Download Press Release - Plant Services

A Stable Ride Through Unstable Power Conditions Via Use of Variable
Frequency Drives
(Q & A with Robert Glickman, Thomas Bernhardt, ABB Inc.)
Q: Will industrial users continue to experience a decline in power quality?
A: (Robert Glickman) Most likely; while there has been an increase in power generation capacity to meet the
expected increases in demand, there has not been a corresponding investment in transmission assets.
In fact, investments in transmission assets have been in a steady decline for many years, undermining their
reliability. And any drop in supply reliability is surely a decrease in power quality.
This lack of investment is related to how energy production and energy transmission is regulated in the United
States. The U.S. power grid has grown as a patchwork of regional capabilities that are regulated by local, state and
federal authorities. Transmission assets are not growing because transmission investment has been more mired in
regulatory uncertainty than power generation. The fuzziness of jurisdictional boundaries has created a game of ‘hot
potato,’ in which States blame the Feds for inaction, and the Feds claim there is nothing they can do. Unfortunately,
deregulation was allowed to occur without clarifying these regulatory and jurisdictional issues.
Deregulation has had the effect of increasing distances between generation sites and power consumption points.
This increased the complexity and number of interactions between regional transmission systems, increasing the use
of overall transmission capacity. The failure to add to existing transmission assets has put added pressure on the
transmission infrastructure.
Liberalized rules for power trading, along with the liberalization of access to transmission grids, has increased the
number of operating constraints on the grid far more often than, historically, has been the case. As you can see
from the graphic, the incidences of transmission constraints have roughly quadrupled over the past four years,
regardless of seasonal load variations.
The situation would improve greatly if the Federal Energy Regulatory Commission made jurisdictional lines clearer.
There is general agreement that Federal regulators must fundamentally change the way in which the U.S. power
transmission system is governed. Over the past several years, the Federal Energy Regulatory Commission has laid
out a series of restructuring plans. The most recent plan calls for the existing regional transmission organizations to
evolve into independent transmission providers that operate regional transmission assets, administer a uniform
series of tariffs, and make markets for transmission services.
Because structural change has been, and remains, difficult to implement, you should anticipate that the system will
remain enmeshed in this process for some time to come.
The future of our energy system is not unplugging from the grid or creating more isolation, but lies in a far more
integrated power system connected by a much smarter gird than we have today.
Any future electrical grid will need more interconnectedness, not less. Creating islands of power will not meet
customer needs.
State-of-the art technologies that can make the grid more reliable include: High Voltage DC Transmission; Flexible
AC Transmission Systems; Gas-insulated Substations; increased use of underground cables and distributed
generation resources; and grid-control technologies such as wide-area monitoring systems. The application of these
technologies needs regulatory clarity, and utilities with financial incentives to make the necessary investments.
We recommend you closely monitor regulatory changes, along with improvements in power and semi-conductor
technologies; in particular, pay attention to improvements in semi-conductor technologies. You will find that the
improvements being made in these devices will help you better deal with some of the characteristics of poor power
quality.
Q: Basis that background, let’s consider the impact of this power-quality situation on the operation of
PWM 6-pulse AC drives. What are the power- quality problems users of AC drives need to be aware of and
what design features are important to minimize their effect?
A: (Thomas Bernhardt) Any power problem that reveals itself as a voltage, current or frequency deviation and
potentially causes equipment failure is a concern.
To determine the overall impact on a drive, the distance to the fault, the impedance of the system upstream of the
fault, the feeder impedance, and the transformer connections between the faulted system and customer electrical
system bus, should also be known.
Let’s look at each of these conditions.
Voltage Sags
Utility system faults frequently are asymmetrical, single-line-to-ground faults that produce voltage sag. Voltage sags
are a momentary decrease in rms voltage magnitude for typically one-half-to-30 cycles, or 8ms-to-one-half second.
These sags normally are described by their magnitude and duration. But they also need to be understood in terms of
the voltage imbalance, non-sinusoidal wave shapes and phase angle shifts that accompany them. I’ll discuss their
effects in a moment.
AC drives have considerable capability to ride through a voltage sag, because they store energy on their dc bus
capacitors and can make use of the energy stored in the load’s inertia.
The drive’s line voltage is monitored at the dc bus. Its’ control logic and fan power is taken from that bus.
Therefore, the drive is independent of line voltage sag as long as the dc bus holds up. The dc bus typically will trip
on Under-voltage at an equivalent line voltage of 65% to 51% of nominal rated voltage.
On its own, the dc bus can deliver full power to a load for about one cycle or 16ms by allowing itself to decrease
from its nominal voltage. As the bus voltage drops, the drive regulator adjusts the PWM pulse width to make up
the reduced magnitude of the output voltage waveform of the inverter. Alternatively, the drive could allow the
motor speed to decrease and use the energy stored in the inertia of the load to maintain bus voltage. Typically, more
energy is stored in the load’s inertia than the dc bus, making it possible to hold bus voltage for a longer duration -approximately one-half-to-5 seconds.
