Zinc Oxide Nonlinear Resistor Test Power Supply System

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Zinc Oxide Nonlinear Resistor Test Power Supply System [Copy link]

Abstract: Zinc oxide nonlinear resistors are widely used in power system overvoltage protection and surge energy absorption. A power supply for testing them is studied. The test power supply uses a capacitive reactor to provide the surge energy required for nonlinear resistor testing. The test results show that the test power supply works reliably and can complete the effective test of zinc oxide nonlinear resistors.

Keywords: nonlinear resistor; test power supply; energy capacity

　

0 Introduction

Zinc oxide nonlinear resistor is a kind of varistor, which has been used for power system protection for more than 30 years. It has a series of advantages such as good protection effect, energy saving and low price. Therefore, it plays an irreplaceable protective role in generator rotor overvoltage protection, residual magnetism absorption and lightning arrester [1][2]. Due to the existence of inductive components in the power system, the occurrence of fault current in power equipment will lead to serious overvoltage phenomenon. Therefore, suppressing overvoltage is extremely important for the safety of equipment and operators [3]. With the rapid development of my country's power industry, the capacity of the power grid is constantly expanding, and the single-unit capacity of the generator is also increasing. In order to ensure the safe operation of the power grid, the rapid demagnetization and overvoltage protection of the generator are becoming more and more important.

ZnO resistors have large energy capacity and good current flow performance, and can play a role in rapid demagnetization. The uniformity of the ZnO resistor structure has a direct impact on its energy capacity, and poor uniformity will reduce its ability to absorb energy. The purpose of testing the power supply system is to simulate the instantaneous energy absorbed by ZnO during rapid demagnetization, monitor the working conditions of the ZnO resistor, and obtain test results and parameters.

1 Basic Principles of Circuit

The test power supply consists of three parts: rectification, commutation, and discharge, as shown in Figure 1. The three-phase AC charges the reactor L through the rectifier bridge. After L is charged, the commutation circuit (K in Figure 1) operates to disconnect L from the rectifier bridge and discharge the ZnO nonlinear resistor to complete the test. The reactor L is the core of the entire power supply, and its reasonable design plays a decisive role in the performance of the test power supply. Therefore, the reactor design is the core of the test power supply design.

Figure 1 Schematic diagram

2 Optimization design of reactor L

The DC power supply in the schematic diagram is obtained by 380V three-phase rectification, that is,

U _d =1.35 U _2L cos α (1)

The energy stored in the reactor (i.e. the energy capacity of the resistor valve under test) is

W = (1/2) LI ² (2)

Where: I is the current that the resistor valve plate under test can withstand in a short time.

The resistance of the reactor is

R _L = U _d / I（3）

_{From equations (1) to (3 ) , we can derive the parameters} L and RL required for designing the reactor .

If the reactor is designed with W = 20 kJ, I = 200 A, then from equations (1) to (3) we can get L = 1 H, R _L = 2.55 Ω.

In the process of designing the reactor, many factors need to be considered. To ensure that the power supply meets the test requirements, L >1H, R _L <2.4Ω. We first adopted a rectangular cross-section design. After many tests, we found that it was difficult to meet the requirements, so we switched to a square cross-section design and finally designed a reactor that met the requirements. The cross-section of the reactor coil is shown in Figure 2.

Figure 2 Cross-sectional view of reactor coil

2.1 Selection of wire model

BVR type conductor is used, and the parameters are as follows:

Cross-sectional area S = 35 mm ² ;

The maximum outer diameter of the conductor is _dm = 12.5mm;

Conductor resistivity ρ = 0.0217 × 10 ^－6 Ω·m;

Coil winding coefficient K = 1.05;

The axial number of layers and radial number of turns of the coil are equal, and the radial number of turns is 32 turns, so

Number of coil turns N = 32 × 32 = 1024;

Axial height a = 12.5 × 32 × 1.05 = 420 mm = 0.42 m;

Radial width b = 12.5 × 32 × 1.05 = 420 mm = 0.42 m;

Coil inner diameter d ₁ =0.76m;

The average diameter of the coil is d = d ₁ + b = 0.76 + 0.42 = 1.18m;

The total length of the coil l =πdN=π×1.18×1024=3796m (take 3800m);

Coil resistance R1 = ρl / s = 0.0217×10-6 ^{× 3800} _/ 35×10-6 ⁼ 2.356Ω.

