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PD IEC TR 61869 — Instrument Transformers Technical Report Series (Guidance for Application of Current Transformers in Protection and Ferroresonance Oscillations)
Part 100: Guidance for application of current transformers in power system protection; Part 102: Ferroresonance oscillations in substations with inductive voltage transformers
⚡ PD IEC/TR 61869 is the UK adoption of technical reports from the IEC 61869 family, developed by IEC Technical Committee 38 (Instrument transformers) and adopted in the UK through Committee PEL/38. This batch contains two “100/102 series” technical reports: Part 100 provides guidance for the application of current transformers (CTs) in power system protection, and Part 102 specifically discusses ferroresonance oscillations in substations with inductive voltage transformers (IVTs). The technical reports provide important engineering guidance for the design, selection and protective application of instrument transformers.
💡 The current transformer (CT) is the “eye” of protection and metering: the transient process during short-circuit faults (including the DC component) can saturate the CT severely, causing protection maloperation or failure to operate. Part 100 provides engineers with a complete application methodology from theoretical equations through duty cycles to the allocation of design responsibilities.
1. Scope and Series Structure 📌
The IEC 61869 family is the international standard series for instrument transformers (replacing IEC 60044), covering current transformers, voltage transformers, and electronic instrument transformers. Its technical reports (TR) provide application guidance. This series contains two documents:
| Document | Subject | Content | Year |
|---|---|---|---|
| PD IEC/TR 61869-100:2017 | CT application guidance | Guidance for the application of current transformers in power system protection | 2017 |
| PD IEC/TR 61869-102:2014 | Ferroresonance oscillations | Ferroresonance oscillations in substations with inductive voltage transformers | 2014 |
⚠️ Technical reports (TR) are informative, guiding documents that do not specify mandatory clauses, but they provide accepted methods and criteria for engineering design and selection. They should be used together with the corresponding normative instrument transformer standards (e.g. IEC 61869-2 for CTs, IEC 61869-3 for VTs).
2. Part 100 — Guidance for Application of Current Transformers in Protection 🔌
Part 100 (2017) systematically explains the design and application methodology for protection CTs. Core content includes:
- Allocation of responsibilities in the CT design process: the CT design process is subdivided into stages based on history and division of labour, clarifying the respective responsibilities of system engineers, protection engineers and CT manufacturers;
- Basic theoretical equations for transient designing: equivalent circuits of the electrical circuit and the CT, fault inception angle, differential equations, and transient behaviour analysis, deriving the DC component of fault current and CT saturation;
- Duty cycles: definition of typical protection CT duty cycles and their application in various protection schemes (line, busbar, transformer differential, etc.);
- CT saturation and correct selection: explanation of the effects of transient saturation, remanence and secondary impedance on protection performance, guiding the selection of suitable accuracy limit factors and ratings;
- Examples and recommendations: case-based selection recommendations and precautions for protection applications.
🚨 The key requirement for a protection CT is “correct transformation of fault current during fault transients”: if the CT saturates severely from the DC component, differential and distance protection can maloperate or fail to operate, directly affecting grid safety. Selection must be verified against the actual system time constant, fault current and duty cycle.
3. Part 102 — Ferroresonance Oscillations with Inductive Voltage Transformers 🔄
Part 102 (2014) discusses ferroresonance in substations with inductive voltage transformers (IVTs). Core content includes:
- Definition of ferroresonance: ferroresonance is a sustained, non-sinusoidal oscillation produced by a circuit containing a nonlinear inductance (such as a VT magnetising inductance) and capacitance under particular conditions;
- Excitation of steady-state and non-steady-state ferroresonance: distinguishing the excitation conditions of steady-state and transient ferroresonance, analysing start-up and maintenance mechanisms;
- Single-phase and three-phase oscillations: simplified circuits for single-phase ferroresonance, the capacitive voltage transformer (CVT) case, and the configuration, generation and typical oscillograms of three-phase ferroresonance;
- Examples of actual configurations: based on real scenarios such as a 245 kV outdoor substation, illustrating the generation and hazards of single-/three-phase ferroresonance under switching operations, single-phase earthing, etc.;
- Mitigation measures: discussion of damping, neutral earthing and parameter matching to suppress ferroresonance.
💡 Ferroresonance is a “hidden hazard” of power systems: it can burn out voltage transformers, blow fuses, cause protection maloperation and even equipment damage, and it often appears intermittently after switching operations or single-phase faults, making diagnosis difficult. Preventing it through parameter matching and damping at the design stage is far more economical than remedial action.
4. Engineering and Application Practice 🛠️
- 🔍 Protection selection: following the Part 100 methodology, verify and select the accuracy limit factor and rating of protection CTs considering system short-circuit capacity, time constant, fault-clearing time and protection type.
- 📊 Design responsibility allocation: clarify the division of labour and inputs/outputs among system, protection and manufacturer in the CT design process to avoid selection errors from unclear interfaces.
- 🔄 Ferroresonance assessment: for substations with IVTs or CVTs, assess ferroresonance risk under switching operations and single-phase faults, and configure damping or improve neutral earthing as necessary.
- 📋 Operational monitoring: watch for abnormal oscillations in VT secondary voltage and current; use waveform records to determine whether ferroresonance has occurred and take timely action.
5. Frequently Asked Questions ❓
❓ Q: What type of document are Parts 100 and 102?
A: Both are technical reports (TR) of the IEC 61869 family — informative, guiding documents. Part 100 addresses protection CT application; Part 102 addresses ferroresonance. They must be used together with the corresponding normative instrument transformer standards.
A: Both are technical reports (TR) of the IEC 61869 family — informative, guiding documents. Part 100 addresses protection CT application; Part 102 addresses ferroresonance. They must be used together with the corresponding normative instrument transformer standards.
❓ Q: Why do fault transients saturate CTs?
A: Short-circuit fault current contains a DC component and a high-amplitude AC component. The DC component magnetises the CT core in one direction, causing flux offset and increased saturation. Saturated CTs distort secondary current, compromising protection criteria.
A: Short-circuit fault current contains a DC component and a high-amplitude AC component. The DC component magnetises the CT core in one direction, causing flux offset and increased saturation. Saturated CTs distort secondary current, compromising protection criteria.
❓ Q: Why is ferroresonance dangerous?
A: The high-amplitude, non-sinusoidal overvoltages and overcurrents of ferroresonance can burn out voltage transformers, blow fuses, cause protection maloperation, and even lead to equipment explosions. It often appears intermittently and is difficult to diagnose.
A: The high-amplitude, non-sinusoidal overvoltages and overcurrents of ferroresonance can burn out voltage transformers, blow fuses, cause protection maloperation, and even lead to equipment explosions. It often appears intermittently and is difficult to diagnose.
🔍 Q: How can ferroresonance be suppressed?
A: Through parameter matching (avoiding resonance between circuit capacitance and magnetising inductance), adding damping (e.g. damping resistors), improving neutral earthing, and using capacitive voltage transformers; risk assessment should be performed at the design stage.
A: Through parameter matching (avoiding resonance between circuit capacitance and magnetising inductance), adding damping (e.g. damping resistors), improving neutral earthing, and using capacitive voltage transformers; risk assessment should be performed at the design stage.