Transformer Grounding & Cable Concepts Explained
Sharing of Transformer Concepts and Terminology
The zero-mode impedance of a load is infinite, and its line-mode impedance is also extremely large, approximately 100 times that of the line-mode impedance of the line.
The capacitance to ground of a cable is 25-50 times that of an overhead line.
The free oscillation frequency of transient capacitive current: 300-1500Hz for overhead lines and 1500-3000Hz for cables.
Performance requirements for an external grounding transformer: Under normal power supply of the power grid, its impedance value is extremely high, and only a tiny magnetizing current flows through the windings; when a single-phase ground fault occurs in the system, the winding presents high impedance to positive and negative sequences, and low impedance to zero sequence. The wiring modes of such transformers are Y0/Δ or Z-type.
Since the high-voltage side of the transformer adopts Z-type wiring, each phase winding consists of two segments, which are respectively located on two core columns of different phases, and the two segments of the coil are connected with opposite polarities. The zero-sequence magnetic fluxes generated by the two-phase windings cancel each other out, resulting in extremely low zero-sequence impedance and extremely small no-load loss, allowing 100% utilization of the transformer capacity. When an arc suppression coil is connected to an ordinary transformer, its capacity shall not exceed 20% of the transformer capacity; while a Z-type transformer can be connected with an arc suppression coil with a capacity of 90%-100%, which can effectively save investment.
In addition to being connected with an arc suppression coil, a grounding transformer can also carry secondary loads and replace a station transformer. When carrying secondary loads, the primary capacity of the grounding transformer should be the sum of the capacity of the arc suppression coil and the capacity of the secondary load; when not carrying secondary loads, its capacity is equal to that of the arc suppression coil.
The purpose of adding a damping resistor is to limit the displacement voltage UN of the neutral point to less than 15% of the phase voltage when series resonance occurs in the system, so as to maintain the normal operation of the system and prevent overvoltage. When a single-phase ground fault occurs in the system, a large current flows through the neutral point, and the damping resistor must be short-circuited at this time.
When using the parallel medium resistance line selection method, a parallel medium resistance box is required, which is connected in parallel at both ends of the arc suppression coil. When the device confirms that a permanent single-phase ground fault occurs in the system, the medium resistance is put into operation to inject active current into the system for line selection, and the resistance is cut off after a short delay.
The higher the dielectric constant, the stronger the conductivity.
Three-phase transformers used in distribution systems mostly adopt the Dyn11 connection mode, which has the following advantages: it can reduce harmonic current, improve power supply quality, has small zero-sequence impedance, can increase single-phase short-circuit current, and is conducive to cutting off single-phase ground faults; it can make full use of the transformer capacity under the condition of three-phase unbalanced load, and reduce transformer loss at the same time.
The wave impedance of the line connected to the primary side of the transformer is usually several hundred ohms, and that of the line connected to the low-voltage side is generally several tens to more than one hundred ohms.
The power frequency damping rate of a normal overhead line is about 3%-5%, which can increase to 10% when the line is damp; the power frequency damping rate of a cable line is about 2%-4%, which can reach 10% when the insulation is aged.
The capacitance to ground per phase of 3-35kV overhead lines is 5000-6000pF/km. The capacitive current of overhead lines in double-circuit lines on the same pole is Ic=(1.4-1.6)Id (where Id is the capacitive current of one circuit in the double-circuit lines; the coefficient 1.6 corresponds to 35kV lines, and 1.4 corresponds to 10kV lines).
For a neutral point resonant grounding system, when a single-phase ground fault occurs, since the zero-sequence impedance is close to infinity, the residual current does not contain 3rd and integer multiple harmonic currents, mainly 5th and 7th harmonic currents.
According to the regulations, when an arc suppression coil is connected to an ordinary transformer, its capacity shall not exceed 20% of the transformer capacity. A Z-type transformer can be connected with an arc suppression coil with a capacity of 90%-100%. In addition to being connected with an arc suppression coil, a grounding transformer can also carry secondary loads and replace a station transformer, thus saving investment costs.
During the operation of a grounding transformer, when a zero-sequence current of a certain magnitude passes through, the currents flowing through the two single-phase windings on the same core column are opposite in direction and equal in magnitude, so that the magnetomotive forces generated by the zero-sequence current are exactly opposite and cancel each other out, resulting in extremely small zero-sequence impedance. Due to the low zero-sequence impedance of the grounding transformer, when a single-phase ground fault occurs in phase C, the ground current I of phase C flows into the neutral point through the earth and is equally divided into three parts into the grounding transformer; since the three-phase currents flowing into the grounding transformer are equal, the displacement of the neutral point N remains unchanged, and the three-phase line voltages still remain symmetric.
The harmonics in the zero-sequence circuit are mainly caused by the nonlinear characteristics of the transformer core. Since the secondary side of transformers in China's distribution networks mostly adopts delta connection, there are generally no 3rd and integer multiple high-order harmonics in the zero-sequence circuit, so the ground fault current basically does not contain these high-order harmonic components, mainly 5th and 7th harmonic components, whose magnitude will change with the load.
For a single-phase ground fault, the equivalent sequence network is a series connection of positive, negative, and zero sequence networks; for a double-phase ground fault, the equivalent sequence network is a parallel connection of positive, negative, and zero sequence networks; for a double-phase short circuit, the equivalent sequence network is a parallel connection of positive and negative sequence networks; for a three-phase short circuit, the equivalent sequence network only includes the positive sequence network.
