January 21, 2017

Cooling Systems Maintenance for Transformers

Presented By:
Kevin Riley
Trantech Radiator Products
TechCon 2017

Abstract

Losses in transformers are inevitable. Electromagnetic losses reflect as heat and consequently as elevated temperatures due to thermal resistance. High temperature accelerates aging of insulation paper; the Arrhenius equation shows that the life of a transformer is reduced by a factor of two with 8°C rise in temperature. The life of a transformer is measured using the degree of polymerization (DP) of the insulating paper and aging, or the remaining life of a transformer is measured as the inverse of DP. With temperature control being crucial in transformers, heat dissipation through cooling apparatuses becomes of utmost importance. As transformer and cooling system performance are directly related to each other, the operating environment is vastly different for each system and are almost never a linear match in degradation and life-cycle rates. Cooling system performance, with or without moving mechanical equipment, deteriorates with age. Even systems without oil-pumps or fans can fail due to weathering, rusting and fouling. Therefore, evaluating transformer cooling systems along with routine diagnostic testing of the transformer is critical.

Introduction

Larger transformers with higher electrical load density need additional external cooling systems. Liquid cooled transformers can be designed to meet the electrical loads and also limit the temperatures to safe operating conditions with passive or active cooling systems. These cooling systems come in various designs and functions that are a part of the original transformer design for proper heat dissipation under load. Maintaining these cooling systems is fundamental to the proper cooling of a transformer in all passive and active cooling systems. With transformer aging and especially in cases of gradual oil temperature rise, the need for periodic maintenance and inspection becomes critical to the overall health of a transformer and its’ ability to perform to original load ratings. Internal and external factors have to be considered in the periodic maintenance and inspection of all types of cooling systems.

Based on the type of cooling system installed on a transformer, periodic maintenance schedules covering both general inspection, DGA comparisons as well as detailed inspections should be established upon installation. Establishing these schedules on existing transformer assets should follow the same guidelines after baseline data on the transformer and cooling system has been established.

General Maintenance

All transformer assets and their cooling systems require general inspection and maintenance for proper operation to maintain oil temperatures. These are characterized as follows:

1. Corrosion
1.1. Corrosion is one of the leading causes of systemic failures in transformer cooling systems that have been in service for 10 years or more according to major studies throughout the industry since 1980. Degradation to substrates and structures, especially in harsh environments leads to leaks, mechanical failures and reduced heat transfer from primary cooling surfaces.
1.2. The effects of corrosion to a primary active cooling system based on the assumption that its coating and substrate should last as long as the transformer tank, therefore requiring no maintenance, leads to expensive repairs and potentially unplanned outages.

2. Contamination
2.1. Contamination to cooling surfaces reduces heat transfer characteristics with any style cooling system. Mineral deposits form at a higher rate on heat transfer systems due to the heating and cooling cycles that deposit natural and man-made elements onto radiating surfaces from rain, snow, pollution or general debris in the air.
2.2. Certain contaminants begin to attack cooling equipment coatings and substrates if not remediated when found. Based on the transformer’s environmental surroundings many coatings and substrates do not offer adequate protection for the life of the transformer; therefore, periodic inspection and maintenance to remove contamination is essential.
2.3. In OFWF systems, contamination may occur inside the tube and shell heat exchanger which cannot be seen. In these cases the particular fouling agent might be detected through water sampling or as in most cases, the inspection of the tube bundle. This will require a scheduled shutdown in most cases.

3. Lubrication
3.1. It is known throughout all industries that lack of lubrication is the single largest contributor to mechanical failures for any equipment.
3.2. Any cooling system requiring fans as part of an ONAF or OFAF method of cooling must be inspected and receive routine maintenance according to the fan manufacturer’s guidelines for bearing lubrication (unless not required through sealed bearing designs).

4. Visual Leak Inspection
4.1. Routine visual inspections for leaks both on the transformer and cooling systems are a necessity. As the transformer ages the frequency of these inspections should increase as the inherent risk of leaks becomes higher with age.
4.2. Leaks pose several risks to transformers if they are severe in nature and always pose a risk to the surrounding environment. Minor leaks are sometimes indicators of potential future issues and signal the need to plan remediation efforts. They may also indicate the need for in-depth inquiries into the cooling system’s current health.

