Analysis of 10KV distribution network closing device

In this paper, a line closing device is designed to accurately measure the parameters of the closing line in order to achieve the power supply interruption during the closing power exchange process and improve the reliability of power supply. The problem of high-voltage precision measurement and phase measurement during the measurement process is solved. At the same time, the device adopts wireless operation, which is simple and convenient.

At the same time, a power flow calculation software is designed and an effective system model is proposed. According to the current system operation status, the impact of loop closing on the system is analyzed, loop closing criteria are provided, and guidance is provided for loop closing operations.

The continuous development of the power grid system has put higher and higher requirements on power supply reliability and power quality. The 10KV distribution network generally adopts a closed-loop design and an open-loop operation mode to supply power. However, the distribution network uses the method of disconnecting the power supply and loading the load in sections during line maintenance and load switching. In order to reduce the power outage time and improve the power supply reliability, the live closing operation of the 10KV line is of great significance to improve the reliability and economy of power supply.

During the closing operation, the voltage at both ends of the closing point has amplitude difference and phase difference, i.e., vector voltage difference, and line impedance and other factors will affect the power flow distribution of the closing system. The device is designed to perform high-precision sampling with isolation and use a wireless handheld terminal for measurement operations. At the same time, the accuracy of the closing software is verified by comparing the amplitude difference and phase difference calculated by the power flow software with the data measured by the device. The software calculates parameters such as the current flowing through the closing point after the closing, providing a reliable criterion for the closing parameters.

1. Principle of Loop Point Parameter Measurement

1. Composition of the measuring device

The measuring device mainly consists of the following three parts: operating rod, measuring terminal and handheld terminal. The operating rod is an 8m telescopic rod used to hang the test device on the line measurement point, the measuring terminal is a cylinder used to sample line parameters and transmit data to the handheld terminal, and the handheld terminal is a handheld plastic box used to operate the test and analyze the measurement data.

2. Basic principles of device testing

The measuring device is hung on the same-phase line at both ends of the loop point through two measuring rods to measure the voltage phase of the line. The whole test process uses a handheld terminal to operate the measuring rod through wireless communication. When the whole device is tested, the operator is far away from the test site and operates wirelessly through the handheld terminal, which is safe and convenient.

Schematic diagram of the loop closing device 1

3. Voltage phase measurement at closing point

The 10kV line voltage phase measurement belongs to the measurement of electrical quantity under a high-voltage environment. Precision measurement under high voltage usually has the problems of difficult sampling and spatial interference. At the same time, the device measurement is a separate independent measurement. When performing phase comparison, it is impossible to use the comparison circuit for measurement, and the precise phase measurement is also relatively difficult. In order to ensure the accuracy of voltage measurement, the device uses electromagnetic sensor isolation measurement. After processing, the high-voltage signal passes through a high-precision traditional voltage level sensor. The sensor output is AD sampled after signal conditioning. At the same time, in order to ensure the accuracy of phase measurement, the measurement adopts synchronous measurement based on the orthogonal method. The synchronization time error is less than 2us, which brings an error of less than 2′, and has little effect on the phase measurement result.

The principle block diagram of the measuring rod is shown in Figure 2:

2. Analysis of the closed-loop system

1. Closed-loop system modeling

The basis of 10kV distribution network closed-loop system analysis is power flow calculation. The usual power flow calculation method is to find out the operating status of the entire network based on the given network structure and operation mode, including the voltage and current of each node in the system, as well as the power distribution and power loss on the line. The entire system is a problem of solving a set of multivariate nonlinear algebraic equations. However, due to the complex distribution of the distribution network system, it is impossible to accurately convert the system parameters. Therefore, the reasonable equivalent conversion of network system parameters and the establishment of network equivalent models are the key to the analysis of the entire closed-loop system, and are also the difficulty of the closed-loop system power flow calculation.

The network model of the closed-loop system is shown in Figure 3. The network structure of a 10kV overhead line is generally the same 220kV line passing through different 110kV substations, and then leading out various 10kV outgoing lines. Therefore, we can make the system equivalent as shown in the following figure:

As shown in the figure, the 220kV line is stepped down to 110kV by transformer T0, and then T1 and T2 share a 110kV busbar. Transformer T1 steps down to 10kV to supply power to the 10 outgoing lines on the left, and transformer T2 steps down to 10kV to supply power to the 10 outgoing lines on the right. The two outgoing lines in the middle are connected by a circuit breaker at the end of the line.

After the system is closed, the system power is redistributed, and the loads of the two substations are transferred to each other through the closing point to reach a new balance. During the entire closing process, the three-stage protection device of the line is required not to operate, and the line cannot be overloaded. Therefore, the current flowing through the closing point after the closing directly reflects the impact of the closing operation on the system.

2. Closed-loop steady-state current analysis

The power S' flowing through the closing point after the loop is closed has the following relationship with the current I' flowing through the closing point: I' = S'/3Up, where Up is the line voltage of the 10kV line. Since S' is not convenient to measure directly from the closing point, it cannot be used as a direct criterion. According to Kirchhoff's current principle, the closing current I can be regarded as a circulating current superimposed on the system.

