POWER SYSTEM RELIABILITY IN INDUSTRIES - PART III

RELIABILITY OF INDUSTRIAL PLANT POWER SYSTEM – PART III
by Doods A. Amora, PEE

VOLTAGE DIPS & SAGS

VOLTAGE DIPS & SAGS IN INDUSTRIAL PLANTS


Voltage Dips & Sags and their impact on plant operation constitute the most frequent and most noticeable reliability problem in the distribution systems of industrial plants. Spoiling Reliability, voltage dips can result in trip-outs of plant equipment - shutting down manufacturing lines leading to production loss and expensive re-start procedures.

These system disturbances can be grouped into three general types:

Voltage Sag is a partial reduction in the magnitude of voltage that often persists for extended periods and is usually related to system loading conditions.

Voltage Dip is a significant reduction in voltage for a relatively short duration, often caused by power system faults, or as frequently in events of large motor starts-up.

Voltage Interruption is a complete loss of input voltage, lasting from seconds to a much longer time.

Inside the industrial plant, voltage dips may be caused by faults somewhere in the other parts of the distribution system and more frequently by large motor starts-up in already loaded transformers. Common in heavy industrial plants, voltage sags are caused by heavily loaded transformers with poor voltage regulation and relatively lengthy distribution lines.


Utility system Voltage Dips are often caused by bad weather conditions (thunderstorm, etc), or utility equipment failures, or faults in some other areas of the utility system; and can last anywhere from a few cycles to seconds or more. Voltage Sags in utility systems on the other hand, are related to voltage drops along transmission lines and more prevalently by system loading conditions, as well.

Voltage Interruption is the complete loss of voltage and usually require a source of energy to replace the utility supply.

There are several mechanisms by which voltage sag or dip can interfere with industrial manufacturing processes, as follows:


Control Error – Loss of control power results in the inability to control the process.

Contactor Dropout – Many industrial controls employ magnetically-latched contactors as motor control devices. A voltage dip or sag can cause a momentary collapse of the magnetic field which holds the contacts closed. When the contacts open, the motor stops.

Voltage Flicker – In the practical sense, flicker is the repetitive variation in intensity of lighting, and is more of a human irritation factor (threshold of perception, or threshold of human objection) than a direct cause of process disruption. However, it can also be used in a more literal sense to describe a set of problems in which lighting is extinguished due to voltage dips.

Machine Dynamics – Since voltage magnitude is essential to transmitting power, voltage dips and sags limit the ability of a power system to distribute power from sources to loads. This limitation in power transfer can lead to generators not being able to maintain stability.

Stall & Re-Acceleration – Motors will stall if the supply voltage is depressed for a prolonged period. Furthermore, motors must reaccelerate when normal voltage is restored. Reacceleration involves higher than normal motor currents which may result in further voltage sag problems.


MITIGATING VOLTAGE SAGS

As system modifications can be implemented to minimize the magnitude and duration of voltage dips & sags; these modifications too, don’t come in cheap. However, if the vulnerability of the system is understood, the system engineer may well identify the only selected critical areas that require modification. Or if several problems can be solved with “one stone hitting several birds”, then these capital outlays would come up in reasonable proportions.


So then, voltage dips & sags generally imply solutions that provide some means of supporting voltage. One good voltage support scheme is the ‘On-Load-Tap-Changer’ (OLTC) usually in large substation power transformers. Other schemes could be the Automatic Voltage Regulators (AVR’s) which are separate apparatuses from the transformer but somehow perform similar function as the OLTC’s. While the OLTC’s are accomplished in the primary winding side of the transformer, the AVR’s are preferred voltage correction at the secondary output side. Smaller voltage support apparatuses could be the Voltage Stabilizers which are for control systems. Again, these don’t come in cheap.

Because of the diversity in sub-distribution system peaks, OLTC’s are very effective in primary substations which are usually upstream of several layers of smaller downstream substations. But at the intermediate voltages of 3.6 kV, 4.16 kV or 6.9 kV where large motors are populating, the performances of OLTC’s and AVR’s may be found wanting because of their inherent time delay responses. Note that the time delay is designed purposively in these equipments. OLTC’s and AVR’s don’t just react instantaneously, as the logic of these apparatuses would first make sure that the voltage variation is real and not just fleeting spikes. This is to prevent hunting or wild operation. They too, have contacts immersed in oil. The contacts wear rapidly and the oil need frequent replacements. And operation must be interrupted more frequently.


