The neutral point grounding resistance cabinet is mainly used to connect
the neutral point of the main transformer and the grounding network in urban and
rural distribution networks. Urban and rural distribution networks mainly refer
to the three voltage levels of 10kV, 35kV, and 66kV. They are widely used in
power systems. It is wide and occupies an important position. With the in-depth
development of urban network transformation, the capacity of 10KV distribution
network has increased rapidly and the network structure has become increasingly
perfect. According to the needs of urban construction, overhead bare conductor
lines are gradually being replaced by cables and insulated conductors. At the
same time, due to overvoltage, Fire accidents in switch cabinets and expensive
household appliances are also common. Therefore, how to effectively and
economically limit the overvoltage problem of distribution network has become
the focus of current power supply and consumption.
The neutral point of the 10KV distribution network can usually be divided
into ungrounded system, resistance grounding system and arc suppression coil
grounding system. Since the choice of grounding method is a comprehensive issue
involving factors such as the insulation level of lines and equipment,
communication interference, relay protection composition, and the safety and
reliability of the power supply network, the power supply system practices of my
country's distribution network and large industrial and mining enterprises vary.
Differently, in the past, most of them used the operation mode of ungrounded
neutral point or grounding through arc suppression coil. In recent years, some
urban power grids have vigorously promoted the operation mode of resistive
grounding.
In the mid-1980s, the 10kV distribution network in cities such as Guangzhou
developed rapidly. A large number of cables were laid in the central area of the
city. The single-phase grounding capacitor current increased rapidly, reaching
more than 60A in 1987. Although arc suppression coils were installed, due to the
large capacitor current and The operating mode changes frequently, and it is
difficult to adjust the arc suppression coil. In addition, some cables with low
insulation levels are used. In order to reduce the overvoltage level and reduce
the possibility of phase-to-phase faults, the neutral point is grounded through
low resistance.
According to the research results of Ouzhuang substation in Guangzhou area,
the neutral point is used to ground through low resistance. When Rn ≤ 10Ω, the
single-phase grounding power frequency voltage can be reduced to about 1.4p.u in
most cases. From the perspective of limiting arc ground overvoltage, when the
arc is ignited and extinguished, the excess charge accumulated in the system can
leak through Rn within half the power frequency cycle after the arc is
extinguished, and the overvoltage amplitude can be significantly reduced.
According to this requirement, the conditions that the low resistance value of
the neutral point should meet are:
Rn≤1/3ωC0
When Rn=10Ω, the arc ground overvoltage can be reduced to less than
1.9p.u.
If the resistance value of the neutral point is too low, the single-phase
grounding current will be large, causing great interference to the communication
line; if the resistance value is too high, the relay protection action will be
unreliable. Generally speaking, when the current in the neutral point resistor
is 100~200A, it will not cause interference to the communication line. Under
this condition, the neutral point resistance value of the 10kV overhead line is
28.80~57.74Ω. For cable-based distribution networks, according to Japanese
experience, when the current in the neutral point resistor is 400 to 800A, there
is little interference with communication lines. Accordingly, the range of the
neutral point resistance value of the 10kV cable distribution network It should
be 7.2~14.4Ω.
From the perspective of ensuring the reliability of the relay protection
action, it should have a higher sensitivity when a single-phase ground fault
occurs. There are two types of grounding relays: one is a grounding overcurrent
relay, and the other is a grounding direction relay that operates according to
the direction of the zero-sequence current.
When using an overcurrent relay, there is no problem with the sensitivity
of protection when a metallic grounding occurs. However, when grounding through
a transition resistor, there are mainly a considerable number of single-phase
ground faults in overhead lines. The resistance at the fault point is relatively
large, and there are some problems with the sensitivity of the protection. For
cable lines, the transition resistance when single-phase grounding is generally
relatively small, which has little impact on the sensitivity of relay
protection.
Finally, starting from the requirement of limiting resonant overvoltage,
when the cable line is particularly long, jωLe=1/jωC may occur, causing
resonance. If the neutral point has an appropriate resistance, the abnormal
voltage on the sound phase can be limited. .
