Line-commutated converter-voltage source converter (LCC-VSC) power transmission technology does not have the problem of communication failure very usually. It therefore can support the long-distance, long-capacity transmission of electric energy. However, factors such as topology, control strategy, and short-circuit capacities make the traditional protection principles not fully applicable to LCC-VSC hybrid transmission systems. To enhance the reliability of hybrid DC systems, a single-ended principle based on transmission coefficients is proposed and produced. First, the equivalent circuit of the LCC-VSC hybrid DC system is analyzed and the expression of the first traveling wave is deduced accordingly. Then, the concept of multi-frequency transmission coefficients is proposed by analyzing the amplitude-frequency, and the characteristics of each element. Finally, the LCC-VSCDC system model is built to verify the reliability and superiority of the principle itself. Theoretical analysis and experimental verification show that the principle has strong interference resistance.

In recent decades, high voltage direct current (HVDC) transmission technology based on line-commutated converter (LCC) converters has become the mainstream solution for DC lines. However, with the continuous construction of high-voltage DC lines, the problem of commutation failure caused by the centralized feeding of multiple DCs has become increasingly prominent, which seriously impacts the security and stability of the power grid. The currently emerging three-terminal hybrid DC transmission technology uses the LCC sending end to connect two modular multilevel converters (MMC) receiving ends. The advantage that the MMC receiving end does not have commutation failure has become a new method to solve the commutation failure problem of multi-circuit centralized DC feeding into the power grid [

DC line protection is the first line of defense to ensure the safe and stable operation of DC lines. At present, the traveling wave protection principle of DC transmission lines includes two categories: Use time-domain information such as amplitude and differential and use frequency domain information such as frequency difference [

Aiming at the insufficient sensitivity of the principle-based on abrupt changes in the time domain, scholars proposed a protection principle constructed by using the energy information in the frequency domain [

It can be, therefore, extrapolated from the above analysis that the principle of the Hybrid DC System system still faces problems such as the reliability of protection, the complexity of the implementation process, and the ability to resist transition resistance, and the speed of protection. Therefore, further in-depth research is still needed to propose a protection principle with selectivity, quickness, sensitivity, and reliability for DC lines. To ensure the reliable and safe operation of the LCC-VSC multi-terminal direct current (MTDC) system, a single-ended protection principle based on the traveling wave transmission coefficient is proposed. This paper’s main work and contributions are: 1) The simplified circuit and the first traveling wave of the LCC-VSC hybrid DC system are analyzed, respectively. 2) The calculation method of transmission coefficient is introduced, and a single-ended protection principle based on multi-frequency transmission coefficient is proposed. 3) Compared with existing solutions, the principle proposed in this paper has better resistance to high resistance (600

In this section, the topology of the LCC-VSC-MTDC system is given first; then its equivalent circuit and the first traveling wave expressions are analyzed. Finally, using transmission coefficients to identify fault types is proposed.

This paper takes the LCC-MMC-MMC parallel three-terminal hybrid DC transmission system as the object to study, and its topology is shown in

According to

Traditional headline expressions such as

Line1

The traveling wave fault circuit of Line1 is shown in

Line2

The analysis process of Line2 is the same as that of Line1. Its equivalent circuit is shown in

If the attenuation-related parameters in the expression are extracted, the eigenvalues that are not affected by the fault resistance can be constructed. Since line attenuation expressions are closely related to line parameters, boundary element attenuation coefficients are closely related to boundary element parameters. Such frequency-varying parameters make the expression difficult to analyze quantitatively. Therefore, this paper proposes a method of fitting transmission coefficients to construct fault identification criteria.

The analysis in

This section proposes to use the Gauss-Newton method to fit the attenuation part to obtain the fault coefficient. The expression of the first traveling wave is a nonlinear curve, which can be written as:

The first-order Taylor expansion

According to

Each time iterative calculation is performed,

At this point, approximating

Set the initial point and threshold, the threshold is the condition to stop iteration;

Calculate the function value

Calculate

Solve the equation

Judge

So, the discrete traveling wave data can be fitted to the following form:

From

Obviously, the transfer coefficient of the forward external fault is larger than that of the internal fault. The transmission coefficient at this time cannot identify the reverse fault, and the traditional solution needs to increase the directional element to solve this problem. This section proposes a scheme using different frequency transmission coefficients to identify reverse faults.