VULNERABILITY TO VOLTAGE DIPS

There are many things that could happen during voltage dips. Generally, for a running motor for instance; as the supply voltage to the motor decreases considerably during voltage dips, the motor speed decreases. Depending on the depth and duration of the voltage dip, motor speed may recover to its normal value as the voltage amplitude recovers. If the voltage dip magnitude and/or duration exceed certain limits, the motor may stall and would be taken out of the system by the means provided for in its controls. Maximum voltage dip magnitude and/or duration, which the motor operation can survive, depend on the motor parameters and the torque-speed characteristic of the driven load.

On the other hand, when a large motor is started-up in an already sagged voltage of an already loaded transformer source, the ensuing resultant voltage dip during motor starting may cause the motor to stall and could not complete the starting process. As discussed above, this scenario would also affect the running motors, as they too may stall - aggravating the problem in a domino effect. In this scenario, currents fly high and something somewhere has to trip to relieve the system from further disturbance.


SYSTEM BEHAVIOR DURING LARGE MOTOR START-UP


Transformer reactions to large motor-starting are manifested in voltage dips. In general terms, transformers in an electrical system are usually larger than the maximum demands they serve, in some instances even larger than the connected loads. In the industrial plant scenario, the obvious reason at first glance for this apparent oversizing is the anticipation for future load growth. Fine…

But more often than not, sizing the transformer with extra kVA capacity unwittingly addresses voltage dip problems, not for load growth for which it is intended originally. That’s why for newly constructed plants where load growth is not yet there, the problem of starting significantly large motors may not surface out. Why? Because the extra kVA capacity intended for load growth is taking care of it.

Now, when the plant is already 25 years old and a series of expansions & load growths had been in place, the transformers may now be loaded near their full load ratings. Fine…the transformer can still handle it. However, effects of voltage regulation (which is normal to transformers) this time would surface out. The voltage difference between loaded and unloaded output of a transformer is voltage regulation. Voltage Regulation in transformers is normal – meaning, as the load increases there is a decrease in voltage output due to the corresponding voltage drop within the primary and secondary windings.

Now if a 3.6 kV transformer (transformer terminal voltage of 3.6 kV by North American Norms) has a voltage regulation of 8% (which is usual); the voltage at the secondary terminals in a fully loaded transformer would then be 3.3 kV. If the same transformer is 80% loaded, the voltage at the secondary terminals of the transformer may have been in the vicinity of 3.37 kV. What happens then when a 2,300 kW or a 6,400 kW motor is started up? There would surely be voltage dip! By how much..? And how much voltage dip can cause problems?


BEHAVIOR OF LARGE MOTOR START-UP

Partly similar to short circuit condition, motors have a high initial inrush current when energized at a low power factor (0.30 lagging for 100 hp & about 0.16 for 1,000 hp motor) from standstill. This sudden increase in the current flowing to the load causes a momentary increase in the voltage dip at the supply transformer terminals, the drop along the distribution system, and a corresponding reduction in the voltage at the utilization equipment.

The magnitude of transient current involved in motor starting is however lower than the short circuit condition. But in effect, switching “on” to energize a large motor can be likened to a “soft short circuit”. Like short circuits, the effect of starting large motors results to huge voltage dips but lower in magnitude than the full short circuit scenario.

The voltage drop at the transformer secondary terminals is proportional to motor starting kVA over the short-circuit capacity of the transformer. When motor starting kVA is drawn from a system, the voltage drop in percent of the initial voltage is approximately equal to the Motor Starting kVA divided by the Sum of this kVA and the Short Circuit kVA (Motor Application & Maintenance Handbook by Robert W. Smeaton, 1969 Edition).


SUBSTATION TRANSFORMERS Vs. LOADS

Because of the effects of Transformer Voltage Regulation and Voltage Dips in starting large motors, North American motors after mid 1960’s are rated at 230, 460, 2,300, 4,000, 6,600 or 13,200 volts for use with distribution systems that are rated at 240, 480, 2400, 4,160, 6,900 or 13,800 volts respectively. Again, note the difference in motor nameplate voltages viz-a-viz the transformer terminal voltages. The apparent higher distribution nominal voltages than motor voltages are deliberately established by transformer and motor manufacturers to deal with the inherent voltage drops in the system such as: internal voltage drop of the transformer as dictated by its voltage regulation capability, voltage drop along the distribution cables and the impedance of the system. This could mean that a transformer could be 4,160v at no-load condition may only be 4,000v or less at the motor terminals when the system is already heavily loaded. While in this condition, when a large motor is started-up somewhere in the system, the more critical will the voltage be as felt by other loads in the system.

