High power frequency online UPS (hereinafter referred to as "power frequency UPS") is a key power guarantee equipment widely used in data centers, industrial control, energy and other fields. Its "power frequency" characteristics (including power frequency isolation transformers) bring high reliability, but harmonic problems and grid compatibility in high-power scenarios are the core challenges that affect its stable operation. The following analysis is conducted from four dimensions: harmonic sources, treatment plans, power grid compatibility connotations, and optimization strategies:
1、 Harmonic sources of high-power power frequency UPS: Nonlinear characteristics and "current distortion" caused by structure
The core structure of power frequency UPS includes "rectifier inverter power frequency transformer", and its harmonics mainly come from the input side rectification link. In high-power scenarios (usually referring to ≥ 100kVA), the harmonic impact is more significant due to the increase in power level. The specific sources can be divided into two categories:
1. Nonlinear conduction of rectifier: the "main generator" of harmonics
The traditional rectifiers of power frequency UPS are mostly "diode uncontrolled rectification" or "thyristor semi controlled rectification", which only conduct near the peak of AC voltage during operation, resulting in input current not being a sine wave, but presenting a "pulse like" distortion - this distortion is essentially a "harmonic current", including odd harmonics such as 3, 5, 7, and 11 (the third and multiple harmonics in three-phase systems are zero sequence harmonics, which are more harmful).
At low power factors, the harmonic content is higher: uncorrected rectifiers typically have a power factor of only 0.6-0.7, and the total harmonic distortion (THDi) can reach 30% -50%;
Harm of high-power amplification: When the UPS power reaches several hundred kVA, the absolute value of harmonic current can exceed 100A, directly causing distortion of the grid voltage waveform.
2. Power frequency transformer and system coupling: "transmission and superposition" of harmonics
The power frequency isolation transformer (used for electrical isolation) built into the power frequency UPS can suppress some high-frequency harmonics, but it will generate a small amount of harmonics (mainly 3rd and 5th) due to the "non-linear magnetization of the iron core"; If multiple UPS systems are operated in parallel (common in high-power scenarios), the harmonic phase differences among the devices can cause the harmonics to "superimpose and amplify" on the grid side (especially on the neutral line, where the third harmonic superposition can cause the current to reach twice that of the phase line, leading to overheating).
2、 Harmonic Control of High Power Frequency UPS: From "Source Suppression" to "External Compensation"
The core of harmonic control is to reduce THDi to the grid standard (such as GB/T 14549 requirement ≤ 10%), which needs to be combined with "source optimization" and "external filtering", taking into account the "large capacity" and "stability" requirements of high-power scenarios.
1. Source suppression: Optimize the internal topology of UPS to reduce harmonic generation
Starting from the rectification process and reducing harmonics through topology improvement is the most direct "fundamental" solution, suitable for the long-term stable operation of high-power UPS.
Active power factor correction (APF+PFC integration):
Replacing traditional rectifiers with "active PFC rectifiers" (such as three-phase Boost PFC), by controlling the current waveform through high-frequency switching transistors (IGBT), the input current tracks the voltage sine wave, which can increase the power factor to above 0.98 and reduce THDi to 5% -10%. In high-power scenarios, it is necessary to use "multi module parallel PFC" (such as 6-pulse and 12 pulse rectification) to cancel out low order harmonics through phase superposition (12 pulses can eliminate 5th and 7th harmonics).
Combination of high-frequency rectification and power frequency isolation:
Some new types of power frequency UPS adopt a hybrid topology of "high-frequency rectification+power frequency transformer" - high-frequency rectification (such as LLC resonant rectification) achieves low distortion current (THDi ≤ 8%) through high-frequency switches, and then isolated by power frequency transformers, retaining isolation advantages and reducing harmonics from the source.
2. External compensation: For the harmonics that have already been generated, use filtering devices to 'cancel' them out
If the UPS itself has not been optimized at the source, or if the on-site harmonics exceed the standard (such as multiple UPS stacking), external equipment compensation is required, which can be divided into "passive" and "active" categories
3. Special scenario: "Zero sequence harmonic control" under three-phase four wire system
High power frequency UPS systems are mostly three-phase systems, and 3rd and 9th order zero sequence harmonics will flow through the neutral line (because the phase of the three-phase zero sequence harmonics is the same, the neutral line current can exceed the phase line after superposition), and targeted treatment is needed:
Adopting the "three-phase four wire system APF": series connection of APF modules on the neutral line to compensate for zero sequence harmonics separately;
Passive solution: Connect a "3rd harmonic filter" (dedicated tuning circuit) between the neutral line and the phase line to divert zero sequence harmonic current.
