A power transformer is one of the most important and expensive components in a power network. They allow efficient transmission and distribution of electricity through different voltage levels. If a power transformer suffers an outage, it may affect the reliability of the entire network and have considerable economic impact on the system.
Using power transformers efficiently requires planning and control. Normally, transformers are designed to operate within its nameplate ratings.
Since an overloaded power transformer will have an operating temperature increase which harms its lifespan, it is essential to model the heat transfer characteristics between windings and oil in order to predict the hot-spot temperature in the transformer as a function of the load, while considering the cooling characteristics.
The overloading may also occur because of economic considerations. If the transformer is maintained properly so that it is always within specifications, it will last longer before needing to be replaced. Accelerated aging is one of the main consequences of overloading power transformers.
Normal life expectancy occurs when the power transformer has a continuous hot-spot temperature of 98°C for non-thermally upgraded paper, and 110 ℃ for the thermally upgraded paper. However, it is permissible to operate the transformer over this temperature for a short time during a period, providing that the transformer has operated for a long time under this temperature. The exact end of life of transformers are unknown, however, according to International Electrotechnical Commission (IEC), there are different predictions for insulation life depending on mechanical properties such as, retained tensile strength and/or retained degree of polymerization.
This type of loading occurs when the utility operator plans to overload the transformer during a specific time that is more typical in utility operation. The hot-spot temperature may rise to 120-130℃ during this type of loading. No-system outage, planned repetitive loads and shorter life expectancy are the characteristics of this type of loading. For this loading type, calculations can be made to define the time period in which the acceptable loss of life can be achieved.
In this type of overloading, the transformer is operated beyond its nameplate rating for a long time, from several hours to several months, carrying emergency loads. It might occur one or two times during the normal life of the transformer. Long-term overloading occurs because of an outage in a power system or contingencies on the transmission system. However, the risk of failure is greater than the planned overloading and the hot spot temperature can rise to 120°C -140°C under operation. For this loading type, calculations can be made in order to evaluate the acceptable loss of insulation life during a specific load cycle.
Short term overloading is the heavy loading of a transformer during a short time that causes the temperature to increase rapidly and exceed the limits defined by the name plate ratings. In this type of loading, the hot-spot temperature may rise to 180°C for a short time period with the severe loss of insulation life. The main characteristics of this type of loading are:
There are certain risks of overloading of transformers. They are mentioned below.
The block diagram representation of transformer overload protection is shown in figure 1. It consists of one current transformer connected on the primary side of the transformer. Two HL-52S relay modules are connected on both the primary and secondary sides.
In the flow diagram, first the current from primary side current transformer has been taken. If current of CT1 is 1A, then normal current flow LED is turned on. If the CT1 current is greater than 1.67A then transformer overload condition has been detected and as a result, a trip signal is given to the primary and secondary HL-52S relay modules to isolate the transformer from the power system. The piezoelectric buzzer and overload indication LED are also turned on in this case.
Moreover, if 1.0A < current < 1.2A then it means that there is no overload situation on the transformer. Usually normal current of 1.2A flows in the CT secondary (CT rating 1A plus 0.2 margin value). The complete flow diagram is shown in figure 2.
The top-level schematic for the overload protection of the transformer is shown in figure 3.
In this application note, one current transformer and two HL-52S relay modules have been used. The description of each component is given below.
The HL-52S Relay module allows a wide range of inputs to control larger loads and devices like AC or DC Motors, electromagnets, solenoids, and incandescent light bulbs. The relay shield uses one QIANJI JQC-3F high-quality relay with rated load 10A/250V AC, 10A/125V AC, 10A/28V DC, 10A/30V DC. The relay output state is individually indicated by a light-emitting diode.
The features of the module are given below.
We will use the HL-52S one channel relay module. The high voltage output connector has 3 pins, the middle one is the common pin and as we can see from the markings one of the two other pins is for normally open connection and the other one for normally closed connection. The relay module is shown in figure 4. Its output contacts are normally closed. They become open when a high logic is applied at input (IN) pin.
The basic construction of relay module is shown in Figures 5 and 6. Figure 5 shows the relay module in the normally closed state when low logic input is applied. In this case, the current flows through the circuit. In figure 6, when a digital high logic is applied at input pin, the switch changes its position and makes a contact with normally open. In this case, the circuit is opened and no current flows.
A current transformer is a type of transformer that is used to measure AC Current. It produces an alternating current (AC) in its secondary that is proportional to the AC current in its primary. Current transformers, together with voltage transformers (VTs) or potential transformers (PTs), which are designed for measurement, are known as Instrument Transformers.
The main tasks of instrument transformers are:
When you need to measure a current that is too large for your ammeter, a current transformer can be used to provide an isolated lower current in its secondary that is proportional to the current in the primary circuit. The induced secondary current is then suitable for measuring instruments or processing in electronic equipment. Current transformers have very little effect on the primary circuit. A CT is connected in series with the transmission line due to its very low input resistance.
