It
is highly recommended that the design engineer show Test Switches on the System
One-Line and include them in the specifications. These are shown on the one-line
as a box with “TS” in it. Test switches are used during protective relay testing
to provide an alternate path to inject current and voltage from a test set, when
commissioning these devices in the field.
When designing a power system, it is necessary to select the ratio and the accuracy class for the CT’s. For protective relaying, the CT must be sized to ensure they do not saturate under fault conditions. This may result in a higher accuracy class with more physical mass or a higher CT ratio being specified. Most of the CTs shown on Figure 1.1-7 are Standard Accuracy Class for the ratios selected. The exception is the single 600:5 CT in transformer T1’s Neutral to Ground Connection. This is shown as a high accuracy CT.
When selecting CTs for metering purposes, such as those connected to the Eaton PXM-6000 Power Quality Meter (see Tab 3 for details) it is best to use the CT ratio as close to the actual load as possible. This is done to increase the accuracy at the low end of the range because the CT’s excitation begins to deteriorate at about 10% of its ratio setting. As an example, a 600:5A fixed ratio CT would begin to lose accuracy at 60 A.
Where loads are light, during construction or during early build out stages, the actual current that must be measured by the meter may be only 100 A. Multi-ratio CTs are frequently used to set the maximum ratio lower. If set at 100:5A, this would improve accuracy down to 10 A for a 100 A load. Conversely, as the end loads grow, the maximum ratio setting can be easily increased by changing the CT tap settings.
The
secondary output of both voltage and current transformers are measured by
protective relays and used in calculations involving preset thresholds.
Voltage monitoring elements of protective relays compare the input from the VTs against a desired set-point to see if the system voltage is over or under that nominal value. If the value exceeds a plus or minus tolerance band around the set-point, an output contact or contacts in the relay change state to signal an alarm or trip the circuit breaker open.
Microprocessor-based relays offer tremendous functionality over the older electromechanical and solid-state predecessors. Many of these devices offer multiple types of voltage and current protective elements.
Protective relay elements are generally denoted by a number or characters as defined in the ANSI/IEEE C37.2
Standard
for Device Function Numbers, Acronyms and Contact Designations.
See Table 1.1-56 in Power Distribution Systems Reference Data Section of this document for Device Function Number information.
These element numbers are shown in a circle on the One-Line. A given relay may have multiple voltage and current elements shown in a common box, such as the EDR 5000-M1 protecting the 52-M1 breaker in Figure 1.1-9.
The numbers in parenthesis define the quantity of each specific element. In many cases this quantity is (3); one for each of the three phases. In some cases, such as the 50/51N function, this is shown as a quantity of (1). The symbol to the right of this relay represents a transition from (3) individual phase elements to a single residual neutral protective element.
The output of each protective function is shown with a dashed line and arrow indicating what action is to be taken if the relay determines the monitored values exceed the preset thresholds. The EDR-5000-M1 Relay’s 50/51 Elements (Instantaneous Overcurrent and Time Overcurrent respectively) are shown tripping a high-speed 86-M1 Lockout Relay. The elements of the ETR-5000-T1 Transformer Differential Relay are shown similarly, also tripping an 86-T1 lockout relay.
In both cases, the associated (86) lockout relay then trips the incoming main breaker “M1” and the transformer secondary breaker “S1”. Lockout relays are used to multiply the tripping contacts for a given function so they can be wired into multiple breaker’s separate control circuits as indicated for the 86-B1 device on the System One Line. Their primary function, however, is to require a manual reset of the Lockout Relay mechanism by trained personnel after the cause of the fault is determined and corrected.
The 27 and 59 functions shown in the EDR-5000 relay monitor undervoltage and overvoltage respectively. Their outputs are shown combined into a single dashed line directly tripping both the incoming main breaker “M1” and the transformer secondary breaker “S1”. This reflects the engineer’s desire to have only one output contact for both the 27 and 59 functions. Because two breakers need to be tripped, this will only require two separate relay contacts instead of four individual output contacts otherwise necessary.
The direct trip shown on the System One-Line purposely does not use an 86 lockout relay, as this under or over voltage disturbance is anticipated to be caused by the utility and not a fault on the end user’s power system. In these instances, a separate contact from the relay may be allocated to start a backup generator or to initiate a Main-Tie-Main Transfer Scheme.
