Directional Comparison Blocking Schemes - Single Component … Schemes - Single... · 2019-06-03 · directional or non -directional ground overcurrent ) used to initiate the block

Directional Comparison Blocking Schemes - Single Component

Failure: Over-Trip Report prepared by TFSP

Issue 1.0

May 2019

Directional Comparison Blocking Schemes - Single Component Failure:Over-Trip

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Document Change History

Issue Reason for Issue Date

1.0 First Release May 2019


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Table of Contents

Table of Contents .......................................................................................................................................... 3

1. Purpose ................................................................................................................................................. 4

2. Scope ..................................................................................................................................................... 5

3. Directional Comparison Blocking Schemes – Description and Applications ......................................... 6

3.1 Description .................................................................................................................................... 6

3.2 Applications ................................................................................................................................... 7

4. DCB Schemes – Single Component Failure Scenarios ........................................................................... 9

5. DCB Schemes – Recommendations to Improve Security .................................................................... 10

5.1 Recommendation to be added to Directory 4 ............................................................................ 10

5.2 Recommendations to improve the design and maintenance practices ..................................... 10

5.3 Recommendations to improve the planning and operational practices .................................... 11

6. Conclusions ......................................................................................................................................... 12

7. References .......................................................................................................................................... 13


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1. Purpose The purpose of this report is to review the concern associated with the potential for unnecessary tripping in the DCB scheme when certain single components fails and a blocking signal is not received during an external fault. The DCB schemes rely on the receiving of a remote blocking signal to inhibit fast tripping by the local forward over-reaching elements. Therefore, the failure of a single component that can prevent the block signal from being either sent or received will expose the protection system using DCB schemes to unnecessary tripping for faults occurring outside its zone of protection and within the forward-reach of its tripping element. DCB schemes are a viable option for designing protection systems for a variety of reasons, which are biased toward dependability. However, it is important for protection systems designers, system planners and operators to be aware of the potential issues associated with DCB schemes and consider appropriate design requirements and contingency analysis for the potential over-trip condition to ensure DCB schemes for a bulk power system element are designed, planned, and operated in a secure manner.

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2. Scope As noted in the NPCC Directory 4, Appendix A, Guideline for Bulk Power System Protection, Section 2.1.3 “Often increased security (fewer unintended operations) results in decreased dependability (more failures to operate), and vice versa. As an example, consideration is given to the consequence of applying permissive protection schemes, which often are more secure, but less dependable, than blocking protection schemes. The relative effect on the bulk power system of a failure of a protection system to operate when desired versus an unintended operation should be weighed carefully in selecting design parameters.” NPCC Document A-10, Classification of Bulk Power System Element, establishes the methodology to classify elements of the Bulk Power System (BPS). Considering that an un-cleared fault on a BPS element can have a significant adverse impact outside of the local area as determined through the A-10 methodology, the intent of Directory 4 requirements is to primarily ensure dependable operation of the protection systems for the BPS elements with due consideration given to security of those protections . This report reviews concerns related to the security of a DCB schemes on the BPS elements that can results in over-tripping while they experience a single component failure at a time (i.e. relay, telecomm, DC supply, etc.) The report proposes actions to improve the security of the protection systems which use DCB schemes.



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3. Directional Comparison Blocking Schemes – Description and Applications

3.1 Description A Directional Comparison Blocking (DCB) communication assisted protection scheme is used primarily to protect transmission circuits and provides high speed tripping for all faults (phase and ground) occurring on the circuit it is designed to protect. In this protection scheme, the relays at each of the terminal utilize forward overreaching zone 2 phase and ground elements to determine if the fault is in the forward direction (towards the protected element) and reverse distance and non-directional ground overcurrent or ground distance elements to determine if the fault is in the reverse direction (behind the protected circuit terminal). If the protective relaying at a given terminal detects the fault in the forward direction, the terminal will trip high speed if no blocking signal is received from the remote terminal(s) within a short time delay. If the fault is external to the protected element, one terminal will see the fault in the reverse direction and initiate a blocking signal to the remote terminal(s). If the block signal is received within the specific short delay the high speed tripping is prevented from being performed. Generic examples of the design and logic for DCB schemes are shown in Figure 1 and 2 respectively.

