As a large amount of new energy installations are connected to electricity transmission and distribution grids and non-linear loads and impulse-loads like large capacity electronic and electric equipment are widely applied, electricity quality has been seriously impacted. FGSVG devices can significantly improve the electricity quality at points where loads are connected to public grid by, for example, increasing power factor, overcoming three-phase imbalance, eliminating voltage flicker and fluctuation and suppression harmonics contamination.
FGSVG series was included into important technology transformation projects by National Development and Reform Commission in 2013 and received “Top 10 Electricity Quality Brands” issued by Saiermedia in 2014. 35V direct-mounting FGSVG device became an industrialization demonstration project under the torch plan of the Ministry of Science and Technology and received the Ministry’s grant for technological innovation projects of middle and small enterprises. This series was listed into Strategic Emerging Industries (SEI) Key Products and Services Catalogue in 2016, awarded with Excellent Energy Saving Prize of Shandong Province, and received the “Polar Star Cup” the most welcomed top 10 brands that supports photo-voltaic and electric applications and “Light Energy Cup” CREC excellent photo-voltaic station part producer of the year” in 2018. In the same year, WindSun’s MW high voltage water cooling SVG passed the type test acceptance of China Electric Power Research Institute.
35kV Step-down SVG with modern electric and electronic technologies, automation, microelectronics and networked communication, cutting-edge instantaneous reactive power theory and synchronous reference frame transformation-based power decoupling algorithm FGSVG series is aimed to provide the required nature and quantity of reactive power, power factor and grid voltage, dynamically tract the changes in grid electricity quality in order to adjust reactive output, and ensures required operating curves, thereby improving grid quality.
FGSVG series that provides easy operation, high performance and high reliability are designed to meet the urgent needs of increasing power factor of electricity transmission and distribution grids, control harmonics and compensate negative sequence current. For this purpose, the following characteristics are provided:
● Modular-based design for easy installation, maintenance and setting.
● Easy on-site commissioning and automatic analysis of PC and CT wirings.
● Fast dynamic response, response time ≤5ms.
● Output current’s total harmonics distortion≤3%.
● Many operating modes, i.e., device reactive power mode, targeted point reactive power mode, targeted point power factor mode, targeted point voltage mode and load compensation mode, satisfy user needs; operating modes and their targeted values can be changed in a real time manner.
● Real time tracking of load changes and dynamic and continuous smooth compensation of reactive power for increasing power factor of the system, suppressing harmonics, compensating negative sequence current and improving electricity quality.
● Voltage flicker suppression is provided to improve voltage quality and stabilize system voltage.
● Carefully designed FGSVG circuit parameters ensures a small heat generation, high efficiency and low operating cost.
● Compact structure requires a small area.
● Main circuit is a chained series of H bridge power units that are comprised of IGBT circuits. Each phase has many identical power units whose output PWM waveforms are superimposed to be a square waveform that approaches sine wave. The square waveform filtered by output reactor becomes a good sine waveform.
● Redundancy design and module design ensure the high reliability of system.
● Complete protections, including over-voltage, under-voltage, over-current, unit over-heating, voltage imbalance, etc. and the ability to record failure waveform in order to help with failure point locating and easy maintenance, ensuring high operating reliability.
●Friendly HMI provides RS485 and other ports for external communication under standard Modbus protocol. Apart from real time displaying digital quantities and analog quantities, recording historical operating events, searching historical curves, monitoring unit states, searching system information and searching historical failures, the HMI also provide system self-test, one-key start/stop, time-shared control, oscilloscope (AD channel forced waveform recording), recording instantaneous voltage/current waveform in the event of failure.
● FGSVG is designed with ports that match with FC to effectively combine static compensation and dynamic compensation, providing more cost effective and flexible solutions for users.
● No transient impulse, no surge and no arc restrike from switching, no discharge is required for switching again.
● AC phase sequence is not a factor that should be considered for connection to the system.
● Parallel installation is permitted for easy capacity extension. Ring fiber optical communication is used for many SVGs that operate in parallel to achieve fast communication that perfectly satisfy real time compensation requirement.
