**b. ITIC Curve Scatter Plot**

The ITIC Curve describes an AC input voltage boundary that typically can be tolerated (Sabin, 2000). Events above the upper curve or below the lower curve are presumed to cause the mis-operation of information technology equipment. The curve is not intended to serve as a design specification for products or ac distribution systems. In this case, the number of events, which are below the lower limit of the ITIC curve, in Figure 2, is six, giving a SAFRI-ITIC of six events.

#### **c. SEMI Curve Scatter Plot**

In 1998, the Semiconductor Equipment and Materials International (SEMI) group, power quality and Equipment Ride Through Task force, recommended the SEMI Standard F-47 Curve to predict voltage sag problems for semiconductor manufacturing equipment. Figure 3 shows the application of the data of Table 1on the SEMI curve.

The results obtained from each combination of curves can be presented with the use of tables, such as UNIDEPE DISDIP or ESKOM Voltage sag Table, (Sabin, 2000).

#### **d. Voltage sag co-ordination chart-IEEEStd.493 and 1346**

The chart contains the supply performance for a given site through a given period, and the tolerance of one or more devices. It illustrates the number of events as a function of event severity. Observing the graph shown in Figure 4, there are 5 events per year where the voltage drops below 40% of nominal Voltage for 0.1 s or longer. Equally there are 5 events per year where the voltage drops below 70% magnitude and 250 ms duration.

Fig. 4. Voltage sag co-ordination chart

The advantage of this method is that equipment behavior can be directly compared with system performance, for a wide range of equipment. The disadvantage of the method is that a two-dimensional function is needed to describe the site. For comparison of different sites a smaller number of indices would be preferred.

#### **3.4.2 Calculation methods**

146 Electrical Generation and Distribution Systems and Power Quality Disturbances

The ITIC Curve describes an AC input voltage boundary that typically can be tolerated (Sabin, 2000). Events above the upper curve or below the lower curve are presumed to cause the mis-operation of information technology equipment. The curve is not intended to serve as a design specification for products or ac distribution systems. In this case, the number of events, which are below the lower limit of the ITIC curve, in Figure 2, is six, giving a SAFRI-

> **0.001 0.01 0.1 1 10 100 1000 Duration (seconds)**

In 1998, the Semiconductor Equipment and Materials International (SEMI) group, power quality and Equipment Ride Through Task force, recommended the SEMI Standard F-47 Curve to predict voltage sag problems for semiconductor manufacturing equipment. Figure

> **0.001 0.01 0.1 1 10 100 1000 Duration (seconds)**

The results obtained from each combination of curves can be presented with the use of

The chart contains the supply performance for a given site through a given period, and the tolerance of one or more devices. It illustrates the number of events as a function of event

tables, such as UNIDEPE DISDIP or ESKOM Voltage sag Table, (Sabin, 2000).

**d. Voltage sag co-ordination chart-IEEEStd.493 and 1346** 

Total Events: 12 Events Violating SEMI Curve: 5

3 shows the application of the data of Table 1on the SEMI curve.

Total Events: 12

Events Violating ITIC Low er Curve: 6 Events Violating ITIC Upper Curve: 0

**0**

**0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1**

**Voltage Magnitude (pu)**

**0.5**

**1**

**1.5**

**Voltage Magnitude (pu)**

**2**

**2.5**

**b. ITIC Curve Scatter Plot** 

Fig. 2. The ITIC curve scatter plot

**c. SEMI Curve Scatter Plot** 

Fig. 3. SEMI Curve scatter plot

ITIC of six events.

#### **a. Method used by Detroit Edison**

The method calculates a "sag score" from the voltage magnitudes in the three phases (Sabin, 2000).

$$S = 1 - \frac{V\_a + V\_b + V\_c}{3} \tag{6}$$

This sag score is equal to the average voltage drop in the three phases. The larger the sag score, the more severe the event is considered to be.

#### **b. Method proposed by Thallam**

A number of site indices can be calculated from the "voltage sag energy" (Thallam, 2000). The "Voltage Sag Energy Index" (VSEI) is the sum of the voltage sag energies for all events measured at a given site during a given period:

$$VSEI = \sum\_{i} \mathbf{E}\_{\text{VS}\_{-i}i} \tag{7}$$

The "Average Voltage Sag Energy Index" (AVSEI) is the average of the voltage sag energies for all events measured at a given site during a given period:

$$AVSEI = \frac{1}{N} \sum\_{i=1}^{N} \mathbf{E}\_{\text{VS\\_}} \tag{8}$$

Power Quality and Voltage Sag Indices in Electrical Power Systems 149

practical. Power quality surveys in the past have just referred to the number of voltage sags per year for a given site. This value could include minor events, which do not affect any

**The Canadian Electrical Association** recommends tracking 4 indices for sag magnitudes (referring to the remaining voltage), of 85%, 70%, 40% and 1%. The latter refers to

**EPRl -Electrotek** suggests the following five magnitudes and three duration ranges to

**a. RMS variation** Frequency for voltage threshold X: with X=90%, 80%, 70%, 50%, 10%: the number of events per year with magnitude below X, and duration between 0.5 cycle

