Transmission lines in the power network role

- Nov 22, 2017 -

Transmission lines in the power network role

    1. What is the power system and power grid?

    Power plants through the generator of mechanical energy into electrical energy  by the transformer and the different voltage levels of lines delivered to the user, these power generation, substation, transmission, distribution and consumption of electrical energy all connected together as the overall power system.

    In the power system to remove the remaining parts of the generator and electrical equipment called the power grid 升 that boost, step-down transformers and various voltage levels of power transmission lines consisting of a network.

    2. How is the rated voltage of the transmission line?

    Can make the normal working voltage of power equipment is called the rated voltage.

    The normal operating voltage of the transmission line 额 shall be equal to the rated voltage of the electrical equipment directly connected to the line  but the voltage at the line end is lower than that at the head end due to voltage drop or voltage loss in the line. Voltage along the point will not be equal. The production of electrical equipment must be standardized, it is impossible to change with the line pressure drop. To make the equipment terminal voltage and grid rated voltage as close as possible, take the average value of the first terminal of the grid as the rated voltage of the grid, that is, the rated voltage Un = (U1 + U2) / 2

Where U1, U2 are the first end of the grid voltage.

    What is the role of the power line in the power grid and what components does it consist of?

    Power lines are an indispensable part of the power grid. Its purpose in addition to conveying and distributing electrical energy, but also can be connected to form several power grids.

    Transmission lines can be divided into two categories. Ie overhead lines and power cable lines. Overhead line is the wire, lightning-erected on the tower.It is composed of wire, ground wire, tower, insulators, fittings, foundation and other components; cable line is composed of power cables and cable connectors.

    4. What is the voltage level of the transmission line?

    The rated voltage of the power line is the rated voltage of the receiving end equipment and the working voltage of the transmitting end of the line corresponds approximately to the rated voltage of the transmitting equipment. This value is called the voltage level of the line.

    5. Why does the rated voltage of the line depend on its transmission power?

    The rated voltage of the power network is the rated voltage of the power line and the rated voltage of the power receiving equipment. The rated voltage of the power line depends on the transmission power. When the transmission power is constant, the higher the line voltage, the smaller the current through it, the smaller the required wire cross-section and the lower the line investment and operating costs. On the contrary, the lower the line voltage is, the larger the current flows and the larger the required wire cross-section. As a result, the insulation of towers, transformers, circuit breakers and the like will increase and the cost will increase. Therefore, corresponding to a certain transmission power and transmission distance, Always find a reasonable route


6 What is the requirement for voltage offset in the power system? What measures should be taken when the voltage offset is too large?

    Voltage offsets in power systems are generally caused by load changes. Line load, the current is large, loss should be increased, the voltage drop will increase.If the wire cross section is too small, the power transmission distance is long, the voltage difference between the first and last line will have to increase, this will make the user Equipment is threatened, so our provisions to rated voltage as a benchmark, the percentage of voltage offset allows the following provisions:

35kV and above users rated voltage of ± 5%, 1OkV and below the user rated voltage of 7%.

    To ensure that the voltage offset within the specified range, should take the following measures:

① using voltage regulation

② increase the wire cross-section of the circuit to reduce the loss

③ install reactive equipment, such as transfer cameras, power capacitors

④ using a high level of voltage transmission power.

    7 grid is divided into categories?

    By the power supply range can be divided into local power grid and regional power grid. 11OkV and below the power supply distance is short, the transmission power is small, known as the local power grid, 11OkV above the power supply grid long distance, transmission power is called the regional power grid.

    According to the grid structure can be divided into open grid and closed grid. Single-user power supply is called open grid, double-loop power supply is called closed grid.

    According to the voltage level can be divided into low-voltage power grid (1kV below), high-voltage power grid (1 ~ 330kV), EHV grid (500kV and above).

    8 What is the United Power System? What are the advantages of the United Power System?

    Transmission lines are an integral part of the power grid, which is an integral part of the power system. Two or more power systems are connected by a power grid to form a combined power system.

    Its advantages are divided into the following five aspects:

    (1) improve power supply reliability. In a large system, there are many generators with more spare units, more lines and more capacity. Some of the line faults will not affect the system power supply, thus improving the reliability of the power supply.

    (2) can fully guarantee the power quality. First, the two indicators of power quality voltage, and second, the frequency, due to the power grid is large, large capacity, individual load changes, line failure blackouts will not cause voltage changes and frequency fluctuations.

    (3) reduce system spare capacity, improve equipment utilization,

    (4) Easy installation of large lines  Reduce costs, unit capacity, large-capacity, high voltage lines must be built, so that low cost per unit kilowatt, small line loss, significant economic benefits.

    (5) Make full use of power resources. Hydropower, thermal power grid operation. Multiple thermal power in winter, summer hydropower more water, so as to reduce costs and improve operational economy.

    9 transmission line rated voltage is how to provide?

    The normal working voltage of the transmission line 额 should be equal to the rated voltage of the electrical equipment connected to the line  However, due to the voltage drop of the line, especially the long-distance power supply, when there is a large load change, there must be a voltage drop due to the impedance of the transmission line Therefore, the voltage U2 at the end of the line is lower than the voltage U1 at the head of the line, and the voltages at all points along the line are not equal. Therefore, the rated voltage of the power grid is defined as UN = (U1 + U2) / 2.

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