Trench, Pfiffner and Ritz Instrument Transformers

What they are and where they are used

Trench, Pfiffner and Ritz instrument transformers feed protective relays, revenue meters and controllers with a scaled-down copy of the grid current and voltage. They are found in high- and extra-high-voltage substations, at generation sites and at large industrial connections.

All three are specialized European manufacturers: Trench (which keeps the original Haefely-Trench design in its current transformers), Pfiffner (Switzerland, with plants in Germany, Turkey and Brazil) and Ritz (Hamburg, Germany). For the operating principle of each device, see high-voltage current transformer, inductive voltage transformer and capacitor voltage transformer.

Variants: Trench IOSK and SAS, Pfiffner JOF and EJOF, Ritz ICO, IVO and CMO

Trench and Pfiffner also make resistive-capacitive voltage dividers, and Trench makes GIS transformers and optical current transformers.

Main features

• Several cores: metering and protection on separate cores, each with its own ratio, class and burden. See terminals and secondaries of the current transformer.
• Insulation: paper-and-oil, paper-and-ester, SF6 or resin, depending on the line; it sets the maintenance and the insulation tests.
• Top-core or tank type: in the top-core type the cores sit at the top; in the tank type, at the bottom. This changes the behavior under mechanical stress and the way it is tested.
• Insulator: porcelain or silicone-rubber composite.

Data on the families that the manufacturers publish

Trench IOSK. A type approval certificate from the Slovak Metrology Institute (in Slovak, 2019, approved type TSK 212/17-143) describes the IOSK as a current transformer with paper-and-oil insulation. It is designed to use a minimum amount of mineral oil and is built by Trench France and Trench Italy. There are three versions, whose suffix is the maximum voltage of the equipment:

In that certificate all three versions have a primary current of 50 to 3,000 A and a secondary of 1 or 5 A. Their short-time thermal current is 50 kA for 3 s and the dynamic current is 125 kA. The ambient temperature ranges from -35 to +40 °C, the burden from 10 to 100 VA, the degree of protection is IP54 and they have up to 7 cores. The accuracy classes are 0.1 to 1.0 (including 0.2S and 0.5S) and, for protection, 5P or 10P according to IEC 61869-2. These are the values in that certificate for 50 Hz; the unit in a specific project has the values on its own nameplate.

Pfiffner JOF. According to Pfiffner’s product page, the JOF is an outdoor current transformer with paper-and-oil insulation using PCB-free mineral oil, for maximum voltages from 24 to 550 kV. It supports primary currents up to 4,000 A, a thermal current up to 63 kA for 1 s and a dynamic current up to 160 kA. It has up to 7 cores and works at frequencies of 16.7, 50 and 60 Hz. The accuracy classes listed are 0.1 to 3, 0.2S, 0.5S, P, PR, PX, PXR, TPX, TPY and TPZ. The active part is in the head; a stainless steel bellows compensates for the change in oil volume and a mechanical indicator shows the level. The primary ratio can be changed 1:2 or 1:2:4 from one side of the head.

Pfiffner EJOF. It is a combined inductive current and voltage transformer for 24 to 170 kV (24, 36, 72.5, 123, 145 and 170 kV). The current transformer is in the head and the voltage transformer in the base. The published data include a primary current up to 5,000 A, a thermal current up to 63 kA, up to 7 current cores and 5 voltage windings. The thermal limiting output of the voltage transformer reaches 1,500 and 3,000 VA. The voltage classes are 0.1 to 3, 3P and 6P. At 170 kV, the insulation level is 325 kV at power frequency and 750 kV impulse; the height is 2,929 mm and the approximate weight is 550 kg. One unit replaces two, which reduces transport, foundations and installation.

Pfiffner EOF. It is the inductive voltage transformer for 24 to 245 kV, with up to 5 secondary windings. Pfiffner states that it mitigates ferroresonance with a low operating inductance of the core and protects the secondary against transient overvoltages with a capacitive coupling shield. To compensate for the oil volume it uses a Viton membrane between 24 and 72.5 kV and a stainless steel expansion cell above 72.5 kV.

Ritz. At an industry trade fair in 2023, Ritz presented current transformers from 72.5 to 245 kV, inductive voltage transformers from 72.5 to 245 kV and capacitor voltage transformers from 72.5 to 550 kV, outdoor and oil-filled. The ICO, IVO, CMO and CVO models in the table above are read from the nameplate; the data for each one is requested from Ritz with the serial number.

