exercise 3 double bus, single breaker scheme - lab … · substations using the double bus, single...

© Festo Didactic 20528-00 87

When you have completed this exercise, you will be familiar with electric power substations using the double bus, single breaker scheme. You will know the procedure used to perform on-load transfers. You will know the advantages and disadvantages of the double bus, single breaker scheme. You will know why switchgear interlocking is of prime importance in electric power substations using the double bus, single breaker scheme.

The Discussion of this exercise covers the following points:

The double bus, single breaker scheme

On-load transfer procedure

Reliability of substations using the double bus, single breaker schemeOutgoing line fault. Incoming line fault. Bus fault. Outgoing line circuit-breaker fault. Incoming line circuit-breaker fault. Bus-coupler circuit-breaker fault. Reliability.

Operating flexibility and maintenance of substations using the double bus, single breaker scheme

Summary of the advantages and disadvantages of the double bus, single breaker scheme

Interlocking in a substation using the double bus, single breaker scheme

The double bus, single breaker scheme

A single-line diagram of an electric power substation using the double bus, single breaker scheme is shown in Figure 39. The double bus, single breaker scheme includes all components in the single bus scheme (circuitry shown in black in Figure 39) plus a second bus, disconnecting switches linking the second bus to each line circuit breaker, and a bus coupler circuit breaker with disconnecting switches (circuitry shown in green in Figure 39).

Each power line in the double bus, single breaker scheme can be connected to either of the two buses by closing one of the two disconnecting switches that connect the corresponding line circuit breaker to the buses. This provides some operating flexibility that allows the power lines to be connected to the buses in any combination required. For instance, the power lines can be grouped onto separate buses or they can all be connected to the same bus. Furthermore, the bus coupler circuit breaker allows any power line to be transferred from one bus to the other, without interruption in the supply of power to loads, by operating the two disconnecting switches that connect the corresponding line circuit breaker to the buses. Such an operation is commonly referred to as on-load transfer or on-load bus selection. The procedure to be followed when performing an on-load transfer is described in the next section of the discussion.

Double Bus, Single Breaker Scheme

Exercise 3

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

The double bus,

single breaker

scheme is also

commonly referred

to as the duplicate

bus scheme.

Exercise 3 – Double Bus, Single Breaker Scheme Discussion

88 © Festo Didactic 20528-00

Figure 39. Single-line diagram of an electric power substation implemented using the double bus, single breaker scheme (duplicate bus scheme).

On-load transfer procedure

An on-load transfer is accomplished by making sure that the disconnecting switches of the bus coupler circuit breaker are closed, making sure that the bus coupler circuit breaker is closed, closing the disconnecting switch that connects to the bus where the power line is to be transferred, and opening the disconnecting switch that connects to the bus from where the power line is transferred. Most importantly, the above operations must be performed in the order they are enumerated to avoid damage to the contacts of the two disconnecting switches during the on-load transfer. In fact, proceeding this way ensures that the contacts of the two disconnecting switches are short-circuited throughout the on-load transfer. Consequently, this maintains the voltage across the contacts of the two disconnecting switches close to zero which, in turn, avoids electric arcs and possible damage to the contacts.

Once an on-load transfer is completed, the bus coupler circuit breaker and the bus coupler disconnecting switches are normally left closed. This allows subsequent on-load transfers to be performed by simply operating the two disconnecting switches that connect a power line to the buses (following the order mentioned in the procedure above). Note that when the bus coupler circuit breaker is initially open and the two buses in the substation are fed by two different ac power sources, synchronism between the voltage at bus 1 and the voltage at bus 2 must be checked before allowing closure of the bus coupler circuit breaker.

Electric power substation

Power lines

Bus 2

Bus 1

Bus coupler circuit

breaker



Reliability of substations using the double bus, single breaker scheme

This section of the discussion deals with the reliability of an electric power substation using the double bus, single breaker scheme. The section starts with a series of subsections, each one describing how a fault at a particular location in the electric power substation affects the supply of power to loads. The section then concludes on the reliability of electric power substations using the double bus, single breaker scheme.

Outgoing line fault

The effect of an outgoing line fault is the same as in a substation using the single bus scheme, i.e., opening the corresponding line circuit breaker isolates the faulty outgoing line, thereby interrupting the fault current, and causes an interruption in the supply of power to the loads fed by this line. On the other hand, power is maintained in the rest of the substation. The interruption in the supply of power lasts until the faulty outgoing line is repaired. For instance, when outgoing line fault F1 occurs in Figure 40, opening line circuit breaker CB1 isolates the faulty outgoing line (outgoing line A) and power is lost at load 1.

Figure 40. Faults at various locations in a substation implemented using the double bus, single breaker scheme.

Bus 2


DS1-A

CB1

DS1-B

Outgoingline A

Outgoing line B

Incoming line A

Incomingline B

Load 1 Load 2 AC power source 1

AC powersource 2

DS2-A

CB2

DS2-B

DS3-A

CB4

DS4-B

DS4-C

CB5

DS5-B

F1 F2

F3

F4 F5

F6

Bus 1

DS3-B DS1-C DS2-C

CB3

DS5-C

DS4-A DS5-A

Bus coupler circuit breaker



Incoming line fault

The effect of an incoming line fault is the same as in a substation using the single bus scheme. Such a fault causes protection located outside the substation to open a circuit breaker that disconnects the ac power source from the faulty incoming line. This prevents the ac power source from feeding the fault. In the substation, the line circuit breaker associated with the faulty incoming line is also opened to isolate this line, and therefore, prevent an ac power source connected to another incoming line (if any) from feeding the fault. When no other ac power source feeds the substation via another incoming line, power is lost at all loads fed by the outgoing lines of the substation. On the other hand, when a second ac power source feeds the substation via another incoming line, power is maintained at the loads fed by the outgoing lines of the substation.

