KR20110034898A - 18-step back-to-back voltage source converter system for hvdc application - Google Patents

Abstract

PURPOSE: A 18-step back-to-back voltage source converter system for HVDC is provided to independently control reactive and active power of an AC system by controlling the size and phase of an output voltage. CONSTITUTION: An open Y-Y transformer(160) boosts or drops AC power. An auxiliary circuit(150) includes a three-level half-bridge(151) and a single phase transformer(155). First and second converters(110,120) are connected in parallel to the open Y-Y transformer and are serially connected to the auxiliary circuit.

Description

18-step Back-to-Back Voltage Source Converter System for HVDC Application}

The present invention relates to a voltage source converter for direct current transmission, and more particularly, two 18-step converters are respectively connected to a transmission end and a power receiving end in series with an AC side in parallel, and the DC side is coupled in parallel to operate at different firing angles. It is a back-to-back converter that can control the active and reactive power of AC system independently.

Recently, due to energy depletion and environmental protection, many countries are conducting a lot of research on power generation and utilization by renewable energy sources. As the amount of power generated from renewable energy sources increases, it is usually connected to the existing power grid by DC transmission method through a large-capacity voltage source converter.

Voltage source converters include PWM converters and multi-step converters. PWM converters have a simple system configuration, but switching losses are large because each switching element is switched several times per cycle. On the other hand, since the multi-step converter switches once per AC cycle, the switching loss is small but the output voltage cannot be adjusted, and the output of each module is overlapped with multiple modules to reduce the harmonic level. Should increase.

In order to increase the number of steps while reducing the appearance of the system, a method of installing an auxiliary transformer between the peripheral transformer and the converter bridge has been used, but this method is difficult to manufacture due to the complicated wiring structure of the auxiliary transformer and it is not cost-effective. Has

As a solution to this drawback, a method of forming an output voltage by superimposing an injection voltage on a constant voltage applied to a DC capacitor by inserting a separate tap-transformer and a separate H-bridge into a DC terminal of the converter is proposed. However, the tap-transformer used for voltage overlapping is difficult to exactly match the balance of the winding ratio in manufacturing, causing harmonics due to the asymmetry of the injection voltage, and the back-to-back coupling of the converter for grid linkage is not easy.

In order to improve the above disadvantages, the present inventor has proposed a 36-step converter using a three-level half-bridge instead of an H-bridge and a normal transformer instead of a tap-transformer. We have proposed a to-back converter. 1 shows a 36-step back-to-back converter according to the prior art (registration no. 10-0795752).

The configuration of a 36-step converter using an auxiliary circuit composed of a three-level half-bridge 15 and a single phase transformer 13 and two six-step bridges 11 and 12 is shown. The six-step bridges 11 and 12 are supplied with the voltages of the DC capacitors 14 and 14 'combined with the voltages formed by the three-level half-bridges 15 and the transformers 13. Thus, the output voltages formed at each of the six-step bridges 11 and 12 are combined by the three-phase transformers 16 and 17 to form a 36-step output waveform. For this converter, each six-step bridge 11, 12 operates at 60 Hz, while the three-level half-bridge 15 in the auxiliary circuit operates at 360 Hz.

However, the proposed 36-step back-to-back converter requires four peripheral transformers and two injection transformers at the power transmission and reception terminals, respectively, which has the disadvantage of increasing the appearance.

In order to apply this converter to the back-to-back DC power transmission, as shown in FIG. 2, four units should be coupled in series through a peripheral voltage transformer and two injection transformers at both ends. Therefore, the size of the system is unavoidable and also causes a cost problem.

The present invention applies a back-to-back converter system having an auxiliary circuit that uses a normal transformer instead of a mid-tap transformer and a three-level half bridge instead of a full bridge, and applies active and reactive power of an AC system connected to the system. To provide a converter system that can control the independently.

Furthermore, in order to apply the conventional back-to-back DC power transmission, four units must be combined in series with the peripheral pressure transformer at the power transmission end and the power receiving end, and two injection transformers are required at both ends. In order to solve the problem that causes the cost problem, the present invention provides a back-to-back converter using two 18-step converters for each of the power transmission and reception terminals.

