Power quality issues

IEEE/HKIE Seminar

Impact of Traction Harmonics to Power System

Delivered by Dr C T Tse Nov-14, 2011 (Mon) FJ303, PolyU

41

Major Abnormalities due to AC traction 1. Voltage fluctuation 2. Voltage dip (Vmin is 17.5kV) 3. Voltage and Current Imbalances 4. Voltage and Current Harmonics 5. Interferences with signalling and communication system (to be discussed in EE537). (6). Low power factor
In order to apply for the economic (bulk) tariff, the traction operation has to comply with the regulation/limits imposed by the power utility with respect to items 1, 3, 4 & 6, at the point of common coupling (PCC), e.g. Fanling 132kV 4

Appears to have no harmonic current limits at 132kV

3

 AC Traction is the only consumer that contributes all the above abnormalities. Remedy Install booster transformers sectionalize the railway system install capacitor compensator/filter at strategic locations
(to be discussed in EE533)

Traction Harmonics
Harmonic Source AC & DC drives Adverse Effect of Harmonics Overheating of conductors Overheating of electrical equipment Mechanical oscillation of electrical machine Telecommunication interference Inaccurate meter readings Disturbance to sensitive electronic equipment False operation of protection equipment Standards Engineering recommendation G5/3, G5/4 IEEE standard 519-1992

AC Locomotives with Tap Changer Control

AC Locomotives with PWM Control

Single-phase AC powering 3-phase motor

Power collected by pantograph and passed to transformer 25kV stepped down and then rectified to acceptable voltage for motor (dc) Current controlled by Tap Changer (instead of conventional resistor)
(DC traction motor has many problems.)
7

Single phase 50Hz AC (after rectification) becomes 3-phase AC with variable voltage and variable frequency (VVVF), supplying 3-ph motors.

Harmonic current in electrified ac system KCR electrification began in 80s but with very poor power factor. Capacitors were installed in Tai Wai 25kV for pf improvement In the early stage, train drives were of tap-changer type and 3rd harmonic (h=3) dominant. The installed cap bank was then modified to add series reactor to become third harmonic filter. Harmonic increased with the introduction of thyristor type drive. With the advances of power electronics, the speed and traction force of new drives are much enhanced but the harmonics are much increased. Dual functions of 3rd harmonic passive filter in East Rail It provides capacitive compensation at 50Hz and also absorbs harmonics of h3.
25kV

System representation at fundamental frequency

Main supply (50Hz) is represented by Thevenin equivalent (EM & XM). Load represented by impedance ZL The shunt filter (connecting in parallel with load) provides capacitive compensation.
(In ac traction, XM includes transformer XT and system XSYS and usually XT>>XSYS)
IM XM EM XL IF XC ZL IL

Main

Filter

Train Load

System representation at frequency 50n (Hz)

Main is represented by single inductance nXT without emf, since it is a 50Hz source only. The harmonic produced by the train is often represented by Norton (IS and YS), and very occasionally by Thevenin (VS and ZS). Since YS is a very complicated function, YS=0 is usually assumed (most pessimistic assumption for Norton).
9 IT nXT nXL IF XC/n IS

Train Harmonics (In subsequent harmonic diagrams, 10 n may be skipped for simplicty.)

Main

Filter

Harmonic current sharing between Transformer and Filter (at 25kV)

Filter Reactance
XC XL IS XL XF

Worked Example on early shunt filter (passive)

A 25kV traction load is of low power factor and is rich in third harmonic. Design a shunt filter tuned at 125Hz which can also provide 4MVAr capacitive compensation at 50Hz. Solution At 50Hz: =250, XL=L and XC=-1/(C) Q=V2/XF XF=XL+XC= V2/Q= 252/(-4)=-156.25 (1) (negative Q stands for capacitive VAr) At 125Hz: XF=nXL+XC/n=0 n2XL+XC=0 (2) and n=125/50=2.5 (2)-(1) gives XL=156.25/(n2-1)=29.76 , XC=-n2XL=-186.01 L=94.74mH, C=17.11F

