ABBdriveX – Page 24 – AC DRIVES SOLUTION.

AQ: System operation

Our PSA unit (meaning Pressure Swing Adsorption) uses 5 adsorption vessels. The process itself is a batch process, but in order to run in a continuous process plant, each of the 5 vessels can complete all the adsorption process but at the same time, each of them is in a different status of the sequence (i.e. gas in, gas out, adsorption, pressurizing, depressurizing, cleaning, etc). The sequence is mainly controlled by time and pressure condition in each step of the sequence, by managing several valves (I think 5 by vessel, but I’m not sure right now).

Panel operator experienced some problems with valve 1 (gas entry) in vessel 2 because it should open but immediately it received the close command. Instruments technician check that orders coming from the DCS were OK, and also check the valves by injecting the open order, so, they and operation staff concluded that “the program has some kind of problem”.

Some time ago, I spent a lot of time studying the operation manual of this unit and the code written to control it and I wrote a document merging both knowledge. In page 9, I described a condition (an exclusive pressure difference between vessel and gas coming in the vessel) avoiding valve 1 opening during adsorption stage. I explained this condition to operation staff and they confirm that the values were right and that the excessive delta P really exist so, the decided to check back the valve 1 (already checked), discovering a problem (the stem moved, but the disk not).

Conclusion:
– If operation staff know properly the process, they know about this condition, but this could be solved with a properly designed HMI (i.e. including and alarm indicating “valve 1 closed by excessive deltaP”).
– The initial inspection of the valve didn’t show anything wrong, but stem and disk were disconnected.
– If we didn’t dig into the code, this problem, solved in less of an hour could take several hours.

AQ: What if

Many years ago we used to call this the “what ifs?”. Part of the design phase is when we model what we think the system is meant to do. Just as important is how the system is meant to react when things are not going well, the abnormal situations or what ifs?

Your client will tell you how their machine or process works, well he will describe how he thinks it works. This is OK as a starting point but we need to consider the scenarios of “what If” something goes wrong? Scenarios is also a good word as scenarios paint a situation that can be described to the customer for his comment.

For example on a compressor control project what if the lube oil pump fails on the compressor, how do we alarm this to the Operator, should we trip the compressor or do we start the back-up oil pump (if there is one). As you look at the system you can pick out various components and generate likely scenarios that you can discuss with the client. Using this approach gives more of a real world feel to your client meetings that are likely to generate a deeper insight into how the system is meant to work.

All scenarios do not have to be centered around abnormal situations but can also relate to things that need to be considered as part of normal operation. For example we might look at how a duty/standby pump system works? One scenario might relate to duty pump failure but another scenario might consider rotating the duty and standby pumps to even out wear and tear? You might also have manual mode and auto mode scenarios to consider?

What you have to remember is that most clients are not control systems experts. They might and probably will struggle with flow charts or any other pseudo code expression type formats that describe how you think the client’s system is meant to work? You have to tailor your approach to match your audience and that is also very important when you produce your documentation, you do not want to lose valuable information just because the client doesn’t fully understand what you are trying to tell him? Also make sure that you spend time with your client. Walk the client through your design, do this face to face as much as possible and do it more than once! Getting feedback on a regular basis helps to eliminate the dreaded word “REWORK”! Also taking this partnership approach builds a good relationship with your client.

For the machine builders your client might be in your own company? Remember same company or not they are your client and your success in no small way depends on your relationship with them.

Add the modes of operation and abnormal situations to your system model and develop the methods of how you will flag these situations to the Operator and Maintenance Engineer. Alarm Management is dealt with by EEMUA 191. If you do nothing else then read this document it will help you to set up alarm workshops, alarm reviews, alarm prioritization and rationalization and how to develop an effective alarm management structure for your system.

AQ: Why your project failed?

I have contracted with lots of different groups and moving within the same company to save failed projects or project in trouble or impossible to implement and helped these groups to achieve company goal. What I have noticed is that less the managers or groups know less they realize more knowledge or experience can help them. Less they know, less they understand they need help because they don’t know what they need. They think they are just fine until it is too late and a group or company goes under because of it.

