Upgrade measuring and metering devices quickly and safely with pluggable connection solutions for current and voltage transformers.
By Alan Sappe and Harry Bentler
According to a report by Michael Beehler, vice president with Burns & McDonnell, investment in the electric transmission and distribution (T&D) industry is booming. This boom is supported by four fundamentals: (1) the need to maintain system reliability; (2) the need to deliver generation to load; (3) the need for environmental and regulatory compliance; and (4) the need to replace T&D assets that have reached the end of useful life.
"Old, conservative and typically late adopting utility models will be challenged as utilities integrate smart solution sets that move at the speed of Moore's Law," Beehler said.
For utility companies, enhancing or upgrading existing protection and control systems with the newest solutions for substation operations is an important aspect in the overall strategy for process optimization. More managers are upgrading older protection and control systems with intelligent electronic devices. These devices provide enhanced functionality and connection to existing hardware.
Increasing demand, emphasis on safety and cost push substation engineers, and switchgear and control panel builders to continually find new ways to optimize their production. The growth of renewable energy, in particular, has led to expanding low- and medium-voltage supply networks. Using pluggable measuring transducer terminals, measuring and protection devices can be upgraded or replaced with prefabricated cables. See Figure 1.
Modular substation plant and system concepts, which are becoming increasingly common, can reduce production and commissioning costs. Modular systems call for on-site pluggable wiring solutions to connect prefabricated sections of a plant. To connect different modular units, prefabricated cable trees are simply plugged into each unit.
Important to substation operators, energy measuring devices are basic elements that can be individually preinstalled to meet the demands of specific applications. Commercial and industrial operations frequently follow specifications used by electric power utilities to measure and control circuits, meter panels and cabinets.
Current transformers (CTs) are extensively used for measuring current and monitoring the operation of the electric grid. Along with voltage leads, revenue-grade CTs drive the electric utility's watt-hour meter on every building with three-phase service and single-phase services greater than 200 amperes. The CT is typically described by its current ratio from primary to secondary. The ratio of a CT refers to the turns ratio of the windings. The secondary of a CT is always stated to be 5 amps at the rated primary winding current. Multiple CTs are often installed as a stack for various uses. Protection devices and revenue metering, for example, can use separate CTs to provide isolation between metering and protection circuits, and allow current transformers with different characteristics-such as accuracy and overload performance-to be used for the varied purposes.
Extreme care must be taken that the secondary of a CT is not disconnected from its load while current is flowing in the primary because the transformer secondary will attempt to continue driving current across the effectively infinite impedance. This would produce a high voltage across the open secondary, which may cause arcing. The high voltage produced will compromise operator and equipment safety and permanently affect the accuracy of the transformer. Before removing a load such as a meter or relay, therefore, the CT should be short circuited.
Shorting or disconnect terminal blocks are an easy, efficient and convenient method of performing CT shorting. The disconnect terminal blocks automatically short circuit the instrument transformer when the load is removed.
Disconnect terminal blocks are an important interface for measurement with the transducer. These terminal blocks are used on a regular basis to take reference measurements and realize switching tasks when meters are upgraded or replaced. Connection to the upstream current and voltage transformers is usually manually wired, which is time consuming. The more wiring that is required, along with traditional measuring techniques, can produce errors by inadvertent interchanging transducer secondary connections.
Pluggable Solutions for Current and Voltage Transformers
Pluggable measuring transducer terminal blocks are one solution to these possible hazards. New component options offer the ability to design measuring or protective equipment so it can be replaced using prefabricated cables. When combined with a corresponding modular plug connector system, these disconnect terminal blocks provide typical isolating, testing and switching functionality, with an added feature of a plug zone for current and potential transformer circuits.
Current Transformers Always in a Safe State
For the pluggable base terminals, connectors are available to measure both current and voltage. A current transducer connector with an integrated short-circuit metal element enables a leading short circuit of the current transformer when withdrawn. See Figure 2.
|Figure 2. The current transformer (CT) connector has a short-circuit switching element. When withdrawn, this element enables a leading short circuit of the current transformer. At the top, the measuring circuit is looped in; at the bottom, the measurement is disconnected and the current transformer is short circuited.|
On the other side, the connector is inserted in the base terminal. This establishes an initial connection to the downstream measuring equipment. At that point, the current transformer short circuit is closed. This function ensures current transformers remain in a safe state. CTs are either short circuited or connected with low measuring current-i.e., 5 amperes.
