Protection relays are used to protect high- or mid- or low-voltage power systems, sense an abnormal condition in the circuit, and command the operation of circuit breakers. Voltage and current inputs from the equipment being protected are connected to the protection relay.
Protecting power-system elements requires accurate measurement of three-phase voltages and currents to provide reliable fault detection and breaker operations to minimize power-system disruptions. Current transformers (CTs) and potential transformers (PTs, also known as voltage transformers) are the most popular sensors for current and voltage measurement in switchgear applications. When a protection relay is connected in power applications, along with measurement of voltage and current, isolation of the protection relay from the high-voltage side is an important requirement for safety of the system and operators. CTs or PTs isolate protection, control and measurement equipment from the high voltages of a power system, and supply the equipment with the appropriate values of current and voltage to the internal analog to digital converter (ADC).
The Fig. 1 diagram provides an approach for measurement of voltage and current inputs in a protection relay application. The primary voltage and currents are stepped down using a voltage or current transformer. The secondary current or voltage is amplified and interfaced to the ADC for sampling the analog inputs. The ADC is interfaced to a signal processing system to process the fault parameter required to perform protection functions.
Figure 1: Data-acquisition AFE for protection relays
PTs are the most common sensors used to measure AC voltage. PTs are basically step-down transformers with an extremely accurate turns ratio and present a negligible load to the supply being measured. The transformers have a large number of primary turns and a smaller number of secondary turns. The number of turns per volt varies with the measurement accuracy requirement resulting in increased size of the PT.
For power and energy measurement the phase shift between the voltage and the current also affect the accuracy, since the power equals voltage multiplied by current multiplied by the cosine of the phase angle between voltage and current. Both PT and CT used in protection relay applications introduce phase shift between the input and the output. Other important requirement is to have linear phase shift over entire dynamic range to minimize non-linearity. Current or voltage transformers with low phase errors over wide range of input current and voltages will be expensive and choosing CT or PT is a challenge due to phase error requirements.
Accuracy requirements for Potential Transformer
Table 1 provides the voltage error and the phase error requirements for different classes of potential transformers at rated voltage used for measurement and protection. For measurement Class 0.5 or 02 is used and for protection Class 3P is used. The accuracy requirements are specified by IEC standards which are used for testing the performance of the voltage transformer. In protection relays, different potential transformer used to measure the AC input voltage has the below accuracy specifications:
Table 1: Voltage transformer accuracy classes (phase angle error expressed in minutes: one degree = 60 minutes)
While popular, PTs have several limitations, including:
- Ratio and phase errors. An ideal PT has a secondary voltage proportional to the primary voltage and exactly in-phase opposition. But practically speaking, because of some primary and secondary voltage drops, you cannot obtain the exact proportional voltage at the secondary or the exact phase shift. Thus, a PT introduces both ratio and phase-angle errors.
- Size and weight. When PTs are designed to ensure greater accuracy; they are made with a special high-quality core operating at lower flux densities in order to have small magnetizing current so that no load losses are minimized. This makes PTs bulky and the weight also increases.
- Output scaling. Scaling the output requires redesigning the transformers or providing multiple taps on the transformer. The size of the transformer also varies with output scaling.
An alternative solution for measurement of AC voltage is to use a resistor divider with isolation amplifier. This solution exceeds the measurement accuracy and isolation levels that conventional potential transformer provides.
You can use a simple resistance-based voltage divider for line/phase voltage measurement. In practical implementations, I recommend designing this divider from several resistors connected serially due to the power dissipation, continuous working voltage and surge-voltage withstand requirements. Choose the resistor divider values (>1MΩ) to ensure safe current levels (<1mA at the maximum input voltage) and select the ratio such that the input scales down to the ADC measurement range at the maximum input voltage.
A resistance potential divider has no phase shift, provides linear output over the entire range, and you can minimize the ratio error with careful resistor-divider selection. The frequency response is >100kHz and the overall size is small. Attenuated output from the Resistor does not provide isolation which can be realized by using an isolation amplifier. The output of the isolation amplifier interfaces to the ADC for measurement. This solution improves measurement accuracy with a negligible phase shift between the input and output compared to PT over wide dynamic range, reduces board size, weight, simplifies design and reduces overall system cost. The outputs are easily scalable by changing the resistor to adjust the required divider ratio.
TI provides isolation-amplifier solutions with basic or reinforced isolation that can be used along with the resistive potential divider for voltage measurement; I list a few of them, along with their critical parameters, in Table 2.
Table 2: Isolation amplifier critical parameters
The Reference Design to Measure AC Voltage and Current in Protection Relays and High-Accuracy ±0.5% Current and Isolated Voltage Measurement Reference Design demonstrate solutions to measure AC voltage inputs using a resistor divider and isolation amplifier ( instead of the conventional potential transformer). These reference designs have the following functional blocks:
- Potential divider. An on-board resistor divider scales the input voltage of 5V-300VAC to 175mVAC. The mVAC output of the potential divider is connected to the isolation amplifier to provide the required isolation between the AC input and the measurement AFE and additionally provide amplification.
- Isolation amplifier with basic isolation and isolated power. The AMC1200 basic isolation-type amplifier with ±250mV input and differential output with a gain of X8 measures the AC input. The common-mode output is scalable to 1.3V or 2.55V.
- Isolation amplifier with reinforced isolation and isolated power. The AMC1301 reinforced isolation-type amplifier with ±250mV input and differential output with a gain of X8.2 measures the AC input. The common-mode output is scalable to 1.4V. The isolation amplifier high side operates at 5V. The required isolated power is generated using a transformer driver and low-dropout (LDO) regulator.
- ADC interface. The isolation-amplifier output interfaces to the differential inputs of the ADS131A04 delta-sigma ADC. The ADS131A04 is a 24-bit delta-sigma ADC with a configurable input range from 0-5V (AMC1200) and ±2.5V (AMC1301).
The isolation amplifier AMC1200 or AMC1301 based voltage measurement AFE in the reference designs meets accuracy requirements for both measurement and protection requirements. The reference designs have been tested for voltage accuracy performance over a wide range of 5-300V AC. Accuracy was achieved for five-cycle measurement (400 samples at 4,000 samples per cycle for 50Hz).
The ratio error was observed to be within ±0.2% from a 5-300V AC input. The phase error was less than 6µs and same for a wide dynamic input range.
Additional Reference Designs
The following reference designs support other applications, including remote terminal units, distribution terminal units, feeder terminal units, molded case circuit breakers and power-quality analyzer. See these links for details:
- High Accuracy ±0.5% Current and Isolated Voltage Measurement Ref Design Using 24-Bit Delta-Sigma ADC
- Reference Design to Measure AC Voltage & Current in Protection Relays w/Delta-Sigma Chip Diagnostics