PCS 9671s x technical manual EN overseas general x r1 11

PCS-9671S provides the main protection elements, such as: current differential protection and restricted earth fault protection.. 87T Transformer differential protection  Biased differe

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Although every reasonable effort is made to present current and accurate information, this document does not purport to cover all details or variations in equipment nor provide for every possible contingency to be met in connection with installation, operation, or maintenance Should further information be desired or should particular problems arise which are not covered sufficiently for your purposes, please do not hesitate to contact us

Preface

About This Manual

The manual describes the protection, measurement and supervision functions with the information of relevant hardware for PCS-9671S Transformer Relay

Safety Information

This manual is not a complete index of all safety measures required for operation of the equipment (module or device) However, it comprises important information that must be followed for personal safety, as well as to avoid material damage Information is highlighted and illustrated

as follows according to the degree of danger:

Indicates an imminently hazardous situation that, if not avoided, will result in death or serious injury

Indicates a potentially hazardous situation that, if not avoided, could result in death or serious injury

Indicates a potentially hazardous situation that, if not avoided, may result

in minor or moderate injury or equipment damage

Indicates that property damage can result if the measures specified are

not taken

Important information about the product, please pay attention to avoid undesired result

Instructions and Warnings

The following hazard statements apply to this device

Disconnect or de-energize all external connections BEFORE opening this

device Contact with hazardous voltages and currents inside this device can cause electrical shock resulting in injury or death

Contact with instrument terminals can cause electrical shock that can result in injury or death

Use of this equipment in a manner other than specified in this manual can impair operator safety safeguards provided by this equipment

Have only qualified personnel service this equipment If you are not qualified to service this equipment, you can injure yourself or others, or cause equipment damage

This device is shipped with default passwords Default passwords should

be changed to private passwords at installation Failure to change each default password to a private password may allow unauthorized access

NR shall not be responsible for any damage resulting from unauthorized access

DO NOT look into the fiber (laser) ports/connectors

DO NOT look into the end of an optical cable connected to an optical

output

DO NOT perform any procedures or adjustments that this instruction

manual does not describe

During installation, maintenance, or testing of the optical ports, ONLY use the test equipment qualified for Class 1 laser products!

Incorporated components, such as LEDs, transceivers, and laser emitters,

are NOT user serviceable Return units to NR for repair or replacement

Equipment components are SENSITIVE to electrostatic discharge (ESD)

Undetectable permanent damage can result if you do not use proper ESD procedures Ground yourself, your work surface, and this equipment

BEFORE removing any cover from this equipment If your facility is not

equipped to work with these components, contact NR about returning this device and related NR equipment for service

Insufficiently rated insulation can deteriorate under abnormal operating conditions and cause equipment damage For external circuits, use wiring

of SUFFICIENTLY RATED insulation that will not break down under

abnormal operating conditions

SEVERE power and ground problems can occur on the communications

ports of this equipment as a result of using non-standard cables Please use the wiring method recommended in the manual for communication terminals

DO NOT connect power to the relay until you have completed these

procedures and receive instruction to apply power Equipment damage can result otherwise

Use of controls or adjustments, or performance of procedures other than

those specified herein, may RESULT IN hazardous radiation exposure

The firmware may be upgraded to add new features or enhance/modify

existing features, please MAKE SURE that the version of this manual is

compatible with the product in your hand

Document Conventions

 The abbreviations and acronyms in this manual are explained in “Appendix A Glossary” The

Glossary also contains definitions of important terms

 Menu path is connected with the arrow "→" and bold

For example: the access path of protection settings is: MainMenu→Settings→Protection Settings

 Settings not in the table should be placed in brackets

For example: the system setting [Opt_SysFreq]

 Cross-references are presented in italics

For example: refer to Figure 1.1-1, refer to Table 1.1-1, reference to Section 1.1

 Binary input signals, binary output signals, analogs, LED lights, buttons, and other fixed meanings, should be written in double quotes and bold

For example: press the button "ENT"

An overview of each manual section and section topics follows

6 System Functions

Describes how to perform fundamental operations such as clock synchronization, communicating with the device, switching active setting group, checking relay status, reading event reports and SER (Sequential Events Recorder) records