The “nominal system voltage” is the terminal voltage of the transformer. Per General Electric (Prolec) Publication, “secondary voltage ratings are approximately 4.2 percent above the standard motor voltages, allowing for voltage drop in the line between the substation and the motor terminals without operating the motor at subnormal voltage. Motors and control operate satisfactorily on voltages 10 percent above or below rating.”

In some IEEE proceedings, a question has been raised why the utilization equipment voltage ratings and system nominal voltage cannot be made the same. Manufacturers’ response is that the performance guarantee for utilization equipment is based on the nameplate rating and not on the system nominal voltage.

SIDEBAR: THE LOWEST ACROSS-THE-LINE STARTING VOLTAGE THAT MOTOR MANUFACTURERS GUARANTEE IS 90% OF THE MOTOR NAMEPLATE VOLTAGE,- NOT ON THE NOMINAL VOLTAGE .RATING.

That’s why most motors are designed to be capable of operating at plus or minus 10% of nameplate voltage. Therefore, the voltage drop on inrush should not be allowed to drop more than 10% of the rated voltage. This means 208v for 230v or 414v for 460 volt motors. Likewise, 2.07 kV for 2.3 kV motors, or 3.6 kV for 4.0 kV motors. It means that a 4.0 kV motor can still operate satisfactorily at 3,600v but any disturbance in the system that brings the system voltage lower, the affected motors may trip off as provided for by its protection – or if not, the motor burns.

Hence, the wisdom of employing “reduced voltage stating methods” are common to large motors in order to reduce the voltage dips during starts-up. Foremost of these mitigating starting methods are: reduced voltage auto-transformer type, soft starters, and variable frequency controllers for squirrel cage induction motors and resistor starting (liquid or hard resistor) types for wound rotor induction motors.


ECONOMIC EVALUATION

While the stability of the incoming supply voltage is fundamentally a technical problem, at the end of the day, it is business sense that makes decisions in implementing a fix. Some solutions to voltage dip and sag problems require the use of exotic technology. Today, terms like: “voltage dip-proofing” or “voltage dip & sag immunization” are already a reality, but then again, they don’t come in cheap – and may be prohibitive!

A major influencing factor concerning the financial loss is whether or not the factory production is continuous. As practiced by many industries, continuous production means that there is market to all products that a company can produce. In continuous production, the production lost during downtime cannot be recovered by working extra time, so loss of production translates directly into loss of profit – that is, the loss is equal to the value of the product not produced as a result of the downtime.

In a non-continuous process, lost production can be recovered by overtime working, although there may well be additional labor & utilities consumption costs.

Whether or not a solution is seen as cost-effective thus depends on the economic criterion that is used to evaluate the solution. The actual economic justification in preventing production interruptions due to voltage disturbances must therefore consider the following elements:


1) How vulnerable is the process to various types of voltage disturbances?

2) What is the net cost of production outages due to these disturbances?

3) How effective is a particular solution in avoiding these outages?

4) How does the cost of the solution compare to the savings which can be realized?


As to the cost associated with voltage interruptions the following elements should be recognized and quantified:

Cost of Lost Production – In the simplest case, this is the incremental margin on products that cannot be sold because they are not manufactured.

Cost of Damaged Product – If the interruption damages a partially completed product, the cost of repairing that product must be recognized. In some cases, the product cannot be repaired, so the value of the raw materials (including the consumed energy and other manufacturing costs up to the point where the disruption occurred) must be accounted for together with the cost of the incremental value added to the product.

In other environments, a major source of concern is lost computer data.

Cost of Maintenance – This is the cost of reacting to a voltage disruption. This includes everything involved in restoring production, including trouble-shooting and correcting the problem, cleanup and repair, disposing of damaged product, and environmental costs.

In some industries (e.g., plastics, glass manufacturing, cement manufacturing, electronics, etc), an interruption may result in the need to invest many days and a significant amount of money in cleaning up the process system before it can be returned to service.

Hidden Costs – This factor may be the most difficult to quantify but it can easily be the most significant. If the impact of the voltage dip or sag is control error, it is possible that the impact on product may not be apparent until the product is in the hands of the consumer. As business nightmare, product recall and the subsequent public relations costs can be significant or may even cause bankruptcies.

It is usually best to identify the problems that are responsible for Plant Reliability, and apply solutions that most efficiently address those gaps.

If the manufacturing plant does not design its system with reliabilty and quality in mind, SOMEONE ELSE WILL.

DAA
Sept, 2008