The present invention to achieve the above technical problem, an open Y-Y transformer for boosting or stepping down AC power of a predetermined size; Auxiliary circuitry comprising a three-level half-bridge and a single phase transformer; Capacitors for DC voltage division; And first and second converters connected in parallel with the open Y-Y transformer and connected in series with the auxiliary circuit.

Preferably, the first and second converters are configured by a pair of six switching elements, and each breakpoint of each pair of the first converter and the breakpoint of each pair of the second converter correspond to each other. And two clamping diodes each connected in series.

The DC voltage dividing capacitor may include a first DC capacitor connected between the other end of the primary side winding of the single phase transformer and the other end of the secondary side winding of the auxiliary circuit and the output end of the first converter; And a second DC capacitor connected between the other end of the primary side winding and the second end of the secondary side winding of the auxiliary circuit of the auxiliary circuit and the ground end of the second converter.

More preferably, the auxiliary circuit includes: first to fourth switching elements connected in parallel with the first and second DC capacitors; A first clamping diode having one end connected to a connection point of the first and second switching elements; And a three-level half-bridge consisting of a second clamping diode connected to the connection point of the third and fourth switching elements.

And the auxiliary circuit, one end of the primary winding of the single phase transformer is connected to the breakpoints of two clapping diodes selected from the first and second converters, and one end of the secondary winding of the single phase transformer is connected to the second winding. And a connection point of the third switching element.

According to the present invention, the number of steps of the output voltage is small, the structure of the transformer for the connection is simplified and the number is reduced, thereby reducing the installation area of the system and it is possible to reduce the installation cost.

Two 18-step converters are connected to the AC side and the DC side in parallel to operate at different firing angles to control the magnitude and phase of the output voltage, independent of the active and reactive power of the connected AC system. Control is possible.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

According to the present invention, two 18-step converters are respectively connected in series with the AC side and the DC side in parallel with each other, and the DC side is operated in different firing angles to independently control the active and reactive power of the connected AC system. It is a back-to-back converter.

3 shows a basic configuration of an 18-step converter which is a basic module of a back-to-back converter according to the present invention.

The converter system according to the present invention includes an open circuit YY transformer 160 for boosting or stepping down AC power of a predetermined magnitude, an auxiliary circuit 150 including a three-level half-bridge 151 and a single-phase transformer 155, and DC voltage division. And a first converter 110 and a second converter 120 connected in parallel with the capacitors 140a and 140b and the open YY transformer 160 and in series with the auxiliary circuit 150.

Here, the first converter 110 and the second converter 120 are composed of a pair of six switching elements, each of the pair of switching points of the first converter 110 and the second converter 120 The breakpoints of each pair of switching elements correspond to each other and are connected by two clamping diodes connected in series.

In addition, the DC voltage dividing capacitors 140a and 140b may include a first terminal connected between the other end of the primary winding of the single phase transformer 155 of the auxiliary circuit 150 and the other end of the secondary winding and the output terminal of the first converter 110. To the second DC capacitor 140b connected between the other end of the primary winding and the other end of the secondary winding and the ground terminal of the second converter 120 of the DC capacitor 140a and the single-phase transformer 155 of the auxiliary circuit 150. It is composed.

And first to fourth switching elements connected in parallel with the first and second DC capacitors 140a and 140b of the auxiliary circuit 150, and having one end connected to a connection point of the first and second switching elements. A three-level half-bridge 150 consisting of a first clamping diode and a second clamping diode, the other end of which is connected to the connection point of the third and fourth switching elements, wherein one end of the primary winding of the single phase transformer 155 is One end of the secondary winding of the single phase transformer 155 is connected to the connection point of the second and third switching elements. .

In the present invention, the converter includes an auxiliary circuit 150 composed of a three-level half-bridge 151 and a single phase transformer 155 in a DC stage and is connected to a system through one peripheral transformer 160.

와 보조브리지가 주입변압기를 통해 공급하는 전압

를 이용하여 다음과 같이 나타낼 수 있다.As shown in FIG. 3, the DC upper and lower voltages of the converter are the voltages of the DC capacitors.

