The size of system voltage deviation, the degree of three-phase voltage imbalance, and the reliability of power supply have an important impact on the safe operation and service life of electrical equipment, and are also directly related to the safety, stability and economic operation of the cement plant power system itself.

1. Power supply voltage deviation

Causes and hazards of voltage deviation:

At any time during system operation, the reactive power supplied by the reactive power supply of each workshop should be equal to the reactive power required by the system, but the load in the cement plant power system may change at any time, and factors such as faults in the power system operation will cause power imbalance in the entire plant. The unbalanced system reactive power means that a large amount of reactive power will flow through the power supply line and transformer. Due to the impedance in the line and transformer, the voltage difference between the line and the transformer will occur. The transmission distance of the power supply and distribution line is too long, the transmission capacity is too large, and the cross-section is too small, which will increase the voltage loss of the line, resulting in voltage deviation.

The output power, working efficiency and service life of all electrical equipment in the cement plant are affected by voltage to varying degrees. Excessive voltage deviation will significantly affect the luminous efficiency and service life of the lighting equipment in the cement plant, and may even directly burn out the bulb. The maximum electromagnetic torque of asynchronous motors used in cement plants is proportional to the square of the terminal voltage. When the terminal voltage of the motor drops by 10% from the rated voltage, its torque is only 81% of the rated torque, which is 19% less than the rated torque. If the voltage drops too much, the motor may stop running or fail to start.

Measures to improve voltage deviation:

Change line parameters to reduce voltage loss. Reducing the resistance or reactance of the line can reduce the voltage loss of the line, thereby increasing the voltage at the receiving end of the line and achieving the purpose of voltage regulation. Therefore, in the design of the power plan layout of the cement production line, the best cable laying path should be designed for each electrical equipment as much as possible, and the cable length should be reduced as much as possible. For electrical equipment far away from the distribution cabinet, cables with slightly larger specifications than those under normal current-carrying conditions should be selected.

Change the transformer ratio. Select appropriate taps according to the specific conditions of the cement plant workshop so that the output voltage of the transformer varies within the range of ±5% of the rated voltage, thereby obtaining a suitable transformer output voltage.

Configure sufficient reactive power supply. For high-voltage motors with a power of more than 1000kW such as cement mills and roller presses, each motor should be equipped with a static phase advancer. For the low-voltage cabinets in the workshop, a reasonable reactive power capacitor compensation device should be configured according to the calculation.

2. Three-phase voltage imbalance

Causes and hazards of three-phase imbalance:

The three-phase imbalance of the cement plant power supply system can be divided into two categories, one is accidental imbalance and the other is normal imbalance. Accidental imbalance is caused by various faults, such as single-phase grounding short circuit or two-phase short circuit. Normal imbalance is the three-phase imbalance of the power system caused by the imbalance of power lines or power consumption links under normal operation. For the long-distance transmission lines of cement plants, the line reactance is much greater than the line resistance. When the three-phase conductors are arranged in an equilateral triangle, the magnetic flux of each phase is equal, and the reactance is equal; when the three-phase conductors are arranged horizontally or vertically, the magnetic flux of the conductors on both sides is greater than the magnetic flux of the middle conductor, so the three-phase reactance is not equal, so whether the three-phase reactance is equal directly determines whether the power supply line is balanced. The imbalance of the power consumption link is mainly caused by the three-phase imbalance of the power supply system caused by the asymmetry of the three-phase load.

When the power supply and distribution system is in a three-phase voltage unbalanced state, the voltage and current will contain a large amount of negative sequence components. The negative sequence current will generate additional power loss, thus increasing the total loss of the line and reducing the economic operation of the power system. For three-phase asynchronous motors, negative sequence voltage will generate braking torque, which will reduce the maximum torque and output power of the asynchronous motor and may also cause motor vibration. For transformers, when the transformer is operating under unbalanced load, the transformer capacity cannot be fully utilized and the insulation life of the transformer will be shortened. Excessive negative sequence components in the three-phase unbalanced system may also cause false operation of the relay protection device.

Measures to improve three-phase imbalance:

Reasonably distribute the asymmetric load in the three phases to make the load of each phase as balanced as possible. For multiple large-capacity single-phase loads, their active power and reactive power should be fully calculated so that the loads distributed in the three phases are as close as possible.

For single-phase loads with extremely large capacity, consider using a separate transformer to power the asymmetric load.

For long-distance and large-capacity power feeding, for example, when the main drop is far away from a transformer in the factory, try to use three-core cables to avoid the three-phase imbalance of the power line caused by the uneven distribution of reactance using single-core cables.

To avoid the losses caused by accidental three-phase imbalance, install arc extinguishing devices.

3. Power supply interruption and power supply reliability

The causes of power supply interruption and the hazards of power supply interruption:

The reasons for power supply interruption in cement plants are multifaceted, mainly divided into the following categories. One is that small animals sneak into the distribution cabinet, causing an accidental power supply interruption caused by phase short circuit of the primary components; another is that some operating personnel are not familiar with the relevant operating rules and regulations, or the supervisor is careless, which causes accidents and power outages; another is that lightning strikes, lightning, etc. cause equipment protection to act, causing system accidents.

The operation of the cement production line is continuous. The production line trip caused by power outages will bring serious consequences to cement companies, such as a decrease in cement production and a decrease in cement quality. Especially during the peak season of cement sales, cement is in short supply, and the losses caused are immeasurable, and the reputation and image of cement companies will also be affected. Therefore, it is of great significance to ensure the reliability and continuity of power supply throughout the plant.

Measures to improve power supply reliability:

Install anti-theft doors in each distribution room and raise the threshold to prevent small animals from entering. Short-circuit failures caused by small animals entering the distribution cabinet are common. Some production lines have even experienced more than one similar incident, causing huge economic losses to cement companies.

Strengthen the pre-job training and business skills training of operators, and resolutely ensure that they are certified to work. At the same time, safety production education should be strengthened to continuously improve the technical level of operators and their ability to quickly handle accidents.