IT
16

IF

XT

frequency
132/25kV
Tx

Passive Filter

Harmonic source

XC

31

Filter reactance XF = XL+XC = L + (-1/C)

13 12

Tuned freq. (n50Hz) n is non-integer

13

IS shared between XT and XF IT =IS XF / (XT+XF) IF =IS XT / (XT+XF)

22 32 11

The tuned (resonant) freq. must be less than the targeted harmonic freq. (i.e. n<h)

The 50Hz filter current (=4MVA/25kV=160A) is fixed irrespective of train load. However, the filter harmonic current IF depends on (a) harmonic source current IS and (b) the main supply Thevenin inductance XT. It is called passive filter, since it consists of two passive elements L&C and its harmonic role is passive (harmonic absorption is predetermined by L & C). 12 The harmonic flows can be easily calculated by excel as follows:

The smaller XF, the less IT flow to the PCC, and the larger IF (filter more harmonic absorption)

11

Harmonic current with Filter tuned at 2.5x50=125Hz

pu on 25kV & 26.5MVA base and assumes 4MVA capacitive compensation XL=29.7626.5/252=1.26pu, XC=-186.0126.5/252=-7.89pu. (Given: XT=0.18pu) Harmonic number n 50Hz Indcutor Reactance (pu) Capacitor Filter XF Tx. Filter Harmonic Current (%) Tx. Source IS XL XC XL+XC XT IF IT IF +IT
1.26 -7.89 -6.63 0.18 0.15
12

Harmonic current with 4-MVA Filter tuned at 2.9x50Hz

32% absorption of the early 125Hz filter is too small (i.e. filter capacity wasted)
55 13

Harmonic number 2
1.79 -3.76 -1.97 0.36 -22.34 122.34 100.00

50Hz 6
7.57 -1.31 6.26 1.08 14.72 85.28

3
2.68 -2.51 0.18 0.54 75.44 24.56 100.00

4
3.58 -1.88 1.70 0.72 29.80 70.20 100.00

5
4.47 -1.50 2.97 0.90 23.28 76.72 100.00

6
5.36 -1.25 4.11 1.08 20.80 79.20 100.00

7
6.26 -1.07 5.18 1.26 19.55 80.45 100.00
13 28 20

2
2.52 -3.94 -1.42 0.36 -33.97 133.97 100.00

3
3.79 -2.63 1.16 0.54 31.83 68.17 100.00

4
5.05 -1.97 3.08 0.72 18.97 81.03 100.00

5
6.31 -1.58 4.73 0.90 15.98 84.02 100.00

7 Inductor
8.83 -1.13 7.71 1.26 14.05 85.95 100.00
11 19 11

XL XC XL+XC XT IF IT IF +IT

0.89 -7.52 -6.63 0.18 0.15

Reactance (pu)

Capacitor Filter XF Transf. Filter

Harmonic Current (%)

Transf. Source IS

100.00

At n=3, XF=1.16 (positive) and the filter absorbs 32% I3 At higher n, XF=nXL+XC/n is also positive, and it absorbs 16% I5, 14% I7 , . But at low n=2, XF=-1.42pu, IF absorbs -34%, i.e. IT is amplified by 34% at Tx
Fortunately, I2 is very small in ac traction, 34% amplification is of no problem.
14

With closer tuned frequency at 145Hz, XF is much reduced to 0.18pu at n=3 The absorption at n3 is increased, e.g. 75% I3, 23% I5, 20% I7 ; and the amplification at n<3 decreases, e.g. IT2 to 22% (previous 34%)
41 13

13

As a conclusion, performances both better than 125Hz filter

19

14

Harmonic current with 4MVA Filter tuned at 6.8x50Hz (1)

