315KVA/630KVA/800KVA Oil-Immersed Transformers Overload Capacity and Time Regulations

Introduction 

Oil-immersed transformers are core equipment in power systems, and their operating status directly affects the stability and reliability of the power grid. In actual operation, whether it's a 315KVA transformer specifically designed for small and medium-sized enterprises, a 630KVA transformer for general industrial use, or an 800KVA transformer used as the main unit in a factory, all inevitably face overload operation due to special scenarios such as peak and valley fluctuations in electricity load, seasonal peak electricity consumption, and emergency power supply during system failures. Overload operation is not absolutely prohibited, but it must strictly adhere to the established capacity limits and time regulations. 

Overload operation is not absolutely prohibited, but it must strictly adhere to established capacity limits and time regulations; otherwise, it will accelerate equipment aging and induce safety hazards. This article, combining industry standards and practical experience, will provide a detailed analysis of the overload capacity, time regulations, and key points for safe operation of oil-immersed transformers, specifically for popular models such as 315KVA, 630KVA, and 800KVA, helping relevant practitioners accurately control equipment operation.

Core Principles and Classifications of Overload Operation 

Overload operation refers to a load exceeding the rated capacity. The core limitation is temperature control—overload causes a sharp increase in winding and core losses, and a surge in temperature, accelerating insulation aging (following the "6-degree rule"). Whether it's a small-capacity 315KVA or a medium-to-large-capacity 630KVA or 800KVA model, this will shorten equipment lifespan and, in severe cases, cause insulation breakdown, fires, and other faults. Based on the actual application scenarios of popular models, overload can be divided into three categories, with significantly different operating principles and consequences.

Normal Periodic Loads

This type of overload commonly occurs during peak daily electricity consumption periods, such as the peak daytime production load of a 315KVA small and medium-sized enterprise (SME) or the daytime electricity fluctuations of a 630KVA commercial park. The core principle is that "cooling during off-peak periods can offset the aging effect of peak periods," allowing for full utilization of capacity without significantly sacrificing equipment lifespan. Models commonly used in SMEs require comprehensive consideration of initial load rate, ambient temperature, and other factors during operation, adhering to standards such as IEC 60076-7.

Long-Term Emergency Periodic Loads

This type of overload primarily addresses seasonal peaks lasting several weeks/months, such as the summer air conditioning load of an 800KVA factory or the winter centralized heating load of a 630KVA residential unit. These significantly accelerate insulation aging and reduce equipment lifespan. For both 800KVA high-capacity models and 630KVA general-purpose models, the limitations for this type of overload are more stringent. It typically requires low ambient temperatures or additional cooling measures, and timely equipment maintenance and lifespan assessment are necessary after the peak period ends.

Short-Term Emergency Load

This type of overload is only applicable to system failures (such as the shutdown of a 630KVA/800KVA transformer operating in parallel) or emergency power supply scenarios, with the aim of ensuring critical loads. Regardless of whether it's a small-capacity 315KVA or a large-capacity 800KVA model, the permissible overload multiple is the highest, but the duration is extremely short, and there are strict upper limits on top oil temperature and winding hot spot temperature. All popular models must undergo a thorough inspection (such as oil chromatography analysis) after this type of overload, and the number of times it is used per year should be strictly limited to no more than twice.

General Overload Capacity and Time Specifications for Popular Models

According to the "Load Guidelines for Oil-Immersed Power Transformers," overload capacity is negatively correlated with duration. The following are general specifications for popular models such as 315KVA, 630KVA, 800KVA, and 1250KVA under conventional cooling methods (specific details are subject to the product manual):

Overload Multiple (Actual/Rated Capacity) Permissible Duration Applicable Scenarios (Compatible with Popular Models)

1.1x (10% Overload) 180 minutes Daily peak load (315KVA/630KVA for SMEs)

1.2x (20% Overload) 150 minutes Short-term load surge (General for 315KVA/630KVA)

1.3x (30% Overload) 120 minutes Seasonal mild peak load (800KVA for industrial use)

1.6x (60% Overload) 45 minutes Seasonal moderate peak load (General for 800KVA/1250KVA)

1.75x (75% Overload) 15 minutes System Fault Emergency Repair (for 1250KVA charging stations)

2.0 times (100% overload) 7.5 minutes Emergency Power Supply (Applicable to all popular models)

2.4 times (140% overload) 3.5 minutes Extreme Emergency (Applicable to all popular models)

3.0 times (200% overload) 1.5 minutes Temporary Power Supply for Fault Repair (Extremely High Risk)

Note: All popular models can operate stably for a long time under rated load, at which point the temperature and insulation aging rate are within a reasonable range, representing the ideal operating condition.