For instance, suppose that incoming line fault F2 occurs in Figure 40 while incoming line A and outgoing line A are connected to bus 1, bus coupler circuit breaker CB3 is closed, and incoming line B and outgoing line B are connected to bus 2. Protection located outside the substation opens a circuit breaker (not shown in Figure 40) that disconnects ac power source 1 from the faulty incoming line (incoming line A), thereby preventing this source from feeding the fault. Then, opening line circuit breaker CB2 isolates the faulty incoming line from the substation and prevents ac power source 2 connected to incoming line B from feeding the fault. Power is maintained at loads 1 and 2 by ac power source 2 via incoming line B, buses 1 and 2, and outgoing lines A and B.

Bus fault

When a fault occurs on either of the two buses, opening the circuit breakers of all power lines connected to the faulty bus and the bus coupler circuit breaker isolates the faulty bus and interrupts the fault current. Power is lost at the loads fed by any outgoing line connected to the faulty bus. Also, power from any incoming line connected to the faulty bus is no longer available (i.e., it cannot be routed to loads via the substation). Finally, power is maintained at the loads fed by any outgoing lines connected to the other bus, provided that there is an incoming line connected to this bus and that ac power is available at this line. Fortunately, however, all power lines affected by the bus fault can be transferred to the other bus immediately, then the open line circuit breakers can be reclosed to restore power to loads and minimize the duration of any interruption in the supply of power. Transferring power lines to another bus is not possible in a substation using the single bus scheme. Consequently, this makes the double bus, single breaker scheme more reliable than the single bus scheme when a bus fault occurs.

For instance, suppose that bus fault F3 occurs in Figure 40 while incoming line A and outgoing line A are connected to bus 1, bus coupler circuit breaker CB3 is closed, and incoming line B and outgoing line B are connected to bus 2. Opening circuit breakers CB1, CB2, and CB3 isolates the faulty bus (bus 1) and interrupts the fault current. Power is lost at load 1 since ac power source 1 and ac power source 2 can no longer supply this load. On the other hand, power is maintained at load 2 by ac power source 2 via incoming line B, bus 2, and outgoing line B. Fortunately, outgoing line A and incoming line A can be transferred to bus 2 immediately by opening disconnecting switches DS1-A and DS2-A and closing disconnecting switches DS1-C and DS2-C. In this situation, the disconnecting switches can be operated with the bus coupler circuit breaker CB3 open because line circuit breakers CB1 and CB2 are open. This type of transfer is referred to as

Whenever a fault occurs on

either of the two buses in a

substation using the double

bus, single breaker scheme,

the bus coupler circuit

breaker should never be

reclosed before the faulty

bus has been repaired

because this would spread

the fault to the other bus

and result in a complete

shutdown of the substation.



an off-load transfer. Then, line circuit breakers CB1 and CB2 can be reclosed to connect incoming line A and outgoing line A to bus 2. This allows power at load 1 to be restored immediately. Note that synchronism between the voltage at bus 2 and the voltage of the ac power source at incoming line A must be checked before closing line circuit breaker CB2.

Outgoing line circuit-breaker fault

The effect of an outgoing line circuit-breaker fault is the same as in a substation using the single bus scheme. Opening the bus coupler circuit breaker and the circuit breakers of all other power lines connected to the bus affected by the faulty outgoing line circuit breaker isolates this bus (consequently isolating the faulty line circuit breaker) and interrupts the fault current. This causes an interruption in the supply of power to the loads fed by the faulty outgoing line and any other outgoing line connected to the bus affected by the faulty outgoing line circuit breaker. Also, power from any incoming line connected to the bus affected by the faulty outgoing line circuit breaker is no longer available (i.e., it cannot be routed to loads via the substation). Finally, power is maintained at the loads fed by outgoing lines connected to the other bus, provided that an incoming line feeds this bus. The interruption in the supply of power to the loads fed by the faulty outgoing line lasts until the faulty outgoing line and circuit breaker are repaired. On the other hand, once the faulty outgoing line circuit breaker has been isolated by opening the proper disconnecting switches, the open circuit breakers can be reclosed to reconnect the corresponding bus and lines to the rest of the substation. In certain cases, this allows power to be restored to some loads immediately.

For instance, suppose that outgoing line circuit-breaker fault F4 occurs following outgoing line fault F1 in Figure 40 while incoming line A and outgoing line A are connected to bus 1, bus coupler circuit breaker CB3 is closed, and incoming line B and outgoing line B are connected to bus 2. Opening incoming line circuit breaker CB2 and bus section circuit breaker CB3 isolates the bus (bus 1) affected by the faulty outgoing line circuit breaker (CB1) and interrupts the fault current. Power is lost at load 1 and power from ac power source 1 is no longer available to feed loads. However, power is maintained at load 2 by ac power source 2 via incoming line B, bus 2, and outgoing line B. Once outgoing line circuit breaker CB1 has been isolated by opening disconnecting switches DS1-A to DS1-C, incoming line circuit breaker CB2 and bus coupler circuit breaker CB3 can be reclosed to reconnect ac power source 1 to the rest of the electric power substation.

