Performance evaluation of custom harmonic filter design for port cranes

Shashank Mathur, Niranjan Patil, Marcelo Lara TMEIC Corporation, Roanoke, VA

Abstract—This study discusses harmonic injection to the grid with crane systems connected to the point of common coupling adhering guidelines from utility, and advantages of custom harmonic filter design.

Keywords—Harmonic filter, Point of Common Coupling, Total Harmonic Distortion, Total Demand Distortion, Automatic Stacking Crane, Quay Crane, Active Front End, Variable Frequency Drive

I. INTRODUCTION

With growing sophistication, complexity and demanding efficiency to operate the port terminals, there is tremendous emphasis on the technology used in electromechanical machinery design for cranes. Due to variation in motion, load, height, speed, torque and wind, the power requirements vary drastically and dynamically. As a result, magnitude and phase angle of the currents and voltages may change, affecting power factor, overshoot, harmonics and related phenomena. As the volume of machines goes up to meet the requirements of the business, degradation of power quality becomes a considerable concern for the grid operators. In addition, the harmonic levels also affect the smooth operation of drives, motors and other electrical equipment as they face thermal challenges due to increased line currents [1], leading to maintenance, reliability and safety issues with increased costs. This paper presents a case study to highlight the advantages of custom harmonic filter design in meeting specific criteria.

II. HARMONICS

A. Variable Frequency Drive

With the growing popularity of variable frequency drives

(VFDs), there is a concern over harmonics injected into the utility grid. The system in Fig. 1 shows active front end (AFE), an IGBT-based controlled rectifier [2] followed by DC bus and inverter stage [5].

Fig. 1. Power stages in crane electrical system

A lower switching frequency to switch the IGBTs would result in higher switching ripple, and thus can introduce harmonics affecting the performance of other electrical components [5].

B. IEEE-519 Guidelines

The IEEE-519 standard provides guidelines for limiting

harmonic injection to maintain the quality of voltage at the point of common coupling. It requires that each user connected at the PCC meet the limits of THD and TDD. The

IEEE limits for voltage and current harmonics shown are dependent on several variables and concepts defined as follows [3] [4]:

PCC: Point of common coupling. This point is defined as the point in the utility service to a particular customer where another customer could be connected.

ISC: Available short-circuit current.

IL: 15- or 30-minute (average) maximum demand current.

TDD: Total demand distortion. IL is used instead of the fundamental current component.

Table 1. Current Distortion Limits (in % of IL) for General Distribution Systems (120-69,000 V)

ISC/IL 3 ≤ h<11 11≤ h<17 17≤ h<23 23≤ h< 35 35≤h≤50 TDD

<20 4.0 2.0 1.5 0.6 0.3 5.0

20-50 7.0 3.5 2.5 1.0 0.5 8.0

50-100 10.0 4.5 4.0 1.5 0.7 12.0

100-1000 12.0 5.5 5.0 2.0 1.0 15.0

>1000 15.0 7.0 6.0 2.5 1.4 20.0

Table 2. Voltage Distortion Limits (in % of V1)

PCC Voltage Individual Harmonic (%) THDV (%)

V ≤ 1 kV 5.0 8.0

1 kV < V ≤ 69 kV 3.0 5.0

69 kV < V ≤ 161 kV 1.5 2.5

161 kV < V 1.0 1.5

IEEE Std 519-2014 recommend users should limit current harmonics [3] as follows:

Daily 99th percentile very short time (3 s) harmonic currents should be less than 2 times the values given in Table 1.

Weekly 99th percentile short time (10 min) harmonic currents should be less than 1.5 times the values given in Table 1.

Weekly 95th percentile short time (10 min) harmonic currents should be less than the values given in Table 1.

Similarly, users should limit line-to-neutral voltage harmonics [3] as follows:

Daily 99th percentile very short time (3 s) values should be less than 1.5 times the values given in Table 2.

Weekly 95th percentile short time (10 min) values should be less than values given in Table 2.

Harmonic filters are used to limit harmonics introduced by nonlinear loads from being injected into the grid. They are used to compensate for harmonics distortion and to improve power quality. They also limit line current and improve power factor to control transformer kVA and neutral upsizing. Efficient design of the filter would ensure harmonic limits specified in the IEEE-519 are met and THD is minimized.

III. CASE STUDY: PORT CRANE HARMONIC FILTER

A. Harmonic Analysis of 11kV system

This case study is based on the data obtained from port

authorities and analysis performed to estimate the harmonic distortion due to operation of multiple Automatic Stacking Crane (ASC) and Quay Crane (QC) systems. The focus is to highlight the custom design approach ensuring compliance of both local and global utility guidelines and to analyze performance of the harmonic filter against worst-case scenarios. It is interesting to observe the difference in harmonic injection between two different cases of load connected to utility bus. This provides an opportunity to study the impact on harmonic injection as the port business expands with the increasing demand to operate more cranes, resulting in increased nodes connected to the PCC contributing to higher harmonic distortion. Comparison between local utility planning levels for harmonic distortion with other standards such as IEEE-519 at 11kV is also discussed, to highlight that local utilities may have more stringent limits than IEEE-519 in some cases.