The train drive also has 7th harmonics, to be absorbed by another 340Hz filter.
Harmonic number 50Hz Inductor Reactance Capacitor (pu) Filter XF Transf. Harmonic Filter Transf. Current (%) Source IS XL XC XL+XC XT IF IT I F + IT
0.15 -6.77 -6.63 0.18 0.15

16

Harmonic current with Filter tuned at 6.8x50Hz (2)

I5 Resonance occurs if capacitive compensation is reduced from 4 to 2.78MVAr
Harmonic number

15

2
0.29 -3.39 -3.09 0.36 -13.17 113.17 100.00

3
0.44 -2.26 -1.82 0.54 -42.26 142.26 100.00

4
0.59 -1.69 -1.11 0.72 -186.00 286.00 100.00

5
0.73 -1.35 -0.62 0.90 323.84 -223.84 100.00

6
0.88 -1.13 -0.25 1.08 130.11 -30.11 100.00

7
1.03 -0.97 0.06 1.26 95.62 4.38 100.00

50Hz Inductor Reactance (pu) XL

0.21 -9.74 -9.53 0.18

2
0.42 -4.87 -4.45 0.36 -8.80 108.80 100.00

3
0.63 -3.25 -2.62 0.54 -26.02 126.02 100.00

4
0.84 -2.44 -1.59 0.72 -82.48 182.48 100.00

5
1.05 -1.95 -0.895 0.900 18301.4 -18201.4 100.00

6
1.26 -1.62 -0.36 1.08 149.92 -49.92 100.00

7
1.47 -1.39 0.08 1.26 93.81 6.19 100.00

Capacitor XC Filter XF Transf. XL+XC XT IF IT IF +IT

Harmonic Current (%)

Filter Transf. Source IS

This 340Hz filter absorbs 96% I7,, but all lower harmonics are amplified, e.g. I5 by 124% Filter may be overloaded with very high I5=324% of IS. Thus, if 7th harmonic filter is to be installed, additional lower harmonic filters may be required. The new 5th harmonic filter will also amplify I4, I3, and I2.
15

Controlled 25kV series resonance to absorb more 7th harmonics (94%).

11

25kV parallel resonance at lower harmonic (I5 is much amplified)

28

16

Harmonic current with Filter tuned at 6.8x50Hz (3)

I3 Resonance occurs if capacitive compensation is increased to 13.5MVAr
Harmonic number 50H z Inductor Reactance (pu) XL
0.04 -2.01 -1.96 0.18 54

Harmonic current with Filter tuned at 4.8x50=240Hz

I3 Resonance occurs if capacitive compensation is increased to 10MVAr
Harmonic number 6 7 Inductor Reactance (pu) XL 50H z
0.12 -2.77 -2.65 0.18

5 2
0.09 -1.00 -0.92 0.36 -64.70 164.70 100.00

5 2
0.24 -1.39 -1.14 0.36 -45.88 145.88 100.00

3
0.13 -0.67 -0.539 0.540 38971.1 -38871.1 100.00

4
0.17 -0.50 -0.33 0.72 183.69 -83.69 100.00 0.22 -0.40 -0.18 0.90 125.75 -25.75 100.00 0.26 -0.33 -0.07 1.08 107.36 -7.36 100.00 0.30 -0.29 0.02 1.26 98.66 1.34 100.00

3
0.36 -0.92 -0.563 0.540 -2378.42 2478.42 100.00

4
0.48 -0.69 -0.21 0.72 141.63 -41.63 100.00 0.60 -0.55 0.05 0.90 95.02 4.98 100.00 0.72 -0.46 0.26 1.08 80.61 19.39 100.00 0.84 -0.40 0.45 1.26 73.86 26.14 100.00

Capacitor XC Filter XF Transf. XL+XC XT IF IT IF +IT

Capacitor XC Filter XF Transf. XL+XC XT IF IT IF +IT

Harmonic Current (%)

Filter Transf. Source IS

Harmonic Current (%)

Filter Transf. Source IS

Controlled series resonance to absorb more 7th harmonics (99%). Parallel resonance at lower harmonic (I3 is much amplified)
17

