Relays--Universal microprocessor overcurrent; Type testing [Industrial Electrical Power Systems]

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Universal microprocessor overcurrent relay

Electromechanical relays are designed specifically for particular protection applications and they usually have a limited setting range. For example, a different relay is necessary when a 'very inverse' characteristic is required or if a setting is required that is outside the range of the standard relay. This means that at the time when an electric power system is being designed and specified, considerable thought must be given to both the type of protection characteristic that will be required and the likely setting of the relay to ensure that the correct relay is specified.

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The concept of many modern microprocessor relays is to provide a protection relay that covers all likely protection requirements in one relay.

This includes wide setting ranges and, in addition, several selectable characteristics and options to cover many protection applications. Microprocessor overcurrent relays are typically selectable for definite time, normal inverse, very inverse, extremely inverse, longtime inverse and sometimes a thermal characteristic as well to cover all likely application requirements. In addition, several output options are often provided to enable the user to select, For example, whether he requires an overcurrent 'starting' output contact or not. From a user's point of view, this delay in decision characteristic and setting range is required to the time of commissioning.

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The concept of a universal relay tends to improve the availability of protection relays from the manufacturers by making them 'stock' item. From a manufacturing point of view, this minimizes the number of relay types that have to be manufactured and held in stock and allows to provide a faster and better service to the users of protection relays.

This also tends to reduce the cost of protection relays by reducing the number of variations.

+--+ summarizes the available characteristics and setting ranges of a modern microprocessor overcurrent relay in comparison to a typical induction disk IDMT overcurrent relay.


Static (Digital) Electromechanical Characteristics Selectable Separate relay

Definite time or--Definite time and Normal inverse or Normal inverse and Very inverse or Very inverse and Extremely inverse or Extremely inverse and Long time inverse or Long time inverse Current inputs 1 A and 5 A 1 A or 5 A Thermal current withstand Continuous : 3 A/15 A 2 × setting current For 10 s : 25 A/100 A - For 3 s : - 20 A/100 A For 1 s : 100 A/300 A -

Overcurrent setting Continuous Plug setting 50-500% 50-200% in 7 steps

Ground fault setting Continuous Plug setting

10-80% 10-40% in 7 steps or 20-80% in 7 steps

Time multiplier Continuous; Continuous 0.05-1.0 0.1-1.0

High-set overcurrent Included Extra add-on 0.5-40 times

High-set time delay Included Extra add-on 0.05-300 s

+--+ Comparison of microprocessor vs electromechanical relays


Technical features of a modern microprocessor relay

Current transformer burden

One of the disadvantages of the IDMT relays of the induction disk type is that they have relatively high CT burdens when compared to static IDMT relays. The ohmic value of these burdens varies with the setting. As the setting is reduced, the burden on the CT is increased. Induction disk relays have a burden typically specified as 3 VA. Modern static relays, on the other hand, have a very low burden of less than 0.02 ? for 5 A input and 0.10 ? for 1 A input, which is independent of the setting. The table below shows the calculated ohmic burden of a 1 A induction relay at the various settings compared to a microprocessor overcurrent relay. (Also refer to the beginning of this section.) The main consequence of the high burden is the poor performance of the CT/relay combination under high fault current conditions, particularly when low CT ratios are used. The high burdens can affect the actual primary setting achieved by the CT/relay combination. The example below shows that, with an electromechanical relay, the actual primary setting increases even though the plug setting is reduced on the relay.

Setting (%) Induction Disk Relay Burden (?) Microprocessor Relay Burden (?)

+--+ Comparison between the CT burdens in ? of equivalent 1 A relays of the induction disk and microprocessor types

With the static relay, almost any primary setting is possible. This means that on a distribution network using static relays, relay coordination is still possible at high fault levels even for a very low relay current setting and low CT ratios.

Accuracy of settings

The current and time-multiplier settings on a microprocessor relay are done with the aid of a digital display, which is part of the measuring unit. The accuracy and repeatability of the settings on this type of relay is far greater than that for electromechanical relays.

Setting accuracys of ±1% and operating accuracys of ±3% of set value for the static relay compare very favorable with the ±7.5% accuracy of the electromechanical device. The accuracy of the electromechanical relay is also dependent on the frequency, and the presence of harmonics further affects accuracy.

