protections catalogue

Copyright © 2010 ComAp s.r.o. Written by Tomas Jelen Prague, Czech Republic

ComAp, spol. s r.o. Kundratka 2359/17, 180 00 Praha 8, Czech Republic Tel: +420 246 012 111, Fax: +420 246 316 647 E-mail: [email protected], www.comap.cz

ComAp Protections Catalogue

December 2010

ComAp

Protections catalogue

Table of contents Table of contents................................................................................................................................. 2 Introduction.......................................................................................................................................... 3 Overvoltage, Undervoltage (ANSI 27, 59) .......................................................................................... 3 Voltage unbalance - amplitude asymmetry (ANSI 47) ........................................................................ 3 Voltage synchronous components evaluation .................................................................................... 4 Positive sequence undervoltage (ANSI 47) ........................................................................................ 4 Negative sequence overvoltage (ANSI 47)......................................................................................... 4 Overfrequency, underfrequency (ANSI 81H, 81L) .............................................................................. 5 "Loss of mains" protections................................................................................................................. 5 Vector shift (ANSI 78) ......................................................................................................................... 5 Rate Of Change Of Frequency (df/dt, ROCOF, ANSI 81R)................................................................ 6 Definite-time overcurrent (ANSI 50).................................................................................................... 7 IDMT overcurrent (ANSI 51) ............................................................................................................... 7 Time overcurrent with voltage control (ANSI 51V).............................................................................. 8 Neutral overcurrent (ANSI 50N, ANSI 51N)........................................................................................ 9 Directional overcurrent (DOC, ANSI 67) ........................................................................................... 10 Reverse power (ANSI 32) ................................................................................................................. 11 Neutral Voltage Displacement (NVD, ANSI 59N) ............................................................................. 11 Synchro check (ANSI 25).................................................................................................................. 11 AC reclosing relay (ANSI 79) ............................................................................................................ 12 Breaker failure................................................................................................................................... 12

Protection catalogue, ©ComAp – December 2010 2

Introduction Generally, there are different systems to be protected. In ComAp controllers, three basic systems are considered:

- protection of the engine - evaluation of water temperature, oil pressure and temperature etc.

- generator protections - mains protections

This document describes protection functionalities used in ComAp mains protections, their explanation and purpose from the application point of view. The most frequent application of the protection relays provided by ComAp is described as "Mains decoupling" (UK terminoogy) or "inter-tie" (US terminlogy), though, in some cases, the same units can be also used as protections of generator or even a different devices, like transformers or motors. Mains-decoupling protections are located at the point of connection of a device generating electrical energy ("generator") and the mains, e.g. public grid operated by a local power utility, also referred to as DNO (Distribution Network Operator). The main task of mains-decupling relay is to disconnect the generator from the grid in case of unacceptable conditions, caused either by a failure in the grid or by the generator itself. Thus, the mains is kept in a controllable state and the property of the DNO, his customers connected to the public mains as well as the property of the generator owner is protected.

The protective functions described in this document are sometimes referred to as an "ANSI codes", i.e. numerical indications of the protective functions and devices. These are based upon the ANSI /IEEE Standard C37.2.

Overvoltage, Undervoltage (ANSI 27, 59) These are the essential protections, based on measurement of the voltage RMS value. These protective functions provide basic criteria whether the protected system is healthy or not. In case of mains protections, the out-of-limits voltage shows mains fail. The failure modes may be many, from short-circuits in vicinity of the installation point through power fluctuations to large failures of the complete grid areas. Often, the voltage setting is done in 2 levels, with "softer" over/under voltage limits and longer delays in the first step, and stricter limits with shorter delay reaching down to 100 ms or even faster reaction times. The 2-level setting allows for so called "Fault ride-through" of the grid part in case of temporary mains fail. Typical setting is shown e.g. in G59/2 standard (see below).

