Analysis on the application of current transformer in low voltage flexible DC microgrid

Low-voltage flexible DC microgrids are bridges between DC-specific devices such as energy storage, electric vehicles, and distributed energy and medium-voltage DC distribution networks, and are an important part of energy Internet technology. With the development of low-voltage flexible DC technology and the access of more and more DC devices, the development of flexible DC microgrids is necessary and urgent. Flexible DC microgrids have the advantages of large power supply capacity, high power quality, high transmission efficiency, high power supply reliability, and convenient access to clean energy. At present, they have become the key development direction in the field of flexible DC technology. Several demonstration projects have been built at home and abroad  , which have greatly promoted the development of technology.

The DC current transformer is an essential part of the low-voltage flexible DC microgrid. It plays an important role in DC power metering, grid control and protection. Low-voltage DC transformers generally require energy supply circuits. Their technology is more complex than that of AC transformers of the same voltage level, making them difficult to manufacture. In addition, there are currently few application scenarios and the procurement cost is high. Since the flexible DC microgrid directly faces users, has a large user base and will have a large scale of application in the future, it is of great significance to select a DC current transformer that meets the scenario requirements and is low-cost to improve the quality of the project and reduce the overall project cost of the flexible DC microgrid. This article analyzes the current development status of DC transformer products, proposes solutions suitable for application scenarios, and provides suggestions for similar application scenarios.

1 Analysis of Typical Application Scenarios of Low-Voltage Flexible DC Microgrid

1.1 Overview of Typical Application Scenarios

A ±375 V flexible DC microgrid is constructed in a certain science and technology park. To meet the needs of park users, the microgrid is equipped with a total capacity of 0.96 MW electric vehicle fast DC charging piles, including 12 60 kW DC charging piles for cars and 2 120 kW (one machine and two guns) DC charging piles for buses. The park load is 1 MW, including 0.5 MW DC load (except DC charging piles) and 0.5 MW AC load powered by DC inverters; a 700 kWp distributed photovoltaic power generation system is built on the roof space of the park to provide part of the clean power supply for the park load; the park is equipped with a 1 MW/1 MWh electrochemical energy storage system to solve the power supply reliability of a single DC bus and the output power fluctuation problems of photovoltaic power generation, to ensure the second reliable independent power supply support for the single DC bus load in the park, to support the operation of the park fast charging piles, to help absorb photovoltaics, to achieve peak shaving and valley filling, and to operate the DC microgrid in an isolated island when the DC distribution network fails. Combined with the above park power supply requirements, a ±375 V flexible DC microgrid was constructed, and its topology diagram is shown in Figure 1 .

Figure 1 Typical flexible DC microgrid topology

 

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The DC microgrid adopts a single busbar connection type. The DC transformer (2 MW) reduces the medium voltage ±10 kV DC distribution network to a low voltage ±375 V and connects to the DC busbar of the DC microgrid. The DC busbar is connected to branches such as 0.7 MWp photovoltaic system, 1 MW/1 MWh electrochemical energy storage system, 1 MW electric vehicle DC charging pile, 0.5 MW DC load, and 0.5 MW AC load connected via DC/AC inverter. Each branch is equipped with a low-voltage DC circuit breaker and a low-voltage DC current transformer.

1.2 Analysis of Typical Application Scenarios

The rated current and minimum operating mode of each branch of the flexible DC microgrid are shown in Table 1 .

Table 1 Rated current of each branch and minimum operating mode

Tab. 1 Rated current and minimum current of each branch

Branch name	Rated current/A	Minimum operation mode
DC low voltage side	267 0	0.1 pu
Energy Storage	133 0	0.1 pu
Photovoltaic	930	0.1 pu
Charging Station	133 0	0.1 pu
DC load	670	0.1 pu
AC load	670	0.1 pu

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Consider the following typical operating conditions: the real-time power of the charging pile is 0.5 MW, the real-time power of the DC load is 0.5 MW, the real-time power of the AC load is 0.5 MW, the output power of the photovoltaic power generation system is 0.5 MW, the charging and discharging power of the energy storage system is 0.5 MW, and the steady-state power of the DC transformer is 1 MW.

