As a type of electrical load, steel mills are the equivalent of jackhammers early in the morning. If you’re not prepared for them, they can cause headaches. Steel mills, with arc furnaces, have a randomly varying demand that can swing as much as 200 MW for a 300 MW steel plant every 30-90 minutes or so. The effects of this load change may be noticed in lights, PC’s, and TV’s. When a mill’s furnace comes on, the voltage dips and rises when it is switched off.  Voltage and the frequency will change and result in a change of light intensity.  In addition, the arcs in each furnace of the mill can result in an imbalanced load that is loaded with harmonics that varies cycle to cycle creating what one may call “dirty” power. This load connected to the grid can affect other customers connected to the grid.

In the grid itself, there is need to provide adequate capacity to supply the range of demand of the steel mill.  A 300 MW mill will require at least two 230 kV circuits, and any upgrades to the network to bring the 300 MW to the load point, including voltage support.  Equipment need to be designed to withstand the maximum switching surge.  The network needs to have sufficient damping of oscillations from each change in demand from the mill.  Frequency dip during load pickup may reach below the threshold at which under-frequency load shedding relays are set, and trigger nuisance trips.

To the electrical emanations from the mill, a large turbine-generator looks just like another load.  Thus, mills introduce harmonics and negative sequence currents that can cause rotor heating. Moreover, mills using cyclo-converter drives may produce sub-synchronous resonance that can excite turbine-generator torsional vibrations.  On the other hand, steel mills feed on the bountiful and agile supply of reactive power from the generator.  Smaller turbines, such as wind mills, are also susceptible to the harmonics coming from steel mills.  Both frequency and voltage dips must be within tolerance of the wind farm protection and low-voltage ride-through capability.

All this makes for a sometimes unfriendly neighbor on the grid.  The magnitude and breadth of the impact from a steel mill varies by grid and, for certain specific effects, by location of the mill in the grid.  There are known medicines or countermeasures for specific types of impacts.  But there is no single cure-all.  Before one can prescribe the type and strength of countermeasures, it is important to conduct a detailed analysis, or as the doctor would say, “We have to do some tests.”

Types of Tests

The battery of tests to evaluate a steel mill’s impact can include: system simulations involving power flows, contingency analysis, short circuit, transient switching, stability and extended dynamics, torsional analysis, harmonic measurements and voltage flicker analysis.  The specific regimen of tests depend on whether the mill exists or is the design stage, the size of the mill loads and it’s relative location on the grid.

The system tests are generally treated as impact studies, but for thoroughness, we recommend a planning approach wherein not just the initial year of operation is tested, but rather over time, in conjunction with the development of the grid.  In many cases, the impacts, especially on power quality are localized and once the prescription for controlling the PQ impacts are made measurements are needed to ensure that the impacts are controlled within acceptable criteria.

The system response performance is tested against the system reliability, voltage flicker, harmonic and frequency criteria which would include acceptable response to probable disturbances such as first contingencies, as well as extreme contingencies.  Fundamental frequency impacts can be determined from long-term dynamic simulations or thru approximation.

For instance, the during the first half second after the load swing, you can apply the following rule of thumb derived from the energy balance:  f = SQRT (1. –  dP * 0.5/ Ke).  For a dP = +/- 200 MW and neighboring plants of  2000 MW-sec of steady state kinetic energy, then the frequency changes are in the order of  +/-1.5 Hz for a 60 Hz system. Subsynchronous frequency impacts require a torsional analysis that evaluates the response of turbine generator mass-damping systems to SSR from mill operations.  The test is to ensure that there is adequate damping in the mechanical system to contain any subsynchronous excitation.

Power quality is measured against several industry standards, including flicker standard IEC-41000 and IEEE 519.

Once the impacts of the mill are known, then we can move on to identifying solutions.

Types of Countermeasures

The proximity of equipment, such as a generator, to the mill make them vulnerable to harmonics developed during the steel-making processes.  To contain the harmonics would require a carefully designed harmonic filter within the steel mill.

If the mill fails the flicker and/or voltage dip standards, a typical solution is to implement a static var device (SVD). The SVD should be sized to fully compensate for the reactive demand of the electric arc furnaces, ladle furnaces, rolling mill and other loads in the mill, including any additional reactive power needed to help control the voltage drop caused by MW and Mvar load changes. The control of the SVD should be an open-loop control between the SVD and furnaces to provide faster response to load changes. A slower closed loop control is often employed for power factor or var demand control. A substantial power plant close to the mill could provide sufficient voltage “stiffness” that the SVD may not be required for flicker control. However if a rolling mill is present, harmonic filters may be required due to the high percentage of drives on variable speed control.

Switching of large loads could cause unacceptable frequency excursions for relatively small grids, grids with low overall inertia or with slow or no frequency control. To provide the necessary frequency control, a lower ramp rate on the load may be one option. Other frequency control options may require changing turbine governor settings, or enabling or changing out controllers in generators.

Detectors and planned automatic response systems may sometimes offer a cheap prescription to a mill system impact.  For example relays to detect unhealthy torsional impacts may trip a generator from further harm.  Harmonic monitors may be set to trip the mill when distortions exceed acceptable levels.

Any other system reliability impact are addressed in the normal fashion of system planning studies, through new transmission, upgrades, voltage support, and the like.

Conclusion

Despite the potential headaches, many steel mills are able to integrate well with the neighborhoods they share the grid with.  Through careful application of specific countermeasures, the impacts can be contained.  It’s always a good idea to do some pre-operational testing prior to bringing one online.

References:

  1. “The impact of large steel mill loads on power generating units,” Solanics, P. Kozminski, K. Bajpai, M. Esztergalyos, J. Fennell, E. Gardell, J. Mozina, C. Patel, S. Pierce, A. Skendzic, V. Waudby, P. Williams, J., Power Delivery, IEEE Transactions, Jan 2000, Volume: 15, Issue: 1, pages 24-30.
  2. “Dynamics of a large induction motor load system,” Undrill, J. Renno, A. Drobnjak, G., Power Engineering Society General Meeting, 13-17 July 2003, Volume 3, page 1403.

Notes:

  1. Although the above has focused on a large arc-furnace mill, a rolling mill can have the same effect. However, the change in the rolling mill load MW’s are generally not as great as an arc-furnace.

For questions, comments and further discussion, contact us at info@pterra.us