If the combination of the load’s inertia and energy stored on the bus are inadequate to prevent the bus from
tripping, the drive will use a programmable automatic restart function to recover. After the line condition has
cleared, the drive will follow a programmed delay, find the motor speed, and accelerate the motor to set speed,
resuming normal operation.
Wye-Delta Transformers
Transformer connections have an interesting effect on the ultimate line voltage seen at the load during an
asymmetric fault. Most ac drives are fed from a three-wire delivery without a neutral. As a result, the input rectifier
stage only sees line-to-line voltages. At the fault location, a single-line-to-ground fault will yield a voltage of zero on
one phase to neutral voltage, but the other two phases are essentially unaffected. If a wye-delta transformer is
between the fault and load, then two of the phase voltages on the delta side affected by the fault go to zero, but
none of the line-to-line voltages go to zero. The drive will see a voltage sag on two of the line voltages. Similarly, a
line-to-line fault will cause phase shifting and can cause a zero voltage condition on the secondary of a transformer.
Most AC drives require about 4% more ac input voltage than their maximum output voltage (i.e. 460V output
requires 480V input). This is due to residual zeros in the output waveform. Continuous low-line conditions may
prevent the drive from delivering full output voltage.
As a point of fact, ABB’s ACS800 drives actually can output 3% more voltage than the drive input in the HEX
waveform mode, so, with a 480V input, the drive will output 495V.
Phase Imbalance
The phase imbalance that is associated with voltage sag is caused by the difference in the inductive reactance to
resistance (X/R) ratios of the source and the faulted feed, and by the propagation of voltage sags due to singlephase faults through a transformer.
During normal operation, the input voltage to a drive is balanced and the diodes in the bridge rectifier are
symmetrically forward-biased. A phase imbalance acts to lower the magnitude of one or more line-to-line voltages,
reducing the peak voltage for one or more phases below the nominal capacitor voltage. This causes the rectifier
diodes not to forward bias. In this state, no energy flows from the ac mains to the capacitor. The capacitor will
continue to discharge until there is an input voltage peak that is high enough to forward-bias the diodes.
Since the capacitor has discharged more than its normal amount, the current drawn from the ac mains to recharge it
to peak voltage will be quite high. In fact, the rms input current on some phase might exceed 200% of normal
rating, and the associated peak current drawn in the high-current phase may be as much as four (4) times the normal
current.
Under these conditions, an ac drive often will trip on Over-current, because the line current was too high; or, on
Under-voltage, because the dc bus dropped below some threshold.
Similar effects will be seen when single-phase loads are unevenly distributed on the customer 3-phase power system.
The solution is to balance these loads more evenly across all three phases, or install an ac line reactor at the ac drive
that is experiencing problems. The reactor should be sized to carry the drive’s full load current, and it should have
an impedance rating anywhere from 2% to 6%.
Single phase input operation of drives for a short duration is usually not a concern. But, continuous single phase
input operation will require that the drive be de-rated or sized by two or more times the actual power requirement
because single phase current is 1.87 times three phase current for the same power.
(Hot standby Caterpillar diesel power generator - above)
Line Impedance
The effect of utility impedance on the operation of a drive is a function of how stiff or soft the line is. Nominally
(i.e. %Z = 1%), the drive short circuit current is set at 100 times the drive rating. On a soft line (i.e. %Z > 10%), the
short circuit rating is 10 times the drive’s rating; and on a stiff line (i.e. %Z < 0.1%), the short circuit rating is 1000
times the drive’s rating (limited only by fault interrupting limits).
Loading variations within the user facility are a second source for impedance issues. When facility loads are
energized, especially loads that have high inrush currents, excessive impedance will cause voltage sags. The effect on
a drive is the same as discussed earlier.
A number of design techniques are typically used to minimize line impedance deregulating effect. It is generally
more productive to make impedance improvements, where possible, in the highest impedance components, such as
transformers and long conductor runs. Some recommended practices include:
-- Avoid using multiple transformers to achieve proper load voltage;
-- Select low impedance transformers or oversize;
-- Design long cable runs at the higher voltages; and
-- Increase conductor size above the minimum ampacity required by thermal design.
Operating ac drives off a generator creates special considerations, because they typically have high impedance – 12to-20%, with respect to their rating. Voltage harmonic distortion should be kept to < 10% when operating drives
on generators to maintain generator regulator stability. To accomplish this, the generator sub-transient reactance
should be < 8%, with respect to the connected non-linear load KVA. The remaining generator capacity can be used
to power linear (sinusoidal) loads.
Voltage Transients
Capacitor switching is a common event on most utility systems. Shunt capacitors are applied on transmission
systems, distribution feeders and at substations. They adjust the line voltage for differences between daytime and
night time loading and may be switched on a daily basis.
Energizing these capacitors causes a transient voltage between the capacitor and the power system inductance.
Voltage transients also are caused by line switching of heavy loads, such as large motors, without the use of soft
switching, switching power factor correction capacitors, welding equipment, clearing line short circuit faults and
lightning strikes.
