EXPERIENCE ON DIAGNOSIS OF MV CABLE IN WIND FARM
Dae-jin Park Chung-hwan Lee Hyeon-seok Lee Jung-ji Kwon Jin-wook Choi Seok-hyun Nam LS cable & system – Republic of KOREA
[email protected] [email protected] [email protected] [email protected] [email protected] [email protected]
ABSTRACT
This paper shows the diagnostic technology of cable system in the wind farm. We diagnose the 22.9kV MV cable system which had occurred breakdown at several times at low wind speed generation.
First of all, partial discharge is measured using on-line PD measurement system and oscilloscope because insulation of cable system is vulnerable in PD signal.
Power quality is measured using power analyser for checking the condition of cable system. Power quality can affect insulation performance by generating the over- heat and additional losses. Power fluctuation, power factor, voltage & current and waveform and current unbalances of each phase are checked for identifying the power quality. We measured the harmonics by 50th orders and analysed the THDs of voltage and current.
Recently, harmonics in voltage and current are increased due to non-linear components and loads in the power system. Especially, wind power system among the renewable energy system makes much harmonics and steep signal like flicker from power electronic devices.
These extremely high frequency signals are accelerating the degradation of insulation and reducing the life of cables than power frequency signal. Therefore harmonics measurements of voltage and current have conducted.
INTRODUCTION
Renewable energy systems such as solar, wind, thermal power are used for the alternative power resources due to environmental issues. Among renewable energy sources, especially wind power system is rapidly increased for competitive price comparing with other renewable energies. However, electric engineer should consider power quality when connecting wind power system with power grid[1]. Wind power system involves in many power electronics devices which can produce the harmonics signal. Also, generated power and power quality are difficult to control because power is directly proportional to varied wind velocity.
This paper shows the diagnostic results of cable system in the wind power system at low wind speed. The power has served from wind farm to the load when wind velocity is fast and has received from the power utility when wind velocity is low. Test circuit is composed of 3phases, 3bundle, 2 circuits and 50m lengths between transformer and VCB. Diagnosis is implemented at 22.9kV VCB panel that is common coupled of wind turbines as shown in figure 1.
Fig 1. System diagram
PARTIAL DISCHARGE MEASUREMENT
The defects of insulation such as void, protrusion and contaminant generate discharge signal of high frequency and these can lead to the insulation breakdown in power facilities. Partial discharge measurement was conducted at VCB panel of TR#1 and TR#2. Table 1 shows the PD test equipment specification. Also oscilloscope is used to identify the waveform of signal.Table 1. PD test equipment
Equipment Specification
PD device MPD 600 (~20MHz)
Sensor HFCT (~80MHz)
Calibrator Pulse generator (10~1000pC)
Oscilloscope 400MHz 5Gs/s
HFCT sensors were installed at ground wire of termination joint and the 8~13MHz range of measuring frequency was used for detecting the discharge signal. PD pattern of all bundle cables were measured at various measurement frequency ranges.
Signals which have measured in PD equipment are noise signal pattern. Noise signal patterns have flat shape and didn’t appear at reverse 180° phase position. As shown in Table 2 and 3, patterns have flat shape and appeared at random phase. The majority of strange noise signals were measured at 8MHz measuring frequency range. Therefore, waveform analysis was conducted for identifying the signal in details
Table 2 PD measurement results of TR # 1 line Phase Measuring frequency range
8 MHz 10 MHz 13 MHz
TR
#1 R
S
T
Table 3 PD measurement results of TR #2 line Phase Measuring frequency range
8 MHz 10 MHz 13 MHz
TR
#2 R
S
T
Figure 2 shows measured waveform at R phase of TR
#1 using oscilloscope. The waveform has several hundred of nano-seconds in rising time. Generally, PD signal has the characteristic of more sharp rising time under serval nano-seconds
Fig 2. R phase waveform by oscilloscope Figure 3 shows simultaneously measured waveform of 3phase. The initial time of waveform is the same position.