Relay tap (A) Effective primary setting (pu)--Actual setting--Theoretical setting

++++ Primary to relay tap setting

This greater accuracy and repeatability of the static relay, generally independent of harmonics, combined with negligible 'overshoot' means that reduced grading intervals are now possible, especially when these relays are used in combination with the faster operating SF6 and vacuum switchgear. This is clear when one recalls that the grading times are dependent on the following:

• Errors in CTs

• Errors in the relay operating time

• Relay 'overshoot' time

• Circuit breaker operating time

• Safety margin.

It’s practical to consider grading intervals of as low as 0.2 s when using microprocessor relays in combination with SF6 or vacuum breakers as compared to 0.4 ~ 0.5 s needed with electromechanical ones.

Reset times

Electromechanical IDMT relays have reset times of up to 10 s at time multiplier settings = 1, which means that during auto-reclose sequences an integration effect can take place and coordination can be lost. This situation can occur when the disk has turned some distance in response to a fault in the network cleared possibly by some other breaker with an auto-reclose feature. If the fault is still present when the breaker recloses and if the disk has not fully returned to its reset position, the relay would take less time than calculated to trip.

Uncoordinated tripping is then possible. The reset times of static relays are negligible.

Starting characteristics

An IDMT relay of the induction disk type is an electromechanical device, which includes mechanical parts such as a disk, bearings, springs, contacts, etc., which are subject to some mechanical inertia. When the current exceeds the setting, the disk only starts to move somewhere between 103 and 110% of the setting and closes for currents between 115 and 120% of the setting.

Static relays have a definite pick-up point within 5% of the current setting and this initiates the timing characteristics. The pick-up usually accompanied by an LED indication, makes it easy to check the accuracy of the current setting during testing of the relay.

On some static relays, this 'start' signal is available on a separate pair of output contacts, which can be used for indication or to initiate a simple busbar protection scheme. This type of busbar protection when used on metal clad MC switchgear is superior to frame leakage protection because it covers both phase-faults in the switchgear and avoids the necessity of insulating the switchgear and cable glands from ground. The principle of this type of protection is illustrated.


++++ Busbar protection scheme using starting contact of the static overcurrent relays I1>, I2>, I3> =Outgoing feeder overcurrent relays

= Busbar incomer overcurrent relay

= Blocking relay

= Blocking timer

=Master trip relay (incomer)

++++ Busbar protection scheme using starting contact of the static overcurrent relays

Dual setting banks

Some digital relays are now designed to provide a dual settings bank, which provides a complete duplication of all the settings and operating switch positions. Setting 1 or setting 2 can be selected at the relay, via the serial communications system or a remote switch, which can be an output contact of another relay or a circuit breaker auxiliary switch.

In many instances, when setting relays, such as the example shown, we are faced with having to set a relay for the lowest or an average value of two possible settings. Now we can have both settings, calculated exactly, and switch from setting 1 to setting 2 at will.

This dual setting bank can also be useful in a ring main circuit, which can be opened at different places, necessitating differing settings when a relay can be in two different places in two radial feeders.

Open ++++ Open ring protection

++++ Parallel feeder protection

High-set instantaneous overcurrent element

In microprocessor overcurrent and ground fault relays, a high-set overcurrent element is provided as a standard feature and often has a timer associated with it to provide a time delay. If not required, it can be set to be 'out-of-service'. Because of the measurement method, the transient overreach is very low and the instantaneous overcurrent setting can be set much closer to the maximum fault current for a fault at the remote end of the feeder. The transient overreach is the tendency of the relay to respond to the DC-offset, which is commonly present in most fault current waveforms. To avoid this problem on electromechanical relays, the setting of the high-set element has to be at least twice the calculated maximum fault current, making the protection less effective.

High-set overcurrent protection is particularly useful on the higher voltage side of a transformer, where it provides fast protection for most faults on the HV side while the time-delayed overcurrent relay provides protection for the faults on the lower-voltage side of the transformer.

Breaker fail protection

Modern microprocessor relays are now provided with breaker fail protection. When the main trip contact of the relay signals a trip to the circuit breaker and if after pre-set delays (say 150 ms) current is still flowing through the relay this indicates that the circuit breaker has not opened. The relay with a second trip contact sends another signal to a second trip coil in the same breaker or a second breaker.

