The following paper was presented at the 2011 IEEE Innovative Smart Grid Conference in Anaheim, CA January 18, 2011.

Overvoltage Protection of Data Concentrators used in Smart Grid Applications

Transient Protection for Pole-Mounted Data Concentrator Hardware

“© 2011 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.”

Overvoltage Protection of Data Concentrators used in Smart Grid Applications

Transient Protection for Pole-Mounted Data Concentrator Hardware

James Schroeder, BSEE, MBA, PE, IEEE Senior Member

Schroeder Consulting Services 249 Lyndel Dr.

Palmyra, PA 17078

Edward Doherty, BSEE, MBA, IEEE Member Mike Nager, BSEE, Senior Member IEEE

Phoenix Contact P.O. Box 4100

Harrisburg, PA 17111

Abstract—“Smart Grid” is a term used to define several phases of activities within the utility industry: from providing communications, monitoring and control capabilities for the energy infrastructure at the macro scale to controlling the energy usage of home appliances at the micro scale. This paper will address the segment of the Smart Grid activity that distributes data during last-mile connectivity between the data concentrator and the end user (home) level. Specifically, this paper will discuss the products and methodology required to protect outdoor link layer hardware from lightning strikes and current surges.

Keywords-lightning protection, Smart Grid communication networks, transient overvoltages

I. INTRODUCTION Data flow within the Smart Grid network takes place at

three levels of connectivity: the core, distribution, and access networks [1]. Below is a summary of each network’s function and the technologies used for that function:

A. Core Network

The core network provides connectivity between substations and utilities head offices. Technologies used for core network connectivity include:

• Wired technology – fiber, BPL (broadband over power lines)

• Wireless technologies – WiMAX, license-exempt broadband wireless

B. Distribution Network

The distribution network provides broadband connectivity between data collected by Smart Grid link layer hardware and distribution devices (e.g., monitors, sensors, SCADA systems) located on the grid and their related databases and analytical servers, which are located at headquarters. Technologies used for distribution network implementation include:

• Wired technology – fiber, BPL (broadband over power lines)

• Wireless technology – WiMAX, GSM, license-exempt broadband wireless

C. Access Network

The access network provides last-mile connectivity between Smart Grid link layer hardware (routers, data hubs, etc.) and smart meters located on the edge of the Smart Grid (at homes, offices, and municipal facilities). Technologies used for access network implementation include:

• Wired technology – PLC (power line communication) • Wireless technologies – ZigBee (IEEE 802.15.4), Wi-

Fi (IEEE 802.11), WiMAX ( IEEE 802.16), GSM, license-exempt broadband wireless.

Several types of Smart Grid link layer hardware are

available in the marketplace. They include: • Silver Spring Network’s eBridge and Access Points • SmartSync’s Grid Router • Cisco’s Integrated Services Routers • Alvarion’s BreezeMAX PRO Outdoor units • Trilliant’s SecureMesh Collector

These examples of link layer hardware share several

characteristics. They can all be used in multiple functions within the network, including neighborhood area networks (NAN), business area networks (BAN) and home area networks (HAN). Additionally, they all require communications and power inputs to function and use pole-mounted hardware. All link layer hardware mounted outdoors is susceptible to lightning strikes and power surges.

II. DISCUSSION As mentioned above, link layer hardware requires a power

input cable and a communications cable (antenna). This power input cable typically requires 120 V AC or 240 V AC. The communications cable port will operate in the frequency ranges specified by the communications standard being used: typically 915 MHz, 1.9, 2.15, or 2.4 GHz for wireless technologies referenced above. Surge protection devices (SPDs) are available to protect both the power and communication lines of the data link layer hardware.

Fig. 1 Speed versus energy characteristics for surge protection devices

A. Basic Principles of Surge Protection SPDs can be characterized as high-speed, voltage-triggered

switches that close during an overvoltage event. This action diverts energy away from the protected devices and to

electrical ground, while limiting potentially h armful voltage differences between the lines being

protected. Effective operation depends on a low impedance ground path.