Controlled series resonance to absorb more fifth harmonics (95%). Parallel resonance at lower harmonic (I3 is much amplified) Higher-order and multi-leg filters have potential resonant hazard. 18

Problems of 3rd harmonic filter design in East Rail

Power system is rich in 3rd harmonics but the magnitude is unknown.
External also sees a low X

Resonance at other frequencies

Recall XT=0.18pu at 50Hz, and X132 XT+XF for all frequencies At 150Hz, both XT & XF are positive, no resonance Above 150Hz, both XT & XF are more positive, no resonance
14 21 19

Filter installed to absorb load harmonics

Filter and consumer load connected to system (filter also absorbs external harmonics)

Possibly with fear of resonance, early filters in KCR tuned not closed to 150Hz had restricted the absorbing capacity of passive filter (32%).

Resonance may be due to very low X25 and/or X132, overloading the filter. If the 3rd harmonic filter is tuned closed to 150Hz, say at 145Hz, XT=0.54 at 150Hz is slightly increased and is positive 14 XF=0.18 is much reduced but remains also positive X25 (XT //XF XF) is small, series resonant design to increase absorption to 75%. (Filter rating is determined by the foreseeable total max train harmonic current IS.) X132 (XT+XF XT) is always large, irrespective of XF. 150Hz resonant at 132kV PCC due to 3rd harmonic filer is impossible.
19
20

Load

XT XSYS X132 XF X25

X132 XT+XF

13

Train
40 13

At 50Hz, XT=0.18pu. System may resonant if XF-0.18pu, i.e. if capacitive compensation is 26.5/0.18=147MVAr At 100Hz, system may resonant if the capacitive compensation is 22MVAr
21

X25 XT//XF
19

The maximum capacitive compensation in KCRC is 4MVAr. Conclusion : Resonance (series or parallel) due to 145Hz filter is impossible. Heavy 3rd harmonics in East Rail can be combated by tuning filter closed to 150Hz, with adequate filter rating. 22

20

Harmonic current (Filter tuned at 2.9x50Hz and with 22MVAr compensation)

Harmonic number 50Hz Inductor Reactance (pu) XL
0.16 -1.37 -1.20 0.18

2
0.33 -0.68 -0.358 0.360 23041.7 -22941.7 100.00

3
0.49 -0.46 0.03 0.54 94.41 5.59 100.00

4
0.65 -0.34 0.31 0.72 70.01 29.99

5
0.81 -0.27 0.54 0.90 62.53 37.47

6
0.98 -0.23 0.75 1.08 59.10 40.90 100.00

7
1.14 -0.20 0.94 1.26 57.21 42.79 100.00

Capacitor XC Filter XF Transf. XL+XC XT IF IT

Harmonic Current (%)

Filter Transf.

Harmonic Problems in West Rail Characteristics of new drive of SP1900 (IKK) train Unity power factor Rich in low harmonics with some high-order harmonics Passive filter (causing over-compensation and overvoltage at 49 50Hz) is inappropriate for installation. In a consultancy study of including IKK train in the East Rail (one IKK with 4 convention MLR), for a scenario of the only IKK train in powering mode: - poor and negative power factor = -0.427, - over-compensation by 3MVAr and over-voltage (V=1.073pu)
Other Problems: High-order (over 50th) harmonics generated by unity pf drives Passive filter tuned at, say, n=50.5 must amplify harmonics h<n, and may lead to resonance at some lower hs. Passive filter cannot be installed to the West Rail. Possible solution: Active Filter directly connected to at 25kV ? (Present G4/5 regulation only covers h<51.)
50

Source IS IF +IT

100.00 100.00

Controlled 25kV series resonance to absorb more third harmonics (94%) 25kV parallel resonance and I2 is much amplified (where X25XT//XF) Uncontrolled 132kV series resonance according to X132XT+XF 19 21 and external (132kV) I3 magnitude is unknown.

