EMC: No Challenge for Medium-voltage Switchgear

By Ansgar Màƒ¼ller, Oliver Nàƒ¶ldner and Andreas Werner, Siemens

The requirements on electromagnetic compatibility (EMC) increase constantly. Complex equipment and sensitive processes in power supply and production industries may carry a high risk potential for the operator in the event of failures. The number of electronic devices used for protection, control, monitoring and communication in the secondary system of switchgear installations is rising, and one of the reasons is the developing smart grid.

1 Inside the box: Movable parts and Shields

EMC Fundamentals

Electronic devices must be able to work reliably inside switchgear. EMC measures help create the right conditions for that reliability, although those measures may vary depending on a number of environmental and equipment configurations, including:

  • Electromagnetic climate. In an intact electromagnetic climate, all equipment is compatible with each other (inner EMC) and with the environment (outer EMC). To the outside, electromagnetic fields influence other equipment in the vicinity, and fields revert influences from there as well. The same environmental also applies to all connected cables and lines. In order to limit unintended interference, disturbance emission must be limited and some disturbance immunity must be achieved. Medium-voltage switchgear (MV switchgear) offers help in this area.
  • High-voltage. With EMC, the high-voltage part of a switchgear is almost passive; the main circuit does not generate interference under normal operation but conducts electromagnetic interference from the grid. Partial discharge (PD) or corona might cause high-frequency emissions, but, in the medium-voltage range, the radio interference voltage—the more easily measurable equivalent of interference radiation—is negligibly low and does not require any proof of emission limit values. The stationary electric and magnetic field originating from high-voltage conductors can have negative effects on the electronic devices in the switchgear. To the outside, however, the electric field is shielded by the earthed metal enclosure, and the outer magnetic field also reduces very quickly in the case of three-phase arrangements.
  • Switching operations. Interference emission due to switching operations can be disregarded as it occurs rarely and stochastically. The duration of such noise (clicks) is within the range due to low number, level and consequences. For this reason, no proof of EMC is required as regards switching operations.
  • Secondary system. The secondary system accommodates interference sources and susceptible devices in close vicinity. There, steady state interference radiators (switched-mode power supplies, converters, processors) and short-term sources of interference (contactors, switching relays, magnet coils, motors) can encounter electronic devices which are potentially susceptible to interferences. Further difficulties are presented by the variety of specifically designed secondary systems of switchgear, such as high-packing device density and proximity to the high-voltage conductor. The fact that electronic devices, which were not developed for a high-voltage environment, are often used inside switchgear, makes EMC a complicated matter.

EMC-compatible Design

The construction of switchgear contributes substantially to the electromagnetic compatibility. Table 1 (top left) shows measures to prevent the propagation of interferences—or, at least, to limit the effects of interferences. It also creates the conditions for appropriate interference immunity. Metal-enclosed switchgear generally offers positive preconditions for electromagnetic compatibility.

Good conducting bonding of the switchgear enclosure’s metal parts and the inner partitions provide a consistent reference potential for all integrated electronic equipment, reducing the effects of coupled disturbances. The interconnection of all passive metal parts—i.e. conductive parts which only adopt voltage in case of fault—to the earth is commonly called grounding.

EMC-compatible design should also look at wiring arrangements. Besides short lengths at the connections of instrument transformers, tripping coils, auxiliary contacts, etc., the high-voltage circuit is always separated from the cables of the secondary system. Within the secondary system, unintended couplings between parallel signal or data cables and power supply cables can be avoided by well-thought-out cable routing (see Table 2, top right).

As a rule, most of the devices used in the secondary system comply with the EMC requirements of their relevant product standard. This may not be enough in a high-voltage environment. Table 3 on Page 53 lists recommended test requirements. Higher disturbance variables can be applied to electronics coupled with a high-voltage conductor, and these should be tested with a higher testing severity. The standard for protection relays offers a template for the selection of requirements. The corresponding level must be selected according to the design and the expected electromagnetic phenomena.

At the switchgear frame, constructional measures should ensure that all components remain permanently connected at low impedance (highly conductive, low inductance). Powder-coated metal plates should be connected with the frame by means of special bolted joints. The surface coating (galvanization, powder coating) should protect the metal plates so that the joints prevail throughout the entire service life.

In the secondary system, a perfect separation between the wiring of auxiliary and control circuits and the main circuit must be observed. This separation should be reliably ensured by the switchgear vessel (stainless steel) or by the screened silicone insulation (busbar, cable T-plug). Additionally, the secondary wires of the instrument transformers should be routed through a metallic duct. Cables coming from outside must also be routed separately from the main circuit.

2 Inside the box: Secondary devices and wires

 

In order to create a good electromagnetic climate in MV switchgear, some measures must be taken concerning switchgear design, selection of electronic devices and their installation. In the metal-enclosed switchgear design, continuous equipotential bonding and grounding—as well as an effective separation between high voltage and secondary system—may be implemented quite easily. Electronic devices should meet minimum requirements to interference emission and interference immunity, and installation and wiring should conform to the applicable rules of electrical equipment engineering. Thus, EMC in medium-voltage switchgear is not a great challenge but a question of detailed design and quality manufacturing.

Dipl.-lng. Ansgar Màƒ¼ller, VDE, is responsible for medium-voltage standardization and technology in the Siemens Energy Distribution Sector in Erlangen. Dipl.-lng.(FH) Oliver Nàƒ¶ldner is responsible for the medium-voltage switchgear NXPLUS C as a product manager in the Siemens Energy Distribution Sector in Erlangen. (NXPLUS C is Siemens’ EMC-compatible gas-insulated switchgear product.) Dipl.-lng.(FH) Andreas Werner is responsible for productivity in product support of medium-voltage switchgear in the Siemens Energy Distribution Sector in Frankfurt. (In German, Dipl.-Ing. indicates someone who’s received a diploma in engineering.)

Authors’ Suggested Reading List:

[1] Directive 2004/108/EC of the European Parliament and of the Council of 15 December 2004 on the approximation of the laws of the Member States relating to electromagnetic compatibility and repealing Directive 89/336/EEC; http://eur-lex.europa.eu

[2] CISPR 14-1: 2009-02, Electromagnetic compatibility: Requirements for household appliances, electric tools and similar apparatus, Part 1: Emission; IEC Central Office, 3 rue de Varembàƒ©, CH-1211 Geneva 20

[3] IEC 62271-1: 2007-10, High-voltage switchgear and control gear – Part 1: Common specifications; IEC Central Office, 3 rue de Varembàƒ©, CH-1211 Geneva 20

[4] IEC 62271-200: 2004-10, High-voltage switchgear and control gear – Part 200: AC metal-enclosed switchgear and control gear for rated voltages above 1 kV and up to and including 52 kV; IEC Central Office, 3 rue de Varembàƒ©, CH-1211 Geneva 20

[5] IEC 60255-26: 2010-04, Measuring relays and protection equipment – Part 26: Electro-magnetic compatibility requirements; IEC Central Office, 3 rue de Varembàƒ©, CH-1211 Geneva 20

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