Voltage unbalance - amplitude asymmetry (ANSI 47) Generally, protection of voltage asymmetry is requested as a supplementary protection to the over- and undervoltage protections. In most of ComAp products, voltage amplitude unbalance is evaluated,


http://en.wikipedia.org/wiki/ANSI_Device_Numbers

http://en.wikipedia.org/wiki/ANSI_Device_Numbers

http://en.wikipedia.org/wiki/Low_voltage_ride_through

which is sensitive to drop of voltage in one or 2 phases. This may occur e.g. in case of single-phase or double-phase short circuit in the vicinity of the generator. As such, it is considered as a mains failure and the DNOs usually give the requested limit and delay values to trip the generator by the voltage unbalance protection. The unbalance function is not sensitive for angle asymmetry. If such requirement is given, some more sophisticated methods have to be used, as described below.

Voltage synchronous components evaluation Generally, by "synchronous components analysis" it is understood a mathematical vector-breakdown of the measured voltage of generally any quality of asymmetry into two perfectly symmetrical components - positive sequence voltage (U1) and negative sequence voltage (U2) and one conphase component - zero voltage (U0):

Any three-phase voltage system may be broken-down to those three components. By expressing solely the phasor magnitude of positive, negative or zero sequence respectively, significant phenomena on the measured voltage may be described:

- In a perfectly symmetrical system, the measured voltage is exactly the same magnitude as the positive sequence voltage. Negative and zero sequence are not present

- With growing asymmetry, either in angle or amplitude of some of the phasors, the positive sequence voltage magnitude decreases and the negative sequence voltage increases.

- With moving the zero point of the measured system away from the geometrical centre of the system, the zero sequence increases, indicating shift in the zero point of the measured system. This zero sequence voltage shift is however different from the Neutral Voltage Displacement method, which is generally used for detecting the ground faults in isolated or partially earthed systems.

Positive sequence undervoltage (ANSI 47) Detecting the decrease of the positive sequence voltage is sometimes used as a combined method for detecting undervoltage and unbalance in the measured system. Denmark is one of the countries, where the DNOs require positive sequence undervoltage instead of second (fast) stage undervoltage. The requirement is to detect drop of the positive sequence voltage under 60-70% of the nominal value of the measured voltage. This way, the system is protected from large voltage drop as well as severe asymmetry, what is a perfect combination for the second-stage voltage protection. Trip time around 50-60 ms is a typical requirement in this case.

Negative sequence overvoltage (ANSI 47) The negative sequence voltage component represents system with opposite phasor rotation then the measured voltage. Applying voltage with content of negative sequence voltage on rotating machinery such as motors or generators induces a "parasite" magnetic field, rotating in the opposite direction then the rotation of the shaft. This may cause mechanical pulses and thus damages of shafts or


mechanical parts of the machines. Content of negative sequence voltage, i.e. asymmetry, in the grid is observed as one of the quality parameters and its increase over given limit is considered as mains failure. Due to its nature and effect on mechanical rotating parts, this method is also used as generator or motor protection. Usual threshold for this type of protection is cca 20% content of negative sequence voltage in the measured signal. Another phenomenon, where symmetrical components are used, is detecting the loss of one fuse in the grid at zero load from the generator. Sometimes, the following test is performed by the DNOs during commissioning: at the non-loaded state of the generator a fuse is pulled on the distribution transformer, connecting the generator to the grid. Given that the current flowing from the generator is zero and the voltage measurement is connected at the generator side of he pulled fuse, this should not cause any decrease of voltage in the affected phase, so none of the over/under voltage or amplitude unbalance would be capable of detecting this failure. What changes in such case, are conditions behind the pulled fuse, which are mostly given by the transformer winding inductiveness and capacity. This change results in angle shift of the affected phase, which can be effectively detected by positive or negative sequence evaluation or their ratio: U2/U1.

