Protection History - Static Protection Devices

Author: Walter Schossig, Germany

Protection History ? Static Protection Devices

When semiconductors became available the second generation of protection devices came out in the 1960s. With the known protection criteria and characteristics of electromechnical relays the mechanical components have been replaced by their static equivalents. Additional new measurement methods and characteristics have been introduced. Known issues of electromechanical relays (contact contamination, problems in mountings, impact of arc resistance...) could be solved and the characteristics could be adapted to the protection object easily.

Fully electronic devices were launched in 1960 for the first time. The first relays had problems with electromagnetic compatibility. As other disadvantages the need for additional trainings of the engineers as well as the huge effort for spares inventory have been claimed. Starting with the 1970s selective protection was available in electromechanical as well as static realization.

The measurement systems at this time have been electromechanical – soft iron, moving coil, bi-metal and electrodynamical measurement elements. In the English speaking countries Ferraris measurement elements have been quite popular. Additional passive elements adapted the measurement systems to the different tasks. Real electronic elements as diodes have been uncommon and electronic amplifiers have been avoided. As we all know the acceptance of electronics in substations dramatically increased since then. In the beginning this was quite different - what to do with electronics?

Protective relays monitor the current, voltage, frequency, or any other type of electric power measurement for the purpose of triggering a circuit breaker to open in the event of an abnormal condition. The term "trigger" was taken from fire arms. Electronic triggers had no problems with contacts or erosion and could be powered with low voltage.

Recognized Advantages
The advantages of electronics became obvious when analogue computers and calculators have been used. Realizing a protection function as a combination of different calculating elements was a huge advantage. Features like:

  • Holding ratio equals one
  • Differential protection without any iron and the possibility to supress current saturation
  • Easy setup of vector groups and transformer ratio
  • High sensitivity for interturn fault protection based on voltage
  • High sensitivity for direction of power based on high accuracy angle calculation (very important for instance in reverse power protection of generators)
  • No contact problems

The different tripping signals of the measurement elements have been combined in tripping matrix. The "programming" of this matrix happened with diodes. The tripping voltage and voltage of the logic have been decoupled.

Most devices of this new generation such as modern semiconductors, new operating amplifiers , controllers and the first integrated circuits (SSI - small scale integration) have been produced in the 1970s. Measurement schemes have been adapted to the new electronic technology. From a functional point of view the new devices met all requirements in the wish list:

  • Decreasing the power consumption of current circuit to a value of less than one VA
  • Possibility to realize a sensitive startup and advanced characteristics
  • To realize a reset ratio of 0.95
  • Realization of 6-measurements systems
  • Implementation of AC voltage memories to be used for estimation of direction
  • Advanced possibilities for setup of values
  • Avoiding any contact issue
  • To achieve high service ability with plugging technique

It was the obvious wish of the vendors to shift from electromechanics to electronics within a short time. But this was not possible. Already the design of the system was different. Electronic relays came with printed cards in Eurocard (for printed circuit boards) format. This was fine for mounting in cubicles but did not fit to the old wall mounted devices. The use of central cubicles was not accepted at this time. Bringing the devices to a wall mounted type could help. Protection engineers in all parts of the world err on the side of conservatism often.

Why should they accept a new and unproved technology? None of the guys in the substation was neither educated nor familiar with electronics. In addition the first devices have been very sensitive against static discharge (CMOS elements used) and the knowledge about electromagnetic compatibility and input coupling was limited at this time. Nevertheless, already at this time, first investigations of utilities and vendors took place checking coupling and interferences. As a result, the German speaking community published a recommendation how to decrease transient overvoltages.
Three additional aspects should be covered becoming relevant with electronic protection:

Power supply
Conventional protection was supplied from station's battery. In huge substations there was very often a 110-V-battery as well as one with 220 VDC. The voltage needed for electronics was much smaller, so interposing transforming elements have been necessary. The stability of the entire protection was depending on the power supply- a real problem. Additionally the power demand of electronic protection could be higher compared to the electromechanical protection. Figure 4 shows the principle of a power supply of BBC's NF92 (1981)

Electromagnetic Interference
In case of disturbances in the grid as well as during control operations in substations interferences occurred. There can be transient signals, signals with operating frequency as well as higher frequencies- depending from the event. The voltage peak can be up to several kV. This was not knew experience, but it became more important when electronics were introduced in switchgears. Voltage peaks had no impact on electromechanical relays- but could damage the electronics. Shielding, earthing, filters have been some of the measures used and had to be tested and verified.

Increasing the reliability with new schemes
The elements and scheme used have an impact on the reliability of the system. The failure rate of the elements is calculated by the vendor and might be checked by the relay manufacturer as well. But this rate is valid under special conditions only. Using the worst case method allows estimation. Figure 2 shows some assumptions.

Static Protection Relays
Already in 1959 TOSHIBA started a program to develop static protection and produced a static busbar protection and overcurrent protection in 1959/1961 (Figures 3, 5, 7).

