POWER QUALITY COMPLEX ESTIMATION AT ALTERNATING CURRENT TRACTION SUBSTATIONS

Purpose. At alternating current traction substations are used three-winding transformers. This scheme comes to significant costs from power quality disturbances at each supplier. The most significant power quality indices at alternating current traction substation are voltage deviation, voltage unbalance and harmonic distortion. The purpose of this article is power quality complex estimation at two traction substations that work on the one district of traction network. Methodology. The measurements were made according to accepted program during 24 hours with 1 sec discretization. A few power quality analyzers PM175 are used to record data with time synchronization. The scheme of measurements means that devices are connected through current and potential transformers at the each level of voltage. Findings. Voltage level at different substation is various due to power losses in primary network. The voltage in one phase of 1-st substation is bigger than in other phases. Voltage THD values according to standard requirements are below norm only for primary 154 kV networks. For another traction and regional networks voltage THD values are above norm value. The voltage unbalances in 154 kV networks are below voltage unbalance in 35 kV networks. Besides the voltage unbalance in 154 kV network is below 2% that complies with the requirements of standard. At the same time we can consider that voltage and current spectrums consist from discrete harmonics with frequencies that multiples of the fundamental frequency. Originality. Voltages at the connection points of traction substations to supply lines are complex stationary random process that determines the voltage mode of the external power supply system of each traction substation. The flows of active and reactive power of the higher harmonics at AC traction substation are directed opposite power of the fundamental harmonic. The power flows on higher harmonics create the disturbing impact on related devices. The total power at AC traction substation consists of 61.8% of the active power, 32.5% of the reactive power of the 1st harmonic and 5.7% of the distortion power in the quadrature components. Practical value. Analysis of the quadrature components suggests that traditional approaches to reactive power compensation at the fundamental frequency will be sufficiently effective. But the influence of the distortion power on reducing energy losses must be also researched. Besides reactive power compensation should be considered with the issue of reducing the basic power quality indices to standardized values.


Introduction
The global electrification of railway transport in Soviet Union force to use three-winding transformers at AC traction substation.The advantage of this scheme is in refusing from individual transformer for regional power supply.In that case all regional suppliers consume electric power from third winding of transformer which called regional.But in real these scheme comes to significant costs from power quality disturbances at each supplier.
The most significant power quality indices at AC traction substation are voltage deviation, voltage unbalance and harmonic distortion.
As a rule regional suppliers have some voltage increasing in one of three phases.Such increasing caused not equaling load of left and right side of traction substation.Currents of left and right sides load two working windings.In the third winding flows the difference of that currents with 1/3 multiplier.Therefore third winding are not fully loaded and it caused power quality problems.
Each power transformer at AC traction substation has voltage regulation device which works under load.But it regulates voltage in all windings together and can't be used for three-phase balancing.Besides the voltage mode at AC traction substation is choose to provide minimal voltage level at the most difficult district of traction network.

Purpose
The purpose of this article is power quality complex estimation at two traction substation that works on the one district of traction network.

Methodology
The measurements were made according to accepted program during 24 hours with 1 sec discretization.A few power quality analyzers such PM175 are used to record data with time synchronization.Simplified scheme of measurements are shown at Fig. 1.The scheme means that devices are connected through current and potential transformers at the each level of voltage.

Total harmonic distortion and three-phase unbalance
Voltage total harmonics distortions (THD) for each phase are similar.Voltage THD values according to standard requirements [4] are below norm only for primary 154 kV networks (Fig. 4).For another traction and regional networks voltage THD values are above norm value.The norm of voltage unbalance is 2% and it doesn't depend on the voltage range.
The voltage unbalance series are shown at Fig. 5.As can be seen the voltage unbalance in 154 kV networks are below voltage unbalance in 35 kV networks.Besides the voltage unbalance in 154 kV network is below 2% that complies with the requirements of standard.The voltage unbalance in 35 kV network is higher than 2% and its maximum reached within 4.7% value.Traditionally the voltage unbalance caused by unbalancing currents in a three-phase system.The primary currents at one substation are shown at Fig. 6 as fields of points.Each point is defining the end of primary current vector at complex plot.Fig. 6.Currents in 154 kV primary network at complex plot It can be seen from Fig. 6 that currents in second phase (B) are lower than in other phase.In this case phase A and C are working.The phase B is the least loaded.Besides the phase A has the lowest power factor because the angle of load in this phase is the higher than in other phases.