Most current transformers have the standard secondary rating of 5 or 1 amps with the primary and secondary currents being expressed as a ratio such as 100/5 or 100/1 respectively. This means that in the case of 100/5 CT, the primary current is 20 times greater than the secondary current so when 100 amps are flowing in the primary conductor it will result in 5 amps flowing in the secondary winding.
By increasing the number of secondary windings (N2), the secondary current can be made much smaller than the current in the primary circuit being measured because as N2 increases, I2 goes down by a proportional amount. In other words, an inverse proportion relates the number of turns and the current in the primary and secondary windings.
A current transformer, like any other transformer, must satisfy the amp-turn equation and this turns ratio is equal to:
From which we get:
Where,
The current ratio is set by the turns ratio, and as the primary usually consists of one or two turns while the secondary can have several hundred turns, the ratio between the primary and secondary can be quite large. For example, assume that the current rating of the primary winding is 100A. The secondary winding has the standard rating of 5A. The ratio between the primary and the secondary currents is 100A-to-5A, or 20:1. In other words, the primary current is 20 times greater than the secondary current.
In GreenPAK™ mixed signal array, there are analog voltage comparators. Therefore, it is necessary to convert the CT current into voltages. For this purpose, we have connected a shunt resistance Rshunt in parallel with the CT. The voltage across Rshunt has been measured and it is fed to the GreenPAK design. It is shown in figure 8.
The value of Rshunt can be found as follows. Consider we have a CT with standard turn ratio 1:50.
The normal current flowing in the primary of the CT is 50 A. The secondary current can be found as follows.
For normal current flowing in the primary winding, the secondary current is 1A. To obtain 1V corresponding to this current, the Rshunt can be found as follows.
For this value of Rshunt, there would be 1V corresponding to 1A current in the secondary winding of CT.
In the GreenPAK design, the output voltage from shunt resistor connected in parallel with CT of 1A rating has been taken and it is applied at three PINs named PIN6, PIN10, and PIN13. One analog comparator is attached to each pin individually. Here, three different conditions have been checked. If the voltage from shunt resistor is approximately equal to 1V then the normal current flow LED indication is turned on. If the voltage is less than 1.2V and greater than 1V then there is a no-overload condition. Moreover, if the voltage is greater than 1.67V then the transformer is over loaded and it needs to be isolated from the power system in order to protect it. In this case, signals to the primary and secondary sides HL-52S relay modules have been provided and they will isolate the transformer. The signals to the overload LED indication and piezoelectric buzzer have also been provided. In this way, the transformer is properly protected from overload conditions using this GreenPAK design. It is shown in figure 9.
In this application note, we designed transformer overload protection using Silego CMIC SLG46721V as a main controlling element. The proposed design isolated the transformer from the power system in case of overload situations in order to avoid the heating of transformer body and a decrease in its expected life. The proposed design can be used in combination with transformer differential protection and overcurrent protection to completely protect the transformer from damage due to various faults.
The complete hardware components used for transformer overload protection are shown in figure 10.
The function of each used component is mentioned below.
Primary Side HL-52S Relay Module: It is used to isolate the primary side of the transformer in case of overloading of the transformer.
Primary Side Current Transformer: It is used to measure the current flowing in the primary side of the transformer.
Transformer Under Study: The transformer that has been used to protect it from overload situations.
DC Battery: It has been used to supply VCC to the primary and secondary sides HL-52S relay modules.
Secondary Side HL-52S Relay Module: It is used to isolate the secondary side of the transformer in case of overload of the transformer.
Piezoelectric Buzzer: It has been used to indicate the overload situation.
The top view of hardware is shown in figure 11.
A ring main CT has been used. The conductor has to pass through it from which the current has to be measured. The connections of CT are shown in figure 12.
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Figure 13. Normal Voltage (1V) LED Indication |
Figure 14. No Overload voltage (1.0 < V < 1.2) LED Indication |
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Figure 15. Overload Voltage LED Indication |
Figure 16. Switching of Relay Contacts (NC to NO) on Primary Side during Overload Situation |
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Figure 17. Switching of Relay Contacts (NC to NO) on Secondary Side due to Overload Situation |
Figure 18. Indication of Overload using Piezoelectric Buzzer |
Name: Muhammad Waseem
Background: Muhammad graduated with a degree in Electrical Engineering from University of Engineering and Technology Lahore, Pakistan in 2017. For more than 4 years his particular sphere of interest has included CMOS integrated circuit design, control system design, programming, the design of power system equipment and Supervisory Control and Data Acquisition (SCADA) systems. He has authored several research papers and technical articles.
Contact: appnotes@silego.com