vsd

The notion of using Variable Speed Drives (VSDs) andProgrammable Logic Controllers (PLCs) in oil and gasfacilities and pipeline pumping station applications is

hardly new. Besides giving pipeline operators a better range ofchoices in controlling flow rates and pressures, VSD andPLCs give the operators the ability to effectively control theirpower usage. With the electrical utility deregulation causingwide swings in regional power rates and the exposure of manyto market demand prices, facility and pipeline owners arewatching their power usage more closely. Many oil and gascompanies are applying Variable Frequency Drives (VFDs) asthe VSD of choice for pipeline pumping and facilities appli-cations for two reasons: • Enhanced VFD/PLC based motor control with energy savings • Enhanced VFD/PLC based motor transfer control with

soft-starting

In the first case, variation of the VFD output frequencycauses proportional variation of the motor and pump speed,giving energy savings over conventional pump stations. Inthe second case, since the frequency output is set by thedrive, it can be used to soft-start and synchronously transfer(or sync-transfer) motors between fixed speed operation at 60Hz and variable frequency operation from 0 to 60 Hz.

In order to understand these benefits let us look at the fol-lowing associated topics:• Medium Voltage VFD Technology Update• Oil and Gas Facilities Operation and Energy Savings

Basics• VFD/PLC Synchronous Transfer Basics• Typical VFD Project Experiences

Medium Voltage (MV) VFD Technology UpdateDevelopments in microprocessor based control systems

have made possible many technological changes and conse-quent cost savings in the oil and gas business. VFDs have alsoseen the same significant improvements.

Over the last ten years, driven by these control technologyimprovements, there has been a slow migration from 6 and 12pulse inverter control systems to PWM based inverter sys-tems. Also the two following VFD inverter topologies havebecome popular with VFD systems designers, namely:• Current-Source Inverters [See diagram 1A] • Voltage-Source Inverters [See diagram 1B]

28 ELECTRICAL LINE May / June 2001

Pipeline

ManagementBy Roy Spencer & Jon Blois

UsingVFDs andSync-TransferControl Systems

Energy

MotorAC-DC

RectifierAC-DCInverter

MotorAC-DC

RectifierAC-DCInverter

Diagram 1A: Current Source - VFD

Diagram 1B: Voltage Source - VFD

May / June 2001 ELECTRICAL LINE 29

Semiconductor Device Improvements – VFDs With Conventional MV BridgesThe switching characteristics of thyristors have substan-

tially improved allowing GTO (Gate Turn-Off) thyristors andtheir recent cousin the Integrated Gated CommutatedThyristors (IGCT) to be used to enhance the performance ofconventional MV bridge based VFDs. These semiconductordevices have made possible such design improvements asactive front-ends, smaller harmonic filters, and the use ofmore sophisticated Pulse Width Modulation (PWM) controltechniques. These device improvements have resulted inimproved input and output power quality[1] albeit with morecomplex control circuits. [See diagram 2: Details of MVBridge Based VFDs]

Semiconductor Device Improvements – VFDs With Multiple Power CellsThe switching characteristics of IGBT’s (Integrated Gate

Bi-polar Transistors) have substantially improved, allowingthe IGBT to be used to enhance the performance of conven-tional MV bridge based VFDs as well as multiple power celldrive topologies. These semiconductor devices have madepossible design improvements that have led to the complete

elimination of the harmonic filters with incremental improve-ments in input and output power quality[1] over the conven-tional MV bridge designs. [See diagram 3: Details ofMultiple Power Cell Based VFDs]

More recently higher voltage and current rated IGBT’s,used extensively in low voltage drives, were made availablefor use in medium voltage VFD systems. The use of IGBTsin medium voltage VFDs brought out the best in switchingcharacteristics along with the simpler control circuits associ-ated with the device. These systems are being applied todaywith terrific power quality improvements[1].

Advantages Of Multiple Power Cell Designs Over MV Bridge Based VFDsAs mentioned above, prior to the availability of medium

voltage, power cell based, modular VFDs, medium voltagebridge designs were the topology of choice.

With the medium voltage bridge arrangement, in order toovercome the individual device voltage and current ratinglimitations, the thyristors were connected in series to extendthe range of the bridge configuration. Both current and volt-age balancing networks were necessary to ensure devicevoltage and current sharing and compensate for the manufac-turing variations in the semiconductors.

