Application du variateur de fréquence dans système de refroidissement à eau par vaporisation pour coulage en continu
2009-12-15 14:45:53
Résumé: cette page introduit l’application du variateur de fréquence multi niveau à cellule en cascade dans le système de refroidissement à eau par vaporisation pour coulage en continu.
Mots clés: ajustement de vitesse à fréquence variable, fourniture d’eau à pression constante, by-pass.

1. Preface

In water treatment station of the plant, continuous casting misting cooling water pumps supply industry water to continuous casting machine. Original design flow is 400 m³/h, pressure is 1.4Mpa. But max flow demand of continuous casting is 370m³/h. Pressure is 1.3Mpa. In direct on line state, the pressure of pump group is too high and adverse to continuous casting production. Meanwhile, in standby state of continuous casting, in order to ensure continuity of water supply, pump group still works in full load. Industry water flows back to industry water tank from relief valve, which leads to much power wasted.

With rapid development of computer technique and emergence of new model power components, medium voltage variable frequency drive has been used widely. It not only solves problems of speed adjustment for fan/pump and soft start, but also saves much energy. Therefore, the plant decided to reform with VFD. After technology and economy analysis, HARSVERT-A was selected.

2. Technics introduction of continuous casting misting cooling water system

Continuous casting misting cooling water pump group is composed of 2 MTC－A－50/04 multi-level pumps, 2 MGF－400C asynchronous motors. One works and the other is spare.

In daily operation, single pump works in direct on line state for 24 hours continuously. When the machine does not cast, industry water flows back to industry tank via bypass valve.

3. Control system design

3.1 Electrical design

Because continuous casting production has high requirement for continuity of water supply, sudden discontinuity would affect product quality seriously. In term of ensuring continuity of water supply, combining actual state of power system, we decide to adopt control mode of constant pressure water supply. Electrical circuit is “one drive two”.

(1) Main circuit principle

In principle drawing, QF1 is breaker of 1# pump and it supplies power to VFD now. QF2 is breaker of 2# pump and power source now. Contactors KM1, KM2, KM3, KM4 and isolators QS1, QS2, QS3, QS4 are in bypass cabinet. Contactors are used to switch line frequency and variable frequency automatically. Generally, medium voltage breaker is closed and only opens for VFD maintenance.

(2) Pump operation mode after reform

In normal operation, one pump works and the other is spare. When VFD is broken, pump will be switched to direct on line automatically. When running motor is broken, the spare pump will start in direct on line automatically.

Switch process: switch variable frequency pump to direct on line first, then start spare pump to 50Hz by VFD. At the moment, outlet pressure of 2 pumps is same without circular flow. At last, secede line frequency pump. VFD adjusts output frequency automatically according to set pressure in order to keep pipe pressure constant.

Electrical interlock of contactors ensures 2 pumps can not both run in direct on line or in variable frequency simultaneously, as well as pump can not run in variable frequency and direct on line simultaneously.

3.2 Network design

Original system adopts integration control for display, control, monitor of parameters, such as temperature, pressure, flow and liquid level. Central monitor system has 2 servers, 3 operation stations, 3 AS (auto control system) stations. 3 sets of S7-400 PLCs monitor all equipments and devices. Communication network adopts Profibus DP in bottom. 2 servers and 3 PLCs connect via optical fiber ring net of 100Mbps. 3 operation stations connect with 2 servers via ethernet. they transfer control commands. A printer is used to print report.

Monitor network

Control system of variable speed is composed of upper PC of original PCS7, PLC and medium voltage VFD. As main controller, original PLC connects with S7-200 PLC inside VFD via hard wire to transfer signals.

3.3 Software design

(1) Application software

PCS7 system is a new generation auto control system with completed structure and perfect performance. It embodies developing trend of the newest technology in current control field.

In software empoldering, we adopt compositive entire data management and uniform configuration tool, namely SIMATIC programme manager. HMI and system configuration use WinCC to implement instant moitor for all kinds of technics. Programme software uses Step 7, adopts module programme. It programmes each work into function unit and invokes them in main programme. It is easy and advantageous to understand and modify for user.

(2) Control function description

There is local control and remote control.

Local control:

User starts VFD on HMI, sets frequency, then starts motor in variable frequency via button on bypass cabinet. motor also can start in direct on line via original control box.

Remote control:

a. auto control: user sets control pressure of network. The set value inputs S7－400 PLC. PLC compares this value to pressure feedback signal from pressure transmitter. After PID module operation, PLC sends signal of VFD operation frequency to adjust motor speed and keep constant pressure. In addition, in auto control mode, there is producing pressure and standy pressure. The latter is far less than the former. Selection of the two is decided according to start/stop signal of continuous casting machine. This ensures production and also saves power farthest, thereby heightening energy saving effect.

b. Manual control: user sets operation frequency manually.

4. Energy saving calculation

4.1 Data statistic of producing and standby

4.2 Power consumption

(1) Variable name

P_d：total power of motor；I：input current of motor；_d：motor efficiency；U：input voltage of motor；cosφ：power factor（direct on line 0.85，variable frequency 0.98）；C_d：yearly power consumption in direct on line；T：yearly work time；δ：time rate of different loads；C_b：yearly power consumption in variable frequency；F₁：power cost in direct on line；F₂：power cost in variable frequency

(2) Formula

P_d =1.732×U×I×cosφ … （1）

F_d= T×∑（Pd×δ）… （2）

C_d＝8500×（1.732UI cosφ×85%＋1.732UI cosφ×15%） （3）

(3) Power consumption in direct on line

C_d＝8500×（1.732×10×13.1×0.85×85%＋1.732×10×12.1×0.85×15%）=1620571.69 kW·h

In direct on line state, power consumption is 1620571.69 kW·h.

4.3 Power consumption in variable frequency

C_b＝8500×（1.732×10×8.1×0.98×85%＋1.732×10×4.0×0.98×15%）=1079934.54 kW·h

In variable frequency state, power consumption is 1079934.54 kW·h.

4.4 Calculation

Power price: 0.65yuan /kW·h, yearly work time is 8500h.

（1）power cost in direct on line state: F₁＝1620571.69×0.65=1053400yuan

（2）power cost in variable frequency state: F₂＝1079934.54×0.65=702000yuan

（3）yearly power saved: ΔC= C_d－C_b = 1620571.69－1079934.54= 540637.15 kW·h；

（4）yearly cost saved: ΔF=F₁－F₂＝105.34－70.20＝351400yuan

（5）energy saving rate: （ΔC/C_d）×100% =（540637.15 / 1620571.69）×100% =33.36%.

5. Concluding remark

Reform with VFD meets design requirement, reduces pipe pressure, improves operation state of continuous casting misting cooling water system. It overcomes problems of high pressure of outlet in direct on line state and serious throttle loss. VFD satisfies requirement of cooling water pressure, heightens efficiency and saves energy. According to preliminary calculation, 200000yuan (USD 29410) is saved in a year. In addition, VFD reform protects pump and motor, reduces maintenance cost of pump and motor, prolongs equipments lifetime, lowers production cost.