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    Frequently Asked Questions
 

What is power quality?

Is a Frequency and voltage are considered good degree when their actual values are within allowed range. This range is determined by criteria of frequency and voltage of power supply for electrical loads including power-cut 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

Short-circuit, 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 short-circuit power (Scc). Modify the discrimination of protective devices.

Interruptions

Short-circuit, 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, zero-time set, shunt circuit breaker, remote management.

Harmonics

Non-linear 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.

Anti-harmonic choke, passive or active filter, hybrid filter, line choke. Increase the Scc. Contain polluting loads. Derate the equipment.

Inter-harmonics

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, pre-insertion resistor, line chokes, static automatic compensator.

Voltage unbalance

Unbalanced loads (large single-phase 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:

                     Vk = kQ / (1-kQ) x 100%

of which:
                VK: fluctuation amplitude
                kq = ΔQ / Sba
                ΔQ: sudden change reactive load (MVAR)
                SBA: Nominal transformer capacity for the given point
Allowed fluctuating voltage is determined by the formula:

                     Vcp = 1 + 6 / n = 1 + Δt/10 (%)

of which:
           n: the frequency of the variation in one hour (times / hour)
          Δt: The average time between the variation (minutes)

                 VK <Vcp

 

 
 

What is voltage symmetry ?

If U1, U2, U0 ,one after the other, are positive, negative and zero sequence components. The asymmetry level is indicated by the two factors :

                 Non symmetry factor : kpdx = U2 / U1
                 Non-balance factor : kkcb = U0 / U1

              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 non-sine factor kks

                          kks: (U - U1) / U1 x 100%

Of which:
                 U1: voltage of basic harmonic
                 U: RMS voltage

               The allowed value is less than 5%

 

 
 

What does bad power quality cause ?

          - Increase the cost of investment capital.
          - Increase operating cost.
          - Reducing equipment efficiency

 

 
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