Discussion on the Configuration of Single-Phase Grounding Protection on the 380 V Low Voltage Side of Distribution Transformer

With the acceleration of economic development, the application of 10/0.4 kV distribution transformers is becoming more and more extensive. Their protection is generally equipped with quick-break protection, overcurrent protection, and high-voltage side single-phase grounding alarm. For the low-voltage side single-phase grounding protection, some companies configure low-voltage side neutral point zero-sequence current transformers and special single-phase grounding protection. Some units do not configure special low-voltage single-phase grounding protection separately, but use high-voltage side three-phase overcurrent protection to take into account. What kind of configuration should be more reasonable? This article analyzes and explains this.

1. Regulatory requirements

From the requirements of GBT 50062-2008 "Design Specifications for Relay Protection and Automatic Devices of Power Installations" for single-phase grounding protection on the low-voltage side of distribution transformers, it can be seen that transformers with directly grounded neutral points on the low-voltage side need to be equipped with single-phase grounding protection. The specific implementation method of the protection needs to be analyzed based on the actual situation of the project.

For transformers with directly grounded neutral points on the low-voltage side, whether it is star-star connection or delta-star connection, it is recommended to use three-phase overcurrent protection on the high-voltage side to provide single-phase grounding protection on the low-voltage side. The reasons are as follows: ① The three-phase overcurrent protection on the high-voltage side, under the premise that the sensitivity meets the requirements, not only plays the role of protecting the transformer from overcurrent on the high-voltage side, but also completes the function of single-phase grounding protection on the low-voltage side of the transformer, which greatly saves the operating cost of the entire transformer and can obtain better economic benefits; ② Taking into account the power supply radius and distribution, this type of transformer is mostly arranged close to the low-voltage load center and relatively far away from the high-voltage switchgear. Some projects are even hundreds of meters away from the high-voltage switchgear. If zero-sequence protection is installed at the neutral point of the low-voltage side to achieve this, the installation position of the protection is difficult, and the trip outlet wiring is relatively long, which brings hidden dangers to the safe operation of the transformer and is not conducive to the economic and reasonable operation of the transformer.

2 Theoretical Analysis

Analysis of single-phase grounding short circuit on the low-voltage side of transformer with different wiring methods. To simplify the calculation, considering that the cable impedance from the high-voltage system to the transformer has little effect on the single-phase grounding short circuit analysis on the low-voltage side, this paper directly calculates the impedance of the high-voltage cable part into the high-voltage system impedance.

For three-phase short circuit, since it is assumed that the system is symmetrical and only has positive sequence components, there is no need to emphasize the concept of sequence impedance; for single-phase ground short circuit, the concepts of sequence impedance and phase protection impedance must be proposed. Since the short circuit point is far away from the generator, it can be considered that the negative sequence impedance of all components is equal to the positive sequence impedance, while the zero sequence impedance is different from the positive sequence and negative sequence impedances and must be calculated separately. For zero sequence impedance, for distribution transformers with star-star connection and delta-star connection, when a single-phase ground short circuit occurs on the low voltage side, the zero sequence current cannot flow in the high voltage winding of the transformer, and the high voltage side is equivalent to an open circuit state for the zero sequence current, so this impedance is regarded as non-existent when calculating the single-phase ground short circuit current.

This article considers the single-phase grounding short circuit of the low-voltage side busbar of the distribution transformer. Therefore, the following three parts of impedance need to be calculated.

(1) High-voltage system. The high-voltage system is estimated as follows:

Where Ss——short-circuit capacity of the high-voltage side system of the transformer, MVA

Un——nominal voltage of transformer low voltage side, 0.38 kV

c———voltage coefficient, when calculating three-phase short-circuit current, take 1.05, when calculating single-phase short-circuit current, take 1.0

Zs———High voltage system impedance calculated to the low voltage side of the transformer, mΩ

Xs———High voltage system reactance calculated to the low voltage side of the transformer, mΩ

Rs——High voltage system resistance calculated to the low voltage side of the transformer, mΩ

For the TN grounding system, the phase protection resistance and phase protection reactance are

Where Xphp.s is the high voltage system reactance calculated to the low voltage side of the transformer, mΩ