Incoming line circuit-breaker fault

The effect of an incoming line circuit-breaker fault is the same as in a substation using the single bus scheme. Opening the bus coupler circuit breaker and the circuit breakers of all other power lines connected to the bus affected by the faulty incoming line circuit breaker isolates this bus (consequently isolating the faulty line circuit breaker) and interrupts any fault current. Obviously, power from the faulty incoming line is no longer available (i.e., it cannot be routed to loads via the substation). Power is lost at the loads fed by any outgoing line connected to the bus affected by the faulty incoming line circuit breaker. Also, power from any other incoming line connected to the bus affected by the faulty incoming line circuit breaker is no longer available. Finally, power is maintained at the loads fed by outgoing lines connected to the other bus, provided that at least one incoming line feeds this bus. Once the faulty incoming line circuit breaker has been



isolated by opening the proper disconnecting switches, the open circuit breakers can be reclosed to reconnect the corresponding bus and lines to the rest of the substation. When a second ac power source feeds the substation via another incoming line, this allows power to be restored to loads immediately. Otherwise, power is lost at all loads fed by the outgoing lines of the substation.

For instance, suppose that incoming line circuit-breaker fault F5 occurs following incoming line fault F2 in Figure 40 while incoming line A and outgoing line A are connected to bus 1, bus coupler circuit breaker CB3 is closed, and incoming line B and outgoing line B are connected to bus 2. Opening outgoing line circuit breaker CB1 and bus section circuit breaker CB3 isolates the bus (bus 1) affected by the faulty incoming line circuit breaker (CB2) and interrupts the fault current. Power is lost at load 1 and power from ac power source 1 is no longer available to supply loads. However, power is maintained at load 2 by ac power source 2 via incoming line B, bus 2, and outgoing line B. Once incoming line circuit breaker CB2 has been isolated by opening disconnecting switches DS2-A to DS2-C, outgoing line circuit breaker CB1 and bus coupler circuit breaker CB3 can be reclosed to reconnect outgoing line A to bus 1. This restores power at load 1 immediately.

Bus-coupler circuit-breaker fault

A bus-coupler circuit-breaker fault occurs when the bus coupler circuit breaker fails to open following a fault on one of the two buses (see faults F3 and F6 in Figure 40). Opening all line circuit breakers in the substation isolates the two buses (consequently isolating the faulty bus coupler circuit breaker) and interrupts the fault current. Power is lost at all loads fed by the outgoing lines of the substation. Also, power from any incoming line is no longer available (i.e., it cannot be routed to loads via the substation). In other words, this results in an outage of the entire substation. Fortunately, once the bus coupler circuit breaker has been isolated by opening the corresponding disconnecting switches, all power lines connected to the faulty bus can be transferred to the other bus immediately, then all open line circuit breakers can be reclosed to restore power to all loads and minimize the duration of any interruption in the supply of power. Transferring power lines to another bus is not possible in a substation using the single bus scheme. Consequently, this makes the double bus, single breaker scheme more reliable than the single bus scheme when a bus-coupler circuit-breaker fault occurs.

For instance, suppose that bus-coupler circuit-breaker fault F6 occurs following bus fault F3 in Figure 40 while incoming line A and outgoing line A are connected to bus 1, bus coupler circuit breaker CB3 is closed, and incoming line B and outgoing line B are connected to bus 2. Opening line circuit breakers CB1, CB2, CB4, and CB5 isolates the two buses (consequently isolating the faulty bus coupler circuit breaker) and interrupts the fault current. Power is lost at loads 1 and 2. Also, power from ac power sources 1 and 2 is no longer available to supply loads. Once bus coupler circuit breaker CB3 has been isolated by opening disconnecting switches DS3-A and DS3-B, outgoing line A and incoming line A can be transferred to bus 2 immediately by opening disconnecting switches DS1-A and DS2-A and closing disconnecting switches DS1-C and DS2-C (this is an off-load transfer). Then, line circuit breakers CB1, CB2, CB4, and CB5 can be reclosed to connect all incoming and outgoing lines to bus 2. This allows power at loads 1 and 2 to be restored immediately. Note that synchronism between the voltage of the ac power source at incoming line A and



the voltage of the ac power source at incoming line B must be checked before closing the second incoming line circuit breaker (i.e., CB2 or CB4).

Reliability

The effect of each of the aforementioned faults on the continuity of the supply of power is summarized in Table 5.

Table 5. Effect of various types of faults on the continuity of the supply of power.

Fault description Effect of fault

Outgoing line fault

Outgoing line circuit-breaker fault

Results in an interruption in the supply of power to the loads fed by the faulty outgoing line. The interruption in the supply of power lasts until the faulty equipment is repaired.

Incoming line fault

Incoming line circuit-breaker fault

Results in an interruption in the supply of power to all loads fed by the outgoing lines of the substation when there is no other incoming line or when no ac power is available at another incoming line of the substation. The interruption in the supply of power lasts until the faulty equipment is repaired.

Bus fault Results in a brief interruption in the supply of power to the loads fed by any outgoing line connected to the faulty bus.

Bus-coupler circuit-breaker fault

Results in a brief interruption in the supply of power to all loads fed by the outgoing lines of the substation.