20

22

Standard for Harmonics

High-order harmonics recorded beyond PCC High order harmonic current were recorded at 132kV s/s beyond PCCs supplying West Rail and also East Rail. These s/s are connected to PCC via 132kV cables. A cable represented by -equivalent has 3 parameters: R, L & C, where Z=R+jX, X=L and B=C at 50 Hz

Total harmonic distortion (THD) on voltage sufficient to use values of up to 19 B B

equivalent of Cable

n>50 is ignored in THD calculation

For hth harmonic, ZhR+jhX, and Bh=hB. Both Zh and Bh will increase with h and cable length. The 50Hz charging current V2B is very high at V=132kV.
24

Necessity to revise existing regulation ?

23

System modeling at 50Hz

Harmonic current flows at 132kV

ISYS

Ih

hXSYS

IB hB

Ih (small) from traction is injected to 132kV system via PCC, and will return via Isys (positive) and IB (negative)

Appro. equivalent circuit at s/s if Zh0

54

System modeling at h harmonic

ISYS at a s/s is much amplified if hXSYS 1/hB (parallel resonance) To meet hXSYS 1/hB, the location of resonance (B), the harmonic order (h), and the time in a day (XSYS) can vary. Fortunately, many Rs in the two Z branches and connected loads will attenuate current amplification in parallel resonance, if any. Note the approximate equivalent circuit does not include 25kV (i.e. not related to filter design), and this 132kV resonance may not be detected in KCR.

36

25

26

Effect of 3rd harmonic current in neutral wire for 3-phase

Harmonics in 3-ph system:
IR1
0 50 100 150 200 250

Harmonic in Automated People Mover (APM) System for Airport The 3-ph 600V supply to APM does not have neutral wire, and I3 is suppressed. Harmonics of 5, 7 &11 are rich and 3-leg filters were already installed. Whilst I5 is absorbed by 5th harmonic filter (<100%), it is amplified by 7th harmonic filter. Similarly, 11th harmonic filter must amplify I5 and I7. Resonance may occur at I5 and I7. Multi-leg filter may not be effective to absorb multi harmonics. A solution is to install only 5th harmonic filter to absorb I5, as well as I7 and I11 (but with larger filter rating). Alternately method: install active filter, since V is low (600V)
28

IR3
300 350 400

1=t for fundamental, n=nt for nth harmonic For the same time span t, n=n1 When 3th harmonics completes one cycle, the fundamental goes through only 120 Under balanced load, the neural wire current IN=IR+IY+IB=0 for fundamental 50Hz But, their 3rd harmonics are in-phase IR3=IY3=IB3 and IN3=3IR3 This also applies to harmonics of 6th, 9th, .

1=120

IY3
0 50 100 150 200 250

IY1
300 350 400

16

If a system has, say, 40% 3rd harmonic, let I1=1, IP=(12+0.42)=1.077, IN=30.4=1.2, IN>IP and the neutral wire may be overloaded.
IB3
0 50 100 150 200 250 300

IB1
350 400

14

50

27

Traditional Concept on Singly tuned Filter

Why engineers do not aware lower harmonic amplification in mulit-leg filter?

Traditional Concept on Double tuned Filter

XF= XL+XC

The filter impedance is Z=R+j(L-1/C) and Z={R2+(L-1/C)2} is always positive At tuned frequency r , rL=1/rC, and Z=R
31

The filter has the lowest Z=R (i.e. highest absorption) at r. However, this tuning concept may be adequate to the filter design at power system, but inadequate at traction substation which has a 132/25kV Tx in parallel with the filter. The absorption concept at >r also applies, but this concept has overlook that negative reactance X at <r will amplify the low-order harmonic flows in the Tx.

Characteristic of a 2-leg filter (5th and 7th)

29

Similar inadequacy of using scalar impedance Z also occurs on double tuned filter, e.g. 2-leg filter.