**b. Instantaneous RMS variation** Frequency for voltage threshold X: with X=90%, 80%, 70%, 50%: the number of events per year with magnitude below X, and duration

**c. Momentary RMS variation** Frequency for voltage threshold X: with X=90%, 80%, 70%, 50%: the number of events per year with magnitude below X, and duration between 0.5

**d. Momentary RMS variation** Frequency for voltage threshold I0%: the number of events per year with a magnitude below 10%, and a duration between 0.5 cycle and 3 sec. **e. Temporary RMS variation** Frequency for voltage threshold X: with X=90%, 8070, 70%, 50%, I0%: the number of events per year with magnitude below X, and duration

The duration ranges are based on the definition of instantaneous, momentary and

System Indices are typically a weighted average of the single-site indices obtained for all or a number of sites within the system. The difficulty lies in the determination of the weighting factors. In order to assess any indices for the system, first monitoring of the quality of supply must take place. When the Electric Power Research Institute (EPRI)-Distribution Power Quality (DPQ) program placed monitoring equipment on one hundred feeders, these feeders needed to adequately represent the range of characteristics seen on distribution systems. This required the researchers to use a controlled selection process to ensure that both common and uncommon characteristics of the national distribution systems were well represented in the study sample. Thus a level of randomness is required. Many devices are susceptible to only the magnitude of the variation. Others are susceptible to the combination of magnitude and durationOne consideration in establishing a voltage sag index is that the less expensive a measuring device is, the more likely it will be applied at many locations, more completely representing the voltage quality electricity users are

**ESKOM** (South African Utility), groups voltage sags into five classes (Sabin, 2000):

equipment.

interruptions rather than sags.

characterize voltage thresholds:

between 0.5 cycle and 0.5 sec.

between 3 sec. and 60 sec.

temporary, as specified by IEEE (IEEE Std. 1159, 1995).

and 60 sec.

sec and 3 sec.

**3.5 System indices** 

experiencing.

class Y: 80% – 90% magnitude, 20 ms - 3 sec duration class X: 40% - 80% magnitude, 20 ms - l50 ms duration class S: 40% - 80% magnitude, 150 ms - 600 ms duration class T: 0 - 40% magnitude, 20 ms - 600 ms duration class Z: 0 – 80% magnitude, 600 ms - 3 sec duration

A sensitive setting will result in a large number of shallow events (with a low voltage sag energy) and this in a lower value for AVSEI.

The sag event frequency index at a particular location and period is suggested as the number of qualified sag events at a location and period (Thallam & Koellner, 2003).

The System sag count index is the total number of qualified voltage sag events over the number of monitor locations. By the expression qualifying events, it implies a voltage less than 90%, with event duration limited to 15 cycles and energy greater or equal to 100.

#### **3.4.3 Non-rectangular events**

Non-rectangular events are events in which the voltage magnitude varies significantly during the event. A method to include non-rectangular events in the voltage-sag coordination chart is also applicable according to the IEEE defined standard (IEEE Std.493, 1997). Alternatively, the function value can be defined as the number of times per year that the RMS voltage is less than the given magnitude for longer than the given duration.

EPRI-Electrotek mentions that each phase of each ms variation measurement may contain multiple components (Thallam, 2000). Consequently, these phase rectangular voltage sag measurements are easily characterized with respect to magnitude and duration. Approximately 10% of the events are non-rectangular. These events are much more difficult to characterize because no single magnitude-duration pair completely represent the phase measurement.

The method suggested for calculating the indices used by EPRI-Electrotek is called the ''Specified Voltage'' method. This method designates the duration as the period of time that the rms voltage exceeds a specified threshold voltage level used to characterize the disturbance.' The consequence of this method is that an event may have a different duration when being assessed at different voltage thresholds as shown in Figure 5.

Fig. 5. Illustration of "specified voltage" characterization

Most of the single site indices relate the magnitude and duration of the sag and the number of events. These events can be grouped in order to make their counting easier and more practical. Power quality surveys in the past have just referred to the number of voltage sags per year for a given site. This value could include minor events, which do not affect any equipment.

**The Canadian Electrical Association** recommends tracking 4 indices for sag magnitudes (referring to the remaining voltage), of 85%, 70%, 40% and 1%. The latter refers to interruptions rather than sags.

**ESKOM** (South African Utility), groups voltage sags into five classes (Sabin, 2000):

class Y: 80% – 90% magnitude, 20 ms - 3 sec duration

class X: 40% - 80% magnitude, 20 ms - l50 ms duration

class S: 40% - 80% magnitude, 150 ms - 600 ms duration

class T: 0 - 40% magnitude, 20 ms - 600 ms duration

class Z: 0 – 80% magnitude, 600 ms - 3 sec duration

**EPRl -Electrotek** suggests the following five magnitudes and three duration ranges to characterize voltage thresholds:


The duration ranges are based on the definition of instantaneous, momentary and temporary, as specified by IEEE (IEEE Std. 1159, 1995).