Specific Maintenance

1. Thermal / Infrared Monitoring
1.1. Periodic thermography scans of cooling system components should occur semi-annually or at the least annually on all of the cooling system components.
1.2. Problems can be detected with the evidence of temperature rise in areas such as pump motors, fan bearings and heat dissipation surfaces. It is critical to collect and chart recorded temperatures for the cooling system for comparative analyzes between scans.
1.3. Problems can also be detected by subnormal temperatures on fins and plates within the cooling system. This is an indicator that blockages may have occurred that are impeding flow of oil to these surfaces for cooling. Generally if a subnormal temperature is found it will be accompanied with recorded higher temperatures in other areas of the cooling surfaces.
1.4. It is critical to establish base line readings at a recorded load and compare future readings at the same load levels to ensure data accuracy. With two and three stage systems it is also critical that all functioning components are operating the same as from previous recordings.

2. Motor Load Current
2.1. At installation, after one month and annually the motor load current or winding insulator resistance on fans and pumps should be tested. It is sometimes difficult to test fan motors for obvious safety reasons, therefore data from IR scans and vibration can be used to detect problems with these components.
2.2. Check pump motor load current on all three phases and record this information. Low motor load current compared to factory testing or previous testing may indicate low oil flow, whereas a high load current may indicate that there is impedance in the pump rotation.
2.3. It is recommended that a complete cooling system inspection be performed if significant load current differences are found in testing as plugged, damaged or leaking cooling fins and surfaces can have a direct effect on flow characteristics within the overall system.

3. Vibration
3.1. Use of vibration analyzing equipment is very common in a predictive maintenance schedule. Baseline readings on pumps and comparative periodic readings are used to diagnose potential wear issues and stresses within that particular pump design.
3.2. Vibration analysis can also be used for potential detection of inactive system component changes. An example is the comparative readings at load bracing and connection points. These types of inspections might indicate loose hardware or structural support issues that can directly affect the cooling system over its lifetime.

4. Oil Sampling / Data
4.1 Elevated readings of oxygen and hydrogen levels in the oil can be indicators of valve and pump seal leaks. These readings can also indicate potential leaks at tube to header sheets and drain valve plugs. A thorough inspection should be performed on the cooling system for potential leaks. A good example would be visual debris buildup around the suction side pump flange in which air might be
introduced to the oil through a gasket leak.
4.2 Elevated readings from an oil sample of moisture and free water can indicate a leak within an OFWF system. In these cases water could potentially migrate to the oil if a leak is present. If the system is a double walled tube assembly there should be water present in the drain or leak detector housing. If the assembly is a single wall tube exchanger the water portion of the heat exchanger should be tested. In these cases it is critical to repair the issue immediately as this can damage or
destroy the transformer.

Inspection Intervals

Various inspection recommendations are given by the transformer manufactures for cooling systems. Although not all inclusive for every transformer model, below in Table 1 is a general recommendation of inspection frequencies:

Special Conditions

There are many types of special conditions that apply to the maintenance of cooling systems on power transformers. The most common special conditions are listed under the following headings and maintenance programs should be adjusted to these conditions:

1. Special Environments
Special environments include chemical production facilities, harsh coastal/marine
environments, mining operations, volcanic activity zones and sites that do not have
adequate clean cooling water. Based on the surrounding environment, maintenance
schedules and frequencies of inspection may need to be increased to offset the exact
special conditions and environment. The transformer manufacturer and/or cooling system
supplier generally will take environmental factors and transformer location into
consideration when designing the cooling system. This does not necessarily apply when
transformers are moved from an existing site to another or for aftermarket transformers.
Specifications of cooling system construction and operation should be verified and
maintenance schedules adjusted for special environments on aftermarket or relocated
transformers.

2. Increased Load Levels (Sustained)
As with any transformer that is operated at high or substantially increased load levels
from historical averages, the inspection and maintenance frequencies must coincide with
load level increases. Adding extra cooling capacity to an existing transformer does not
always assure that the transformer and associated components can withstand the increased
loads. As a general rule, the inspection frequency should increase after a 10% average
load increase.

Conclusion

In conclusion, there are many factors to consider when maintaining a transformer cooling system. Whether it is an active or passive cooling system it is critical to monitor the overall health of these apparatuses. Heat exchangers all work for the same reason at maintaining or lowering oil temperature however, the type and components within each system may vary widely. The function of any cooling system is critical to the life of a transformer. For this reason, scheduled and periodic maintenance is a necessity.

CASE EXAMPLES FROM THE FIELD

Example 1

Coastal deterioration of fins (Figure 1a). Fins blocked near gypsum mine (Figure 1b). In both cases there was little to no maintenance for fin and tubes on these oil coolers.

After cleaning with low pressure steam (Figure 2a). After chemical solution cleaning (Figure 2b).

Example 2

Figure 3a is an example of a shell and tube cooler after 10 years of service at a hydroelectric plant. Unfiltered lake water had been used to cool the transformer oil with no water tube maintenance. Transformers were consistently experiencing high temperature faults. Figure 3b is an example of mineral deposits collecting for 13 years on unmaintained water tubes.