In brief, an outgoing line fault or an outgoing line circuit-breaker fault in an electric power substation using the double bus, single breaker scheme always causes an interruption in the supply of power to some loads that lasts until the faulty equipment is repaired. Also, when there is no other incoming line or when no ac power is available at another incoming line of the substation, an incoming line fault or an incoming line circuit-breaker fault causes an interruption in the supply of power to all loads that lasts until the equipment is repaired. In other words, the effect of each of these faults in a substation using the double bus, single breaker scheme is the same as in a substation using the single bus scheme with bus section circuit breaker. On the other hand, a bus fault or a bus-coupler circuit-breaker fault in a substation using the double bus, single breaker scheme only causes a brief interruption in the supply of power to some of the loads or all loads fed by the substation because the affected power lines can be transferred to the healthy bus immediately after the fault. In comparison, a bus fault or a bus-section circuit-breaker fault in a substation using the single bus scheme causes an interruption in the supply of power to the loads fed by any outgoing line connected to the faulty bus section that lasts until the faulty equipment is repaired. Consequently, this makes the double bus, single breaker scheme a little more reliable than the single bus scheme (with or without bus section circuit breakers).



Operating flexibility and maintenance of substations using the double bus, single breaker scheme

In an electric power substation using the double bus, single breaker scheme, each power line can be connected to either of the two buses by closing one of the two disconnecting switches that connect the corresponding line circuit breaker to the buses. This provides some operating flexibility that allows the power lines to be connected to the buses in any combination required, either for operational purposes or maintenance purposes.

Maintenance of any line circuit breaker or line disconnecting switch in a substation using the double bus, single breaker scheme causes the corresponding line to be lost for a while. In the case of an outgoing line circuit breaker or disconnecting switch, this automatically results in an interruption in the supply of power to the loads fed by this line. In other words, the maintenance of the line circuit breakers and line disconnecting switches is as problematic as in a substation using the single bus scheme (with or without bus section circuit breakers).

Nevertheless, maintenance in a substation using the double bus, single breaker scheme is somewhat easier than in a substation using the single bus scheme (with or without bus section circuit breakers). For instance, transferring all power lines of the substation to the same bus and opening the bus coupler circuit breaker isolates one of the two buses in the electric power substation. This allows maintenance of the isolated bus to be performed without interrupting the supply of power to any of the loads fed by the substation. Furthermore, maintenance of any disconnecting switch connected to the bus which is being maintained can be performed without losing more than one power line, i.e., the power line associated with the disconnecting switch on which maintenance is being performed. This power line is lost because the circuit breaker and the other two disconnecting switches associated with the line must be opened to de-energize the disconnecting switch that requires maintenance. In comparison, maintenance of the bus (or any section of the bus) or any disconnecting switch connected to the bus (or the bus section under maintenance) in a substation using the single bus scheme causes all power lines connected to the bus (or the bus section under maintenance) to be lost.

Obviously, protective grounding must be used when performing the maintenance of a bus or the maintenance of a disconnecting switch connected to the bus that is under maintenance. Bus maintenance requires grounding of the de-energized bus, using a portable grounding and short-circuiting device. The maintenance of a disconnecting switch connected to the bus that is under maintenance also requires grounding of the line-side end of the line switchgear using either a grounding switch (if available) or a portable grounding and short-circuiting device. This is shown in Figure 41. Notice that this allows the maintenance of the line circuit breaker and disconnecting switch to be performed at the same time as the maintenance of the disconnecting switch connected to the bus.



Figure 41. Protective grounding required when performing maintenance on a bus and a disconnecting switch connected to the bus that is under maintenance.

Portable grounding and short-circuiting device

Equipment under

maintenance

Bus 2

Grounding switch (closed)

Power line

Bus 1



Summary of the advantages and disadvantages of the double bus, single breaker scheme

The double bus, single breaker scheme provides a good operating flexibility that allows each power line in the electric power substation to be connected to either of the two buses without interrupting the supply of power to loads. Furthermore, bus redundancy makes the scheme moderately reliable (i.e., more reliable than the single bus scheme) since any bus fault or bus-coupler circuit-breaker fault only results in a brief interruption in the supply of power to loads. Also, bus maintenance can be done without interrupting the supply of power to loads, and the maintenance of any disconnecting switch connected to a bus can be done without losing more than one power line. In other words, bus maintenance and the maintenance of any disconnecting switch connected to a bus are not as problematic as in a substation using the single bus scheme. Finally, substation expansion (e.g., addition of a line) is generally easy, as equipment can be added to one bus while all power lines are connected to the other bus. On the other hand, however, the cost of a substation using the double bus, single breaker scheme is high, due to the presence of the second bus and the extra disconnecting switch required for each power line. Also, the good operating flexibility of the scheme allows multiple configurations of the power lines that makes both substation operation and protection arrangement relatively complex (i.e., definitely more complex than in a substation using the single bus scheme). Finally, maintenance of any outgoing line circuit breaker or disconnecting switch cannot be performed without interrupting power to loads fed via this outgoing line, making this aspect of maintenance as problematic as in a substation using the single bus scheme. The following table summarizes the main advantages and disadvantages of the double bus, single breaker scheme.

Table 6 summarizes the main advantages and disadvantages of the double bus, single breaker scheme.

Table 6. Main advantages and disadvantages of the double bus, single breaker scheme.

Advantages Disadvantages

Good operating flexibility

Moderately reliable (more reliable than single bus scheme)

Bus maintenance can be done without interrupting supply of power to loads

Maintenance of any disconnecting switch connected to a bus can be done without losing more than one power line.

Easy to expand

High cost

Substation operation is relatively complex

Protection arrangement is relatively complex

Maintenance of any outgoing line circuit breaker or disconnecting switch interrupts supply of power to corresponding loads

Exercise 3 – Double Bus, Single Breaker Scheme Procedure Outline


Interlocking in a substation using the double bus, single breaker scheme

In the previous exercise, you saw that some interlocking is required between each line circuit breaker and the corresponding line disconnecting switches in an electric power substation to prevent errors in the operation of the switchgear from causing damage to the disconnecting switches and a long interruption in the supply of power to some loads. You also saw that some other type of interlocking, i.e., checking the synchronism between two ac power sources, is also required before allowing closure of certain circuit breakers to prevent errors in the operation of the switchgear that could affect the supply of power to loads.

