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Longmen Steel 27 MW Blast Furnace Blower Variable-Frequency Soft-Starting Project

Metallurgy & Steel | 1,800 m³ Blast Furnace / Blower Driven by a 27 MW Synchronous Motor | One Variable-Frequency Soft Starter Serving Two Motors + Bumpless Synchronous Transfer

Project Background

Shaanxi Longmen Steel's blast furnace uses blowers driven by 27 MW synchronous motors. The blowers operate in duty/standby configurations to support continuous blast furnace production. The project must address starting impact, synchronous control and reliable transfer from variable-frequency starting to line-frequency operation for ultra-high-power synchronous motors.

Project Challenges

  • Direct-on-line or conventional reduced-voltage starting of a 27 MW synchronous motor can cause significant grid impact and increase the capacity required from the upstream power supply system.
  • Multiple blower sets use duty/standby configurations. The soft-starting system must serve multiple electrically excited synchronous motors from different manufacturers.
  • The soft-starting process must coordinate with the excitation system and complete phase locking and transfer to the line-frequency supply near 50 Hz.

Dayu Solution

A high-voltage variable-frequency soft-starting system uses one starter to serve two motors. The drive participates in excitation current control through a 4-20 mA signal. After the motor accelerates to 50 Hz, the system enters the phase-locking stage. Once the input and output voltage phase angles and amplitudes meet the transfer conditions, the line-frequency operating circuit closes, followed by opening of the VFD incoming and outgoing circuits. The motor then operates on the line-frequency supply.

Key Configuration

  • Blast furnace: No. 5 blast furnace, 1,800 m³.
  • Main motor: 27 MW electrically excited synchronous motor.
  • System: Multiple blower sets in duty/standby configurations, with multiple variable-frequency soft starters, each serving two motors.
  • Control: Coordination with the excitation system, frequency skipping to avoid the blower surge region, phase-locked synchronization and transfer to line-frequency operation.

Application Value

After commissioning, all transfers following variable-frequency soft starting were successful, and the equipment operated stably. During starting, current rose from zero as speed increased and did not exceed 1.2 times the VFD rated current. Skip frequencies of 13.20 Hz, 19.26 Hz and 31.92 Hz were set to avoid the blower surge region. The system also reduced starting noise and eased upstream power supply capacity requirements.