I give you an example of one of the project I worked on at Nortel. I was assigned to write project specification for a product working with a director group with 100 designers and testers. During my research to get information to write the product specification I discover deficiency in the hardware they wanted to use that would cause the system reliability and availability unacceptable to the customer and did not meet customer requirement. I proposed design change in one of the interface card and firmware used in the system. The management did not agree with me on this item so I refused to write the specification the way they want it to not expose this deficiency. We had a large meeting with the president of the company with 20 people in that meeting looking at two different presentation to see if they need to change direction or stay on course and move me out of the way to another project activity. I am not the greatest in politics and making things look good when they are not.

The result was that I was moved to different project for 1 year implementing and releasing one more product that made the company lots of money. After a year development, they complete the project and released it to the customer. The customer starts validating the product and had lots of the test cases failing in the area that I proposed to change.
This was a large project and lots of money involve. The customer rejected the product and they went back on the drawing board after getting lots of this equipment on order for this project. The management came back to me and one of my team members to come back to the team and help.

Me and my team member both having experience over 15 years at that point came back and have a solution designing a new complex interface card with microcode firmware and some software to save the project. I and he had to work for 4 months for day and night having design review between two of us at 3 am in the cafeteria to get it done (defining specification to validated working product).

This project was completed and customer accepted this solution. The director group was dismantled and all people in the group were laid off and absolved in other groups in the company and some in the same group. The management groups were smart people with good intention, they were with software background and good intention for the project. They just did not have the knowledge and background to manage the system and hardware level because of lake of knowledge and experience in that area.

You see this in lots of companies when a software designer or manager is successful in their area, they get promoted and manage groups that are out of their area and lots of time they destroy groups or project because of not having the background to identify good or bad direction to go. You will always have engineers to not agree with each other and managements have to make decision to go one way or another. The wrong decision in these cases can destroy a project or company. Not all engineers can present a case in 1 hour to sell you their point of view, Remember they are not lawyer or sales man, they are engineers. So what do you do, Follow the sales man or lawyer to save your project or the reason to drive your decision and if you don’t have the knowledge or experience to lesson to the reason then you will make the wrong decision.

AQ: UPS systems commissioning test and inspection procedures

The UPS systems commissioning test and inspection procedures are to conform to;

• BS EN 50091-1:1993 – Specification for Uninterruptible Power Supplies (UPS). General and Safety Requirements, AND

• IEC 62040-3 (Draft Edition – 2) in particular the Efficiency test procedures outlined in its “Annexure-J”.

These procedures to include:

1. Visual Inspection:
a. Visually inspect all equipment for signs of damage or foreign materials.
b. Observe the type of ventilation, the cleanliness of the room, the use of proper signs, and any other safety related factors.

2. Mechanical Inspection:
a. Check all the power connections for tightness.
b. Check all the control wiring terminations and plugs for tightness or proper seating.

3. Electrical Pre-check:
a. Check the DC bus for a possible short circuit.
b. Check input and Bypass power for proper voltages and phase rotation.
c. Check all lamp test functions.

4. Initial UPS Startup:
a. Verify that all the alarms are in a “go” condition.
b. Energize the UPS module and verify the proper DC, walkup, and AC phase on.
c. Check the DC link holding voltage, AC output voltages, and output waveforms.
d. Check the final DC link voltage and Inverter AC output. Adjust if required.
e. Check for the proper synchronization.
f. Check for the voltage difference between the Inverter output and the Bypass source.
g. Perform full-load, step-load, and battery discharge tests using supplier furnished load bank.

AQ: Power industry engineers

The power industry has many tentacles. Energy production is one key subset, the design, manufacture, installation and operation of hydro, nuclear, fossil, renewables, etc is continuing to grow especially in the renewable area. Then there is the transmission of energy which includes the design/manufacture/construction/maintenance of substations, protection and control systems, overhead and underground lines, series and shunt compensation, etc. Last there is the distribution of the energy to the customers at the lower voltages which includes many of the transmission opportunities but introduces other niche areas like power quality, smart metering, distributed generation, etc.

