ieee brownbook - harmonics

IEEE Brown Book: Harmonics DIGSILENT PowerFactory

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IEEE Brown Book Harmonics ExampleThe harmonics example has been reproduced from the IEEE Brown Book [1] .

In this example, the effect of two non-linear loads within the confines of a small industrial plant (see

Figure 1) is assessed using harmonic analysis. The system in question contains multiple voltage levels

of 33kV and 6.6kV in the industrial plant and 220kV at the utility supply point. The plant load and

power factor correction are both connected at the 33kV and 6.6kV voltage levels, along with two

harmonic current sources that model the non-linear behaviour of a rectifier fed motor.

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DIgSILENT

PowerFactory 12.1.172

IEEE Std 399-1997 Harmonic Case Studies

Harmonics - Case Study 1 IEEE Brown Book Pg290

Project: IEEE Graphic: Grid Date: 4/17/2002 Annex:

SL - P -- static load real power

C -- power factor correction capacitorHF -- harmonic filter

SL - Q -- static load reactive power

Motor -- motor load with harmonic injection

SL - P

SL - P

T2 30MVA 33/6.6kV

Motor

C HF

C HF

SL - QMotor

6.6kV

33kV

220kV PCC

T1 100MVA 220/33kV

Utility Supply

SL - Q

DIgSILENT

Figure 1: Industrial plant SLD for harmonic analysis and filter design study

The static load on both buses is modeled separately as a parallel combination of the real and reactive

power drawn. This is to ensure the correct impedance frequency response is modeled in the example.

The motors on both the 33kV and 6.6kV buses are modeled as real power loads at fundamental

frequency with harmonic current injections. By double clicking on a motor load and going to the

harmonics page, the current magnitudes are given as a percentage of the fundamental frequency current

drawn.

There are a number of case studies in the IEEE Brown Book to highlight the problems in this industrial

plant and the filter solutions that can be implemented to alleviate them. Mainly the problems are due to

the substantial harmonic current injections from the motor drives. The presence of the power factor

correction capacitors on both the 33kV and 6.6kV buses means that there are going to be parallel

resonances within the plant frequency response. If these parallel resonances are large in magnitude and


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are at a frequency near or at a harmonic current that is produced as a result of the non-linear motor

drives, then substantial harmonic voltage distortion levels may result.

Harmonics in power systems, particularly in industrial plant like this one, can lead to many problems.

These problems can be due to the both the harmonic currents and the resulting harmonic voltage levels

they produce. In industrial plant they can lead to pulsating torques in rotating machinery and in some

cases may even cause smaller units to stall. In plant containing power factor correction capacitors the

increased voltage levels across the capacitors can seriously reduce their lifetime and may result in

failure.

Case Study 1In the first study example, Case Study 1, the 33/6.6kV transformer breakers are opened, disconnecting

the 6.6kV bus from the supply. The motor drive, static load and power factor correction capacitor are

all connected to the 33kV bus, which is supplied from the modeled utility source. A frequency scan is

performed to show the impedance seen from the harmonic source at the 33kV bus (see Figure 2). This

can be viewed in DIgSILENT on the Freq Scan 33kV page in the example, by left mouse clicking on

the Z(f) button on the button bar menu, and executing the frequency scan function. The results show

that with the 33/6.6kV transformer breakers opened, there is a parellel resonance at approximately

500Hz.

1200.0962.00724.00486.00248.0010.000 [Hz]

50.000

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33kV\33kV Bus: Network Impedance, Positive Sequence in Ohm

514.000 Hz40.714 Ohm

Freq Scan 33kV

Date: 4/17/2002

Annex: /8

DIgSILENT

Figure 2: Impedance vs frequency scan from the 33kV bus with the 6.6kV bus not connected

Even though the parallel resonant frequency is not at one of the characteristic harmonics of the motor

drive (these are the six-pulse rectifier characteristic harmonics at the 5th, 7th, 11th, 13th, 17th and 19th), it

will still present a significant impedance to the harmonic injection currents at the 5th, 7th, 11th and 13th,

particularly the 7th harmonic at 420Hz.


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To view the effect of motor drive operation on the voltage distortion levels at the 33kV and 220kV

buses, the voltage waveforms at these buses are detemined and a harmonic analysis performed. In the

example, the voltage waveforms can be viewed on the 33kV Vdist and 220kV Vdist pages, and the

harmonic voltage spectra viewed on the 33kV bus:A spectra (see Figure 3) and 220kV bus:A spectra

pages. To view these waveforms and spectra the Calculate Harmonic Load Flow option is chosen on

the Button Bar menu and executed.