Overfrequency, underfrequency (ANSI 81H, 81L) Frequency is a global parameter, showing overall quality of the mains voltage on a large scale, e.g. complete electrical system. In fault-free conditions, frequency does not vary, unless a severe failure of a grid area occurs, causing breakdown of the electricity system in that area. Hence, the frequency shift is a good method to indicate mains failure. Setting of the frequency is sometimes requested in two stages, similar to the over and under voltage, to allow "Fault ride-through" of the grid part in case of temporary mains fail. Typical setting is shown e.g. in G59/2 standard (see below). Specific cases of the unintentional islanding are the automatic reclosing and situations where the imbalance between power production and consumption in the islanded area is very big. Consequences of such situations are usually very fast and could not be safely protected only by frequency and voltage protections. For this purpose, a specific type of very fast "Loss of mains" protections, is applied.

"Loss of mains" protections Specific cases of the unintentional islanding are the automatic reclosing and situations where the imbalance between power production and consumption is very large. These situations usually require very fast solution and could not be safely protected only by frequency and voltage protections. In case of auto-reclosing sequence within the mains, the connected generators can get out of synchronism during the reclosing period. Other risk of damage can occur to the generator, by step-loading it by the complete load of the islanded area above its design capacity. For these purposes, a specific type of fast protections is provided. In UK, it is generally referred to as "Loss of Mains" (LOM) protections. The requested LOM protections are Vector shift and/or ROCOF, which is mostly requested in the UK, in other European countries, Vector shift is requested more often. With increasing number of distributed generation installations, many of them being inverter-based devices (solar plants, wind turbines), the requirements to the protective relays are sustainably growing, asking for more and more strict and sophisticated methods to detect the islanding situations, like e.g. active anti-islanding methods.

Vector shift (ANSI 78) Vector shift is one of the LOM protections. It provides very fast detection of mains failure (in tens of ms), based on the principle of shift of the synchronous generator displacement angle. The displacement angle is an angle between magnetic field of the rotor and the rotating magnetic field of the stator winding and relates strongly to the load of the generator. In case that this load changes, the displacement angle immediately "jumps". Compared to the frequency change, which probably also occurs, this jump is an immediate phenomena and is detected as a shift of the measured voltage sine curve - Vector shift or Vector jump. It allows almost immediate disconnection of very fast failures and thus prevention of severe damages which could not be prevented within the delay of frequency or voltage protections.


http://en.wikipedia.org/wiki/Low_voltage_ride_through

Vector shift reaction times are usually requested up to 30ms. Typical setting is shown e.g. in G59/2 standard (see below).

Rate Of Change Of Frequency (df/dt, ROCOF, ANSI 81R) ROCOF is second most frequently requested method of LOM detection. In principle, the method uses similar evaluation method like Vector shift, but the physical phenomena detected is different. It calculates the change of speed of the generator, caused by sudden change of its load, together with unintentional loss of mains, which is normally capable of keeping the frequency on a stable level. The frequency change is expressed as tangent in time [Hz/s]:


ROCOF is a fast protection, similar to Vector shift, however, unlike Vector shift, the calculation requires a certain time for evaluation. In ComAp protection relays, the evaluation time for ROCOF protection can be adjusted in number of sine curve cycles that are taken into evaluation from 1 to tens of voltage cycles (ROCOF filter). This allows setting up the ideal ratio between the evaluation speed and the protection sensitivity. Typical setting of the abovementioned protections is shown e.g. in G59/2 standard:

Definite-time overcurrent (ANSI 50) Overcurrent protections are generally considered as basic protections of any electrical device. In the applications of generator parallel operation, the overcurrent protections are used to protect either the generator itself or the grid equipment from damage by short-circuit phenomena. The mechanism of damage is given by thermal effects of the current flowing through the protected device (generator, transformer, circuit breaker, ...). Basically, the thermal failure needs certain time to develop (time needed for the heat to grow inside of the conductors and propagate through the device), which is the base for the IDMT overcurrent protection. However, for currents overreaching certain critical value, the damage could develop very fast with no control, therefore immediate tripping, i.e. tripping with "definite time" is requested.

