Overview: The Solkor Relay Family
The Reyrolle Solkor name covers multiple generations of pilot-wire differential protection relays spanning more than 50 years of product history. Engineers encountering old switchgear with Solkor relays often ask whether these relays are suitable for transformer protection. The short answer depends critically on which Solkor variant is installed, the CT ratios on both sides, and the size of the transformer relative to the feeder rating.
| Model | Primary Function | Protection Scheme | Transformer in Zone? |
|---|---|---|---|
| Solkor R | Pilot-wire line differential (87L) | Circulating-current, balanced-voltage | Tapped feeder — limited conditions |
| Solkor Rf | High-speed line differential | Balanced-voltage with bias | No — line only |
| Solkor N | Feeder differential | Circulating current | No |
| Duo-bias | Transformer differential (87T) | Biased differential, through-fault stabilisation | Yes — purpose-designed |
Solkor R — Circulating-Current Pilot Differential
The Solkor R (serial prefix A3B series) is a circulating-current differential relay designed for use with a metallic pilot pair connecting CTs at each end of a feeder. Operation relies on the algebraic summation of pilot currents; a fault inside the zone produces a net differential current that overcomes the relay operating coil restraint.
Resistance Stud Settings
On the Solkor R front panel, typically 5 tapped resistor studs allow adjustment of the relay's through-fault stabilising resistance. These studs set the value of RS — the stabilising resistance inserted in the relay operating circuit to prevent maloperation on external faults with CT saturation.
| Stud Position | Typical Resistance (Ω) | Purpose |
|---|---|---|
| 1 | ~40 Ω | Minimum stabilising — short pilots, matched CTs |
| 2 | ~90 Ω | Low pilot resistance application |
| 3 | ~190 Ω | Medium pilot loop resistance |
| 4 | ~340 Ω | Long pilot, higher loop resistance |
| 5 | ~490 Ω | Maximum — long pilots or high-resistance loop |
A relay found set at the maximum 490 Ω stud (all studs cumulative) indicates either long pilot cables or a high-impedance pilot loop. The total stud resistance must satisfy:
R_S ≥ V_k / (2 × I_f_max) − R_CT − R_pilot/2
where Vk is the CT knee-point voltage, If_max is the maximum through-fault current, RCT is CT secondary winding resistance, and Rpilot is the total pilot loop resistance. Verify this calculation against the site CT test certificates before commissioning.
Transformer Feeders in the Solkor R Zone
The Solkor R can accommodate a tapped transformer within the differential zone under the following conditions, described in the Reyrolle application guidance:
- Tapped load ≤ 20–25 % of main feeder rating. Magnetising inrush from a large transformer will generate differential current and can cause spurious trip if this ratio is exceeded.
- No interposing CT compensation for vector group. The Solkor R has no means of compensating delta/star phase shift or zero-sequence blocking. Both CTs must see the same current waveform — only radial step-down transformers with matched winding configurations are suitable.
- Inrush is below operate threshold. The Solkor R has no second-harmonic restraint (87T feature). On a tapped transformer energisation, the magnetising inrush current appears as differential current with no blocking. Small distribution transformers (< 1 MVA, lightly loaded pilot) are acceptable; large power transformers are not.
- CT ratios at each end are matched for the main feeder. The tapped transformer load throws a balance error proportional to its load share; this must remain below the relay pickup threshold.