The EDR-5000 Relay and the ETR-5000 Relay are programmable multi-function devices with many protective elements that can be utilized simultaneously. In a more fully developed protection scheme, certain protective elements (such as the 50/51 functions) can be used in both relays to back each other up in the event of a failure. Figure 1.1-10 shows the many protective elements available in the EDR-5000 Feeder Protective Relay.
Eaton’s “E Series” relays include an ANSI 74 element to monitor the trip coil of the circuit breaker or lockout relay they are tripping. This circuit ensures the integrity of the device to operate correctly when a trip signal is applied. The example One-Line should show the relay circle with the “74” in it next to the “86” lockout relay and breaker “52” symbols. These were purposely not shown on the drawing as it would make it more crowded and difficult to read.
Figure 1.1-9 shows some additional important information about the equipment
required in the dashed box that comprises utility switchgear “USG-1A”. This
switchgear is defined as 15 kV Class with a 95 kV basic impulse rating. The bus
is rated to handle 1200 A even though the actual ampacity flowing through it
will be under 500 A. The equipment will be operating at 13.8 kV and have a
short-circuit rating of 50 kA Symmetrical.
Because this One-Line is for educational purposes, a hypothetical short-circuit value at “Point A” from the Utility is shown for reference at 11.65 kA. In actuality, this value would be part of a short-circuit study. If using a program such as SKM to calculate the downstream short-circuit values, the cable lengths and conduit types as well as the transformer impedance would factor into the calculations.
When designing a power system, it is necessary to select the ratio and the accuracy class for the CT’s. For protective relaying, the CT must be sized to ensure they do not saturate under fault conditions. This may result in a higher accuracy class with more physical mass or a higher CT ratio being specified. Most of the CTs shown on Figure 1.1-7 are Standard Accuracy Class for the ratios selected. The exception is the single 600:5 CT in transformer T1’s Neutral to Ground Connection. This is shown as a high accuracy CT.
When selecting CTs for metering purposes, such as those connected to the Eaton PXM-6000 Power Quality Meter (see Tab 3 for details) it is best to use the CT ratio as close to the actual load as possible. This is done to increase the accuracy at the low end of the range because the CT’s excitation begins to deteriorate at about 10% of its ratio setting. As an example, a 600:5A fixed ratio CT would begin to lose accuracy at 60 A.
Where loads are light, during construction or during early build out stages, the actual current that must be measured by the meter may be only 100 A. Multi-ratio CTs are frequently used to set the maximum ratio lower. If set at 100:5A, this would improve accuracy down to 10 A for a 100 A load. Conversely, as the end loads grow, the maximum ratio setting can be easily increased by changing the CT tap settings.
Voltage
transformers are used to step higher voltages down to safe levels for inputs to
relays and meters. Traditionally, voltage transformers (VTs) utilize a higher
primary voltage winding that is a fixed ratio to the 120 Vac secondary winding.
Examples shown on the One-Line are 14,400 V:120 V (a ratio of 120:1) or 4200
V:120 V (a ratio of 35:1). Voltage transformers are often referred to as potential
transformers or PTs. They are illustrated symbolically as shown in Figure 1.1-8.
 |
Figure 1.1-8. Voltage Transformer Symbol
|
Voltage monitoring elements of protective relays compare the input from the VTs against a desired set-point to see if the system voltage is over or under that nominal value. If the value exceeds a plus or minus tolerance band around the set-point, an output contact or contacts in the relay change state to signal an alarm or trip the circuit breaker open.
Microprocessor-based relays offer tremendous functionality over the older electromechanical and solid-state predecessors. Many of these devices offer multiple types of voltage and current protective elements.
Protective relay elements are generally denoted by a number or characters as defined in the ANSI/IEEE C37.2
 |
| Figure 1.1-9. Protective Relay Element Symbols |
See Table 1.1-56 in Power Distribution Systems Reference Data Section of this document for Device Function Number information.
These element numbers are shown in a circle on the One-Line. A given relay may have multiple voltage and current elements shown in a common box, such as the EDR 5000-M1 protecting the 52-M1 breaker in Figure 1.1-9.
The numbers in parenthesis define the quantity of each specific element. In many cases this quantity is (3); one for each of the three phases. In some cases, such as the 50/51N function, this is shown as a quantity of (1). The symbol to the right of this relay represents a transition from (3) individual phase elements to a single residual neutral protective element.
The output of each protective function is shown with a dashed line and arrow indicating what action is to be taken if the relay determines the monitored values exceed the preset thresholds. The EDR-5000-M1 Relay’s 50/51 Elements (Instantaneous Overcurrent and Time Overcurrent respectively) are shown tripping a high-speed 86-M1 Lockout Relay. The elements of the ETR-5000-T1 Transformer Differential Relay are shown similarly, also tripping an 86-T1 lockout relay.
