VFD/PLC Based Synchronous Transfer Control

VFD/PLC Based Synchronous Transfer Control

 In any system where one VFD controls multiple motors, the problem becomes “How do we efficiently move pumping units from the 50Hz utility bus to the VFD output or variable frequency bus and vice-versa”?

VFD/PLC based sync-transfer controls have the following components:

• VFD per system

• PLC per system

• VFD contactor per motor

• Bypass contactor per motor

During synchronous transfer control system operation it is common to set the VFD output frequency to match the 50 Hz operation of the electrical utility. This allows paralleling of the utility 50 Hz bus and the VFD output bus by closing both the VFD and bypass contactors. Once the output frequency matches the utility’s operating frequency (with equal voltage magnitude and phase angle), the latter motor bypass and VFD output contactors may be simultaneously operated to quickly transfer the motor from the VFD output to the 50 Hz utility bus and vice-versa.

This process allows the VFD to sequentially control multiple motors using the closed circuit, sync-transfer operation. The latter is the basis of “VFD/PLC based sync-transfer control” where one VFD controls many motors. The VFD and associated PLC based control system is programmed to implement the synchronous transfer control process by the use of two basic commands:

UP Transfer - transfers the motor from the VFD to the Utility bus.

DOWN Transfer - transfers the motor from the Utility bus to the VFD.

UP Transfer Command Process [See figures 1 through 4]

 

The PLC based control system UP transfer command involves the following operations - assuming all motors are stopped on receipt of an operator command the PLC and VFD operate as follows:

1. PLC closes motor1 VFD contactor

2. VFD accelerates motor1 to the operator set point

3. PLC receives a start motor2 command

4. PLC sends the UP transfer request for motor1 to the VFD

5. VFD accelerates motor 1 to 50 Hz and confirms synchronization OK

6. PLC closes motor1 bypass contactor transferring motor1 to the 50 Hz bus

7. PLC opens motor1 VFD contactor and closes motor2 VFD contactor

8. VFD ramps its output and accelerates motor2 to the operator set point

Note: This process continues until all requested motors are operating.

DOWN Transfer Command Process

The PLC based control system DOWN transfer command involves the following operations - assuming at least two motors are operating on receipt of an operator command the PLC and VFD operate as follows:

1. PLC receives stop motor2 command from the operator

2. PLC requests VFD to decelerate motor2 to minimum speed

3. PLC opens VFD contactor

4. PLC sends DOWN transfer request for motor1 to VFD

5. VFD accelerates to 50 Hz and confirms synchronization OK

6. PLC closes motor1 VFD contactor and opens motor1 bypass contactor

7. VFD decelerates motor1 to the operator set point

Note: This process continues until only one motor is operating on the VFD.

Soft Start Advantage of Synchronous Transfer Control

A great benefit of the VFD is the ability to avoid across the line starts by ramping the unit motors from zero speed to the minimum speed over several seconds. The soft start advantage of VFDs allows large motors to be sequentially started while maintaining compliance with the local utility flicker limits and minimizing the mechanical strain on the electric motors.

BIGGEST MISTAKES IN INSTRUMENTATION

BIGGEST MISTAKES IN INSTRUMENTATION

Mistake #1: Selecting the wrong sensor

Technology mismatch Although it’s generally obvious what quantity needs to be measured in a flow, temperature, or pressure control application, it’s not always obvious what kind of flow meter, temperature sensor, or pressure gauge is best suited to the job. A mismatch between the sensing technology and the material to be sensed can lead to skewed measurements and severely degraded control.

 Mistake #2: Installing sensors incorrectly

The best sensor can yield disappointing results if not installed correctly.

Mistake #3: Poor Grounding

Instruments must be grounded to provide a reference voltage for the data signals they generate. Relying on earth ground is risky since not all of the earth shares the same electrical potential. The resulting currents will interfere with the sensors’ signals.

While it’s generally a good practice to insulate a sensor from the thermodynamic effects of its surroundings it’s absolutely critical to establish electrical isolation. The most common electrical problems due to poor installation are ground loops. Ground loops occur when an extraneous current flows through the instrumentation wiring between two points that are supposed to be at the same voltage, but aren’t (see Figure ). The resulting electrical interference can cause random fluctuations in the sensors’ output and may even damage the sensors themselves. As the name implies, ground loops most often occur when instruments and their cables are grounded improperly or not at all. Interestingly, the best way to isolate a plant’s instruments from ground loop currents is to connect them together at one master grounding point. If that’s not possible, a grid of grounding points must be spread throughout the plant, making sure that all points on the grid are at the same electrical potential. Insecure connections and inadequate wires can cause a voltage imbalance in the grid and ground loops between the instruments connected to it.

