Transformer neutral earthing¶
Model description¶
This case study examines the effect of earthing transformer neutrals (VTs) and including reactors in VT neutrals on the magnitude of single-phase short-circuit (SC) current to earth. The model represents a 110 kV CHPP switchgear (CHPP) with six generators of 50 MW each, from where electricity is transmitted through two 50 km long double-circuit overhead lines (overhead lines) to the power system. At the moment of time 0.5 s, a single-phase earth fault of 0.5 s duration occurs at the 110 kV busbar of the switchgear.
The main purpose of limiting single-phase fault currents is to bring their values into compliance with permissible values and to increase the reliability of equipment operation. The following scenarios are considered to compare the main methods of single-phase fault current limitation [1-3]:
Neutrals of all TRs are earthed;
Neutrals of only two TRs are grounded, the other four are ungrounded;
Reactors are included in all neutral points of TRs.
The process of starting and setting up the model from the script development environment using command control, processing of simulation results, visualisation of simulation results and suggested scripts for independent work with the model are also shown. Logging of current and voltage values is carried out and their time plots are shown. The model adjustment for each of the scenarios is carried out by changing the positions of switches in the subsystems "Earthing", connected to the neutral outputs of the TR. Model appearance:
The power system and generators are modelled by the Voltage Source (Three-Phase) block, the steady state is set by specifying the effective line voltage and phase shift. The forward and zero-sequence impedances of the power system are modelled by the Coupled Lines (Three-Phase) block. Short circuits are modelled by the Fault (Three-Phase) block, in the settings of this block using the Failure mode drop-down menu it is possible to select the type of short circuit. Transformers are modelled with Two-Winding Transformer (Three-Phase).html). Overhead lines are modelled by the Double-Circuit Transmission Line block, which is a U-shaped substitution diagram taking into account the mutual induction between circuits. The system parameters are:
| Element | Parameter |
|---|---|
| Power system | Equivalent EMF $E = 115\angle-44.9° кВ$ |
| Generators 1-6 | Equivalent EMF $E = 10.5\angle0° кВ$ |
| Overhead Line 1,2 | $L = 50км$ AC 240/32 |
| TR 1-6 | TD-63000/110 |
Experience when earthing all neutrals of TR¶
Import the necessary module for working with LaTeX graphs and lines:
using Plots
gr()
Loading the model:
model_name = "grounded_neutral_network_effect";
model_name in [m.name for m in engee.get_all_models()] ? engee.open(model_name) : engee.load( "$(@__DIR__)/$(model_name).engee");
The model is set up for the first experiment when all neutrals of the TR are grounded by changing the values of the constant blocks connected to the switches. If the constant value is zero, the breaker is in closed state, otherwise it is open. Thus, for earthing of the TP neutrals it is necessary to set the following values of constants:
# Замыкание всех нейтралей на землю
for i in 1:6
engee.set_param!(model_name*"/Заземление-"*string(i)*"/Constant "*string(i), "Value" => 0);
engee.set_param!(model_name*"/Заземление-"*string(i)*"/Constant_ react-"*string(i), "Value" => 1);
end
Running the loaded model:
results = engee.run(model_name);
To import the simulation results, logging of the required signals has been enabled in advance and their names have been set. Convert the instantaneous values of currents and voltages of the variable results into separate vectors:
# вектор времени симуляции
sim_time = results["i_a_kz"].time;
# ток в месте КЗ
i_fault = hcat(results["i_a_kz"].value,results["i_b_kz"].value,results["i_c_kz"].value);
# напряжение нейтрали ТР 6
Un = results["Un"].value;
# напряжение фазы B в месте КЗ
v_b_kz = results["v_b_kz"].value;
Graph of short-circuit currents at the fault location at the measuring point 2:
plot(sim_time, i_fault./1e3, label = [L"I_a" L"I_b" L"I_c"], title = "Токи КЗ",
linecolor = [:orange :green :red], ylabel = "I, кА", xlabel="Время, c", size = (700,440))
println("Ударный ток = "*string(round(maximum(i_fault)))*" А")
print("Действующее значение периодической составляющей = "*string(round(maximum(i_fault[8000:10000,:])/sqrt(2)))*" А")
When all transformer neutrals are earthed, the values of single-phase short-circuit currents are quite high. It is advisable to take measures to reduce these values in order to minimise the wear of the circuit breakers and the overloading of the equipment. For this purpose it is possible to apply partial earthing of transformer neutrals, which will lead to increase of total resistance of the network on zero sequence and decrease of single-phase short-circuit currents. The advantage of earthing all neutrals is the absence of overvoltages on TR neutrals and undamaged phases.
Experience with earthing of four TR neutrals:¶
Setting up the model for the experience of earthing four neutral points of TR is done by changing the values of the constant blocks connected to the circuit breakers. The following values of constants must be set for earthing of the TR neutrals:
# разземление части нейтралей
for (i,j) in zip(1:6,[0 0 1 1 1 1])
engee.set_param!(model_name*"/Заземление-"*string(i)*"/Constant "*string(i), "Value" => j);
engee.set_param!(model_name*"/Заземление-"*string(i)*"/Constant_ react-"*string(i), "Value" => 1);
end
Run the loaded model and import the results:
results = engee.run(model_name);
# вектор времени симуляции
sim_time = results["i_a_kz"].time;
# ток в месте КЗ
i_fault = hcat(results["i_a_kz"].value,results["i_b_kz"].value,results["i_c_kz"].value);
# напряжение нейтрали ТР 6
Un = results["Un"].value;
# напряжение фазы B в месте КЗ
v_b_kz = results["v_b_kz"].value;
Graph of short-circuit currents at the fault location at the measuring point 2:
plot(sim_time, i_fault./1e3, label = [L"I_a" L"I_b" L"I_c"], linecolor = [:orange :green :red],
title = "Токи КЗ", ylabel = "I, кА", xlabel="Время, c", size = (700,440))
println("Ударный ток = "*string(round(maximum(i_fault)))*" А")
print("Действующая пероидическая составляющая = "*string(round(maximum(i_fault[8000:10000,:])/sqrt(2)))*" А")
By earthing the neutrals of the four TRs, the short-circuit currents were reduced by a factor of approximately 1.5.