The above analysis shows that the transfer coefficient can be used to identify forward external faults and internal faults. However, this criterion cannot identify reverse external faults. Take the line Line1 measuring point of the LCC-VSC-MTDC system as an example (

The busbar system makes the forward external fault transfer coefficient more significant than the threshold. Since the attenuation characteristics of Line1 are unknown, it is not possible to compare its differences with boundary elements. The transfer coefficient of the reverse external fault is not necessarily higher than the threshold. At this time, the threshold cannot identify a reverse external fault. Therefore, it is necessary to compare the attenuation characteristics of the line and each boundary element.

Reference [

The attenuation characteristics of the line are closely related to its parameters. Building a line frequency-dependent model in PSCAD can get its amplitude-frequency characteristics, and the results are shown in

Comparing

A summary of the amplitude-frequency characteristics of boundary elements and lines is shown in

Frequency | Element | |||
---|---|---|---|---|

Line |
Current limiting reactor (75 mH∼200 mH) | Smoothing reactor |
Bus | |

0.1% | 0.1% | 0.1% | 0.1% | |

0.1%∼1% | 0.1% | 0.1% | 0.1%∼90% | |

1%∼2.5% | 0.1%∼2% | - | 0.1% | |

2.5%∼8% | 2%∼16% | - | 0.1% | |

8%∼19% | 16%∼20% | 30%∼50% | 0.1% | |

19%∼80% | 20%∼40% | 50% | 0.1% |

It is worth noting that all attenuation percentages in

The analysis shows that when the internal fault occurs, the transmission coefficient is only affected by the line distribution parameters, and the value is small; when the external fault occurs, the transmission coefficient is affected by the line distribution parameters and boundary elements, and the value is large. When considering Line1 as an example, the fault identification principle of the multi-frequency transmission coefficient is analyzed.

The influence of the current-limiting reactor on the (5∼10) kHz fault signal is greater than that of the line, and the bus bar has almost no effect on the (5∼10) kHz signal, so the transmission coefficient of a single frequency cannot judge the reverse fault. Therefore, the LCC-VSC-MTDC system selects the second frequency band (1∼100) Hz to cooperate with the (5∼10) kHz high-frequency band to identify faults. Only when the transmission coefficients of the two frequency bands meet the requirements the protection identification element act. Therefore, the fault identification components of the proposed protection scheme are:

Therefore, the process of the fault identification component is: 1) Calculate the transmission coefficient threshold according to the line parameters; 2) Use the Gauss-Newton method to calculate the fault transmission coefficient; 3) Use

The start-up criterion must be sensitive enough to ensure immediate start-up in any internal fault. Existing references have matured into this research and will not be repeated in this section [

According to

In order to test the correctness and superiority of the protection principle, the LCC-VSC-MTDC simulation model was built in PSCAD/EMTDC. The thresholds calculated according to the line parameters are: Line1(

Set the internal faults

Calculating the transfer coefficient based on the fault current results in

Since the LCC-VSC-MTDC system adopts symmetrical bipolar wiring, this section takes the positive fault as an example to analyze. The simulation results are shown in

Frequency | LCC-VSC-MTDC | |||||
---|---|---|---|---|---|---|

(5∼10) kHz | >187.2 | >2813.1 | - | - | ≈121/157 | |

(1∼100) Hz | ≈1.05 | ≈1.05 | - | - | ||

(5∼10) kHz | <162.5 | <162.5 | - | - | ||

(1∼100) Hz | <11.8 | <11.8 | - | - | ||

(5∼10) kHz | >287.1 | >196.4 | ≈1.05 | <81.5 | ||

(1∼100) Hz | - | - | >105.3 | >105.3 | ||

(5∼10) kHz | >169.9 | ≈1.04 | <105.9 | <105.9 | ||

(1∼100) Hz | >112.4 | >109.38 | <10.2 | <10.2 | ||

(5∼10) kHz | - | - | >1621.7 | >138.5 | ||

(1∼100) Hz | - | - | ≈1.05 | ≈1.05 |

The transmission coefficients constructed in this chapter relate only to line parameters and boundary elements. Therefore, the high-resistance fault cannot affect the fault identification criterion. In order to verify the influence of fault distance and fault resistance on the protection principle, faults in different positions and stages are set up. The simulation results are shown in