Generally noise signal was measured at the same initial time to the 3 phase sensor. Therefore, we conclude that noise signal generated from outside of facility and PD signal is not detected in the cable system.
Fig. 3. Waveform measurement of 3phase
POWER QUALITY MEASUREMENT Active power measurement
Test has conducted at the low wind speed during a day because almost cable breakdown happened at low wind speed condition in this system. As shown in figure 4, power is not generated from 9 PM to 1 AM by wind turbine. It means that active power is supplied from power grid(power utility) at that time. Therefore power flow direction is changed frequently according to wind velocity.
Fig. 4. Active power variation of wind power system
Voltage and current quality
Voltage fluctuationVoltage and current quality are regarded as significant factors due to making damage and mis-operation of electric facilities. Therefore, magnitude of voltage and current are measured using power analyser.
Fig. 5. Voltage (U0) fluctuation of each phase
1000 ns/div
Same initial time of 3phase Waveform analysis is conducted as shown in fig.
2.
Figure 5 shows voltage fluctuation of 13.9kV(U0) power system. Voltage fluctuation has occurred below 1% in the power line. Voltage fluctuation is limited by 2% at all time to connect wind power system with grid in korea.
Therefore voltage fluctuation is satisfied with korea standard. Figure 6 shows voltage unbalance between phases. Voltage unbalance is under 2% in this system. As the results, voltage quality is stable.
Fig. 6. Voltage unbalance rate Current unbalance between bundle cables
Cable lay-out and load condition are influence on current unbalance. Current unbalance can make the overheating of specific phase of cable. This system consists of 3 bundle cables in each phase. Figure 7 shows installed bundle cables in VCB panel. Bundle cable system may cause the current unbalance due to unbalance of line impedances and EMF. Therefore current unbalance could be occurred even though same phase of cable.
Fig. 7. Current magnitude of cable line
We have measured all cable currents to check current unbalance. Figure 8 shows current magnitude of A phase 3 bundle cables.
Fig. 8. Current unbalance at same phase
Current unbalance was occurred at bundle cables as table 4. The most severe current unbalance was occurred at A phase of 3 bundle cables about 23.1%. Unbalance of B phase 3 bundle cables were the smaller than other phases due to stable line impedance.
Table 4. Current unbalance ratio between bundle cables.
Ave. current unbalance [%]
A phase B phase C phase
23.1% 6.5% 17.7%
Current unbalance between phases
Current unbalance between phases occurs maximum 30%
as shown in figure 9. This phenomenon may degrading and overheating the specific cable by flowing high current. Therefore to reduce the current unbalance, modification of cable lay- out or changing bundle cables to single installation is recommended.
Fig. 9. Current unbalance between phases
Harmonics measurement
Renewable energy system produces harmonics elements due to the presence of nonlinear components. These harmonics are making negative effect to the power facilities. Voltage harmonics may increase dielectric stress and give damage to power components. Current harmonics may increase the over-heating of cable and mis-operation of protection relay. Therefore, operator should comply with the harmonics limit to connect with grid[2]. Harmonics level is limited in accordance with IEEE 519[3]. We have measured harmonics to investigate how much harmonics are generated by wind power system in at the point of common coupling.
Voltage harmonics
Voltage harmonics is limited under 3% of individual harmonics and 5% of THDv in 13.9kV(U0) system as table 5.
Table 5. Voltage harmonics limits in IEEE 519 Bus voltage Individual harmonics Total harmonics
V≤1kV 5% 8%
1kV<V≤69kV 3% 5%
69kV<V≤161kV 1.5% 2.5%
161kV<V 1.0% 1.5%
A1: inner A2: middle
A3: outer C3 : outer
C2 : middle C1:inner
B2: middle B3: outer B1: inner
Voltage harmonics was measured through VT(Voltage Transformer) in VCB panel by power analyser. THDv had measured under 2.5% during a day, so it is the lower than 5% of IEEE standard as shown in figure 10.