3、 Grid compatibility of high-power power frequency UPS: "dual requirements" for bidirectional adaptation
Grid compatibility refers to the bidirectional balance between the "adaptability of UPS to the grid" and the "friendliness of UPS to the grid" - ensuring stable operation of UPS during grid fluctuations while avoiding interference to the grid caused by UPS operation.
1. UPS's "adaptability" to the power grid: resistance to power grid disturbances
The power grid may have issues such as voltage fluctuations (± 10%), frequency fluctuations (± 0.5Hz), and voltage distortion (THDu ≤ 5%). High power frequency UPS needs to improve its adaptability through design optimization
Wide input voltage range: By using "thyristor voltage regulation" or "Buck Boost circuit", the input voltage adaptation range can be extended to ± 20% (such as 380V grid can adapt to 304-456V), reducing the frequency of "switching to battery mode" caused by voltage fluctuations;
Anti harmonic interference design: Install an "EMI filter" on the input side of the UPS (to suppress harmonic input from the grid side), and optimize the inverter control algorithm (such as using "repetitive control+PI control") to avoid distortion of the inverter output waveform caused by grid harmonics;
Frequency adaptive: Real time tracking of grid frequency through phase-locked loop (PLL). When the frequency fluctuates within ± 1Hz, the inverter synchronously tracks to avoid frequent "off grid/on grid" switching.
2. The "friendliness" of UPS to the power grid: compliance with grid regulations
During UPS operation, it is necessary to avoid causing "secondary pollution" to the power grid. The core is to meet the limit requirements of the power grid for "harmonics, reactive power, and impulse current", and to combine standards and actual indicators:
Harmonic limit: It must comply with GB/T 14549 "Harmonics in Public Power Grids", where in ≤ 100MVA power grids, the third harmonic current limit on the 220kV side is ≤ 20A, and the fifth harmonic current limit is ≤ 12A (high-power UPS needs to control the harmonic current within the limit through treatment);
Reactive power control: By using PFC technology, the input power factor is increased to 0.95 or above (to avoid absorbing a large amount of reactive power into the grid, which can cause a drop in grid voltage);
Surge current suppression: When starting a high-power UPS (especially during the charging phase of the rectifier), a "soft start circuit" (such as a current limiting resistor or thyristor) should be used to limit the starting current to within 1.5 times the rated current (to avoid voltage drops caused by grid surges).
4、 Compatibility optimization in high-power scenarios: collaboration from device to system
The grid compatibility of high-power power frequency UPS needs to be optimized through multiple links including "equipment design," "on-site configuration," and "system integration." Key strategies include:
1. Equipment selection: Prioritize "low harmonic+wide adaptability" models
When selecting, focus on two indicators:
Input THDi: Select the model with built-in active PFC (THDi ≤ 8%) to avoid traditional diode rectifier models (THDi ≥ 30%);
Input voltage/frequency range: Priority should be given to wide range models with "voltage ± 25%, frequency ± 2Hz" (such as industrial grade UPS), suitable for scenarios with large fluctuations in the power grid.
2. System configuration: The filtering device and UPS operate in conjunction
When multiple UPS are connected in parallel, a "same model+same phase" configuration is adopted (to avoid superposition caused by harmonic phase difference), and a "centralized APF" is installed on the parallel busbar side (to uniformly compensate for total harmonics);
If the power grid itself has severe harmonics (such as nearby electric arc furnaces and frequency converters), a "isolation transformer+passive filter" should be connected in series on the input side of the UPS (to isolate the power grid harmonics while suppressing the external transmission of UPS harmonics).
3. Standard compliance: Anchoring domestic and international core specifications
In addition to GB/T 14549 (Harmonics), it must also comply with:
GB/T 7260.3 "Uninterruptible Power Supply Equipment Part 3: Requirements for Batteries" (ensuring no grid impact during battery switching);
IEC 62040-3 (International Standard): Requirements for UPS input current harmonics and electromagnetic compatibility (EMC) (to be met in export scenarios).
summary
The essence of "harmonic control" and "grid compatibility" for high-power power frequency UPS is the core issue of "balancing equipment operation and grid safety": harmonic control needs to achieve low distortion (THDi ≤ 10%) through "source PFC optimization+external hybrid filtering", and grid compatibility needs to start from both "resistance to grid disturbances" and "compliance with grid regulations". In practical applications, a comprehensive plan should be developed based on power levels (such as prioritizing APF multi module parallel connection for hundreds of kVA), power grid environment (such as wide adaptability and isolation design for industrial power grids), and cost budget (higher cost-effectiveness of hybrid filtering), ultimately achieving the goal of "UPS stable operation and no secondary pollution in the power grid".