Switchgear interlocking is required in an electric power substation using the double bus, single breaker scheme as in any other electric power substation. In fact, switchgear interlocking in an electric power substation using the double bus, single breaker scheme is of prime importance because errors in the operation of the switchgear are more likely to occur, mainly because the substation operation is more complex due to the multiple configurations of the power lines that are possible. Also, switchgear interlocking in a substation using the double bus, single breaker scheme is rather complex due to the numerous situations that can occur. Further study of interlocking in a substation using the double bus, single breaker scheme, however, is beyond the scope of this manual.

The Procedure is divided into the following sections:

Set up and connections

Familiarization with the SCADA window of the electric power substation

Starting up the electric power substation

On-load transfer

Bus fault

Bus coupler circuit-breaker fault

Bus maintenance

Set up and connections

In this section, you will set up a circuit representing one phase of an electric power substation using the double bus, single breaker scheme.

1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform this exercise. Install the required equipment in the Workstation.

2. Make sure that the ac and dc power switches on the Power Supply are set to the O (off) position, then connect the Power Supply to a three-phase ac power outlet.

Make sure that the power switch on the DC Power Supply/Ethernet Switch is set to the O (off) position, then connect the Power Input to an ac power outlet.

Connect the Power Input of the Data Acquisition and Control Interface to a 24 V ac power supply. Turn the 24 V ac power supply on.

PROCEDURE OUTLINE

PROCEDURE

Exercise 3 – Double Bus, Single Breaker Scheme Procedure


3. Connect the equipment as shown in Figure 42 to set up one phase of an electric power substation using the double bus, single breaker scheme.

a Connecting the equipment to set up only one of the three phases of the electric power substation reduces the amount of equipment required and makes equipment connections easier and faster. However, this does not affect study of the substation operation since all three phases have the same behavior.

Use modules Circuit Breakers and Disconnecting Switches 1 and Circuit Breakers and Disconnecting Switches 2 to implement the electric power substation. Use the Line Inductors module to implement the two line inductors at incoming lines A and B. These line inductors emulate short (about 20 km [12.4 miles]) power lines between the ac power source and the electric power substation. E1, E2, I1, I2, I3, and I4 are voltage and current inputs of the Data Acquisition and Control Interface (DACI). Load resistors R1 and R2 are implemented with the Resistive Load module.

a The resistance value to be used for resistors R1 and R2 depends on your local ac power network voltage. A table below the circuit diagram in Figure 42 indicates the resistance value to be used for ac power network voltages of 120 V, 220 V, and 240 V. Make sure to use the resistance value corresponding to your local ac power network voltage. Appendix C of this manual lists the switch settings and connections to perform on the Resistive Load module in order to obtain various resistance values.

4. Connect the USB port of the Data Acquisition and Control Interface to a USB port of the host computer.

5. Connect the Power Input of each of the two Circuit Breakers and Disconnecting Switches modules to the 120 V output of the DC Power Supply/Ethernet Switch.

6. Connect the Ethernet port of each of the two Circuit Breakers and Disconnecting Switches modules to one of the ports on the DC Power Supply/Ethernet Switch.

7. Connect a USB port of the host computer to one of the ports on the DC Power Supply/Ethernet Switch via the USB-to-Ethernet adapter (included with the DC Power Supply/Ethernet Switch).

8. Turn the DC Power Supply/Ethernet Switch on. Wait a few seconds, then notice that all open (O) LEDs on the front panels of the two Circuit Breakers and Disconnecting Switches modules are lit, thereby indicating that all circuit breakers and disconnecting switches in the electric power substation are open.



Local ac power network Load resistors

R1, R2

( ) Voltage

(V)

Frequency

(Hz)

120 60 240

220 50 880

240 50 960

220 60 880

Figure 42. Electric power substation using the double bus, single breaker scheme (one phase only).

Bus 2


Bus 1

Outgoingline A

Outgoing line B

Incoming line A

Incomingline B

Loadresistor R1

AC power source

Load resistor R2

L

N

Line inductor

Line inductor

25 mH 25 mH

E1 E2

I1 I2 I3 I4

To neutral

To neutral

DS1-A

CB1

DS1-B

DS2-A

CB2

DS2-B

DS3-A

CB4

DS4-B

DS4-C

CB5

DS5-B

DS3-BDS1-C DS2-C

CB3

DS5-C

DS4-A DS5-A




Familiarization with the SCADA window of the electric power substation

In this section, you will familiarize yourself with the SCADA window of the electric power substation.

9. Turn the host computer on, then start the Electric Power Substation SCADA Application by performing the following two steps.

Start the Electric Power Substation SCADA Application Launcher by double-clicking the corresponding icon on the host computer desktop.

Launch the Electric Power Substation SCADA Application by clicking the Launch Application button in the Electric Power Substation SCADA Application Launcher. The Electric Power Substation SCADA Application Launcher should disappear (in fact, the corresponding window is minimized) and the Electric Power Substation SCADA Application window should appear.

10. In the Electric Power Substation SCADA Application window, click the Double Bus Single Breaker Scheme button to select this substation switching scheme. The single-line diagram of the corresponding electric power substation should appear on your host computer screen.

a For the remainder of this exercise procedure, the Electric Power Substation SCADA Application window is simply referred to as the SCADA window.