To verify the superiority of the proposed scheme, two typical single-ended protection principles are compared and the proposed protection principle. Reference [

Fault resistance | 100 |
200 |
400 |
600 |
---|---|---|---|---|

Proposed protection | ||||

Time-domain protection scheme [ |
√ | √ | × | × |

Frequency domain protection scheme [ |
√ | √ | √ | × |

Signal noise ratio is often used to indicate that normal signals are polluted by noise. The reliability problem that high-frequency noise affects the traditional protection needs to be paid attention to. In this chapter, discrete wavelet transform is used to extract the transmission coefficient of the first traveling wave signal. The db3 of Daubechies Wavelet is chosen as the mother wavelet of this paper, and a 7-layer wavelet transform is carried out. Assuming

The above formula shows that when the Li’s exponent

To verify the above analysis, 10 dB∼40 dB of noise is superimposed on all traveling wave signals to obtain the traveling wave current with noise as shown in

References [

SNR | 10 dB | 20 dB | 30 dB |
---|---|---|---|

Proposed protection | |||

Time-domain protection scheme [ |
× | √ | √ |

Frequency domain protection scheme [ |
× | × | √ |

To analyze the gap and contribution of the proposed schemes, the performance of different schemes is compared in this section. At present, the main protection of the HVDC transmission projects in operation adopts the single-ended traveling wave protection scheme (scheme 1 [

Since fault resistance and noise are the main factors affecting the anti-interference ability of the protection scheme, different fault resistance and noise are added to several schemes, respectively. The simulation results are shown in

In conclusion, the proposed scheme has a stronger anti-interference ability than the existing scheme.

The analysis in

In order to verify the scalability, a typical transmission system model is established, as shown in

System | Interference | ||||
---|---|---|---|---|---|

(1–100) Hz | (5–10) kHz | (1–100) Hz | (5–10) kHz | ||

LCC-HVDC | 0 dB + 0 |
0.97 | 159.7 | 1.03 | 52.6 |

20 dB + 400 |
1.12 | 161.3 | 1.42 | 60.8 | |

10 dB + 600 |
1.15 | 167.8 | 1.48 | 69.1 | |

VSC-HVDC | 0 dB + 0 |
0.75 | 121.0 | 0.87 | 49.5 |

20 dB + 400 |
0.98 | 135.1 | 0.93 | 51.0 | |

10 dB + 600 |
1.01 | 149.5 | 1.24 | 58.7 | |

Result | √ | √ | √ | √ |

The protection scheme proposed in this chapter applies to single-ended electrical signals, and its action time composition is as follows:

Among them, the starting element is judged by four sampling points at the beginning of the fault. When the sampling frequency is 20 kHz, the time to activate the element is 0.2 ms. The data calculation time includes the calculation and conversion time of the calculation formula in hardware. Taking the TMS30F2812 as an example, its period is 6.67 nanoseconds. Therefore, the time of all calculation formulas does not exceed 0.1 ms, and the conversion time is about 1μs and can be ignored. Therefore, the data calculation time is approximately equal to 0.1 ms. In addition, the photoelectric conversion and measurement delay time is a microsecond, which is selected as 0.1 ms in this paper. Therefore, the total time for

This section selects the wavelet transform modulo maximum method to select the data window. If the time of the first modulo maximum value is

In summary, the maximum action time of the proposed scheme is 3.46 ms. The simulation results in

A novel transmission coefficient-based protection principle is used to ensure the reliable operation of hybrid DC systems. First, the equivalent circuit of the hybrid DC system is resolved. Accordingly, the expression of the first traveling wave of the system is deduced. Then, the concept of multi-frequency transmission coefficient is proposed by analyzing the amplitude-frequency characteristics of different boundary elements. The calculation method and frequency selection of the transmission coefficient is also analyzed. Finally, a single-ended protection principle is proposed, and its reliability and superiority are verified. The advantages and innovation of the proposed protection principle are shown as follows: 1) Innovation: The first traveling wave expression of LCC-VSC-MTDC is analyzed, and the principle of multi-frequency single-ended protection is proposed accordingly. 2) Advantage: The novel scheme can withstand higher fault resistances (600

The authors acknowledge the reviewers for providing valuable comments and helpful suggestions to improve the manuscript.