Index Terms—Euclidean distance, entropy weight method,
flexible direct current power grid, pilot protection, waveform
similarity.
Introduction
To integrate and accommodate large-scale renewable generations such as
wind power and solar power, multi-terminal flexible DC grid (MTFDC)
based on modular multi-level converter (MMC)-HVDC has been widely used
as a promised solution due to its independent power regulation and
flexible operation capabilities, small switching loss and strong
expansibility [1-5]. However, the stable and
reliable operation of MTFDC is heavily dependent upon the fast and
reliable DC line protection [6]. The protection
scheme of the DC line can be categorized as two types: single-ended
quantity protection and double-ended quantity protection according to
the extracted electrical quantity of converter station from one-end or
both-end of the DC line [7-23].
Typical single-ended quantity protection schemes on the MTFDC grid
mainly include boundary protection, transient voltage protection and
voltage traveling wave protection [7-13], and so on. In [7-8],
the protection principles proposed are based on the change rate of
voltage measured at the line side of DC inductors. However, the
reliability under high-resistance faults cannot be guaranteed and the
required sampling frequency is much high. In [9], a protection
method using the change rate of the DC inductor voltage is proposed. In
[10], a single-ended transient-voltage-based protection strategy for
a flexible DC grid was proposed. The proposed method has higher
reliability and stronger ability to endure high transition resistance,
but the high transition resistance is only 300Ω. In [11], the method
based on the high-frequency components of transient voltages on DC lines
is proposed, which has fast operation speed and good robustness.
However, an accurate theoretical basis is not analyzed thoroughly. In
[12], according to the second-order difference of backward voltage
traveling wave, a protection scheme is proposed. The high-resistance
problem is overcome, but the required time window is more than 3ms. In
[13], the method based on transient high-frequency energy of line
current is proposed. Lumped parameter-based DC line model cannot reflect
the propagation process of traveling waves on long transmission lines.
According to the above analysis, the single-end protection can identify
internal fault and external fault quickly without communication only by
using the one-end data of the DC line, but it also has some
disadvantages such as weak resistance to high transition resistance and
low sensitivity.
As for double-ending protection, many achievements mainly focus on
traveling waves, line boundary elements, transient
impedance[14-25], etc. In [14], a protection
scheme is proposed, which is based on the time difference between the
initial forward and backward traveling waves reaching the relays on both
ends of the DC line. In [15-16], the protection methods can be
implemented by the voltages of DC inductors on both ends of the DC line.
In [17], the protection principles are both based on the ratio of
the transient voltages (ROTV) at both sides of the DC inductor. To
ensure the reliability under high-resistance faults, the ROTVs at both
ends of the DC line are required. In [18], a protection method based
on the transient measured impedance is proposed. In [19], short-time
energy is used to identify DC single-pole ground fault in a
multi-terminal flexible DC system. In [20-21], the ratio of
electrical fault quantity at both ends of the line is selected to
identify internal and external faults. In [21], protection based on
incremental reactive power coefficients (IRPCs) is proposed. The
protection principles can be carried out by the both-end data of the DC
line, which can meet the absolute selectivity of fault identification.
However, due to the communication delay, some algorithms require strict
synchronization of double-terminal data and high error measurement
ability, which limits the rapidity of protection. Besides, a large
short-circuit current will be generated in an ultrashort time after DC
fault occurs in multi-terminal power transmission systems, which can
damage the semiconductor devices and even lead to the blocking of the
converter station if the fault cannot be cleared quickly[22]. In [24], a new protection scheme for
flexible distribution system based on cosine similarity of DC current
waveform is proposed. The proposed protection scheme can identify
internal faults in less than 0.3 millisecond with good reliability and
high sensitivity. And it is robust to distributed capacitance, and not
requires strict communication synchronization. In [25], an adaptive
reclosing scheme based on the transient current waveform similarity
matching is proposed. The protective reach of reclosing criterion
proposed can cover the entire transmission line, with high sensitivity
and well applicability.
In [26], a new scheme of traveling wave longitudinal protection
based on measuring wave impedance Euclidean distance is put forward, the
protection principle can identify internal and external faults with
reliably, sensitively and quickly, and has a strong ability to withstand
transition resistance and noise interference. In [27], load curve
clustering method based on Euclidean Dynamic Time Warping Distance and
Entropy Weight is introduced, which can be used in power system
protection principle.
In view of the above reasons, in order to improve the ability of the
modular multi-level converter to quickly clear and isolate faults in the
multi-terminal flexible direct current grid, it is necessary to study a
new type of protection methods for DC grid.
This paper analyzes the characteristics of current faults in the
multi-terminal flexible direct current grid, introduces the
comprehensive distance of current fault waveform, and proposes a line
protection method based on the similarity of current fault waveform. The
proposed method has good resistance to high resistance, and its rapidity
and adaptability are further improved.
The remainder of this paper is organized as follows. In Section 2, the
topology of the four-terminal MMC flexible DC grid is introduced; In
Section 3, the current fault characteristics are analyzed; In Section 4,
the measurement methods of the current fault component waveform
similarity are introduced; In Section 5, the protection criteria and
protection principal flow chart of are introduced; In Section 6,
simulation verifications are carried out in MATLAB; And conclusions are
given in Section 7.
Topological Structure of Modular Multi-terminal Flexible DC
Grid with Multi-level Converter
This paper analyzes the characteristics of Zhangbei four-terminal ring
flexible DC power grid. The system is a four-terminal symmetrical
bipolar MMC flexible DC transmission line with a DC voltage level of
±500kV, and the length of each line is marked according to the actual
length. Topology and fault location are shown in Fig.1. A series of
small inductors are added to the DC side of each station to construct
the boundary conditions. According to the actual situation, the
parameter is 0.3H.