Observe that the single-line diagram of the electric power substation displayed in the SCADA window corresponds to the electric power substation (see diagram in Figure 42) that you set up using the two Circuit Breakers and Disconnecting Switches modules.

Observe that each circuit breaker symbol in the SCADA window indicates the current state (open) of the corresponding circuit breaker in the electric power substation. Similarly, each disconnecting switch symbol in the SCADA window indicates the current state (open) of the corresponding disconnecting switch in the electric power substation.

Observe that the letter "R" appears next to each circuit breaker symbol in the SCADA window to indicate that the corresponding circuit breaker in the electric power substation is ready to close.

Finally, observe that displays in the SCADA window indicate the values of voltage at bus 1 and bus 2, the values of current at incoming lines A and B, and the values of current at outgoing lines A and B.



Starting up the electric power substation

In this section, you will perform manipulations required to start up the electric power substation.

11. On the Power Supply, turn the ac power source on.

12. Successively close disconnecting switches DS2-A and DS2-B in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box.

Close circuit breaker CB2 in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box. This connects incoming line A to bus 1. Observe that bus 1 is now energized by the ac power source connected to incoming line A. The value of the voltage at bus 1 (i.e., the ac power source voltage) is indicated in the SCADA window.

a Parasitic voltage is produced across bus 2 even if it is not yet energized, due to some capacitive coupling. Ignore this voltage at this time.


Close circuit breaker CB1 in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box. This connects outgoing line A to bus 1. Observe that power is now routed to load resistor R1 via incoming line A, bus 1, and outgoing line A. The value of the current flowing through incoming line A and outgoing line A is indicated in the SCADA window.

14. Successively close disconnecting switches DS4-B and DS4-C in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box.

Close circuit breaker CB4 in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box. This connects incoming line B to bus 2. Observe that bus 2 is now energized by the ac power source connected to incoming line B. The value of the voltage at bus 2 (i.e., the ac power source voltage) is indicated in the SCADA window.

15. Successively close disconnecting switches DS5-B and DS5-C in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box.

Close circuit breaker CB5 in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box. This connects outgoing line B to bus 2.



Observe that power is now routed to load resistor R2 via incoming line B, bus 2, and outgoing line B. The value of the current flowing through incoming line B and outgoing line B is indicated in the SCADA window.

On-load transfer

In this section, you will learn how to perform on-load transfers of a power line from one bus to the other.

16. Examine the manipulations in the next step of this exercise procedure. What are these manipulations intended for?


Close circuit breaker CB3 in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box. This interconnects bus 1 and bus 2.

Close disconnecting switches DS5-A in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box.

Open disconnecting switches DS5-C in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Open button in the switchgear control dialog box.

18. Which bus is outgoing line B of the electric power substation connected to?


20. Make sure that disconnecting switches DS3-A and DS3-B in the electric power substation are closed. Also make sure that circuit breaker CB3 in the electric power substation is closed. This ensures that bus 1 and bus 2 are interconnected.

Close disconnecting switches DS5-C in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box.



Open disconnecting switches DS5-A in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Open button in the switchgear control dialog box.

21. Which bus is outgoing line B of the electric power substation connected to?

Bus fault

In this section, you will insert a fault at bus 1 of the electric power substation and observe the effect that this fault has on the currents flowing through the incoming lines. You will then open circuit breakers in the electric power substation to isolate the faulty bus, observe what happens to the currents flowing through the incoming and outgoing lines, and assess the effect on the supply of power to the loads. Finally, you will determine what operations can be performed immediately to limit the consequences of this fault.

22. On the Power Supply, turn the ac power source off.

On the Fault Module, make sure that the Fault switch is set to the open (O) position, then connect the Power Input to the ac power source. Connect fault contact K1-A and the Current-Limiting Resistor of the Fault Module to bus 1, as shown in Figure 43. This allows a fault to be inserted at bus 1. The Current-Limiting Resistor is connected in series with fault contact K1-A to limit the value of the fault current.

On the Power Supply, turn the ac power source on.



Figure 43. Connection of the Fault Module required to insert a fault at bus 1 of the electric power substation.

Bus 2

Bus 1

Outgoingline A

Incoming line A

Loadresistor R1

AC power source

L

N

Line inductor

25 mH

E1

I1 I2

To neutral

DS1-A

CB1

DS1-B

DS2-A

CB2

DS2-B

DS3-A DS3-B

DS1-C DS2-C

CB3



K1-A

Current-limiting resistor



23. On the Fault Module, set the Fault switch to the closed (I) position to insert a fault at bus 1. Observe that the values of the currents flowing through incoming lines A and B increase considerably.

Which circuit breakers in the electric power substation must be opened to isolate bus 1 and interrupt the fault currents flowing through incoming lines A and B?

24. Successively open the circuit breakers that you mentioned above by clicking the corresponding symbol in the SCADA window, then clicking the Open button in the switchgear control dialog box.

Have the fault currents flowing through incoming lines A and B been interrupted?

What are the consequences of opening these circuit breakers?

What operations can be performed immediately to limit the consequences of this fault while the faulty bus (bus 1) is being repaired?

25. Successively open disconnecting switches DS1-A and DS2-A in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Open button in the switchgear control dialog box.

Successively close disconnecting switches DS1-C and DS2-C in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box.



Reclose incoming line circuit breaker CB2 by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box.

Reclose outgoing line circuit breaker CB1 by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box.

