2. Challenges in implementation of smart grids

As shown in Figure 1, the key features of the smart grid offer many advantages and prospects in the power industry, thereby revitalizing the socioeconomic strategies of this sector. However, the extensive applications of emerging technologies, if not considered, have vulnerabilities that may result in disasters such as long-term blackouts, economic failure, and so on. Table 1 provides a brief survey of some of the challenges of smart grid technology [10].

Extensive research on this technology, which aims to overcome many challenges, has been launched by various universities. The system parameters and

problems

Security Exposed to Internet attacks (spams, worms, virus, and others), question of national security Reliability Failure during natural calamities, system outages, and total

unscheduled power flow and dispatch

Long-term and unpredictable intermittent sources of energy,

Decoupling causes system stability issues and causes reduced

network for reliable protection, control, and communication

Generation demand equilibrium and power system stability

System instability during sags, dips, or voltage variation such as over-voltages, under-voltages, voltage flickers, and other

Transmission line congestion and huge investments

inertia due to high level of wind penetration

Complexity Complex customary design module and networks Non-flexibility Unique designs for all individual networks; no ease of

handling, and smuggling

Reliability Grid automation Need for strong data-routing system with secure and private

with grid complexity

Security Malware, data interception, data corruption, illegal power

Privacy Sharing of data causes privacy invasion, identity spoofing, eavesdropping, and other problems

blackout

Innovative Differential Protection Scheme for Microgrids Based on RC Current Sensor

adaptation

The Rogowski coil (RC) current sensor works in the same way as a conventional AC core current transformer. They are not closed loops, making the coils open and flexible, and can be wrapped around the conductors [11–13]. RCs can immediately respond to the changing currents, down to a few nanoseconds, due to their low inductance. RC has no iron core to saturate, making it highly linear even when exposed to large currents. Linearity also allows high-current RCs to be determined using smaller reference currents. No danger is observed in opening the secondary winding [14]. Power construction costs and temperature compensation are simple [15–17]. Besides, RC does not use the magnetic core to support two windings. RC is designed with two coils that are electrically connected in the opposite direction, eliminating the electromagnetic field from the outer ring routing. For obtain the current sensor quality, the RC design must meet two important criteria; the first criterion, the mutual inductance M must have a constant value for any of the main conductor locations inside the coil loop, which can be achieved if the coil core has a constant cross section S, is perpendicular to the median line and is constructed with

configuration of the power grids were investigated more intelligently.

3. Rogowski coil current sensor

Grid

Power quality Disturbance

Challenges of smart grid technology.

Reconfiguration

identification

Technology Challenges Issues

DOI: http://dx.doi.org/10.5772/intechopen.85473

Wind/PV generation and forecasting

Power flow optimization

Power system stability

Self-healing action

Renewable energy integration

Energy systems storage

Consumer motivation

Table 1.

115

Figure 1. Future smart grid processing technology.


Innovative Differential Protection Scheme for Microgrids Based on RC Current Sensor DOI: http://dx.doi.org/10.5772/intechopen.85473

#### Table 1.

all means of energy generation and distribution to meet the future demands of

a variety of needs and options at different costs. Furthermore, smart grids will provide advanced monitoring and control by employing intelligent equipment such as digital sensors, electronic switches, smart energy metering, and creative and advanced communication systems. Its data acquisition and control systems include interactive software, real-time control, and power flow analysis. All different types of renewable energy sources will be interconnected with the energy grid system to improve quality, reliability, and stability by using intelligent and advanced devices. Providing advanced technology such as the smart grid requires a smart and intelligent protection system to improve the efficiency of power delivery to customers, and to reduce outages. Employing the smart grid allows energy consumers to be active participants by providing information and options to control the electric

Quality of power delivery is a significant goal of the smart grid that will provide

The microgrid is used to provide customers with economical and reliable power resources and to make effective use of them through the formation of a smart grid structure during the disturbance. However, the protection of microgrid is a challenging task [3–5]. This chapter discusses the application of differential protection schemes. Issues related to protection include bidirectional power flow; it also handles the decrease in fault current levels [6–8]. The power system must operate safely at all times. The main requirements for power system protection include speed, selectivity, sensitivity, safety, reliability and dependability. The reliability requirements of the protection system ensure that appropriate and operable protective measures taken even when certain parts of the protective device may fail [9].

As shown in Figure 1, the key features of the smart grid offer many advantages and prospects in the power industry, thereby revitalizing the socioeconomic strategies of this sector. However, the extensive applications of emerging technologies, if not considered, have vulnerabilities that may result in disasters such as long-term blackouts, economic failure, and so on. Table 1 provides a brief survey of some of

2. Challenges in implementation of smart grids

the challenges of smart grid technology [10].

energy and its technologies [1].

Micro-Grids - Applications, Operation, Control and Protection

demand balance [2].

Figure 1.

