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Power Quality Analysis

The Power quality Analysis Study is conducted in order to observe Voltage Sag, which is a reduction of AC voltage at a given frequency for the duration of 0.5 cycles to 1 minute’s time, that are usually caused by system faults, and are also often the result of switching on loads with heavy startup currents. The common causes of sags include starting large loads and remote fault clearing performed by utility equipment. Similarly, the starting of large motors inside an industrial facility can result in significant voltage drop (sag). Voltage Sags cause the majority of malfunctions of equipment. Relays and contractors can drop out if a dip is 60% for longer that 1 cycle. Potential damage is dependent on the ability of the equipment to sustain lower voltage for short periods. Information technology is particular sensitive to a dip. Electronic drives, converters and equipment with an electronic input stage are also sensitive to dips. An asynchronous motor can draw a current higher than it’s starting current at dip recover.

Voltage Swell which is an instantaneous voltage increase (opposite to Sag)caused by single line ground failures (SLG), upstream failures, switching off a large load or switching on a large capacitor, which usually last for a short period, do not have significant impact on Equipment, except for some safety problem.

Voltage interruptions are a network’s isolation from any source of supply. Because of energy stored in a network, a specific voltage above zero, exists for a short period after the interruption commences. The interruptions can cause disruption in production by increasing the number of rejects or material wastage. In some areas, interruptions can increase the risk of equipment damage or even injury. Information technology is affected in two ways. First, current data can be lost and the system can be corrupted. Second, after interruption is over, the re-boot process, especially on a large and complex system, can last for several hours. Because of these reasons, critical computer systems and telecommunication equipment are supplied with UPS power.

Transients which is a short, highly damped momentary voltage disturbance are classified as, impulsive overvoltage and oscillatory overvoltage. The common problem being the Oscillatory transient overvoltages that are caused by switching, ferroresonance or can arise as a system response to an impulsive overvoltage. Switching overvoltages have high energy and are classified as low (<5kHz), medium (5kHz<f<500khz) and=”” high=”” frequency=”” (=””>500kHz) transients.The Transient overvoltages cause the immediate failure or degradation of a transformer, capacitor or semiconductor or causes cable isolation that can lead to faulty operation. Electronic drives may fall out. Also, magnification of MW transients caused by capacitor bank switching may occur under some circumstances producing 2-4 p.u. over voltages.

Conduction of voltage variation study:

The Voltage variation study is done at the selected Power Supply nodes where the impacts of Voltage variations are reported to be significant,

Voltage Sags:

For the assessment of Voltage Sags, the rms voltage is calculated over a single cycle or a half cycle and is refreshed each 10 ms i.e. every half of a cycle. This value is denoted as Urms(1/2). Every 10 ms a new rms value is presented for comparison with the dip threshold.

Voltage dip is characterised by:

  • Dip threshold
  • Starting time of a dip
  • Dip duration
  • Retained voltage (uret)

Dip threshold can be set by the user and represents part of nominal Un or declared Uc (or Udec in some standards) voltage and can vary from 0.9 Uc for troubleshooting to 0.65 Uc for contractual purposes.

Voltage dip attributes:

The dip starts when Urms(1/2) drops below the dip threshold. The dip ends when Urms(1/2) rises above the dip threshold. The difference between end and start time is dip duration and is reported in seconds or in cycles. Retained (residual) voltage uret is the lowest Urms(1/2) value recorded during a dip.

The minimum set of attributes which describes a dip is a pair [uret, duration], although some instruments store more data such as the average voltage during the dip period or the shape of Urms(1/2) voltage. The example in figure can be described as dip[209V,160ms] or dip[209 V,8c].

Instead of using Un or Uc as reference, a sliding reference voltage can be used for the calculation of a dip threshold. This option is useful for avoiding problems with transformer ratios when measurement is taken on both the LV and MV side of a system. Also, the retained voltage can be reported as a % or p.u. of rms value before dip.The end threshold is typically 1% higher than the start threshold. This is due to a problem, which can arise if a measured value is near to the start of a dip threshold.

Voltage swell:

Voltage Swell have same attributes as for the Voltage sag.

Voltage interruptions:

Voltage interruption is detected as a Urms(1/2) drop below an interruption threshold. The interruption threshold can vary but is usually set to 1%, 5% or 10% of the declared voltage. The duration of an interruption is measured in the same manner as a measurement of dip duration after setting an interruption threshold.

Because of the measurement technique a short circuit fault can appear as a short interruption in one section of the network and a dip in another. Interruptions are classified in two groups:short interruptions, long interruptions. The Short interruptions are introduced by a fault condition in a network, which causes switchgear to operate.Complex schemes of operations are used for reclosing purposes. The duration of a short interruption is limited to 1 minute or 3 minutes depending upon the reclosing operation, the standard used or the contract between supplier and customer.

Long interruptions are interruptions in excess of the short interruption duration limit. They arise when a fault condition cannot be terminated with a control sequence and the final tripping of a circuit breaker occurs.

Anomalies – voltage sags, Swells and interruptions

The detection of voltage dips, swells and interruptions is performed during a measurement period and is obtained in the periodics recording.

Voltage transients:

Transient recording is done with the fastest sampling to capture signals in the order of 25 kHz. For a single signal enabled for capturing, there are 1000 samples in a 50 Hz signal period. Triggering is performed with a
triggering mode (a difference between two samples), where the difference between the two samples is that a signal’s slope and trigger is set if the slope is higher than the set threshold (du/sample or di/sample).

The voltage variation problems are compensated by modern DVRs which address the following disturbances:-

a) Voltage dips and swells
b) Voltage variations,
c) Voltage distortion,
d) Voltage flicker,
e) Voltage unbalance,
f) Some level of transient over voltages

Higher power units of DVR can be built by paralleling number of slave units to a master, from one to more than 10 units. The following are the features of Typical modern DVR to address the Voltage Variation Problems.

a. No battery or alternative energy storage is required, minimizing the maintenance cost and increasing the reliability
b. Continuous voltage regulation within the ± 0.5%.
c. Compensation of long lasting dips (-50% up to 30 seconds)
d. Avoids relays and brushes
e. Time response less than 3 milliseconds
f. Capable to operate with industrial regenerative loads (four-quadrant converter, …)
g. Improves the voltage distortion.
h. Flicker compensation.
i. Non stop of process operation in case of failure.
j. Easy to parallel additional equipment
k. Independent phase compensation
l. Voltage balancing capability
m. Balanced and unbalanced dip compensation
n. Automatic Bypass
o. Efficiency 97.5% for low voltage and 98.5% for medium voltage.
p. Overload capacity: 150% during 1 second
q. Dips logging and system monitoring