Electrical Power Systems Quality, Second Edition phần 2 ppsx

causing a voltage sag with duration of more than 1 cycle occurs within

the area of vulnerability. However, faults outside this area will not

cause the voltage to drop below 0.5 pu. The same discussion applies to

the area of vulnerability for ASD loads. The less sensitive the equip￾ment, the smaller the area of vulnerability will be (and the fewer times

sags will cause the equipment to misoperate).

3.2.3 Transmission system sag

performance evaluation

The voltage sag performance for a given customer facility will depend on

whether the customer is supplied from the transmission system or from

the distribution system. For a customer supplied from the transmission

system, the voltage sag performance will depend on only the transmission

system fault performance. On the other hand, for a customer supplied

from the distribution system, the voltage sag performance will depend on

the fault performance on both the transmission and distribution systems.

This section discusses procedures to estimate the transmission sys￾tem contribution to the overall voltage sag performance at a facility.

Section 3.2.4 focuses on the distribution system contribution to the

overall voltage sag performance.

Transmission line faults and the subsequent opening of the protec￾tive devices rarely cause an interruption for any customer because of

the interconnected nature of most modern-day transmission networks.

These faults do, however, cause voltage sags. Depending on the equip￾ment sensitivity, the unit may trip off, resulting in substantial mone￾tary losses. The ability to estimate the expected voltage sags at an

end-user location is therefore very important.

Most utilities have detailed short-circuit models of the intercon￾nected transmission system available for programs such as ASPEN*

One Liner (Fig. 3.7). These programs can calculate the voltage through￾out the system resulting from faults around the system. Many of them

can also apply faults at locations along the transmission lines to help

calculate the area of vulnerability at a specific location.

The area of vulnerability describes all the fault locations that can

cause equipment to misoperate. The type of fault must also be consid￾ered in this analysis. Single-line-to-ground faults will not result in the

same voltage sag at the customer equipment as a three-phase fault.

The characteristics at the end-use equipment also depend on how the

voltages are changed by transformer connections and how the equip￾ment is connected, i.e., phase-to-ground or phase-to-phase. Table 3.1

summarizes voltages at the customer transformer secondary for a sin￾gle-line-to-ground fault at the primary.

Voltage Sags and Interruptions 51

*Advanced Systems for Power Engineering, Inc.; www.aspeninc.com.

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The relationships in Table 3.1 illustrate the fact that a single-line￾to-ground fault on the primary of a delta-wye grounded transformer

does not result in zero voltage on any of the phase-to-ground or

phase-to-phase voltages on the secondary of the transformer. The

magnitude of the lowest secondary voltage depends on how the

equipment is connected:

■ Equipment connected line-to-line would experience a minimum volt￾age of 33 percent.

■ Equipment connected line-to-neutral would experience a minimum

voltage of 58 percent.

This illustrates the importance of both transformer connections and

the equipment connections in determining the actual voltage that

equipment will experience during a fault on the supply system.

Math Bollen16 developed the concept of voltage sag “types” to describe

the different voltage sag characteristics that can be experienced at the

end-user level for different fault conditions and system configurations.

The five types that can commonly be experienced are illustrated in Fig.

3.8. These fault types can be used to conveniently summarize the

52 Chapter Three

Figure 3.7 Example of modeling the transmission system in a short-circuit program for

calculation of the area of vulnerability.

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Voltage Sags and Interruptions 53

0.58 1.00 0.58 0.00 1.00 1.00

0.58 1.00 0.58 0.33 0.88 0.88

0.33 0.88 0.88 — — —

0.88 0.88 0.33 0.58 1.00 0.58

TABLE 3.1 Transformer Secondary Voltages with a Single-Line-to-Ground

Fault on the Primary

Transformer

connection Phase-to-phase Phase-to-neutral Phasor

(primary/secondary) Vab Vbc Vca Van Vbn Vcn diagram

Sag Type D

One-phase

sag, phase

shift

Sag Type B

One-phase

sag, no phase

shift

Phase

Shift

Angle

None

Sag Type C

Two-phase

sag, phase

shift

Sag Type E

Two-phase

sag, no phase

shift

Sag Type A

Three-phase

sag

Note: Three-phase sags

should lead to relatively

balanced conditions;

therefore, sag type A is a

sufficient characterization

for all three-phase sags.