FGSVG has a control-gear, a power cabinet where there are three-phase power units and a reactor. In the control-gear, there are a main control box that is connected to power unit through fiber optics and a secondary control logic circuit. Inside the power cabinet are a three-phase power unit, each phase of which is a series of several power units depending on grid voltage. The three-phase power unit is then connected to the grid through a reactor.
Each power unit is a cascaded H bridge circuit. Several power units are connected in series, with their outputs superimposed to be a high voltage output, and then connected to the grid through a reactor. Modular-based design makes power units interchangeable.
FGSVG’s control system collects grid voltage and current signals, calculates the current commands for all power units by using instantaneous reactive power control theory, and sends these commands to respective power units through fiber optic. Each phase of the power cabinet is a series of several power units whose output voltages are superimposed by using carrier phase-shifting technology, to form a multi-level high voltage output. Power cabinet is then connected to the grid through a reactor to output controllable reactive current.
In an AC circuit, current and voltage phases have three combinations. Voltage and current are in the same phase in a pure resistive circuit; voltage leads current in an inductive circuit; voltage lags current in a capacitive circuit. The basic principle of FGSVG is to parallel a controllable H bridge circuit to the grid through a reactor, and then makes the H bridge circuit absorbs or produces such reactive current as to the satisfaction of the grid by adjusting the amplitude and phase of its output voltage or directly controlling its output current, in order for dynamic reactive power compensation. The principle is also shown in the following table.
Operating state | Waveform and phase | Description |
Zero load |  | U1=Us，IL=0，SVGneither absorbs nor generates reactive power. |
Output capacitive reactive power |  | U1>Us，ILis a leading current whose amplitude can be continuously controlled by adjusting U1, there by continuously adjusting the reactive power generated by SVG. |
Output inductive reactive power |  | U1<Us，ILis a lagging current. In this case, the reactive power absorbed by SVG can be controlled continuously. |
1．Low voltage ride through
With continuous innovation, WindSun has developed a full-performance low voltage ride through technology for FGSVS, which can provide rated capacitive current regardless of three-phase short circuit, inter-phase short circuit or voltage decrease to 90% or 20% of its rated value. For low voltage ride through problem under unbalanced grid voltage, WindSun has proposed double phase locked loop and double PG control theories.
In 2017, WindSun’s FGSVG has passed the comprehensive tests performed by Electric Equipment Quality Test Center. Keep phase A voltage constant; decrease phase B and phase C voltages drop to 20% instantaneously for 2 seconds, then increase their voltages to rated values; repeat the said procedure for 3 times within 1 minute. The test result is shown in the following table and complies with the requirement.
2．Harmonics Compensation
In industrial and mining enterprises where middle-frequency furnaces and electrolytic aluminum are used or produced, connection of large capacity and non-linear loads into the grid results in injection of low-frequency harmonic current into the grid, which materially affects the stability of other equipment. With discrete Fourier transform-based fast algorithm and instantaneous reactive power algorithm, WindSun has developed high voltage dynamic reactive power compensation devices that can compensate harmonics. Due to low carrier frequency (for increase operating efficiency), the lower the harmonics frequency (not higher than 25Hz), the higher the compensation efficiency.
3．Negative Sequence Compensation
A lot of unbalanced loads and some single-phase large capacity loads, such as electric railways, which exist in electric system, produce negative sequence and reactive current, which result in huge electricity loss, threaten the safety and increased the cost of electric system.
WindSun triangle chain SVG is an ideal compensation solution that provides fast response and controls both reactive power and negative sequence power. By analyzing the phasor diagram of compensation circuit geometrically, the triangle solution obtains the phasor of phase current necessary for negative sequence compensation, then derives dq/△ transformation matric based on instantaneous reactive power theory, thereby getting the phase current command required for negative sequence compensation.
4．Parallel for Coordinated Control
Growing electricity system capacity demands higher capacity of reactive power compensation device. Limited by IGBT technology and user’s phase-based production, several SVGs may be operating on the same bus in parallel, a circumstance that requires SVGs in parallel can automatically balance their powers.