30

Another common error in filter design

Discrepancy of Traditional Concept

Filter impedance Z=R+jX is a complex number, a 2-D vector. To fully depict Z variation with frequency f, a 3-D graphic is necessary. But 3-D analysis is complicated and difficult. To depict Z-f relationship by 2-D, traditional concepts use 1-D of Z=|Z|, but the abrupt sign change of X is overlook. In the present presentation, the small R is ignored and ZjX is simplified to 1-D. Finally the Z-f relationship becomes a 2-D problem. The advantage is that the abrupt sign change of X and the harmonic current absorption/amplification can be estimated using simple excel program.

With passive filter, the 50Hz capacity compensation will be excess when less train at powering mode and may lead to over-compensation and over-voltage.

The max. allowable voltage for KCR ac traction drive is 27.5kV. Suppose each cap has a voltage rating of Vcap=4.5kV. For Vmax=27kV, the number of cap in series 25kV bus appears to be s=Vmax/Vcap=27/4.5=6
29

12

However, the actual voltage across the cap is VC>Vmax since VC and VL are of opposite sign
11

Vmax=27kV
52

VC

It should be s=Vmax/Vcap+XLCp
(Details to be provided in EE510)

37

VL

31

Filter design based on s=Vmax/Vcap may lead to capacitor insulation failure under higher voltage stress.

32

Harmonics in dc traction system

In 3-ph system, DC supply obtained from full wave rectifier is common.

12-pulse Rectifier
A a1

Two Secondary
c2
30

a2

Primary
DC ripples B c1 b1 Vbc Vbc

b2

60
D1

360
D6

D5

60

If a 3-ph Tx has two sets of secondary windings of star and delta connections, the secondary line voltages will have an angle difference of 30. If a rectifier is fed by these two secondary windings, the rectifier output VDC will be of 360/30=12 pulses, and the DC ripple is smaller than that of the 6-pulse rectifier. For 12-pulse rectifier, harmonic current Ih with h=12k1 (i.e. 11,13, 23, 25, 35, 37 ) will exist at the Tx primary, and Ih/I1=1/h is simply the reciprocal of h which is rather small at high h values. (I1 is the fundamental 50Hz Tx current.) In 24-pulse rectifier, Ih for h= 11, 13, 35 & 37 are further suppressed. 34

D3 VRY D4

D2

The DC output voltage has six sections in one cycle (i.e. the so-called 6-pulse rectifier), and each section is of 360/6=60. DC ripple can be reduced by more pulse rectifiers
33

24-pulse rectifier in DC traction system In MTR, the each 1.5kV source is a pair of Tx rectifier of 12-pulse each. These Tx are connected to 33kV systems (of both CLP and HEC), in which they are split to avoid power circulation.
Each Tx has zig-zag primary winding, such that one Tx winding of 7.5 phase shift and the other +7.5 (by means of phase shift change switch), i.e. an angle difference of 15 Then VDC will be of 360/15 =24 pulse.
Each Tx has 2 secondary windings (star and delta).
35

Harmonic suppression by 24-pulse rectifier

The two primary current have an angle difference 1=7.5-(-7.5)= 15 at 50Hz, and h=7.5h-(-7.5h)=15h at harmonic frequency, given by:
h 15h RF 11 165 0.13 13 195 0.13 23 345 (-15) 0.99 25 375 (15) 0.99 35 37 Ih + 525 555 (165) (195) 0.13 0.13

Two 12-pulse rectifiers

82.5=7.5h

Ih+ + Ih
-82.5

For h=11, 7.5h=82.5, 82.52=165 Reduction factor (RF) RF = cos82.5=0.13, and similarly for h=13, 35 & 37. Thus, only 23th and 25th harmonics can only be rich in the 24-pulse rectifier with magnitude Ih/I1=1/h.

Ih
Current sum of 2 Tx for h=11

In MTR, the high harmonic injection to PCC is very unlikely, but the hazard of harmonic resonance beyond PCC for all h (due to B of 33kV cable & 33kV cap) still exists.