A significant factor in the development of the more techno-logically advanced power cell based VFD systems, was thecoordination of the power semiconductors control circuits overmultiple power cells. The improved performance of the newpower cell based VFDs, over those with a conventional MVbridges, has resulted in a dramatic increase in their use, espe-cially in remote pipeline and similar oil and gas installations.

The uptime advantages of power cell based VFDs are:• VFD maintenance allows for a cell repair-by-replacement

method rather than requiring the immediate replacement ofindividual semiconductor devices.

• In many drives where higher availability is desired, additional

M

Input Filter forPower Factor andHarmonic Correction

3-Phave MV Input

Input TransformerOutput Filter

Series Thyristors or GTOs

Conventional MVBridge Based - 12/6

pulse CurrentSource VFD

Perfect Harmony Liquid Cooled VFD System, NEMA 12 cabinet construction, typical for Enbridge pump stations; 7500 Hp, 4200VAC VFD system,overall length: 356"(30ft)

Diagram 2


cells can be added to the VFD system so that if any cell fails,the coordinated power cell control can bypass the faulty celland keep the VFD in service. This gives the drive configura-tion a level of redundancy in the event of a device or compo-nent failure.

• Some new drives can even change the coordinated PWMcontrol to all cells, thereby effectively shifting the drive out-put neutral point to maintain phase balance. This clever tech-nique permits the removal of only the failed cell and leavesthe other cells able to contribute power to the drive’s output.

Oil and Gas Facilities Operation And Energy Savings Basics

For many applications equipment sharing, such as VFDs orPCVs (Pressure Control Valves), makes perfect sense. Thisis true in most pumping stations with multiple series con-nected pumps. Most pump stations are operated by simplystarting one pump, then the next, and so forth, until there issufficient head developed. Such stations also use a PCV totrim the total pump unit pressure to the desired station dis-charge pressure set point, the latter station discharge pressureset point being set by an operator based upon the desiredpipeline flow requirements. Throttling using a PCV is a nec-essary evil, in that it provides a desired outlet pressure but ata great energy cost.

In the traditional PCV based station the total head is theaddition of one or more units plus a half-head unit. In the caseof the VFD based station, the half-head unit is replaced by

M

Input transformer Power Cells

OR

POWERCELL A1

POWERCELL A2

POWERCELL A3

POWERCELL A5

POWERCELL A4

POWERCELL B5

POWERCELL B4

POWERCELL B3

POWERCELL B2

POWERCELL B1

POWERCELL C1

POWERCELL C2

POWERCELL C3

POWERCELL C4

POWERCELL C5

Diagram 3

May / June 2001 ELECTRICAL LINE 31

varying the speed of one or more of the full head units.Utilizing a VFD, instead of a PCV, improves the flexibility ofa half-head pumping unit without having to buy the energyfor the entire unit. It does this by varying the speed of a sin-gle full-head or half-head unit rather than throttling. Thuswith the VFD no energy is lost due to the elimination of pres-sure drop across the PCV.

VFD/PLC Synchronous Transfer BasicsThere have been many pipeline pumping station installa-

tions with sync-transfer VFD systems. Even though muchstation and pumping unit design has been done and much ofthis previous design is applicable, there is a need for anappropriate amount of engineering for each project. This isusually necessary because of the site to site variations in theelectrical power supply and the hydraulic pumping systems.

Note - the purpose of a pumping station is to deliver theenergy into the hydraulic system. Often the power involvedin the movement of fluid is large. If the system controllingsuch large amounts of power is improperly configured, pres-sure shock waves in the liquids pipeline can result and liter-ally cause a pipeline rupture. Similarly an improperly con-trolled closed circuit, sync transfer of two high power motorcircuits can cause a similar and spectacular energy release.

PLC Based System Types & RequirementsControl systems that implement both VFD sync-transfer

controls and station controls sometimes are combined into

one PLC system. Alternatively, in the case where the “stationcontrols” are developed by a different control specialist thanthe “sync-transfer controls”, a separate VFD-PLC system isused to reduce the programming coordination required.

Because station operation procedures vary from companyto company and sometimes between sites, safety limits andunit lockouts must be defined before the PLC programmingis started. Also a rigorous commissioning procedure must beused to ensure the closed circuit sync transfer controls oper-ate correctly.