The four major types of SPDs are the suppressor diode (also known as silicon avalanche diode or SAD), metal oxide varistor (MOV), gas tube (gas discharge tube or GDT), and the spark gap. For a given application, they can be used either alone or in combination to provide the necessary protection and response time. Fig. 1 shows component characteristics in terms of energy handling capability versus speed of response.

As indicated in Fig. 1, the energy versus response spectrum

is bounded by the fast response, low energy capabilities of the suppressor diode on one end of the spectrum to the slow response, high energy characteristics of the spark gap at the other end of the spectrum.

In order to evaluate the protection of pole-mounted hardware, we will discuss lightning strike information and the concepts associated with Lightning Protection Zones (LPZs).

B. Lightning Strike Information It has been documented that the current magnitude

associated with lightning strikes can vary from approximately 2.0 kA to 200.0 kA or higher. This magnitude variation is a function of several factors beyond the scope of this paper. Various agencies have done studies on the frequency of lightning strikes that occur worldwide and on a country-by-country basis.

Fig. 2 – Source Vaisala-GAI [2]

The map in Fig. 2 documents the findings from the National Lightning Detection Network. It shows the cloud-to-ground strikes over a ten-year period within the United States. Note the increased susceptibility to lightning strikes in the southeastern and Midwestern portion of the country, which can experience more than 14 flashes per square kilometer per year. Current magnitude and frequency of lightning strikes are two main parameters to consider when designing a surge protection device system. Other factors include potential unplanned maintenance cost, replacement equipment cost and availability, as well as system downtime consequences. C. Lightning Protection Zone Definitions

A primary tool used in the surge protection device industry for the quantification of SPD requirements for pole-mounted link layer hardware is IEC 62305-4. This standard defines protection zones for electrical and electronic systems against lightning. The protection zones are established using the “rolling sphere” concept as shown in Figure 3. [3]

Lightning Protection Zones (LPZs) for pole-mounted link layer hardware typically include zones LPZ 0B and LPZ 1 shown in Table I.

Fig. 3 – Lightning Protection Zone applied to a pole-mounted hardware

TABLE I DEFINITION OF LIGHTNING PROTECTION ZONES (LPZ)

Zone Definition LPZ 0A Zone where a direct lightning flash and electromagnetic

hit is possible. The internal equipment may be subjected to full lightning surge current. Lightning current test pulse of first stroke 10/350 µs.

LPZ 0B Zone protected against direct hit, but unattenuated electromagnetic field is present. This zone is determined by an external lightning protection system consisting of air termination, down conductor and earth termination system. Current test pulse of first stroke 10/350 µs.

LPZ 1 Zone where a direct hit is not possible and the currents in conductive components are lower than in LPZ 0A and LPZ 0B. Surge current is limited by current sharing and by SPDs at the boundary. Spatial shielding may attenuate the lightning electromagnetic field.

LPZ 2 Zone where the surge current may be further limited by additional SPDs at the boundary and current sharing. Additional spatial shielding may be use to provide additional attenuation to the lightning electromagnetic field.

TABLE II ELECTRICAL PARAMETERS FOR SPD USED IN COMMUNICATIONS EQUIPMENT

AT LPZ BOUNDARIES LPZ 0B - 1

Parameter Symbol Rating Maximum Continuous Operating Voltage UC 60 V Nominal Operating Current IN ≤ 1.5 A Nominal Surge Current (8/20µs) line-line I SN 100 A Nominal Surge Current (8/20µs) line-PE I SNT 2 KA/sig. pr. Total Nominal Discharge Current (8/20µs) line-line

I SNT 10 KA

Voltage protection level, line-line UP 9 V Voltage protection level, line-PE UP 700 V Voltage protection level line-line @ 1KV/µs rate of rise

UP ≤85 V

Voltage protection level line-PE @ 1KV/µs rate of rise

UP ≤700 V

Insertion loss @ 250 MHz ąE ≤2 db Capacitance line-line C 12 pf @ 1

MHz Capacitance Line-PE C 2 pf @ 1 MHz Data Transmission Speed GBit ≤10 GBIT/s Characteristic Impedance Zo 50 Ω Category tested in accordance with IEC 61643-21:2000

C2

D. SPD Parameter Definitions

Key SPD parameters include surge current ratings, voltage protection levels, and speed of operation for SPD devices.

