Abstract: Currently, transformers are equipped with differential protection and ratio-controlled differential relays are introduced to ensure the safe operation of power systems. Therefore, this paper introduces the braking characteristic curves of transformer differential protection and field testing methods.
Keywords: Transformer; Differential protection; Braking characteristics; Test methods
1. Introduction
Transformers are one of the main electrical devices in modern power systems. Because transformer failures have a significant impact, it is crucial to strengthen the debugging of their relay protection devices to improve the safe operation of the power system. The most important configuration in transformer protection devices—differential protection—is designed to protect against phase-to-phase and turn-to-turn short circuits within the transformer's internal coils and leads, as well as ground faults on the leads and coils of a directly grounded neutral system. Furthermore, due to the high selectivity and sensitivity of differential protection, we should also consider its ability to withstand unbalanced currents generated by inrush currents and external short circuits, and it should not malfunction during transformer over-excitation.
2. Introducing ratio braking characteristic curve into differential protection
Under normal load conditions, the error of the current transformer is very small, and the unbalanced current in the differential protection circuit is also very small. However, as the external short-circuit current increases, the current transformer may saturate, and the error will increase accordingly, as will the unbalanced current. When the current exceeds the protection operating current, the differential protection will maloperate. Therefore, to prevent maloperation of the differential protection when a fault occurs outside the transformer zone, we hope to introduce a relay whose operating characteristic is that its operating current increases proportionally with the increase of the unbalanced current, and even increases faster than the unbalanced current, thus preventing maloperation. Therefore, we introduced a ratio-restrained differential relay into the differential protection. In addition to using the differential current as the operating current, it also introduces the external short-circuit current as the restraining current. When the external short-circuit current increases, the restraining current also increases, causing the relay's operating current to increase accordingly, thereby effectively preventing maloperation of the differential protection when a fault occurs outside the transformer zone. The restraining characteristic curve is shown in Figure 1.
As shown in Figure 1, the protection relay can reliably avoid the unbalanced current during external faults and effectively prevent the protection from maloperating when faults occur outside the transformer area. Therefore, the accuracy of the braking characteristic curve of the differential protection is the key to the correct operation of the protection device. Thus, the testing of the braking characteristic curve is the focus of the commissioning of the entire protection device.
3. Test method for braking characteristic curve
In the past, due to limitations in testing equipment, we easily overlooked the testing of ratio braking characteristics in practical work, assuming that the braking coefficient device was inherent and did not need testing. This often resulted in malfunctions of the protection device due to insufficient debugging. However, with the continuous advancement of field testing equipment, we must do this work well. Conventional protection braking characteristic curves can be obtained by applying operating current and braking current to the differential winding and braking winding respectively, and after repeated tests. However, with the widespread application of transformer microprocessor protection in power systems, how do we test the ratio braking characteristic curve of microprocessor protection? Based on experience summarized from multiple field debugging of transformer microprocessor protection, the microprocessor differential protection braking system can only obtain the ratio braking characteristic curve by applying current to the fault outside the high and low voltage sides, simulating the fault zone, and calculating the operating current and braking current. A simple, reliable, and highly accurate testing method is introduced here for reference. The test wiring is shown in Figure 2.
For ease of calculation, we can first assume that the transformer connection groups are Y and y0, the current compensation coefficient of the current transformer ratio is 1, and divide them into two categories according to the different transformer windings on site.
The first type is differential protection with two winding braking characteristics.
Two current sources (designated as phases IA and Ia, with a 180° angle between them) are used to connect the currents of phases IA and Ia to the high and low voltage sides of the protection device, respectively. The currents of the two phases are adjusted so that IA = Ia, at which point Id = 0. Then, an external fault is simulated. The current of phase Ia remains constant, while the current of phase IA increases, causing the differential current Id to increase until the protection operates. At this time: the operating current Id = IA - Ia; the braking current Ir = IA + Ia; then the braking coefficient (i.e., the slope of the curve in Figure 1) Kb1 is: Kb1 = (Id - Icd) / (Ir - IB). Where Icd is the minimum operating current; IB is the inflection point current.
Repeat the above experiment, fixing different Ia values and adjusting different IA values to enable protection actions, and the curve can be obtained. This allows the calculated Kb1 value to match the set Kb1 value.
Category 2: Differential protection with multi-winding braking characteristics.
At this point, the operating current is the sum of the currents on each side with the same polarity, and the braking current is the maximum value of the currents on each side. When an external fault occurs, the differential current is an unbalanced current, and the braking current is the fault current on the side with the maximum value. In this case, the test method is the same as the first type. It can be assumed that Ia is constant, and the differential current Id is increased by decreasing the current IA, that is: operating current Id = Ia - IA; braking current Ir = Ia; then the braking coefficient Kb1 = (Id - Icd) / (Ir - IB). The calculated braking coefficient Kb1 is consistent with the device setting value Kb1.
The testing methods described above only consider the case where the transformer windings are Y and y0, and the current compensation coefficient of the current transformer ratio is 1. However, in field operations, we may encounter situations where the transformer windings are Y and Δ, and the current compensation coefficient of the current transformer ratio is not 1. In such cases, we need to consider the influence of other compensation coefficients.
4. Conclusion
Through on-site testing of the braking coefficient of microprocessor protection for different types of transformers, the author found that this method is simple, easy to implement, and highly accurate, and is worth promoting.