VFD/PLC Based Synchronous Transfer ControlIn any system where one VFD controls multiple motors,

the problem becomes “How do we efficiently move pumpingunits from the 60Hz utility bus to the VFD output or variablefrequency bus and vice-versa”?

VFD/PLC based sync-transfer controls have the followingcomponents:• VFD per system• PLC per system• VFD contactor per motor• Bypass contactor per motor

During synchronous transfer control system operation it iscommon to set the VFD output frequency to match the 60 Hzoperation of the electrical utility. This allows paralleling ofthe utility 60 Hz bus and the VFD output bus by closing boththe VFD and bypass contactors. Once the output frequency

matches the utility’s operating fre-quency (with equal voltage magnitudeand phase angle), the latter motorbypass and VFD output contactors maybe simultaneously operated to quickly

transfer the motor from the VFD outputto the 60 Hz utility bus and vice-versa.This process allows the VFD to sequen-tially control multiple motors using theclosed circuit, sync-transfer operation.The latter is the basis of “VFD/PLCbased sync-transfer control” where oneVFD controls many motors.

The VFD and associated PLC basedcontrol system is programmed toimplement the synchronous transfercontrol process by the use of two basiccommands:

UP Transfer - transfers the motorfrom the VFD to the Utility bus.DOWN Transfer - transfers themotor from the Utility bus to theVFD.

UP Transfer Command Process[See figures 1 through 5]The PLC based control system UP

transfer command involves the follow-ing operations - assuming all motors arestopped on receipt of an operator com-mand the PLC and VFD operate as fol-lows:1. PLC closes motor1 VFD contactor2. VFD accelerates motor1 to the oper-

ator set point3. PLC receives a start motor2 com-

mand4. PLC sends the UP transfer request

for motor1 to the VFD5. VFD accelerates motor 1 to 60 Hz

and confirms synchronization OK 6. PLC closes motor1 bypass contactor

transferring motor1 to the 60 Hz bus7. PLC opens motor1 VFD contactor

and closes motor2 VFD contactor8. VFD ramps its output and acceler-

ates motor2 to the operator set point Note: This process continues until all

requested motors are operating.

DOWN Transfer Command ProcessThe PLC based control system

DOWN transfer command involves thefollowing operations - assuming at leasttwo motors are operating on receipt ofan operator command the PLC andVFD operate as follows:1. PLC receives stop motor2 command

from the operator2. PLC requests VFD to decelerate

motor2 to minimum speed 3. PLC opens VFD contactor4. PLC sends DOWN transfer request

for motor1 to VFD

5. VFD accelerates to 60 Hz and con-firms synchronization OK

6. PLC closes motor1 VFD contactorand opens motor1 bypass contactor

7. VFD decelerates motor1 to theoperator set point

Note: This process continues untilonly one motor is operating on theVFD.

Soft Start Advantage OfSynchronous Transfer ControlA great benefit of the VFD is the abil-

ity to avoid across the line starts byramping the unit motors from zerospeed to the minimum speed over sev-eral seconds. The soft start advantage ofVFDs allows large motors to be sequen-tially started while maintaining compli-ance with the local utility flicker limitsand minimizing the mechanical strainon the electric motors. [see figure 6]

VFD Project ExamplesLike many great ideas, the “real

world” implementation requires a sig-nificant amount of attention to the con-trol of the system in order to realizethese benefits. However the capital costsavings can be compelling with largeVFD equipment costs sometimes over amillion dollars per drive.

In the early 1990s, notable pipelinecompanies like Enbridge (then IPL) andBP (then Amoco) invested in the appli-cation of VFDs for improvements intheir pipelines. In 1994, Enbridgedeployed nine 3700 HP, Ansaldo Ross-Hill VFDs on their new Line 13 pipelinerunning from Hardisty, Alberta toClearbrook, Minnesota. The new pump-ing stations were in configurations ofone, two or three pumping unitsdepending on their location on thepipeline. The deployment and operationof these new systems brought about anew set of “real world” problems. New


Figure 1

One VFD, Many Motors

1

3φ

2 3 4

VFD

Figure 2

Soft Start a Unit

1

3φ

2 3 4

VFD

Pump StationPLC Controls

Figure 6

Why Use Sync-Transfer?

•Capital Costs of VFDs are less•Synch Transfer Operation works!