Describes the abbreviations adopted in this manual

Document Revision History

PN: ZL_PCS-9671S_X_Technical Manual_EN_Overseas General_X

Current version: R1.11

Corresponding Version

Document Software

R1.00 R1.00 2019-10-25  Form the original manual

R1.10 R1.00 2019-12-09  Update of technical data

R1.11 R1.11 2020-06-11

 Update of technical data

 Improved some description of 87T in Section 3.1

 Added PWR plug-in module NR6311A

 Added CPU plug-in module NR6106AH

 Added CPU plug-in module NR6106AP

 Added BIBO plug-in module NR6661A

Figure 1.1-1 Two-winding transformer 1-1

Figure 1.1-2 Three-winding transformer 1-1

Figure 1.1-3 Typical application of a 3-winding transformer 1-2

1

1.1 Application

PCS-9671S can be applied for a two-winding transformer, three-winding transformer,

auto-transformer in any voltage level PCS-9671S provides the main protection elements, such as:

current differential protection and restricted earth fault protection Ancillary functions of fault

diagnostic, disturbance records, event records and communication function are integrated in the

device

PCS-9671S is adaptive to the following 2/3-winding transformers

Figure 1.1-1 Two-winding transformer

Figure 1.1-2 Three-winding transformer

The function diagrams for protecting a 3-winding transformer is shown below

87T Transformer differential protection

 Biased differential protection with three slopes

 Biased DPFC differential protection

 Unrestrained instantaneous differential protection

 Optional inrush current distinguished principles: harmonic criterion or waveform distortion

 Optional harmonic blocking modes: self-adaptive 1Pblk1P mode, 2PBlk3P mode, 1Pblk3P mode

 Overexcitation detection: fifth harmonic or third harmonic criterion

 Optional transfer methods: △→Y or Y→△

 Independent CT saturation criterion

 Differential CT circuit failure supervision

64REF Restricted earth-fault protection

 CT transient characteristic difference detection

 CT saturation detection based on 2 nd and 3 rd

harmonics

1

50/51P Phase overcurrent protection

 2 stages with independent logic by default for each side

 Optional definite-time characteristic and inverse-time characteristic for each stage

 Trip purpose or alarm purpose for each stage

 Harmonic control element for each stage

50/51G Earth fault protection

 2 stages with independent logic by default for each side

 Optional measured zero-sequence current or calculated zero-sequence current

 Optional definite-time characteristic and inverse-time characteristic for each stage

 Selectable trip purpose or alarm purpose for each stage

 Harmonic control element for each stage

2 Measurement and Metering Functions

 Positive sequence currents, zero sequence currents and phase angles

3 Supervision Functions

 CT circuit failure supervision (CTS)

 Self-diagnostic

 Powerful faults recording (max buffer for 10,000 sampled points at 4.8 or 9.6 kHz)

 Event Recorder including 1024 disturbance records, 1024 binary events, 1024

supervision events, 256 control logs and 1024 device logs

 Disturbance recorder including 64 disturbance records with waveforms (The file format of

disturbance recorder is compatible with international COMTRADE file.)

 Single line diagram representation in display

4 Communication Functions

 Support of various protocols

Modbus, DNP3.0, IEC 60870-5-103, IEC 61850 Ed.1 & Ed.2, IEC 61850 MMS Server,

IEC 61850-8-1 GOOSE, IEC 61850-9-2LE SV, IEC 62439 Parallel Redundancy Protocol

(PRP), IEC 62439 High-availability Seamless Ring (HSR) Redundancy Protocol, IEEE

802.1w Rapid Spanning Tree Protocol (RSTP)

 Up to four 10Base-T/100Base-TX copper Ethernet ports

 Up to four 100Base-FX optical Ethernet ports

 Two RS-485 serial ports for communication or printer

 One RS-485/TTL serial port for clock synchronization

 4 Programmable operator pushbuttons with user-configurable labels

 Up to 15 programmable target LEDs with user-configurable labels

 1 RS-232 or RS-485 rear ports for printer

 Language switchover—English+ selected language

 Configuration tool—PCS-Studio

6 Additional Functions

 User programmable logic

 Switching system phase sequences function (ABC or ACB)

 Clock synchronization

 IRIG-B: IRIG-B via RS-485 differential level, TTL level or optical fibre interface