B. Local Utility Requirements

The harmonic distortion varies with the loading condition. To establish the criteria to evaluate the performance, local utility has provided with harmonic limit requirements at the point of common coupling. The worst-case scenario is based on the following assumptions:

One of the 11kV Local Utility Feeders is out. Evaluations are based on Utility-1 and Utility-3.

11kV Local Utility Feeder supplies all loads.

Main substation bus ties are closed.

Harmonic voltages are calculated at Main Substation.

Harmonic currents are calculated for 11kV Local Utility Feeder.

Table 3. Local utility requirements of harmonic injection

H(n) Current Voltage (Amps) (%V) VTHD Worst (V2)

2 3.67 0.100 24.147 3 3.07 0.125 76.217 5 7.77 0.529 1410.300 7 4.30 0.410 967.71

11 4.48 0.671 49.304 13 2.83 0.501 14.05 17 1.32 0.306 14.794 19 0.94 0.242 826.45 23 0.81 0.253 262.67

Utility-1 feeder power flow one-line output results are shown in Fig. 2 with 2.959 MVA generation. Equation (1) is used to calculate average maximum demand current. Short-circuit current ISC as per EasyPower simulation is 7.401 kA. From equation (2), current distortion is found to be in the range between 20 and 50. So, the current distortion limits are referenced from Table 1, based on this range.

IL = ���

√� �� =

�.��� � ���

√� � �� � ��� = 155.3 A (1)

���

�� =

�.��� � ���

���.� = 47.65 (2)

Fig. 2 EasyPower one-line diagram with Power Flow output.

Table 4. Local utility requirements vs. IEEE-519

H(n) Current (Amps) Voltage (%V)

Local Utility IEEE-519 (���

��

:20-50) Local Utility IEEE-519

2 3.67 2.72 0.100 3.0 3 3.07 10.87 0.125 3.0 5 7.77 10.87 0.529 3.0 7 4.30 10.87 0.410 3.0

11 4.48 5.435 0.671 3.0 13 2.83 5.435 0.501 3.0 17 1.32 3.88 0.306 3.0 19 0.94 3.88 0.242 3.0 23 0.81 1.55 0.253 3.0

It is observed that in this case, the local utility has lower- limit requirements on both voltage and current harmonic injection to the grid. Table 4 shows comparative results of the local limits with IEEE-519 limits based on certain IL that falls in the current distortion limit of 7% (of IL) for lower order harmonics. Interestingly, even the worst-case current distortion limit per IEEE-519, i.e., 4% for lower order harmonics, is higher than the local utility.

C. EasyPower Results

A detailed model was developed, with simulations

performed using the EasyPower tool to extract harmonic contribution from individual crane systems, ASCs and QCs, and the harmonics injected at the PCC were observed with different crane loads connected. The one-line diagram in Fig. 3 below depicts an example of one of the QC systems connected to the 11kV bus with components such as cable, main and auxiliary transformer, boom, gantry, hoist and trolley drives, and custom notch filter. In some cases, tuning reactors may be used to tweak the filter parameters. The performance of the custom design is summarized in Table 5 and Table 6 for two cases involving a different number of connected loads, with requirements met in both. The worst-case voltage THD was found to be 0.4711%, which is safely under the THD limit.

Fig.3. EasyPower one-line diagram for a QC system with custom harmonic filter.

Table 5. Case 1: Utility-1 and Utility-3 with 7 ASC and 4 QC

H(n) Current Voltage

(Amps) Met (%V) Met 2 1.198 OK 0.032 OK 3 1.485 OK 0.060 OK 5 4.656 OK 0.314 OK 7 2.741 OK 0.259 OK

11 0.418 OK 0.062 OK 13 0.169 OK 0.030 OK 17 0.015 OK 0.003 OK 19 0.010 OK 0.003 OK 23 0.005 OK 0.001 OK

Table 6. Case 2: Utility-1 and Utility-3 with 12 ASC and 5 QC

H(n) Current Voltage

(Amps) Met (%V) Met

2 1.654 OK 0.045 OK 3 1.959 OK 0.079 OK 5 5.058 OK 0.341 OK 7 2.993 OK 0.283 OK

11 0.430 OK 0.064 OK 13 0.194 OK 0.034 OK 17 0.015 OK 0.003 OK 19 0.010 OK 0.003 OK 23 0.005 OK 0.001 OK

It is inferred from the two cases that as load increases, system’s harmonic injection contribution also increases. To meet the stringent harmonic requirements that local utilities might have, and with expansion of the crane system at port terminals, a custom design of harmonic filter, such as a notch filter, as used in the case study, would provide better performance as compared to the standard LCL filter.