WindSun has invented a simple and efficient ring fiber optic control system for SVG parallel. Paralleled SVGs form a ring through fiber optics and fiber optic quantity does not restrict the quantity of SVGs that are paralleled. Within the ring, SVGs of different power levels can be connected. The ring can automatically balance reactive power output and make the most use of all SVGs. Any of these SVGs can be set as the master and others as salves. A slave failure does not affect the operation of other SVGs.
5．Automatic Analysis of PC and CT Wirings
Through on-site PT and CT, SVG collects system voltage and current signals. PT wiring directly affects SVG’s phase lock and grid connection; and CT wiring directly affects SVG’s calculating the target point’s reactive power and power factor. WindSun SVG is able to analyze PT and CT wirings automatically, a function that makes grid connection commissioning easy and efficient.
6．Control System’ s Self-test
Control system used for FGSVG consists of multiple DSPs +FPGA and is complicatedly structured. To ensure the system’s reliable operation, DSP is used to perform self-test for PCBs in the control box. If any PCB goes wrong, the problem can be quickly located. This can enhance working efficiency and product intelligence.
7．Fiber Optic Series Communication
To have low voltage isolated from high voltage, fiber optics are used to connect SVG controller and power units. The typical fiber optic connection between SVG controller and power units is that two fiber optic cables connect each power unit to the controller.WindSun has developed a fiber optic series communication technology that has been filed for patent. SVG controller and power units are connected to be many communication circuits in series through fiber optics to greatly reduce the quantity of fiber optic ports and fiber optic cables, thereby increasing productivity and lowering failure rate.
8．Fast Response
FGSVG has passed the tests performed by third-parties including Nanjing Branch and Wuhan Branch of China Electric Power Research Institute, with a response time of less than 5ms.
Waveforms of Response Time (Abrupt Down) (upper: system; middle: load; bottom: tested sample)
Rated working voltages: ：10kV；
● Rated capacity：±5～±25Mvar；
● Output reactive power: continuous change from rated inductive reactive power to rated capacitive reactive power;
● Response time：≤5ms；
● Over-load capacity: 1 minute for 1.2 times of rated capacity；
● Total harmonics distortion of output voltage (before grid connection)：≤5%；
● Total harmonics distortion of output current：≤3%；
● System voltage imbalance protection and setting range:4%～10%；
● Efficiency: ≥99.2% (at rated working condition)；
● Working temperature：－20℃～+40℃；
● Storage temperature：－40℃～+65℃；
● HMI: Color touch panel for which Chinese and English languages can be selected；
● Relative humidity: monthly average RH is not higher than 90% (25℃) , no condensation;
● Cooling method: forced air cooling, water cooling, air conditioning (optional);
● Installation method: indoor, container (optional);
● Elevation:<2000 m (product can be customized for elevation of higher than 2000 m);
● Seismic intensity: ≤ scale 8 .
FGSVG series can be widely applied to petroleum and chemistry, electric system, metallurgy, railway electrification and urban construction for providing various asynchronous motors, hoists, transformers, thyristor converters, VFDs, induction furnaces, middle-frequency furnaces, arc furnaces, resistance furnaces, quart melting furnaces, electric locomotives, cranes, punching machines, lifts, elevators, wind power generators, lighting apparatus, welding machines and other equipment with highly qualified and reliable reactive power compensation and filter solutions.
This series can enhance power transmission capacity, reduce electricity loss, compensate reactive power, suppress harmonics, inhibit voltage flicker, stabilize grid voltage, balance three-phase system, change system’s damping property and increase system stability.
Note: Capacity (Mvar) means the rated maximum capacity adjustable from inductive reactive power to capacitive reactive power. With C2.0/10 as an example, it is a direct mounting device whose power level is 10 kV and capacity is ±2 Mvar, which means it can continuously adjust the reactive power from +2000 kvar (inductive) to -2000 kvar (capacitive).