Digital display

Most of the present day microprocessor relays are now provided with alpha-numerical data display. The display is used for the following:

  • • Accurate relay settings: The settings are adjusted by means of potentiometers or the software, but the actual value of the setting is accurately displayed on the display window.
  • • Measured values: Information such as the measured values of the various parameters can be displayed in a cyclic order by selecting the sequence of display or by default.
  • • Memorized fault information: When the replay operates for a fault, the values of the measured parameters and times are stored in memory. This information can later be recalled to assist in the analysis of the cause of the fault.
  • • Indications and status information: Other functions and information such as the number of starts, blocking information, etc. can be displayed for motor control applications.

Memorized fault information

Microprocessor relays, which provide memorized fault information, have been available for some years but this information had been initially limited to the maximum value of the measured current or the most recent fault. The advancement in digital technology nowadays enable much more comprehensive fault analysis with up to quite a number of memorized values of the three-phase currents, zero sequence current, maximum demand current (15 min) duration of the start of the low-set and high-set overcurrent. There are possibilities to have these data stored in hard discs connected on a continuous manner for later retrieval.


Relay Duration of event (n), starting of stage l0>>; 9 Latest recorded, event (n), current on phase L1 Latest recorded, event (n), current on phase L2 Latest recorded, event (n), current on phase L3 Maximum demand current value for 15 min Duration of event (n), starting of stage l>

Duration of event (n), starting of stage l>>

Duration of event (n), starting of stage l0>

Latest recorded, event (n), neutral current l

Event (n-1), current on phase L1 Event (n-1), current on phase L2 Event (n-1), current on phase L3 Duration of event (n-1), starting of stage l>

Duration of event (n-1), starting of stage l>>

Duration of event (n-1), starting of stage l0>

Event (n-1), current on phase L1 Duration of event (n-1), starting of stage l

Event (n-2), current on phase L1 Event (n-2), current on phase L2 Event (n-2), current on phase L3 Duration of event (n-2), starting of stage l>

Duration of event (n-2), starting of stage l>>

Duration of event (n-2), starting of stage l0>

Duration of event (n-2), starting of stage l0>>

Event (n-2), current on phase L1

Event (n-3), current on phase L1 Event (n-3), current on phase L2 Event (n-3), current on phase L3 Duration of event (n-3), starting of stage l>

Duration of event (n-3), starting of stage l>>

Duration of event (n-3), starting of stage l 0>

Duration of event (n-3), starting of stage l 0>>

Event (n-3), current on phase L1

Event (n-4), current on phase L1 Event (n-4), current on phase L2 Event (n-4), current on phase L3 Duration of event (n-4), starting of stage l>

Duration of event (n-4), starting of stage l>>

Duration of event (n-4), starting of stage l >

Duration of event (n-4), starting of stage l0>>

Event (n-4), current on phase L1

Nbr of l> stage starts since the latest reset Nbr of l>> stage starts since the latest reset Nbr of l0> stage starts since the latest reset Nbr of l0> stage starts since the latest reset

++++ Example of memorized fault information


Auxiliary power requirements

Electromechanical IDMT overcurrent relays don’t require an external source of auxiliary power to operate the relay. They take their power requirements from the CT and this is the main reason for their high burden mentioned earlier. However, this 'zero battery drain' during quiescent conditions has allowed municipal engineers to fit tripping batteries and chargers of limited capacity at small stations for tripping purposes only.

With the introduction of static relays, which require an auxiliary power supply to drive the electronic circuits and the output relays, users were reluctant to change these small battery and charger arrangements to accommodate additional power requirements. This is not normally a problem in larger stations because the station battery usually has sufficient capacity for the relay auxiliary supply, typically at voltages of 30 V DC, 110 V DC or 220 V DC. Some manufacturers overcame this problem by building a CT power supply card as an extra option. However, this tends to defeat one of the main advantages of static relays, which is their low CT burden.

Consequently, in microprocessor relays a lot of effort has been made to reduce the auxiliary supply requirements as much as possible by using circuit techniques, such as CMOS, which requires very little power. Auxiliary power requirements of 3 W and lower can be achieved depending on the type of relay.

