. Different Principles

a. SVC can be seen as a dynamic reactive power source. Based on the grid’s connection needs, it can either supply capacitive reactive power to the grid or absorb the grid’s excess inductive reactive power. This is achieved by connecting a capacitor bank, typically a filter bank, to the grid. When the grid doesn’t require much reactive power, any excess capacitive reactive power is absorbed by a parallel-connected reactor. The reactor current is controlled by a thyristor valve group. By adjusting the phase angle of thyristor triggering, the RMS value of the current flowing through the reactor can be altered. This ensures that the SVC at the grid access point provides just enough reactive power to stabilize the voltage within the specified range, thereby compensating for the grid’s reactive power.

b. SVG employs a high-power voltage inverter as its core. By adjusting the amplitude and phase of the inverter’s output voltage, or directly controlling the amplitude and phase of the AC side current, SVG quickly absorbs or emits the necessary reactive power. This enables rapid and dynamic regulation of reactive power.

2. Different Response Speeds

The response speed of SVC generally ranges from 20-40ms, whereas SVG’s response does not exceed 10ms, allowing for more effective suppression of voltage fluctuations and flicker. With the same compensation capacity, SVG provides the best results in mitigating voltage fluctuation and flicker.

3. Different Low Voltage Characteristics

SVG behaves like a current source, with its output capacity minimally affected by bus voltage. This quality gives SVG a significant advantage in voltage control applications. The lower the system voltage, the more necessary dynamic reactive power regulation becomes. SVG’s superior low-voltage characteristics mean that its output of reactive current is independent of system voltage. It can be considered a controllable, constant current source that continues to deliver rated reactive current even when system voltage drops, demonstrating robust overload capacity. In contrast, SVC exhibits impedance-type characteristics, with output capacity heavily influenced by bus voltage. As the system voltage decreases, SVC’s capacity to output reactive current diminishes proportionally, lacking the ability to handle overloads. Consequently, SVG’s reactive power compensation is unaffected by system voltage, while SVC’s compensation capacity linearly decreases as the system voltage falls.

4 Different Operation Safety Performance

SVC takes thyristor-adjusted reactance and multiple capacitors as the main means of reactive power compensation, which is very prone to resonance amplification phenomenon, leading to safety accidents, and when the system voltage fluctuates greatly, the compensation effect is greatly affected, and the operation loss is large; SVG supporting capacitors do not need to set up a filter bank, and the resonance amplification phenomenon does not exist, and the SVG is an active-type compensation device, and it is a current source device composed of IGBT, which is a switchable device, thus avoiding the resonance phenomenon and greatly improving the operation safety performance. SVG is an active compensation device, which is a current source device composed of switchable device IGBT, thus avoiding the resonance phenomenon and greatly improving the operation safety performance.

5. Different Harmonic Characteristics

SVC employs Silicon Controlled Rectifiers (SCR) to manage the reactor’s equivalent fundamental impedance. This not only makes it susceptible to system harmonics but also causes it to generate a significant number of harmonics. To mitigate this, SVC must be paired with a filter bank to eliminate its own harmonic emissions. On the other hand, SVG utilizes three-level single-phase bridge technology, capable of producing five-level voltage waveforms in a single phase, and employs carrier phase-shifting pulse modulation methods. This approach makes SVG less influenced by system harmonics and even enables it to suppress them. SVG significantly reduces the harmonic content in the compensation current by incorporating techniques such as multiplication, multi-level, or pulse-width modulation, offering an advantage over SVC.

6. Different Space Requirements

SVG occupies a space that is 1/2 to 2/3 smaller than that of SVC when providing the same compensation capacity. SVG’s use of fewer reactors and capacitors substantially decreases both the size and footprint of the device. In contrast, SVC’s reactors are not only larger but also require more space for installation, resulting in a larger overall footprint.

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