A DC microgrid simulation test model was built in the simulation software [11] . The simulation test considered the most serious fault condition under typical operating conditions, that is, a short-circuit fault between the ±375 V sides of each port, with a short-circuit resistance of 10 mΩ. The simulation calculation results are shown in Table 2 :

Table 2 Short-circuit current of each branch when each port is short-circuited

Tab. 2 Short-circuit current of each branch under terminal short circuit kA

Short circuit location	DC transformer low voltage side branch	Photovoltaic branch	Charging pile branch	Energy storage branch	DC load branch	AC load branch
DC transformer ±375 V side pole	7.33	2.42	0	3.21	0	1.6
Photovoltaic outlet pole	4.21	10.79	0	3.78	0	2.0
Between poles at the charging pile exit	4.19	2.45	11.42	3.66	0	1.4
Interpolar space at energy storage outlet	4.22	2.56	0	8.78	0	2.2
±375 V busbar short circuit	4.29	2.31	0	2.31	0	1.6
Inter-pole at DC load outlet	4.23	2.41	0	3.24	11.22	1.9
AC load outlet between poles	4.23	2.54	0	3.17	0	9.8

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From the above simulation results, the selection requirements of low voltage DC current transformer can be obtained:

1) Since DC equipment can operate within the range of the minimum operating point (0.1 p.u) to the rated current (1.34 p.u), the DC equipment should be controllable within the range. DC transformers and energy storage devices have constant current control functions and should meet the requirements of constant current control for current transformers: the accuracy of the current transformer from the minimum operating point (0.1 p.u) to the rated current (1.34 p.u) should not be less than 0.2%.

2) When a fault occurs between poles of each branch, the maximum short-circuit current is 11.42 kA, which is 20 times the rated current of the branch. Therefore, it is necessary to ensure that the DC current transformer is not saturated in this case to avoid false protection.

3) The current transformer should have a fast enough response speed to avoid false protection caused by delay.

4) The application scenario is low-voltage DC switchgear, and the cost should be within a reasonable range.

According to the above requirements, the selection of DC current transformer was carried out.

2 Selection and Analysis of Low Voltage DC Current Transformer

This paper considers the current industrialization status of current transformers, selects all-optical fiber current transformers, resistance shunts, giant magnetoresistance effect sensors and zero-flux Hall current transformers used in high-voltage DC projects as comparison objects, analyzes their principles, compares various current transformer parameters, and obtains the basis for selecting low-voltage DC transformers.

2.1 Principle Analysis

2.1.1 All-fiber mutual inductor

The all-fiber mutual inductor measures the line integral of the magnetic field strength around the current. Since the magnetic field strength is proportional to the current, the current can be measured. Its basic principle is Ampere’s circuit theorem and Faraday’s magneto-optical effect [ 2 , 3 ] . The all-fiber mutual inductor is currently used for polar line current measurement in UHV DC projects. The light source, optical fiber, beam splitter, optical modulator, electronic components and other equipment in the mutual inductor are easily affected by the ambient temperature and have high manufacturing process requirements. In addition, the amount of the mutual inductor is relatively small, resulting in the high cost of the all-fiber mutual inductor. The schematic diagram of the all-fiber mutual inductor is shown in Figure 2.

Figure 2 Schematic diagram of all-fiber mutual inductor

 

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The all-fiber mutual inductor consists of three parts: a laser generator, a fiber-optic sensor, and an optical signal processing system. The laser generator generates optical signals; the optical fiber sensor element converts the magnetic field intensity around the conductor into the phase difference between two coherent beams of light; the optical signal processing system converts the optical signal into an electrical signal, and realizes the modulation and demodulation of the signal to calculate the measured current.

All-fiber mutual inductors require a power supply circuit. Currently, the reliability of laser power supply circuits is low, and the reliability of existing ultra-high voltage DC all-fiber mutual inductors is subject to the reliability of laser power supply circuits.

All-fiber sensors do not have the problems of resistance heating and core saturation of other types of transformers. They have a large measurement range and high measurement accuracy.

2.1.2 Resistor Shunt

The resistor shunt is composed of a four-terminal resistor of manganese-nickel-copper alloy, which has two current terminals and two voltage measurement terminals [ 4 , 5 , 6 ] . When the primary current flows into the current terminal and the resistance value of the resistor is known, the voltage drop at the voltage measurement terminal can be measured to obtain the current value. The resistor shunt is currently used to measure the pole line current of UHV DC projects. Because the manufacturing process is simpler than that of the all-fiber mutual inductor and the amount used is larger, its cost is lower than that of the all-fiber mutual inductor.