114

Future smart grid processing technology.

Challenges of smart grid technology.

Extensive research on this technology, which aims to overcome many challenges, has been launched by various universities. The system parameters and configuration of the power grids were investigated more intelligently.

### 3. Rogowski coil current sensor

The Rogowski coil (RC) current sensor works in the same way as a conventional AC core current transformer. They are not closed loops, making the coils open and flexible, and can be wrapped around the conductors [11–13]. RCs can immediately respond to the changing currents, down to a few nanoseconds, due to their low inductance. RC has no iron core to saturate, making it highly linear even when exposed to large currents. Linearity also allows high-current RCs to be determined using smaller reference currents. No danger is observed in opening the secondary winding [14]. Power construction costs and temperature compensation are simple [15–17]. Besides, RC does not use the magnetic core to support two windings. RC is designed with two coils that are electrically connected in the opposite direction, eliminating the electromagnetic field from the outer ring routing. For obtain the current sensor quality, the RC design must meet two important criteria; the first criterion, the mutual inductance M must have a constant value for any of the main conductor locations inside the coil loop, which can be achieved if the coil core has a constant cross section S, is perpendicular to the median line and is constructed with a constant turn density n. Second, the effects of adjacent conductors carrying a large current to the coil of the output signal should be minimal. The following formula defines mutual inductance M:

$$\mathcal{M} = \mu\_o \cdot n \cdot \mathcal{S} \tag{1}$$

• Accuracy: Rogowski coils can be used for measurement, protection, and

Innovative Differential Protection Scheme for Microgrids Based on RC Current Sensor

• Linearity (no saturation): Rogowski coils are linear over a wide range of

required. Calculation of precision limit factors is also unnecessary.

• No accuracy versus burden calculation: Rogowski coils are used with

sensors with smaller dimensions.

DOI: http://dx.doi.org/10.5772/intechopen.85473

transformers, and switches.

4. Proposed method of protection

Differential relay currents at the time internal fault.

Figure 4.

117

• Safety: Low secondary (transmitted) voltage.

control. Traditionally, separate secondary windings are used for measurement and protection. The linearity range of sensors enables the combination of sensors for metering and protection in a single device, thereby resulting in

currents. No calculation specific to the control for different primary currents is

microprocessors that have high input impedance. Therefore, calculating the accuracy and burden (no precision and load calculation) is not needed.

• Size: Rogowski coil devices are compact and can be easily combined with voltage sensors in one device, which is known as Combi sensor. Furthermore, RCs can be integrated into other equipment such as circuit breakers, power

• Weight: Rogowski coils are lightweight, especially compared to conventional current transformers with large cores. A higher weight/size advantage arises from the use of combined units, including both current and voltage sensors.

Figures 4 and 6 shows a single-phase (internal and external) fault differential protection system. Obviously, a pair of current sensors by using RC surrounded the protected area. Since this was a natural trend, differential protection provided protection for system equipment wire carrying current from the RC called a pilot wire.

When the core has a constant cross-section S, μ<sup>o</sup> is the permeability of the free space, and the winding line is perpendicular to the midline m, with a constant density n. The output voltage is proportional to the measured rate of change of current, as shown in Eq. (2) [18]:

$$v(t) = -\mu\_o n \text{S} \frac{di(t)}{dt} = -M \frac{di(t)}{dt} \tag{2}$$

Where M is the mutual inductance of the coil, also called the sensitivity of the RC. The CT iron core has a nonlinear characteristic and is therefore saturated when a high current or a direct current component is present in primary current. When CT is saturated (that is, the CT ratio error increases), which adversely affects the performance of the relay. Figure 2 displays the equivalent circuit of a current transformer.The current phase angle between the primary coil and the secondary voltage is almost 90° (due to the coil inductance Ls). Figure 3 shows the equivalent circuit of RCs.

Rogowski coils are linear and can be used in measurement applications. The oscillatory response of RC can be represented by voltage response and natural frequency, as described below:

$$\mathbf{V}\_o(t) = -\mathbf{M}\frac{di(t)}{dt} = -\mathbf{M}\frac{di(t)}{dt}\,\boldsymbol{x}^{-\xi w\_n t}\sin\left(\alpha\_n\sqrt{\mathbf{1}-\boldsymbol{\xi}^2}\right)\mathbf{t},\tag{3}$$

where ω<sup>n</sup> is the natural frequency, and ε is the damping coefficient. As a result, RCs can replace conventional CTs for measurement and protection. IEEE Std C37.235TM-2007 [19] provides guidance on the application of RC sensors when used for protection purposes, review the essential characteristics.

Figure 2. Current transformer equivalent circuit.

Figure 3. Rogowski coil equivalent circuits.

Innovative Differential Protection Scheme for Microgrids Based on RC Current Sensor DOI: http://dx.doi.org/10.5772/intechopen.85473