Fig. 10. Total harmonic distortion(THD) of voltage Also, individual voltage harmonics were measured as figure 11. The 17th harmonics is the most highest about 1.2%. All voltage harmonics orders were measured below 3%. Therefore THDv and individual harmonics are satisfied with IEEE standard value.
Fig. 11. Individual of voltage harmonic Current harmonics
Current harmonics were also measured in accordance with IEEE 519. Odd current harmonics are limited as table 6 and even harmonics are limited to 25% of odd harmonics level. ISC/IL factor of this system is blew 20 because IL is 2000A and ISC is 25kA. And TDD is restricted by 5% when ISC/IL is under 20%. TDD is measured at each phases.
Table 6. Current harmonics limits in IEEE 519
Current harmonics should be checked by TDD (Total demand distortion) value not a THD. Because current harmonic ratio is calculated by dividing current value.
For example, THDi is calculated by over 30% due to low measured current value in this system but TDD is calculated below 3% due to calculating by demand load current value.
Fig. 12. Total demand distortion(TDD) of voltage Individual current harmonics was measured during a day. Figure 13 shows the individual current harmonics when TDD is the highest value. Dominant harmonics are 5th, 13th 17th orders. Almost harmonics are not exceeding IEEE standard value. But 17th harmonics of C phase is measured by 2.2%, it exceeds the limit of 1.5%
Fig. 13. Individual of current harmonic
To identify how much current waveform has distorted, waveform was measured through oscilloscope as shown figure 14 and 15. Voltage waveform is shown as sinusoidal without harmonics. But current waveform has distorted by harmonics because it is measured at low wind speed.
Fig. 14. Voltage waveform Isc/IL 3≤h<11 11≤h<17 17≤h<23 23≤h<35 35≤h≤50
h<20 4.0 2.0 1.5 0.6 0.3 20<h<50 7.0 3.5 2.5 1.0 0.5 50<h <100 10.0 4.5 4.0 1.5 0.7 100<h <1000 12.0 5.5 5.0 2.0 1.0 h>1000 15.0 7.0 6.0 2.5 1.4
C phase B phase A phase
Fig. 15. Current waveform
CONCLUSION
This paper shows results of power quality measurement in the wind power system at low wind speed. Power quality of wind power system varies continuously because of varied wind speed and non-linear load. Also, active power and power flow direction are changed when wind speed is low.
Partial discharge signal was not detected in the cable system, only noise signal was detected. And waveform analysis is implemented to identify that measured signal is noise by using oscilloscope.
And voltage fluctuation isn’t also severe below 1%.
Voltage quality is satisfied in accordance with IEEE standard. Also voltage unbalance between phases is under 1.5% in this system. Current unbalances were detected at bundle cables of each phase. We assumed that unbalance is caused by cable lay-out of bundle cable. Therefore single cable installation or layout modification is recommended to reduce current unbalance.
Voltage and current harmonics are measured by power analyser. THDv and individual voltage harmonics are satisfied with IEEE 519. Also, TDD values don’t exceed limit of IEEE 519. But the 17th current harmonic exceeds the limit of IEEE 519. Another individual harmonics are satisfied with limit value
In future work, we will check the power quality when wind speed is high and chemical analysis will conduct.
REFERENCES
[1] E. Muljadi, C. P. Butterfield, J. Chacon, and H.
Romanowitz, 2006, “Power quality aspects in a wind power plant", IEEE Power Engineering Society General Meeting.
[2] Kuo-Hua Liu, Li Wang, 2007,“Analysis of Measured Harmonic Currents and Voltages Contributed by a Commercial Wind Power System”, IEEE Power Engineering Society General Meeting
[3] IEEE Power and Energy Society, 2014, "IEEE recommended pactice and requirements for harmonic control in electric power systems", IEEE std 519.