The resistor shunt is connected in series in the loop, and the DC current value is measured on the primary side. After the signal is processed and converted, it is connected to the merging unit through optical fiber, and the merging unit supplies energy to the primary side through optical fiber. The measurement principle of the resistor shunt is shown in Figure 3.

Figure 3 Schematic diagram of resistor shunt current transformer

 

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The resistor shunt requires a power supply circuit. Currently, the reliability of the laser power supply circuit is low. In existing UHV DC projects, the reliability of the resistor shunt transformer is subject to the reliability of the laser power supply circuit.

The resistor shunt is made of manganese-copper alloy resistor and must work within the optimal temperature range. If a large current passes through the resistor, the temperature rise will reduce the measurement accuracy. Therefore, the accuracy of the resistor shunt is reduced when the current is large, and the maximum measurement range is set to 6 p.u.

2.1.3 Giant Magnetoresistance Effect Sensor

The phenomenon that the resistance of a material changes under a certain magnetic field is called the “magnetoresistance effect”. Under certain conditions, the resistivity of a material under the action of a magnetic field decreases by more than 10 times compared to the magnetoresistance value of ordinary magnetic metals and alloy materials. This phenomenon is called the “giant magnetoresistance effect” (TMR) [ 7 ] . The principle of the giant magnetoresistance effect sensor is shown in Figure 4.

 

Figure 4 Schematic diagram of giant magnetoresistance effect sensor 1

 

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When current flows through a conductor, the magnitude of the magnetic field around the conductor is proportional to the magnitude of the current in the conductor. As shown in Figure 5, four TMR resistors are used to form a Wheatstone bridge structure, in which resistors R1 and R3 are shielded by NiFe material and cannot sense changes in the magnetic field, and resistors R2 and R4 are not shielded. Under the action of the external magnetic field, the resistance values of the shielded R1 and R3 do not change, while the resistance values of the unshielded R2 and R4 change, causing the bridge output to change. The output of the bridge reflects the magnitude of the external magnetic field, thereby reflecting the magnitude of the bus current. Giant magnetoresistance effect sensors are currently used in flexible DC systems for distribution networks. Giant magnetoresistance effect sensors require a laser power supply circuit.

Figure 5 Schematic diagram of giant magnetoresistance effect sensor 2

 

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The giant magnetoresistance effect sensor is made of resistors and must work within the optimal temperature range. If a large current passes through the resistor, the temperature rise will reduce the measurement accuracy. Therefore, the accuracy of the giant magnetoresistance effect sensor decreases when the current is large, and the maximum measurement range is set to 6 p.u.

2.1.4 Zero-flux Hall Current Transformer

The basic principle of the zero-flux Hall current transformer is based on complete magnetic flux potential balance, and its schematic diagram is shown in Figure 6. J is the detection winding, J is the dynamic detection unit, and K generates a self-balancing adjustment current. The magnetic potential balance equation of the loop is

Figure 6 Schematic diagram of zero flux Hall current transformer

 

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(1)

I0 is the primary current, and its excitation magnetic potential generates an induced voltage at both ends of NJ . The dynamic detection unit J detects the induced voltage at both ends of NJ , and generates a self-balancing adjustment current I1 through K , which is supplied to the N1 winding ; I1 generates a magnetic flux opposite to I0 to achieve magnetic potential balance [ 8 , 9 ] . Finally, the current I1 is all supplied by K. J dynamically detects the voltage at both ends of NJ at any time. When the voltage difference is small enough, the magnetic flux in the core is zero flux. If the detection value deviates from the allowable value, K is adjusted at high speed so that the core always remains in the zero flux state. At this time, the sensor accuracy is high.

Zero-flux Hall current transformers are currently used to measure neutral line current in UHV DC projects. Because the technology is mature and does not require a laser power supply circuit, their cost level is the lowest.

The zero-flux Hall current transformer saturates the iron core through a large current. The maximum unsaturated current is lower than that of other types of sensors. Due to the closed-loop sensing principle, the measurement accuracy is lower than that of other types of sensors.

2.2 Comparison of DC current transformer parameters

The parameter comparison of DC current transformer is shown in Table 3 .