1.00 5.00 7.00 11.0 13.0 17.0 19.0 [-]

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6.0000

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33kV\33kV Bus: Harmonic Distortion A in %

7.000 9.324 %

5.000 5.995 %

11.000 5.144 %

13.000 2.919 %

17.000 1.374 %

19.000 1.042 %

33kV bus:A spectra Date: 4/17/2002

Annex: /5

DIgSILENT

Figure 3: 33kV bus voltage spectra for 33kV connected motor load with the 6.6kV bus not connected

The harmonic voltage spectra results in the 33kV bus:A spectra and 220kV bus:A spectra pages have

the IEEE harmonic voltage limits superimposed on the harmonic voltage distortion results. They show

that the voltage distortion at the 220kV bus is within the IEEE defined limits but that at the 33kV bus,

the 5th, 7th and 11th harmonic voltages exceed the limits, and the 13th harmonic is at the limit. Clearly

under this scenario a solution to alleviate the harmonic problem at the 33kV bus, is neceesary.

Case Study 2In the second study example, Case Study 2, the 33/6.6kV transformer breakers are closed, and the

effect of the 6.6kV bus load assessed without the contribution of the motor drive load at 6.6kV. Firstly,

the impedance assessment at the 33kV bus (see Figure 4) now shows that the inclusion of the 6.6kV

bus capacitor results in two parallel resonant frequencies at approximately 330Hz and 715Hz, with the

largest parallel resonance occurring at the higher frequency. This is fortunate as the higher order

harmonics in a six-pulse converter which the harmonic motor drives harmonic injection currents are

modeling, tend to rapidly diminish in magnitude as the order increases.


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1200.0962.00724.00486.00248.0010.000 [Hz]

50.000

40.000

30.000

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10.0000

0.000

33kV\33kV Bus: Network Impedance, Positive Sequence in Ohm

714.000 Hz27.474 Ohm

330.000 Hz14.351 Ohm

Freq Scan 33kV

Date: 4/17/2002

Annex: /8

DIgSILENT

Figure 4: Impedance vs frequency scan from the 33kV bus with the 6.6kV connected

However, the same frequnecy scan performed at the 6.6kV bus (see Figure 5) shows that the lower

frequency parallel resonance at approximately 350Hz is the most significant, which is coincident with

the higher magnitude lower order harmonics.

1200.0962.00724.00486.00248.0010.000 [Hz]

4.0000

3.2000

2.4000

1.6000

0.800

0.000

6.6kV\6.6kV Bus: Network Impedance, Positive Sequence in Ohm

346.000 Hz 2.238 Ohm

730.000 Hz 1.177 Ohm

Freq Scan 6.6kV

Date: 4/17/2002

Annex: /7

DIgSILENT

Figure 5: Impedance vs frequency scan from the 6.6kV

The harmonic spectra results at the 33kV bus (see Figure 6) now indicate that the 5th, 7th, 11th, and 13th

harmonic voltage distortion limits have been exceeded, and that at the 6.6kV bus (see Figure 7) the 5th,

7th and 11th harmonic voltage distortion limits have been exceeded.


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1.00 5.00 7.00 11.0 13.0 17.0 19.0 [-]

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33kV\33kV Bus: m:HD:A

5.000 6.621 %

7.000 3.264 %

11.000 5.387 % 13.000

4.516 %

17.000 1.705 %

19.000 1.225 %

33kV bus:A spectra

Date: 4/17/2002

Annex: /5

DIgSILENT

Figure 6: 33kV bus voltage spectra for 33kV connected motor load with the 6.6kV bus connected

1.00 5.00 7.00 11.0 13.0 17.0 19.0 [-]

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6.6kV\6.6kV Bus: m:HD:A

5.000 10.200 %

7.000 8.535 %

11.000 5.041 %

13.000 2.477 %

17.000 0.454 %

19.000 0.249 %

6.6kV bus:A spectra

Date: 4/17/2002

Annex: /4

DIgSILENT

Figure 7: 6.6kV bus voltage spectra for 33kV connected motor load

These results indicate that the inclusion of the 6.6kV bus elements worsen the voltage distortion

problem by introducing a further parallel resonant frequency and increasing the effective driving point

impedance value as seen from the 33kV bus. Additionally, the frequency scan results indicate that the

motor drive may be better placed at the 33kV bus, due to the relatively lower magnitude impedance at

lower frequencies, seen from the 33kV bus by the harmonic injection currents.


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Case Study 3In the third study example, Case Study 3, the effect of the 6.6kV bus load is assessed without the

contribution of the motor drive load at the 33kV bus. The harmonic spectra results for the 6.6kV and

33kV buses indicate that while the distortion levels at the 33kV bus have generally improved in terms

of the Total Harmonic Distortion (THD) levels, the distortion voltage magnitudes at the 6.6kV bus

could be considered excessive in terms of good plant operating practice, (see Figure 8) with the THD

level exceeding 16%. Additionally the result in Figure 8 confirms that the motor drive would be best

placed at the 33kV bus due to the increased distortion levels at the lower order harmonic voltages at the

6.6kV bus.