Mistake #4: Generating gibberish

Noise

Ground loops are not the only source of noise that can distort a sensor’s readings. Radio frequency interference (RFI) is even more common in plants that use walkie talkies, pagers, and wireless networks extensively. RFI also results whenever a current changes, such as when an electro mechanical contact or a static discharge generates a spark.

The sources of RFI noise must be eliminated or at least kept away from the plant’s instrumentation if at all possible. Replacing electro mechanical equipment with solid state devices will eliminate arc-generated RFI.

Mistake #5: Quitting to Soon

Calibration

Most instrumentation engineers know that a sensor must be calibrated in order to associate a numerical value with the electrical signal coming out of the transmitter. Yet all too often, the instruments are calibrated just once during installation then left to operate unattended for years. The result is an insidious problem known as drift. A sensor’s output tends to creep higher and higher (or lower and lower), even if the measured variable hasn’t changed.

Deposition on the sensing surfaces, corrosion in the wiring, and long term wear on moving parts can all cause an instrument to begin generating artificially high (or low) readings. As a result, the controller will gradually increase or decrease its control efforts to compensate for a non-existent error.

Planning for the road ahead

Natural Gas Liquefication Process: LNG

Natural Gas Liquefication Process:

A liquefied natural gas plant (LNG plant) is roughly divided into five processes:

1 pretreatment,

2 acid gas Removal,

3  Dehydration,

4 liquefaction and

5  Heavy oil separations.

(1) In the pretreatment process, undesired substances are removed from the gas taken from a gas field. Then the gas is separated using a separator/slug catcher into oil and water which are then weighed.

(2)Natural gas taken from a gas field contains environmental pollutants like hydrogen sulfide (H₂S) and carbon dioxide (CO₂). These impure substances are absorbed and removed from natural gas with an amine absorber (acid gas removal or AGR). With the use of a sulfur removal unit (SRU), sulfur is extracted from the hydrogen sulfide in the removed pollutant.

(3)An adsorbent is used to remove water from the natural gas from which impure substances have been removed so that ice will not form during the subsequent liquefaction process.

(4)Traces of harmful mercury are removed before liquefaction.

(5)The heavy compounds separation process is the core of an LNG plant in which natural gas is cooled and liquefied to –160°C or less using the principle of refrigeration. Because gas is cooled and liquefied to an extremely-low temperature during the process, an enormous amount of energy is consumed. How much this energy can be reduced is important, so various ingenious processes have been proposed and commercialized.

Major liquefaction processes are as follows:

1)  C3-MR method: The C3-MR method is currently the main method. Propane and mixed coolants (nitrogen, methane, ethane and propane) are used as the coolant (APCI), and an improvement on this method called the AP-X method is also used for large LNG plants.

2)  AP-X method: As liquefaction trains get larger, they approach a limit on the size of heat exchanger that can be produced and transported. This process can increase LNG production capacity by adding LNG sub-coolers with nitrogen coolant used according to the C3-MR method, without increasing the size of the main heat exchanger (APCI).

3)  Cascade method: This method sequentially uses propane, ethylene and methane as the coolant (Phillips).

4)  DMR method: This method uses two kinds of mixed coolants (an ethane and propane mix and a nitrogen-methane, ethane and propane mix) (Shell).

5) SMR method: This method is called the PRICO process and uses only one kind of mixed coolant (Black & Veatch).

  

Voltage Drop on 4 to 20 mA loop

Voltage drop on 4 – 20 mA loop:

Current loops are ideal for data transmission because of their inherent insensitivity to electrical noise. Designing 4-20 mA current loops is just managing the voltage drops around the loop. The voltage drops occur in the wire, the transmitter and load resistor.

Formula for calculating the DC voltage-drop is V = 2xIxRxL, where:
V = Voltage-Drop, in volts.
I = Current, in amperes.
R = Resistance, in ohms/km.
L = Distance, in km.

Table 1 Copper Wire Resistance @ 20°C (68°F)

Ohms per American 1,000 feet (305 meters)

Wire Gauge         Solid Wire           Stranded Wire

14                           2.53Ω                                     2.73Ω

16                           4.02Ω                                    4.35Ω

18                           6.39Ω                                    6.92Ω

20                           10.15Ω                                  10.90Ω

22                           16.14Ω                                  16.50Ω

24                           25.67Ω                                  27.70Ω

The voltage drop in wire is directly proportional to the current flowing through it by Ohms law, current x resistance equals voltage. The transmitter consumes 7 to 15 VDC of loop voltage, depending on model, to power itself.

Finally, the controller’s load resistor converts the loop current to a voltage for easy signal processing. Most commonly a 250 Ω resistor is used to convert the loop current back to a voltage. At 4 mA, a 250 Ω resistor drops 1 volt; at 12 mA, a 250 resistor drops 3 volts and at 20 mA, a 250-Ohm resistor drops 5 volts. The load resistor can be internal to the controller or external.