電力品質分類

download report

Transcript 電力品質分類

Power Quality
Any power problem manifested in voltage, current or frequency
deviation that results in failure or misoperation of customer
equipment.
IEEE (Working Group on Distribution Voltage Quality) :
電力系統擾動的相對程度;以電力公司為立場,所定義之電
力品質可以簡述為電力系統對污染源(用戶)之接受度。
CEA (Canada Electric Association) :
在某一電力條件情況下, 電力設備性能的滿意程度;以用戶
為立場,所定義之電力品質可以簡述為用戶對電力公司供電
品質之滿意度。
換句話說,電力品質必須得到電力公司與用戶雙方皆能接受
或滿意,至於滿意與否之界定就必須由電力品質相關因素之
管制標準來規範。
電力品質分類
污染因素
污染源
影響對象
電力諧波
非線性負載:如電力電子 減短電力設備壽命,嚴重過壓或
類、鐵心激磁類與電弧爐 過流可能導致設備故障
電壓閃爍
驟變負載如電弧爐
電燈閃爍使眼睛不適、發電機激
磁系統不穩定
電壓驟降與
電壓昇
雷擊、鹽害與人為事故
瞬間停電
電力中斷
維修、人為事故與天災
保護系統動作導致區域停電
三相不平衡
單相負載:如單相高週波 馬達過熱、電腦螢光幕扭曲與通
爐、高速鐵路等
訊干擾
電壓突波與
電流突波
雷擊、開關切換與電容器 瞬間過壓或過流可導致設備故障
切換
電磁場
家電、行動電話與輸配電 保護電驛誤動作與通訊干擾
線
頻率變動
大型發電機、負載或輸電 汽電廠解聯、發電機頻率電驛動
線跳脫
作等
不良電力品質範例
含 3 級 、5 級 諧 波
AMPLITUDE
份
2
1
0
-1
2
-2
0.04
2
0.02
0.04
3 級諧波
含 2 級 、Sec.
諧波(harmonic)
1
0
-1
-2
0.04
0
0.02
TIM E (SEC)
0.04
電壓閃爍(flicker)
40
kV
2
SEC)
AMPLITUDE
級諧波
0
0
-40
0
30
m sec
電壓驟降(sag)
60
不良電力品質範例
70.5
70
69.5
電壓(kV)
69
68.5
68
67.5
67
Vab
Vbc
Vca
66.5
66
0
50
100
150
時間(微妙)
三相不平衡與突入電壓(three-phase unbalance)
突入電壓與電流(inrush current)
頻率波動
200
In IEC 61000-2-5:1995, the IEC classifies electromagnetic phenomena into several groups
as shown in Table 1. The IEC standard addresses the conducted electrical parameters shown
in Table 1. The terms high frequency and low frequency are not defined in terms of a specific
frequency range, but instead are intended to indicate the relative difference in principal
frequency
content
of
the
phenomena
listed
in
these
categories.
IEEE Std
1159-2009
Transient
An event that is undesirable but momentary in nature
Part of the change in a variable that disappears during
transition from one steady state operation condition
to another
Surge
It can be classified into two categories:
Impulsive,oscillatory
Impulsive transient
1.An impulsive transient is a sudden, non-power frequency change in the
steady state condition of voltage, current or both, that is unidirectional in
polarity
2.They are characterized by rise and decay times.
3.lightning
4.They are generally not conducted far from the occurred source
5. It can be divided into three parts
1.2  50s 2000V
Rise from zero to its
peak value of 2000 V
in 1.2 s , then decay
to half its peak value
in 50 s
台
灣
落
雷
日
數
與
頻
率
實
測
圖
Oscillatory transient
1.An oscillatory transient is a sudden, non-power frequency change in the
steady state condition of voltage, current or both, that includes both
positive and negative polarity values
2.They are characterized by spectral content, duration and magnitude .
3. It can be divided into high, medium and low frequency three parts
HF(>500kHz, 5us, 0-4pu):the result of a local system responses to an
impulsive transient
MF(5-500kHz, 20us, 0-8pu):fig.2-2,cable switching or impulsive transient
LF(<5kHz):occurred at utility sub-transmission and distribution systems,
capacitor bank energization: result is oscillatory voltage
transient with frequency 300-900Hz. Peak magnitude can
reach 2.0pu(typically 1.3-1.5), duration 0.5-3 cycles
Long-Duration Voltage Variations
1.
Long-duration variations encompass root-mean-square(rms) deviations
at power frequencies for longer than 1 min
2.
Over-voltage: rms voltage greater than 1.1pu, duration longer than 1min
switching off a large load, energizing a capacitor bank, weak system,
inadequate voltage control(TX OLTC, Gen AVR..)
3.
Under-voltage: rms voltage less than 0.9pu, duration longer than 1min
4.
Sustained Interruption: rms voltage drop to zero, duration longer than
1min(outage:it is the same as interruption for utility. For user,any
interruption of power that shuts down a process. This could be as little
as one-half of a cycle.)
Short-Duration Voltage Variations
Short-duration voltage variations are caused by fault
conditions, the energization of large loads which require
high starting current, or intermittent loose connects in power
wiring.
Interruption:Interruption can be the result of power system faults,
equipment failure, and control malfunctions.
The duration of an interruption due to a fault is determined by the
operating time of the protective devices
Sag(Dip):Sag are usually associated with system faults but can also be
caused by energization of heavy loads or starting of large motors
a 20 percent sag will be considered an event during which the rms voltage
decreased by 20 percent to 0.8pu
Voltage sag caused by the starting
of large motor stating
Swell: Unfaulted phases during a SLG fault, switching off a large load,
or energizing a large capacitor bank.
Swell are characterized by their rms magnitude and duration
Ungrounded system: unfaulted phases will be 1.73pu during a SLG
Grounded system:there will be little or no voltage rise on unfaulted phases
Faults at different points along four-wire, multigrounded feeders will have
varying of voltage swell as shown in the figure
Voltage Imbalance
Imbalance (sometimes called unbalance) in a three-phase system is defined as the ratio of the
magnitude of the negative sequence component to the magnitude of the positive sequence
component, expressed as a percentage. This definition can be applied for either voltage or current.
Typically, the voltage imbalance of a three-phase service is less than 3%. The current imbalance
can be considerably higher, especially when single-phase loads are present.
Measuring instruments often use a definition of voltage imbalance based on ANSI C84.12006, which defines imbalance as the ratio of the maximum deviation of a voltage from the
average voltage to the average voltage, expressed in percent, using phase-to-phase voltage
measurements
VUR 

