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Medium voltage VFD application in Huludao Pingshan Waterworks
Abstract: combining with actual state of Pingshan Waterworks, this paper introduces cell-cascade multi-level VFD application in the water plant.
Key words: medium voltage big power VFD cell-cascade multi-level remote control energy saving

In recent years, medium voltage variable frequency technology is applying in key industries increasingly.
Huludao is serious short in water. Pingshan Water Plant is lifeblood of this city. Water-lack makes water adjustment very important. There are 4 sets of 10KV/710KW pumps with rated flow 1800m3/h and lift 120m. Pingshan Water Plant is 60km away from Huludao city and there are several hillsides of 100m on the way. So pipe pressure has reached to 0.9Mpa. Flow is adjusted according to user demand and water storage. General range is 2000~3800m3/h. In general, 2 pumps should start and 3 pumps start together sometimes. Valve adjustment is difficult to control and it wastes energy. After lots of inspections and researches, we decide to choose Leader&Harvest VFD. After several months of operation, we find the VFD has advantages of energy saving, easy operation and good stability.

II. Variable speed scheme
According to actual state, we design following scheme: 1# and 4# pumps are variable frequency pump. During normal work, one works and the other is spare. Switch between the two pumps is manual. Both two pumps must not work in variable frequency state or direct on line state simultaneously. Thereby, we design mechanical interlock in bypass switch cabinet. Principal drawing:

KA1, KA2 are medium voltage breakers
KG1-KG4 are isolator switches.

III. Energy saving
1. Actual site of pump
Pingshan Pump Station transfers water to purifying plant of Huludao city. After being treatmented, the water is transferred to waterworks.
2. Energy saving analysis

Pump character curve

In variable speed mode, flow changes to Q2 from Q1. Speed is adjusted to N2 from N1. Work point lowers to B from A. In valve control mode of constant speed N1, pump is from A to C. Pump power of B and C are: PC=H3×Q1, PB =H2×Q1;If efficiency of B and C is little different, about η approximately, energy saving

3. Actual operation
In direct on line state, pump output in full load is 1800m3/h. We make statistic for 3268m3/h and 2600m3/h in two modes.
Mode A: two pumps run in direct on line.
When they are running at 3268m3/h, accumulative flow is 50190m3, power consumption is 21450kW.h, namely 0.4274kW.h/t. At 2600m3/h, accumulative flow is 62270m3. Power consumption is 28950kW.h, namely 0.464kW.h/t.
Mode B: one pump runs in variable frequency state and one pump runs in direct on line state. When they are running at 3268m3/h, accumulative flow is 47525m3, Power consumption is 18375kW.h, namely 0.3866kW.h/t. At 2600m3/h, accumulative flow is 68100m3. Power consumption is 29550kW.h, namely 0.4339kW.h/t.
In mode A, pump works at 3268m3/h for half year and at 2600m3/h for half year. Compared with mode B, cost saved is: (0.4274-0.3866)×3268×24×365/2×0.45+(0.4649-0.4339)×2600×24×365/2×0.45=262802+158862=421664yuan (USD 62009).

III. Valve link
HARSVERT-A series VFD equips with Siemens S7-200 PLC. Valve link function: during pump acceleration, outlet pressure rises gradually. When the pressure is more than "minimum pressure of opening", valve starts to open fully. When pump stops, valve closes synchronously. When pump starts, if valve does not open fully, it presents "valve not open fully". When pump stops, if valve does not close tightly, it presents "valve not close". In actual operation, we set "minimum pressure of opening"as 0.9Mpa. By this way, in process of starting pump or stopping pump, no need to adjust valve, which reduces adjustment mistake and impact on pipe.

IV. Easy operation
HARSVERT-A VFD has perfect function of upper PC monitor. Personnel can operate VFD in control room. HMI of VFD and remote monitor interface display flow, pressure, outlet pressure synchronously. It reduces work intensity and heightens efficiency.

V. Advanced remote monitor
For managers of water plant, they often need to know VFD work state. Remote dial-up function of VFD makes it reality. They just need to install dial-up software in their computer. They dial special line of VFD to see VFD interface when it is necessary. The software designs operation limitation for them, so no mistake operation affects VFD work. If VFD is broken, Leader&Harvest can get exact operation data to help them solve the problem through dial-up function

VI. Attention
1. When constant speed pump and variable speed pump work in parallel, speed range of pump should be not too big in order to avoid internal friction. it is usually not less than 50% of rated speed, 70~100% is better. When speed is less than 40~50% of rated speed, pump efficiency lowers obviously.
2. Conventional pump is designed as per full speed. It usually works in high speed for long term and just experiences other speed during start-up. So it is difficult to ensure no resonance on other points besides line frequency. Driven by VFD, pump may work stably in each point besides to line frequency. So resonance test for pump in whole speed adjustment range is necessary. If there is resonance point, it should be input to VFD to prevent VFD from running in these special points for long time.
3.Ambience temperature should be less than 40℃. Installation space should be big enough, otherwise heat would not be discharged and over heat would lead to VFD stop for protection. When ambience temperature is more than 40℃, air conditioner is necessary. When installation space of VFD is limited, ventilation device should be designed to ensure VFD normal operation.

VII. Concluding remarks
Medium voltage VFD will be used in more and more water systems. In addition to its energy saving effect, it can be connected to DCS to constitute a big close loop control system. It makes equipment run reasonably, heightens automatization level and gain considerable social benefit.

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