What the accuracy classes mean

The IEC 61869 series replaced IEC 60044: part 1 sets general requirements, part 2 covers current transformers, part 3 inductive voltage transformers, part 4 combined transformers, part 5 capacitor voltage transformers and part 9 the digital interface. For metering, classes 0.2 and 0.5 are the usual ones for billing, and the «S» classes (0.2S and 0.5S) keep their accuracy at low currents, from 1 % of rated; that is what matters in a revenue meter that sees little load. For protection, 5P and 10P are the maximum composite errors of 5 % and 10 % at the accuracy limit. The PR, PX and TPX/TPY/TPZ classes are used where transient behavior and remanent magnetism matter, as in busbar and line differential protection. TPY limits the remanent flux, TPZ barely reproduces the DC component, and PX is specified by its knee-point voltage and winding resistance, not by an accuracy class. The class is chosen by the relay, not by the manufacturer (current transformer).

Safety and specific risks

• CT with an open secondary: it produces dangerous voltages and can damage the core and the insulation; unused cores are short-circuited and grounded.
• VT with a shorted secondary: the opposite case; a voltage transformer with its secondary short-circuited overheats and is damaged. It is protected with a fuse or a miniature circuit breaker.
• Ferroresonance in inductive voltage transformers: it can burn them out when they are switched with a circuit breaker fitted with grading capacitors or on floating busbars; see ferroresonance in voltage transformers.
• VT secondary fuse failure: the relay may think there is a fault or lose the voltage; the supervision protection is explained in PT and CT supervision, fuse failure 60.

Aging and diagnosis of paper-and-oil insulation

Paper-and-oil units age through moisture, oil oxidation and partial discharges. The power factor and capacitance of the main insulation, measured with the equipment de-energized, detect moisture and contamination; a sustained increase from the initial measurement points to deterioration (power factor and tan delta test). Because the oil volume of an instrument transformer is small, oil sampling is limited and is done only following the manufacturer’s guidance, since an incorrect extraction can affect the seal (dielectric oil analysis). Capacitor voltage transformers are also tested section by section of the capacitor divider.

Replacing a unit and compatibility

When replacing a current transformer, the ratio, the number of cores, the class of each core, the connected burden and the height of the base must match those of the relay, the meter and the structure. As for frequency (50 or 60 Hz), it is advisable to confirm that the nameplate of the replacement states that of the grid. Trench, Pfiffner and Ritz publish tables of dimensions and weights for each voltage level; the base and the phase spacing are checked against the yard drawing (primary engineering).

Communications and integration

An instrument transformer does not communicate: it delivers analog signals to the relay, the meter or a merging unit that digitizes them for IEC 61850. Trench optical transformers deliver the digital signal directly.

Frequently asked technical questions

• Manual for the Trench IOSK 145 and IOSK 245: it is requested from the manufacturer with the serial number; the data for testing it are on the nameplate and in the factory test report.
• Ritz instrument transformers: Ritz makes units from low to high voltage; the nameplate says whether it is a current, voltage or combined transformer.
• Open current secondary: it is dangerous; every unused core is left short-circuited and grounded.
• What is tested: ratio and polarity, excitation and knee point, winding resistance, insulation and power factor, and, in oil-filled units, the oil.

What IEC 61869 says

IEC 61869-2 sets how the designation of each class is written. For the insulator and the terminals, see creepage distance and mechanical loads on the terminals.

How we help

We do not sell instrument transformers. At Tiza Group we test current and voltage transformers of any brand, in medium, high and extra-high voltage. The tests cover ratio, polarity, excitation, insulation, tan delta and burdens. We also verify their connection to relays and meters and support commissioning, in substations and in industry, in Colombia or elsewhere on request. See current and voltage transformer testing within testing and commissioning. If you are going to replace a unit, we design the change in primary engineering. Write to us through contact.

Related guides

• Current transformer
• High-voltage current transformer
• Inductive voltage transformer
• Transformation ratio test
• Power factor and tan delta test
• Hitachi Energy (ABB) IMB and EMF instrument transformers
• Arteche instrument transformers
• Creepage distance on instrument transformer insulators
• Mechanical loads on instrument transformer terminals

References

• IEC 61869-2, current transformers; IEC 61869-3, inductive voltage transformers; IEC 61869-5, capacitor voltage transformers.
• Slovenský metrologický ústav, type certificate no. 143/1/212/17 revision 1, Trench IOSK (Slovak).
• Pfiffner Group, product pages for JOF, EJOF and EOF.
• Trench Group, instrument transformer pages, «Conventional Fluid Current Transformer» and «Eco Fluid Current Transformer»; gas-insulated transformer catalog 72–800 kV (SAS, SVS, SVAS); Pfiffner Group, corporate site and Pfiffner Switzerland; Ritz Instrument Transformers, high-voltage transformers.

Frequently asked questions