 PPS: Pulse per second (PPS) via RS-485 differential level or binary input

 PPM: Pulse per minute (PPM) via RS-485 differential level or binary input

 IEEE 1588: Clock message based on IEEE 1588 via optical fibre interface

 SNTP (PTP): Unicast (point-to-point) SNTP mode via Ethernet network

 SNTP (BC): Broadcast SNTP mode via Ethernet network

 Message (IEC103/Modbus/DNP3.0): Clock messages through IEC103 protocol, Modbus protocol and DNP3.0 protocol

1

 High reliability and redundancy design for drive systems of the sampling circuit and the output

circuit ensure that overall reliability of the device is high Real-time sampling based on dual AD

can mutually check and detect the potential abnormality in the sampling circuit in time The

control power supply of the output relay is independent with the control circuit of trigger signals,

which can prevent from undesired operation caused by the abnormality of drive circuit of

output relays

 Various function modules can satisfy various situations according to the different requirements

of users Flexible and universal logic programming, user-defined configuration of BI/BOs,

buttons and LEDs and powerful analog programming are supported

 Modularized hardware design makes the device be easily upgraded or repaired by a qualified

service person It can be mixed with different I/O modules, with online self-check and

monitoring function, and the device can be restored from abnormal operation only need to

replace a single abnormal module

 Support memory check and error correction function, ensure high reliability and safety

 Support the internet communication protocol of native PRP/HSR and RSTP

 Fully compatible with IEC 61850 edition 1 & edition 2, support MMS service, IEC 62351

communication service, GOOSE communication in station level & process level, SV

communication with multi-sampling rate

 Full comply with cyber security standards, including IEC62443, IEC62351, IEEE1686,

NERC-CIP, support role based access control (RBAC), security audit, security encryption

communication and security tool, improve the cyber security capability of devices

 Powerful COMTRADE fault and disturbance recording function is supported The whole

recording time is automatically configurable by the fault duration, which is convenient to fault

analysis and replay The recording sample rate is up to 9.6kHz

 Settable secondary rated current (1A/5A) and settable voltage threshold of binary input

 Support LCD, control and multifunction button

 Support flush mounting, semi-flush mounting, surface mounting, wall mounting and other

mounting methods

 Cross screw IO, CT/VT terminals can support AWG12 specification connector and 4mm2 lead

 Protection class of front side is up to IP54

 PCS-Studio is the application software on the user's PC for the interface with PCS S series

devices providing all the related functionality It ranges from device configuration to full

substation design of bay integration

 Support IEEE1588, IRIG-B clock synchronization

 Support actual system phase sequence, either ABC or ACB, incorrect connection of actual

phase sequence can automatically be verified and relevant protection functions can be

blocked

1

 Equipped with high-speed large capacity output relay, its operation speed is less than 1ms and its break capacity is up to 10A The real-time supervision for output drive circuit can detect the abnormality in advance

 Support setup up to 40 users and allow each user to own different password and access authority

 The tripping output contacts can be configured by tripping matrix, which is flexible, convenient and suitable to any mode of tripping

 Multiple inrush current blocking options are provided Self-adaptive inrush current blocking criterion can ensure the relay fast operation for transformer energized on to a slight fault, meanwhile it will avoid the unwanted operation in the case of the energization inrush current caused by energizing transformer with no load, the recovery inrush current caused by cutting off the transformer external fault, and the sympathetic inrush current

 Biased DPFC differential protection is regardless of load current and is sensitive to small internal fault current within the transformer Its anti CT saturation performance is also strong

2.4.2 Copper Ethernet Port 2-5

2.4.3 Optical Fibre Port 2-6

2

2.10.1 Clock Performance 2-11 2.10.2 Fault and Disturbance Recording 2-11 2.10.3 Binary Input Signal 2-11

2.11 Protective Functions 2-11

2.11.1 Transformer Current Differential Protection (87T) 2-11 2.11.2 Restricted Earth Fault Protection (64REF) 2-12 2.11.3 Phase Overcurrent Protection (50/51P) 2-12 2.11.4 Earth Fault Overcurrent Protection (50/51G) 2-13

2

2.1 Electrical Specifications

“System phase sequence”, which can be set by PCS-Studio, this setting

informs the device of the actual system phase sequence, either ABC or ACB CT inputs on the device, labeled as A, B and C, must be connected

to system phase A, B and C for correct operation

Burden <0.05VA/phase @1A, <0.25VA/phase @5A

2.1.2 Power Supply

Standard

IEC 61000-4-11:2017 IEC 61000-4-29:2000

Permissible AC ripple voltage Max 15% of the nominal auxiliary voltage

Burden Quiescent condition Max 15W (default hardware configuration)