Zone 2

Zone (R)

DC(+)

Tx

Rx

Zone 2

Zone (R)

DC(+)

Rx

Tx

Non-conditional Timed Trip

ReverseLooking Zone

CommunicationsTransmitter

Delay Timer(50ms)

CommunicationsReceiver

DirectionalComparison

Fast Trip

Station A

Delay Timer(50ms)

DirectionalComparison

Fast Trip

CommunicationsTransmitter

ReverseLooking Zone

Non-conditional Timed Trip

Station B

Zone 1

Zone 1

Zone 2

Zone 2

Reverse Zone

Reverse Zone

Line (L1)A B

Figure 1: Design Example for a DCB scheme


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Figure 2 – Logic Example for a DCB scheme

Figure 2 shows an example of DCB logic and timer delay settings. In addition to relying on the communication channels, DCB schemes also use directional distance or directional ground overcurrent elements to achieve the high speed tripping of the faults occurring on the protected circuits. The local reverse looking protection elements (distance or directional or non-directional ground overcurrent) used to initiate the blocking signal must be set to overreach the remote forward looking elements (distance or directional ground overcurrent) to make sure that the blocking signal will be initiated for all reverse faults occurring outside of the protected circuit that are detected remotely by the overreaching forward looking elements. Some DCB schemes use low set non-directional overcurrent elements to initiate a blocking signal to ensure a blocking signal is initiated for all ground faults. The blocking signal is cut off by a higher set (typically 2x) forward directional overcurrent element if the fault is in the forward direction. The use of ground overcurrent elements provides greater sensitivity than ground distance elements but the reach of directional elements is more defined.

3.2 Applications It should be noted that the DCB schemes are cost effective solutions to provide high speed tripping for faults occurring on BPS circuits. They are particularly useful in those situations where the full redundant communication media required by other types of protection schemes are cost prohibitive. Below are some examples of using DCB schemes that are cost effective solutions over other types of protection schemes that would require expensive communication media.

• DCB schemes are traditionally used by at least one protection group when Power Line Carrier is one of the redundant communication channels used to exchange signals between the terminals. DCB logic is well suited for use with PLC communications because the PLC signal is transmitted over the power line conductor. A short or fault on the conductor will cause the PLC signal to be interrupted, but the DCB logic will still operate to clear the fault. Similarly, DCB logic may be well suited for use with other


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communications medium, such as optical fiber, which is located on the transmission line structures or right-of-way of the protected line, and where a single event may cause a power line fault and a loss of the communications medium. DCB schemes will still allow high speed tripping for faults occurring on the protected circuit while the PLC equipment experiences a failure or loss of power. DCB scheme are also cost effective because they do not require a separate communications medium over long distances, only equipment at the terminals is required.

• DCB schemes are cost effective protection solution for certain three terminal circuits when the fully redundant communication media required by other protection schemes are cost prohibitive. The DCB schemes are cost effective solutions for three terminal applications where one terminal is a weak source and the planning studies allow using of the sequential clearing logic (this simpler solution will help meet the PRC-023 relay loadability requirements).

• DCB is one of a set of redundant protection schemes when common communication channels are used, and telecommunication route diversity cannot be achieved (Directory 4, Appendix A, Section 2.11.4).



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4. DCB Schemes – Single Component Failure Scenarios

The security of the DCB schemes is affected by any single component failure that prevents the blocking signal from being received within the allowable short time delay (depending on communication, channels performance is within the range 16 to 50 milliseconds). When the blocking signal is disabled or not received, a high speed trip may be initiated for faults occurring outside of the protected circuits, resulting in additional circuit(s) being removed from service. Depending on the DCB design implementation, a single component failure may result in one or more circuit(s) being over-tripped in addition to the one(s) experiencing fault(s). One over-trip may be considered acceptable as long as there are appropriate alarms (as described in Section 5.3) that will trigger operators’ actions.

The design implementation that allows for more than one over-trip for a single component failure is not acceptable and specific provision will be recommended as an addition to Directory 4. This is described in more detail in Section 5.

Loss of the local DC supply (battery) can affect all DCB schemes supplied by that battery. Under this failure condition the remote terminals could over-trip because the local block-signal will not be initiated. If this scenario exists on the system, it is an operating condition that should be assessed and action needs to be taken in a timely manner. It should also be noted that the loss of the DC supply is not considered a planning event in the NERC TPL-001-4 standard. [5]


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5. DCB Schemes – Recommendations to Improve Security

In order to limit the risk of over-tripping associated with single component failure in a DCB scheme, the following design mitigations solutions, control actions and planning assessments are recommended.

5.1 Recommendation to be added to Directory 4 Add a criteria statement to D4 with respect to limiting the impact of single component failure in DCB schemes to the Element it is designed to protect. Single component failure in a DCB scheme, with the exception of the battery, shall not cause more than one circuit to over-trip (e.g. It will not be acceptable to have a non-redundant telecomm equipment used by more than one protection group since the failure of that telecom equipment used for sending/receiving block signals may cause multiple DCB schemes to over-trip.)