Rphp.s———High voltage system resistance calculated to the low voltage side of the transformer, mΩ

(2) Distribution transformer. The positive sequence impedance is calculated as follows:

Where ΔP——Transformer short-circuit loss, kW

Ur———Rated line voltage, kV

SrT——transformer rated capacity, MVA

ud%——Transformer impedance voltage percentage

The negative sequence impedance of the transformer is equal to the positive sequence impedance. The zero sequence impedance of the transformer connected to Y, yn0 is much larger than the positive sequence impedance, and its value is provided by the manufacturer through testing; if there is no test data for the zero sequence impedance of the transformer connected to D, yn11, its value can be taken as equal to the positive sequence impedance value.

The phase protection resistance and phase protection reactance of the Y, yn0 connected transformer are:

D, yn11 The phase protection resistance and phase protection reactance of the transformer are

(3) Low voltage busbar. The conductor resistance is

Where Rθ is the DC resistance when the conductor temperature is θ ℃, Rθ=ρθCjL/A

ρθ————Resistivity when the conductor temperature is θ ℃, ρθ =ρ20［1 + α(θ － 20)］

Cj——twisting coefficient, single-strand conductor is 1, multi-strand conductor is 1.02

ρ20———The resistivity of the conductor when the temperature is 20℃, the aluminum core is 2.82 μΩ·μm, and the copper core is 1.722 μΩ·μm

α———Resistance temperature coefficient, copper and aluminum are both 0.004

θ———actual working temperature of the conductor, ℃

kjf———Skin effect coefficient, obtained by looking up the table

klj———Proximity effect coefficient, the busbar is 1.03

In order to simplify the calculation, when calculating the conductor reactance, the line capacitance is ignored and only the line inductance is calculated.

For a 50 Hz system, the busbar inductive reactance is calculated as follows:

Where Dj is the geometric mean distance, cm

b——busbar thickness, cm

h———busbar width, cm

The above is the calculation method of line impedance (positive sequence, negative sequence) of the line and busbar. The calculation method of zero-sequence resistance and zero-sequence reactance of the phase line and protection line is the same as the calculation method of positive and negative sequence resistance and reactance, except that the geometric mean distance Dj is replaced by D0 when calculating the zero-sequence reactance X(0)ph of the phase line and the zero-sequence reactance X(0)p of the protection line. D0 = the distance from the center of the phase line L1, L2, L3 to the center of the protection line PE or PEN line.

For a single-phase ground short circuit, the phase protection resistance and phase protection reactance of the busbar are:

3 Calculation Example

The following calculation example illustrates the configuration principles and characteristics of single-phase grounding protection on the low-voltage side of the transformer.

The transformer of a workshop substation is SCB9-1000 kVA, 10/0.4 kV, ud = 6%, ΔPk = 7.6 kW, overload factor is 3, and the system short-circuit capacity Ssmin = 200 MVA on the high-voltage side of the transformer. The connection forms of the transformer are D, yn11 and Y, yn0 respectively. The low-voltage 380 V busbars are placed vertically in parallel, the busbar spacing is 350 mm, and the neutral line is 200 mm away from the side busbar. Other parameters are shown in Figure 1. The configuration of single-phase ground short-circuit protection at the low-voltage busbar of the transformer and the calculation of sensitivity are carried out as follows.

Figure 1 Schematic diagram of transformer

According to the above parameters, the system phase resistance Rs, phase reactance Xs, phase protection resistance Rphps, and phase protection reactance Xphps are calculated by equations (1) to (5); the transformer phase resistance RT, phase reactance XT, and phase protection resistance RphpT and phase protection reactance XphpT of the D,yn11 connection type are calculated by equations (6) to (8), (11), and (12). For the Y,yn0 connected transformer, the zero-sequence impedance is much larger than the positive-sequence impedance, and its value is provided by the manufacturer through testing. Therefore, its phase protection resistance RphpT and phase protection reactance XphpT are obtained by looking up the table based on the data provided by the manufacturer; the low-voltage bus phase resistance Rm, phase reactance Xm, phase protection resistance Rphpm, and phase protection reactance Xphpm at 20°C are calculated by equations (13) to (16). The calculation results are shown in Table 1.

