Power Quality Engineering - Products > Hybrid Reactive Power & Harmonic Filter

CASE STUDY 3 - 3000hp Metal Shredder Medium Voltage Dynamic Reactive Power Compensation

Overview
A large metal recycling plant near Pittsburgh, PA processes thousands of tons of waste metal each year with a 3000hp (2.237MW) metal shredder. Although the largest amount of scrap metal is from automobiles, the plant also processes many other forms of metal scrap including refrigerators, other similar domestic metal waste, and larger industrial steel waste such as piping, steel drums, etc. Scrap metal shredding is an extremely important process in the disposal and recycling of household and industrial metal products.

Typical Metal Shredder

The vast majority of metal shredding machines are operated at medium voltage (2.4kV to 13.2kV) with motors varying in size from 2000hp (1.5MW) to as large as 8000hp 6MW). Under full- load conditions, these machines can operate at 165% of rated power with considerable dynamic load fluctuations. This case study presents before and after measurements of an installation with a 3000hp shredder connected to the network through a dedicated 5MVA 13.2 / 4.16kV transformer.

A Real Time Reactive Power Compensation (RTRPC) dynamic power compensation system rated at 4.5MVAr was installed and connected directly to the shredder supply using a 5MVA 600V/4.16kV step-up transformer (See Figure 1, page 2) The RTRPC was supplied in two equally rated parts to offer redundancy for annual maintenance and breakdown purposes.

3000hp Metal Shredder Load Profile
The shredder idles at approximately 30% of full load (1000hp), but power levels increase dramatically with the onset of shredding. Typical load increases are 150% to 200% within 3 to 4 cycles (50-67msec). The degree of dynamic load fluctuation is directly proportional to the type of material being processed, i.e.: heavier material requires higher power consumption.

Reactive power (kVAr) consumption also fluctuates dynamically; varying 200- 250kVAr every 3 to 4 network cycles during even lightly loaded periods. When the shredder is operating at peak load, (measured at 9825hp, 7.325MW), the reactive power demand peaks at 4.4MVAr. (See Fig.3 on page 3) Ultimately these power fluctuations result in unacceptable degrees of voltage modulation (flicker or sag) which can be transferred to the local network and affect other consumers nearby.

Peak kilowatt demand of the shredder is impossible to reduce. Therefore, without increasing the supply transformer capacity to at least match the peak kVA consumption (8.3MVA) and adding dynamic compensation systems, it is not possible to completely eliminate voltage sags. However, if the reactive power component alone is dynamically removed, then the total consumed power (kVA) will be minimized and the voltage modulation can be limited to an acceptable level.

RTRPC System Description
The RTRPC systems for this application were designed to operate on a “Load Share” basis. Each
2.25MVAr powered system is responsible for 50% of the shredder load. This method extends the lifespan of the system(s) and increases overall reliability. Either system can be re-programmed in case of breakdown or maintenance shut down to operate in “Full Load” mode. If half of the system is out of service, the remaining unit is capable of responding to 70% of the shredder’s reactive power demand.

Each RTRPC system connects or disconnects steps based on sophisticated control algorithms which consider True Power Factor (PF) taking into consideration all harmonics up to and including the 63rd harmonic order, reactive energy demand (kVAr) and voltage at 4.16kV. The PF and kVAr are monitored and used for reactive compensation, while voltage control functions may also be utilized depending upon utility requirements.

2 x 2.25MVAr RTRPC Systems with 5MVA 'Step-up' Transformer and MV Switchgear

Figure 1 : Basic Electric Diagram of System Installation

To simultaneously prevent outages caused by over-current trips of the shredder’s 1200A main circuit breaker and to meet utility voltage requirements, the RTRPC system is connected in parallel between the breaker and the shredder. The current transformers for monitoring the system power for dynamic reactive power control are installed on the power cables feeding the shredder. Information gathered is fed to each RTRPC system controller (2) to enable either combined load share control or independent standalone operation.

Further control functions of each RTRPC system can be enabled to allow the system(s) to compensate for the main upstream transformer and further reduce system losses.

Performance of the RTRPC: Varying Load Conditions
Figure 2 is a combination of two recorded measurement sequences. The first is a 12 second duration measurement without the RTRPC in operation, and the second, a 10 second sequence with the system running. Each measurement period uses a sampling rate of 60 samples per second.

The measurements detail the dynamic nature of a metal shredder load as it moves from “idling” to “shredding” operation. In the first period of measurement (without RTRPC), the overall real power consumption (kW) surges by more than 2.6MW in less than 6 seconds. In the second measurement period (with RTRPC), the kW consumption surges even more dramatically, increasing more than 3.5MW in less than 2 seconds. In each case, the kW requirement fluctuates every 2-3 cycles by +/-250kW while the reactive power (kVAr) consumption fluctuates by +/-500kVAr at the same rate.

Figure 2 : Shredder Operating Modes With & Without the RTRPC Systems

As can be seen, the dynamic load increases directly impact the supply voltage in a negative manner. The no-load voltage at the shredder averages approximately 4.25kV. The first measurement period starts with the shredder lightly loaded and quickly ramps up to a relatively high load (3.5MW), at which time the voltage at the shredder sags to 3.96kV (overall voltage sag of 290V, or 7%). Although the overall kW and kVAr consumption is slightly higher during the second measurement period with the RTRPC system(s) functional, the voltage sag is significantly reduced to only 80V (2% below no-load voltage).

Figures 3 & 4 on the following page depicts a 2 minute measurement period during which the highest load recorded on the 3000hp shredder after installation of the RTRPC systems occurs. During this event, the demand peaked at 9825hp, 327.5% of the rated power of the motor. Although apparent power (kVA) consumption at the motor peaks at 8.3MVA, the utility (mains) see over 1.0MVA less demand due to the RTRPC systems.

Further, even though power consumption peaked at more than 3 times the motor rating, voltage sags were still limited to less than 3% of 4160V, the motor’s nominal operating voltage. Figure 4 expands the peak load of Figure 3 to show more detail. It depicts the effectiveness and extremely accurate operation of the RTRPC systems. The utility kVA demand is reduced to equal the kW consumption of the shredder motor, effectively reducing average kVAr demand to zero and maintaining the average power factor at unity (1.0).

Figure 3 : Peak Recorded Shredder Load – 3000hp Motor

Figure 4 : Expanded Peak Shredder Load

Contact our application engineers for further details click here.