26

36

Summary of major observations

Two types of resonance associated with ac traction harmonic at 25kV are: At series resonance, -XLXC and max IF=IS, VC+VL is much smaller than IF IT VCor VL, implying voltage resonance. 55 XC IS XM However, as train harmonic IS is foreseeable, voltage XL resonance is controllable by tuning at n (where n<h), and the amount of filter absorption of IF can be controlled. So long taking s=Vmax/Vcap+XLCp in the filter design, equipment insulation 32 failure due to voltage resonance should not occur. At parallel resonance, IT IF -XM XC+XL, and IF>>IS (can be > 100) current resonance XC IS Both VL &VC can be of large values voltage resonance Since XM consists of Tx and 132kV system circuits as well as fault level, when will this resonance occur is uncertain. From analyses, parallel resonance should not occur for 3th harmonic filter, even 50n is tuned close to 150Hz (to increase absorption), but not for filter of harmonics 5th, 7th .., nor for multi-leg filter. 132kV series resonant (slide-21) and parallel resonances (slide-26) are all uncontrollable.
37

Comments on harmonic impact on the 4 local railway systems (1)

1. East Rail (25kV 1-ph) Mainly low order harmonics and h=3 dominant. The problem can be solved with passive filter tuned closed to 150Hz and with proper filter rating according to foreseeable total harmonic current IS. Harmonics issues becomes complicated with IKK (trains of unity power factor) that overcompensation and over voltage will occur, but the advantage is that 3rd and 5th harmonics from IKK will be anti-phase to (i.e. cancel) those from conventional train. Capacitors will likely experience overvoltage due to over-compensation and inadequate capacitor design of s=Vmax/Vcap. 2. West Rail (25kV 1-ph) Because of the unity power factor drive of IKK, both low- and high-order harmonics exist. With the conflicts of (a) over-compensation and overvoltage and (b) low harmonic amplification (or even resonance), passive filter cannot be installed. Since the technique of HV active filter is not yet mature, no pragmatic measures can be recommended. 38

XM

XL

Comments on harmonic impact on the 4 local railway systems (2) 3. Automated People Mover (600V 3-ph) Rich in low-order harmonic with the absence of 3 and 3-multiples. Multi-leg filter (5, 7 & 11) has been installed, and lower order harmonics must be amplified. A proper filter design has to look after the filter loading increase due to lower harmonic amplifications, and to avoid the hazard of 5th and 7th harmonic resonances. 4. Mass Transit Rail (1.5kV DC) Mainly h=23 & 25 with magnitude of I1/h. Magnitudes of h=11, 13, 35 & 37 are further reduced to 13% of I1/h by the 24-pulse rectifier Tx. Very heavy harmonic injection to PCC should not occur. In all above 4 cases, there is a potential hazard of parallel resonance beyond PCC, even the traction harmonic injection is small. If it really happens, the role of responsibility should be the supply utility, not MTRC.
39

END of Presentation Q&A

The excel program for the harmonic calculations, the PDF files (colored) of the present slides on traction harmonics and the previous slides on traction Imbalance can be downloaded from: ftp://ftp.ee.polyu.edu.hk/cttse/seminar
Animations by Icons: e.g.
11 12 1

Go to slide-11 Return to slide-12

40

Annex-1 Power Quality equipments in EE Power systems Lab

Installed at EE in 2003 with full support from ABB (HK)
Except the 2 motors, all the PQ equipments and supply panels (MINIC/MNS) are freely designed/transported/installed by ABB(HK) for EE Dept of PolyU for teaching/research purposes.

