Winding temperature simulation system
Author: Michael Miller
One item often overlooked during the testing and commissioning of a transformer is the verification of the oil-winding temperature gradient within the winding temperature simulation system.
Unless the transformer came with some type of fiber optic temperature probes mounted directly to the windings you are going to have to rely on a system designed to simulate the actual temperature of the windings and not just the temperature of the insulating oil that surrounds and cools those windings.
Before conducting the simulation, you will need to locate and/or verify the following:
Top oil temperature indication for a reference or starting point
Some type of transformer load reference
A device to fine tune that load reference
A way of applying a temperature bias based on the transformers load
The actual gradient value which comes from the manufacturer’s heat run or temperature rise test at a known load point.
1. Top oil temperature
Using another oil temperature indicator, have the face plate read high-voltage, low-voltage, or TV Winding Temperature. These are normally referred to in the ANSI/IEEE standard device classification as a 26 device. In this case we are using it as a transformer thermal relay so its true designation will be a 49 device as described by IEEE standards below.
IEEE Std C37.2™- 2008 IEEE Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations are as follows:
3.1.26 Device number 26 - apparatus thermal device A device that functions when the temperature of the protected apparatus (other than the load-carrying windings of machines and transformers as covered by device function number 49), or that of a liquid or other medium, exceeds a predetermined value; or when the temperature of the protected apparatus or that of a liquid or other medium, exceeds a predetermined value or decreases below a predetermined value.
3.1.49 Device number 49 - machine or transformer thermal relay A device that functions when the temperature of a machine armature winding or other load-carrying winding or element of a machine or power transformer exceeds a predetermined value.
2. Load reference
Load reference is done by the use of a current transformer (CT). This (Hot-Spot) CT will be a single ratio CT and that ratio is specified by the transformer manufacturer to somewhat match the current of the phase or leg it is monitoring.
3. Device for fine-tuning the load reference
The load reference will be in the form of the secondary current from the above CT. Though this secondary current will remain proportional to the loading of the transformer it’s not of much use at this point. You will need to adjust the secondary current to give the exact temperature bias needed by using some type of calibration unit: either an adjustable resistor or a small multi-tap auto transformer. This device will serve as the calibrating mechanism for the whole system.
4. Applying the temperature bias
The easiest way to affect the reading of a temperature probe is to heat it, using the heater coil inside the wells of thermal plates on the transformer tank walls or tank cover.
You can tell this well is for the winding temperature if it has some form of conduit running to it. Inside the conduit are the conductors that will carry the now calibrated CT secondary current to the heater coil.
In the case of the Qualitrol thermal plates, the winding temperature probe wells are marked with a (WT) for winding temperature and an (LT) for liquid temperature. Be certain not to confuse them when checking the calibration of the indicators, themselves.
5. Actual temp
The temperature gradient is found in the factory test reports and should already be applied by the setting of the calibration unit.
The table below is from a temperature rise test of a 55MVA transformer during the FAT (factory acceptance tests).
a. Terminals X2-X0 tell us which numbers to reference.
b. Average winding temp at shutdown, subtracted by, Average oil temperature at shutdown, plus the factories 3°C constant equals a Gradient of 14.85°C This is saying that this winding is running 14.85°C hotter than the top oil temperature when the transformer is at full load. This unit is a 55MVA / 15MVA, 19.1kV / 4.16kV, Dyn1 Aux unit.
Now you are ready to check the system and make sure everything is working properly.
You will need a way of simulating the secondary current of an energized CT. There are many choices including a top-of-the-line, Doble F-2253 all the way down to a poor man's special (light bulbs, clamp-on amp meter and a dimmer switch).
Regardless of what current source you use, you will need to know the correct amount of current to apply to the winding simulation system and its heat source.
To find this out, you need to know a few things about the transformer:
The Hot-Spot CTs location (Figure 5)
The Hot-Spot CTs ratio (Figure 5)
The current at a known load on that CT (Figure 6).
Now we will calculate the actual secondary current that the Hot Spot CT will produce taking these items into consideration.
Current on LV(X) at 15 MVA is shown to be 2,081.8 amps
Hot Spot CT ratio is 2090:5 amps
Here is how you calculate the secondary current:
Take the known current multiplied by CT secondary (2081.8 * 5)
Then divide the product by the CT primary (10,409 / 2090 )
The answer = 4.98 amps.
Now, you know that when the LV(X) is at 15 MVA, the secondary current of the Hot Spot CT will be 4.98 amps. Your current source has to be able to supply the 4.98 amps to the terminals where the Hot Spot CTs S1 and S2 leads land. You also know that according to the manufacturer's temperature rise test, the temperature gradient on the LV winding is 14.85°C.
Now, it is time to calculate how much current you will need to actually apply to the winding temperature simulations systems heat source by adjusting the calibration unit.
First, look at the devices shown in Figure 1 and Figure 2 and how they are adjusted to get the current we need.
For the adjustable resistor: you will use the formula:
R = ( 4.9*Ih ) / ( Ict – Ih )
R = Resistance value to set the adjustable resistor.