It’s not as simple as stating you want a PHD in the power industry with hands on experience without first knowing the ins and outs of the business. As has been previously mentioned, get your BS in EE with a slant toward power. Get a job in a utility and learn the business top to bottom so you can actually make an intelligent decision on what area of the business floats your boat. Once you know that then pursue an advanced degree in that specific area (the real bonus is most companies will pay for it).

AQ: Operate low speed generator and high speed generator in the same terminal

Can we operate low speed generator and high speed generator in the same terminal? Is there a mechanical effect?

First, specify that this is an isolated system with two generators feeding the same bus. Operation of an isolated system is different than a grid connected system, and the mode setting of the governors have to be set to accommodate this. Depending upon the prime mover type and governor model, improper tuning will manifest itself in speed variations. The size of the two machines relative to each other, as well as their size relative to the load, can have measurable impact as well. The best way to tell whether it is mechanical or electrical in nature is to look at the time-frame of the phenomena relative to the time constants of the various control and response loops.

Second, “…In large power system, generators are not connected in the same terminal…” is not generally true, there are many power plants where multiple generators feed the same bus before the power is utilized.

Third, “…frequency oscillation is about 1.5-2 Hz…”, if you mean that the frequency swings between 48 and 52 Hz routinely, that usually indicates a governor setup/tuning problem or a non-uniform load.

Fourth, reactive current compensation takes place in quadrature from real power and should have minimal effect on real power and only affect the terminal voltage if not set properly. Droop compensation is the means for ensuring that the AVRs do not fight with each other since you cannot have two independent controllers attempting to control the same control variable.

Fifth, regarding different types of prime movers, some are inherently more likely to induce mechanical vibrations, especially reciprocating engines, especially if they are not all of the same size and/or number of cylinders. The same is true of the loads, non-uniform, cyclic loads can cause very severe problems especially on isolated systems where the load is a significant percentage of the prime movers’ output power. The analysis of, and solution to, such problems is an interesting area of study.

AQ: Industrial Ethernet vs. Fieldbus technologies

Where we really need digital communication networking, in my personal opinion, is down at the sensor/transmitter and positioner/actuator/valve level to take the place of 4-20 mA and on/off signals. Down at the level 1 of the Purdue reference model you need a fieldbus, not one of the “H2” types of fieldbus, but one of the “H1” types of fieldbus. When first introduced, these technologies were not as fast and not as easy to use has they could have been, but after many years of refinement these technologies are finally becoming sufficiently easy for most plants to use.

An “H1 fieldbus” is the most practical way to digitally network sensors/transmitters and positioners/actuators/valves to the DCS. Options include FOUNDATION fieldbus H1, PROFIBUS-PA, CompoNet, ASI, and IO-link. These protocols can take the place of 4-20 mA and on/off signals.

Note that “H1 fieldbus” should not be confused with the very different “H2 fieldbus” category of protocols used at level 1-1/2 of the Purdue reference model to connect remote-I/O,

AQ: DC Drives QUIZ

1. List three types of operations where DC drives are commonly found.

2. How can the speed of a DC motor be varied?

3. What are the two main functions of the SCR semi conductors used in a DC drive power converter?

4. Explain how SCR phase angle control operates to vary the DC output from an SCR.

5. Armature-voltage-controlled DC drives are classified as constant-torque drives. What does this mean?

6. Why is three-phase AC power, rather than single phase, used to power most commercial & industrial DC drives?

7. List what input line & output load voltage information must be specified for a DC drive.

8. How can the speed of a DC motor be increased above that of its base speed?

9. Why must field loss protection be provided for all DC drives?

10. Compare the braking capabilities of nonregenerative & regenerative DC drives.

11. A regenerative DC drive requires two sets of power bridges. Why?

12. Explain what is meant by an overhauling load.

13. What are the advantages of regenerative braking versus dynamic braking?

14. How is the desired speed of a drive normally set?

15. List three methods used by DC drives to send feed back information from the motor back to the drive regulator.

16. What functions require monitoring of the motor armature current?

17. Under what operating condition would the mini mum speed adjustment parameter be utilized?

18. Under what operating condition would the maxi mum speed adjustment parameter be utilized?

19. IR compensation is a parameter found in most DC drives. What is its purpose?

20. What, in addition to the time it takes for the motor to go from zero to set speed, does acceleration time regulate?

AQ: DC drive typical applications

DC drive technology is the oldest form of electrical speed control. The speed of a DC motor is the simplest to control, & it can be varied over a very wide range. These drives are designed to handle applications such as:

Winders/coilers – In motor winder operations, maintaining tension is very important. DC motors are able to operate at rated current over a wide speed range, including low speeds.