Voltage transients are sudden, one-shot, sub-cycle voltage disturbances of tens to hundreds of microseconds in
duration and could be over 1KV.
Today, you can purchase drives that have considerable built in transient energy absorption capability: I’ll provide
you with some recommendations in a moment.
Voltage Swell or Over-voltage
Voltage-source PWM drives use a dc capacitor to smooth the rectified line voltage and act as an energy source for
the inverter section of the drive. Line voltage to the drive is monitored at the dc capacitor or bus. Over-voltage trip
points are normally set at 130 to 135% of nominal rated voltage. An Over-voltage trip inhibits drive operation, but
does not remove the drive from the line. And, by regulating the motor voltage via PWM modulation, the automatic
restart function of the drive can be programmed to recover from an Over-voltage trip, once the normal line
condition is restored.
Frequency Changes and Operation on a Generator
AC drives have a wide fundamental line frequency range: 47 to 63 hertz. The rate of change should be < 10
hertz/second.
The frequency delivered by a generator is related directly to the speed of the generator. As the generator is loaded
the speed of the generator may drop along with its delivered frequency.
Ground Placement
Drive power circuits are usually floating with respect to ground except for MOVs and EMI/RFI filters. Ground
reference can be at any potential at or within the line voltages. EMI filters should only be used on symmetrical
grounded neutral systems.
Much of the published literature related to power quality suggests that a high percentage of power quality problems
are, in fact, related to wiring issues within the user’s facility, with specific reference to grounding and bonding.
These problems are eliminated easily through proper training of engineering and maintenance personnel.
AC Drives and Power Conditioning
Power line disturbances cannot be avoided totally. Different solutions exist to mitigate voltage, current and
frequency deviations that are produced by the utility system. The first possible solution is to stabilize the overall
voltage on the MV facility level. We are not addressing this solution here.
A second solution is to stabilize voltage deviations and their corresponding current and phase imbalances directly
on the load or equipment side, which, in most cases, is low voltage.
A number of low-voltage devices exist that have the capability to deal with a range of power quality conditions.
Examined individually, each of these devices may either fully correct the power quality condition – as reflected by a
green coding; or, cannot fully correct a power quality condition – as reflected by a red coding.
AC drives, in their own right, can – and should -- be viewed as a power conditioning technology. The degree to
which they will correct power quality conditions is largely a function of the design of the drive and the ratings of its
semiconductors.
Recommended Protective-Functions
Let me conclude with a set of recommended protective-functions and ratings that, if followed, will provide you with
the most robust drive performance when power quality conditions are present at the drive input.
Specify:
-- Phase-to-phase and phase-to-ground line voltage transient protection;-- A broad voltage operating range. We
recommend a range from 65% to 130% of nominal voltage;
-- Power-loss ride through, utilizing kinetic energy recovered from the rotating mass; and
-- Ground testing at the factory to 2500V rms power circuit to ground with MOVs disconnected.
Recommended Ratings
Specify drive components with high voltage stress ratings:
-- 1600 PIV rectifier diodes; and
-- Line-to-line and line-to-ground Metal Oxide Varistor transient protection rated 120 to 370 joules, up to 8000 amp
max, providing up to 160,000 watt absorption at the rectifier voltage rating for 1 to 2 milliseconds.
As the MOVs have only a 1-to-2 watt line-to-line power rating, care must be taken to avoid subjecting the MOVs to
a continuous string of transients. An effective way to eliminate nuisance tripping is to isolate the drive from the
power system with series inductors (chokes or line reactors). The additional series inductance of the choke or line
reactor reduces the magnitude of the voltage transient. ABB drives have 3% line reactors on drives > 60HP and
equivalent 3% to 5% bus reactors on drives 60HP and below.
-- Specify control power operation down to an equivalent line voltage of 95VAC.
-- Specify rated power without speed reduction to the load for up to 15 milliseconds before tripping occurs during
brownouts and full power outages.
-- Specify current ratings @ 110% for 1 minute (out of 10); 180% for 2 seconds (out of every 60); and 350%
instantaneous over-current trip.
-- Specify frequency variation with a 47-to-63 hertz tolerable range and rate of change > 10 hertz/second.
AC Drives play a critical role in power conditioning, as power moves from utility grid to motors on mechanical
equipment.
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About the Authors:
Robert Glickman has 20 years experience in utilities and power systems consulting. Prior to his consulting work he
was with the Los Angeles Department of Water and Power where he started the department’s Power Quality
Group. He is currently part of ABB’s Electrical Systems Consulting group.
Tom Bernhardt has 40 years experience in drive design, application and in engineering management. He is currently
principal engineer within the ABB Inc., Low-Voltage Drives business unit.
For more information please contact:
ABB Media Relations – U.S., Power
Technologies
Mary Flieller
919-856-3806 phone
919-856-3810 fax
[email protected]
ABB Media Relations – U.S, LV Drives
Ken Graber
262-780-3873 phone
262-785-8501 fax
[email protected]