Bird on Wire Sensing for Capacitor Bank Control

LineWatch™ M, a CO7 technologies product, is a Medium Voltage (MV) sensor that directly monitors both current and voltage on a medium voltage distribution feeder. The sensor (see Figure 1) is referred to as a "bird on wire" device because the sensor is clamped onto a live MV line and has no hard connection to ground. Instead, the capacitive coupling between plates on the outside of the sensor body and ground is used to measure voltage. Accuracy +/-0.5% in voltage and +/-0.5% in current is realized by the CO7's present generation of sensors. These sensors are inductively powered from the line being monitored, operating on line currents carrying currents ≥5A.

Centralized Capacitor Bank Control Use Case

In one use case, the sensors are used as essentially stand-alone monitoring devices. The sensors wirelessly report instantaneous quantities (i.e. current, voltage, real and reactive power) along with accumulated quantities (real and reactive energy, volt-hours, amp-hours) at one minute intervals to a nearby Collector. The Collector communicates via either a mesh radio or a cellular backhaul. The Collector implements a DNP3 interface with Secure Authentication over either IPv4 or IPv6 networks using either TCP/IP or UDP as the transport layer protocol.

The existing communications facilities allow the sensors to deliver measurements to virtually any SCADA system. That centralized SCADA system (or another third-party application) then uses the feeder measurements provided by the CO7 technologies LineWatch™ M sensors to make centralized capacitor bank switching decisions.

Local Capacitor Bank Control Use Case

However, not all utilities implement centralized Volt/VAR optimization. In most cases, capacitor banks are controlled locally based on voltage and current sensors co-located with the bank. These sensors are typically either Line Post sensors or a set of Potential Transformers and Current Transformers (PT/CTs). CO7 Technologies offers a version of the LineWatch™ M system that directly replaces either Line Post or PT/CT sensing methodologies, providing real-time outputs that mimic the outputs of either competing sensor type.

The sensors achieve this functionality by employing a different Collector than for the centralized use case. In this scenario, each medium voltage (MV) sensor communicates with a dedicated collector, which generates 60Hz signals proportional to the measured voltage and current. To mimic a Potential Transformer, the collector generates a nominal 120V 60Hz signal.

To mimic Line Post sensor voltage measurements, the collector generates a 0-10Vrms signal. In both cases, the collector generates a 0-10Vrms signal proportional to the measured current. The sensor is sampled at a higher rate, every 5-20 seconds depending on configuration. The collector takes instantaneous measurements of current, voltage, and relative phase and uses them to smoothly adjust the amplitude and phase of the generated voltage and current signals.

These sensors have been successfully integrated with capacitor bank controllers from leading manufacturers, and multiple pilot installations are currently in operation.

Cost Advantage

Relative to PT/CT installations, the initial cost of these sensing equipment is lower and the installations effort is greatly reduced. Installation of PTs and CTs, along with the the associated structural modifications to utility poles, could take a crew two days of installation time.

Figure 1: CO7's LineWatch M sensor

Furthermore, feeder downtime may be required for CT installation. In contrast, a typical LineWatch™ M sensor installation takes about 15 minutes. The sensors can be installed with a hot stick on a live line, as shown below in Figure 2. One crew is able to install LineWatch™ M sensors at multiple sites in a single day. The accuracy of the LineWatch™ M sensors is higher than the competing Line Post sensors.

Typical Line Post sensors are specified at +/-1%, whereas the (~0.5% accuracy), and the CO7

sensors have a much smaller phase shift (necessary for accurate power and power factor measurements).

The 0.5% accuracy of the CO7 current sensors (based on Rogowski coils) with high phase accuracy greatly exceeds the 2% accuracy quoted for most Line Post sensors.

The payback for higher sensing accuracy is better quality capacitor bank switching decisions.

Conclusions

CO7’s LineWatch™ M product line provides a cost-effective sensing solution for both centralized. Local capacitor bank control. In the centralized use case, the sensors are general

feeder voltage and current sensors.

In local use case, the sensors are dropin replacements for either PT/CT sensors or Line Post sensors.

LineWatch