Quiescent condition Max 1W

Additional for each contact

0.004W @24Vdc 0.008W @48Vdc 0.015W @110Vdc 0.32W @220Vdc 0.41W @250Vdc

Additional for each plug-in binary output (BO) module

Quiescent condition Max 0.1W

Additional for each relay while

2.1.3 Binary Input

Settable pickup voltage and dropout voltage

On value (default set) 69.3~132Vdc 78.75~160Vdc 138.6~264Vdc 157.5~300Vdc

Off value (default set) <55Vdc <62.5Vdc <110Vdc <125Vdc

Maximum permissible voltage 300Vdc

Withstand voltage 2000Vac, 2800Vdc (1min)

Number Up to 34 (ring ferrule), 41 (pin ferrule) binary inputs

Settable pickup voltage and dropout voltage

Maximum permissible voltage 300Vac

Withstand voltage 2000Vac, 2800Vdc (1min)

Number Up to 34 (ring ferrule), 41 (pin ferrule) binary inputs

Settable pickup voltage and dropout voltage

2

Maximum permissible voltage 300Vdc

Withstand voltage 2000Vac, 2800Vdc (1min)

Number Up to 34 (ring ferrule), 41 (pin ferrule) binary inputs

2.1.4 Binary Output

Tripping/signaling contact

Max system voltage 250Vac, 300Vdc

Pickup time (Typical value) <5ms

Dropout time (Resistive load) <6ms

Breaking capacity (L/R=40ms)

0.5A@48Vdc 0.35A@110Vdc 0.30A@125Vdc 0.20A@220Vdc 0.15A@250Vdc

Cyclic capacity (2.5 cycle/second,

L/R=40ms)

0.5A@48Vdc 0.35A@110Vdc 0.30A@125Vdc 0.20A@220Vdc 0.15A@250Vdc

Short duration current

30A@3s 50A@1s Durability (Loaded contact) 10000 operations

Number Up to 18 (ring ferrule), 22 (pin ferrule) tripping/signal contacts

2

Heavy-capacity tripping contact

MOV protection (Max voltage) 350Vdc, 275Vac

Pickup time (Typical value) <1ms

Dropout time (Resistive load) <10ms

Breaking capacity (L/R=40ms)

10A@48V 10A@110V 10A@125V 10A@250V

Cyclic capacity (4 cycle/second,

followed by 2 minutes idle for thermal

dissipation)

10A@48V L/R=40ms 10A@110V L/R=40ms 10A@125V L/R=40ms 10A@250V L/R=20ms

Short duration current

30A@3s 50A@1s Durability (Loaded contact) 10000 operations

Number Up to 4 (ring ferrule) heavy-capacity tripping contacts

2.2 Mechanical Specifications

Weight per device Approx 7.36kg (6U, 1/3 × 19")

Location of terminal Rear panel of the device

Device structure Plug-in modular type @ rear side, integrated front plate

Protection Class

Front side

IP52 IP54 (valid for surface mounting mode of 6U, 1/3 × 19'' case with

2

sealing strip)