Table 1 Low voltage busbar parameters at 20 ℃

According to the data in Table 1, the impedance, short-circuit current, overcurrent protection and low-voltage single-phase grounding protection values ​​of the transformer low-voltage side bus fault are calculated as shown in Table 2.

From the example analysis, it can be concluded that for distribution transformers with the same capacity, the same voltage ratio and the same percentage of reactance, but with different connection types, the short-circuit current, protection form and setting value on the high-voltage side are the same, and the calculation is exactly the same. The main difference between the transformer with D, yn11 connection and the transformer with Y, yn0 connection is the zero-sequence impedance of the transformer. Since the transformer with Y, yn0 connection is electrically connected to the low-voltage system, its zero-sequence impedance value is much higher than the positive-sequence impedance, which leads to large phase protection resistance and phase protection reactance values; while the zero-sequence impedance value of the transformer with D, yn11 connection is not much different from the positive-sequence impedance, and the value can be taken as equal to the positive-sequence impedance value when estimating, and the corresponding phase protection resistance and phase protection reactance values ​​are equal to the phase resistance and phase reactance values.

Table 2 Calculation value table of low voltage side bus fault

It can be seen from Table 2 that the phase protection resistance RphpT = RT = 1.22 mΩ and the phase protection reactance XphpT = XT = 9.52 mΩ of the D, yn11 connection transformer; while the phase protection resistance RphpT = 3.39 mΩ, the phase resistance RT = 1.22 mΩ, the phase protection reactance XphpT = 42.71 mΩ and the phase reactance XT = 9.52 mΩ of the Y, yn0 connection transformer. This is exactly what leads to the great difference in the single-phase grounding fault current of the 380 V low-voltage busbar of the D, yn11 connection and the Y, yn0 connection transformer. D, yn11 connection I22k1 = 1 99* A; Y, yn0 connection I22k1 = 4 862 A. D, yn11-connected low-voltage busbar single-phase grounding fault current is relatively large, and the sensitivity of the high-voltage side three-phase overcurrent protection and low-voltage side neutral point single-phase grounding protection fully meets the requirement Ksen = 1.81 > 1.5, and there is no need to separately configure a dedicated neutral point current transformer and protection relay; due to the Y, yn0-connected low-voltage busbar single-phase grounding fault current is small, resulting in the high-voltage side three-phase overcurrent protection and low-voltage side single-phase grounding protection function The sensitivity does not meet > 1.5 (Ksen = 0.51), therefore, it is necessary to separately configure the low-voltage side single-phase grounding protection.

4 Conclusion

Through the above calculation example analysis: for transformers with D, yn11 connection, the three-phase overcurrent protection on the high-voltage side can be used as single-phase grounding fault protection on the low-voltage side, and the sensitivity fully meets the requirement of >1.5. There is no need to separately configure a dedicated neutral point current transformer and protection relay, which saves equipment configuration and wiring, and also reduces operating costs; for transformers with Y, yn0 connection, due to the large phase protection impedance of the transformer, the short-circuit current of the low-voltage single-phase grounding fault is small, and the sensitivity of using the three-phase overcurrent protection on the high-voltage side as single-phase grounding fault protection on the low-voltage side does not meet the requirement of >1.5. Single-phase grounding protection must be configured on the low-voltage side separately, which increases equipment and wiring, and increases operating and maintenance costs. 10/0.4kV distribution transformers with two wiring types, D, yn11 and Y, yn0, are widely used in industrial and civil construction fields. With the increase of power load, the impact of single-phase grounding fault on the low-voltage side increases. Combined with the rapid increase of electronic loads in recent years, the impact of harmonics generated has also increased. Although some compensation has been carried out on the spot, the impact on the system cannot be underestimated. According to the applicable occasions of 10/0.4kV distribution transformers with two wiring types, D, yn11 and Y, yn0, and combined with the analysis of this article, it is recommended to use distribution transformers with D, yn11 wiring type in places where there are no special requirements for wiring types, and use three-phase overcurrent protection on the high-voltage side as single-phase grounding protection on the low-voltage side, thereby improving the practicability and economy of distribution transformer protection, and also improving the operating efficiency of the transformer, making it safe, effective and reasonable.