The above operations have transferred (off-load transfer) incoming line A and outgoing line A from bus 1 to bus 2 of the electric power substation. Observe that these operations allow electric power to be routed to loads via incoming line A and electric power to be recovered at load resistor R1 while the faulty bus (bus 1) is isolated from the electric power substation for repair. Also, observe that the current flowing through each incoming line and each outgoing line has a normal value.

Does having two buses in an electric power substation (as when the double bus, single breaker scheme is used) improve the reliability of the substation when a bus fault occurs? Explain briefly.

26. On the Fault Module, set the Fault switch to the open (O) position to remove the fault at bus 1 (as when repair of a faulty bus is completed). Consequently, bus 1 becomes available and can be reconnected to power lines.

27. Reclose bus coupler circuit breaker CB3 by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box.

Perform an on-load transfer of incoming line A to bus 1. To do so, close disconnecting switch DS2-A in the electric power substation (by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box). Then, open disconnecting switch DS2-C in the electric power substation (by clicking the corresponding symbol in the SCADA window, then clicking the Open button in the switchgear control dialog box).

Perform an on-load transfer of outgoing line A to bus 1. To do so, close disconnecting switch DS1-A in the electric power substation (by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box). Then, open disconnecting switch DS1-C in the electric power substation (by clicking the corresponding symbol in the SCADA window, then clicking the Open button in the switchgear control dialog box).

The electric power substation is now in the same state as it was before the bus fault.



Bus coupler circuit-breaker fault

In this section, you will produce a bus-coupler circuit-breaker fault in the electric power substation and observe the effect that this fault has on the currents flowing through the incoming lines. You will then open other circuit breakers in the electric power substation to isolate the faulty bus coupler circuit breaker, observe what happens to the currents flowing through the incoming and outgoing lines, and assess the effect on the supply of power to loads. Finally, you will determine what operations can be performed immediately to limit the consequences of the fault.

28. In the SCADA window, click the symbol of circuit breaker CB3, then click the check box next to the Insert fault option in the switchgear control dialog box. Clicking this check box emulates a faulty circuit breaker, i.e., it makes circuit breaker CB3 ignore open commands and close commands.

Make sure that a blinking "X" appears in the check box next to the Insert fault option in the switchgear control dialog box. This confirms that the fault is inserted in the circuit breaker.

Click the Open button in the switchgear control dialog box. Observe that circuit breaker CB3 fails to open.

If your local ac power network voltage is 120 V, the nominal current of the line inductors

will be exceeded significantly during the course of the following two manipulations.

Proceed rapidly when performing these manipulations to prevent excessive overheating

from causing damages to one of the two line inductors in the circuit.

29. On the Fault Module, set the Fault switch to the closed (I) position to insert a fault at bus 1. Observe that the values of the currents flowing through incoming lines A and B increase considerably.

Which circuit breakers in the electric power substation must be opened to isolate bus 1 and interrupt the fault currents flowing through incoming lines A and B?

Successively open the circuit breakers that you mentioned above by clicking the corresponding symbol in the SCADA window, then clicking the Open button in the switchgear control dialog box. Observe that fault current no longer flows through incoming line A. Also observe that bus coupler circuit breaker CB3 fails to open and that, consequently, fault current continues to flow through incoming line B. This results in a bus-coupler circuit breaker fault.



In this situation, which other circuit breakers in the electric power substation must be opened to isolate the faulty bus coupler circuit breaker and interrupt the fault current flowing through incoming line B?

30. Successively open the circuit breakers that you mentioned above by clicking the corresponding symbol in the SCADA window, then clicking the Open button in the switchgear control dialog box.

Has the fault current flowing through incoming line B been interrupted?

What are the consequences of opening these circuit breakers?

What operations can be performed immediately to limit the consequences of this fault while the faulty bus and bus coupler circuit breaker (bus 1 and circuit breaker CB3) are being repaired?

31. Successively open disconnecting switches DS3-A and DS3-B in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Open button in the switchgear control dialog box. This isolates the faulty bus section circuit breaker (CB3) from the electric power substation.

Successively open disconnecting switches DS1-A and DS2-A in the electric power substation by clicking the corresponding symbol in the SCADA



window, then clicking the Open button in the switchgear control dialog box. This isolates the faulty bus (bus 1) from the electric power substation.

Successively close disconnecting switches DS1-C and DS2-C in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box.





The above operations reconnected all power lines to bus 2 of the electric power substation. Observe that these operations allow electric power to be routed to loads via both incoming lines, and electric power to be recovered rapidly at all loads (load resistors R1 and R2) while the faulty bus and bus coupler circuit breaker (bus 1 and circuit breaker CB3) are isolated from the electric power substation for repair. Also, observe that the current flowing through each incoming line and each outgoing line has a normal value.

Does having two buses in an electric power substation (as when the double bus, single breaker scheme is used) improve the reliability of the substation when a bus-coupler circuit-breaker fault occurs? Explain briefly.

32. On the Fault Module, set the Fault switch to the open (O) position to remove the fault at bus 1.

In the SCADA window, click the symbol of circuit breaker CB3, then click the check box next to the Insert fault option in the switchgear control dialog box. Clicking this check box makes the circuit breaker operate normally, i.e., it makes circuit breaker CB3 respond normally to open commands and close commands.

Make sure that no blinking "X" appears in the check box next to the Insert fault option in the switchgear control dialog box. This confirms that no fault is inserted in the circuit breaker.



Open bus section circuit breaker CB3 by clicking the Open button in the switchgear control dialog box.

The faults affecting bus 1 and circuit breaker CB3 have been removed (as when repair of the faulty equipment is completed). Consequently, bus 1 becomes available and power lines can be reconnected to this bus.