Table 3 Parameter comparison of DC current transformers

Tab. 3 Comparison between several DC current transformers

parameter	All-fiber	Resistor Shunt	Magnetoresistance effect	Zero Flux Hall Current Transformer
Accuracy	0.2%	0.1 Id.～1.34 Id<0.5	0.1 Id.～1.34 Id<0.2%	0.5
		1.34 Id.～3 Id<1.5%	1.34 Id.～3 Id<1.5%
		3 Id.～6 Id<10%	3 Id.～6 Id<3%
Maximum unsaturated current	30 pu	6 pu	6 pu	1.5 pu
Delay	10 μs	400 μs	50 μs	1 μs
Cost	high	medium	medium	Low
±375 V commercialization	none	have	have	have
Requires laser power	√	√	√	×
High current shock	allow	allow	allow	Unable to withstand more than 5 times impact

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2.3 Analysis of Current Transformer Application in Typical Scenarios

Comparing the DC current transformers listed in Section 2.2, the following analysis can be obtained:

1) All-fiber mutual inductors can simultaneously meet the requirements of the measurement accuracy of the minimum operating point and the maximum unsaturated current range. However, the all-fiber mutual inductor has a complex process and a small amount, which leads to its high cost. In addition, there is no ready-made ±375 V product, which is not suitable for the application scenario proposed in this article.

2) The accuracy of the zero-flux Hall current transformer is less than 0.2, and it cannot withstand current shocks greater than 5 times. It is not suitable for the application scenarios proposed in this article. It can be used in scenarios where the measurement accuracy requirements are not high within the range from the minimum operating point to the rated current, or the short-circuit current is not large.

3) Both the resistor shunt and the magnetoresistance effect sensor cannot simultaneously meet the requirements of low current accuracy and maximum unsaturated current range.

Based on 3, first select the current transformer based on the principle of meeting the maximum fault current unsaturation.

Since the DC transformer and energy storage equipment have a constant current operation mode within the range of (0.1 p.u) to (1.34 p.u), the current transformer accuracy of the low-voltage side port of the DC transformer and the energy storage converter port of the energy storage branch is required to be 0.2%, but it cannot simultaneously meet the accuracy requirements of the maximum fault current being unsaturated and the minimum operating point (0.1 p.u). Therefore, these two branches are equipped with current transformers dedicated to control to meet the control accuracy requirements between the minimum operating point (0.1 p.u) and the rated operating point (1.34 p.u). Both the resistance shunt and the magnetoresistance effect sensor can resist large current shocks and will not be damaged by large current shocks in the short-circuit state.

Therefore, for the application scenarios proposed in this paper, resistance shunts or magnetoresistance effect sensors are selected as current measurement devices. For branches with high control accuracy requirements between the minimum operating point (0.1 p.u) and the rated operating point (1.34 p.u), such as the low-voltage side of the DC transformer and the energy storage branch, protection and control are configured with independent current transformers to simultaneously meet the requirements of small current control accuracy and maximum unsaturated current range; for branches with low control accuracy requirements, protection and control share a common current transformer, which should meet the maximum unsaturated current range.

3 Conclusion

This paper is based on the current status of current transformer industrialization and combines application scenarios for analysis, and draws the following conclusions:

1) In the application scenario of this article, the maximum short-circuit current is 20 times the rated current, the minimum operating current is 1/10 of the rated current, and the DC transformer and energy storage device have constant current control requirements and high control accuracy requirements. Therefore, a resistor shunt or magnetoresistance effect sensor is selected as the current measurement device; for branches with high short-circuit current levels and high control accuracy requirements such as DC transformers and energy storage, two current transformers are installed, and one current transformer is used for protection and control respectively; for branches with high short-circuit current levels and low control accuracy requirements, only one current transformer is installed, and protection and control are shared.

2) For low-voltage flexible DC microgrid applications where the measurement accuracy requirement is not high within the range from the minimum operating point to the rated current, or the short-circuit current is not large, a zero-flux Hall current transformer can be used to reduce the project cost. At the same time, the Hall device does not require a laser power supply circuit, which can improve the operating reliability of the current transformer.

3) Since user-side equipment such as photovoltaics, energy storage, and electric vehicles all exhibit DC characteristics, if the scope of application increases in the future, all-optical mutual inductors can be selected for mass production, which can meet engineering requirements while significantly reducing construction costs.

 

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