MINIC & MNS: Power Supply to PQ and other labs

41

42

Overall view of PQ equipments

44

43

43

44

Active Filter (in parallel with load)

Third Harmonic Filter THF (at neutral & in series with load)

Motors & Drives

30kW & 11kW motors Inverter or Variable speed drive, driving 30kW motor

XC XL

3-phase Load
N

Active Filter
R Y B Local 3-ph Load N THF 45 46

Soft starter driving 11kW motor

Effect of Active filter

CT-1 measures total current with filter CT-2 measures total load current without filter

The Active Filter senses the current via the current transformer (CT-1) On-line computes the measured harmonic current: magnitude and angle AF injects harmonic current (equal but opposite to local harmonics) until TOTAL harmonic current of CT-1 reach specified values
47 48

Annex-2 Traction Harmonics and Research on Active Filter

Problem with trains of unity/leading power factor A proper filter design has to look after both the capacitive compensation at 50Hz and the anticipated total harmonic current for all foreseeable scenarios. A passive filter will absorb harmonics generated by trains, but it will inevitably generate MVAr to be absorbed by train load. The shortcoming of trains with unity power factor is the incapability to absorb MVAr, resulting in system overcompensation and over-voltage. Thus, unity power factor may not be beneficial to a system if a passive filter has to be installed.

Necessity of Active Filter

With advance of Power Electronics, more rich in high order and multi order harmonics Advantages: Without lower harmonic amplification nor resonance Programmable capability to handle dynamic range of harmonics Immune to external harmonics Applicable to lagging/unity/leading power factor load Restrictions of existing Active Filter design Harmonic order below h=50 Voltage below 1 kV
49
22 28

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Proposed Research Proposal of more advanced Active Filter Higher voltage level (11kV and then 25kV) Faster dynamic response to combat the high order harmonics. Optimal selections and design of the power electronic converters and the coupling transformer Suitable operational voltage to improve efficiency and improve response Can handle both single-phase and three-phase applications Can handle shallow voltage dip of short durations (say less than 200ms) in low power installations.

Annex-3: Revision on Circuit Theory

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Simple Circuit Analyses

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(if XL or XC is large.)

17

but XL and XC values at resonant freq are finite, and resonant V magnitude is restrictive.
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In general, parallel resonance (with both V and I) is more severe than series resonance (with V only), due to the possibly large internal current.
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Example of series circuit for 2.950=145Hz Filter

Base values: Sb=26.5MVA, Vb=25kV, Ib=Sb/Vb=1.06kA, At 50Hz, XL1=0.89pu, XC1=-7.52pu, V1=25kV (i.e. 1pu) VL1=XL1/(XL1+XC1)V1 = -0.134pu (-3.35kV), 32 VC1=XC1/(XL1+XC1)V1= 1.134pu (28.35kV)
(subscript 1 stands for 50Hz fundamental)
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Biography of Speaker
Dr. C.T. Tse was the Associate Professor in the Electrical Engineering Department, the Hong Kong Polytechnic University (PolyU). Before joining the Hong Kong Polytechnic in 1990, Mr. Tse was the Planning Engineer of System Planning Branch in CLP. His main duty was to look after power system stability and the abnormal loads, such as arc furnace and traction. During his 20-year service in PolyU, Dr. Tse has engaged in 7 consultancy investigations associated traction power supply (3 with KCR, 2 with MTR, one with KCR/MTR and one with an overseas 1.5kV DC project). One of his research works was supported by MTRCL via the PolyU Teaching Company Scheme in 1996 As the Visiting Associate Professor with the EE Dept after retirement since September 2010, three of his taught MSc subjects (EE510, EE533 & EE537) related with traction systems are continued.
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At 150Hz, XL3=3XL1=2.68pu, XC3=XC1/3=-2.51pu, and close to resonance For injection of even a very large I3=10A (i.e. 0.00943pu)
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VL3=I3XL3= 0.025pu, VC3=I3XC3=-0.023pu

Adverse effect due to series resonance is marginal with foreseeable I3 Only slight increase from VL1 & VC1
1

Total VL=(VL12+VL32) = 0.136pu, (3.4kV) total VC=(VC12+VC32) = 1.134pu (28.35kV)

By proper design of the voltage and current ratings for both L & C, this 145Hz filter should be effective to absorb 3rd harmonic current.