Homepage » Analysis of Multiple Metal Oxide Varistor (MOV) in Parallel in Surge Protective Device

Created by: Glen Zhu | Updated Date: May 28^{th}, 2024

To study the three parallel Metal Oxide Varistor (MOV), different surge protective device (SPD) lifespan parameters were caused by inconsistent MOV voltages due to insufficient screening. Now a series of experiments are being designed with the following test conditions:

(1) Use three parallel Metal Oxide Varistor (MOV) of the same model produced by the same manufacturer. After screening, control the voltage difference between the three Metal Oxide Varistor (MOV) at 1V, 5V, and 10V. The voltage values of the Metal Oxide Varistor (MOV) used in the experiment are shown in the table. Group A has an error of 1V, group B has a difference of 6V, and group C has a difference of 10V.

Metal Oxide Varistor voltage (V) | ||||

Serial number | 1 | 2 | 3 | Parallel connection |

Group A | 619 | 619 | 620 | 589 |

Group B | 615 | 621 | 620 | 588 |

Group C | 610 | 619 | 620 | 592 |

Table 1 – Initial parameter values of Metal Oxide Varistor (MOV) selected during the experiment

**Experimental Phenomena and Analysis**

1. Test results of Metal Oxide Varistor (MOV) voltage

According to the experimental principle, the final test data is now obtained as shown in Table 2.

Metal Oxide Varistor voltage (V) | ||||

Serial number | 1 | 2 | 3 | Parallel connection |

Group A | 610 | 612 | 615 | 600 |

Group B | 584 | 616 | 612 | 591 |

Group C | 562 | 607 | 606 | 574 |

Table 2 – Metal Oxide Varistor (MOV) voltage values after 25-time impacts

1.2 Analysis of Voltage Variation of Group C Metal Oxide Varistor (MOV)

Now, the C group with the most obvious trend of change will be taken as the analysis object, and the change trends of Metal Oxide Varistor (MOV) voltage values for three individual chips, C1, C2, and C3, are plotted in Figure 1.

From Figure 1, it can be seen that:

(1) Due to the slight difference in diversion amplitude, according to the theory in the literature, for each piece of diversion: I_{n}=I_{a}+(U-U_{an})/R_{zn} (1) where R_{zn} is the dynamic resistance value, I_{a} is the impulse current value, U is the overall residual voltage value, and U_{an} is the residual voltage value per piece. Therefore, due to different aging effects on each piece, with an initial Metal Oxide Varistor (MOV)voltage difference of 10V, after 3 sets of impacts, this pressure difference has narrowed down to 5V.

(2) According to IEC61643-11, the variation range of U_{1mA} is ±20%. The C1 chip Metal Oxide Varistor (MOV) reaches the limit value of Metal Oxide Varistor (MOV) voltage degradation first. After exceeding the limit value, the C1 chip Metal Oxide Varistor (MOV) degrades at a rate of approximately 1% per group of impacts. Under the U_{1mA} indicator, the degree of degradation of C1 Metal Oxide Varistor (MOV) after testing is about 2.7 times that of the other two chips.

It can be concluded that when Metal Oxide Varistor (MOV) with lower initial voltage are used in parallel, they will deteriorate first and to a greater extent than the other Metal Oxide Varistor (MOV).

To study the inconsistency of Metal Oxide Varistor (MOV) voltage caused by insufficient screening in a 3-parallel configuration, resulting in different product life parameters. Now design a series of experiments, with the following experimental conditions:

(1) Use three parallel Metal Oxide Varistor (MOV) of the same model produced by the same manufacturer. After screening, control the voltage difference between the three Metal Oxide Varistor (MOV) at 1V, 5V, and 10V. The voltage values of the test Metal Oxide Varistor (MOV) are shown in the table. Group A has an error of 1V, group B has a difference of 6V, and group C has a difference of 10V.

Metal Oxide Varistor (MOV) voltage (V) | ||||

Serial number | 1 | 2 | 3 | Parallel connection |

Group A | 619 | 619 | 620 | 589 |

Group B | 615 | 621 | 620 | 588 |

Group C | 610 | 619 | 620 | 592 |

Leakage current (μA) | ||||

Serial number | 1 | 2 | 3 | Parallel connection |

Group A | 10.1 | 9.66 | 11.1 | 31.8 |

Group B | 16.4 | 8.94 | 9.10 | 37.4 |

Group C | 12.3 | 9.46 | 8.05 | 33.0 |

Table 3 – Initial parameter values of the Metal Oxide Varistor (MOV) film selected for the experiment

(2) Perform an 8/20μs waveform test on each group of parallel-connected Metal Oxide Varistor (MOV), with a current impulse I_{n}=40kA. Conduct the test in cycles of 5 times, with cooling and measurement of the voltage, leakage current, and nonlinear coefficient α for both the entire unit and each Metal Oxide Varistor (MOV) during intervals between cycles.

(3) Compare the inter-group variation rate of the overall DC parameter values after parallel connection.

(4) Compare the changes of each Metal Oxide Varistor (MOV) within the group.