BBC's overcurrent relay IHX103 (Figure 1) was developed for 16 2/3 -Hz railway-systems and reached tripping times of 0.6 ms. Other examples of static protection were SIEMENS 7SD31 (Phase-comparison used since 1964, at 220- and 110-kV-grid of KEW Mark AG in Hagen.)

Reyrolle launched the IDMT relay R3As15 in 1967; S & H presented the synchrocheck relays TC31 and TC32 in 1969. SIEMENS single system distance protection 7SL24 was presented in 1971 (Figure 6) and used in ENEL's 110-kV-grid in Palermo substation. ASEA produced static distance relays RAZFE in 1975 (Figure 8). Figures 9, 10 and 15 show device and scheme of AEG's overcurrent relay SMK381.

Figure 10 shows the shunt used in this relay. The voltage is proportional to the currents and connected to the operating amplifiers IC1 up to IC3. The inputs are inverted and connected to potentiometer P1 defining the trigger value for all three phases. If the voltage is higher the operating amplifier switches on, with the positive half wave at the output being a pulse of differing length. This pulse is used as phase sensitive startup signal in the card AT6. The outputs IC1 up to IC3 are OR-connected, the pulses are extended and operating the relay H1. This signal could be used for indication or staring the autorecloser. IC4 up to IC6 are operating the high current system. Timing element (Figure 10) consists of a pulse generator, connected to capacitance C1, flip flop K2 / K3 and Inverter IV with memory Z2. The signal + is amplified by isolating amplifier V1. This allows a stable output time. Load resistance can be set up with plug pin S into the connector S1 or S2 (S3 = ∞).

The pulse generator delivers depending from chosen resistance R1 or R2 the pulses with two defined frequencies. The pulses are stored in counter Z2. Potentiometer P3 defines the number (≙ delay) and trips the auxiliary relay K2 / H.

Blackburn,J.L. and Rockefeller,G.D. of Westinghouse presented a paper describing "Solid-State Relaying for Transmission Lines.“ The first high impedance busbar protection produced in series came from MFO for the station Rathausen (CKW, Switzerland) in 1968.

A real revolution was ASEA's RAZOG relay with its polygonal characteristics (Figure14). It was possible to leave circle characteristic and switch to startup characteristic in the R/X coordinate system (maximum ratio 1:8, see PACW Spring 2008)

The measurement system consists of compensating circuit, a setting-up-element and the phase comparison. Stage 1 consists of raw setup and fine tuning possibility (1 – 2%). Stages 2 and 3 are defined as a multiple of stage 1. The phase comparison was working without any contacts and mounted in 4-unit-card. The entire system is pluggable consisting of the new COMBIFLEX-system in 19“ racks.

VEIKI from Hungary started to produce static thermal motor protection EMV3t in series in 1971 (Figure 11).
In the year after the frequency relay EFR1, the distance relay ETV came in 1973 as well as overcurrent ETIVA10/20. (Figure 13). The AEG Telefunken single phase distance relay SD35 (Figure12) and the six system SD335 have been launched in 1978. SD 35 was used in medium voltage, and SD335 in high voltage.

Two different types of construction have been used: modular system in 19" racks or the classical construction with single devices. Modular systems could be realized very compact, especially in case of complex protection systems. The connections within the rack can be short and are shielded. The advantage of the single-relay-solution has been simple mechanical and electrical interfaces as with electromechanical protection. The user could design the protection system instead of the vendor.
The devices could be produced smaller and smaller- an example from the 1980s shows BBC series 900 with the over-current relay IKT943 (Figure16).
BBC's other series was called MODURES®-the modular system. Figure 17 shows the distance protection LZ91 and earth fault detection relay EOR2. In the medium voltage secondary unit containing protection, control and measurement have been united- Figure 23 shows medium voltage switchgear in Mannheim (Germany). The high voltage distance relay LZ96 was developed in 1980. The startup characteristic could be adapted to the load (Figure19).

Electronical relays have been more and more accepted- already at the end of the 1960s they have been used to protect huge turbo generators with a power of up to 300 MW.

The Czech ZPA was involved in generator protection development in 1970s. Figure 18 shows an impedance relay for generators, Figure 22 a cubicle for an entire unit. With such cubicles the area of the front wall could be reduced to 40%- and the mass to 50%. A complete feeder protection with multi-shot autoreclosing SDND101 by GEC produced in the 1970s is shown in Figure 21. Reyrolle's static distance relay THR - produced in 197-5 is shown in Figure 20.

A typical example of a single cubicle for a 380-kV-feeder with redundant power supply is shown in Figure 24.
As mentioned 19" racks became popular all over the world. But in Eastern Germany an own system was developed- they called it EGS. Ralph Lüderich and Hans-Hinrich Merkel, of the Deutsche Reichsbahn developed an own protection for the 15 kV and called it EFR1 in 1967. The electronic protection EFR2 followed in 1973 utilizing MOS-technology, unipolar and bipolar circuits and transistors. In 1975 in Western Germany the static protection was SDB15 and 7SL16, in Eastern Germany EFS1 and EFS2 . Railway protection will be covered in another issue.

Power. Flexible. Easergy.
Protecting your electrical assets? today and tomorrow