Title of the invention : I DENTI FICATION M ETHOD FOR I DENTI FYI NG A RESONANCE OF A POWER GRI D, AN D GRI D-CON N ECTED U N IT Technical field

0001

The invention relates to identification method for identifying a resonance of a power grid, particularly to detecting the occurrence of resonance and its frequency for stabilizing the operation in grid-connected units.

Background Art

0002

Resonance at the Point of Common Coupling on the power grid has to be within the limits specified e.g . by a grid code regulation . This resonance occurs as different frequency waveforms on the top of fundamental frequency voltage and current, which is 50 Hz or 60Hz. The resonance on the grid is caused by variations of power grid impedance due to changes of power grid network configurations, switching of grid-connected elements such as load , capacitors, reactor banks and generators of power producing units connected to the grid .

0003

Use of resonance damping control in the grid-connected units for suppressing undesired resonances is possible. However, the power grid impedance becomes more changeable and unpredictable as larger scale renewable energies are installed at the power grid . This uncertainty makes original control parameters (coefficients used in controller) of resonance damping control in the grid-connected units unsuitable , and the resonance problem still occurs.

0004

Since the power grid resonance is a problem , there is a need for a method for identifying occurrence of power grid resonance and estimating control parameters for the resonance damping control of the grid-connected unit, which suppresses the resonance against the impedance changes on power grid.

0005

It would be advantageous to achieve improvements in the identification method of power grid resonances. In method of the prior art disclosed by Patent Literature 1 (WO2014/202077A 1 ), the identification of the power grid resonance is accomplished by Fourier Transform (FT), and the control parameters for the damping control of the grid-connected unit are estimated and updated after the calculation of FT.

Citation List

Patent Literature

0006

Patent Literature 1 : WO2014/202077A1

Summary of Invention

Technical Problem

0007

However, it takes longer time to estimate and update the control parameters. During this delay time, the unsuppressed resonance could become high so that the grid-connected units need to be disconnected from the power grid .

0008

Accordingly, it is an object of the present invention to provide an identification method and a grid-connected unit for identifying a resonance of a power grid within a shorter delay time.

Solution to Problem

0009

I n order to solve the above mentioned problem , an identification method for identifying a resonance of a power grid where a grid-connected unit is connected , the method includes, measuring an electrical quantity at a point of common coupling , calculating an envelope of a waveform of the electrical quantity, and detecting an occurrence of the resonance based on the envelope.

001 0

Moreover, in order to solve the above mentioned problem, a grid-connected unit connected to a power grid comprises a power converter having a switching device; a detecting unit measuring an electrical quantity at a point of common coupling ; a controller controlling the power converter with turn-on and turn-off of the switching device on the basis of the electrical quantity, wherein the controller detects an occurrence of the resonance based on a n envelope of the electrical quantity and then the controller performs a damping control to the power converter so that the resonance is damped.

Advantageous Effects of I nvention

0011

The identification method and grid-connected unit according to the present invention result that a resonance of a power grid is identified within a shorter delay time with detecting the resonance based on the envelope.

001 2

Other objects, features and advantages of the invention will appear from the following description with the accompanying drawings.

Brief description of Drawings

001 3

Fig . 1 shows a grid-connected unit connected to a power grid .

Fig . 2 shows an i mplementation of the grid-connected unit.

Fig . 3 illustrates a functional block diagram of the controller.

Fig . 4 shows a configuration of an identification method for power grid resonance. Fig . 5 shows corresponding waveforms in an identification method for power grid resonance.

Fig . 6 shows the various specified time interval .

Description of Embodiments

0014

Fig . 1 shows a grid-connected unit [100] connected to a power grid [ 1 02] through a power grid impedance [1 01 ] . This grid-connected unit [100] is mainly composed of a DC-to-AC converter (inverter) , and it is core component for applications as a static synchronous compensator (STATCOM) , a power conditioning system (PCS) for a photovoltaic (PV) and a wind farm (WF) , and other grid-connected applications. The power grid impedance [1 01 ] typically behaves as an inductance at fundamental frequency because of power transmission/distribution network. However, in the power transmission/distribution network, factors as network configurations and other grid-connected elements in the network also affect the behaviors of this power grid impedance [1 01 ]. Fig . 2 shows an implementation of the grid-connected unit [1 00] , which includes a power converter [21 0], which is composed of semiconductor power switching devices such as insulated gate bipolar transistors (IGBTs) [211 ] , a DC capacitor [21 2], an isolation transformer [21 3] , a current transformer (CT) [214], a voltage transformer (VT) [21 5], a controller [21 6], and a Pulse-Width Modulation (PWM) modulator [21 7] . The point that this grid-connected unit connects is referred to as Point of Common Coupling (PCC) [201 ] hereafter. The CT [214] detects a current (phase current ) i _c on the AC side of the power converter 21 0. The VT [21 5] detects a voltage v _s at PCC [201 ] .