**Analysis of Voltage Variation in Groups A, B, and C Metal Oxide Varistor (MOV)**

The overall voltage change curve of the combined Metal Oxide Varistor (MOV) in groups A, B, and C is shown in Figure 2.

As shown in Figure 2:

(1) The overall Metal Oxide Varistor (MOV) voltage value of the 3 parallel connected pieces is about 30V lower than each piece.

(2) Although each group is connected in parallel with Metal Oxide Varistor (MOV) of different voltage differences, the overall voltage difference of the three groups of samples is within 4V. In the first 8 impacts, the differences are minimal, indicating that the performance quality of a single Metal Oxide Varistor (MOV) has little impact on the overall performance in the early stage of impact.

(3) The degradation of Group C samples occurred first due to the parallel connection of a Metal Oxide Varistor (MOV) with a voltage lower than the standard 10V. The degradation rate of Group B samples was moderate. After the experiment, the degree of degradation in Group C samples was the highest, being 1.7 times that of Group A.

Therefore, the combination of Metal Oxide Varistor (MOV) with lower voltage in parallel will reduce the overall performance and lifespan.

**Analysis of Other Phenomena and Inference Changes**

From the data, it can be seen that the initial Metal Oxide Varistor (MOV) voltage of A3, B3, and C3 slices in each group is 620V. The final values of Metal Oxide Varistor (MOV) also show different changes. Plot the trends of their degradation as shown in Figure 3.

A3, B3, and C3 all show deterioration after multiple impacts. Among them, the parallel-connected C group with a voltage difference of 10V has the fastest and most severe degradation in C3 chips, followed by B3. From this inference, it can be concluded that a parallel connection with a lower voltage Metal Oxide Varistor (MOV) will degrade the performance of the remaining chips.

**Analysis of Leakage Current Changes**

According to the experimental principles, the final test data obtained are shown in Table 4.

Leakage current (μA) | ||||

Serial number | 1 | 2 | 3 | parallel connection |

Group A | 11.7 | 10.4 | 10.8 | 33.4 |

Group B | 20.4 | 9.74 | 9.35 | 39.8 |

Group C | 25.3 | 12.1 | 13.7 | 51.2 |

Table 4 – Leakage Current Variation after 25-time Impacts

It can be seen from the table that the total leakage current after parallel connection is slightly greater than the sum of 3 pieces; The direction of the leakage current test is opposite to the change curve of Metal Oxide Varistor (MOV) voltage, and the trend is the same as shown in Figure 4.

**Simulation solution**

(1) The source impedance of the generator is 0.431Ω and remains unchanged, analyzing the relationship between residual voltage and current change with different quantities of parallel-connected Metal Oxide Varistor (MOV).

(2) The charging voltage is fixed at 15.214kV, and common parallel situations of 1, 3, and 5 cells are simulated to compare and analyze the changes in residual pressure and current flow.

(3) Collect residual pressure and current waveform for intuitive comparative analysis.

**Simulation Results and Analysis**

The residual voltage and current waveform collected by the circuit shown in Figure 5 with five parallel simulation circuits are shown in Figures 6 and 7.

From the above figure, the following conclusions can be drawn:

(1) The difference in current-carrying capacity under different parallel situations is very obvious. When 5 pieces are connected in parallel instead of 1 piece, the current carrying capacity increases by 1.6kA, and compared to 3 pieces, it increases by 0.36kA.

(2) From the graph, it can be seen that the residual voltage when a single flow is passing is 2.1497kV, compared to when five flows are connected in parallel, the residual voltage decreases significantly to 1.3948kV. It crosses the insulation withstand level II and III boundary of 1.5kV.

(3) The parallel use of Metal Oxide Varistor (MOV) can reduce dynamic impedance, directly leading to an increase in current flow and a decrease in residual voltage, with a significant change range of about 35%.

**Analysis of the trend of current flow and residual pressure**

**Simulation Plan**

(1) Using the circuit in 2.2, the charging voltage remains fixed at 15.214kV unchanged, simulating the cross-sectional residual voltage and current change trends for parallel situations of 1, 3, 6, 12, 18, and 24 pieces.

(2) Collect residual pressure and current waveform, process the data statistically, and draw trend line analysis.

**Simulation Results and Analysis**

From a large amount of simulation data analysis, it can be seen that:

The trend of changes in flow and residual pressure is opposite, but the rates of change are similar.

(1) Analyzing Figure 8, it can be observed that under a constant charging voltage, as the number of parallel Metal Oxide Varistor (MOV) increases, the current through the Metal Oxide Varistor (MOV) continues to rise while the residual voltage decreases. The trend line is monotonically nonlinear.

(2) When the number of parallel cells is 1-5, the absolute values of current and residual pressure change slopes are greater compared to 5-24 cells.

(3) From the graph, it can be seen that the optimization range of current and residual pressure obtained from 3 parallel connections is equivalent to the optimization range of 3-24 connections.