4.9 = Constant.
Ict = Hot Spot CT secondary current at known load.
Ih = Heater current required.
Now fill in the formula with our known values:
R = (4.9 * 0.730) / (4.98 - 0.730)
R = (3.577) / (4.25)
R = 0.841Ω.
The adjustable resistor should set to a resistance value of 0.841ohms.
For the multi-tap auto transformer:
The connection diagram with input and output ratios will help you understand the multi-tap auto transformer.
Current from the Hot-Spot CT is injected into the X1 and X2 terminals (normally) and the adjusted current is picked up on any two tap combinations based on the output current needed.
X1 (tap1) to X2 (tap 5) have 75.5 turns.
Tap 2 to tap 7 has a total of (14+13+31.5+3+3=) 64.5 turns.
That is a ratio of 1.171:1
4.98 amps feeding into X1 & X2 = 4.252 amps on taps 2 and 7.
4.98 amps feeding into X1 & X2 = 4.45 amps on taps 2 and 8.
(Caution, be sure your injecting current into the correct side of the transformer.)
Here is why:
4.98 amps into taps 4 & 5 = 13.36 amps on taps 1 and 8!
Now that you know how to adjust your current output up or down, you will need to find the correct current to apply to the heater coil. First, you will need to look at the heater coil and see which heater you have and what its resistance is. Each of the Qualitrol heaters will have their own characteristic curve.
Once you match up your heater to one of the three charts below you will then need to find the temperature rise or gradient in the Y axis of the chart and trace it across to the solid curved line and then trace straight down to the bottom or X axis for the desired current to be injected into the heater.
Keeping with your current gradient of 14.85°C - In the transformer you see a 609-1 heater element. Refering to Figure 10, follow the 14.85°C across, then trace down you will see a heater coil current of 4.4 amps. The dashed lines show the possible error or deviation from the chart. There will be some trial and error when setting this for the first time.
Based on this information, if you are injecting 4.8 amps into taps 1 and 5, you will get 4.45 amps out of taps 2 and 8. Using this information, you can adjust the current up or down depending on what the gauge reads.
Note: For oil bulb type temperature indicators, you will need to let it “cook” for at least 30 minutes to get a good gradient reading. Qualitrol recommends a full three hours to allow the gauge to settle out. In the first half hour you will know if you are close. Figure 13 below illustrates a time constant curve showing how fast the gauge will respond to the heat generated by the heater coil.
For analog type temperature gauges: Series 104
Check your transformer to see what devices it is using for its winding temperature simulation system.
Calculate the secondary current of the hot spot CT.
Locate or calculate your transformers temperature gradient.
Verify the settings to the matching unit, resistor or transformer.
With the transformer de-energized, lift the hot spot CT secondary wires and related ground wires and shorting screws.
Land your current source wires in their place.
Record your top oil and winding temperature before injecting current.
Inject the calculated hot spot secondary current.
In 30 minutes check your winding temperature. If it’s close, proceed. If not, kill the power and readjust the matching unit. Give the bulb time to cool off before starting the test again.
Once you have re-adjusted the matching unit and the bulb has cooled back down, inject the same current again into the simulation system.
At the 30 minute mark, if it’s close, keep going and in three hours record your “heated” winding temperature.
Now subtract your “un-heated” temperature from your “heated” temperature and this will be your measured gradient.
Looking at the test unit again:
The unheated winding temperature indicator read 23°C.
The same gauge heated for three hours now reads 38°C.
38°C – 23°C = 15°C
The gradient was 14.85°C
That is a deviation of 0.15°C. Generally a deviation of +/- 2.5°C is acceptable.
For the electronic temperature monitors:
509, TTC1000, Weschler Advantage, etc.
Refer to steps 2, 3, and 5 above.
Verify the gradient, hot spot CT ratio and transformer load current are correctly entered into the controllers software.
Inject current as described above, but you will pass current through the window of the split core CT of the electronic temperature monitor (ETM). A wire is usually looped from S1 to S2 for this.
The electronic monitors do not need the three hour cook time. In the software you can change the time constant to zero for testing but change it back when you are done.
The gradient is displayed instantly.
DANGER / HAZARDOUS VOLTAGE: Always use extreme caution when working with electricity or electrical equipment. Always properly de-energize and ground the equipment before maintenance. Maintenance work should only be performed by qualified and properly trained personnel. The use of unauthorized parts in the repair or maintenance of the equipment or tampering by unqualified personnel will result in dangerous conditions which could result in death, severe injury or equipment damage.
The information contained herein is general in nature and the instructions contained herein do not purport to cover all details or variations in equipment. The instructions are not intended to provide every possible contingency which may be encountered during installation, operation or maintenance. Users should always use sound practices in application, installation, operation and maintenance of the equipment. Should further information be desired or should particular problems arise that are not covered sufficiently for the user's purposes, additional information should be requested from your local Siemens representative. Siemens reserves the right to make changes in the information shown herein or to make improvements at any time without notice or obligations.