5.2 Recommendations to improve the design and maintenance practices

For DCB scheme using power line carrier as a communication medium, consideration could be given to the use of Directional Comparison Unblocking Schemes (DCUB) rather than DCB, if the system configuration allows. “The DCUB schemes transmit a continuous blocking signal except during internal faults when the un-block (trip) signal is initiated” [4]. With two independent redundant protection groups, a single component failure in one group will not cause the over-trip and the high speed tripping is still being performed by the redundant protection group. The use of DCUB scheme in place of a DCB scheme requires additional considerations other than just a change in the logic. DCB schemes use on/off PLC while DCUB schemes require frequency shift keying (FSK) PLC. This requires equipment changes and may be restricted due to frequency congestion. To limit the impact of single component failure of DCB schemes, consideration could be given to the utilization of separate communication paths (e.g. main and alternate paths for one protection group) for the transmission of the DCB blocking signal for the protection group. This may only be practical in fiber based communication systems.

When mutual agreement exists between protection system owners and system operators, consideration could be given to add design provision of remote control capability to allow operators to respond to an alarmed failure of a DCB component.

During design work for DCB schemes the following good practice principles should be considered:


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a. The carrier coordination timer setting should be based on the communication

channel’s performance and appropriate margin should be included. b. Use of correct logic for the Directional Polarization for the Carrier Directional

Overcurrent Ground Elements that includes the effect of the mutual coupling. c. For certain system configurations with certain fault locations, the manufacturer

recommended logic and simplified AUTO settings for the directional control of the Carrier Directional Ground Overcurrent Elements may cause the DCB reverse looking element to fail to detect the reverse fault and send carrier blocking signal to the remote end. Therefore, the proper directional control logic and calculated element settings based on short circuit simulation should be considered since the default logic/settings may not cover all conditions for all particular applications.

The NERC PRC-005 standard requires that maintenance activities will be performed more frequently for the protection systems components not capable of self-monitoring reducing the risk of failure [2]. It is recommended that the maintenance cycles included in the standard for the components that are not monitored and can affect the security of the DCB schemes when they fail, to be strictly followed or increased if the performance characteristics decay over time.

5.3 Recommendations to mitigate risks associated with operation.

Directory 4 requires that the main protection system components failures be alarmed such as DC supply, relays, telecomm equipment etc. The operators from a central location will be able to initiate control actions for single component failures that are alarmed – e.g. to open the terminal where the communication failure alarm was initiated or to remotely block a certain DCB scheme if the design was done in accordance with the recommendation included in this report subsection 5.2 third paragraph. An assessment should be jointly performed by entities’ operationand P&C groups to identify the high risk conditions that will require enhancing the contingency lists to include some of the DCB over-trip scenarios. At stations with more than one DCB scheme powered by a single DC supply, additional monitoring is necessary. This monitoring would include the ability to detect and alarm for an “open cell” condition that may otherwise go unnoticed with conventional battery charger based alarm monitoring technology (High/Low Voltage, loss of AC, etc.).

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6. Conclusions

• The DCB schemes allow high speed tripping while they are cost effective. They are a viable option in those cases where the full redundant communication media required by other types of protection schemes are cost prohibitive.

• The recommendations included in the section DCB Schemes – Recommendations to Improve Security along with the maintenance requirements included in the NERC standard PRC-005 should reduce the risk of over-tripping for DCB schemes while they experience a single component failure.

• An assessment should be jointly performed by entities’ operation and P&C groups to identify the high risk conditions that will require enhancing the contingency lists to include some of the DCB over-trip situations.



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7. References [1] NPCC – Regional Reliability Reference Directory #4 Bulk Power System Protection Criteria https://www.npcc.org/Standards/Directories/Directory%204_TFSP_Rev_20151001_GJD.pdf [2] NERC – Standard PRC-005-6 - Protection System, Automatic Reclosing, and Sudden Pressure Relaying Maintenance http://www.nerc.com/pa/Stand/Reliability%20Standards/PRC-005-6.pdf [3] NERC – Standard PRC-023-3 - Transmission Relay Loadability http://www.nerc.com/pa/Stand/Reliability%20Standards/PRC-023-3.pdf [4] P. M. Anderson, Power System Protection - The Institute of Electrical and Electronics Engineers, Inc., New York 1999 [5] NERC - TPL-001-4 — Transmission System Planning Performance Requirements http://www.nerc.com/pa/Stand/Reliability%20Standards/TPL-001-4.pdf


https://www.npcc.org/Standards/Directories/Directory%204_TFSP_Rev_20151001_GJD.pdf

http://www.nerc.com/pa/Stand/Reliability%20Standards/PRC-005-6.pdf

http://www.nerc.com/pa/Stand/Reliability%20Standards/PRC-023-3.pdf

http://www.nerc.com/pa/Stand/Reliability%20Standards/TPL-001-4.pdf

Documents

Directional Comparison Blocking Schemes - Single Component … Schemes - Single... · 2019-06-03 · directional or non -directional ground overcurrent ) used to initiate the block