Modular connectors can be used for voltage paths or potential transformers. For optimum electrical and mechanical reliability, plug connector systems should be designed to meet international standards such as International Electrotechnical Commission (IEC) 61984 and IEC 60947-7-1 for plug connectors with terminal blocks.
Manual current and reference measurements can be performed using a screwless longitudinal disconnect element. Field operators can swivel and lock the disconnect element into the desired switch position with a standard screwdriver. Symbols on the device are marked to clearly show switching positions. Additional switching locks can be used on both sides of the disconnect point to prevent accidental misuse. Continuous function slots on both sides of the disconnect point allow placement of potential distribution bridges and test sockets to meet test and measurement requirements.
Advantages in the Field
Pluggable disconnect terminal blocks enable safe and reliable commissioning, maintenance and replacement of meter panels and measuring devices. A pluggable solution eliminates the need for time-consuming wiring while the system is operational. In addition, current transformer sets can be installed with prefabricated short-circuit connectors. Measuring and protective equipment can be connected on-site shortly before commissioning the system.
To properly install such a connection system, transformer connectors are positioned in the corresponding location in the base terminal strip. Coding prevents connectors from being incorrectly inserted and offers reverse polarity protection. See Figure 3. The contact to the base terminal strip is established using strain relief and interlocking components, which prevents accidental opening of the circuit. Additional advantages include:
• Shorter installation time using prefabricated cables and cable trees,
• Fast commissioning and quick replacement of measuring devices,
• Flexible and modular system configuration to meet field operator requirements, and
• Optimized meter calibration process using standard test terminal strips and connectors.
|Figure 3. The coding system prevents incorrect insertion and also offers additional reverse polarity protection.|
Substation, plant and design engineers can simplify system configuration with engineering planning software. This tool can eliminate many manual documentation tasks typically required.
The software can be used to identify and specify all rail-mounted control cabinet components and generate circuit diagrams in common engineering platforms such as Eplan, Elcad and Ruplan, as well as data on terminal points, cables and labeling. Importing files is easy and incorporates relevant information into the planning and documentation process.
Cost-saving Potential for Measuring Devices
Modern pluggable disconnect terminal blocks can be used wherever electricity is measured using current and voltage transformers. For billing functions at electric power utility transmission and distribution, pre-metering areas within industrial companies, and in low- and medium-voltage switchgear systems, pluggable disconnect blocks serve as an effective alternative to conventional wiring.
A system with modular base terminals and connectors allows universal use. Application-specific circuit tasks can be combined. See Figure 4. Modular terminal block systems offer a high degree of efficiency in the production of substation plants, control systems and in the commissioning of these critical functions.
|Figure 4. Using a pluggable range CT terminals, current and voltage measurements can be simply and transparently configured.|
It is strongly recommended that only trained and certified technicians operate CTs, measuring devices and control systems. Mistakes can be costly and cause serious injury. National Electrical Code (NEC) and National Fire Protection Association (NFPA) safety rules should be followed.
Growing concerns for utilities such as aging infrastructure, changing workforce and new network architectures contribute to the increasing need for innovative pluggable connection systems for current and voltage transformers. Modularity and flexibility, with a strong emphasis on safety, drive the rationale for implementing these systems.
About the authors: Alan Sappe is product specialist for Phoenix Contact USA and Harry Bentler is terminal blocks product manager for Phoenix Contact GmbH & Co. KG, Blomberg, Germany. For more information about terminal blocks for electric power utility, visit: http://www.phoenixcontact.com/meseries.
UTME 4 Measuring Transducer Terminal Blocks-Brief Overview
Phoenix Contact's UTME 4 series with a nominal cross section of 4 mm2 comprises three different base terminals:
• Pluggable current transformer terminal UTME 4-CT/1P with short-circuit plug, 2- or 3-pin;
• Pluggable potential transformer terminal UTME 4/1P with Combi connector system Universal Powerline Bus (UPB) 4; and
• Compact UTME 4 test disconnect terminal block with screw connections at both sides and transverse bridges for manual wiring.
This product line is supplemented by a comprehensive range of practical accessories, which allow all switching and measuring processes to be simply performed. See Figure 5.
|Figure 5. The extensive range of accessories for the terminal block series can be used as required. Applicable standards: • IEC 61984:2008: plug connectors-safety requirements and tests; and • IEC 60947-7-1: low-voltage switchgear and control gear; Part 7-1: ancillary equipment-terminal blocks for copper conductors.|