Graph of voltage between ground and neutral of TR 6:
plot(sim_time, Un./1e3, label = L"U_n", title = "Напряжение нейтрали", ylabel = "U, кВ", xlabel="Время, c", size = (700,440))
println("Действующее напряжение на нейтрали = "*string(round(maximum(Un)/sqrt(2)))*" В")
println("Коэффициент замыкания на землю = "*string(round(maximum(v_b_kz)/(110e3*sqrt(2/3)),digits=3)))
The reverse side of partial earthing of TP neutrals is the appearance of voltage on the ungrounded neutrals. It is necessary to take this fact into account when selecting measures to limit single-phase short-circuit currents and not to allow exceeding long-term permissible levels of industrial frequency voltages on the neutral of TRs. Grounding through reactors can be used to reduce the values of voltages on TR neutrals. Another problem is the increase of voltages of undamaged phases (in this example, phases B and C). The value of the earth fault coefficient is greater than 1.4, which is unacceptable under the condition of operation of valve arresters and surge arresters.
Experience with earthing of all TRs through reactors¶
Reactors with inductive resistance of 10 Ohm were chosen for block TRs. Adjustment of the model for the experience at grounding of TR neutrals through reactors is carried out by changing the values of the blocks of constants connected to the switches. For connection of reactors it is necessary to set the following values of constants:
# подключение реакторов
for i in 1:6
engee.set_param!(model_name*"/Заземление-"*string(i)*"/Constant "*string(i), "Value" => 1);
engee.set_param!(model_name*"/Заземление-"*string(i)*"/Constant_ react-"*string(i), "Value" => 0);
end
Running the loaded model and importing the results:
results = engee.run(model_name);
# вектор времени симуляции
sim_time = results["i_a_kz"].time;
# ток в месте КЗ
i_fault = hcat(results["i_a_kz"].value,results["i_b_kz"].value,results["i_c_kz"].value);
# напряжение нейтрали ТР 6
Un = results["Un"].value;
# напряжение фазы B в месте КЗ
v_b_kz = results["v_b_kz"].value;
Short-circuit current plot at the short-circuit point at the measuring point 2:
plot(sim_time, i_fault./1e3, label = [L"I_a" L"I_b" L"I_c"], title = "Токи КЗ",
linecolor = [:orange :green :red], ylabel = "I, кА", xlabel="Время, c", size = (700,440))
Graph of voltage between earth and neutral TR 6:
plot(sim_time, Un./1e3, label = L"U_n", title = "Напряжение нейтрали", ylabel = "U, кВ", xlabel="Время, c", size = (700,440))
println("Действующее напряжение на нейтрали = "*string(round(maximum(Un)/sqrt(2)))*" В")
println("Коэффициент замыкания на землю = "*string(round(maximum(v_b_kz)/(110e3*sqrt(2/3)),digits=3)))
Connection of reactors in the neutral of the TR leads to an increase in the total resistance of the zero sequence relative to the fault location, due to which the values of single-phase fault currents are reduced. Compared to partial earthing of TR neutrals, this method allowed to significantly reduce overvoltages on the neutral and undamaged phases with insignificant increase of short-circuit currents.
Addendum¶
Try changing the following model parameters yourself and investigate how this affects the simulation results:
- decrease and increase the overhead line length by 25 km;
- the number of grounding neutrals of TRs;
- double the resistance of reactors.
Conclusions¶
In this example, the tools for command control of the Engee model and uploading of the simulation results were used, and work with the Plots module was shown. Measured currents and voltages were imported into the Workspace from the result variable, then plotted on time plots and as text. The influence on single-phase short-circuit currents of earthing of TR neutrals and inclusion of reactors in them is shown and analysed.
The use of reactors in TR neutrals turned out to be the most effective from the point of view of reduction of single-phase short-circuit currents and limitation of overvoltages on neutrals and undamaged phases. However, this measure requires additional capital and operating costs, so when choosing a method of limiting single-phase short-circuit currents, each case should be considered individually, taking into account all the features of the network scheme, equipment characteristics and requirements of regulatory documents.
References¶
- RD 34.20.176. Guidelines for limiting single-phase short-circuit currents in 110 - 220 kV power grids of power systems" (approved by the Ministry of Energy of the USSR on 10.12.1984) URL: https://meganorm.ru/Data2/1/4294817/4294817287.htm (accessed on 02.12.2024).
- STO 34.01-21.1-001-2017. Distribution electric networks with voltage 0,4-110 kV. Requirements for technological design. URL: https://www.rosseti.ru/upload/iblock/c59/3yblo2sg3d5w1jd1qzun11h05ypjx66f/СТО%2034.01-21.1-001-2017v2022.pdf (дата обращения 09.12.2024)
- Order of the Ministry of Energy of the Russian Federation from 15.01.2024 No. 6 "On Approval of Methodological Guidelines for the Technological Design of AC Substations with the Highest Voltage of 35 - 750 kV". URL: http://publication.pravo.gov.ru/document/0001202407020008 (date of circulation 09.12.2024)