0016

I n Fig . 2 , the grid-connected unit [1 00] accomplishes the DC-to-AC power conversions by operating the power converter [21 0] with different combinations of turn-on and turn-off states of IGBTs [211 ] , therefore creating a chopping waveform from voltage of a DC capacitor [21 2] with a typical chopping frequency of a few kHz. A sinusoidal waveform can be derived at the side of PCC [201 ] because a frequency component at the chopping frequency is filtered out by effects of the isolation transformer [21 3] . These turn-on and turn-off states are determined by the PWM modulator [21 7] with the signal generated by the controller [216] .

0017

Fig . 3 illustrates a functional block diagram of the controller [216] principle for controlling the grid-connected unit [1 00] in terms of the current reference _c _ _dq corresponding to the desired amount of current injected into the PCC [201 ]. The current reference i" _c _dq is output from the other controller. The current i _c and the voltage v _s are measured by the CT [214] and VT [21 5] respectively, and these information are processed with Automatic Current Regulator (ACR) [321 ] to realize the current regulation. Additionally, the functions shown by the blocks in Fig.3 are brought by a processing unit such as a micro-computer performing a computer program.

0018

The ACR [321] performs current regulation with proportional and integral (PI) controller, which provides zero steady-state error at DC component. However, parameters such as voltage and current in a 3-phase system are time-varying with fundamental frequency, i.e. the frequency of the power grid. In order to obtain better current regulation, the parameters in the 3-phase system are transformed into parameters in a 2-phase system, where the values are represented in a reference frame rotating with the frequency of the power grid. Therefore, Θ detection [311] detects the phase angle of the rotating reference frame for transformations [312-314], and the measured voltage v _s and current i _c are transformed into the values v _{s d(?} and i _c _dq in the rotating reference frame.

0019

Additionally, a "vector control" using d-q axis is applied to the controller [216] in the embodiment. Therefore, in Fig.3, each of the currents (i' _c _dq, ic_dq, i'cdp_dq) and voltage (v _s _ _dq ) represented by symbols with subscript "dq" has a d-axis component and a q-axis component.

0020

The difference between the current reference and the measured current i _c _ _dq is supplied to the ACR controller [321], and the decoupling compensation [322] calculates values of coupling effects, which is caused by the 3-phase to 2-phase transformation, in the 2-phase system of the rotating reference frame for decoupling control between the d-axis component and the q-axis component. In addition, the measured voltage v _s _ _dq is processed by a low-pass filter [323] to avoid voltage distortion of this feed-forward voltage affecting performance of the current regulation . The output of ACR [321 ] , the decoupling compensation [322] , and the low-pass filter [323] are added to generate the voltage reference in the 2-phase system . This 2-phase voltage reference (d-axis, q-axis) is performed with 2-phase to 3-phase transformation [314] to derive the 3-phase voltage reference for PWM modulator [21 7] in Fig. 2.

0021

I n Fig . 1 , power grid impedance [ 101 ] is affected by many factors as power grid network configurations and grid-connected elements such as load, capacitors, reactor banks and generators. These effects could result in resonance problem at the PCC [201 ]. Once this resonance occurs, different frequency waveforms appear on the fundamental frequency voltage and current. Consequently, the voltage and current of the grid-connected units are distorted .

0022

Resonance at the PCC [201 ] should be suppressed within the limits specified e.g . by a "grid code regulation". Typically the use of resonance damping control for suppressing undesired resonances is possible. In Fig. 3, a resonance damping control [331 ] uses the measured voltage v _s _ _dq which is processed by a filter having a transfer function shown in [MATH 1 ], to derive a resonance damping reference i' _C dp_dq- Herein K _dp and Q _res are control parameters of resonance damping control [331 ] . K _dp and ci _res are given by an identification method of power grid resonance [341 ] mentioned later. It is noted that the resonance damping control 331 can be implemented in other ways.

0023 0024

However, the power grid impedance [101] becomes more changeable and unpredictable as larger scale renewable energies are installed at the power grid. This uncertainty makes original control parameters in resonance damping control [331] unsuitable, and the resonance problem may still occur. In this situation, if the resonance damping control [331] does not respond to the resonance quickly, the unsuppressed resonance could become high so that the grid-connected unit 100 need to be disconnected from the power grid [102].