max Va  Vavg , Vb  Vavg , Vc  Vavg
V
d 2  2 100%
V1
Vavg
V
d 0  0 100%
V1
100%
1
V0 
V   1 1
 1 3

V2 
1
V  Vb  Vc
Vavg  a
3
1  V 
a

a
a 2  Vb 

a 2 a  Vc 
1
This figure shows an example of a one-day trend of imbalance measured at one point
on a residential feeder. The trace labeled “V Neg-Seq Imb Total” is using the preferred
definition of imbalance. The trace of “V Zero-Seq Imb Total” is the ratio of the zerosequence-to-positive-sequence voltage. This is sometimes helpful to understand
sources of asymmetry in the power system, particularly due to single phase loads
connected phase to neutral. The trace labeled “V Imb Avg LN” is using the ANSI-type
maximum deviation from the average definition, but on line-to-neutral quantities.
Waveform Distortion
1.Dc offset can occur as the result of a geomagnetic
disturbance, the effect of half wave rectification. It will
result in the saturation of transformer in normal operation
2.Harmonics and inter-harmonics
3.Notching
4.Noises are unwanted electrical signals with broadband
spectral lower than 200kHz. It can be caused by power
electronic devices,control circuits,arcing equipment,
rectifiers and SPS. Noise will disturb electronic devices such
as microcomputer and programmable controllers.
Harmonic
Notching
Notching is a periodic voltage disturbance caused by the
normal operation of power electronics devices when current
is commutated from one phase to another
Voltage Fluctuation
Voltage fluctuations are systematic variations of the voltage envelope or a series
of random voltage changes, the magnitude of which does not normally exceed
the voltage ranges specified by ANSI C84.1- 2006 of 0.95 pu to 1.05 pu. Any
load that has significant cyclic variations, especially in the reactive component,
can cause voltage fluctuations.
Loads that exhibit continuous, rapid variations in load current magnitude can
cause voltage variations referred to as “flicker.” The term flicker is derived from
the impact of the voltage fluctuation on lighting intensity. Voltage fluctuation is
an electromagnetic phenomenon, and flicker is an undesirable result of that
phenomenon.
Arc furnaces are the most common cause of voltage fluctuations on the
transmission and distribution system. (Lamp flicker is measured with respect to
the sensitivity of the human eye.)
Power Frequency Variations
They are seldom occurred in large power system except
1.a large block of load or generation trip due to transmission system faults
2.Some customers with generation isolated from the utility system(Governor)
3.Voltage notching may be mistaken for frequency deviation to result in error
for zero crossing devices.