In both cases, the associated (86) lockout relay then trips the incoming main breaker “M1” and the transformer secondary breaker “S1”. Lockout relays are used to multiply the tripping contacts for a given function so they can be wired into multiple breaker’s separate control circuits as indicated for the 86-B1 device on the System One Line. Their primary function, however, is to require a manual reset of the Lockout Relay mechanism by trained personnel after the cause of the fault is determined and corrected.
The 27 and 59 functions shown in the EDR-5000 relay monitor undervoltage and overvoltage respectively. Their outputs are shown combined into a single dashed line directly tripping both the incoming main breaker “M1” and the transformer secondary breaker “S1”. This reflects the engineer’s desire to have only one output contact for both the 27 and 59 functions. Because two breakers need to be tripped, this will only require two separate relay contacts instead of four individual output contacts otherwise necessary.
The direct trip shown on the System One-Line purposely does not use an 86 lockout relay, as this under or over voltage disturbance is anticipated to be caused by the utility and not a fault on the end user’s power system. In these instances, a separate contact from the relay may be allocated to start a backup generator or to initiate a Main-Tie-Main Transfer Scheme.
The EDR-5000 Relay and the ETR-5000 Relay are programmable multi-function devices with many protective elements that can be utilized simultaneously. In a more fully developed protection scheme, certain protective elements (such as the 50/51 functions) can be used in both relays to back each other up in the event of a failure. Figure 1.1-10 shows the many protective elements available in the EDR-5000 Feeder Protective Relay.
Eaton’s “E Series” relays include an ANSI 74 element to monitor the trip coil of the circuit breaker or lockout relay they are tripping. This circuit ensures the integrity of the device to operate correctly when a trip signal is applied. The example One-Line should show the relay circle with the “74” in it next to the “86” lockout relay and breaker “52” symbols. These were purposely not shown on the drawing as it would make it more crowded and difficult to read.
 |
Figure 1.1-10. EDR-5000 Protective Relay Elements Available
|
Because this One-Line is for educational purposes, a hypothetical short-circuit value at “Point A” from the Utility is shown for reference at 11.65 kA. In actuality, this value would be part of a short-circuit study. If using a program such as SKM to calculate the downstream short-circuit values, the cable lengths and conduit types as well as the transformer impedance would factor into the calculations.
The
“USG-1A” switchgear on the One-Line is shown with a 50 kA rating when other
lower ratings such as 40 kA and 25 kA are available at 15 kV. This has been
done as an example to future design engineers who may be involved in urban areas
with medium-voltage services. These MV services typically have higher available
short-circuit capacity. In most cases, the serving utility may have specific
specifications for the switchgear and breakers used as medium-voltage service
equipment.
One such requirement is the breaker’s close and latch rating. Where higher fault current values exist, some utilities specify this value at 130 kA peak, which is more in line with the older 1000 MVA rated design. Because the 40 kA K=1 design’s close and latch rating is only 104 kA, the 130 kA close and latch rating for the 50 kA breaker would dictate it being used instead. It is very important to be cognizant of nuances in all utility specifications to avoid costly problems or delays in energization.
Certain utilities mandate the type of cable termination that must be provided in the switchgear such as the (3) single-phase pot-heads illustrated on Figure 1.1-9.
Utilities may also require neon glow tubes or other live line indicators to be located on all three incoming phases. These devices are intended to caution personnel that the incoming circuit may be energized.
However, it is always best to follow Occupational Safety and Health Administration (OSHA) approved practices and assume that the circuit is live until a calibrated voltage reading probe attached to a hot-stick determines otherwise.
Most utilities and institutions involved in the distribution of medium-voltage power use portable ground cables that are applied only after no voltage presence has been confirmed. This requires that ground studs be mounted in the switchgear in order to facilitate their OSHA compliant grounding procedure.
One such requirement is the breaker’s close and latch rating. Where higher fault current values exist, some utilities specify this value at 130 kA peak, which is more in line with the older 1000 MVA rated design. Because the 40 kA K=1 design’s close and latch rating is only 104 kA, the 130 kA close and latch rating for the 50 kA breaker would dictate it being used instead. It is very important to be cognizant of nuances in all utility specifications to avoid costly problems or delays in energization.