IEC standard 61643-21:2000 established specific electrical parameters, performance requirements and testing methods for SPDs connected to communications equipment. Table II shows the electrical parameters for a typical SPD used in a communications equipment application. The table also defines parameters.

III. PRODUCT SELECTION CRITERIA DATA LINK LAYER HARDWARE

A. Communications Port Based on the definition of the LPZs and SPD performance

parameters discussed above, Table III shows typical rating for SPDs used to protect ZigBee/Wi-Fi and GSM network applications [5]. ZigBee and Wi-Fi communication ports usually use an RJ45 connector with Category 5 or 6 cable, while GSM modems typically use a coax connector and cable.

Where: Uc = maximum voltage (d.c. or r.m.s. ), which may be continuously applied to SPD terminals without causing any degradation in the transmission characteristics of the SPD. Up = parameter that characterizes the performance of the SPD in limiting the voltage across its terminals. This value is greater than the highest measured value of impulse-limiting voltage and is specified by the manufacturer. In = Nominal current handling capability under normal operating conditions. Is = The SPD must handle 100% of this surge current ( 8/20µS waveform) without a significant change in protection level 8/20 µS Waveform = Surge current impulse waveform used to evaluate nominal surge current ratings according to IEC 60060-1. Shown in Figure 4.

iîiî

tµst

µs88

2020

1.01.00.90.9

0.50.5

0.10.10.0

Fig. 4 Surge current impulse waveform, 8/20 µS

TABLE III ELECTRICAL PARAMETERS FOR SPDS USED IN DATA LINK LAYER HARDWARE

AT LPZ BOUNDARIES LPZ OB – 1 (IEC 61643-21:2000)

Parameter Symbol Rating ZigBee and Wi-Fi

Networks

GSM Networks

Maximum Continuous Operating Voltage

UC 60 V 60 V 10V

Nominal Operating Current

IN ≤ 1.5 A ≤ 1.5 A 5.0 A

Nominal Surge Current (8/20µs) line-line

I SN 100 A 100 A 20 KA

Nominal Surge Current (8/20µs) line-PE

I SNT 2KA/sig. pr.

2KA/sig. pr. 20 KA

Total Nominal Discharge Current (8/20µs) line-line

I SNT 10 KA 10 KA NA

Voltage protection level, line-line

UP 9 V 9 V NA

Voltage protection level, line-PE

UP 700 V 700 V ≤ 20V

Voltage protection level line-line @ 1KV/µs rate of rise

UP ≤85 V ≤85 V NA

Voltage protection level line-PE @ 1KV/µs rate of rise

UP ≤700 V ≤700 V ≤10 V

Insertion loss @ 250 MHz

ąE ≤2 db ≤1 db 0.2 dB (1.7GHz

to 2.3GHz)

Capacitance line-line C 12 pf @ 1 MHz

Typ. 12 pf @ 1 MHz

NA

Capacitance Line-PE C 2 pf @ 1 MHz

Typ. 2 pf @ 1 MHz

<2 pf @ 1 MHz

Data Transmission Speed

GBit ≤10 GBIT/s

≤10 GBIT/s ≥10 GBIT/s

Characteristic Impedance

Zo 50 Ω ≥50 Ω 50 Ω

Category tested in accordance with IEC 61643-21:2000

C2 C2

TABLE IV ELECTRICAL PARAMETERS FOR TYPICAL TYPE 2 SPD USED IN POWER CABLE

PORT

Parameters Power Nominal Voltage Un 120 V AC Arrester rated voltage Uc 150 V AC/ 200 V

DC Nominal frequency fn 50/60 Hz Discharge current to PE at Uc <0.45mA Max. discharge surge current Imax (8/20) µs 40 kA Nominal discharge surge current In (8/20) µs 20 kA Lightning test current (10/350) µs, peak value Iimp 3 kA Response time <24 ns

B. Power Input Port The power cable to the link layer hardware would typically

use voltages in the 110-120 V AC range or 220 – 240 V AC range. A typical product selection for this application would be a surge protection type 2 SPD that uses a high-capacity varistor, provides thermal fusing and a visual fault warning. It should also be noted that Type 2 SPDs require a backup fuse, typically with a maximum rating of 125 A. Table IV shows the performance characteristics of a typical type 2 SPD [6].