To simplify matters further, universal power supplies for relays have been developed to operate over a wide voltage range and cover several 'standard' voltages. For example an 80-265 V universal power supply is suitable for 110 V DC or 220 V DC station batteries and will operate right down to 80 V. This type of power supply is independent of polarity and can be supplied from AC or DC. It uses a pulse width modulation (PWM) technique, which is self-regulating, short circuit and overload protected. It’s also protected against ripple and transients in the auxiliary supply voltage. In practice, battery voltages in a substation can vary over a wide range. During the 'boost' charging cycle the voltage can be up to 30% higher than normal and has often been the cause of power supply overheating in early day static relays. Conversely, during low charge situations the voltage can fall as low as 80% of nominal. The universal power supply can easily accommodate these wide fluctuations without any additional heating or loss of performance. A block diagram is shown.

Regulated output:

Input 80-265 V AC or DC Feedback--Smoothing capacitor—Rectifier--Isolation transformer--Thyristor Rectifier--Smoothing capacitor--Pulse width regulator

++++ Block diagram of pulse width modulated (PWM) self-regulating power supply

Some of the benefits of PWM self-regulated universal power supply units are as follows:

  • • The same relay can be used in several applications for a wide range of battery voltages resulting in one power supply for all standard battery voltages from 30 to 220 V.
  • • Battery fluctuations due to the charger don’t affect the replay performance.
  • • Low battery voltage, within reasonable limits, does not affect the relay performance.

In small stations where no station battery is available or economic, auxiliary supply can be arranged from a capacitor storage unit fed from both the CT and the PTs. This unit will provide auxiliary power to the relay even when no current is flowing in the primary circuit. Because of the relays, universal power supply, fluctuations of voltage due to the variations in the supply don’t affect relay performance. A typical connection of a capacitor storage unit supplying a relay using a PWM self-regulating power supply is shown. The capacitor storage unit also provides the energy to trip the circuit breaker where there is no tripping battery.

Auxiliary voltage for the protective relay-- Tripping contact of the protective relay.

++++ Capacitor storage unit is supplied from both the CTs and a CT to provide auxiliary power for the relay and for tripping the circuit breaker

Flexible selection of output relay configuration

With the help of six output relays (two heavy duty and three medium duty) and a completely flexible software-switching program, we can choose to have any function to operate any combination of output relays - including the various 'start' operations.

Signal diagram--Remote settings--Relay reset; SGB/5 SGB/6 RESET TRIP

++++ Flexible selection of output relay configurations

Type testing of static relays

Type tests

To ensure that static protection relays of all types comply with reasonable requirements and are suitable for applications to power system networks, many national standard organizations such as BEAMA, ANSI, SEN, etc. introduced stringent testing requirements for static relays. These requirements are now included in the international recommendations by IEC. The following electrical type tests are normally applied by manufacturers to ensure that relays comply with the requirements of IEC 255:

• Insulation test voltage: 2 kV, 50 Hz, 1 min IEC 255-5

• Impulse test voltage: 5 kV, 1.2/50 micro s, 0.5 J IEC 255-5

• High-frequency interface test: 2.5 kV, 1 MHz 255-6

• Spark interference test: 4-8 kV SS 436 15 03.


Perhaps the most important feature introduced by microprocessor relays is that of continuous self-supervision. One of the classical problems of the older protection relays lies in the absence of any ready means to identify a fault in the relay. As protective relays are, for most of their lives, in a quiescent state, regular secondary injection tests are necessary to prove that the relays are operational.

The microprocessor relays, on the other hand, utilize their capacity during quiescent periods to continuously monitor their circuits and will provide an alarm if a failure occurs. The digital readout can be used to diagnose the problem. This enhances protection system reliability on a continuous basis and intervals between manual inspections can be prolonged.

Digital devices tend to work either 100% or not at all. Consequently, it’s very easy to check a microprocessor relay on a regular basis and achieve a very high certainty that the relay is operational. By pressing the button requesting the display of the phase current, a reading that matches the ammeter on the panel confirms the following:

  • The CTs are healthy
  • The wiring from the CT to the relay is okay
  • The relay is working

If necessary, a trip-test can be done from the relay to ensure that the relay output trip contacts are working and that the breaker trip coil and mechanism are okay.

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Friday, November 10, 2023 12:43