Rear side, connection terminals IP20

2.3 Ambient Temperature and Humidity Range

Baud rate 4.8kbit/s, 9.6kbit/s, 19.2kbit/s, 38.4kbit/s, 57.6kbit/s, 115.2kbit/s

Max transmission distance 500m

Twisted pair Screened twisted pair cable

2.4.2 Copper Ethernet Port

Transmission standard 100Base-TX

Transmission distance Max.100m

Protocol IEC 60870-5-103:1997, DNP 3.0 or IEC 61850

2

2.4.3 Optical Fibre Port

Characteristic Glass optical fiber

Transmission standard 100Base-FX

Transmission distance Max 2km

2.4.4 Print Port

Baud rate 4.8kbit/s, 9.6kbit/s, 19.2kbit/s, 38.4kbit/s, 57.6kbit/s, 115.2kbit/s

2.4.5 Clock Synchronization Port

2

Characteristic Glass optical fiber

Peak optical input power

Logic level HIGH

Logic level LOW

Maximum -40dBm Maximum -10dBm, Minimum -24dBm

2.5 Type Tests

2.5.1 Environmental Tests

Damp heat test, cyclic IEC60068-2-30:2005

2.5.2 Mechanical Tests

Shock and bump IEC 60255-21-2:1988 Class Ⅰ

2.5.3 Electrical Tests

Dielectric tests Test voltage 2kV, 50Hz, 1min

Impulse voltage tests Test voltage 5kV

2

Electrostatic discharge test

IEC 61000-4-2-2008 class Ⅳ For contact discharge: 8kV For air discharge: 15kV

Radio frequency interference tests

IEC 60255-26:2013 class Ⅲ Frequency sweep

Radiated amplitude-modulated 10V/m (rms), f=80~1000MHz, 1400~2700MHz Spot frequency

Radiated amplitude-modulated 10V/m (rms), f=80MHz/160MHz/450MHz/900MHz

Fast transient disturbance tests

IEC 60255-26:2013 Power supply, I/O, Earth: class Ⅳ, 4kV, 5kHz, 5/50ns Communication terminals: class Ⅳ, 2kV, 5kHz, 5/50ns

Surge immunity test

IEC 60255-26:2013 Power supply, AC input, I/O port: class Ⅳ, 1.2/50us Common mode: 4kV

Differential mode: 2kV

Conducted RF electromagnetic

disturbance

IEC 60255-26:2013 Power supply, AC, I/O, Comm Terminal: Class Ⅲ , 10V (rms), 150kHz~80MHz

Spot frequency 10V (rms), f=27MHz/68MHz Power frequency magnetic field

Immunity

IEC 61000-4-8: 2009 class Ⅴ, 100A/m for 1min, 1000A/m for 3s

Pulse magnetic field immunity

IEC 61000-4-9:2016 class Ⅴ, 6.4/16μs, 1000A/m for 3s Damped oscillatory magnetic field

immunity

IEC 61000-4-10:2016 class Ⅴ, 100kHz & 1MHz–100A/m Conducted emission IEC 60255-26:2013

230MHz~1000MHz: 47dB (μV/m) quasi peak @10m, 57dB (μV/m) quasi peak @3m

Above 1GHz

1GHz~3GHz: 56dB (μV/m) average, 76dB (μV/m) peak @3m

3GHz~6GHz: 60dB (μV/m) average, 80dB (μV/m) peak @3m

Auxiliary power supply performance

50ms for interruption without rebooting without energy storage board (Typical configuration) 500ms for interruption without rebooting with energy storage board (Typical configuration)

 EMC: 2014/30/EU, EN60255-26:2013

 Products safety (LVD): 2014/35/EU, EN60255-27:2014

 IEC 61850: Edition 2, Parts 6, 7-1, 7-2, 7-3,7-4 and 8-1

 IEC 61850: Edition 2, GOOSE Performance Class P1 (3ms)

2

 IEEE 1588: IEEE Std C37.238TM-2017

 DNP: DNP 3.0

 PRP: IEC 62439-3 Ed.3 (IS 2016)

 HSR: IEC 62439-3 Ed.3 (IS 2016)

2.7 Liquid Crystal Display (LCD)

2.8 Terminals

2.8.1 Ring Ferrule

Grounding (Earthing) Connection BVR type, 0.8~4mm 2 lead M3 0.6~0.8 N·m

2.8.2 Pin Ferrule

Power supply Screw terminals, 0.3~3.3mm 2 lead M2.5 0.4~0.6 N·m Contact I/O Screw terminals, 0.3~3.3mm 2 lead M2.5 0.4~0.6 N·m

2.9 Measurement Scope and Accuracy

External time synchronization IRIG-B (200-98), PPS, IEEE1588 or SNTP protocol

2.10.2 Fault and Disturbance Recording

Duration & Recording position

Settable pre-disturbance, post-disturbance and maximum recorded duration

2.10.3 Binary Input Signal

Resolution of binary input signal ≤1ms

Binary input mode Potential-free contact

2.11 Protective Functions

There are some symbols mentioned in the following sections and the meaning of them is given

here

In——CT rated secondary current

p.u.——per unit value of settings and currents for current differential protection