Sync TransferPLC Controls

VFD Station Synch transfer Station

SG

VF

D

SG

VF

D

SG

VF

D

SG

VF

D

SGSG

SGSG

SGSG

SGSG

VF

D

contactors

Figure 4

Pick Up Next Unit

1

3φ

2 3 4

VFD

Figure 5

And So Forth ...

1

3φ

2 3 4

VFD

Figure 3

Sync and Transfer to Utility

1

3φ

2 3 4

VFD

controls for the operators meant newoperations procedures and training.New power utilization characteristicsmeant changes to the pipeline energymodel and new rate agreements with theutilities. The introduction of VFD gen-erated harmonics meant the need forharmonic filters and the associated elec-trical system strains for any unfilteredharmonics. Electrical maintenance per-sonnel needed new maintenance proce-dures and training. Even the warehous-ing of spares added new complications.On the mechanical side, the pumps werenow operating in new speed ranges sothe pump performance curves were dif-ferent. In some cases fluid resonance inthe pump casing became a factor at cer-tain operating speeds. Even with thesenew factors in their business, the valueof VFDs at the pumping station toEnbridge and other pipeline companieswas sufficient to call for the deploymentof more VFDs in pumping stations inthe years to follow.

I Want A New Pumping Station;And I Want It Now!In a marketplace that demands

“Faster, Better, Cheaper” the applica-tion of sync-transfer can make animpossible project possible. On March17, 1997, a Husky Oil managementteam called for the design, procurementand construction of a new pumping sta-tion located between Lloydminster andWainwright, Alberta. [2] Because of anincrease in shipping oil nominations,they needed the station in service by theend of June 1997, (approximately 100days later), otherwise they would betrucking it. The station needed to beabout 5000 HP of new pumping capac-

ity. Because of the station’s location onthe electrical grid, the transmissionlines would not take an across the linestart. So a VFD or soft-start would haveto be used. With two pumping units, aVFD for each unit was cost prohibitiveand made the equipment procurementtiming twice as difficult. The applica-tion of the sync-transfer VFD configu-ration in this case lowered the equip-ment cost close to that of using soft-starts.

The primary issue in this situation, asin most situations, is the need to assessthe application requirements in contextwith the end user’s primary business.Sadly, even the customers themselvessometimes get caught up in a projectside issue and overlook important busi-ness issues. In the case of this pumpingstation, the long and short of the cus-tomer’s business was to ship oil costeffectively. Side issues of a very shortproject execution time and the weakconnection point on the electrical utilitygrid overshadowed the more funda-mental issues of pipeline flow require-ments, pump characteristics andpipeline operating pressure constraints.

An evaluation with Husky to assesstheir basic business needs, then defin-ing schedules, then the electricalrequirements led to an aggressive, butattainable business solution. An ableproject team was assembled. ARobicon Harmony 3500 HP VFD wasprocured and engineering was done toinclude the sync-transfer contactorsand controls. In the end these projectobjectives were met and the pumpingstation is in service today, takingadvantage of the VFD and theenhanced controls.

ConclusionIn Canada and United States there are

hundreds if not thousands of multipleunit pumping stations that are prospec-tive VFD sync-transfer opportunities.These sites could also reap the benefitsof improved pipeline control and energycost savings like the applications dis-cussed. (In many cases the energy sav-ings alone could pay for the implemen-tation of the VFD and associated PLCcontrol system.) The number of viableopportunities has increased appreciablysince there have been so many techno-logical improvements in the VFDs, VFDcontrol and associated application. Ω

Roy Spencer is the Canadian SalesManager for ASI-Robicon and is on theBoard of Directors of JTG Technology. Royhas his Bachelors Degree in ElectricalEngineering from Trent University and hisMasters Degree in Electrical Engineeringfrom the University of Alberta.

Jon Blois is a Senior Engineer at HinzAutomation Inc. Jon is a RegisteredProfessional Electrical Engineer inCalifornia, and he also teaches in theElectrical Engineering department ofthe University of Alberta.

References [1] “Power Quality” is defined by the

IEEE 519 industry accepted standard.

[2] “Building a 5000HP, VFD Controlled,Pump Station in 105 Days ‘Fact orFantasy’”, by Spencer, Granger, Blois,International Pipeline Conference,Calgary, 1999

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