And auxiliary bridges supply voltage through injection transformer

It can be expressed as follows.

[식 1]

[Equation 1]

[식 2]

[Formula 2]

에 의해서 조절이 가능함을 알 수 있다. 한편 주입전압

는 보조브리지회로의 스위칭 패턴과 주입변압기의 권수비에 의해 결정된다. 따라서 주입전압

는 0,

k,

인 3-레벨 값으로 정해지고, 보조브리지회로는 120˚마다 동작하기 때문에 기본주파수의 3배에 해당하는 펄스를 형성하게 된다. 보조회로에 사용되는 변압기의 권선비 k는 출력 전압파형의 고조파수준을 최소화하도록 결정하여야 한다.According to the above [Formula 1] and [Formula 2], the output AC voltage of the converter

It can be seen that can be adjusted by. Meanwhile injection voltage

Is determined by the switching pattern of the auxiliary bridge circuit and the turn ratio of the injection transformer. Therefore, injection voltage

Is 0,

k,

It is defined as the in-level value, and the auxiliary bridge circuit operates every 120 degrees, thereby forming a pulse corresponding to three times the fundamental frequency. The turns ratio k of the transformer used in the auxiliary circuit should be determined to minimize the harmonic level of the output voltage waveform.

에

를 가하거나 감한 파형을 나타낸다. 도 4의 (d)와 도 4의 (e)는 출력 선간전압과 상전압을 나타낸 것이다.Figure 4 shows the waveform of each part of the 18-step converter proposed in the present invention. Figure 4 (a) shows the output waveform of the auxiliary bridge circuit, it can be seen that there are three pulses per cycle of AC and operates every 120 degrees. 4 (b) and 4 (c) show voltage waveforms applied to the upper and lower converter parts, respectively.

on

Add or subtract the waveform. 4 (d) and 4 (e) show output line voltages and phase voltages.

On the other hand, since the magnitude of the voltage injected by the auxiliary bridge affects the harmonics of the output voltage, the turn ratio of the injection transformer should be selected to the optimum value.

When the output voltage shown in Fig. 4E is developed by Fourier series, it is as follows.

[식 3]

[Equation 3]

Here, the magnitude of each harmonic component including the fundamental wave can be expressed by the following [Equation 4].

[식 4]

[Equation 4]

₀와

을 각각 20˚로 할 때 크기가 0이 되어야 한다. 이때 k 값을 구하면 k=0.574 이다.The component that most affects the output waveform of the inverter is the 5th harmonic, so to replace it, substitute n = 5 in the above formula.

₀ and

When each is 20˚, the size should be 0. In this case, the value of k is k = 0.574.

FIG. 5 shows a conceptual diagram of a back-to-back HVDC system. As shown in FIG. 5, a back-to-back converter connects two alternating current systems having different frequencies, or direct current transmission or a large-scale wind farm. Used to link to Therefore, the back-to-back converter should be able to independently control the active and reactive power of the two AC systems connected and control the bidirectional power current.

If the converter operates in PWM mode, the magnitude and phase of the AC output voltage can be adjusted to allow independent control of active and reactive power. However, when operating in multi-step, the magnitude of AC output voltage is fixed and independent control of active and reactive power is impossible.

과 컨버터_2의 점호각

를 상이하게 동작시켜 연계된 교류계통의 유·무효전력을 독립적으로 제어하는 방식을 제안한다.In the present invention, after the two 18-step converters are connected in series with the AC side and in parallel with the DC side, the firing angle of the converter_1

Firing angle of the converter and converter_2

By operating differently, we propose a method to independently control the active and reactive power of the connected AC system.

과 하단점호각

의 값을 적절히 조절하면 컨버터(200)의 출력전압벡터

는 전원전압

와 임의의 전력각

를 이루고 그 크기를 조절하는 것이 가능하다. 따라서 상·하단 점호각

과

를 조절하여 연계된 교류계통의 유·무효전력을 독립적으로 제어하는 것이 가능하다.(A) of FIG. 6 shows a single phase equivalent circuit of one end of a transmitting and receiving end of the back-to-back converter 200 according to the present invention. And when the upper and lower firing angles of the converter 200 are different, the vector diagram of the power supply voltage, the voltage of each converter, and the converter synthesis voltage is shown in Fig. 6B. As can be seen in Figure 6 top firing angle

And bottom firing angle

When the value of is properly adjusted, the output voltage vector of the converter 200

Power supply voltage

And arbitrary power angle

It is possible to achieve and adjust its size. Therefore, upper and lower firing angle

and

It is possible to control the active and reactive power of the connected AC system independently.