2.2 Check whether the inductance value meets the requirements

The calculation formula for inductance is as follows:

L = N ² dΦ（4）

Where: Φ is a coefficient determined by the coil structure, which can be found in the inductance calculation manual as Φ=16.26;

N is the number of coil turns;

d is the average diameter of the coil;

μ ₀ is the magnetic permeability of air, and its value is 4π×10 ^－7 .

The coil inductance is

L = N ² dΦ = ×1024 ² ×1.18×16.26×10 ^-7 =1.006H,

The coil inductance meets the requirements.

3 Commutation circuit principle

The commutation circuit is shown in Figure 3. It is required that when the reactor L is fully charged, that is, when the DC current reaches I , the main circuit is cut off to allow the reactor to discharge to the zinc oxide resistor valve plate. The commutation circuit uses the method of reverse blocking the thyristor by the LC oscillation circuit.

Figure 3 Circuit diagram of commutation circuit

When L is fully charged, the logic control on the secondary side activates relay ZJ, V3 _is turned off, and V2 _is turned on. At this time, the forward current exists and V1 is still turned on. The oscillation circuit composed of L1 and C _begins to oscillate, _{and capacitor} C begins to discharge through L1 . The current direction is opposite to that of the main circuit current. When the current value flowing through V1 _drops to 0, V1 _will be forced to turn off, and the commutation process ends. This requires C to complete charging before L.

Since the time constant of the reactor L is τ _L = L / R _L , the charging time t ≈ 4τ _L . Then the time constant of the capacitor C is τ _C = τ _L / 4 = R ₁ C .

In order to ensure that the oscillation circuit can reliably block the main circuit, its peak oscillation current is set to 1.5 I , that is,

I _{m L 1C} = U _d / ωL ₁

Oscillator circuit frequency ω = 1/

L ₁ = CU _d ² / I _{m L 1} C ₂

In Figure 3, _R5 is a zinc oxide nonlinear resistor that provides overvoltage protection for the main circuit, and its voltage level is higher than that of the resistor to be measured. When the nonlinear resistor to be measured fails, R5 can limit the high voltage across the reactor.

4 Experiments

In order to verify the feasibility of the above test power supply system, a test was conducted, and the test circuit parameters are as follows:

1) The energy of the zinc oxide resistor valve plate under test is W=20kJ;

2) Test the DC side current of the power supply I (>=) 200A;

3) Electrical parameters of the reactor: L (>=) 1H, R _L (<=) 2.55Ω;

_{4 )} Commutation circuit L1 = _0.43mH , C = 150μF, R1 = 550Ω.

_{The test shows that when the DC} side current reaches 200A, the AC side current amplitude is 245A. Based on this, the current transformer on the AC side is adjusted to make the current relay actuate, the control circuit drives the relay ZJ to actuate, and the L1C oscillation circuit begins forced commutation. The test shows that the capacitor C is charged before L , and its reverse oscillation current can force the thyristor to turn off, disconnecting the main circuit, and forced commutation can proceed smoothly, so that the reactor energy is injected into the zinc oxide nonlinear resistor valve plate, and the test can be completed.

Figure 4 is a diagram of commutation current. Figure 5 is a diagram of reactor current.

Figure 4 Commutation current diagram

Figure 5 Reactor current diagram

5 Conclusion

After the test, this power supply meets the requirements of testing zinc oxide nonlinear resistors, and the test results are consistent with the calculations. By adjusting the parameters, the working conditions of the system can be adjusted. The test system is feasible and has a reasonable and simple structure.

　

About the Author

Zhang Chenghui, professor, is a young and middle-aged expert in the field of power electronics and power supplies in China. He has published nearly 100 papers in famous academic journals at home and abroad.

Du Chunshui, researcher, researcher at B&R Modern Motion Control Laboratory.

Ma Tongxing is a graduate student who is engaged in the research of multi-motor synchronous control at the B&R Modern Motion Control Laboratory of the School of Control Science at Shandong University.