There is a voltage difference ΔU at both ends of the closing point before the loop is closed. The circulating current formed after the loop is closed and ΔU satisfy the following relationship: Itotal=ΔU/Z, where Z is the loop impedance through which the circulating current flows, including the T1 transformer impedance R1+X1, the closing line impedance ZLD1, ZLD2 and the T2 transformer impedance R2+X2, that is, Z=R1+X1+ZLD1+ZLD2+R2+X2.

The equivalent circuit diagram of the closed-loop circulation is as follows:

Since the loop current formed after the ring closing does not flow through the lines outside the ring closing line, the load of the 110kV substation except the load of the ring closing line can be transferred and converted into a load branch. Therefore, the model of the entire system can be further simplified. As shown in Figure 5: SLD1 and SLD2 are the loads of the ring closing line, S1 is the load of the remaining outgoing lines of the left substation, and S2 is the load of the remaining outgoing lines of the right substation.

3. Analysis of loop closing criteria

Through the simplification of system model analysis and the study of closed-loop current calculation, we can see that the size of the closed-loop current is related to the following factors:

(1) Line load conditions;

(2) Line impedance;

(3) Transformer ratio and impedance.

The processing of the closed-loop load model of the distribution network is to use mathematical methods to transfer loads according to the load structure characteristics of the distribution network, improve the accuracy of the equivalent impedance parameters when modeling the system, and improve the accuracy of the power flow calculation.

The three factors that affect the closing result are ultimately reflected in the voltage difference on both sides of the closing point before the closing. After the closing line is selected, the line impedance parameters are basically fixed. Therefore, after the closing line is selected, the vector voltage difference ΔU of the lines at both ends of the closing point can be used as the closing criterion. The closing voltage difference ΔU and closing current under the current system state are calculated by the power flow calculation software, and the voltage difference ΔUmax corresponding to the maximum closing current allowed under the current network model is simulated by changing the load parameters.

4. Loop Closure Criteria Constraints

Condition 1: The maximum loop current allowed by the line has the following relationship with the current load current of the line and the allowable current of the line: Iclose max

Condition 2: The maximum current during the closing operation should avoid the third-stage line protection action value. The maximum transient current superimposed in the system during the closing operation is 1.8 times the closing steady-state current, that is, Ish=1.8×Iclose, so the line current should satisfy the following relationship: IⅢ>Ish+ILD, where IⅢ is the third-stage line protection action value.

Condition 3: Based on the maximum allowable closing current obtained from the line allowable current carrying capacity and the line protection action value, the allowable closing voltage difference ΔUmax is calculated using the power flow calculation software. If the on-site measured value is less than ΔUmax, the line is closed.

3. Loop closing site measurement and test operation

The measuring device has been tested to have an amplitude measurement accuracy of 0.5% and a phase accuracy of 0.1°, which can ensure that the measurement error has no effect on the closed-loop operating system. In the field test, we and the Yongchuan Power Supply Bureau of Chongqing Electric Power Company selected the closed-loop line formed by the Shengguang II line and the Yongyu line, which are representative of the line connection form, to conduct device measurement test operations and closed-loop operations to verify the availability of the entire system.

Test time: July 12, 2012

After measuring two sets of data for each phase, retrieve the load condition of the closing operation line at this time. Here, only the data of phase A is listed. The line load current read is the current of the phase A outgoing line. At 6:55, the load current of the Yongyu line is 18A, and that of the Shengguang II line is 66A. At this time, measure the voltage and phase of the lines on both sides of the closing point.

Table 1

Phase A

Measured time 6:556:56

Shengguang II phase voltage (kV) 6.1956.14

Yongyu line phase voltage (kV) 6.076.07

Phase difference (°) -0.53-0.76

Amplitude difference (kV) 0.125 0.069

Vector difference (kV) 0.1370.107

After the measurement, apply for loop closing operation, measure the voltage and phase after loop closing, and retrieve the line load current after loop closing. The A phase current of Shengguang II line after loop closing is 60.1A, and the A phase current of Yongyu line is 30.6A.

Table 2

Phase A

Measurement time 7:107:25

Shengguang II phase voltage (kV) 5.9946.022

Yongyu line phase voltage (kV) 6.0566.063

Phase difference (°) 0.10.1

Amplitude difference (kV) 0.059 0.041

Vector difference (kV) 0.059 0.041

The loop was released 20 minutes after the loop was closed. At 07:27, the line status was closed, and the line current was read: the A-phase current of Shengguang II line was 66.8A, and the A-phase current of Yongyu line was 28.1A. At 07:30, the line status was released, and the line current was read: the A-phase current of Shengguang II line was 67.5A, and the A-phase current of Yongyu line was 20A.

Table 3

Phase A

Measurement time 7:25 7:30

Shengguang II phase voltage (kV) 6.0226.081

Yongyu line phase voltage (kV) 6.0636.022

Phase difference (°) 0.10.35

Amplitude difference (kV) 0.0410.059

Vector difference (kV) 0.0410.069

Analysis of the above data led to the following conclusions: Due to the large unevenness of the transformer contacts and line load conditions in the early morning test, the measured voltage difference at both ends before closing the loop was large, and the estimated closing current was also relatively large. However, the contactors such as the isolating switch at the closing point were severely corroded and had a large contact impedance, causing the impedance of the entire closing loop to be much larger than the ideal one. Therefore, the closing current at that time was very small, about 10A.

The on-site closing and opening operations were successful and the system worked stably, which verified the accuracy of the measuring device and the convergence of the closing criteria obtained from the tide closing system analysis.

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