Id——differential current

3I0d——residual differential current

ΔId——DPFC differential current

2.11.1 Transformer Current Differential Protection (87T)

2.11.1.1 Biased Differential Protection

Tolerance of 2 nd and 3 rd harmonic settings 0.01

Tolerance of 5 th harmonic settings 0.02

Tolerance of operating current ≤5% of operating current or 0.02p.u., whichever is greater

Operating time (without blocking criteria) 50Hz: ≤ 30ms (Id>2 times current setting, internal fault)

2

60Hz: ≤ 25ms (Id>2 times current setting, internal fault)

2.11.1.2 Instantaneous Differential Protection

Tolerance of current setting ≤2.5% of setting or 0.02p.u., whichever is greater

2.11.1.3 DPFC Current Differential Protection

Tolerance of operating current ≤10%

Operating time

50Hz: ≤30ms (ΔId >3 times current setting (the current setting is fixed as 0.2p.u in program), internal fault) 60Hz: ≤30ms (ΔId >3 times current setting (the current setting is fixed as 0.2p.u in program), internal fault)

2.11.2 Restricted Earth Fault Protection (64REF)

Tolerance of operating current ≤5% of operating current or 0.02×In, whichever is greater

2.11.3 Phase Overcurrent Protection (50/51P)

Tolerance of current setting ≤1%×setting or 0.01×In, whichever is greater

2

Tolerance of pickup time ≤25ms (at 2 times current setting)

Tolerance of operating time

Definite-time: ≤1%×setting or 25ms (at 2 times current setting)

Inverse-time: ≤ 5 ％ of the calculated value + 1 ％ of current tolerance, or 35ms (1.2≤I/Ip≤30)

Reset time

Definite-time: ≤1%×setting or 30ms Inverse-time: ≤5% of the calculated value or 30ms

2.11.4 Earth Fault Overcurrent Protection (50/51G)

Tolerance of current setting ≤1%×setting or 0.01×In, whichever is greater

Tolerance of pickup time ≤25ms (at 2 times current setting)

Tolerance of operating time

Definite-time: ≤1%×setting or 25ms (at 2 times current setting)

Inverse-time: ≤ 5 ％ of the calculated value + 1 ％ of current tolerance, or 35ms（1.2≤I/Ip≤30）

Reset time

Definite-time: ≤1%×setting or 30ms Inverse-time: ≤5% of the calculated value or 30ms

3

3.5.2 I/O Signals 3-59 3.5.3 Settings 3-59

List of Figures

Figure 3.1-1 Current compensation calculation process 3-7 Figure 3.1-2 Operation characteristic of sensitive biased differential element 3-9 Figure 3.1-3 Operation characteristic of conventional biased differential element 3-10 Figure 3.1-4 Operation characteristic of biased/instantaneous differential elements 3-11 Figure 3.1-5 Operation characteristic of DPFC biased differential protection 3-14 Figure 3.1-6 Schematic of waveform symmetry principle 3-16 Figure 3.1-7 Logic of inrush current blocking current differential protection 3-19 Figure 3.1-8 Logic of enabling transformer differential protection 3-25 Figure 3.1-9 Pickup logic of transformer differential protection 3-25 Figure 3.1-10 Logic diagram of instantaneous differential element 3-25 Figure 3.1-11 Logic diagram of biased differential element 3-26 Figure 3.1-12 Logic diagram of DPFC biased differential element 3-26 Figure 3.1-13 Logic diagram of CT circuit failure alarm 3-26 Figure 3.2-1 Application for two-winding transformer with one CB at one side 3-29 Figure 3.2-2 Application for two-winding transformer with two CBs at one side 3-29 Figure 3.2-3 Application for auto-transformer 3-29 Figure 3.2-4 REF principle 3-32 Figure 3.2-5 Operation characteristic of REF 3-33 Figure 3.2-6 Logic of enabling restricted earth fault protection 3-36 Figure 3.2-7 Pickup logic of restricted earth fault protection 3-36 Figure 3.2-8 Logic diagram of restricted earth fault protection 3-36 Figure 3.3-1 Logic of enabling phase overcurrent protection 3-38 Figure 3.3-2 Pickup logic of phase overcurrent protection 3-38 Figure 3.3-3 Logic of harmonic control element 3-39 Figure 3.3-4 Definite-time operating curve of phase overcurrent protection 3-40 Figure 3.3-5 Inverse-time operating curve of phase overcurrent protection 3-41