Reclose bus coupler circuit breaker CB3 by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box.

Perform an on-load transfer of incoming line A to bus 1. To do so, close disconnecting switch DS2-A in the electric power substation (by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box). Then, open disconnecting switch DS2-C in the electric power substation (by clicking the corresponding symbol in the SCADA window and clicking the Open button in the switchgear control dialog box).

Perform an on-load transfer of outgoing line A to bus 1. To do so, close disconnecting switch DS1-A in the electric power substation (by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box). Then, open disconnecting switch DS1-C in the electric power substation (by clicking the corresponding symbol in the SCADA window and clicking the Open button in the switchgear control dialog box).

The electric power substation is now in the same state as it was before the bus-coupler circuit-breaker fault.

Bus maintenance

In this section, you will isolate (i.e., de-energize) and ground one of the two buses in the electric power substation so that maintenance of this bus can be performed safely. You will then isolate (i.e., de-energize) and ground a power line in the electric power substation so that maintenance of switchgear associated with this line can be performed safely.




35. Perform an on-load transfer of outgoing line B to bus 1. To do so, close disconnecting switch DS5-A in the electric power substation (by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box). Then, open disconnecting switch DS5-C in the electric power substation (by clicking the corresponding symbol in the SCADA window and clicking the Open button in the switchgear control dialog box).

Perform an on-load transfer of incoming line B to bus 1. To do so, close disconnecting switch DS4-A in the electric power substation (by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box). Then, open disconnecting switch DS4-C in the electric power substation (by clicking the corresponding symbol in the SCADA window and clicking the Open button in the switchgear control dialog box).

Open bus coupler circuit breaker CB3 by clicking the corresponding symbol in the SCADA window, then clicking the Open button in the switchgear control dialog box.

Successively open disconnecting switches DS3-A and DS3-B in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Open button in the switchgear control dialog box.

Make sure that bus 2 is de-energized, i.e., check that the voltage across bus 2 (indicated in the SCADA window) is much lower than normal.

a The voltage across bus 2 is not zero because parasitic voltage is produced across the bus even when it is de-energized, due to some capacitive coupling.

Connect bus 2 to ground (a grounding terminal is available on the front panel of the Power Supply) to make sure the voltage across bus 2 is zero.

36. Does this demonstrate that maintenance of bus 2 in the electric power substation can be performed safely and without interrupting the supply of power to any of the loads connected to the substation?


Exercise 3 – Double Bus, Single Breaker Scheme Conclusion


38. Open line circuit breaker CB5 in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Close button in the switchgear control dialog box.

Successively open disconnecting switches DS5-A and DS5-B in the electric power substation by clicking the corresponding symbol in the SCADA window, then clicking the Open button in the switchgear control dialog box.

Make sure that outgoing line B is de-energized. This can be confirmed by using a multimeter (if available) to measure the voltage across outgoing line B to make sure that it is much lower than normal.

a The voltage across outgoing line B is not zero because parasitic voltage is produced across the line even when it is de-energized, due to some capacitive coupling.

Connect outgoing line B to ground (a grounding terminal is available on the front panel of the Power Supply) to make sure the voltage across this line is zero.

39. Does this demonstrate that maintenance of switchgear associated with outgoing line B (including the disconnecting switch that is connected to the bus that is under maintenance) can be performed safely and without losing more than one power line?

In this exercise, you were introduced to the operation of electric power substations using the double bus, single breaker scheme (duplicate bus scheme). You learned that this scheme includes all the components in the single bus scheme, plus a second bus, disconnecting switches linking the second bus to each line circuit breaker, and a bus coupler circuit breaker with disconnecting switches. You also learned that this scheme provides some operating flexibility that allows the power lines to be connected to the two buses in any combination required. Furthermore, you saw that this scheme allows any power line to be transferred from one bus to the other without interruption in the supply of power to the loads.

CONCLUSION

Exercise 3 – Double Bus, Single Breaker Scheme Review Questions


You demonstrated that the double bus, single breaker scheme is a little more reliable than the single bus scheme since a bus fault or a bus coupler circuit-breaker fault only causes a short interruption in the supply of power to loads. You saw that maintenance in a substation using the double bus, single breaker scheme is easier than in a substation using the single bus scheme. For instance, maintenance of either one of the two buses can be performed without interrupting the supply of power to any of the loads fed by the substation (something which is impossible in a substation using the single bus scheme). Also, maintenance of any disconnecting switch connected to the bus which is being maintained can be performed without losing more than one power line (several or all power lines are lost when performing similar maintenance in a substation using the single bus scheme).

Finally, you learned how to isolate (i.e., de-energize) and ground a bus and a power line in a substation using the double bus, single breaker so that maintenance of this equipment can be performed safely.

1. Describe an electric power substation implemented using the double bus, single breaker scheme.

2. Explain what an on-load transfer is in an electric power substation using the double bus, single breaker scheme.

3. To perform an on-load transfer, what verification must be performed before allowing closure of the bus coupler circuit breaker when the two buses in the substation are fed by two different ac power sources?

REVIEW QUESTIONS

Exercise 3 – Double Bus, Single Breaker Scheme Review Questions


4. Explain why substations using the double bus, single breaker scheme are a little more reliable than substations using the single bus scheme.

5. Is maintenance in a substation using the double bus, single breaker scheme somewhat easier (i.e., less problematic) than in a substation using the single bus scheme (with or without bus section circuit breakers)? Explain briefly.

exercise 3 double bus, single breaker scheme - lab … · substations using the double bus, single...

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