It can be seen that the parallel connection of different numbers of Metal Oxide Varistor (MOV) has a significant impact on the actual current flow and residual voltage results. Considering engineering practicality and cost control, the parallel form with 2-5 pieces is the most reasonable and economical design.

**Changes in the voltage-current characteristics**

**Simulation Plan**

(1) Using the circuit described in 3.2, simulate the parallel connection of modules 1 and 6 separately, with current flow through them being 5kA, 10kA, 20kA, and 30kA respectively.

(2) Collect the residual peak value, and plot the data to draw the volt-ampere characteristic curve for analysis.

**Simulation Results and Analysis**

According to the simulation scheme, the drawn current-voltage characteristic curve is shown in Figure 9.

From the graph, we can draw the following conclusions:

(1) 6 pieces of Metal Oxide Varistor (MOV) connected in parallel also exhibit nonlinear U-I characteristics, without residual voltage inflection points due to the rise or fall of the impulse amplitude, proving that the optimization of residual voltage by parallel Metal Oxide Varistor (MOV) exists at each impulse amplitude.

(2) From the perspective of trends analysis, the greater the impact amplitude value, the greater the reduction in residual stress optimization.

**1. Metal Oxide Varistor (MOV) impact verification**

**1.1 Experimental Principles and Plans**

The purpose of the Metal Oxide Varistor (MOV) test is to verify the above theories and simulation analyses, mainly focusing on the following inferences:

(1) Using 3-5 pcs Metal Oxide Varistor (MOV) in parallel can significantly reduce residual voltage by about 35% and increase current flow compared to using only 1 Metal Oxide Varistor (MOV).

(2) The influence of many parallel Metal Oxide Varistor (MOV) on residual voltage and current flow results in monotonic nonlinearity.

(3) Parallel circuits have nonlinear voltage-current characteristic curves.

Regarding inferences (1) and (2), the Haefely PSURGER30.2 generator with a source impedance of 0.432W is used. The EPCOS standard Metal Oxide Varistor (MOV) B32K385/EPC is selected as the test sample. The expected short-circuit current of the equipment is 33.5kA, and under this current amplitude shock, it can make one piece of Metal Oxide Varistor (MOV) achieve a continuous current flow of 30.01kA.

Test the impact resistance of Metal Oxide Varistor (MOV) with 1, 3, 5, 12, 18, and 24 parallel connections. Conduct five impacts for each parallel connection method and perform statistical analysis on the average values. Ensure that the selected test sample U_{1mA} for each piece is within the range of 620±5V to achieve current sharing among the parallel connections.

**1.2 Test Result Analysis**

The data obtained from verification experiments for Inference 1 and 2 are shown in Table 5.

Project | Parallel situation | |||||

1piece | 3pieces | 5pieces | 12pieces | 18pieces | 24pieces | |

Residual pressure /kV | 2.11 | 1.51 | 1.36 | 1.22 | 1.13 | 1.11 |

electric current /kA | 30.01 | 31.26 | 31.63 | 32.00 | 32.12 | 32.23 |

Table 5 – Test data of the different numbers of slices Metal Oxide Varistor (MOV) at 30kA

As can be seen from the above table,

(1) The actual test shows that the residual pressure of 3-5 parallel Metal Oxide Varistor (MOV) is reduced by 34.59% compared to using only one piece, and the current flow is increased by 1.62kA, proving the validity of the simulation inference. The waveforms of residual pressure and current flow tested are shown in Figure 10.

Note: The waveform is taken from a Tektronix oscilloscope, and waveform restoration is performed using WaveStar for Oscilloscopes software. The current ratio is 100V/A.

(2) It can be seen from the above table that the changes in residual pressure and flow rate follow a monotonically nonlinear trend. By superimposing the measured residual pressure data with simulation results, a curve as shown in Figure 11 is plotted, indicating a high degree of consistency between the two sets of data.

Experimental verification of inference (3): Conduct impact tests with current amplitudes of 5, 10, 20, and 30 kA on parallel-connected Metal Oxide Varistor (MOV) consisting of one and six pieces. The residual voltages are recorded in the table below:

Impact current（kA） | 5 | 10 | 20 | 30 |

Actual measurement（1 piece）kV | 1.33 | 1.52 | 1.84 | 2.11 |

Actual measurement（6 pieces）kV | 1.00 | 1.09 | 1.22 | 1.30 |

Residual pressure drop（ΔU）kV | 0.33 | 0.43 | 0.62 | 0.81 |

Table 6 – Residual Pressure at Different Amplitudes

(1) The graph shows that the measured and simulated current-voltage characteristics coincide.

(2) The blue diamond-shaped curve reflects the residual pressure difference between 6 pieces and 1 piece under different impact amplitudes. The change in residual pressure difference shows an approximately linear increasing trend, proving that the larger the impact amplitude, the more significant the optimization of residual pressure.

(3) Calculate the slopes of the three segments of the trend line ΔU to obtain slope values K respectively as: 0.02, 0.019, and 0.019. It can be seen that the slope remains unchanged, confirming that the voltage drop for six cells in parallel compared to one cell follows a linear change rate with a slope of K.

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