IDMT overcurrent (ANSI 51) Because the thermal failure of devices in the electrical system needs certain time to develop, it is not always necessary to trip the overcurrent immediately. The duration, for which the device may withstand exposure to a certain current, has indirect relation to the current magnitude. This allows coordination of the tripping times within the mains in setting the time-dependant (IDMT = inverse definite minimum time) overcurrent characteristics. The IDMT overcurrent protection is usually implemented together with definite-time overcurrent to provide complex current protection. Calculation of the trip delay is done according to the trip curve:


The delay, i.e. the IDMT curve shape is prescribed by the DNO, based on the calculations of the mains parameters in the point of the generator connection. The mains protections are coordinated and allow isolating the faults by the protections which are closer to the short-circuit point and the current, flowing through them is the highest. This way, disconnection point is controlled, allowing the generator to withstand distant faults and contribute to recover the mains voltage after the fault is isolated. In ComAp products, the IDMT curve shape is given by the following formula:

Sometimes, it is possible to meet with requirements for different IDMT curve shapes, according to IEC standards. The most common are "normal", "inverse" and "very inverse" curves.

Time overcurrent with voltage control (ANSI 51V) ANSI 51V is a modification of the IDMT overcurrent protection. It uses the same mechanism of the delay calculation according to the IDMT trip curve, but the delay is further adjusted according to the measured voltage:


This protection functionality uses the fact that voltage in the point of short-circuit drops to zero or very low values and with growing distance from the fault, it increases up to the mains nominal voltage value. Thus, increased sensitivity to the short-circuits localization nad protection coordination is provided. ANSI 51V is requested e.g. by some DNO in Northern America. In ComAp products it is available in InteliPro.

Neutral overcurrent (ANSI 50N, ANSI 51N) Earth-fault relay measures and evaluates the current, flowing through the ground wire. This current corresponds to the vector-summation of all 3 phase-currents and in fault-free state is zero. In case that ground fault in the system occurs, the summation is not equaled to zero, because some part of the fault current flows through the earth-fault path and not through the phase wires. The earth-fault relay is used to indicate mains failure and trips the generator from the mains in order to avoid its contribution to the fault. It also provides effective protection of the internal faults in the generator. The trip is either provided with definite time delay for higher values of the measured earth-fault current or with delay given by the IDMT curve, similar to the ANSI 51 protection, allowing protection coordination in the given area. Depending on grounding method of a particular system, different connection schemes and current limits are applied. The typical connections of earth fault protection in the ComAp products are as follows:

G

I1k I1l I3k I3lI2k I2l

K Lk l

L1L2L3

N

Ink Inl

K Lk l K L

k l

L K

l k

The simplest arrangement covers all zones from the generator windings to the final circuits in the load network.

GL1L2L3

N

Ink Inl

k lk lk lk l

This arrangement covers earth faults in the load network only.

GL1L2L3

N

Ink Inl

L K

l k

k lk lk lk l

K L

This arrangement necessary for restricted earth fault protection. The location of the neutral earthing point in relation to the protection current transformers in the neutral conductor determines whether four or five current transformers are employed.

GL1L2L3

N

Ink Inl

k lk lk lk l

This arrangement necessary for restricted earth fault protection. The location of the neutral earthing point in relation to the protection current transformers in the neutral conductor determines whether four or five current transformers are employed.


http://ecmweb.com/power_quality/electric_ground/

Directional overcurrent (DOC, ANSI 67) Directionally oriented protections, DOC being one of them, are sensitive for location of the fault with relation to the measurement point. This way, directional protection is capable to detect whether the fault happened "in front of it" or "behind", having it "look" in the direction of the mains connection or to the generator.

Directional overcurrent protection, applied in the point of generator connection to mains, is considered a loss of mains protection, however it does not substitute the traditional loss of mains protections like Vector shift or ROCOF. The typical application is a generator with its own load consumption (e.g. peak shaving, soft transfer stand-by, or other applications). The generator is usually used to support the local consumption with no export to the mains. In case of mains transition into an island mode, the generator, running in parallel with the islanded area, starts to supply its consumption, generating current in opposite direction. DOC protection relay is used to avoid this situation, and trip the generator from the islanded mains, combining the overcurrent protection together with its directional character. Compared to the "reverse power protection", DOC protection detects also reactive currents within the given angle, what increases its sensitivity for tripping if reactive components are present in the islanded area (transformers, capacitors, cables, compensators, ...).

