Mark B. Goff, MSEE, PE
Tennessee Valley Authority
Collin Jay Rosson, EIT
Tennessee Valley Authority
Cassandra H. Goff, MSEE, PE
This paper discusses the most efficient actions to take after a transformer trip by relay operation. As the power grid is pushed harder, it is vitally important to keep the power flowing. This entails having the shortest downtime possible while maintaining safety and reliability. Utilities often approach this scenario from one of the two extremes. One approach is to re-energize without any investigation and the second approach is to run every test possible. This paper discusses how to efficiently evaluate the problem and take a balanced approach to determine if the transformer can or cannot be re-energized.
The pressure in today’s world is to keep the power flowing. In order to keep up with power demand, utilities need a robust plan for maintenance and capital improvements. But sometimes the system has a hiccup. In this case, the hiccup is a protective relay operation. While in some cases it is obvious that the transformer cannot be placed back in service. If this is not the case then it becomes an issue to determine the condition of the transformer. This is known as the grey zone. In order for Management to make the best decision, Engineering must collect data and provide the analysis of the data. Also, this decision needs to be made quickly because timeliness relates to grid reliability and loss of revenue.
The first step in the process is to look for physical damage or signs of an animal/debris that caused a phase-to-phase or phase-to-ground fault. Next, look at which relay(s) initiated the trip. The two most common transformer relays are the Differential Relay and Sudden Pressure Relay (SPR). The Differential Relay can be thought of as a relay that sums all the energy going into the protected zone and compares with the sum of energy leaving the protected zone. These values should be almost equal. If not, then energy is going through an unintended path. The differential zone may include portions of the bus and not just the transformer. The Sudden Pressure Relay which is sometimes referred to as the Rapid Rise Relay which looks at a sudden increase of pressure inside the transformer tank. This sudden change in pressure happens when a fault occurs inside the transformer tank which creates a shock wave of pressure. Therefore, the fault zone is limited to an event that occurred inside the tank. Note: for transformers with a Load Tap Changer (LTC) they often have a pressure relay (PR) that trips on a set increase of pressure in the LTC compartment opposed to a rapid change in pressure on the main tank. For the purposes here concerning testing and actions, the SPR and PR can be placed in the same category. See the table below for possible combinations and recommendations.
Listed are a few questions to stimulate the search:
- Short-term past events include:
- Load changes
- Movement of the Load Tap Changer if applicable
- Other Relay / Functional testing taking place at the time of the trip
- Check for gas in the Gas Accumulation Relay and/or Buchholz Relay
- Check the Pressure Relief Relay (flag raised or oil release)
- Check the Relay Event Report for anomalies and fault current values
- Check the Digital Fault Recorder for anomalies and fault current values
- Mid-term past events include:
- Returning to service after maintenance activities
- Recent wiring changes like lift/land
- Recent Relay setting changes
- Recent Relay replacement
- Longer term past events include:
- Extreme temperature changes (hot to cold or cold to hot)
- Deteriorating trends in the electrical test or oil analysis data
Once a few questions are considered, the move is toward isolating the transformer for testing. A transformer fault can be broken down into three major categories. 1) Winding to Winding or Winding to Ground Insulation Breakdown 2) Turn to Turn Insulation Breakdown 3) High Resistance Winding / Conductor Path. There are other special tests such as Bushing Test, Sweep Frequency Response Analysis, Dielectric Frequency Response, Leakage Reactance, Lightning Arrester and performing test on all taps but they should be reserved for only a deep analysis of the transformer and should be put off until the next outage. The extra test adds very little value when knowing if the transformer can be re-energized. The goal is to take a balanced approach to make an educated data-driven decision.
There are four test categories required to fully determine the operability of the transformer:
Insulation Power Factor or Insulation Resistance test: By applying a voltage in between windings and applying a voltage in between windings and ground, it can be determined if the main insulation system has been compromised. A typical power factor test set can apply up to 10 kV AC and an Insulation Resistance test set can apply up to 5 kV DC. Either method is valid but usually, the power factor method is preferred since past data is readily available and can operate at a higher voltage thus stressing the system more.
DGA Analysis: The DGA analysis is the best test to confirm an issue inside the transformer tank. The only weakness in the DGA is the fact that sometimes a turn-to-turn fault does not produce a significant amount of gas. The DGA salient points are Hydrogen, Carbon Monoxide / Carbon Dioxide, Hot Metal Gases, and Acetylene. An increase in Hydrogen will be present in any fault.
Carbon Monoxide and Carbon Dioxide increases if the fault involves paper or some part of the cellulose insulation. The hot metal gases show up if the fault involves hot metal in contact with the oil. Acetylene is associated with an arc that has occurred in the oil.
Note: When taking the DGA sample:
- Take the sample from multiple locations if possible or wait at least 4 hours after the trip to give the gases a chance to diffuse to all parts of the oil
- Take at least 2 syringes of oil from each sample location to have a backup
- If possible, test on-site with a multi-gas portable DGA analyzer
- A turn-to-turn fault generally produces much less gas than other types of faults
Excitation Test / Ratio Test: The Excitation Test / Ratio Test is the key to finding an issue where the fault is turn to turn. The excitation test stresses the insulation in between turns and will show up an increase in the excitation current. The Transformer Turn Ratio will shift the ratio out of the normal tolerance of +/- 0.5 % from the calculated ratio.
Winding Resistance Test: A failure mode that only shows up as a high resistance (no issues with the DGA) is rare. However, if the transformer design has more than one winding connected in parallel internally, it is possible that one of the parallel windings became disconnected or burns free. In this case there is no change in the ratio due to the windings that are still connected. Also, if the parallel winding burns in two, there is no shorted turn or path to ground. So, only the resistance increased due to the loss of a parallel winding.
If no issues are found from the above testing of the transformer, move on to testing the tripping circuits and CT wiring.
Possible causes and test:
- Water in the conduits ± Isolate the tripping circuit and Megger 500V for 1 minute
- Vibration caused the wiring insulation to deteriorate – Isolate the tripping circuit and Megger 500V for 1 minute
- Test the SPR relay and seal in module where applicable
- Inspect the CT wiring at the tank wall penetrations
- Inspect the CT test blocks for moisture and corrosion
- Test the Differential relays
- Test the current transformers feeding differential relays
- Where applicable – Test the Gas Accumulation Relay, Buchholz Relay, Pressure Relief Device, Oil level Gauges, Oil Temperature Gauge, and Winding Temperature Gauges
- Identify and test other relays that may be wired to trip the transformer (i.e., Winding Hot Spot or Buchholz)
The main focus of the paper is to determine a methodology and tests to see if a transformer can or cannot be quickly placed back in service. If one of the tests fails, then look carefully through the other test to find a second failure or degraded results. As it has been stated before “Out of the mouth of two or three witnesses let a thing be established”. Once all of the transformer tests are complete, the data and information can be given to management such that a decision can be made for the path forward.