In the equivalent circuit of FIG. 6A, the active and reactive power can be expressed by the following equation.

[식 5]

[Equation 5]

[식 6]

[Equation 6]

와

를 얻기 위한 두 방정식을 도출할 수 있다.Combine the above [Equation 5] and [Equation 6], using the root formula of the quadratic equation,

Wow

We can derive two equations for

[식 7]

[Equation 7]

[식 8]

[Equation 8]

와 전력각

에 대해 상·하단 컨버터(210, 220)의 점호각

과

의 관계를 도출하기 위한 수식은 도 6의 (b)의 벡터도를 사용하여 전 개할 수 있다.

와

또는

와

사이의 각을

라고 하면, 다음 두 관계식을 얻을 수 있다.Output voltage

And power angle

Firing angle of upper and lower converters (210, 220)

and

The equation for deriving the relationship of can be developed using the vector diagram of FIG.

Wow

or

Wow

Angle between

In this case, the following two relations can be obtained.

[식 9]

[Equation 9]

[식 10]

[Equation 10]

과

에 관한

의 관계, 그리고

과

에 관한

의 관계는 하기 [식 11]과 [식 12]로 표현된다.

의 최대값은

의 1.15배로 가정한다.

and

To about

Relationship, and

and

To about

The relationship of is represented by the following [Equation 11] and [Equation 12].

The maximum value of is

Assume 1.15 times

[식 11]

[Equation 11]

[식 12]

[Equation 12]

과

는 하기 [식 13]과 [식 14]로 표현될 수 있다.By combining the above [Equation 11] and [Equation 12], the firing angle of the upper and lower converters (210, 220)

and

Can be represented by the following [Equation 13] and [Equation 14].

[식 13]

[Equation 13]

[식 14]

[Equation 14]

과

의 값을 일률적으로 결정할 수 있다.Thus, for a given P and Q

and

The value of can be determined uniformly.

TABLE 1

와

의 값을 보여준다.[Table 1] shows the values of P and Q when the line voltage is 220V and the coupling inductance is 10% of the 10KVA reference capacity.

Wow

Shows the value of.

과

의 변화를 Q에 따라서 보여준다. 무효전력 Q가 유도성에서 용량성으로 이동함에 따라,

과

의 값이 0ㅀ축에 가까워진다.7 relates to P

and

The change of is shown by Q. As reactive power Q moves from inductive to capacitive,

and

The value of close to the 0 ㅀ axis.

In order to apply the back-to-back converter according to the present invention to a voltage type HVDC system, system operation characteristics and performance analysis of the controller were performed using PSCAD / EMTDC. The power circuit was constructed using circuit elements, switches and transformers, and the controller was constructed using the built-in control module.

8 shows a schematic diagram of a system in which a 10-step back-to-back converter according to the present invention is applied to an HVDC.

The circuit constants used in the simulation for the system of FIG. 8 are shown in Table 2 below.

TABLE 2

Voltage source HVDC systems operate by controlling the firing angle to adjust the magnitude of the terminal voltage across the converter and to redirect the power. The control target of each converter changes according to the direction of power transmission. That is, when power is transmitted from system A to converter B, converter A 200 performs constant DC voltage control and reactive power control, and converter B 300 controls active power and reactive power. When power is transferred from system B to A, reverse control is performed. Therefore, the controller of each converter has the same configuration as shown in FIG.

의 실측값은 기준값

를 따른다. 유효전류의 기준값

는 컨버터 B의 유효전류 실측값

로부터 얻어진다. 무효전류

와 유효전류

의 실측값은 제어 알고리즘을 통해 기준값

와

를 따른다. 교류 전류 제어기는 컨버터에 일반적으로 사용되는 제어기와 동일한 구성을 갖는다. d-q 변환된 교류 시스템 전압의 기준값

와

는

와

의 값을 결정하기 위해 사용된다.