Detail waveform researching
The voltage and current waveforms are very different from the sine (Fig. 7).Besides the current waveform are lags from the voltage waveform due to inductive component in traction load.
Before Fourier analysis check the discreteness of spectrum is needed.This check may be performed by comparing RMS values that defined by various methods.

Fig. 7. Voltage and current waveforms in traction network
The RMS values may be evaluated by integration of waveform and discrete Fourier transform methods.So the conditions that used to check spectrum discreteness may be written as follows  ( ) where ( ) u t , ( ) i t -instantaneous voltage and cur- As can be seen from table 1 the conditions (1)-( 2) are satisfied with 0.001% error.This means that we can consider that voltage and current spectrums consist from discrete harmonics with frequencies that multiples of the fundamental frequency.The relative spectrums of voltage and current at AC traction substation consist of odd harmonics (Fig. 8).The fundamental harmonics are shown not in scale.When spectrums of voltage and currents will be compared it can be seen that in current spectrum with frequency increasing the harmonics values are decreasing.But in voltage spectrum some harmonics are gained and some are weakened.For example, in current spectrum the harmonic with a frequency of 150 Hz has a maximum.But in voltage spectrum the harmonic with maximum value has a frequency of 250 Hz.This indicates for non-linear resistance characteristics of traction network as a function of frequency.

Findings
Using these spectra, we can calculate the components of the balance of the total power in an AC circuit.Notable is the fact that the flows of active and reactive power of the higher harmonics are directed opposite power of the fundamental harmonic (Fig. 9).
This suggests that the AC electric rolling stock is a load at the fundamental frequency and a noise generator on the higher harmonics.
In percentage terms power flows on higher harmonics are less than 1 % and create more disturbing impact on related communication devices, automation and remote control than the effect of the losses increasing.
The results of power components evaluation showed as keeping the balance in the quadrature components of active power P, reactive power of the 1st harmonic Q 1 and distortion power D (Fig. 10).The obtained result shows that the total power consists of 61.8% of the active power, 32.5% of the reactive power of the 1st harmonic and 5.7% of the distortion power in the quadrature components.

Originality and Practical value
The voltage and current spectrums at AC traction substation may be considered as spectrums that consist from discrete harmonics with frequencies that multiples of the fundamental frequency.
The AC electric rolling stock is a noise generator on the higher harmonics because the flows of active and reactive power of the higher harmonics are directed opposite power of the fundamental harmonic.
Analysis of the quadrature components suggests that traditional approaches to reactive power compensation at the fundamental frequency will be sufficiently effective.But the influence of the distortion power on reducing energy losses must be also researched.Besides reactive power compensation should be considered with the issue of reducing the basic power quality indices to standardized values.

Conclusions
1.The most significant power quality indices at AC traction substation are voltage deviation, voltage unbalance and harmonic distortion.
2. Voltages at the connection points of traction substations to supply lines are complex stationary random process that determines the voltage mode of the external power supply system of each traction substation.
3. Voltage waveform distortions according to standard requirements are below norm only for primary networks.On the other connection the dis-turbances are outside the allowable range.The situation is similar for the voltage unbalance.
4. The flows of active and reactive power of the higher harmonics at AC traction substation are directed opposite power of the fundamental harmonic.The power flows on higher harmonics create the disturbing impact on related devices.
5. The total power at AC traction substation consists of 61.8% of the active power, 32.5% of the reactive power of the 1st harmonic and 5.7% of the distortion power in the quadrature components.

Fig. 1 .
Fig. 1.The scheme of measurements at AC traction substation Voltage deviations in primary network 154 kV of each traction substation are shown at Fig. 2 during 24 hours.Voltage level at different substation is various due to power losses in primary network.Besides, the voltage in one phase of 1-st substation is bigger than in other phases.Voltage regulating devices helps to align voltages at other points of coupling either 35 kV or 27.5 kV.Voltage deviations in these networks of one traction substation are shown at Fig. 3.When voltage equalization in 35 kV is needed the compensation current may flows throw traction network.In this case compensation current causes additional useless power losses.

Fig. 9 .Fig. 10 .
Fig. 9. Active and reactive power spectrums In calculations the total power was determined by the product of the RMS values of current and voltage.