The Impact of Nighttime Voltage Increases on Transformer Losses and Comprehensive Management Strategies
Abstract: Research has confirmed that the phenomenon of nighttime voltage increases in distribution grids significantly increases transformer no-load losses. When the operating voltage exceeds the rated value by 5%, the iron losses of a typical distribution transformer will increase by more than 10%. By adopting a coordinated control scheme combining on-load voltage regulation and dynamic reactive power compensation, energy-saving and loss-reduction objectives can be effectively achieved.
I. Formation Mechanism of Nighttime Voltage Rise
The load factor of distribution grids generally drops to the 0.3-0.5 range at night. At this time, inductive voltage drops along the lines weaken, while capacitive effects to ground become prominent, leading to voltage rise at the end of the line. Theoretical analysis and measured data indicate that voltage deviation can reach 7%-10% of the rated voltage under light load conditions. Fluctuations in distributed power generation exacerbate this phenomenon. Actual measurement data from a 330 MW photovoltaic cluster shows that when output suddenly drops in the evening, the voltage transient amplitude at the connection point exceeds 8%. The issue of voltage regulation lag is also prominent. Traditional on-load voltage regulators have an action delay of 120-300 seconds, resulting in an average duration of 23 minutes for nighttime voltage exceeding standards.
II. Characteristics of the impact of voltage elevation on transformer losses
Total transformer losses consist of no-load losses (iron losses) and load losses (copper losses), following a specific mathematical model. Actual operational data from the S13-400kVA transformer shows: at a rated voltage of 400V, no-load losses are 560W; when the voltage rises to 423V (an increase of 5.75%), iron losses increase to 692W, representing an actual increase of 23.6%. This value exceeds the theoretical calculation by 11.8%, indicating that the hysteresis effect significantly exacerbates loss growth.
Although the reduction in load decreases the proportion of copper losses to 15%-30%, the proportion of iron losses simultaneously increases to 70%-85%. The net effect is an increase in total losses of 8%-12%, while the temperature of insulation hotspots rises by 6-8°C, accelerating the aging process of insulation materials by approximately 40%.
III. Multi-level Collaborative Voltage Control Scheme
A hierarchical control architecture is established to achieve precise voltage regulation: the 220kV main transformer reduces the high-voltage side voltage by 1.25% via an on-load tap changer (OLTC); the 110kV busbar receives instructions from the distribution network automatic voltage control system (AVC); the 10kV feeder is equipped with a static var generator (SVG) to dynamically absorb reactive power, ultimately establishing a voltage control closed-loop at the distribution transformer side.
Key technological applications include:
- OLTC pre-regulation technology based on load forecasting, stabilizing the high-voltage side voltage at night within the 102%-105% rated voltage range; after implementation in a certain regional grid, the voltage qualification rate improved to 99.97%
- SVG devices achieve continuous reactive power regulation from -1 to +1 Mvar, with a response time of less than 20 ms and a voltage fluctuation suppression rate exceeding 85%
- The intelligent distribution transformer terminal (TTU) system has three core functions: real-time monitoring of voltage distortion rate (THD_u < 1.5%), automatic disengagement of capacitor switching when voltage exceeds 107% of the rated value, and reverse regulation of distribution transformer tap settings.
IV. Engineering Practice and Benefit Verification
Operational data from a certain industrial park renovation project indicates: the average nighttime voltage decreased from 10.58 kV to 10.22 kV (a reduction of 3.4%), and the transformer loss rate decreased from 1.82% to 1.51% (a reduction of 17%). After project implementation, annual energy savings reached 136.7 MWh, and the transformer insulation lifespan was extended from 21.3 years to 25.1 years. Calculated at an electricity price of 0.8 yuan/kWh, the payback period is 2.3 years.
V. Conclusions and Implementation Recommendations
1. The non-linear increase in transformer iron losses caused by nighttime voltage elevation is the primary cause of non-technical losses in distribution grids.
2. It is recommended to adopt a three-tiered collaborative control mode combining "OLTC pre-voltage adjustment + SVG dynamic compensation + TTU fine control."
3. New construction projects should prioritize the use of SH18-type amorphous alloy transformers, which reduce no-load losses by 65% compared to S11 standards.
4. Voltage management should strictly adhere to the IEEE C57.91 standard, with the control range limited to 95%-105% of the rated voltage.