과

의 값은 식 (13) 과 (14)에 의해서

와

로부터 얻어진다.DC power when power is transferred from converter A 200 to converter B 300.

Measured value is the reference value

Follow. Reference value of active current

Is the actual current measured value of converter B

Is obtained from. Reactive current

And active current

The measured value of is based on the control algorithm

Wow

Follow. The AC current controller has the same configuration as the controller generally used in the converter. dq Reference value of the converted AC system voltage

Wow

Is

Wow

It is used to determine the value of.

and

The value of is given by equations (13) and (14).

Wow

Is obtained from.

[Table 3]

[Table 3] shows the simulation scenarios used in the operation analysis of the proposed back-to-back converter. The controller starts its operation 0.5 seconds after the simulation starts. From 0.5 seconds to 3.0 seconds, power was transferred from System A to B, and then from System B to Power at 3.0 seconds. Also, in order to analyze the active power and reactive power control performance of the system, the reference values of active power and reactive power were changed for each mode (M1 ~ M5).

, 상단 점호각

, 그리고 하단 점호각

의 변화를 나타낸 것이다.Simulation results for verifying overall system operation are shown in FIGS. 10 to 12. The controllers of systems A and B have the same structure, all control values are the same and the signs are reversed. 10A and 10B show power angles of output voltages.

, Top firing angle

, And bottom firing angle

The change is shown.

11 (a) and 11 (b) show the converter output voltages and the synthesized output voltages. 12 (a) shows the control performance of the DC link voltage. The DC link voltage was initially charged to 350V to allow the system to start operating in 0.5 seconds. The measured DC link voltage well follows the reference value of 350V without large transients. FIG. 12B shows active power and reactive power transmitted from the AC system A to the converter A. FIG. It is confirmed that active power and reactive power of system A are controlled independently. FIG. 12C shows active power and reactive power transmitted from the converter B to the AC system B. As shown in FIG. The active power has almost the same magnitude as the value in FIG. 12B, but the reactive power is different because each converter performs independent control. As can be seen from this result, the proposed system is capable of independent control of active power and reactive power.

Furthermore, for operation verification through hardware implementation of the 18-step back-to-back converter according to the present invention, hardware was fabricated and experimented as shown in FIG. 13. The experimental set was configured to connect three-phase power to both sides with a power simulator and an 18-step back-to-back converter according to the present invention. Table 4 below shows the experimental circuit constants used in the scale model.

The control board was composed of main board and expansion board. The main processor used DSP (TMS320vc33), a real processor developed by Texas Instruments. The main board used an EPLD (EP1K50QC208), a 50,000 gate developed by Altera, and the expansion board used EPLD (EP1K50QC208) for PWM logic.

[Table 4]

In order to confirm whether the back-to-back converter system according to the present invention performs the active and reactive power control in the actual hardware, the experiment was performed according to the scenario of the following [Table 5].

의 관계를 가지고 있다. 도 14의 (d)는 상ㆍ하단 컨버터의 출력전압과 각 컨버터의 합성전압, 보조변압기에 의한 주입전압을 나타낸다. 상ㆍ하단 컨버터의 출력전압이 합해져 컨버터의 합성전압이 됨을 확인할 수 있고, 보조변압기에 의해 기본주파수의 3배에 해당하는 3-레벨 전압이 주입되고 있음을 확인할 수 있다. The waveforms obtained by changing the active and reactive current reference values according to the scenario and measuring the active and reactive power on both sides are shown in FIG. It can be seen that both sides A and B are following the desired reference values smoothly and the reactive power is controlled independently. Figure 14 (b) is a waveform of the DC-link voltage on both sides of the back-to-back converter. DC-link voltage control is performed by the A side or B side converter according to the transmission direction of the active power, and when the control starts, it can be seen that the reference value is well followed without a large transient. 14C is a waveform of power angle shown in the operation principle vector diagram.

