## Sunday, April 17, 2011

Voltage Sag Types are generally based on the individual voltages (both magnitude and angle) for each of the three phases during sags or dips. Usually, three-phase voltage sags are categorized by either the ABC classification or the symmetrical components classification. However, voltage sag type according to the ABC classification is frequently used due to its simplicity as it is based on a simplified network model. Consequently, the classification is based on incomplete assumptions and cannot be used to obtain the characteristics of measured sags. The symmetrical component classification is more general and gives a direct link with measured voltages but is harder to understand and a translation to the ABC classification may be suitable for many applications. In addition, the ABC classification was developed to analyze the propagation of sag or dip from transmission to distribution levels, when a disturbance propagates through a transformer.

Factors That Affect Voltage Sag Type

Specifically, at the equipment terminals, these factors affect the voltage sag type:

1.     Fault type

Voltage sags are primarily caused by system faults. Each fault type has a different effect to the voltages at the fault point, which subsequently defined the voltage sag types.

Ø  Single-Line-to-Ground (SLG) Fault
Ø  Line-to-Line (LL) Fault
Ø  Double-Line-to-Ground (LLG) Fault
Ø  Three Phase (3P) Fault

2.     Transformer Winding Connection

Transformer winding connections are classified into three types to explain the transfer of three-phase unbalanced voltage sags, as well as the change in voltage sag type, from one voltage level to another.

Ø     Type 1 – Transformers that do not change anything to the voltages. The primary voltages (per unit) are equal to the secondary per unit voltages.  The only transformer configuration that falls under this type is the WYE Grounded-wye grounded (Ynyn).

Ø     Type 2 – Transformers that remove the zero-sequence voltage. Basically, the secondary voltage (pu) is equal to the primary voltage (pu) minus the zero-sequence component. The DELTA-delta (Dd), DELTA-zigzag (Dz) and the WYE-wye (with both windings ungrounded or with only one star point grounded) belong to this type.

Ø     Type 3 – Transformers that changes line and phase voltages. DELTA-wye (Dy), WYE-delta (Yd) and the WYE-zigzag (Yz) fit under this type.

The ABC classification is derived based on the combination of the three factors discussed above. The table below summarizes the combination and the resulting voltage sag type. Combination of Factors and the Resulting Voltage Sag Type
Examples:

Ø     For a SLG fault, Type 2 Transformer and Wye-connected Load - voltage sag type D is to be expected.
Ø     For LLG fault, No transformer and Delta-connected load - the voltage sag type is F.

Moreover, based on the table presented above, the following can be noted:

• Type A voltage sags are due to three-phase faults, regardless of transformer winding and load connections.
• Type B voltage sags are only caused by SLG faults.
• Types C & D are produced from either SLG or LL faults.
• Types E, F & G are only expected if the fault is LLG.
• The transformer winding and load connections varies the voltage sag type as the disturbance propagates through the power system.

Voltage Sag Types - ABC Classification

There are seven basic voltage sag types according to the ABC classification. These are shown (equation and phasor forms) in the figure below considering phase A as the reference. The pre-event voltage in phase A is indicated by E1, recalling to the equivalence between phase A voltage and the positive-sequence voltage in a balanced system. The voltage in the phase or between phases, which experienced the sag is denoted by V. Voltage Sag Types - ABC Classification
Note: Before fault (dashed line) and during fault (solid line)

From the figure above, the following can be observed:

• Voltage Sag Type A results to all three voltages down by the same amount.
• Type B has the faulted phase voltage reduced.
• Type C sag causes the two affected phase voltages to change along the imaginary axis only (both in magnitude and angle).
• For type D, the two affected phase voltages change in the real axis only with an accompanying drop in magnitude in the remaining phase.
• Type E results to reduced voltage magnitude in the two affected phases.
• Type F is similar to Type D, except that the voltage change is along both real and imaginary axes.
• Voltage sag type G is similar to Type C but with the voltage change in both axes. In addition, the remaining phase also experiences a decrease in voltage.

References:
Bollen, M. (2000). Understanding Power Quality Problems: Voltage Sags and Interruptions.
Leborgne, R. (2005). Voltage Sags Characterisation and Estimation 