Transformer Protection — Correct Relay Selection
For dedicated transformer differential protection (ANSI 87T), the correct Reyrolle relay is the Duo-bias. Modern equivalents from Siemens Protection Devices (the successor to Reyrolle) are the SIPROTEC 7UT series.
| Feature | Solkor R | Reyrolle Duo-bias | SIPROTEC 7UT85 |
|---|---|---|---|
| 2nd harmonic inrush restraint | No | Yes | Yes (adaptive) |
| Through-fault stabilisation | Resistor studs | Bias winding | Numerical bias characteristic |
| Vector group compensation | No | Interposing CTs | Software-configurable |
| Zero-sequence blocking | No | Interposing CTs | Built-in |
| 5th harmonic overexcitation | No | No | Yes |
| Pilot requirement | Metallic pair | None (local 3-CT) | None |
| Typical operate time | 20–40 ms | 40–60 ms | <15 ms |
Injection Intertripping — Alternative Interpretation
A second legitimate reason to find a Solkor R associated with a transformer feeder is as part of an injection intertripping scheme. In this arrangement:
- The Solkor R pilots carry a superimposed audio-frequency signal (typically 800 Hz or 1 kHz) injected by a separate transmitter at one end.
- A receiver at the remote end detects loss of the injected tone (pilot fault or fault operation at the remote breaker) and issues a trip command to the local breaker.
- The transformer feeder breaker is interlocked into this scheme for remote isolation — it does not rely on the differential protection function of the relay itself.
Confirm which scheme applies by checking for injection transmitter/receiver units on the relay panel and reviewing the control schematic for pilot supervision connections.
Commissioning Checks for Legacy Solkor R on Transformer Feeders
- Identify relay serial prefix and variant from the nameplate — A3B prefix confirms original Solkor R.
- Record all five stud resistance values with a Wheatstone bridge or precision LCR meter; verify they sum to the nameplate maximum (typically 490 Ω cumulative).
- Measure pilot loop DC resistance: calculate minimum RS setting using the CT parameters and maximum through-fault current from network studies.
- Confirm the tapped transformer MVA is <25 % of the feeder CT rating by reviewing feeder protection coordination study.
- Perform secondary injection: inject differential current and confirm operate time <60 ms at 2× pickup; inject through-fault current and confirm no operation.
- Perform transformer energisation test (where safe): close onto the transformer with the relay in service and monitor for spurious trips using an event recorder with 1 ms resolution.
- Review Siemens Protection Devices Reyrolle documentation archive for the original Solkor R application guide — request via Siemens technical support if the relay predates public document availability.
Can a Reyrolle Solkor R relay protect a transformer?
Only under limited conditions: the tapped transformer must be ≤25% of the main feeder rating, the CT ratios must match the feeder (not the transformer), and the transformer winding vector group must not introduce a phase shift. The Solkor R has no second-harmonic inrush restraint or zero-sequence blocking, so it is not a substitute for a dedicated 87T relay like the Reyrolle Duo-bias.
What do the five resistance studs on the Solkor R front panel control?
They set the total stabilising resistance RS in the relay operating circuit — typically 40 Ω, 90 Ω, 190 Ω, 340 Ω, and 490 Ω cumulative. The correct setting is calculated from CT knee-point voltage, CT and pilot resistance, and maximum external fault current to prevent maloperation under through-fault CT saturation.
What is the modern replacement for the Reyrolle Duo-bias transformer protection relay?
The Siemens Protection Devices (formerly Reyrolle) SIPROTEC 7UT series (7UT82, 7UT85, 7UT86) are the direct numerical successors, providing biased differential (87T), inrush restraint, vector group compensation, and restricted earth fault (REF) in a single IED.
What pilot wire resistance can the Solkor R tolerate?
The maximum pilot loop resistance the Solkor R can accommodate depends on the stud setting; at maximum 490 Ω, the relay is typically suited to pilot loops up to 1,000–1,500 Ω depending on CT knee-point voltage. Always recalculate using site-specific CT parameters — do not rely on nominal figures from the relay nameplate alone.
How do I tell if a Solkor relay is wired as a differential scheme or an intertripping scheme?
Trace the pilot pair back to both ends: a differential scheme connects CT secondaries directly into the relay pilots; an injection intertripping scheme has a separate audio-frequency transmitter/receiver unit connected across the pilots, with the Solkor relay providing pilot supervision. Check for tone injection equipment (typically a metal box with a coaxial or terminal-strip pilot connection) on each end of the protected feeder.