Table 7. Standard LCL filter harmonic performance

H(n) Current (Amps) Voltage (%V)

Limit Result Limit Result

2 3.67 1.05 0.100 0.03 3 3.07 1.44 0.125 0.06 5 7.77 6.7 0.529 0.45 7 4.30 6.22 0.410 0.59

11 4.48 0.44 0.671 0.07 13 2.83 0.06 0.501 0.01 17 1.32 0.01 0.306 0 19 0.94 0.01 0.242 0 23 0.81 0 0.253 0

Table 8. Custom notch filter harmonic performance

H(n) Current (Amps) Voltage (%V)

Limit Result Limit Result 2 3.67 1.07 0.100 0.03 3 3.07 1.47 0.125 0.06 5 7.77 3.5 0.529 0.24 7 4.30 2.49 0.410 0.24

11 4.48 0.39 0.671 0.06 13 2.83 0.13 0.501 0.02 17 1.32 0.01 0.306 0 19 0.94 0.01 0.242 0 23 0.81 0 0.253 0

As load increases, the fifth, seventh, etc., harmonics start to become significant. In one scenario of crane load, as shown in Table 7. the seventh harmonic injection has exceeded the limit requirements of the local utility, when LCL filter is used. In addition, the fifth harmonic injection of both current and voltage is found to be very close to the respective limits. To operate under the safe margins of utility standards, it is evident that custom notch filter would perform better in this scenario as shown in Table 8. A custom design equipped to reduce injection of fifth, seventh and higher order with a notch filter can be used to meet the limits when the standard LCL filter design might not and when it might affect the performance of other equipment, causing issues with heating, safety, reliability and maintenance.

IV. DC DRIVE HARMONICS

Historically ports have had cranes with DC drives powering DC motors for various motions. Due to obsolescence, ease of maintenance, better power factor and less harmonics, AC drives and motors are preferred. However, this replacement is gradual which leaves ports having a mix of DC and AC cranes. DC and AC drives have different harmonic spectrum.

Fig.4. DC and AC cranes: Custom HF not connected

2 AC CRANES5 DC CRANES

DC Cranes witout Filters

~30th High Pass Filters

DC drives inject significant lower order harmonics, while AC drives contribute moderate higher order harmonics around the switching frequency. This case study depicts the merits of custom harmonic filter design to mitigate harmonics generated by DC and AC cranes.

Fig. 4. shows an EasyPower model of a port power system that follows IEEE-519 standard for guidelines to limit harmonics. It shows 5 DC cranes and 2 AC cranes connected at the PCC. Without a custom harmonic filter, the lower order harmonics will dominate and might exceed the harmonic limits specified by the grid operators as shown in Fig. 5. DC harmonics may cause other issues on AC cranes due to providing a low impedance path for the DC harmonic currents to sink which may lead to equipment failure and downtime.

Fig.5. Lower order harmonics from DC cranes exceed limits

It is observed that 5th and 7th harmonics shown in the Fig.5. significantly exceed the limits. For this problem, custom notch filter design as discussed above in detail, is added to the crane electrical model and simulations are performed again to collect current harmonic spectrum at PCC.

Fig.6. DC and AC cranes: Custom HF connected

Notch filter for 5th and 7th harmonics is connected at the medium voltage bus of the DC crane as shown in Fig.6. After adding the custom harmonic filter, it is observed that the lower order harmonics are significantly reduced as shown in Fig.7. and falls well under the IEE-519 limits. This confirms the performance of custom harmonic filter design for the case when port has both DC and AC cranes.

Fig.7. Lower order harmonics mitigated using Custom Filter

V. CONCLUSION

To ensure power quality for all customers, grid operators are provided with recommended practices, standards and requirements to control harmonic injection. A comparative study shows that local utilities may have tighter limits than IEEE. This paper emphasizes the importance of custom design as compared to standard design of harmonic filters by presenting case studies depicting higher distortion in currents and voltages with increased loads, which is a challenge as the port expands business and thus demands the connection of more electrical machines and non-linear loads to support its operation. For ports that are transitioning into using AC cranes replacing DC, may need to carefully consider the impacts of harmonics distortion from each type and interference of harmonics from one another. Custom harmonic filter design is capable of providing effective harmonic mitigation for ports.

REFERENCES

[1] Psomopoulos, Constantinos & Kaminaris, S.D. & Ioannidis, G.C. &

Apergis, Antonios & Zilakos, Sarantos & Pizanis, Petros & Gampletsas, Andreas. (2015). “Experimental Investigation of Harmonic Distortion in Port Crane Systems”.

[2] J. Mazumdar, W. Koellner and A. Holweck, "Design and Implementation Issues of Active Front End Based Systems in Mining Draglines," 2007 IEEE Industry Applications Annual Meeting, New Orleans, LA, 2007, pp. 1760-1765. doi: 10.1109/07IAS.2007.269.

[3] IEEE Standard 519-2014, Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.

[4] S. Mark Halpin and Reuben F. Burch, IV, “Harmonic Limit Compliance Evaluations Using IEEE 519-1992”.

[5] Application Decision Making: Active Harmonic Filters vs. Active Front Ends, Mesta Electronics.