Have a relationship. Fig. 14D shows the output voltages of the upper and lower converters, the synthesized voltages of the respective converters, and the injection voltages of the auxiliary transformers. It can be seen that the output voltages of the upper and lower converters are combined to form a synthesized voltage of the converter, and that the three-level voltage corresponding to three times the fundamental frequency is injected by the auxiliary transformer.

TABLE 5

In the present invention, two 18-step converters are respectively connected to the transmission and reception terminals in alternating current and in parallel with the direct current side, and operated at different firing angles, thereby controlling the magnitude and phase of the output voltage. A new structure of back-to-back converter with independent control is proposed.

Such a back-to-back converter according to the present invention has a lower number of steps of the output voltage compared to the conventional system, but the harmonic level is rather high, but the structure of the linking transformer is simple and the number thereof is small and the installation cost is low. In addition, the harmonic level according to 18-step can be reduced by the filter.

The above description is merely illustrative of the technical idea of the present invention, and those skilled in the art to which the present invention pertains may make various modifications and changes without departing from the essential characteristics of the present invention. Therefore, the embodiments described in the present invention are not intended to limit the technical idea of the present invention but to explain, and the technical idea of the present invention is not limited by these embodiments. The protection scope of the present invention should be interpreted by the following claims, and all technical ideas within the scope equivalent thereto should be construed as being included in the scope of the present invention.

1 shows a 36-step back-to-back converter according to the prior art (registration number 10-0795752),

Figure 2 shows a DC power transmission system applying a converter according to the prior art,

3 shows a basic configuration of an 18-step converter which is a basic module of a back-to-back converter according to the present invention.

Figure 4 shows the waveform of each part of the 18-step converter according to the present invention,

5 shows a conceptual diagram of a back-to-back HVDC system,

6 shows an operation principle of a back-to-back converter according to the present invention,

7 shows a change in the firing angle of each converter in the present invention,

8 shows a schematic diagram of a system applying a 10-step back-to-back converter to an HVDC according to the present invention;

FIG. 9 shows a configuration of a converter controller of system A of FIG. 8,

FIG. 10 shows the change of the power angle of the output voltage and the firing angle of each converter in the system of FIG.

11 shows each converter output voltage and the composite output voltage,

12 shows a DC link voltage and transmission effective and reactive power of each converter,

Figure 13 shows an embodiment for a hardware implementation of an 18-step back-to-back converter according to the present invention,

FIG. 14 shows experimental results of the embodiment of FIG. 13.

<Description of Major Symbols in Drawing>

110: first converter, 120: second converter,

140a, 140b: capacitor for direct voltage division,

150: auxiliary circuit, 160: open Y-Y transformer,

200: Converter A system, 300: Converter B system.

Claims (5)

An open Y-Y transformer for boosting or stepping down AC power of a predetermined magnitude; Auxiliary circuitry comprising a three-level half-bridge and a single phase transformer; Capacitors for DC voltage division; And And first and second converters connected in parallel with the open Y-Y transformer and connected in series with the auxiliary circuit. The method of claim 1, The first and second converters, It consists of six pairs of two switching elements, And a break point of each pair of the first converter and a break point of each pair of the second converter are connected to each other by two clamping diodes connected in series. The method of claim 2, The DC voltage division capacitor, A first DC capacitor connected between the other end of the primary side winding and the second end of the secondary side winding of the auxiliary circuit of the auxiliary circuit and the output end of the first converter; And And a second DC capacitor connected between the other end of the primary side winding and the other end of the secondary side winding of the auxiliary circuit of the auxiliary circuit and the ground end of the second converter. The method of claim 3, wherein The auxiliary circuit, First to fourth switching elements connected in parallel with the first and second DC capacitors; A first clamping diode having one end connected to a connection point of the first and second switching elements; And And a three-level half-bridge consisting of a second clamping diode, the other end of which is connected to the connection point of the third and fourth switching elements. The method of claim 4, wherein The auxiliary circuit, One end of the primary winding of the single phase transformer is connected to a breakpoint of two clapping diodes selected from the first and second converters, And one end of the secondary winding of the single phase transformer is connected to a connection point of the second and third switching elements.

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