Number of Phases

12 3

Figure 3.8 Voltage sag types at end-use equipment that result from different types of

faults and transformer connections.

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expected performance at a customer location for different types of

faults on the supply system.

Table 3.2 is an example of an area of vulnerability listing giving all the

fault locations that can result in voltage sags below 80 percent at the cus￾tomer equipment (in this case a customer with equipment connected

line-to-line and supplied through one delta-wye transformer from the

transmission system Tennessee 132-kV bus). The actual expected per￾formance is then determined by combining the area of vulnerability with

the expected number of faults within this area of vulnerability.

The fault performance is usually described in terms of faults per 100

miles/year (mi/yr). Most utilities maintain statistics of fault perfor￾mance at all the different transmission voltages. These systemwide

statistics can be used along with the area of vulnerability to estimate

the actual expected voltage sag performance. Figure 3.9 gives an exam￾ple of this type of analysis. The figure shows the expected number of

voltage sags per year at the customer equipment due to transmission

system faults. The performance is broken down into the different sag

types because the equipment sensitivity may be different for sags that

affect all three phases versus sags that only affect one or two phases.

3.2.4 Utility distribution system sag

performance evaluation

Customers that are supplied at distribution voltage levels are impacted

by faults on both the transmission system and the distribution system.

The analysis at the distribution level must also include momentary

interruptions caused by the operation of protective devices to clear the

faults.7 These interruptions will most likely trip out sensitive equip￾ment. The example presented in this section illustrates data require￾ments and computation procedures for evaluating the expected voltage

sag and momentary interruption performance. The overall voltage sag

performance at an end-user facility is the total of the expected voltage

sag performance from the transmission and distribution systems.

Figure 3.10 shows a typical distribution system with multiple feed￾ers and fused branches, and protective devices. The utility protection

scheme plays an important role in the voltage sag and momentary

interruption performance. The critical information needed to compute

voltage sag performance can be summarized as follows:

■ Number of feeders supplied from the substation.

■ Average feeder length.

■ Average feeder reactance.

■ Short-circuit equivalent reactance at the substation.

54 Chapter Three

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Voltage Sags and Interruptions 55

TABLE 3.2 Calculating Expected Sag Performance at a Specific

Customer Site for a Given Voltage Level

Voltage at

Bus monitored

Fault type Faulted bus voltage bus (pu) Sag type

3LG Tennessee 132 0 A

3LG Nevada 132 0.23 A

3LG Texas 132 0.33 A

2LG Tennessee 132 0.38 C

2LG Nevada 132 0.41 C

3LG Claytor 132 0.42 A

1LG Tennessee 132 0.45 D

2LG Texas 132 0.48 C

3LG Glen Lyn 132 0.48 A

3LG Reusens 132 0.5 A

1LG Nevada 132 0.5 D

L-L Tennessee 132 0.5 C

2LG Claytor 132 0.52 C

L-L Nevada 132 0.52 C

L-L Texas 132 0.55 C

2LG Glen Lyn 132 0.57 C

L-L Claytor 132 0.59 C

3LG Arizona 132 0.59 A

2LG Reusens 132 0.59 C

1LG Texas 132 0.6 D

L-L Glen Lyn 132 0.63 C

1LG Claytor 132 0.63 D

L-L Reusens 132 0.65 C

3LG Ohio 132 0.65 A

1LG Glen Lyn 132 0.67 D

1LG Reusens 132 0.67 D

2LG Arizona 132 0.67 C

2LG Ohio 132 0.7 C

L-L Arizona 132 0.7 C

3LG Fieldale 132 0.72 A

L-L Ohio 132 0.73 C

2LG Fieldale 132 0.76 C

3LG New Hampshire 33 0.76 A

1LG Ohio 132 0.77 D

3LG Vermont 33 0.77 A

L-L Fieldale 132 0.78 C

1LG Arizona 132 0.78 D

2LG Vermont 33 0.79 C

L-L Vermont 33 0.79 C

3LG Minnesota 33 0.8 A

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