1.00 5.00 7.00 11.0 13.0 17.0 19.0 [-]

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6.6kV\6.6kV Bus: m:HD:A

5.000 15.254 %

7.000 9.632 %

11.000 3.596 % 13.000

3.317 % 17.000 1.525 %

19.000 1.146 %

6.6kV bus:A spectra

Date: 4/17/2002

Annex: /4

DIgSILENT

Figure 8: 6.6kV bus voltage spectra for 6.6kV connected motor load

Case Study 4In the final case study in the Harmonics example, Case Study 4, a filter design is implemeneted at the

33kV bus, to modify the resonant behaviour of the plant and reduce the harmonic voltage distortion

levels.

Filter design in situations where there are harmonic distortion problems can be approached using either

single tuned harmonic filters or C-type high pass filters. In this example a single tuned filter is applied

with a low Q factor, and is designed to provide reactive power and minimise losses at the fundamental

frequency. Additionally the effective impedance of the original resonance should be modified such that

the voltage distortion is maintained within either prescribed limits or levels that allow adequate plant

operation.


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In the example, the filter is connected at either the 33kV or 6.6kV buses, along with the motor drive

with the harmonic current injections. This is to finalise whether the motor drive installation is better

suited at the higher or lower voltage levels within the plant.

The design of the filter is not elaborated on within this text, however in summary its design

incorporates the original power factor correction capacitance, and a series connected tuning reactor.

The filter is designed to alleviate the effects of the parallel resonance in Study Case 1, evident at the

33kV bus when the 6.6kV bus is not connected. It can be seen from Figure 2 that the parallel resonance

occurs at approximately 500Hz. The filter could be tuned to eliminate this resonance directly, however

this approach leads to the occurrence of lower order resonant behaviour. Given that there are two

significant harmonic currents at the 5th and 7th harmonics below the original resonant frequency, the

risk is that the new parallel resonance may occur at these frequencies. For this reason single tuned

filters should be designed to remove specific harmonic currents rather than parallel resonances.

Additionally it is recommended that single tuned filters be tuned to resonate 5% below the harmonic

current to be eliminated. This is to account for variation in the utility system impedance which may

occur due to switchings or outages and affect the frequency of the parallel resonance.

In this example, the filter is tuned to approximately 5% below the frequency of the 5th harmonic

current, with a low Q factor, in order that a substantial component of the higher order harmonics

produced by the motor are also filtered.

In the DIgSILENT Study Case 4 example, the 33/6.6kV transformer breakers are opened as in Study

Case 1, and on the 33kV bus, the power factor correction capacitor is removed from service, while the

harmonic filter HF is connected. The example is then re-run with the 33kV motor load connected to

assess whether the harmonic voltage limits have been breached with the addition of the harmonic filter.


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1.00 5.00 7.00 11.0 13.0 17.0 19.0 [-]

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33kV\33kV Bus: m:HD:A

5.000 1.170 %

7.000 2.657 %

11.000 2.934 %

13.000 2.938 % 17.000

2.882 % 19.000 2.837 %

33kV bus:A spectra

Date: 4/17/2002

Annex: /5

DIgSILENT

Figure 9: 33kV bus voltage spectra for 33kV motor load and harmonic filter connected, with the 6.6kV bus

disconnected

The results from Figure 9 indicate that at each harmonic, the IEEE voltage standards for bus voltages

below 69kV have been met. However, the Total Harmonic Distortion (THD) levels have only been

reduced from approximately 12% to 6% and in some situations this level of distortion may prove to be

too high. This may necessitate the design and implementation of a second single tuned filter, tuned

around the 7th harmonic frequency, to filter more of the higher order harmonics.

1.00 5.00 7.00 11.0 13.0 17.0 19.0 [-]

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6.6kV\6.6kV Bus: Harmonic Distortion A in %

5.000 1.928 %

7.000 6.118 %

11.000 1.714 % 13.000

1.088 %17.000 0.558 %

19.000 0.429 %

6.6kV bus:A spectra

Date: 4/18/2002

Annex: /4

DIgSILENT

Figure 9: 6.6kV bus voltage spectra for 33kV motor load and harmonic filter connected, with the 6.6kV bus

connected


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Figure 10 shows the harmonic voltage spectra at the 6.6kV bus for the 33kV connected motor load and

harmonic filter. The distortion levels have significantly reduced when compared with the unfiltered

case in Figure 7, although the 7th harmonic voltage level may prove onerous for smaller induction

motor operation.

Reference[1] IEEE „Brown Book“ (IEEE-Std 399), pgs 265 to 312, 1997

ieee brownbook - harmonics

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