90°

-90°

0°

DOC threshold

Trip area

150°

Base angle -135°

-60°


DOC protection is requested under certain conditions as a mains decoupling protection e.g. in UK or Ireland as a part of G10 engineering recommendation to the Distribution Code.

Reverse power (ANSI 32) Application of reverse power protection is very wide. In the mains-decoupling applications, mostly the "mains reverse power" is used. The protection assures that the active power export at the point of mains-connection will not overreach the preset limit. The protective functionality is very similar to the Directional overcurrent, though it is not sensitive to reactive currents, where the content of active power is below the limit. The DNO may request the reverse power protection to assure that the generator is tripped in the situation that, for any reason, starts to export energy. Very common application of reverse power is protection of the generator mechanical drive from motoric operation, which is an undesirable and often dangerous mode for some types of mechanical drives.

Neutral Voltage Displacement (NVD, ANSI 59N) NVD protective function is used in medium or high voltage systems with isolated or indirectly grounded zero-point. Under healthy conditions, the sum of the three phase-to-earth voltages balances to zero. When an earth-fault occurs, it does not cause a short-circuit, because of the isolation of the system zero point. However, the fault provides connection of one phase with earth what represents hazard for the system safety: ground-fault in any other phase would then mean a phase-to phase short-circuit, phase-to-ground voltage of the other phases may fluctuate to multiples of its nominal, providing increased stress to the isolation system, etc. One of the effects of the fault is a rise in the geometrical centre of the system, so called neutral voltage displacement. This neutral voltage displacement is measured in a specific "open-delta" connection of voltage transformers (see the picture below). In some cases, it is required to clear the fault by tripping the appropriate circuit breaker. However, the NVD protection itself does not allow detecting the location of a fault and because the earth-fault is not a severe failure, the system may stay connected until the fault is located by some other method and NVD is used for an earth-fault alarm only.

NVD is not considered as mains-decupling protection, though it may be required on larger machines or HV alternators connection to mains. In ComAp, it is offered in IP-C, D and E and in InteliPro with possibility of setting whether the NVD it contributes to the common trip of the relay or uses its own separate output for signalization of the NVD alarm.

Synchro check (ANSI 25) This function checks state of synchronism at both sides of the circuit breaker. In mains-decoupling applications, it is requested by some DNOs as a supplementary function to the AC reclosing functionality to assure that the synchronous conditions are met before the circuit breaker is automatically closed by the protection relay, or unblocked to be closed by some other device. In



InteliPro, L2-L3 phase-to-phase voltage is measured on the generator side of the circuit breaker and compared with the corresponding measured mains voltage. Synchronous conditions are evaluated based on the preset window of voltage, frequency and angle match.

AC reclosing relay (ANSI 79) The automatic reclosing mechanism is generally used in situation, where temporary nature of the failure is expected. The protection relay trips the breaker according to the standard protection settings. In case that the breaker As soon as the trip conditions disappear, i.e. the protection senses fault-free situation, the tripping output deactivates and after a reclosing delay, the reclosing output issues a signal to automatically reclose the breaker. There may be several reclosure attempts done in case of unsuccessful reclosure cycle. The reclosing function is typical for feeder protection, where the fault can be expected at the feeder and its disconnection from the mains may result in successful clearance of the fault, making the reclosing cycle successful. It is also requested by some DNOs in mains-protection applications.

Breaker failure Breaker failure is a supplementary protective function assuring that the circuit breaker, tripped by any other protective function is securely open. This is evaluated based on the CB feedback. The function issues a separate output, providing a backup trip which either triggers a backup mechanism of the same circuit breaker, or trips another back-up breaker.

protections catalogue

Documents

neutral conductor

idmt curve

neutral voltage

temporary

negative sequence

reverse power

neutral earthing

positive sequence