C. Device-Mounting Technique Many commercially available products, including SPDs,

are designed for mounting on a DIN rail inside the enclosure. The name DIN rail is based on the Deutsches Institut für Normung (DIN) (translation: German Institute for Standardization), which defines the dimensions and tolerances of the rail. This allows manufacturers to design mounting methods for products destined for assembly onto the rail [7]. Manufacturers that use DIN mounting in their link layer hardware find it easy to add components or customize their designs by using other devices that mount on the DIN rail, including terminal blocks and power distribution blocks, fuses, relays, and power supplies.

D. Hardware Grounding Technique In addition to providing mechanical support, the DIN rail

ideally serves as a single-point ground for SPDs and other devices used to distribute power and signals within the enclosure. The grounding methodology of surge protection devices is very important to ensure proper functioning during an overvoltage condition[8]. Surge protection devices should be bonded to the enclosure and using either a short (i.e., low impedance), high ampacity wire or connecting directly to a grounded DIN rail. This ensures that the surge current is safely and effectively routed to ground without creating voltage differentials between components within the link layer hardware.

E. Environmental Specifications The ambient operating temperature is an important

environmental parameter to consider when selecting the components, including SPDs, that will be placed into link layer hardware enclosures. External locations must withstand more extreme temperatures on both ends of the temperature spectrum, so -40ºC to + 80ºC is commonly specified.

IV. CONCLUSION Smart Grid is a term that is used to define several phases of

activities within the utility industry: from providing communications, monitoring and control capabilities for the energy infrastructure at the macro scale to controlling the energy usage of home appliances at the micro scale. This paper has addressed the segment of the Smart Grid that distributes data during last-mile connectivity from the link layer hardware to the home. The paper has discussed the application of surge protection devices in protecting outdoor link layer hardware from lightning strikes and current surges. The paper has reviewed the definition of Lightning Protection Zones, discussed surge protection device performance parameter definitions, and provided selection criteria for SPDs. In addition, typical products have been chosen as a function of the communication network being used for both the network and power cable inputs. Using the decision criteria and methodologies outlined in this paper will ensure the protection of outdoor data link layer hardware from lightning strikes and other overvoltages.

REFERENCES

[1] Alvarion, Inc., White Paper 215135 Rev A. “Optimizing smart power grids with WiMAX and broadband wireless connectivity solutions,” 2009. www.alvarion.com

[2] Lightning data provided by U.S. National Lightning Dection Network, National Weather Service www.weather.gov/om/lightning/stats/08_Vaisala_NLDN_Poster.pdf

[3] International Electrotechnical Commission, International Standard 62305-4, “Protection against lightning – Part 4: Electrical and electroinc sysems within structures.”

[4] IEC Standard 61632-21:2000, “Low voltage surge protective devices – Part 21: surge protective devices connected to telecommunications and signaling networks – performance requirements and testing methods.”

[5] Phoenix Contact, specifications for the ZigBee/Wi-Fi applications for model DT-LAN-CAT.6+ and for GSM applications Phoenix Contact model CN-LAMBDA/4-2.0-BB, 2010.

[6] Phoenix Contact, specifications for Type 2 SPD values from Phoenix Contact model VAL-MS 120 ST, 2010. www.phoenixcontact.com

[7] A. Offner, “DIN rail in the electrical control cabinet and junction box,” IEEE SC2 Committee Presentation, Tucson AZ: November 2008.

[8] M. Nager, “Understanding Surge Suppression,” Plant Engineering. November 2004, pp. 39-43.