
Frequently
Asked Questions 



What
is power quality? 
Is a
of frequency
and voltage of power supply for electrical loads including powercut devices and
protectors. 



Issues
related to the power quality ? 
Type of disturbance 
Origins 
Consequences 
Examples of mitigation solutions 
Voltage variations and fluctuations

Large load variations (welding
machines, arc furnaces, etc.) 
Fluctuation in the luminance of lamps
(flicker) 
Electromechanical reactive power
compensator, real time reactive compensator, series electronic
conditioner, tap changer. 
Voltage
dips 
Shortcircuit, switching of large loads
(motor starting, etc.). 
Disturbance or shutdown of process: loss
of data, incorrect data, opening of contactors, locking of drives,
slowdown or stalling of motors, extinguishing of
discharge lamps. 
UPS, real time reactive compensator,
dynamic electronic voltage regulator, soft starter, series electronic
conditioner. Increase the shortcircuit power (Scc).
Modify the discrimination of protective devices. 
Interruptions 
Shortcircuit, overloads, maintenance,
unwanted tripping. 
Disturbance or shutdown of process: loss
of data, incorrect data, opening of contactors, locking of drives,
slowdown or stalling of motors, extinguishing of
discharge lamps. 
UPS, mechanical source transfer, static
transfer switch, zerotime set, shunt circuit breaker, remote
management. 
Harmonics 
Nonlinear loads (adjustable speed
drives, arc furnaces, welding machines, discharge
lamps, fluorescent tubes, etc.). 
Overloads (of neutral conductor,
sources, etc.), unwanted tripping, accelerated ageing, degradation of
energy efficiency, loss of productivity. 
Antiharmonic choke, passive or active
filter, hybrid filter, line choke. Increase the Scc. Contain polluting
loads. Derate the equipment. 
Interharmonics 
Fluctuating loads (arc furnaces, welding
machines, etc.), frequency
inverters. 
Interruption of metering signals,
flicker. 
Series reactance 
Transient
over voltages

Operation of switchgear and
capacitors, lightning 
Locking of drives, unwanted tripping,
destruction of switchgear, fire, operating
losses. 
Surge arrester, surge diverter,
controlled switching, preinsertion resistor, line chokes, static
automatic compensator. 
Voltage
unbalance 
Unbalanced loads (large
singlephase loads, etc.) 
Inverse motor torque (vibration) and
overheating of asynchronous machines. 
Balance the loads. Shunt electronic
compensator, dynamic electronic
voltage regulator. Increase the Scc. 




Criteria
of power quality ? 
The frequency
deviation The voltage deviation The voltage fluctuation The voltage symmetry The sine degree of voltage waveform




What
is the voltage deviation ? 
It is the difference between actual voltage value
and nominal one . This value is indicated as a percentage (%) of nominal
voltage. The voltage deviation of different electrical households has different
values
Voltage levels at points in the grid should be determined by the method of
operation and according to maximum and minimum load regimes . In normal
conditions, the voltage deviation is allowed to fluctuate within approximately ±
5% compared with normal voltage and is determined at measuring device position or
at other one by the two sides agreement
In case of unstable grid, the allowed voltage value variation is 10% to +5%.
(According to Part I  General Regulations of the electrical facilities
Regulations  11 TCN  18  2006  The Ministry of Industry  Viet Nam) 



What
is voltage fluctuation ? 
It is a variation of the voltage occurring in
relatively short time .Its speed does not exceed 1% per second. Load is affected
by not only voltage fluctuation amplitude but also its appearance frequency.
Voltage fluctuation amplitude can be applied by formula:
V_{k}
= k_{Q} / (1k_{Q}) x 100%
of which:
V_{K}:
fluctuation amplitude
k_{q} = Δ_{Q} / S_{ba}
Δ_{Q}: sudden change reactive load (MVAR)
S_{BA}: Nominal transformer capacity for the given point
Allowed
fluctuating voltage is determined by the formula:
V_{cp}
= 1 + 6 / n = 1 + Δ_{t}/10 (%)
of which:
n: the
frequency of the variation in one hour (times / hour)
Δ_{t}: The average time between the variation (minutes)
V_{K}
<V_{cp}




What
is voltage symmetry ? 
If U_{1},
U_{2}, U_{0} ,one after the other, are positive, negative and
zero sequence components. The asymmetry level is indicated by the two factors :
Non – symmetry factor : k_{pdx}
= U_{2} / U_{1}
Nonbalance factor : k_{kcb} = U_{0} / U_{1}
The allowed value is
less than 5%




What
is the frequency deviation ? 
It is the difference between actual frequency value
and nominal one. This value is indicated as a percentage (%) of nominal
frequency.
In normal conditions, the frequency of the power system is allowed to fluctuate
within ± 0.2 Hz compared with the nominal frequency of 50Hz.
In the case of power system is unstable, allowed frequency deviation is ± 0.5 Hz
(According to Part I  General Regulations of the electrical facilities
Regulations  11 TCN  18  2006  The Ministry of Industry  Viet Nam) 



The
sine degree of voltage waveform 
The change of
current and voltage when do not comply with sine law will result in harmonics
The sine level is evaluated by nonsine factor k_{ks}
k_{ks}: (U 
U_{1)} / U_{1} x 100%
Of which:
U_{1}: voltage
of basic harmonic
U: RMS voltage
The allowed value is less than 5%







What is Tuned
and Detuned ? 