Certain utilities mandate the type of cable termination that must be provided in the switchgear such as the (3) single-phase pot-heads illustrated on Figure 1.1-9.
Utilities may also require neon glow tubes or other live line indicators to be located on all three incoming phases. These devices are intended to caution personnel that the incoming circuit may be energized.
However, it is always best to follow Occupational Safety and Health Administration (OSHA) approved practices and assume that the circuit is live until a calibrated voltage reading probe attached to a hot-stick determines otherwise.
Most utilities and institutions involved in the distribution of medium-voltage power use portable ground cables that are applied only after no voltage presence has been confirmed. This requires that ground studs be mounted in the switchgear in order to facilitate their OSHA compliant grounding procedure.
As
shown on the System One-Line, there are ground studs on the incoming and
outgoing sides of both the “USG-1A”, (13.8 kV) and “PSG-1A”(4.16 kV) switchgear.
Applying these portable ground cables requires a safe disconnection of power in
the zone to be grounded to ensure personnel safety. Consequently, a Key
Interlock Scheme would be required to prevent grounding unless the respective
breakers in the zone were withdrawn from their connected position and locked
open.
The symbol representing the key interlock shown on the One-Line next to the “M1” and “S1” breaker is the box with the circle and the letters “LO” inside it. The “LO” nomenclature indicates that the key “M1A” or key “S1A” respectively is only removable when the device (breaker or fused switch) is in the Locked Open position.
Key interlocks are also shown on the MVS switches in two of the (4) Primary Selective Step-Down substations fed from MV Feeder Breaker F3A, as well as on the medium-voltage switch “CUP-F1A”. This is done to prevent paralleling of the two different sources involved in the Primary Selective Scheme.
The 750 kVA pad-mounted transformers on the One-Line, feeding “Residence Hall A” and “B”, are shown with internal vacuum fault interrupters (VFIs) as their overcurrent protection. The VFIs offer many of the benefits of a circuit breaker, such as disconnection of all three phases simultaneously, and may be used with external protective relays such as EDR-3000 Distribution or ETR-3000 Transformer Differential Relay. The VFI option is available for fluid-filled transformers in both pad-mounted and unit substation configurations.
Key interlocks are also used in the Main-Generator-Tie “Bus A” half of double-ended 480/277 Vac secondary unit substation as shown in Figure 1.1-11 and Figure 1.1-12. This scheme permits only one source to feed “Bus A” of the doubleended switchgear at a time.
This arrangement, while functional in physically blocking multiple sources such as “MB-F1A”, “GB” and “MB-F1B” from being paralleled, does not permit Bus “B” of the double-ended substation to be alternately fed from the “MV-F1A” breaker or the “GB” breaker. This may or may not be the intent of the design engineer. In either case, the engineer must think through the intent of the key interlock scheme and develop the logic accordingly.
The symbol representing the key interlock shown on the One-Line next to the “M1” and “S1” breaker is the box with the circle and the letters “LO” inside it. The “LO” nomenclature indicates that the key “M1A” or key “S1A” respectively is only removable when the device (breaker or fused switch) is in the Locked Open position.
Key interlocks are also shown on the MVS switches in two of the (4) Primary Selective Step-Down substations fed from MV Feeder Breaker F3A, as well as on the medium-voltage switch “CUP-F1A”. This is done to prevent paralleling of the two different sources involved in the Primary Selective Scheme.
The 750 kVA pad-mounted transformers on the One-Line, feeding “Residence Hall A” and “B”, are shown with internal vacuum fault interrupters (VFIs) as their overcurrent protection. The VFIs offer many of the benefits of a circuit breaker, such as disconnection of all three phases simultaneously, and may be used with external protective relays such as EDR-3000 Distribution or ETR-3000 Transformer Differential Relay. The VFI option is available for fluid-filled transformers in both pad-mounted and unit substation configurations.
Key interlocks are also used in the Main-Generator-Tie “Bus A” half of double-ended 480/277 Vac secondary unit substation as shown in Figure 1.1-11 and Figure 1.1-12. This scheme permits only one source to feed “Bus A” of the doubleended switchgear at a time.
This arrangement, while functional in physically blocking multiple sources such as “MB-F1A”, “GB” and “MB-F1B” from being paralleled, does not permit Bus “B” of the double-ended substation to be alternately fed from the “MV-F1A” breaker or the “GB” breaker. This may or may not be the intent of the design engineer. In either case, the engineer must think through the intent of the key interlock scheme and develop the logic accordingly.
To be continued.....
No comments:
Post a Comment