0025

For this purpose, the identification method of power grid resonance [341] is implemented to provide a fast identification for the resonance and it updates the estimated control parameters d _res and K _dp to the resonance damping control [331] quickly. Therefore, the grid-connected unit [100] can stabilize the voltage and current at PCC [201] against the resonance caused by uncertainty in the power grid impedance [101].

0026

Fig.4 shows a configuration of an identification method for power grid resonance [341]. The block of envelope detection and resonance identification [401] identifies the occurrence of the resonance by analyzing envelope of the measured current i _c [411], and on the basis of the analyzed results (waveforms [412], [511] in Fig. 5), control parameters estimation [402] estimates control parameters including resonance frequency [414] and control gain [416]. The block of updating control parameters [403] take charge of updating control parameters (co _res [415], K _dp [417]) to the resonance damping control [331].

0027 Fig.5 shows corresponding waveforms in an identification method for power grid resonance [341]. The measured current i _c [411] operates at the fundamental frequency, which is 50Hz or 60Hz before the resonance occurs. Once the resonance occurs, a different frequency waveform occurs on the top of fundamental frequency current. Namely, the waveform having a resonance frequency higher than the fundamental frequency is superimposed to the fundamental frequency current waveform. The block [401] (Fig. 4) takes off the fundamental component from the measure current i _c [411] and obtains waveform [412], and then this block [401] also generates envelope waveform [511] based on the waveform [412], and the method monitors variation of this envelope [511] during a specified time interval [512]. Consequently, the block [401] detects the resonance as the variation of the envelope waveform [511] continues increasing during the specified time interval [512], and then changes identification status of resonance [413] (cf. Fig.4).

0028

The envelope [511] is calculated with a well-known means as follows. The measured current i _c [411] is squared and then a square root of the squared i _c is calculated. A high frequency (resonance frequency) component of the square root is eliminated with a LPF (Low Pass Filter).

0029

On the basis of analyzed result, the block of control parameters estimation [402]

(Fig.4) estimates the resonance frequency S _res [414] and the control gain K _dp [416] by using the waveforms [412] and [511]. For example, the resonance frequency is estimated on the basis of the frequency of waveform [412], and the control gain is estimated on the basis of the amplitude of waveform [412] or [511].

0030 The resonance frequency is estimated with means as follows. Zero-crossing points in the waveform [412 ] in Fig .5 are detected . Then , time intervals between any two adjacent zero-crossing points are calculated . One time interval corresponds to a half cycle of oscillating waveform [41 2] . Consequently, the resonance frequency is estimated with using the calculated time intervals.

0031

As the resonance is identified, the updated resonance frequency [41 5] and the updated control gain [417] are updated by the estimated resonance frequency [414] and the estimated control gain [416] to replace original control parameters. After the update of control parameters, the resonance is suppressed by the resonance damping control [331 ] (Fig . 3) with the quickly updated control parameters [41 5] and [41 7] .

0032

The time interval [51 2] in Fig .5 is specified by a constant, while the time interval could be specified according to the corresponding values of measured current i _c [41 1 ] for an accurate detection of the occurrence of resonance. Fig. 6 shows the various specified time interval [512] . For example, the peak value of measured cu rrent i _c [41 1 ] could be applied to specify the time interval [51 2]. As the peak value is high (upper waveform in Fig . 6) , the specified time interval [512] decreases, therefore identification time of the resonance is shorter. On the other hand , if the peak value is low (lower waveform in Fig . 6), the specified time interval [51 2] increases, and then identification time of the resonance is longer.

0033

With the above-mentioned embodiments, the resonance of the power g rid is identified within a shorter delay time. Additionally, the occurrence of the resonance is detected with a shorter time, and the control parameters are updated with a shorter time. Therefore, the grid-connected unit responses to the change of the power grid impedance quickly and it avoids the disconnection of g rid-connected unit caused by the resonance.

0034

While the invention has been illustrated and described in detailed in the drawings and description , such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood by those skilled in the art in practicing the claimed invention , from a study of the drawings, the disclosure, and the appended claims.

0035

For example, a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a GTO (Gate Turn-Off Thyristor) are applied to the power converter [21 0] in Fig . 2 instead of the IGBT. Additionally, the resonance frequency and the control gain can be estimated on the basis of other measured electrical quantity such as the measured voltage v _s instead of the measured current / _c .

Reference Signs List

0036

1 00 Grid-connected unit

101 Power grid impedance

102 Power grid

201 Point of common coupling

21 0 Power converter

21 1 IGBT 212 DC capacitor

21 3 Isolation transformer

214 Current transformer

21 5 Voltage transformer

216 Controller

21 7 Pulse-width modulation modulator

41 1 Measured current

412 Waveform

512 Specified time interval