Key Factors Affecting the Overload Capacity of 315KVA/630KVA/800KVA Popular Models

The above specifications are general standards. In actual applications, adjustments must be made based on the following factors; otherwise, the system may fail:

Cooling Method

Cooling capacity directly determines overload tolerance. The 315KVA small-capacity transformer with natural cooling (ONAN) has the weakest overload capacity. For the 630KVA general-purpose model with air cooling (ONAF) and the 800KVA large-capacity model with forced oil circulation air cooling (OFAF), the overload multiple can be appropriately increased or the duration extended after starting the standby cooler. Conversely, if the cooling system malfunctions (e.g., fan stops, oil pump clogs), the permissible overload limit must be reduced regardless of the capacity.

Ambient Temperature

Ambient temperature affects heat dissipation efficiency. In low-temperature environments (such as winter), the overload limits for all popular models can be appropriately relaxed. In high-temperature environments (such as outdoor operation in summer), especially for the 800KVA large-capacity model, the heat dissipation efficiency decreases more significantly, requiring a strict reduction in the overload multiple or a shortened duration to avoid heat accumulation.

Initial Load Rate and Operating History

 If the transformer is already operating at high load (e.g., 80%-90% of rated capacity) before overload, regardless of whether it's a 315KVA or 800KVA model, the insulation system has accumulated a certain amount of heat. In this case, the allowable overload time needs to be shortened. If it has been operating under light load for a long time, the insulation system has sufficient cooling "reserve," and it can operate normally according to the standard. However, sudden heavy load after light load should be avoided to prevent a decrease in insulation strength.

Equipment Condition

Older 315KVA/630KVA/800KVA transformers and equipment with deteriorated insulation performance (e.g., oil deterioration, reduced insulation resistance) will have significantly reduced overload capacity, requiring further strict limitations beyond the standard requirements. New equipment or models that have undergone comprehensive overhaul can operate according to the rated standard.

Overload Safety Measures

To ensure overload safety, in addition to adhering to time regulations, the following measures must be implemented:

Real-time Monitoring of Key Parameters

Install and monitor key parameters such as load current, top oil temperature, and ambient temperature in real time. For 800KVA high-capacity models, it is recommended to install fiber optic temperature sensors to monitor winding hotspot temperatures. For 315KVA/630KVA models, monitoring equipment can be configured as needed, and over-limit alarm thresholds can be set. An early warning will be triggered immediately if the temperature approaches the upper limit or exceeds the time limit.

Ensuring Normal Cooling System Operation

Regularly inspect the cooling system and clean the radiators; in case of overload, activate all backup cooling devices in advance to improve heat dissipation efficiency.

Configuring Comprehensive Protection Devices

Install overcurrent protection, gas relays, temperature trip devices, etc., to automatically cut off the circuit when overload timeouts or temperature exceeds the limit, preventing the accident from escalating.

Develop Emergency and Maintenance Plans

For short-term emergency overload scenarios, establish clear emergency response procedures, such as emergency load reduction for 315KVA small and medium-sized enterprises and switching to backup transformers in 800KVA plants. After each overload operation, especially short-term emergency overloads, conduct comprehensive inspections, including oil chromatography analysis and insulation resistance testing, on all popular models to assess the extent of equipment damage. If necessary, contact the manufacturer for professional repairs.

Summary

The overload capacity of popular oil-immersed transformers such as 315KVA/630KVA/800KVA is a crucial performance characteristic for handling special loads, but it is by no means an excuse for "unrestricted use." The time limits corresponding to different overload multiples need to be flexibly adjusted based on actual factors such as cooling methods, ambient temperature, and equipment condition. Unnecessary overloads should be avoided as much as possible during actual operation. When overloads are necessary, the time must be controlled, the temperature monitored, and protective measures implemented to balance short-term power supply needs with the long-term lifespan of the equipment.

Ultimately, all overload operations must comply with national/industry standards and the technical guidelines of the equipment manufacturer to ensure grid safety and stable equipment operation. For detailed overload parameters and customized operation solutions for specific models of oil-immersed transformers such as 315KVA/630KVA/800KVA, please consult CLHT POWER's professional technicians at any time.