When the neutral point is grounded through low resistance, especially when
a distribution network dominated by overhead lines is grounded in a single
phase, the number of trips will be greatly increased. If the ring network power
supply cannot be realized or the line is not equipped with reclosing, the number
of power outages will increase. will increase and reduce the reliability of
power supply. However, for cable-based power distribution, because of its
extremely low failure rate, this problem is not prominent.
Based on my country's specific conditions, it is recommended that
distribution networks with cable-based capacitance currents above 150A can adopt
low-resistance grounding methods, and the corresponding fault current levels are
400 to 1000A. For a 10kV system, the neutral point grounding resistance value
can be RN=10~20Ω.
(3) Implementation standards
DL/T780-2001 "Neutral point grounding resistor for power distribution
system"
GB6450 "Dry-type power transformer"
DL/T620-1997 "Overvoltage protection and insulation coordination of AC
electrical devices"
GB311.2-6 "High Voltage Test Technology"
GB1208-1997 "Current Transformer"
GB4208-93 "Enclosure protection level (IP code)"
IEC289
IEEE32-1972 Standard "Technology, Terminology and Testing of Neutral Point
Grounding Devices"
Other relevant national standards and power industry standards
(4) Type
●Distribution network small resistance grounding resistance cabinet
●Resistance grounding resistance cabinet in distribution network
●High resistance grounding resistance cabinet for distribution network
●Generator neutral point resistance grounding resistance cabinet
(5) Serialization of variety products
●Applicable voltage: 3.3kV～35kV;
●Allow flow: 5A～5000A;
●Nominal resistance: 0.1Ω～1500Ω;
●Allow flow time: 10s, 30s, 60s, 10min, continuous operation;
●Inlet and outlet lines: top in and bottom out, bottom in and bottom out,
side in and side out, side in and bottom out;
●Installation location: indoors or outdoors;
●Allowable temperature rise: 760℃ when the flow time is 10s, 30s, and 60s;
610℃ when the flow time is 10min; 385℃ during continuous operation;
●Current transformer CT: optional
Analysis of the auxiliary circuit diagram of a small-power resistance
multimeter:
In actual work, in order to analyze the circuit principle, when drawing the
electrical schematic diagram based on the actual object, it is often necessary
to measure the actual resistance value of the small resistance resistor, such as
the constantan resistor (generally milliohm) used to detect the load current in
high-end switching power supplies. level), high-power small-value resistors for
over-current protection (some are below 0.1Ω), feedback resistors (generally a
few tenths of ohms) connected in series with the current amplifier tube (E pole
or S pole) in high-power amplifier circuits .
The following is an auxiliary circuit diagram for accurately measuring
small resistance resistors with an ordinary multimeter.
Since the minimum range of electrical resistance of ordinary digital
multimeters is 200Ω, due to accuracy limitations, it is often impossible to
accurately measure the specific resistance values of these resistors, nor to
judge their consistency, which is often difficult. To this end, the auxiliary
circuit shown in Figure 1 was trial-produced and combined with the DC low
voltage range of the multimeter (200mV, 2V, 20V) to achieve accurate measurement
of small resistance resistors.
Add a certain current to the resistor RX under test through a constant
current source, and then use a multimeter to measure the voltage at both ends of
Rx. The measured voltage value is divided by the constant current flowing
through the resistor under test Rx, and the resistance of the resistor under
test can be obtained. . Theoretically, the greater the current flowing through
the resistor to be measured, the easier it is to accurately measure the
resistance of the small-value resistor Rx. However, if the current is too large,
it will cause serious heating of the constant current source and affect the
stability of the current, resulting in the measured resistance. The value is
inaccurate; second, the low-power resistor does not allow excessive current to
flow. For this purpose, this circuit uses LM317 (U1), resistors R1, R2, and
potentiometer Rp1 to form a simple 100mA constant current source.
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