Crane/hoist – DC drives offer several advantages in applications that operate at low speeds, such as cranes & hoists. Advantages include low-speed accuracy, short-time overload capacity, size, & torque providing control. A typical DC hoist motor & drive used on hoisting applications where an overhauling load is present.

Generated power from the DC motor is used for braking & excess power is fed back into the AC line. This power helps reduce energy requirements & eliminates the need for heat-producing dynamic braking resistors. Peak current of at least 250 percent is available for short-term loads.

Mining/drilling -The DC motor drive is often preferred in the high-horsepower applications required in the mining & drilling industry. For this type of application, DC drives offer advantages in size & cost. They are rugged, dependable, & industry proven.

AQ: Non-regenerative & Regenerative DC Drives

Non-regenerative DC drives, also known as single-quadrant drives, rotate in one direction only & they have no inherent braking capabilities. Stopping the motor is done by removing voltage & allowing the motor to coast to a stop. Typically nonregenerative drives operate high friction loads such as mixers, where the load exerts a strong natural brake. In applications where supplemental quick braking and/or motor reversing is required, dynamic braking & forward & reverse circuitry, may be provided by external means.

Dynamic braking (DB) requires the addition of a DB contactor & DB resistors that dissipate the braking energy as heat. The addition of an electromechanical (magnetic) reversing contactor or manual switch permits the reversing of the controller polarity & therefore the direction of rotation of the motor armature. Field contactor reverse kits can also be installed to provide bidirectional rotation by reversing the polarity of the shunt field.

All DC motors are DC generators as well. The term regenerative describes the ability of the drive under braking conditions to convert the generated energy of the motor into electrical energy, which is returned (or regenerated) to the AC power source. Regenerative DC drives operate in all four quadrants purely electronically, without the use of electromechanical switching contactors:

Quadrant I -Drive delivers forward torque, motor rotating forward (motoring mode of operation). This is the normal condition, providing power to a load similar to that of a motor starter.
Quadrant II -Drive delivers reverse torque, motor rotating forward (generating mode of operation). This is a regenerative condition, where the drive itself is absorbing power from a load, such as an overhauling load or deceleration.
Quadrant III -Drive delivers reverse torque, motor rotating reverse (motoring mode of opera tion). Basically the same as in quadrant I & similar to a reversing starter.
Quadrant IV -Drive delivers forward torque with motor rotating in reverse (generating mode of operation). This is the other regenerative condition, where again, the drive is absorbing power from the load in order to bring the motor towards zero speed.

A single-quadrant nonregenerative DC drive has one power bridge with six SCRs used to control the applied voltage level to the motor armature. The nonregenerative drive can run in only motoring mode, & would require physically switching armature or field leads to reverse the torque direction. A four-quadrant regenerative DC drive will have two complete sets of power bridges, with 12 con trolled SCRs connected in inverse parallel. One bridge controls forward torque, & the other controls reverse torque. During operation, only one set of bridges is active at a time. For straight motoring in the forward direction, the forward bridge would be in control of the power to the motor. For straight motoring in the reverse direction, the reverse bridge is in control.

Cranes & hoists use DC regenerative drives to hold back “overhauling loads” such as a raised weight, or a machine’s flywheel. Whenever the inertia of the motor load is greater than the motor rotor inertia, the load will be driving the motor & is called an over hauling load. Overhauling load results in generator action within the motor, which will cause the motor to send cur rent into the drive. Regenerative braking is summarized as follows:

During normal forward operation, the forward bridge acts as a rectifier, supplying power to the motor. During this period gate pulses are withheld from reverse bridge so that it’s inactive.
When motor speed is reduced, the control circuit withholds the pulses to the forward bridge & simultaneously applies pulses to reverse b